Hydrologic Regulation and Water Purification

Declining hydrologic services related to water supply and quality represent a major threat to the well-being of human populations in many regions of the worldReference Scanlon, Jolly, Sophocleous and Zhang⁴³, and have been directly linked to the expansion of intensively managed annual crops at the expense of diverse perennial vegetationReference Tilman, Fargione, Wolff, D'Antonio, Dobson, Howarth, Schindler, Schlesinger, Simberloff and Swackhamer⁴⁴. In the Midwestern US, landscape-scale conversion from perennial to annual crops has resulted in significantly increased streamflow and base flowReference Schilling and Libra⁴⁵, increased sedimentReference Knox⁴⁶ and nutrientReference David, McLsaac, Darmody and Omonode⁴⁷ delivery to streams, and the formation of hypoxic zones in the Gulf of MexicoReference Rabalais, Diaz, Levin, Turner, Gilbert and Zhang⁴⁸.

Perennial plants can restore water purification and flow regulation services to these agriculturally dominated landscapes through several mechanisms. In humid temperate climates such as the Midwestern US, where excess water is often a problem, especially in the early growing season before annual crops have become established, the deeper roots and longer growing season typical of perennial vegetation results in greater total water uptake and evapotranspiration compared with annual cropsReference Anderson, Udawatta, Seobi and Garrett^49, Reference Brye, Norman, Bundy and Gower⁵⁰. Additionally, perennial vegetation can improve soil structure and hydraulic properties by increasing the number and size of macroporesReference Seobi, Anderson, Udawatta and Gantzer^51–Reference Liebig, Johnson, Hanson and Frank⁵³, building organic matterReference Liebig, Johnson, Hanson and Frank^53, Reference Tufekcioglu, Raich, Isenhart and Schultz⁵⁴ and improving soil porosityReference Hernandez-Santana, Zhou, Helmers, Asbjornsen, Kolka and Tomer^55, Reference Wang⁵⁶. Combined, these changes in plant water fluxes and soil properties contribute to greater soil water storage capacityReference Schilling, Jha, Zhang, Gassman and Wolter⁵⁷, higher infiltration ratesReference Bharati, Lee, Isenhart and Schultz⁵⁸ and reduced runoffReference Steward, Yang, Lauwo, Staggenborg, Macpherson and Welch⁵⁹. Perennial grassesReference Blanco-Canqui, Gantzer, Anderson and Alberts^60, Reference Helmers, Zhou, Asbjornsen, Kolka, Tomer and Cruse⁶¹, treesReference Leguedois, Ellis, Hairsine and Tongway⁶² and tree–grass mixturesReference Lee, Isenhart and Schultz⁶³ have also been shown to more effectively trap and retain sediment compared with annual crops.

Critical to developing effective perennialization strategies is a sound understanding of how much of the landscape needs to be converted to perennial cover, where such conversions are best located and which species mixtures are most effective. Studies based on both modelingReference Dosskey, Helmers, Eisenhauer, Franti and Hoagland^64, Reference Sahu and Gu⁶⁵ and empirical measurementsReference Liu, Mang and Zhang^66, Reference Zhang, Liu, Zhang, Dahlgren and Eitzel⁶⁷ suggest a nonlinear relationship between the amount of perennial cover and different hydrologic services, resulting in diminishing returns beyond a certain amount. Such nonlinear relationships have been demonstrated for sediment trapping efficiencyReference Helmers, Zhou, Asbjornsen, Kolka, Tomer and Cruse^61, Reference Lee, Isenhart and Schultz^63, Reference Schmitt, Dosskey and Hoagland^68, Reference Abu-Zreig, Rudra, Lalonde, Whiteley and Kaushik⁶⁹, retention of nitrogen (N), phosphorus (P)Reference Schmitt, Dosskey and Hoagland^68, Reference Mayer, Reynolds, McCutchen and Canfield^70, Reference Zhou, Helmers, Asbjornsen, Kolka and Tomer⁷¹ and pesticidesReference Patty, Real and Gril⁷², and runoff reductionReference Hernandez-Santana, Zhou, Helmers, Asbjornsen, Kolka and Tomer^55, Reference Wang⁵⁶. Combined, these findings underscore the notion that relatively small amounts of perennial cover may produce significant ES benefits, while minimizing negative impacts on crop production (Fig. 1).

The location of perennial vegetation in the landscape is critical in determining the lateral and vertical movement of water through watersheds. For example, Sahu and GuReference Sahu and Gu⁶⁵ used a modeling approach to show that strips occupying 10–20% of the catchment area were more efficient in reducing NO₃–N when placed along the contour than along the riparian zone. Jiang et al.Reference Jiang, Anderson, Kitchen, Sadler and Sudduth⁷³ found that perennial plants located on the backslope position showed greater improvement in hydraulic properties compared with summit and footslope positions, suggesting that targeting perennial plantings to slope positions that are most vulnerable to degradation may provide the greatest benefit–cost returns. Another important consideration is that hydrologic flows generally are not distributed uniformly along buffer strip edgesReference Dosskey, Helmers, Eisenhauer, Franti and Hoagland^64, Reference Tomer, James and Isenhart⁷⁴, and soil water content may also vary with topographic positionReference McGee, Peterson and Westfall⁷⁵. Thus, strategically locating perennial vegetation to maximize flow interception can optimize hydrologic regulation and water purification benefitsReference Tomer, Dosskey, Burkart, James, Helmers and Eisenhauer⁷⁶. Nonetheless, placement of buffer strips on the landscape under current practices is often dictated by economic factorsReference Yang and Weersink⁷⁷, and more work is needed to incorporate approaches for identifying landscape positions for maximizing ES gains into policy and management decisions.

Few studies have assessed the interactive effects of both amount and location of perennial cover on the hydrologic functioning at the watershed scale. One study in central Iowa assessed the effects of varying amounts (10% or 20% of total watershed area) and locations (20% all at the bottom or 20% distributed in contour strips) of native perennial prairie vegetation on sediment and nutrient loss from experimental watershedsReference Geza, Barfield, Huhnke, Stoecker, Storm and Stevens⁷⁸. During an extreme rainfall year (2010), watersheds having 100% annual crops had up to 35-fold greater sediment loss (6412 kg ha⁻¹) compared with watersheds having perennial strips (180–314 kg ha⁻¹)Reference Helmers, Zhou, Asbjornsen, Kolka, Tomer and Cruse⁶¹. In a relatively dry year (2009), the differences among treatments were also substantial: 4114 kg ha⁻¹ sediment lost from watersheds having 100% crops compared with 129–173 kg ha⁻¹ from watersheds with perennial strips. Similar results were observed for N: during a wet year (2008) 100% cropped watersheds had 8-fold greater N export (83 kg ha⁻¹) relative to watersheds with perennial strips (8.1–12.3 kg ha⁻¹); the difference was 6-fold during a dry year (2009). Patterns observed for P losses were even more pronounced, with 100% cropped watersheds having 11-fold and 16-fold greater P export in 2008 and 2009, respectively, compared with watersheds containing perennial strips. Annual total runoff was 1.6 times greater for watersheds with 100% crops compared with watersheds with the perennial stripsReference Hernandez-Santana, Zhou, Helmers, Asbjornsen, Kolka and Tomer^55, Reference Wang⁵⁶. Across all measured variables and years, watersheds with 10% perennial cover at the bottom unexpectedly performed slightly better than watersheds with 20% perennial cover distributed as contour strips, and the reason for this is believed to be due to a larger area in perennial cover at the footslope position for the watersheds with 10% perennial cover at the bottom. The 10% perennial cover at the bottom treatment had the greatest perennial cover at the footslope position rather than intermixed within the watershed so likely provided the greatest protection in reducing sediment and nutrient loss. However, over time the contour strips distributed within the watershed may provide a naturally terracing effect, which could improve the performance of this treatment. Overall, the findings from this study highlight the potential short-term (2–4 yr) benefits provided by strategically integrating small amounts (as little as 10% of the entire watershed) of perennial strips within annual cropping systems. More long-term monitoring is needed to understand the long-term functioning of these strips once they become well established.

Although we are unaware of other watershed scale field studies addressing interactions between both amount and location of perennial cover, several other approaches have been used to identify landscape locations for targeting perennial cover to maximize benefits. Geza et al.Reference Geza, Barfield, Huhnke, Stoecker, Storm and Stevens⁷⁸ used the Soil and Water Assessment Tool (SWAT) to evaluate environmental and economic trade-offs between placing grass filter strips along field drains or targeting their placement to environmentally sensitive areas. While the authors found that placement of the filter strips along field drains was more cost effective, they concluded that the targeted approach would be more environmentally beneficial due to the greater potential for enhancing soil conservation. Terrain analysis has been used to identify sites with large wetness indicesReference Tomer, James and Isenhart⁷⁴, and soil survey and topographic and streamflow data have been combined to develop a system for prioritizing locations for maximizing sediment trapping and surface runoff and/or shallow groundwater flow interceptionReference Tomer, Dosskey, Burkart, James, Helmers and Eisenhauer⁷⁶. Geographic information systems and multiple criteria ranking procedures have been used to identify the intersection of hydrologically sensitive and pollutant source areas to develop decision-making processes for effective buffer placementReference Qiu⁷⁹. While these studies have generated similar conclusions—that targeting perennial cover to key areas in agricultural landscapes maximizes environmental benefits—supporting empirical data are lacking for a range of climatic and edaphic conditions.

The selection of particular species or functional groups (e.g., trees versus grasses) can also affect the hydrologic functioning of perennial vegetation, but information is relatively scarce. Studies comparing the hydrologic functions of different species of herbaceous perennials suggest that species may vary substantially in their ability to reduce overland flowReference Wu, Huang, Teng and Sardo⁸⁰ and herbicide leachingReference Belden and Coats^81, Reference Smith, Putnam, Phaneuf, Lanza, Dhankher and Clark⁸², and to increase sediment trapping efficiencyReference Blanco-Canqui, Gantzer, Anderson and Alberts^60, Reference Meyer, Dabney and Harmon^83, Reference Wilson, Cruse and Burras⁸⁴ and water uptakeReference Brye, Norman, Bundy and Gower⁸⁵. The effectiveness of grass buffers versus mixed grass–tree buffers appears to vary, with some studies suggesting a similar capacity to remove N and PReference Duchemin and Hogue^9, Reference Daniels and Gilliam⁸⁶, while others report greater N and P removalReference Seobi, Anderson, Udawatta and Gantzer^51, Reference Lee, Isenhart and Schultz⁶³ and greater infiltration ratesReference Schilling, Jha, Zhang, Gassman and Wolter^57, Reference Udawatta, Krstansky, Henerson and Garrett⁸⁷ under mixed tree–grass riparian buffers compared with grass-only buffers. A paired watershed study found that after 9 years, grass buffer strips significantly decreased runoff by 8.4%, yet there was no significant effect of agroforestry buffer strips, relative to the controlReference Veum, Goyne, Motavalli and Udawatta⁸⁸. One reason for these contrasting results may be due to the different ages of the tree-based buffers as trees require more time to fully establish before they exhibit positive effects on hydrologic functionsReference Gajic, Dugalic, Sredojevic and Tomic⁸⁹. Further, research is lacking on the potential competitive interactions between trees and adjacent crops in temperate agroforestry systems, as water, nutrient and light resources available for crop growth may become more limitingReference Jose, Gillespie and Pallardy⁹⁰. Overall, improved understanding is needed of how differences in functional type and species composition of perennial vegetation affect hydrologic performance to better select the most suitable mixtures for achieving specific perennialization objectives.

In other climatic regions worldwide, the presence of perennial vegetation in agricultural landscapes can enhance hydrologic services, but impacts vary widely depending on site-specific agroecosystem–climate interactions. For example, in the humid tropics, many traditional agroecosystems comprise combinations of different perennial and annual plants with high structural and compositional complexity, and are often designed to both meet local subsistence needs while simultaneously producing a few specialty high-value market cropsReference Kumar and Nair⁹¹. Two classical market-oriented production systems, shade coffee and cacao plantations, which target the production of perennial crops while maintaining relatively high native species diversity, have been shown to maintain similar hydrologic services compared with the native forestsReference Godsey and Elsenbeer^92–Reference Zimmermann, Elsenbeer and De Moraes⁹⁴. Hanson et al.Reference Hanson, Steenhuis, Walter and Boll⁹³ showed that following conversion of primary forests to grassland, soils had lower saturated hydraulic conductivity and more frequent overland flow compared with soils under traditional shade-grown coffee plantations, while the latter maintained high infiltration capacities and readily conducted water vertically, similar to conditions under primary forest. However, as a result of increasing market pressures and land-use intensification, many shade-grown agroforestry systems are being converted to less diverse and more structurally homogeneous sun-grown plantationsReference Muschler and Bonnemann^95, Reference Perfecto, Rice, Greenberg and Van Der Voort⁹⁶ or, in some cases, to annual or short duration crops such as sugarcaneReference Lindsay and Rocha^97, Reference Thomlinson, Serrano, Lopez, Aide and Zimmerman⁹⁸ or oil palmReference Danielsen, Beukema, Burgess, Parish, Bruhl, Donald, Murdiyarso, Phalan, Reijnders, Struebig and Fitzherbert⁹⁹. These transitions from diverse perennial to simplified monocultural systems are often accompanied by negative consequences for hydrologic servicesReference Martinez, Perez-Maqueo, Vazquez, Castillo-Campos, Garcia-Franco, Mehltreter, Equihua and Landgrave¹⁰⁰. Notably, this example underscores the interlinked nature of biodiversity and perenniality: shade-grown coffee systems are typically managed for multiple products in addition to coffee, many for local consumption (e.g., fruits, herbs and medicinals), whereas a transition to sun-coffee generally involves more intensive management of a single product—coffee. Although difficult to disentangle, especially given the lack of controlled studies, it is likely that many of the ES that are enhanced by perennial systems are inextricably linked with the concomitant increase in biodiversity.

In contrast to humid climatic regions, in arid environments the extraction of too much water by perennials can be problematic for water supply, as water use by forests and shrublands is almost always greater than by croplands or grasslandsReference Zhang, Kang and Zhang¹⁰¹ and planting trees typically reduces annual water yieldReference Farley, Jobbagy and Jackson¹⁰². In China, where large-scale revegetation initiatives in arid regions have resulted in the planting of millions of hectares of treesReference Yang, Zhang, Jia and Ci¹⁰³, a recent study found that the total water loss due to transpiration from a poplar plantation was 100% greater than that of adjacent shrublandReference Wilske, Lu, Wei, Chen, Zha, Liu, Xu, Noormets, Huang, Wei, Chen, Zhang, Ni, Sun, Guo, McNulty, John, Han, Lin and Chen¹⁰⁴. Controversy over tree planting and water supply is especially acute concerning exotic species, with much of the debate focusing on fast-growing eucalyptus and pine speciesReference Casson¹⁰⁵. Crucial to determining the overall cost–benefit relationship of planting trees in drylands will be quantifying the net trade-offs between increased transpiration, on the one hand, and improved infiltration and water storage capacity and reduced runoff on the otherReference Illstedt, Malmer, Elke and Murdiyarso¹⁰⁶. Notably, trees in semi-arid regions may provide many ES that outweigh their increased water use, including fuelwood, timber poles, nutritional products and windbreaksReference Davidson¹⁰⁷, improving soil properties such as porosityReference Oyedele, Awotoye and Popoola¹⁰⁸, infiltrationReference Hansson^109, Reference Torquebiau and Kwesiga¹¹⁰, water storage capacityReference Gebresamuel, Singh and Dick¹¹¹, and nutrient cycling and soil fertilityReference Rao, Mafongoya, Kwesiga and Maghembe¹¹². Trees may also increase water content in drylands by ameliorating microclimate and reducing soil evaporationReference Sanou, Zougmore, Bayala and Teklehaimonot^113, Reference Kho, Yacouba, Yaye, Katkore, Moussa, Iktam and Mayaki¹¹⁴, or by hydraulic lift or redistributionReference Bayala, Heng, van Noordwijk and Ouedraogo^115–Reference Ludwig, Dawson, de Kroon, Berendse and Prins¹¹⁷.

In semi-arid areas of Africa, the positive effects of trees in cropping systems are exemplified by farmers’ use of Faidherbia (Acacia) albida, a leguminous tree with a reverse phenological pattern relative to most other trees (e.g., leaves drop in the wet season and flush in the dry season), which may reduce competition for resources with crops during the growing season. Crop yields in grain fields with interplanted F. albida trees have increased by over 200%, even without additional fertilizer inputs, while these mixed tree–food crop systems also provide multiple benefits such as fuelwood, fodder and enhanced food security during severe droughtReference Garrity, Akinnifesi, Ajayi, Weldesemayat, Mowo, Kalinganire, Larwanou and Bayala^118, Reference Syampungani, Chirwa, Akinnifesi and Ajayi¹¹⁹. Similarly, in dry temperate climate regions in China, widespread incorporation of perennial plants into annual row crops is occurringReference Qiao, Chen, Hong, Li and Liu^120, Reference Zhao, Shanm, Deng, Zhao, Zhang and Wang¹²¹, even though most of these lands do not have irrigation. These perennial plantings are being used to reduce sediment and pollutant exportReference Wang, Yin and Shan¹²², stabilize dunesReference Qiao, Chen, Hong, Li and Liu¹²⁰, conserve soil water and nutrientsReference Guo, Huang, Jiang and Lv¹²³, enhance N-fixationReference Xin¹²⁴ and supplement local economic incomesReference Wilske, Lu, Wei, Chen, Zha, Liu, Xu, Noormets, Huang, Wei, Chen, Zhang, Ni, Sun, Guo, McNulty, John, Han, Lin and Chen^104, Reference Bao, Wu and Zhang¹²⁵. Another positive effect of planting deep-rooted perennials in some drylands, such as southern Australia and the African Sahel, is to alleviate problems of salinization of soils and groundwater by lowering water tablesReference Scanlon, Jolly, Sophocleous and Zhang^43, Reference Gordon, Peterson and Bennett¹²⁶. Studies have also demonstrated the influence of different typesReference Pan, Ma and Shangguan^127, Reference Wang, Yin and Shan¹²⁸ and speciesReference Dass, Sudhishri, Lenka and Patnaik^129, Reference Vigiak, Ribolzi, Pierret, Sengtaheuanghoung and Valentin¹³⁰ of perennial vegetation on enhancing hydrologic functions and reducing sediment and nutrient losses in these regions; however, relatively little work has specifically examined the importance of landscape position.

The impacts of incorporating perennial vegetation within cropping systems on hydrologic services are clearly being documented across different regions and agroecosystems, and these studies are highlighting the importance of considering both the local climate–vegetation interactions and management objectives when selecting suitable perennialzation approaches. Opportunities for significant hydrologic benefits through perennialization may be particularly strong in humid regions where enhanced hydrologic regulation is a priority, while significant ES gains are also being achieved in dryland regions through strategic placement of small amounts of perennial vegetation. More work is required to better understand how varying the spatial location, type and amount of perennial vegetation can differentially influence hydrologic outcomes, especially at the watershed scale, in order to maximize benefits under diverse climatic conditions.

Climate Regulation and Climate Change Mitigation and Adaptation

Although agroecosystems can provide important climate regulation services through carbon (C) sequestration and storage, they can also increase greenhouse gas emissions as a consequence of biomass burning or accelerated decomposition during forest conversion processes, as well as from chemical and energy inputsReference Canadell, Le Quere, Raupach, Field, Buitenhuis, Ciais, Conway, Gillett, Houghton and Marland^131, Reference Grace, Robertson, Millar, Colunga-Garcia, Basso, Gage and Hoben¹³². Whether an agroecosystem is a net carbon sink or source is directly influenced by management practices and their interaction with local biophysical and climatic conditionsReference Bailey, Motavalli, Udawatta and Nelson¹³³. As diverse perennial plant communities typically store more carbon than annual or short-rotation cropping systemsReference Zan, Fyles, Girouard and Samson^10, Reference Tolbert, Todd, Mann, Jawdy, Mays, Malik, Bandaranayake, Houston, Tyler and Pettry¹³⁴, perennialization provides an opportunity for mitigating climate change while sustaining agricultural productivity, especially when targeting those species combinations and landscape positions that maximize potential carbon sequestration benefits.

In the Midwestern US, the native tallgrass prairie ecosystem historically stored large amounts of carbon belowground relative to the annual cropping systems they replaced. Conversion of prairie to agriculture resulted in an estimated loss of 24–89% of the original soil organic C and total NReference Jelinski and Kucharik^135–Reference West and Post¹³⁸. Estimated annual accumulation rates of soil organic C following reestablishment of perennial grasslands on former agricultural lands vary greatly, from 19.7 to 78 g C m⁻² yr⁻¹ in the upper 10 cm soil horizonReference Knops and Tilman^136, Reference Baer, Rice and Blair^139–Reference McLauchlan¹⁴², with average reported rates around 30 g C m⁻² yr⁻²Reference Post and Kwon^143, Reference Schlesinger¹⁴⁴. However, rebuilding soil organic pools to pre-agriculture levels will require time. For the Great Plains region, McLauchlanReference McLauchlan¹⁴² estimated that reestablishment of perennial grasslands on former agricultural lands could rebuild soil organic C pools to levels equivalent to unplowed native prairie within 55–75 years, while Matamala et al.Reference Matamala, Jastrow, Miller and Garten¹⁴⁵ estimated that within a century restored tallgrass prairie vegetation in Illinois would reach 50% of its soil C storage potential. Differences in C accrual rates across sites and soil depths due to variation in root distribution, rhizosphere activityReference Culman, DuPont, Glover, Buckley, Fick, Ferris and Crews^146, Reference Wilson, Barnes, Koen, Ghosh and King¹⁴⁷ and vertical retranslocation of soil CReference David, McLsaac, Darmody and Omonode^47, Reference Culman, DuPont, Glover, Buckley, Fick, Ferris and Crews¹⁴⁶ may account for some of this variability.

Notably, C storage benefits are quickly lost when perennial vegetation reverts to annual crops, leading to rapid releases of CO₂ and other greenhouse gases to the atmosphereReference Grandy and Robertson^140, Reference Sequin, Arrouays, Balesdent, Soussana, Bondeau, Smith, Zaehle, de Noblet and Viovy¹⁴⁸. Gelfand et al.Reference Gelfand, Zenone, Jasrotia, Chen, Hamilton and Robertson¹⁴⁹ examined the greenhouse gas consequences of converting land under the Conservation Reserve Program (CRP) to continuous corn, corn–soybean or perennial grass for biofuel production. They found that projected C debt repayment periods under no-till management ranged from 29 to 40 years for corn–soybean and continuous corn, respectively, whereas the use of existing CRP grasslands for cellulosic feedstock production would avoid C debt entirely and provide modest climate change mitigation immediately.

Landscape position can also influence the rate of C sequestration and storage potential of perennial vegetation. Some studies suggest that C sequestration potential may be maximized by placing perennial cover in topographical positions with higher moisture availability, such as footslopes and riparian areasReference Jelinski and Kucharik^135, Reference Weaver¹⁵⁰. On the other hand, because cultivation of sensitive areas such as shoulder and backslope positions may accelerate erosion of soil organic matterReference Aguilar, Kelly and Heil^151, Reference Pennock and van Kessel¹⁵², targeting topographical positions that are vulnerable to erosion can promote C retention in the soil. More work is needed to refine understanding of the site-specific features that influence the relationship between topographic position and C sequestration.

Carbon sequestration potential in soils and plant biomass may also vary depending on both species identityReference De Deyn, Quirk, Yi, Oakley, Ostle and Bardgett¹⁵³ and functional compositionReference Fornara and Tilman¹⁵⁴ of the vegetation. In general, trees and shrubs sequester and store greater amounts of C compared with herbaceous plants due to their larger total above- and below-ground biomassReference Peichl, Thevathasan, Gordon, Huss and Abohassan¹⁵⁵. In the Midwestern US, riparian buffer systems consisting of mixtures of native herbaceous and woody species sequestered significantly greater amounts of C relative to crop fields or cool-season grass buffersReference Tufekcioglu, Raich, Isenhart and Schultz⁵⁴. Nevertheless, grassland vegetation can be highly effective at accumulating C by depositing large amounts of organic matter deeper in the soil horizon where it is protected from decompositionReference Matamala, Jastrow, Miller and Garten^145, Reference Bashkin and Binkley¹⁵⁶. Conversely, woody plants typically deposit a large fraction of total organic matter inputs on the ground surface, where decomposition occurs more rapidlyReference Post and Kwon¹⁴³. Knowledge of how different species and functional groups affect rates of C sequestration, patterns of partitioning of C between above- and belowground biomass, and soil organic C accrual and depletion rates is important to take into account when using perennial vegetation as part of climate change mitigation strategiesReference Laungani and Knops¹⁵⁷.

Not only do perennials store additional C relative to annual or simplified cropping systems, perennial plant communities may also enhance agroecosystem resilience and stability to environmental fluctuations, especially when they include high levels of biodiversityReference Lin^22–Reference Tilman, Hill and Lehman^24, Reference Wolters, Silver, Bignell, Coleman, Lavelle, Van der Putten, de Ruiter, Rusek, Wall, Wardle, Brussaard, Dangerfield, Brown, Giller, Hooper, Sala, Tiedje and van Veen¹⁵⁸. In the Midwestern USA, conservation agriculture practices are increasingly promoting the use of diverse mixtures of native tallgrass prairie species rather than plantings of a single native or introduced species typically used in the past (e.g., Bromus inermis, brome; Bouteloua cuttipendula, side oats; and Panicum virgatum, switchgrass).

Research suggests that grass and forb species typical to the native tallgrass prairie possess a variety of ecophysiological traits and adaptive strategies that enhance resilience and resistance to climatic extremes, such as high water use efficiencyReference Knapp^159–Reference Nippert, Fay, Carlisle, Knapp and Smith¹⁶¹, the capacity to senesce and resproutReference Novoplansky and Goldberg^162, Reference Xu and Baldocchi¹⁶³, deep rootsReference Tufekcioglu, Raich, Isenhart and Schultz^164, Reference Weaver¹⁶⁵ and high ecological plasticityReference Asbjornsen, Mora and Helmers^166, Reference Hayes and Seastedt¹⁶⁷. In addition, greater vertical and horizontal heterogeneity in more diverse plant communities allows for niche partitioning and complementary use of resourcesReference Nippert and Knapp¹⁶⁸, which on the landscape scale can help buffer ecosystems against extreme climate fluctuationsReference van Nes and Scheffer¹⁶⁹. In contrast, annual crops in temperate humid climates are particularly vulnerable to extreme weather conditions, due to a combination of shallow rooting, a relatively short (3–4 months) growth period, high water demand, and high sensitivity to drought and heat stressReference Benjamin and Nielsen^170–Reference Traore, Carlson, Pilcher and Rice¹⁷⁴. Perennial plant communities with high species diversity may also have a greater probability of containing species with traits adapted to environmental changes, thus conferring greater ‘insurance’ relative to plant communities with low species diversity or monoculturesReference Hooper and Vitousek^175, Reference Naeem and Li¹⁷⁶. Some experimental support for enhanced resilience in diverse perennial plant communities in the context of climate change is emergingReference De Boeck, Lemmens, Bossuyt, Malchair, Carnol, Merckx, Nijs and Ceulemans^177–Reference Van Peer, Nijs, Reheul and De Cauwer¹⁸⁰, especially in terms of greater temporal stabilityReference Tilman, Hill and Lehman²⁴. Nonetheless, the relationship between biodiversity and the sustainability of ES is complexReference Lin, Perfecto and Vandermeer^27, Reference Swift, Izac and van Noordwijk¹⁸¹, and more work is needed to better design integrated annual–perennial landscapes capable of adapting to future climate change while sustaining diverse ES.

In the humid tropics, conversion of forests and traditional agroforestry systems to more intensive agricultural systems has contributed substantially to global greenhouse gas emissions and changes in C stocksReference DeFries and Rosenzweig^182, Reference Gibbs, Brown, Niles and Foley¹⁸³. In Central America and Mexico, agroforestry systems that maintain a significant tree component, such as shade coffee and cacao plantations, home gardens and tree farms, are particularly effective at sequestering and storing C, while providing diverse agricultural productsReference Segura, Kanninen and Suarez^184, Reference Soto-Pinto, Anzueto, Mendoza, Ferrer and de Jong¹⁸⁵. Kirby and PotvinReference Kirby and Potvin¹⁸⁶ found that while native forests in Panama managed with selective logging stored more than twice as much C than forests managed under traditional agroforestry practices, agroforests stored three times more C than pasture. Expanding agroforests into pasture could provide additional biodiversity and livelihood benefits that the most common reforestation system in the region—monoculture teak plantations—does not provide. A meta-analysis examining the potential for soil organic C accumulation through afforestation found that planting trees on croplands resulted in more pronounced increases in soil organic C stocks relative to pastures or grasslands, and that broadleaved tree species had a greater capacity to accumulate soil organic C than coniferous speciesReference Laganiere, Angers and Pare¹⁸⁷.

In dry climatic regions where plant growth is relatively slow and total plant biomass is low, the potential for sequestering and storing C per unit land area is generally less than in moist climatic regions. However, the total area of degraded drylands that could potentially be restored to perennial vegetation cover is so large that it constitutes a significant potential C sinkReference Berndes¹⁸⁸. Studies in dryland regions of Africa have shown that the amount of soil organic matter below tree canopies is significantly greater compared with open areas due to high inputs of leaf litter and higher root massReference Bayala, Balesdent, Marol, Zapata, Teklehaimanot and Ouedraogo^189–Reference Takimoto, Nair and Nair¹⁹¹. In the case of evergreen agricultural systems in semi-arid Africa (discussed previously), incorporating Faidherbia albida trees has been shown to accumulate between 2–4 Mg ha⁻¹ yr⁻¹ of above- and below-ground C, which represents a 10-fold increase over conventional conservation farming systemsReference Kaonga and Bayliss-Smith^192–Reference Makumba, Akinnifesi, Janssen and Oenema¹⁹⁴. Targeting of woody perennials in dryland regions for enhanced C storage should focus on the preservation or introduction of widely dispersed trees within agroecosystems in order to maximize total ES benefits, while maintaining a favorable balance between carbon–water trade-offs.

In semi-arid regions of Africa, the traditional agroforestry parklands are also used by local farmers to adapt and reduce vulnerability to climatic fluctuations and other extreme weather conditions. These parklands typically comprise several species and genera that constitute important sources of medicine, fruits, oils, leaves, nuts and spices that are main components of the local dietReference Bayala, Heng, van Noordwijk and Ouedraogo^115, Reference Bayala, Teklehaimanot and Ouedraogo^195–Reference Jonsson, Ong and Odongos¹⁹⁷, while also providing fuelwood, feed and an additional income sourceReference Kumar and Nair⁹¹. Similarly, shade coffee agroforestry systems in the humid tropics have been shown to provide better protection for crop plants from extremes in microclimate and soil moisture relative to low-shade (10–30%) systems, thus representing an important adaptive strategy for farmers to future climatic extremesReference Lin¹⁹⁸. As woody trees and shrubs are often better able to survive extreme weather events such as droughts or floods, they are particularly important for meeting the nutritional and health needs of local people during times of food scarcity.

Although the potential for C sequestration in agricultural soils worldwide has been estimated to be relatively modest, representing approximately 3–6% of fossil fuel C emissions, practices that enhance C storage within agricultural landscapes are considered an important component of climate change mitigation strategiesReference Hutchinson, Campbell and Desjardins¹⁹⁹. Globally, a meta-analysis by Ogle et al.Reference Ogle, Breidt and Paustian²⁰⁰ found that soil organic C was most sensitive to management impacts in tropical moist regions, followed by tropical dry, temperate moist and, lastly, temperate dry regions. Further, the tropics lose nearly two times as much C and produce less than one-half the annual crop yield compared with temperate regionsReference West, Gibbs, Monfreda, Wagner, Barford, Carpenter and Foley²⁰¹. This situation underscores the importance of avoiding clearing more tropical forests for agriculture while balancing agricultural intensification with targeted opportunities for C sequestration through perennialization of existing agricultural landsReference DeFries and Rosenzweig¹⁸². Another promising approach to promoting greater C sequestration on agricultural lands in both temperate and tropical regions is the use of perennial plants as biofeedstock for cellulosic ethanol production (potentially reducing the use of fossil fuels) combined with application of the charcoal by-product—the biochar—to agricultural soils to increase C storage over long time periods, given that biochar is highly recalcitrant to degradation (see the ‘Provisioning Services’ section for a more detailed discussion)Reference Laird^202, Reference Atkinson, Fitzgerald and Hipps²⁰³.

Biotic Regulation

Perennialization can increase landscape complexity and resource heterogeneity, thereby promoting habitats for diverse communities of beneficial organisms that help control pests and pathogens, and provide pollination services in adjacent crop fieldsReference Fiedler, Landis and Wratten²⁰⁴. Perennials also contribute to food web services by providing habitat for organisms that are a food source for auxiliary biota in the productive or semi-natural sub-systems, and for biota that feed on crop pestsReference Pluess, Opatovsky, Gavish-Regev, Lubin and Schmidt-Entling²⁰⁵.

As perennial vegetation is generally subject to lower levels of disturbance and is more stable over time than are ephemeral annual crop systems (with some exceptions, such as severe over-grazing and hay and grass silage production systems subjected to frequent harvests), various forms of perennial cover can provide critical elements of ‘conservation biological control’ strategies that seek to retain and facilitate populations of predators and parasitoids that attack arthropod pests in adjacent croplandReference Bianchi, Booij and Tscharntke^206–Reference Landis, Wratten and Gurr²⁰⁸. Both within-field and around-field perennial vegetation can maintain and increase the impacts of natural enemies of crop pests through the provision of alternative prey species, pollen and nectar used as protein and energy sources, and habitat for over-wintering and reproductionReference Fiedler and Landis^11, Reference Gurr, Wratten and Luna^209–Reference Woltz, Isaacs and Landis²¹². Within-field diversification with perennial plants can be set up directly by individual farmers, whereas around-field diversification with perennials can reflect the cumulative impacts of multiple landowners and land managers on landscape structure.

A number of studies indicate that increasing the proportion of landscapes occupied by woodlots, hedgerows, old-field fallows and pastures can enhance biological control of crop pests in fields embedded within those forms of perennial vegetationReference Bianchi, van Wingerden, Griffioen, van der Veen, van der Straten, Wegman and Meeuwsen^213, Reference Thies and Tscharntke²¹⁴. Tscharntke et al.Reference Tscharntke, Steffan-Dewenter, Kruess and Thies²¹⁵ reported that parasitism of rape pollen beetles (Meligethes aeneus F.) was greater in German landscapes in which non-crop vegetation occupied >20% of total land area compared with landscapes in which non-crop vegetation occupied <20% of the land area. In the less diverse landscapes, parasitism of rape pollen beetles was greater at the edges of fields than in the centers; in contrast, in the more vegetatively diverse landscapes, parasitism rates did not differ between field edges and centers. Scale issues were addressed in a study conducted by Gardiner et al.Reference Gardiner, Landis, Gratton, DiFonzo, O'Neal, Chacon, Wayo, Schmidt, Mueller and Heimpel²¹⁶ at sites within four states in the north central US. Densities of predatory coccinellid beetles and rates of biological control of soybean aphid (Aphid glycines) increased as the proportion of landscapes occupied by forest and grassland rose, and the proportion occupied by corn and soybean dropped, with diversity and composition evaluated at a scale of 1.5 km around focal fields explaining the greatest proportion of variation in coccinellid densities and biological control rates. Thus, for certain host-specific crop pests and their natural enemies, there is likely to be a need for coordinated management activities in establishing and maintaining perennial vegetation at the landscape scale.

Similarly, perennial vegetation can provide a higher abundance and greater diversity of refuges for pollinator populations that are essential for many fruit and vegetable crops. In agriculture, especially among pollen-limited crops, promoting pollination services is an important means of increasing productivity. In a review of pollination services for the leading food crops in 200 countries, Klein et al.Reference Klein, Cunningham, Bos and Steffan-Dewenter²¹⁷ found that of a total of 107 crops traded on the world market that are directly consumed by humans (but excluding crops that are solely passively self-pollinated, wind-pollinated or parthenocarpic), 40% of these crops are highly to moderately dependent on animal pollinators. Significant declines in different pollinator species have been documented in the Midwestern US, raising several ecological and economic concernsReference Grixti, Wong, Cameron and Favret^218–Reference Hendrix, Kwaiser and Heard²²⁰. Grixti et al.Reference Grixti, Wong, Cameron and Favret²¹⁸ noted that half of the bumblebee species found historically in Illinois have been locally extirpated or have suffered declines, supporting observations of broader declines in North America. Major declines in the bumblebee fauna coincided with large-scale agricultural intensification. Bee richness was positively related to plant richness and abundance of potential nesting resources, and bee community composition was significantly related to plant richness, soil characteristics potentially related to nesting suitability, and canopy coverReference Grundel, Jean, Frohnapple, Glowacki, Scott and Pavlovic²¹⁹. Conservation of agriculturally important pollinators may be promoted by applying wildlife-friendly approaches to agriculture, such as increasing agricultural land set-asides and hedgerowsReference Hendrix, Kwaiser and Heard²²⁰.

In humid temperate regions, perennial cover crops growing beneath ‘main’ crops have been shown to contribute to biological control, but they can also reduce crop yields through competition for water and nutrients, making their use problematic in many situationsReference Bugg^221–Reference Schmidt, O'Neal and Singer²²⁴. An alternative form of within-field diversification involves the use of narrow strips of perennial vegetation that provide habitat for predators and parasitoids of pests in closely adjacent cropped areas. This approach has been studied in wheat fields where perennial grasses function as ‘beetle banks,’ conserving coleopteran predators that forage on cereal aphidsReference Collins, Boatman, Wilcox, Holland and Chaney^225, Reference MacLeod, Wratten, Sotherton and Thomas²²⁶, and in vineyards where strips of flowering plants provide nectar and pollen sources and a corridor from riparian areas for predators attacking thrips and leafhoppersReference Nicholls, Parrella and Altieri²²⁷. Strips of alfalfa (Medicago sativa) have been shown to conserve predatory beetles, bugs, lacewings and spiders, and increase their densities in adjacent strips of cotton, thereby exerting greater control of mirid and lepidopteran pestsReference Mensah, Sequeira, Gurr, Wratten and Altieri²²⁸. Strips of perennial grasses, legumes and other forbs left unsprayed with pesticides formed refuges for predatory ground beetles, permitting rapid recolonization of chemically disturbed cornReference Lee, Menalled and Landis²²⁹.

Compared with arthropod pests and their natural enemies, the effects of perennial vegetation on crop diseases have received much less attention from the scientific community. In artificially constructed assemblages of perennial prairie species, increases in plant species diversity reduced the severity of diseases caused by wind- and rain-dispersed pathogens on four target plant species (Septoria liatridis on Liatris aspera, Uromyces lespedezae-procumbentis on Lespedeza capitata, Ersiphe cichoracearum on Monarda fistulosa, and Colletotrichum spp. on Schizachyrium scoparium)Reference Knops, Tilman, Haddad, Naeem, Mitchell, Haarstad, Ritchie, Howe, Reich, Siemann and Groth²³⁰, suggesting that diverse mixtures of perennial species interplanted with crops might offer protection against certain pathogens. Such protection could occur as the result of non-crop vegetation acting as a barrier against propagule dispersal, hosting antagonistic and competitive microbes, altering microclimate conditions or inducing resistance within cropsReference Finckh, Wolfe, Cooke, Jones and Kaye²³¹. Perennial grain crops may also be bred for high levels of disease resistance. Hayes et al.Reference Hayes, Newell, DeHaan, Murphy, Crane, Norton, Wade, Newberry, Fahim, Jones, Cox and Larkin²³² noted that wheatgrass species (Thinopyrum and Agropyron spp.) conveyed resistance to many diseases when crossed with wheat to produce perennial hybrids.

Cox et al.Reference Cox, Garrett and Bockus²³³ noted that typical cultural practices for reducing soil- and residue-borne pathogens, such as annual crop rotations, delayed planting dates and tillage, are not feasible in perennial crop systems, and that as a consequence, pathogens that survive in soil, crop residues, live shoots and roots may become an increasing threat to perennial crops and adjacent areas of susceptible annual crops over time. To impede the build-up of pathogen populations, Cox et al.Reference Cox, Garrett and Bockus²³³ suggested three approaches: planting mixtures of perennial crop cultivars or species that vary in disease resistance to particular species and strains of pathogens; using grazing to remove plant residues; and burning plant residues, a natural phenomenon in perennial grass systems that can decrease the incidence and severity of a number of diseases. These methods remain to be field-tested in perennial crop and mixed perennial–annual crop associations, however.

The pest problem is especially important in the humid tropics, where the high rainfall and low seasonality favor two key groups of natural plant enemies—insects and fungiReference Givnish²³⁴. Consequently, the potential for losing crops to disease is far greater than in more temperate climates. Conserving both the associated fauna within tropical agroforests and surrounding forest fragments with high quality agricultural matrices is critical to maintaining pest regulation servicesReference Philpott and Armbrecht²³⁵. For example, levels of ant biodiversity exhibit sharp declines with the intensification of coffee and cacao production systems compared with traditional agroforestry systemsReference Philpott, Perfecto and Vandermeer²³⁶. High diversity and abundance of ants provide important ES in coffee agroecosystems since ants are an important predator groupReference Philpott and Armbrecht^235, Reference De la Mora, Livingston and Philpott²³⁷. Another predator studied in the coffee agroforest, birds, helps control coffee berry borers and thus increases coffee yield and farm income, a potentially important conservation incentive for producersReference Johnson, Kellermann and Stercho^238, Reference Kellermann, Johnson, Stercho and Hackett²³⁹. Landscape heterogeneity may allow mobile predators to provide pest control broadly, despite localized variation in farming intensities. Similarly, forest remnants provide nearby coffee plantations with a diversity of bees and other pollinators that can increase both the amount and stability of pollination services by reducing dependence on a single introduced pollinatorReference Brosi, Daily, Shih, Oviedo and Duran^240–Reference Ricketts²⁴², and improve both quantity and quality of coffee yields by reducing the frequency of misshapen seedsReference Ricketts²⁴².

Recent studies in the dry tropical zone have documented the importance of hedgerows (often planted for soil conservation purposes along sloping land) for enhancing biological pest control by serving as refugia for predators. Girma et al.Reference Girma, Rao and Sithanantham²⁴³ found that corn associated with hedgerows comprised of nine different species experienced significantly lower stalk borer (Busseola fusca and Chilo spp.) and aphid (Rhophalosiphum maidis) infestations than pure stands of corn. The study also found, however, that hedgerow effects on pest infestations of crops cannot be generalized but depend on the specific arthropods and their refugia preferencesReference Girma, Rao and Sithanantham²⁴³.

Trees have been shown to affect pest infestations in dry tropical agroecosystems by acting as barriers to movement of insects, masking the odors emitted by other components of the system, and sheltering herbivores and natural enemiesReference Rao, Singh and Day²⁴⁴. In sub-Saharan Africa, where the corn stalk borer, Busseola fusca (Full) (Noctuidae), and the sorghum stem borer, Chilo partellus Swinh (Pyralidae), pose major threats to food production, Khan et al.Reference Khan, Pickett, van den Berg, Wadhams and Woodcock^245, Reference Khan, Midega, Hutter, Wilkins and Wadhams²⁴⁶ reported positive effects of trap crop plants (Napier grass, Pennisetum purpureum, and Sudan grass, Sorghum sudanensis) and intercrop plants (molasses grass, Melinis minutiflora, and Desmodium spp.) for controlling stem borer outbreaks through release of attractant and repellent semiochemicals.

Currently, it is difficult to view perennial vegetation as a panacea for crop pest and disease problems, though promising opportunities for improved management strategies clearly exist. The generally positive influence of perennial vegetation on biological control is not necessarily consistent from year to yearReference Menalled, Costamagna, Marino and Landis²⁴⁷. Moreover, in some cases perennial vegetation can exacerbate pest problems by serving as an alternate host to crop pests, as is the case for soybean aphid, which resides on buckthorn (Rhamnus spp.) growing in hedgerows and woodlands adjacent to soybean fields in the Midwestern USAReference Ragsdale, Voegtlin and O'Neil²⁴⁸. Much more needs to be learned concerning sources of spatial and temporal variability in the effects of perennial vegetation on natural enemies of crop pests and crop disease dynamics, and a much more detailed understanding needs to be developed concerning how individual perennial species affect natural enemies of arthropod pests, crop pathogens and crop yields.

Soil Quality and Nutrient Cycling

Soil quality is the capacity of a soil to sustain biological productivity, maintain environmental quality, and promote plant and animal healthReference Doran, Parkin, Doran, Coleman, Bezdicek and Stewart^249, Reference Karlen, Mausbach, Doran, Cline, Harris and Schuman²⁵⁰. Consequently, soil quality is a required supporting service for agroecosystems and human societies. Over the past several decades, soil scientists have assembled sets of physical, chemical and biological indicators of soil qualityReference Doran, Parkin, Doran, Coleman, Bezdicek and Stewart²⁴⁹. Physical indicators include rates of water flow into and through soil; soil structural characteristics, such as porosity, bulk density and aggregate size distribution; and measures of stability, such as aggregate stability in water and erosion rate. Chemical indicators include quantities and qualities of soil organic C and N, nutrient availability to plants and soil reaction (pH). Biological indicators include the mass of organisms in soil and measures of their physiological activity, including rates of respiration and enzymatic reactions.

Regular additions of organic matter, especially root materials, play a critical role in the maintenance of soil qualityReference Russell, Cambardella, Laird, Jaynes and Meek^251, Reference Weil, Magdoff, Magdoff and Weil²⁵². In general, in humid, temperate environments, woody and herbaceous perennial plants add more root material and organic matter to soil than do annual cropsReference Liebig, Johnson, Hanson and Frank^53, Reference Tufekcioglu, Raich, Isenhart and Schultz⁵⁴ and consequently they are more effective at maintaining soil quality. As noted in the previous sections on hydrologic regulation and climate regulation, rainfall infiltration, hydraulic conductivity and soil C storage are greater for soil occupied by trees and herbaceous perennials than annual row crops.

Adding perennial crops to rotation sequences dominated by annual crops can have beneficial effects on soil quality and function. Residues of alfalfa, red clover and other perennial forage legumes can release enough N to satisfy much or all of the nitrogen demand of subsequent cereal crops, such as corn nitrogenReference Fox and Piekielek^253, Reference Morris, Blackmer and Elhout²⁵⁴. Russell et al.Reference Russell, Laird and Mallarino²⁵⁵ compared cropping systems based on annual crops (continuous corn or a corn–soybean rotation) with systems that contained alfalfa as well as corn and soybean and found that systems containing alfalfa had greater available N, higher rates of potential net N mineralization, and more microbial biomass C. Soil total N and organic C concentrations were also higher in systems containing alfalfa. Thus, cropping systems that contain perennial legumes in sequence with annual crops can have lower requirements for fertilizer and higher rates of soil C storageReference Gregorich, Drury and Baldock^256, Reference Gregorich, Rochette, VandenBygaart and Angers²⁵⁷.

More efficient N use by perennials can potentially reduce the environmental impacts associated with nutrient losses. Nitrogen losses to drainage water are greater for corn and soybean than for perennial grasses and herbaceous legumes such as alfalfaReference Randall, Huggins, Russelle, Fuchs, Nelson and Anderson²⁵⁸. Moreover, trees can have even higher N and P removal efficiency than perennial grassesReference Tufekcioglu, Raich, Isenhart and Schultz^54, Reference Zhang, Liu, Zhang, Dahlgren and Eitzel⁶⁷. In Iowa, N immobilization rates in poplar stands and switchgrass sites averaged 37 and 16 kg N ha⁻¹ yr⁻¹, respectivelyReference Tufekcioglu, Raich, Isenhart and Schultz⁵⁴.

For humid tropical regions, which typically have soils of low fertilityReference Richards, Walsh, Baillie and Greg-Smith²⁵⁹, it is especially important to develop land-use systems that promote efficient nutrient cyclingReference Schroth, Elias, Uguen, Seixas and Zech²⁶⁰. Land-use systems based on tree crops (e.g., multistrata agroforestry systems) have clear advantages over annual cropping systems for the maintenance of soil fertility because they provide permanent soil cover and root systemsReference Schroth, Elias, Uguen, Seixas and Zech²⁶⁰. For example, when shade coffee agroforestry systems are converted to more intensive sun-grown systems by removing the shade trees, N fertilizers are usually applied at high rates to maintain productivity. Research assessing the consequences of these changes for nitrate leaching and water quality has yielded contrasting resultsReference Babbar and Zak^261–Reference Harmand, Avila, Dambrine, Skiba, de Miguel, Renderos, Oliver, Jimenez and Beer²⁶³. Babbar and ZakReference Babbar and Zak²⁶¹ reported lower N leaching rates from shaded than from unshaded coffee cultivation in Costa Rica (9 versus 24 kg ha⁻¹ yr⁻¹, respectively), though plantations differed in the density and pruning regime of the coffee plants, making the data difficult to interpret. Harmand et al.Reference Harmand, Avila, Dambrine, Skiba, de Miguel, Renderos, Oliver, Jimenez and Beer²⁶³ found that drainage was slightly reduced in a shaded compared with an unshaded coffee plantation, but N losses in surface runoff were low in both cases. In general, shade coffee systems appear to have greater potential to enhance other soil fertility properties such as nutrient and organic inputs from litterfallReference Beer, Muschler, Kass and Somarriba^262, Reference Jaramillo-Botero, Santos, Fardim, Pontes and Sarmiento²⁶⁴ and earthworm densityReference Leon, De Melo, Soto, Johnson-Maynard and Lugo-Perez²⁶⁵ than their unshaded counterparts.

In arid agricultural regions of East Africa, retaining savanna trees as part of annual cropping systems can enhance soil fertilityReference Belsky²⁶⁶ and, in turn, understory crop productionReference Ong and Leakey^267, Reference Van Noordwijk and Ong²⁶⁸. Perennialization can also help mitigate or even reverse processes of soil loss and desertification, common in many drylands, by reducing the movement of sand, retaining soil organic matter and promoting establishment of new vegetation. For example, in the Loess Plateau region of China, Qiao et al.Reference Qiao, Chen, Hong, Li and Liu¹²⁰ demonstrated that mixed tree plantations interspersed within cropland can, over time, help stabilize and facilitate the restoration of native vegetation. Yuan et al.Reference Yuan, Bingner and Locke²⁶⁹ studied the effects of mixing wheat with three perennials (Stipa bungeana, Lespedeza davurica and Artemesia capillaries) in the same region, and found that the perennials significantly increased the total crop production, especially root growth.

Overall, the influence of perennials on soil fertility is synergistically coupled with their impact on hydrologic and C sequestration services, and thus, multiple ES can be obtained through perennialization processes in cropland. However, as with hydrological and C sequestration services, more work is needed to improve understanding of how different species mixtures, amounts and landscape positions influence soil quality properties, in both temperate and humid agroecosystems.

Provisioning Services

Food, fiber and fuel production to meet subsistence or market needs is the overwhelmingly dominant goal of agriculture. Crops in individual fields are dependent on services provided by nearby ecosystems, whether native or managed, and nearby ecosystems are often influenced by their agricultural neighborsReference Swinton, Lupi, Robertson and Hamilton⁷. As discussed above, perennialization can contribute toward maximizing ES benefits from agricultural landscapes. Below, we review opportunities for accomplishing this goal, with an emphasis on the additional provisioning services that perennial plants also provide.