Forests are the most widespread terrestrial ecosystem on Earth. In 2015, natural forests accounted for 93% (3.7 billion ha) of this global forest area (FAO 2016), albeit only 26% of these are primary forest (i.e. old-growth or ancient woodland). Since 1990, 31 million ha of primary forest have been modified or cleared, and a net loss of 129 million ha of natural forest has occurred (–0.13%/year) (FAO 2016). This deforestation has largely been in tropical South America and Africa, where forests have been cleared and converted for agricultural uses, resulting in habitat loss and carbon emissions.

We review the literature dealing with mediterranean climate, vegetation, phenology and ecophysiology relevant to the understanding of tree-ring formation in mediterranean regions. Tree rings have been used extensively in temperate regions to reconstruct responses of forests to past environmental changes. In mediterranean regions, studies of tree rings are scarce, despite their potential for understanding and predicting the effects of global change on important ecological processes such as desertification. In mediterranean regions, due to the great spatio-temporal variability of mediterranean environmental conditions, tree rings are sometimes not formed. Often, clear seasonality is lacking, and vegetation activity is not always associated with regular dormancy periods. We present examples of tree-ring morphology of five species (Arbutus unedo, Fraxinus ornus, Quercus cerris, Q. ilex, Q. pubescens) sampled in Tuscany, Italy, focusing on the difficulties we encountered during the dating. We present an interpretation of anomalies found in the wood structure and, more generally, of cambial activity in such environments. Furthermore, we propose a classification of tree-ring formation in mediterranean environments. Mediterranean tree rings can be dated and used for dendrochronological purposes, but great care should be taken in selecting sampling sites, species and sample trees.

Ring widths of five Mediterranean forest tree species (Arbutus unedo, Fraxinus ornus, Quercus cerris, Quercus ilex and Quercus pubescens) growing close to a natural source of CO2 in Tuscany, Italy and at a nearby control site were compared. At the CO2-enriched site, trees have been growing for decades under elevated CO2 concentrations. They originated from parent trees that also grew under elevated CO2 in natural conditions, and they have been continuously exposed to elevated CO2 throughout their growth. Tree-ring series from each of the species were prepared. Assigning calendar dates to rings was difficult but possible, and ring-width series were built for all species. The ring-width data were analysed using a two-sided t-test to assess if there was a difference between the radial growth at the CO2-enriched site and the control site. The cumulative basal area at the same cambial age at both sites was also compared using a Wilcoxon test. Radial growth of trees at the CO2-enriched site was not significantly different from growth at the control site. For each species, year by year, radial growth at the CO2-enriched site was tested against the control site and significant differences were found in only a few years; these differences were not synchronous with extreme climatic events. The expected increase in above-ground productivity, as one of the ecosystem responses to increasing CO2 during drought stress, was not observed in this Mediterranean woody plant community, despite being water-limited. Other resource limitations, such as low nutrient availability (common in the Mediterranean region), may have counteracted the positive effect of elevated CO2 under drought stress, or trees may have acclimated to the high CO2.

The lichens growing on gravestones in 142 Scottish graveyards have been examined. Measurements were restricted to Section Rhizocarpon thalii. These data permit the development of lichenometric growth curves on acidic igneous, basic igneous, sandstone and slate substrates in most areas of Highland Scotland. The colonisation of gravestones, which is extremely erratic, takes place after a minimum of eight years. The ‘great period’ of growth lasts for approximately 20 years after the erection of the gravestone. The lichen factor (growth after 100 years) is correlated with the growth after 25 and 250 years indicating that it is a representative index of the growth rates. Growth rates are non-linear, decreasing with time. Calculated lichen factors for acidic igneous substrates range from 33 to 104 mm. The distributions of different types of gravestones are non-uniform in both time and space, making the comparison of growth rates on different rock types impractical. The results indicate that there may be a gradual decrease in the growth rates from W to E, reflecting the decreasing maritime influence towards the E.