Introduction

Whether in the temperate zone or tropics, tree species composition and forest productivity are strongly associated with soil characteristics (e.g. Figure 11.1). Although these community- and ecosystem-level processes necessarily arise from variation in individual tree performance, the influences of specific soil resources on particular tree demographic processes have yet to be fully elucidated (Kobe 1996).

It is important, for several reasons, to understand how soil resources govern tree performance. First, and perhaps most salient to the theme of this volume, human activity exerts a potentially strong effect on soil resource availability through atmospheric deposition of nitrogen (N) (Figure 11.2), which accelerates soil acidification and the leaching of phosphorus (P) and base cations (calcium (Ca), magnesium (Mg), and potassium (K)) (Izuta et al. 2004; Matson et al. 1999; Perakis et al. 2006). Inorganic N deposition takes two major forms: nitrate (NO3–) from the combustion of fossil fuels and ammonium (NH4+) from agricultural activity. Background levels of N deposition are typically <1 kgN ha–1 yr–1, as measured in remote non-industrialised areas of the world (Hedin et al. 1995). In North America, N deposition can be more than 20 kgN ha–1 yr–1 (Gradowski & Thomas 2008; Weathers et al. 2006). Levels in Europe are much higher, with maximum levels reaching at least 43.5 kgN ha–1 yr–1recently (Stevens et al. 2011) and 75 kgN ha–1 yr–1 in the 1990s (Dise & Wright 1995). Atmospheric deposition of N is not exclusively a temperate issue, and levels of N deposition are expected to increase in the tropics with further industrial and agricultural development (Matson et al. 1999). Even though soil N levels are higher in tropical than temperate forests, additional inputs of N through deposition could lead to lower plant diversity and increased bulk carbon storage, as well as losses of base cations and P (Cusack et al. 2011; Lu et al. 2010; Matson et al. 1999).