Basin-wide variations in Amazon forest structure and function are mediated by both soils and climate

Abstract

<jats:p>Abstract. Forest structure and dynamics vary across the Amazon Basin in an east-west gradient coincident with variations in soil fertility and geology. This has resulted in the hypothesis that soil fertility may play an important role in explaining Basin-wide variations in forest biomass, growth and stem turnover rates. Soil samples were collected in a total of 59 different forest plots across the Amazon Basin and analysed for exchangeable cations, carbon, nitrogen and pH, with several phosphorus fractions of likely different plant availability also quantified. Physical properties were additionally examined and an index of soil physical quality developed. Bivariate relationships of soil and climatic properties with above-ground wood productivity, stand-level tree turnover rates, above-ground wood biomass and wood density were first examined with multivariate regression models then applied. Both forms of analysis were undertaken with and without considerations regarding the underlying spatial structure of the dataset. Despite the presence of autocorrelated spatial structures complicating many analyses, forest structure and dynamics were found to be strongly and quantitatively related to edaphic as well as climatic conditions. Basin-wide differences in stand-level turnover rates are mostly influenced by soil physical properties with variations in rates of coarse wood production mostly related to soil phosphorus status. Total soil P was a better predictor of wood production rates than any of the fractionated organic- or inorganic-P pools. This suggests that it is not only the immediately available P forms, but probably the entire soil phosphorus pool that is interacting with forest growth on longer timescales. A role for soil potassium in modulating Amazon forest dynamics through its effects on stand-level wood density was also detected. Taking this into account, otherwise enigmatic variations in stand-level biomass across the Basin were then accounted for through the interacting effects of soil physical and chemical properties with climate. A hypothesis of self-maintaining forest dynamic feedback mechanisms initiated by edaphic conditions is proposed. It is further suggested that this is a major factor determining endogenous disturbance levels, species composition, and forest productivity across the Amazon Basin.</jats:p>

Soil-induced impacts on forest structure drive coarse woody debris stocks across central Amazonia

“…Physical properties, such as shallow soils with high bulk density, poor aggregation and severe anoxic conditions can restrict deep root growth. In addition, such soil conditions limit tree establishment and tend to increase tree mortality rates (Gale and Barfod 1999;Gale and Hall 2001;Quesada et al 2012). From all edaphic properties studied, effective depth and anoxia seem to be the most related to CWD in our study area (Table 6).…”

“…Amazonia holds a great diversity of tree species (ter Steege et al 2000), and its forests vary substantially in both vegetation dynamics Quesada et al 2012), and structure Malhi et al 2006;Nogueira et al 2008;Feldpausch et al 2011). Our current understanding suggests that CWD stocks generally decrease from north-eastern to south-western Amazonia (Baker et al 2007;Chao et al 2009a).…”

“…Different levels of soil physical restrictions appeared to significantly correlated with forest structure (Table 5), implying an important influence on how, and for how long, living biomass is stored in forest ecosystems (Quesada et al 2012). Soil restrictions may decrease average residence time of trees (Quesada et al 2012), resulting in a population of thinner and shorter trees that individually store less biomass (also with more individuals per hectare). On the other hand, we hypothesise, forests on soils without physical limitations tend to be populated by larger trees, simply because they can live longer.…”

“…Although these studies successfully associated CWD stocks with forest structure and dynamics, to our knowledge there has been no analysis of a potential effect of edaphic properties on CWD stocks. Since edaphic factors, such as effective soil depth and structure are important factors controling forest structure and dynamics (Quesada et al 2012;Jirka et al 2007), they are likely to be related to both the production and the stocks of CWD. We hypothesise that because poor soil physical conditions impose constraints on tree growth and survival, they result in increased stem turnover rates, and, in turn, limit the maximum size that trees can attain.…”

“…Soils represent an important environmental gradient in Amazonia, with a wide variety of soil types across the Basin and with diverse chemical and physical conditions (Quesada et al 2010;. Variations in soil physical properties across the Basin have been related to a large proportion of the variation in tree turnover rates and mean forest wood density, with disturbance levels and vegetation structure of Amazonian forests being related to different soil types (Quesada et al 2012). …”