Edaphic, structural and physiological contrasts across Amazon Basin forest-savanna ecotones suggest a role for potassium as a key modulator of tropical woody vegetation structure and function

Sampling along a precipitation gradient in tropical South America extending from ca. 0.8 to 2.0 m a(-1), savanna soils had consistently lower exchangeable cation concentrations and higher C/N ratios than nearby forest plots. These soil differences were also reflected in canopy averaged leaf traits with savanna trees typically having higher leaf mass per unit area but lower mass-based nitrogen (N-m) and potassium (K-m). Both N-m and K-m also increased with declining mean annual precipitation (P-A), but most area-based leaf traits such as leaf photosynthetic capacity showed no systematic variation with P-A or vegetation type. Despite this invariance, when taken in conjunction with other measures such as mean canopy height, area-based soil exchangeable potassium content, [K](sa), proved to be an excellent predictor of several photosynthetic properties (including C-13 isotope discrimination). Moreover, when considered in a multivariate context with P-A and soil plant available water storage capacity (theta(P)) as covariates, [K](sa) also proved to be an excellent predictor of stand-level canopy area, providing drastically improved fits as compared to models considering just P-A and/or theta(P). Neither calcium, nor magnesium, nor soil pH could substitute for potassium when tested as alternative model predictors (Delta AIC > 10). Nor for any model could simple soil texture metrics such as sand or clay content substitute for either [K](sa) or theta(P). Taken in conjunction with recent work in Africa and the forests of the Amazon Basin, this suggests in combination with some newly conceptualised interacting effects of P-A and theta(P) also presented here - a critical role for potassium as a modulator of tropical vegetation structure and function.