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 ma$^{-1}$, 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 $^{13}$C isotope discrimination). Moreover, when considered in a multivariate context with P$_{A}$ and soil plant available water storage capacity (θ$_{P}$) 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 θ$_{P}$. ... mehrNeither calcium, nor magnesium, nor soil pH could substitute for potassium when tested as alternative model predictors (ΔAIC > 10). Nor for any model could simple soil texture metrics such as sand or clay content substitute for either [K]$_{sa}$ or θ$_{P}$. 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 θ$_{P}$ also presented here – a critical role for potassium as a modulator of tropical vegetation structure and function.