Productivity-efficiency tradeoffs in tropical gallery forest-savanna transitions: linking plant and soil processes through litter input and composition

Vegetation gradients in Central Brazil encompass sharp transitions from savanna to forests, representing an iconic example of how interactions between plants and soils regulate biogeographical boundaries. Here we describe how canopy productivity regulates nutrient inputs to soils, affecting fertility and influencing ecosystem distribution. Based on soil and litter systematically collected during 12 months along a gallery forest-savanna transition, we determined associations between canopy cover (leaf area index-LAI) and the (re)cycling of essential macronutrients. This evaluation was aimed at aggregating information about biogeochemical controls of ecosystem distribution/productivity, to support conservation and management efforts in the region. We confirmed two hypotheses: (i) nutrient inputs via litterfall are significantly higher in forest than in adjacent savanna, and (ii) litter quality varies with canopy productivity and litter nutrient concentrations influence soil fertility reinforcing forest and savanna as alternate stable states. These observations delineate a productivity-efficiency tradeoff in which savannas communities are more efficient in the use of limiting nutrients, yet, less productive than forest communities. The relative importance of different nutrients, apparent on recovery rates in the litter regressed against LAI, revealed that the expansion of forest ecosystems is limited by P > Mg > K > N, with highest conservation observed for P. Differences in Ca input were also significant among ecosystems, but depended solely on the amount of deposited litter, with no differences in recovery rates observed between forests and savannas. A tradeoff-based framework could be used to predict ecotonal stability in the region, with transitions between savannas and forests marked by clear changes in species composition, productivity, litter deposition, and soil fertility.