Coupling carbon allocation with leaf and root phenology predicts tree–grasspartitioning along a savanna rainfall gradient

[摘要] The relative complexity of the mechanisms underlying savanna ecosystemdynamics, in comparison to other biomes such as temperate and tropicalforests, challenges the representation of such dynamics in ecosystem andEarth system models. A realistic representation of processes governingcarbon allocation and phenology for the two defining elements of savannavegetation (namely trees and grasses) may be a key to understandingvariations in tree–grass partitioning in time and space across the savannabiome worldwide. Here we present a new approach for modelling coupledphenology and carbon allocation, applied to competing tree and grass plantfunctional types. The approach accounts for a temporal shift betweenassimilation and growth, mediated by a labile carbohydrate store. This iscombined with a method to maximize long-term net primary production (NPP) byoptimally partitioning plant growth between fine roots and (leaves +stem). The computational efficiency of the analytic method used here allowsit to be uniquely and readily applied at regional scale, as required, forexample, within the framework of a global biogeochemical model.

We demonstrate the approach by encoding it in a new simple carbon–watercycle model that we call HAVANA (Hydrology and Vegetation-dynamics Algorithmfor Northern Australia), coupled to the existing POP (Population OrdersPhysiology) model for tree demography and disturbance-mediatedheterogeneity. HAVANA-POP is calibrated using monthly remotely sensedfraction of absorbed photosynthetically active radiation (fPAR) andeddy-covariance-based estimates of carbon and water fluxes at five tower sitesalong the North Australian Tropical Transect (NATT), which ischaracterized by large gradients in rainfall and wildfire disturbance. Thecalibrated model replicates observed gradients of fPAR, tree leaf areaindex, basal area, and foliage projective cover along the NATT. The modelbehaviour emerges from complex feedbacks between the plant physiology andvegetation dynamics, mediated by shifting above- versus below-groundresources, and not from imposed hypotheses about the controls on tree–grassco-existence. Results support the hypothesis that resource limitation is astronger determinant of tree cover than disturbance in Australian savannas.

[学科分类] 地球化学与岩石