On which timescale.s do optimal adjustments to vegetation function confer resilience? A case study in South-Eastern Australia

Abstract:

Droughts have been implicated as the driver behind recent vegetation die-off across a variety of hydroclimates and, with climate change, are expected to drive greater mortality in the future. To predict ecosystem resilience to future drought requires a predictive capacity currently lacking in state-of-the-art land surface models (LSMs), which rely on empirical factors and relationships to capture the impacts of water stress on the vegetation. By contrast, novel approaches that optimise stomatal conductance with respect to instantaneous plant photosynthetic and hydraulic functions have been shown to reduce the biases of LSM gas exchange predictions during drought. These approaches provide an opportunity for developing mechanistic optimality theory, e.g. on plant trait covariation or on drought deciduousness. However, a question remains: on what timescale.s does vegetation function change to maximise resource investment? Here, we explore timescales of optimality within a simple LSM: (i) instantaneous (regulating canopy gas exchange); (ii) weekly (adjustment of leaf biomass; e.g. leaves can be shed to mitigate drought stress), (iii) seasonal (regulating the investment of nitrogen in photosynthetic capacity); and (iv) annual (legacy of seasonal water stress on plant hydraulic conductance). We use observations from a temperate woodland in South-Eastern Australia to test which optimisation timescales are best supported and whether they can operate together. This insight will help to characterise plausible dynamics and magnitude of resilience to drought conferred by optimal vegetation function across South-Eastern Australia.