CID Research Fellow and Graduate Student Working Paper No. 51

August 2011
Linking Leaf Senescence to Ecosystem Productivity: Physiological Mechanisms and Ecological Processes
Juan Pablo Giraldo
Global food security and conservation of tropical forest ecosystems will rely on understanding the impact of predicted increase in drought frequency and water demand on crop productivity and forest function. The end of leaf lifespan during water stress has a strong impact on plant productivity as leaves are the primary gateways of energy capture from sunlight. The aim of this paper was to bridge the author’s research on the mechanisms regulating the onset of leaf senescence to the development of drought tolerant crops and the improvement of ecosystem dynamic model predictions of tropical forest carbon uptake under a changing climate.

For his PhD, Giraldo studied the role of the plant vascular system in controlling leaf senescence via xylem-transported compounds. In his SSP fellowship, he addressed the question of how this physiological mechanism can contribute to the development of drought tolerant varieties in breeding and genetic modification programs. Based on his greenhouse and field experiments, Giraldo proposes (1) using xylem-transported cytokinins as biomarkers for selection and breeding of drought tolerant varieties, and (2) identifying genotypes with low response of leaf hydraulic conductance to water stress for delaying leaf senescence under drought conditions.
A second question was how to incorporate the physiological mechanisms underlying tree leaf senescence into ecosystem dynamic models for improving our understanding of tropical forest carbon exchange under a changing climate. This approach will shed light on the current controversy about the impact of the worst drought in 100 years on Amazon gross primary productivity. The author proposes that representing the diversity of vegetative phenology in tropical seasonal forests and the seasonal dynamics in plant vascular conductance in structured ecosystem models will lead us to capture the complex responses of tree carbon uptake to water stress.
Keywords: leaf senescence, hydraulic conductance, drought tolerance, tropical forests, climate change
JEL subject codes: Q16, Q23, Q25

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