Role of carbon reserves in tree and forest mortality

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Forest die-off is predicted to become more frequent with climate change. Our ability to predict these events is poor because we lack knowledge about the underlying physiological mechanisms of tree mortality. Specifically, nonstructural carbohydrate carbon reserves (sugars and starch) are involved in almost all critical physiological processes in plants, yet it remains hotly debated what role these carbon reserves play in the survival and mortality of trees. My long-term research objective is to understand the underlying processes controlling forest dynamics and forest ecosystem development. With the proposed research program I will test the overarching hypothesis that during times of stress, when carbon assimilation (photosynthesis) is limited, shifts in reserve allocation could lead to low carbon reserves in specific tissues or organs which in turn will play a critical role in the performance and potentially in the mortality of trees. Over the next five years, I will train 3 PhD and 2 M.Sc. students, as well as several undergraduates to investigate the following hypotheses in carbon reserve allocation of plants: 1. Carbohydrate allocation is a top-down process (e.g. sinks requiring carbon reserves closes to the source (leaves) will receive their reserves first). 2. Forest decline in tall trees is a consequence of top-down allocation, where roots will be affected negatively first (starved) by reduced carbon supply from the crown. 3. Stored reserves can be remobilized from storage tissues, but it is controlled by the distance of the storage source such that a) stores closest to location of need are remobilized first, and b) stored carbon in one organ is largely unavailable to other organs. This program will also be linked to a larger international effort to build a global meta-data set that will enable us to explore what the role functional tree traits, including carbon reserves, play in their growth and survival across a wide range of forest types and climates. My program will combine field and controlled greenhouse studies with mature and juvenile trees using state-of-the-art analytical equipment and protocols. The results will be a unique characterization of the importance of carbon reserves in trees which will fundamentally advance our understanding of the mechanisms and strategies involved in the storage of carbon reserves and their importance in stress tolerance in long-lived plants. Overall, this research program will establish the foundational scientific knowledge required to improve upon the prediction of forest mortality in response to a changing climate for Canadian forests and beyond.