What kills the largest trees in tropical forests?

How and why trees die remains a mystery. Although we know what can kill trees, we struggle to identify what actually does kill trees in nature. Overcoming this knowledge gap is a pressing issue now because tree mortality rates appear to be increasing. [Authors: E.M. Gora and A. Esquivel-Muelbert]
Published in Ecology & Evolution
What kills the largest trees in tropical forests?

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Trees mitigate climate change by absorbing carbon from the atmosphere and storing it as wood. Tree death is the beginning of a process that unlocks this woody carbon and transfers it back to the atmosphere.  As tree death rates increase, this limits the ability of forests to absorb and store carbon. Thus, tree death is an essential piece of the puzzle in the ongoing efforts to understand the global carbon cycle, both now and in the future. To help address this knowledge gap, we reviewed size-dependent patterns of tree mortality, and provided a hypothesis framework for understanding and investigating these patterns in our recent paper - Implications of size-dependent tree mortality for tropical forest carbon dynamics.

 Tropical forests are an important component of this equation.  Tropical forests contain 60% of terrestrial biomass carbon despite comprising only ~10% of total land area. Intact moist tropical forests – that is, forests that are not deforested or degraded by humans - are the largest contributor to this carbon sink.  However, even these intact forests are at risk. A massive network of forest plots has revealed that tree mortality rates are also increasing in intact tropical forests.

 Within a forest, not all trees are equally important to the carbon budget.  Unlike most animals, trees continue to grow throughout their lifetime, producing tremendous variation in size within a single community of trees.  As a result, the largest 1% of trees in a forest typically contain ca. 50% of that forest’s aboveground biomass and woody carbon.  Accordingly, the consequences of death for the largest trees is disproportionately important to carbon losses in a forest. However, these large trees are rare and we know very little about them. We know even less about what kills them.

 Our paths intersected because we were both working on tree mortality. Specifically, we have been investigating tree death across the tropics looking at large scale patterns and the impact of lightning on tree death. From this work, we were both familiar with the struggle to draw conclusions about drivers of the death for the largest trees, primarily because of their rarity and the stochastic nature of tree death. However, we had not yet realized the depth or scope of this problem. This changed when we met during an organized oral session (a type of symposium) at the Ecological Society of America conference in 2019.  The goal of this session was to highlight the major advances and challenges to our understanding of death for large tropical trees.  The presentations in this session revealed that the challenges outpaced the advances.  This session and our conversations during the conference helped us realised how little we truly knew about the drivers of large tree death.

 During that meeting, we decided we needed to take a more detailed look into the causes of death for these tropical giants. Starting in January 2020, we began to have regular chats as we systematically reviewed the literature related to size-dependent patterns of tree mortality. We focused on the physical and physiological mechanisms of size-dependent mortality for each driver and then evaluated their empirical support. Synthesizing the existing data, we slowly put together a theoretical framework of how drivers of tropical tree death should vary across tree size. We presented this framework and our synthesis of the literature to multiple colleagues for comments before submitting it to Nature Plants.

 Based on the literature, our framework predicts that abiotic factors play a bigger role in tree death as tree size increases. However, the more we read about the topic the more it became clear that there is a lot of work to be done. Whilst our field is now starting to understand the role of droughts across the vast diversity of tropical trees, we know virtually nothing about the role biotic drivers, such as pathogens or herbivores, in the death of large tropical trees. Our goal now is to take this collaboration to the field and start testing the predictions from our framework to understand how the giants of tropical forests die. 

This line of inquiry is a testament to the intellectual and professional benefits of keeping an open mind and attending symposia. We began this work with distinct perspectives and approaches to studying tropical tree mortality.  Working together and combining our perspectives made this project possible and facilitated our intellectual development. We encourage other early career researchers to reach outside of their existing collaborative networks where they also may have positive experiences.

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