Periodic Reporting for period 4 - TreeMort (Redefining the carbon sink capacity of global forests: The driving role of tree mortality)
Periodo di rendicontazione: 2022-04-01 al 2023-07-31
Evidence increasingly suggests widespread increases in tree mortality rates across many of the world's forests. The local impacts of increased tree mortality can be substantial and varied, but they also have a global implication; taken together they may threaten the enormous uptake of carbon dioxide by the world's forests, which currently offsets a net 20% of human-caused emissions. Yet understanding tree mortality remains a challenge. Compared to growth, about which we understand a great deal, tree death is much more difficult to observe because it happens relatively infrequently. This observational challenge is magnified when trying to understand the behaviour of forests at the continental or global level. Yet such large-scale understanding is necessary in order to quantify the impact of forests on the carbon cycle and on climate. Without this, the computational models used to understand how forests function within the Earth system have had very limited information on which to base estimates of mortality rates. As a result, projections of present and future tree mortality rates from these models contain a striking lack of consensus. This has a very serious implication: Depending on the rates of tree mortality, the world's forests may continue their strong uptake of carbon dioxide emissions, this uptake may diminish, or forests may even start to release more carbon dioxide than they take up. The size of this uptake is central to assessments of how much carbon dioxide humans can continue to emit with an expectation of keeping global temperature change within defined boundaries, such as the 1.5° and 2° targets. A large reduction in forest carbon uptake would result in smaller carbon dioxide emission budgets and thus require more rapid action to reach net zero emissions.
The TreeMort project aims to provide a solution to this problem. It seeks to:
- Provide observation-based constraints on recent tree mortality rates across the world's major forest biomes.
- Identify the contributions of different forms of tree mortality and formulate a mechanistic understanding that is appropriate to make large-scale assessments.
- Integrate this new understanding into computational models of global foretss to reduce substantially the uncertainty in the role of forests in the present and future carbon cycle.
The TreeMort project aims to provide a solution to this problem. It seeks to:
- Provide observation-based constraints on recent tree mortality rates across the world's major forest biomes.
- Identify the contributions of different forms of tree mortality and formulate a mechanistic understanding that is appropriate to make large-scale assessments.
- Integrate this new understanding into computational models of global foretss to reduce substantially the uncertainty in the role of forests in the present and future carbon cycle.
Based on an unprecedented synthesis of field observations of tree mortality, we quantified the rate of tree mortality across the world’s forests. We found that it did not differ systematically across biomes, nor was it closely related to forest productivity. Tree mortality did, however, play a very important role in explaining the spatial variation in the forest carbon sink. Using observations and modelling we showed that this mortality can be broken down between disturbances which kill a whole stand of trees, such as fires, windstorms and clearcut harvest, and events where one or a few individuals die. By developing novel disturbance models we found that much of the disturbances in the temperate and tropical forests could be attributed to harvest events, rather than natural causes. Changes to the disturbance-regime, caused by harvest and land-use change actions have substantially reduced the length of time carbon remains in forest vegetation (by 32% in northern-hemisphere temperate forests over 2001-2014). Using a novel fusion of modelling with observations of forest age, we showed for the first time that legacies of past disturbance currently account for a quarter of the global forest carbon sink (0.53 Gt carbon per year), offsetting about 5% of annual emissions from fossil fuel burning. These results clearly indicate that considering the effect of disturbances past and future is crucial to be able to properly assess the role of the world’s forests in taking up carbon. Many of the techniques developed in this project can be applied more broadly across the emerging class of vegetation models which explicitly simulate the lifecycle of trees.
Despite their often-dramatic impact on the landscape, however, over 2001-2014 our modelling showed that disturbances were responsible for a relatively modest 12.5% of tree mortality. Thus, mortality of individual trees dominates in the vast majority of forests. Using a new representation of plant hydraulics in our vegetation modelling we see that increases in individual tree mortality in the Amazon rainforest over 1984-2010 can be clearly attributed to drought stress. However, increasing tree mortality observed in Europe over 1986-2010 could largely be explained by increasing rates of forest growth, caused by higher carbon dioxide in the atmosphere, enabling increased rates of harvest. Our results show the power of process-based models to simulate tree mortality across large scales and attribute it to its underlying causes when the underlying mechanisms are appropriately accounted for. Building these mechanisms into the ensemble of vegetation models that underlie carbon cycle and climate projections therefore stands to substantially strengthen the confidence in future projections from these models.
We published our results in major international scientific journals including Nature, Science, PNAS, Nature Geoscience, Global Ecology and Biogeography, Environmental Research Letters. We also presented them in 36 international scientific conference presentations and 6 popular science articles, as well as on national radio and television.
Despite their often-dramatic impact on the landscape, however, over 2001-2014 our modelling showed that disturbances were responsible for a relatively modest 12.5% of tree mortality. Thus, mortality of individual trees dominates in the vast majority of forests. Using a new representation of plant hydraulics in our vegetation modelling we see that increases in individual tree mortality in the Amazon rainforest over 1984-2010 can be clearly attributed to drought stress. However, increasing tree mortality observed in Europe over 1986-2010 could largely be explained by increasing rates of forest growth, caused by higher carbon dioxide in the atmosphere, enabling increased rates of harvest. Our results show the power of process-based models to simulate tree mortality across large scales and attribute it to its underlying causes when the underlying mechanisms are appropriately accounted for. Building these mechanisms into the ensemble of vegetation models that underlie carbon cycle and climate projections therefore stands to substantially strengthen the confidence in future projections from these models.
We published our results in major international scientific journals including Nature, Science, PNAS, Nature Geoscience, Global Ecology and Biogeography, Environmental Research Letters. We also presented them in 36 international scientific conference presentations and 6 popular science articles, as well as on national radio and television.
Our published assessments of rates of stand-replacing disturbances and their interactions with the carbon cycle provide the first global quantification of an individual component of tree mortality and a major step forward in understanding of the drivers of forest carbon turnover. Likewise, our calculations of the contribution of past disturbances to the carbon sink showcase a novel approach to put numbers on this important component of the global carbon cycle. Our work on modelling of disturbance provides a suite of approaches that are applicable in dynamic vegetation models at the scale of biomes or the globe, facilitating the inclusion of these crucial processes in carbon cycle and climate assessments.
The unprecedented size and scope of the forest inventory dataset assembled has enabled us to test theories of how tree mortality rates vary across the world’s forests and quantify their considerable importance in explaining the spatial variation of the forest carbon sink. It has also allowed to develop new tree mortality risk algorithms which are based on theory as well as grounded in massive data, which is expected to substantially increase their reliability. Our work on modelling of drought stress has allowed us to create pioneering maps of how the most successful strategy for trees, and the diversity of plausible strategies, varies across the tropical forests. This is a key step to being able to simulate how tree diversity affects the resilience of forests to drought. Finally, we have been able to make some of the first cause-attribution assessments, namely by using our new knowledge to simulate underlying mortality processes, we were able to attribute the causes of trends in observed tree mortality. By demonstrating that our understanding of the mechanisms controlling tree mortality produces simulated forests that behave like ones in the real world substantially increases our confidence in using these models to make projections into the future.
The unprecedented size and scope of the forest inventory dataset assembled has enabled us to test theories of how tree mortality rates vary across the world’s forests and quantify their considerable importance in explaining the spatial variation of the forest carbon sink. It has also allowed to develop new tree mortality risk algorithms which are based on theory as well as grounded in massive data, which is expected to substantially increase their reliability. Our work on modelling of drought stress has allowed us to create pioneering maps of how the most successful strategy for trees, and the diversity of plausible strategies, varies across the tropical forests. This is a key step to being able to simulate how tree diversity affects the resilience of forests to drought. Finally, we have been able to make some of the first cause-attribution assessments, namely by using our new knowledge to simulate underlying mortality processes, we were able to attribute the causes of trends in observed tree mortality. By demonstrating that our understanding of the mechanisms controlling tree mortality produces simulated forests that behave like ones in the real world substantially increases our confidence in using these models to make projections into the future.