Periodic Reporting for period 2 - TreeMort (Redefining the carbon sink capacity of global forests: The driving role of tree mortality)
Reporting period: 2019-08-01 to 2020-09-30
The TreeMort project aims to provide a solution to this problem. By combining newly available sources of data with appropriate conceptualisation of processes and state-of-the-art ecosystem modelling, 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 for application at continental-to-global scales.
- Integrate this new understanding into global terrestrial ecosystem models to reduce substantially the uncertainty in the role of forests in the present and future carbon cycle.
We have produced a first consistent estimate of occurrence frequencies of stand-replacing disturbance events across the world's forests. These events include large patches of tree death from causes such as wildfire, storm damage, biotic outbreaks and harvest. Applying this information in a vegetation model, we were able to identify that these disturbances contribute about 12% of total carbon turnover from tree mortality globally, but with large regional variation (Figure 1). We were also able to identify those regions in which forest biomass is strongly controlled by such disturbances and those in which it plays a minor role. By assimilating information on forest age structure, based on forest inventories, into the same vegetation model, we were able to calculate a new estimate of the impact of past disturbances on the current uptake of carbon by the world's forests, finding that a quarter of forest carbon uptake is due to disturbance legacies.
As the type of disturbance has major implications for both estimating possible changes in disturbance rate and the fate of the carbon in dead trees, we have further worked to breakdown the disturbance rate by cause. We have individually delineated the more than 400 million such disturbance patches that occurred globally between 2000 and 2018, broken down rates by size (Figure 2) and assessed the contributions of fire, land-use change and human activities for different forest biomes between 2002-2016. These results provide the basis for a more nuanced assessment of the current role of stand-replacing disturbances in global forest carbon cycling, providing the baseline from which future assessments can be made.
We have assembled and standardised a global forest dynamics dataset, following over time 10 million individual trees in 45 different countries, from tropical, temperate and boreal biomes. For the tropical forests, 627 long-term plots, managed and quality-controlled by ForestPlots.net and contributed by many hundreds of tropical ecologists, especially from Latin America, have provided a large observational dataset of tree mortality and carbon turnover. This has been complemented by data from a further 29 networks across all biomes, including both research plots and national forest inventories. This dataset provides the basis for quantifying the rates and understanding the drivers of the remaining 88% of carbon turnover from tree mortality that happens below stand-scale. Two analyses on subsets of this dataset for Europe and the Amazon have shown that tree mortality in most European forests is dominated by harvest, whilst in remote Amazon forests there is a relatively even split between those trees that die broken or fallen from physical disturbances versus those which die standing, albeit with significant regional differences in both cases. These results provide hard quantification of the contribution of different drivers of tree death at the continental and finer scales, as well as establishing the groundwork for the analysis of the full global dataset.
Complementary to this, we have assembled and standardised a global database of 19 plant hydraulic traits and developed a methodology to quantitatively identify different functional strategies employed by trees to withstand drought stress. These functional strategies are designed to be employed in a novel model of tree hydraulics suitable for application at the global scale. We have contributed to the development of this model and combined with the functional strategies it forms a strong basis for a global scale estimate of the role of hydraulic failure in tree mortality - believed to be an increasingly important contributor to tree death.