Pervasive shifts in forest dynamics in a changing world

Shifting forest dynamics Forest dynamics are the processes of recruitment, growth, death, and turnover of the constituent tree species of the forest community. These processes are driven by disturbances both natural and anthropogenic. McDowell et al. review recent progress in understanding the drivers of forest dynamics and how these are interacting and changing in the context of global climate change. The authors show that shifts in forest dynamics are already occurring, and the emerging pattern is that global forests are tending toward younger stands with faster turnover as old-growth forest with stable dynamics are dwindling. Science, this issue p. eaaz9463 BACKGROUND Forest dynamics arise from the interplay of chronic drivers and transient disturbances with the demographic processes of recruitment, growth, and mortality. The resulting trajectories of vegetation development drive the biomass and species composition of terrestrial ecosystems. Forest dynamics are changing because of anthropogenic-driven exacerbation of chronic drivers, such as rising temperature and CO2, and increasing transient disturbances, including wildfire, drought, windthrow, biotic attack, and land-use change. There are widespread observations of increasing tree mortality due to changing climate and land use, as well as observations of growth stimulation of younger forests due to CO2 fertilization. These antagonistic processes are co-occurring globally, leaving the fate of future forests uncertain. We examine the implications of changing forest demography and its drivers for both future forest management and forecasting impacts of global climate forcing. ADVANCES We reviewed the literature of forest demographic responses to chronic drivers and transient disturbances to generate hypotheses on future trajectories of these factors and their subsequent impacts on vegetation dynamics, with a focus on forested ecosystems. We complemented this review with analyses of global land-use change and disturbance datasets to independently evaluate the implications of changing drivers and disturbances on global-scale tree demographics. Ongoing changes in environmental drivers and disturbance regimes are consistently increasing mortality and forcing forests toward shorter-statured and younger stands, reducing potential carbon storage. Acclimation, adaptation, and migration may partially mitigate these effects. These increased forest impacts are due to natural disturbances (e.g., wildfire, drought, windthrow, insect or pathogen outbreaks) and land-use change, both of which are predicted to increase in magnitude in the future. Atmospherically derived estimates of the terrestrial carbon sink and remote sensing data indicate that tree growth and potentially recruitment may have increased globally in the 20th century, but the growth of this carbon sink has slowed. Variability in growth stimulation due to CO2 fertilization is evident globally, with observations and experiments suggesting that forests benefit from CO2 primarily in early stages of secondary succession. Furthermore, increased tree growth typically requires sufficient water and nutrients to take advantage of rising CO2. Collectively, the evidence reveals that it is highly likely that tree mortality rates will continue to increase, whereas recruitment and growth will respond to changing drivers in a spatially and temporally variable manner. The net impact will be a reduction in forest canopy cover and biomass. OUTLOOK Pervasive shifts in forest vegetation dynamics are already occurring and are likely to accelerate under future global changes, with consequences for biodiversity and climate forcing. This conclusion is robust with respect to the abundant literature evidence and our global assessment of historical demographic changes, but it also forms the basis for hypotheses regarding the patterns and processes underlying the shifts in forest dynamics. These hypotheses will be directly testable using emerging terrestrial and satellite-based observation networks. The existing evidence and newly made observations provide a critical test of Earth system models that continue to improve in their ability to simulate forest dynamics and resulting climate forcing. Ultimately, forest managers and natural resource policies must confront the consequences of changing climate and disturbance regimes to ensure sustainable forests and accrue their associated benefits. A conceptual diagram of the components of forest dynamics and the disturbances that drive them. In the far-left panel, a mature ecosystem is responsive primarily to localized mortality, and the primary drivers of demography are chronically changing variables such as CO2, temperature, and vapor pressure deficit (VPD). In the next panel, the system is disturbed by fire, insect outbreak, or another large-scale perturbation that removes most of the overstory trees, and species adapted to rapid postdisturbance recruitment become established. In the third panel, recruitment and growth dominate demographic processes, with mortality increasing over time as competition leads to self-thinning. In the last panel, a mature ecosystem is dominated by species that have replaced the original community in response to chronic environmental changes, leading to a novel ecosystem. Forest dynamics arise from the interplay of environmental drivers and disturbances with the demographic processes of recruitment, growth, and mortality, subsequently driving biomass and species composition. However, forest disturbances and subsequent recovery are shifting with global changes in climate and land use, altering these dynamics. Changes in environmental drivers, land use, and disturbance regimes are forcing forests toward younger, shorter stands. Rising carbon dioxide, acclimation, adaptation, and migration can influence these impacts. Recent developments in Earth system models support increasingly realistic simulations of vegetation dynamics. In parallel, emerging remote sensing datasets promise qualitatively new and more abundant data on the underlying processes and consequences for vegetation structure. When combined, these advances hold promise for improving the scientific understanding of changes in vegetation demographics and disturbances.