Abstract. We developed a new demographic vegetation model, BiomeE, to improve the representation of vegetation demographic dynamics and ecosystem biogeochemical cycles in the NASA Goddard Institute of Space Studies’ ModelE Earth system model. This model includes the processes of plant growth, mortality, reproduction, vegetation structural dynamics, and soil carbon and nitrogen storage and transformations. The model combines the plant physiological processes of ModelE’s original vegetation model, Ent, with minor adaptations to fit the new allometry and vegetation structure with the plant demographic and ecosystem nitrogen processes represented from Geophysical Fluid Dynamics Laboratory (GFDL)’s LM3-PPA. For global applications, we added a new set of plant functional types to represent global vegetation functional diversity, including trees, shrubs, and grasses, and a new phenology model to deal with seasonal changes in temperature and soil water availability. Competition for light and soil resources is individual based, which makes the modeling of transient compositional changes and vegetation succession possible. BiomeE will allow ModelE to simulate long-term biogeophysical and biogeochemical feedbacks between the climate system and land ecosystems. BiomeE simulates, with fidelity comparable to other models, the dynamics of vegetation and soil biogeochemistry, including leaf area index, vegetation structure (e.g., height, tree density, size distribution, crown organization), and ecosystem carbon and nitrogen storage and fluxes. Further, BiomeE will also allow for the simulations of transient vegetation dynamics and eco-evolutionary optimal community assemblage in response to past and future climate changes by incorporating core ecological processes, including demography, competition, and community assembly.
Abstract Our understanding of Earth's carbon climate system depends critically upon interactions between rising atmospheric CO 2 , changing land use, and nitrogen limitation on vegetation growth. Using a global land model, we show how these factors interact locally to generate the global land carbon sink over the past 200 years. Nitrogen constraints were alleviated by N 2 fixation in the tropics and by atmospheric nitrogen deposition in extratropical regions. Nonlinear interactions between land use change and land carbon and nitrogen cycling originated from three major mechanisms: (i) a sink foregone that would have occurred without land use conversion; (ii) an accelerated response of secondary vegetation to CO 2 and nitrogen, and (iii) a compounded clearance loss from deforestation. Over time, these nonlinear effects have become increasingly important and reduce the present‐day net carbon sink by ~40% or 0.4 PgC yr −1 .
Tropical forests contribute a major sink for anthropogenic carbon emissions essential to slowing down the buildup of atmospheric CO 2 and buffering climate change impacts. However, the response of tropical forests to more frequent weather extremes and long-recovery disturbances like fires remains uncertain. Analyses of field data and ecological theory raise concerns about the possibility of the Amazon crossing a tipping point leading to catastrophic tropical forest loss. In contrast, climate models consistently project an enhanced tropical sink. Here, we show a heterogeneous response of Amazonian carbon stocks in GFDL-ESM4.1, an Earth System Model (ESM) featuring dynamic disturbances and height-structured tree–grass competition. Enhanced productivity due to CO 2 fertilization promotes increases in forest biomass that, under low emission scenarios, last until the end of the century. Under high emissions, positive trends reverse after 2060, when simulated fires prompt forest loss that results in a 40% decline in tropical forest biomass by 2100. Projected fires occur under dry conditions associated with El Niño Southern Oscillation and the Atlantic Multidecadal Oscillation, a response observed under current climate conditions, but exacerbated by an overall decline in precipitation. Following the initial disturbance, grassland dominance promotes recurrent fires and tree competitive exclusion, which prevents forest recovery. EC-Earth3-Veg, an ESM with a dynamic vegetation model of similar complexity, projected comparable wildfire forest loss under high emissions but faster postfire recovery rates. Our results reveal the importance of complex nonlinear responses to assessing climate change impacts and the urgent need to research postfire recovery and its representation in ESMs.
Symbiotic nitrogen (N) fixation provides a dominant source of new N to the terrestrial biosphere, yet in many cases the abundance of N-fixing trees appears paradoxical. N-fixing trees, which should be favored when N is limiting, are rare in higher latitude forests where N limitation is common, but are abundant in many lower latitude forests where N limitation is rare. Here, we develop a graphical and mathematical model to resolve the paradox. We use the model to demonstrate that N fixation is not necessarily cost effective under all degrees of N limitation, as intuition suggests. Rather, N fixation is only cost effective when N limitation is sufficiently severe. This general finding, specific versions of which have also emerged from other models, would explain sustained moderate N limitation because N-fixing trees would either turn N fixation off or be outcompeted under moderate N limitation. From this finding, four general hypothesis classes emerge to resolve the apparent paradox of N limitation and N-fixing tree abundance across latitude. The first hypothesis is that N limitation is less common at higher latitudes. This hypothesis contradicts prevailing evidence, so is unlikely, but the following three hypotheses all seem likely. The second hypothesis, which is new, is that even if N limitation is more common at higher latitudes, more severe N limitation might be more common at lower latitudes because of the capacity for higher N demand. Third, N fixation might be cost effective under milder N limitation at lower latitudes but only under more severe N limitation at higher latitudes. This third hypothesis class generalizes previous hypotheses and suggests new specific hypotheses. For example, greater trade-offs between N fixation and N use efficiency, soil N uptake, or plant turnover at higher compared to lower latitudes would make N fixation cost effective only under more severe N limitation at higher latitudes. Fourth, N-fixing trees might adjust N fixation more at lower than at higher latitudes. This framework provides new hypotheses to explain the latitudinal abundance distribution of N-fixing trees, and also provides a new way to visualize them. Therefore, it can help explain the seemingly paradoxical persistence of N limitation in many higher latitude forests.
