Fisher, R A., Charles D Koven, W Anderegg, B O Christoffersen, M C Dietze, C E Farrior, J A Holm, George C Hurtt, R G Knox, P Lawrence, J W Lichstein, M Longo, A M Matheny, David Medvigy, H C Muller-Landau, T L Powell, S P Serbin, H Sato, J K Shuman, B E Smith, and Anna T Trugman, et al., January 2018: Vegetation demographics in Earth System Models: A review of progress and priorities. Global Change Biology, 24(1), DOI:10.1111/gcb.13910. Abstract
Numerous current efforts seek to improve the representation of ecosystem ecology and vegetation demographic processes within Earth System Models (ESMs). These developments are widely viewed as an important step in developing greater realism in predictions of future ecosystem states and fluxes. Increased realism, however, leads to increased model complexity, with new features raising a suite of ecological questions that require empirical constraints. Here, we review the developments that permit the representation of plant demographics in ESMs, and identify issues raised by these developments that highlight important gaps in ecological understanding. These issues inevitably translate into uncertainty in model projections but also allow models to be applied to new processes and questions concerning the dynamics of real-world ecosystems. We argue that stronger and more innovative connections to data, across the range of scales considered, are required to address these gaps in understanding. The development of first-generation land surface models as a unifying framework for ecophysiological understanding stimulated much research into plant physiological traits and gas exchange. Constraining predictions at ecologically relevant spatial and temporal scales will require a similar investment of effort and intensified inter-disciplinary communication.
Trugman, Anna T., and David Medvigy, et al., January 2018: Sensitivity of woody carbon stocks to bark investment strategy in Neotropical savannas and forests. Biogeosciences, 15(1), DOI:10.5194/bg-15-233-2018. Abstract
Fire frequencies are changing in Neotropical savannas and forests as a result of forest fragmentation and increasing drought. Such changes in fire regime and climate are hypothesized to decrease the stability of tropical carbon storage, but there has been little consideration of the widespread variability in tree fire tolerance strategies. To test how aboveground carbon stocks change with fire frequency and community composition, we update the ED2 model with (i) a fire survivorship module based on tree bark thickness (a key fire-tolerance trait across woody plants in savannas and forests), and (ii) plant functional types representative of trees in the region. With these updates, the model is better able to predict how fire frequency affects population demography and aboveground woody carbon. Simulations illustrate that the high survival rate of thick-barked, large trees reduces carbon losses with increasing fire frequency, with high investment in bark being particularly important in reducing losses in the wettest sites. Additionally, in landscapes that frequently burn, bark investment can broaden the range of climate and fire conditions under which savannas occur by reducing the range of conditions leading to either complete tree loss or complete grass loss. These results highlight that woody biomass carbon stocks in the tropics depend not only on changing fire frequencies, but also on tree fire survival strategy. Incorporation of a bark investment strategy in vegetation models holds promise for improving predictions of landscape-level carbon dynamics and savanna distribution, particularly in the context of global climate change.
Oh, Youmi, B Stackhouse, M C Y Lau, X Xu, Anna T Trugman, J Moch, T C Onstott, C J Jørgensen, L D'Imperio, B Elberling, C A Emmerton, V L St. Louis, and David Medvigy, May 2016: A scalable model for methane consumption in arctic mineral soils. Geophysical Research Letters, 43(10), DOI:10.1002/2016GL069049. Abstract
Recent field studies have documented a surprisingly strong and consistent methane sink in arctic mineral soils, thought to be due to high-affinity methanotrophy. However, the distinctive physiology of these methanotrophs is poorly represented in mechanistic methane models. We developed a new model, constrained by microcosm experiments, to simulate the activity of high-affinity methanotrophs. The model was tested against soil core thawing experiments and field-based measurements of methane fluxes, and was compared to conventional mechanistic methane models. Our simulations show that high-affinity methanotrophy can be an important component of the net methane flux from arctic mineral soils. Simulations without this process overestimate methane emissions. Furthermore, simulations of methane flux seasonality are improved by dynamic simulation of active microbial biomass. Because a large fraction of the Arctic is characterized by mineral soils, high-affinity methanotrophy will likely have a strong effect on its net methane flux.
Trugman, Anna T., N J Fenton, Y Bergeron, X Xu, L R Welp, and David Medvigy, September 2016: Climate, soil organic layer, and nitrogen jointly drive forest development after fire in the North American boreal zone. Journal of Advances in Modeling Earth Systems, 8(3), DOI:10.1002/2015MS000576. Abstract
Previous empirical work has shown that feedbacks between fire severity, soil organic layer thickness, tree recruitment, and forest growth are important factors controlling carbon accumulation after fire disturbance. However, current boreal forest models inadequately simulate this feedback. We address this deficiency by updating the ED2 model to include a dynamic feedback between soil organic layer thickness, tree recruitment, and forest growth. The model is validated against observations spanning monthly to centennial time scales and ranging from Alaska to Quebec. We then quantify differences in forest development after fire disturbance resulting from changes in soil organic layer accumulation, temperature, nitrogen availability, and atmospheric CO2. First, we find that ED2 accurately reproduces observations when a dynamic soil organic layer is included. Second, simulations indicate that the presence of a thick soil organic layer after a mild fire disturbance decreases decomposition and productivity. The combination of the biological and physical effects increases or decreases total ecosystem carbon depending on local conditions. Third, with a 4°C temperature increase, some forests transition from undergoing succession to needleleaf forests to recruiting multiple cohorts of broadleaf trees, decreasing total ecosystem carbon by ∼40% after 300 years. However, the presence of a thick soil organic layer due to a persistently mild fire regime can prevent this transition and mediate carbon losses even under warmer temperatures. Fourth, nitrogen availability regulates successional dynamics; broadleaf species are less competitive with needleleaf trees under low nitrogen regimes. Fifth, the boreal forest shows additional short-term capacity for carbon sequestration as atmospheric CO2 increases.
Williams, A P., Richard Seager, M Berkelhammer, A K Macalady, M A Crimmins, T W Swetnam, and Anna T Trugman, et al., December 2014: Causes and Implications of Extreme Atmospheric Moisture Demand during the Record-Breaking 2011 Wildfire Season in the Southwest United States. Journal of Applied Meteorology and Climatology, 53(12), DOI:10.1175/JAMC-D-14-0053.1. Abstract
In 2011, exceptionally low atmospheric moisture content combined with moderately high temperatures to produce record-high vapor-pressure deficit (VPD) in the southwestern United States (SW). These conditions combined with record-low cold-season precipitation to cause widespread drought and extreme wildfires. Although interannual VPD variability is generally dominated by temperature, high VPD in 2011 was also driven by lack of atmospheric moisture. May–July 2011 dew point in the SW was 5.1 standard deviations below the long-term mean. Lack of atmospheric moisture was promoted by already very dry soils and amplified by a strong ocean-to-continent sea-level pressure gradient and upper-level convergence that drove dry northerly winds and subsidence upwind of and over the SW. Subsidence drove divergence of rapid and dry surface winds over the SW, suppressing southerly moisture imports and removing moisture from already dry soils. By the 2050s, model projections developed for the fifth phase of the Coupled Model Intercomparison Project (CMIP5) suggest that warming trends will cause mean warm-season VPD to be comparable to the record-high VPD observed in 2011. CMIP5 projections also suggest increased interannual variability of VPD, independent of trends in background mean levels, due to increased variability of dew point, temperature, vapor pressure, and saturation vapor pressure. Increased variability in VPD translates to increased probability of 2011-type VPD anomalies, which would be superimposed on ever-greater background VPD levels. While temperature will continue to be the primary driver of interannual VPD variability, 2011 served as an important reminder that atmospheric moisture content can also drive impactful VPD anomalies.
