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.
Khanna, Jaya, and David Medvigy, et al., April 2018: Regional hydroclimatic variability due to contemporary deforestation in southern Amazonia and associated boundary layer characteristics. Journal of Geophysical Research: Oceans, 123(8), DOI:10.1002/2017JD027888. Abstract
Amazonian deforestation causes systematic changes in regional dry season precipitation. Some of these changes at contemporary large scales (a few hundreds of kilometers) of deforestation have been associated with a ‘dynamical mesoscale circulation’, induced by the replacement of rough forest with smooth pasture. In terms of decadal averages, this dynamical mechanism yields increased precipitation in downwind regions and decreased precipitation in upwind regions of deforested areas. Daily, seasonal, and interannual variations in this phenomenon may exist, but have not yet been identified or explained. This study uses observations and numerical simulations to develop relationships between the dynamical mechanism and the local‐ and continental‐scale atmospheric conditions across a range of time scales. It is found that the strength of the dynamical mechanism is primarily controlled by the regional‐scale thermal and dynamical conditions of the boundary layer, and not by the continental‐ and global‐scale atmospheric state. Lifting condensation level and wind speed within the boundary layer have large and positive correlations with the strength of the dynamical mechanism. The strength of these relationships depends on time scale and is strongest over the seasonal cycle. Overall, the dynamical mechanism is found to be strongest during times when the atmosphere is relatively stable. Hence, for contemporary large scales of deforestation this phenomenon is found to be the prevalent convective triggering mechanism during the dry and parts of transition seasons (especially during the dry‐to‐wet transition), significantly affecting the hydroclimate during this period.
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.
Buermann, W, C Beaulieu, B Parida, David Medvigy, G J Collatz, Justin Sheffield, and Jorge L Sarmiento, March 2016: Climate-driven shifts in continental net primary production implicated as a driver of a recent abrupt increase in the land carbon sink. Biogeosciences, DOI:10.5194/bg-13-1597-2016. Abstract
The World's ocean and land ecosystems act as sinks for anthropogenic CO2, and over the last half century their combined sink strength grew steadily with increasing CO2 emissions. Recent analyses of the global carbon budget, however, uncovered an abrupt, substantial (~ 1 PgC yr−1) and sustained increase in the land sink in the late 1980s whose origin remains unclear. In the absence of this prominent shift in the land sink, increases in atmospheric CO2 concentrations since the late 1980s would have been ~ 30 % larger than observed (or ~ 12 ppm above current levels). Global data analyses are limited in regards to attributing causes to changes in the land sink because different regions are likely responding to different drivers. Here, we address this challenge by using terrestrial biosphere models constrained by observations to determine if there is independent evidence for the abrupt strengthening of the land sink. We find that net primary production has significantly increased in the late 1980s (more so than heterotrophic respiration) consistent with the inferred increase in the global land sink, and that large-scale climate anomalies are responsible for this shift. We identify two key regions in which climatic constraints on plant growth have eased: northern Eurasia experienced warming, and northern Africa received increased precipitation. Whether these changes in continental climates are connected is uncertain, but North Atlantic climate variability is important. Our findings suggest that improved understanding of climate variability in the North Atlantic may be essential for more credible projections of the land sink under 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.
Khanna, Jaya, and David Medvigy, December 2014: Strong control of surface roughness variations on the simulated dry season regional atmospheric response to contemporary deforestation in Rondônia, Brazil. Journal of Geophysical Research: Atmospheres, 119(23), DOI:10.1002/2014JD022278. Abstract
The atmospheric effects of Amazon deforestation have frequently been studied in the context of small scales (≈1 km) and very large scales (hundreds of km). However, analysis of intermediate-scale deforestation (tens of km) has received less attention, despite the fact that it better represents the contemporary landscape in some parts of the Amazon. In this study, the dynamic and thermodynamic effects of contemporary intermediate-scale deforestation in Rondônia, Brazil are investigated through variable-resolution GCM simulations carried out with the Ocean-Land-Atmosphere Model. In particular, the atmospheric response to surface roughness changes brought about by deforestation is emphasized. This study shows that reductions in surface roughness associated with intermediate-scale deforestation give rise to a mesoscale circulation. This circulation is capable of convective triggering, but it also weakens the turbulent exchange of energy between land and atmosphere. Furthermore,this mesoscale circulation has distinct impacts on the hydroclimates of the western and eastern halves of Rondônia, increasing shallow cloudiness in the former while suppressing it in the latter. These results show that the atmospheric responseto contemporary intermediate-scale deforestation in Rondônia is likely to be more influenced by differences in surface roughness between forest and forest clearings than by the differences in the surface energy partitioning.
Jeong, S-J, David Medvigy, Elena Shevliakova, and Sergey Malyshev, January 2013: Predicting changes in temperate forest budburst using continental-scale observations and models. Geophysical Research Letters, 40(2), DOI:10.1029/2012GL054431. Abstract
A new framework for understanding the macro-scale variations in spring phenology is developed by using new data from the USA National Phenology Network. Changes in spring budburst for the U.S. are predicted by using Coupled Model Intercomparison Project phase 5 outputs. Macro-scale budburst simulations for the coming century indicate that projected warming leads to earlier budburst by up to 17 days. The latitudinal gradient of budburst becomes less pronounced due to spatially-varying sensitivity of budburst to climate change, even in the most conservative emissions scenarios. Currently existing inter-species differences in budburst date are predicted to become smaller, indicating the potential for secondary impacts at the ecosystem level. We expect that these climate-driven changes in phenology will have large effects on the carbon budget of U.S. forests and these controls should be included in dynamic global vegetation models.
Medvigy, David, R L Walko, M J Otte, and S Avnery, November 2013: Simulated changes in Northwest US climate in response to Amazon deforestation. Journal of Climate, 26(22), DOI:10.1175/JCLI-D-12-00775.1. Abstract
Numerical models have long predicted that the deforestation of the Amazon would lead to large regional changes in precipitation and temperature, but the extratropical effects of deforestation have been a matter of controversy. This paper investigates the simulated impacts of deforestation on Northwest United States December-January-February climate. Integrations are carried out using the Ocean-Land-Atmosphere Model (OLAM), here run as a variable-resolution atmospheric GCM, configured with three alternative horizontal grid meshes: (1) 25 km characteristic length scale (CLS) over the US, 50 km CLS over the Andes and Amazon, and 200 km CLS in the far-field; (2) 50 km CLS over the US, 50 km CLS over the Andes and Amazon, and 200 km CLS in the far-field; (3) 200 km CLS globally. In the high-resolution simulations, deforestation causes a redistribution of precipitation within the Amazon, accompanied by vorticity and thermal anomalies. These anomalies set up Rossby waves that propagate into the extratropics and impact western North America. Ultimately, Amazon deforestation results in 10-20% precipitation reductions for the coastal Northwest US and the Sierra Nevada. Snowpack in the Sierra Nevada experiences declines of up to 50%. However, in the coarse-resolution simulations, this mechanism is not resolved, and precipitation is not reduced in the Northwest US. These results highlight the need for adequate model resolution in modeling the impacts of Amazon deforestation. It is concluded that the deforestation of the Amazon can act as a driver of regional climate change in the extratropics, including areas of the western US that are agriculturally important.
Beaulieu, C, Jorge L Sarmiento, Sara E Mikaloff-Fletcher, Jie Chen, and David Medvigy, January 2012: Identification and characterization of abrupt changes in the land uptake of carbon. Global Biogeochemical Cycles, 26, GB1007, DOI:10.1029/2010GB004024. Abstract
A recent study of the net land carbon sink estimated using the Mauna Loa, Hawaii atmospheric CO2 record, fossil fuel estimates, and a suite of ocean models suggests that the mean of the net land carbon uptake remained approximately constant for three decades and increased after 1988/1989. Due to the large variability in the net land uptake, it is not possible to determine the exact timing and nature of the increase robustly by visual inspection. Here, we develop a general methodology to objectively determine the nature and timing of the shift in the net land uptake based on the Schwarz Information Criterion. We confirm that it is likely that an abrupt shift in the mean net land carbon uptake occurred between 1986-1993 (with a probability between 55-61%), with 1988 being the most likely time for the shift. After taking into account the variability in the net land uptake due to the influence of volcanic aerosols and the El Niño Southern Oscillation, we find that it is most likely that there is a remaining step increase of about 1 Pg C/yr at the same time (with a probability of 55-60%). Thus, we conclude that neither the effect of volcanic eruptions nor the El Niño Southern Oscillation are the causes of the sudden increase of the land carbon sink. By also applying our methodology to the atmospheric growth rate of CO2, we demonstrate that it is likely that the atmospheric growth rate of CO2 exhibits a step decrease between two fitted lines in 1988-1989, which is most likely due to the shift in the net land uptake of carbon.
Jeong, S-J, David Medvigy, Elena Shevliakova, and Sergey Malyshev, March 2012: Uncertainties in terrestrial carbon budgets related to spring phenology. Journal of Geophysical Research: Biogeosciences, 117, G01030, DOI:10.1029/2011JG001868. Abstract
In temperate regions, the budburst date of deciduous trees is mainly regulated by temperature variation, but the exact nature of the temperature dependence has been a matter of debate. One hypothesis is that budburst date depends purely on the accumulation of warm temperature; a competing hypothesis states that exposure to cold temperatures is also important for budburst. In this study, variability in budburst is evaluated using 15 years of budburst data for 17 tree species at Harvard Forest. We compare two budburst hypotheses through reversible jump Markov Chain Monte Carlo. Then, we investigate how uncertainties in budburst date mapped into uncertainties in ecosystem carbon using GFDL's LM3 land model. For 15 of 17 species, we find that more complicated budburst models that account for a chilling period are favored over simpler models that do not include such dependence. LM3 simulations show that the choice of budburst model induces differences in the timing of carbon uptake commencement of ~11 days, in the magnitude of April-May carbon uptake of ~1.03 g C m-2 day-1, and in total ecosystem carbon stocks of ~2 kg C m-2. While the choice of whether to include a chilling period in the budburst model strongly contributes to this variability, another important factor is how the species-dependent field data gets mapped into LM3's single deciduous plant functional type (PFT). We conclude budburst timing has a strong impact on simulated CO2 fluxes, and uncertainty in the fluxes can be substantially reduced by improving the model's representation of PFT diversity.
