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1. Modeling the short-term fire effects on vegetation dynamics and surface energy in southern Africa using the improved SSiB4/TRIFFID-Fire model

   https://doi.org/10.5194/gmd-14-7639-2021  

   Huang, Huilin ; Xue, Yongkang ; Liu, Ye ; Li, Fang ; Okin, Gregory S. ( January 2021 , Geoscientific Model Development)

   Abstract. Fire causes abrupt changes in vegetation properties and modifies fluxexchanges between land and atmosphere at subseasonal to seasonal scales. Yetthese short-term fire effects on vegetation dynamics and surface energybalance have not been comprehensively investigated in the fire-coupledvegetation model. This study applies the SSiB4/TRIFFID-Fire (the SimplifiedSimple Biosphere Model coupled with the Top-down Representation of InteractiveFoliage and Flora Including Dynamics with fire) model to studythe short-term fire impact in southern Africa. Specifically, we aim toquantify how large impacts fire exerts on surface energy throughdisturbances on vegetation dynamics, how fire effects evolve during the fireseason and the subsequent rainy season, and how surface-darkening effectsplay a role besides the vegetation change effects. We find fire causes an annual average reduction in grass cover by 4 %–8 %for widespread areas between 5–20∘ S and a tree cover reductionby 1 % at the southern periphery of tropical rainforests. The regionalfire effects accumulate during June–October and peak in November, thebeginning of the rainy season. After the fire season ends, the grass coverquickly returns to unburned conditions, while the tree fraction hardlyrecovers in one rainy season. The vegetation removal by fire has reduced theleaf area index (LAI) and gross primary productivity (GPP) by 3 %–5 % and5 %–7 % annually. The exposure of bare soil enhances surface albedo andtherefore decreases the absorption of shortwave radiation. Annual meansensible heat has dropped by 1.4 W m−2, while the latent heat reductionis small (0.1 W m−2) due to the compensating effects between canopytranspiration and soil evaporation. Surface temperature is increased by asmuch as 0.33 K due to the decrease of sensible heat fluxes, and the warmingwould be enhanced when the surface-darkening effect is incorporated. Ourresults suggest that fire effects in grass-dominant areas diminish within1 year due to the high resilience of grasses after fire. Yet fire effectsin the periphery of tropical forests are irreversible within one growingseason and can cause large-scale deforestation if accumulated for hundredsof years. 

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2. Tree cover shows strong sensitivity to precipitation variability across the global tropics

   https://doi.org/10.1111/geb.12707  

   Xu, Xiangtao ; Medvigy, David ; Trugman, Anna T. ; Guan, Kaiyu ; Good, Stephen P. ; Rodriguez‐Iturbe, Ignacio ( January 2018 , Global Ecology and Biogeography)

   Vegetation is sensitive to mean annual precipitation (MAP), but the sensitivity of vegetation to precipitation variability (PV) is less clear. Tropical ecosystems are likely to experience increased PV in the future. Here we assessed the importance, magnitude and mechanism of PV effects on tree cover in the context of covarying environmental drivers such as fire, temperature and soil properties.

   Tropical land.

   2000–2010.

   Trees.

   We compiled climate, soil and remotely‐sensed tree cover data over tropical land. We then comprehensively assessed the contribution of PV at different time‐scales to tropical tree cover variations and estimated the sensitivity of tree cover to PV changes by conducting rolling‐window regression and variance decomposition analyses. We further adopted a mechanistic modelling approach to test whether water competition between trees and grasses can explain the observed effect of PV.

   We find that PV contributes 33–56% to the total explained spatial variation (65–79%) in tree cover. The contribution of PV depends on MAP and is highest under intermediate MAP (500–1,500 mm). Tree cover generally increases with rainy day frequency and wet season length but shows mixed responses to inter‐annual PV. Based on the estimated sensitivity, tropical tree cover can decrease by 3–5% overall and by up to 20% in Amazonia under a 20% decrease in rainy days. Mechanistic modelling analysis reproduced the continental differences in tree cover along an MAP gradient.

   Under intermediate rainfall regimes (500–1,500 mm), PV can be a more important determinant of tropical tree cover than conventionally proposed drivers such as MAP and fire. The effect of PV likely results from the sensitivity of tree–grass competition to the temporal distribution of water resources. These results show that climate variability can strongly shape the biosphere.

    

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3. Modeling long-term fire impact on ecosystem characteristics and surface energy using a process-based vegetation–fire model SSiB4/TRIFFID-Fire v1.0

   https://doi.org/10.5194/gmd-13-6029-2020  

   Huang, Huilin ; Xue, Yongkang ; Li, Fang ; Liu, Ye ( January 2020 , Geoscientific Model Development)

   null (Ed.)

   Abstract. Fire is one of the primary disturbances to the distribution and ecologicalproperties of the world's major biomes and can influence the surface fluxesand climate through vegetation–climate interactions. This study incorporatesa fire model of intermediate complexity to a biophysical model with dynamicvegetation, SSiB4/TRIFFID (The Simplified Simple Biosphere Model coupledwith the Top-down Representation of Interactive Foliage and Flora IncludingDynamics Model). This new model, SSiB4/TRIFFID-Fire, updating fire impact onthe terrestrial carbon cycle every 10 d, is then used to simulate theburned area during 1948–2014. The simulated global burned area in 2000–2014is 471.9 Mha yr−1, close to the estimate of 478.1 Mha yr−1 inGlobal Fire Emission Database v4s (GFED4s), with a spatial correlation of0.8. The SSiB4/TRIFFID-Fire reproduces temporal variations of the burnedarea at monthly to interannual scales. Specifically, it captures theobserved decline trend in northern African savanna fire and accuratelysimulates the fire seasonality in most major fire regions. The simulatedfire carbon emission is 2.19 Pg yr−1, slightly higher than the GFED4s(2.07 Pg yr−1). The SSiB4/TRIFFID-Fire is applied to assess the long-term fire impact onecosystem characteristics and surface energy budget by comparing model runswith and without fire (FIRE-ON minus FIRE-OFF). The FIRE-ON simulationreduces tree cover over 4.5 % of the global land surface, accompanied bya decrease in leaf area index and vegetation height by 0.10 m2 m−2and 1.24 m, respectively. The surface albedo and sensible heat are reducedthroughout the year, while latent heat flux decreases in the fire season butincreases in the rainy season. Fire results in an increase in surfacetemperature over most fire regions. 

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4. Spatial transitions in tree cover are associated with soil hydrology, but not with grass biomass, fire frequency, or herbivore biomass in Serengeti savannahs

   https://doi.org/10.1111/1365-2745.13303  

   Holdo, Ricardo M. ; Onderdonk, Daphne A. ; Barr, Annabelle G. ; Mwita, Meshak ; Anderson, T. Michael ; Oliveras, ed., Imma ( October 2019 , Journal of Ecology)

   Although there is a well‐known association between tree cover and soil texture in savannahs, the hydrological drivers of tree cover variation have not been systematically explored, particularly in parallel with factors such as fire, herbivory, and tree–grass interactions. The relationship between hydrological factors and tree cover is important for resolving the relative contribution of bottom‐up versus top‐down factors in structuring savannah vegetation.

   We quantified soil moisture dynamics across eight 1‐km transects spanning tree cover gradients from open to woody savannah in Serengeti National Park in Tanzania using soil moisture sensors coupled with dataloggers. We mapped tree cover at two spatial scales through supervised classification of high‐resolution satellite imagery. We simultaneously produced water retention curves in open and woody habitats within each transect to compare soil hydrological properties and to convert volumetric water content (θ) from dataloggers to plant‐available water over the course of an annual cycle. We also quantified grass biomass at 100 locations per transect, estimated fire frequency from MODIS satellite data, and quantified herbivore occupancy with paired camera traps situated in open and woody habitats within each transect.

   We found a positive relationship between tree cover and soil moisture drainage rate, and found that open habitats had more negative water potentials than woody habitats for a given value ofθ. In contrast, we found no evidence for a consistent relationship between grass biomass or fire frequency and tree cover. We found evidence for higher browser occupancy in woody than open habitats, but no habitat effects on herbivores as a group (browsers plus grazers), suggesting that herbivory is unlikely to be the dominant factor explaining variation in tree cover.

   Synthesis. Our results suggest that variation in tree cover is partly driven by hydrological (edaphic) factors unrelated to fire, herbivory, tree–grass interactions or mean annual precipitation at these spatial scales in Serengeti. We contrast our findings with previous work attributing tree cover shifts in Serengeti to precipitation gradients.

    

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5. Climate–fire interactions constrain potential woody plant cover and stature in North American Great Plains grasslands

   https://doi.org/10.1111/geb.12752  

   Scholtz, R. ; Fuhlendorf, S. D. ; Archer, S. R. ( August 2018 , Global Ecology and Biogeography)

   Disturbances such as fire operate against a backdrop of constraints imposed by climate and soils to influence grass–woody plant abundance. However, little is known of how these factors interact to determine the upper limits of woody cover and stature in grasslands, in which shrub/tree abundance has been increasing globally.

   Kansas, Oklahoma, Texas, USA.

   2004–2014.

   Angiosperms and gymnosperms.

   Using a database of 1,466 sites and quantile regression to derive precipitation‐based upper limits to woody cover and height within grasslands of the central/southern Great Plains, USA, we assessed how soil texture and climate‐related fire probabilities [two groups; low fire probability, P(F_low), versus high fire probability, P(F_hi)] might influence realization of the climate potential.

   Soil texture had no substantive influence on regional‐scale woody cover, but taller plants were predicted on sandy soils. Woody plant height potential increased linearly with increasing annual precipitation, becoming asymptotic atc. 800 mm for both the P(F_low) and the P(F_hi) fire groups, after which P(F_low) areas were predicted to support taller plants. Potential woody cover also increased linearly with annual precipitation untilc. 800 mm, after which predictions of maximum % cover were similar under both fire groups.

   Precipitation was the overriding factor constraining potential woody cover and height, particularly in drier regions, with fire playing a minor role at these regional scales. In contrast to height potential, cover potential remained similar for both P(F_low) and P(F_hi) sites. Dynamic adjustments in woody plant architecture and allocation to foliage and stems, wherein areal cover is maintained when height is suppressed has implications for remote sensing, primary production and biogeochemical processes. Our analyses indicate drier grasslands [< 800 mm mean annual precipitation (MAP)] undergoing woody plant encroachment have the potential to become shrublands (e.g. short woody plants, low cover), whereas wetter areas have the potential to become woodland or forest (taller woody plants, high cover).

    

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