Aim: Past analyses of satellite-based fire activity in tropical savannas support the intermediate fire-productivity hypothesis (IFP), which posits a close correlation with estimates of total net primary productivity in drier savannas and declines towards the extremes. However, these analyses ignore the distinct roles played by herbaceous and woody vegetation in fire ignition and spread.We hypothesize that, as herbaceous vegetation provides the primary fuel, fire activity in African savannas is asymptotically correlated with herbaceous production. Conversely, woody production affects fires indirectly through effects on herbaceous production and its connectivity. In contrast to the IFP, we propose the fuel, cure and connectivity (FCC) conceptual model for tropical fire activity. The FCC model makes explicit the distinct role of herbaceous and woody fuels, avoiding the confounding interpretation of the role of total production, while providing opportunities to quantify fuel curability, effects of trees on herbaceous fuel growth and connectivity, and human management.Location: Sub-Saharan Africa (SSA). Time period: 2003-2015.Major taxa studied: Woody and herbaceous vegetation. Methods:We used boosted regression tree analysis to test competing models explaining fire activity: (a) aggregate fuel loads; and (b) partitioned woody and herbaceous fuel loads; both derived from MODIS leaf area index.Results: Herbaceous fuel load was consistently most influential, providing more explanatory power than overall biomass in fire activity. Fuel curability rated second, then human population density (HPD), and woody biomass was least important. We observed an asymptotic relationship between herbaceous fuel load and fire activity consistent with the FCC model; trees promote fires at low densites but suppress fires at higher densities; fires were rare in wetter regions, emphasizing the need for fuel to cure; and fires were concentrated in areas of low human population, underscoring the crucial role of land management. Conclusions:The proposed FCC framework provides a more nuanced understanding of fire activity in tropical ecosystems, where herbaceous biomass is the key determinant of fire activity. biomass where fuel is no longer limiting. We anticipate that average herbaceous fuel availability increases near-linearly with mean annual rainfall (Deshmukh, 1984;Shorrocks & Bates, 2015). However, the availability of sufficient time for fuels to cure (i.e., to dry enough to burn after an ignition event) will be correlated with dry season length (DSL) and thus inversely proportional to mean annual rainfall. Woody biomass, in contrast, may facilitate herbaceous growth in drier environments but tends to suppress herbaceous growth and reduce connectivity in wetter environments (Archibald, Staver, & Levin, 2012;Dohn et al., 2013). Thus, in contrast to the IFP model, which posits a unimodal response of fire to total productivity, the FCC model posits a family of positive sigmoidal relationships (depending on the variable influence of...
Savannas are widespread global biomes covering ~20% of terrestrial ecosystems on all continents except Antarctica. These ecosystems play a critical role in regulating terrestrial carbon cycle, ecosystem productivity, and the hydrological cycle and contribute to human livelihoods and biodiversity conservation. Despite the importance of savannas in ecosystem processes and human well‐being, the presence of mixed woody and herbaceous components at scales much finer than most medium‐ and coarse‐resolution satellite imagery poses significant challenges to their effective representation in remote sensing and modeling of vegetation dynamics. Although previous studies have attempted to separate woody and herbaceous components, the focus on greenness indices and fractional cover provides little insight into spatiotemporal variability in woody and herbaceous vegetation structure, in particular, leaf area index (LAI). This paper presents a method to partition 1 km spatial resolution Moderate Resolution Imaging Spectroradiometer (MODIS) aggregate green leaf area index (LAIA) from 2003 to 2015, into separate woody (LAIW) and herbaceous (LAIH) constituents in both drought seasonal savannas and moist tropical forests of Sub‐Saharan Africa (SSA). In our analysis, we use an allometric relationship describing the variation in peak within‐canopy woody LAI of dominant tree species (LAIWpinc) across gradients in mean annual precipitation, coupled with independent estimates of woody canopy cover (τw), to constrain seasonally changing LAIW. We present the LAI partitioning approach and highlight the broad spatial and temporal patterns of woody and herbaceous LAI across SSA. The long‐term average 8 day phenologies of woody and herbaceous LAI (averaged across 2003–2015) are available for evaluation, research, and application purposes.
Vegetation buffers local diurnal land surface temperatures, however, this effect has found limited applications for remote vegetation characterization. In this work, we parameterize diurnal temperature variations as the thermal decay rate derived by using satellite daytime and nighttime land surface temperatures and modeled using Newton’s law of cooling. The relationship between the thermal decay rate and vegetation depends on many factors including vegetation type, size, water content, location, and local conditions. The theoretical relationships are elucidated, and empirical relationships are presented. Results show that the decay rate summarizes both vegetation structure and function and exhibits a high correlation with other established vegetation-related observations. As proof of concept, we interpret 15-year spatially explicit trends in the annual thermal decay rates over Africa and discuss results. Given recent increases in availability of finer spatial resolution satellite thermal measurements, the thermal decay rate may be a useful index for monitoring vegetation.
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