Abstract
Globally, the spatial distribution of vegetation is governed primarily by climatological factors (rainfall and temperature, seasonality, and inter-annual variability). The local distribution of vegetation, however, depends on local edaphic conditions (soils and topography) and disturbances (fire, herbivory, and anthropogenic activities). Abrupt spatial or temporal changes in vegetation distribution can occur if there are positive (i.e., amplifying) feedbacks favoring certain vegetation states under otherwise similar climatic and edaphic conditions. Previous studies in the tropical savannas of Africa and other continents using the MODerate Resolution Imaging Spectroradiometer (MODIS) vegetation continuous fields (VCF) satellite data product have focused on discontinuities in the distribution of tree cover at different rainfall levels, with bimodal distributions (e.g., concentrations of high and low tree cover) interpreted as alternative vegetation states. Such observed bimodalities over large spatial extents may not be evidence for alternate states, as they may include regions that have different edaphic conditions and disturbance histories. In this study, we conduct a systematic multi-scale analysis of diverse MODIS data streams to quantify the presence and spatial consistency of alternative vegetation states in Sub-Saharan Africa. The analysis is based on the premise that major discontinuities in vegetation structure should also manifest as consistent spatial patterns in a range of remote sensing data streams, including, for example, albedo and land surface temperature (LST). Our results confirm previous observations of bimodal and multimodal distributions of estimated tree cover in the MODIS VCF. However, strong disagreements in the location of multimodality between VCF and other data streams were observed at 1 km scale. Results suggest that the observed distribution of VCF over vast spatial extents are multimodal, not because of local-scale feedbacks and emergent bifurcations (the definition of alternative states), but likely because of other factors including regional scale differences in woody dynamics associated with edaphic, disturbance, and/or anthropogenic processes. These results suggest the need for more in-depth consideration of bifurcation mechanisms and thus the likely spatial and temporal scales at which alternative states driven by different positive feedback processes should manifest.
Highlights
Global distributions of vegetation structure, species composition, and functional composition are governed primarily by climate and phylogenetic processes [1,2,3,4,5]
We examine the histograms of MODerate Resolution Imaging Spectroradiometer (MODIS) vegetation continuous fields (VCF), albedo, and land surface temperature (LST) in 200 mm rainfall zones (e.g., 0–200 mm/yr, 200–400 mm/yr, etc.) for all of Sub-Saharan Africa, replicating earlier analyses that used VCF tree cover stratified by rainfall to infer bifurcation dynamics [18,19,24,25,26,27]
We posed the following two questions: (1) are the apparent forest-savanna and savanna-grassland bifurcations detected in earlier VCF analysis present in albedo and surface temperature data?; and (2) to what extent are emergent bifurcations in VCF, albedo, and land surface temperature dependent on the spatial scale of analysis? Our results show that apparent bifurcation in the VCF data, diagnosed via statistical tests for multi-modality, far exceeds bifurcation detections in albedo and land surface temperature data
Summary
Global distributions of vegetation structure, species composition, and functional composition are governed primarily by climate and phylogenetic processes [1,2,3,4,5]. Landscapes with similar climate are not necessarily similar in vegetation structure (e.g., tree cover, density, and height), and patch mosaics emerge at different spatial scales, reflecting different edaphic conditions and disturbance histories. More interesting in the context of this paper, is the degree to which spatial variability in a landscape is (a) proportional to the frequency, intensity, and time since disturbance (e.g., a harvest event that removes a fraction of tree cover in a particular location) and subsequent regrowth (“linear patch dynamics”), or is (b) amplified by positive feedbacks that lead to alternative state dynamics in response to disturbance events (“bifurcating patch dynamics”). The classic example of an amplifying feedback producing bifurcating patch dynamics (between forest and savannas in mesic systems, and savanna and grasslands in drier savannas) is the so-called fire-trap, where an initial loss (or gain) of tree cover promotes (or suppresses) grass growth, providing more (or less) fuel for fires and increased (or decreased) tree mortality in a continuing cycle of tree loss (or gain) [16,17,18,19,20,21,22,23]
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