Alternative Vegetation States in Tropical Forests and Savannas: The Search for Consistent Signals in Diverse Remote Sensing Data

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.

Fire regimes, fire experiments and alternative stable states in mesic savannas

“…Nevertheless, it is also clear that the results of Veenendaal et al (2018) are not in agreement with several hypotheses recently presented regarding the effects of fires as influenced by time of burning and/or precipitation regime (Laris et al, 2016(Laris et al, , 2017. Perhaps more importantly, however, much of the 'well-established' evidence that Laris and Jacobs present to support their arguments (Staver et al, 2011a,b) should, in our view, be considered as little more than hypotheses arising from in silico interpretations of a remote sensing product with clear data fidelity issues: for extensive discussions of this issue see papers by Hanan et al (2013), Staver & Hansen (2015), Veenendaal et al (2015), Lloyd & Veenendaal (2016), Gerard et al (2017), Wuyts et al (2017, Gross et al (2018), Kumar et al (2019) and Adzhar et al (2021). This contrasts with the Veenendaal et al (2018) study which aimed to test the ASS theory for the forest-savanna transition using actual observational field data from all 11 quantifiable fire trials available to us.…”

MODIS Vegetation Continuous Fields tree cover needs calibrating in tropical savannas

“…Special care should be taken in savannas, a biome that has long been noted as being challenging for EO products to characterize, as solitary trees in the landscape tend to be missed by global tree cover products (Jung et al, 2006;Brandt et al, 2020). The poor performance of MODIS VCF and Landsat TCC in savannas in particular (Gaughan et al, 2013;Gross et al, 2018;Kumar et al, 2019) emphasizes the importance of continuous independent validation and re-calibration of these products. The ecosystem functions of savannas can vary drastically with just a slight difference in tree cover (Gaughan et al, 2013), and even slight errors may create issues in how we interpret the state and dynamics of the biome, which in turn affects how the land is managed.…”

Remote Sensing of Savannas and Woodlands: Editorial

“…Next come four articles on various aspects of savanna and woodland vegetation in Brazil [23][24][25][26]. These are followed by two articles on African environments that include a mix of savannas and woodlands and other vegetation [27,28]. The final two articles address functional ecological aspects of two different types of northern hemisphere oak savannas in North America [29] and Europe [30].…”

“…The two articles based in Africa explore aspects of savanna and woodland vegetation structure and temporal persistence using MODIS data. Kumar et al [27] examine apparent differences in spatial structure of woody vegetation across sub-Saharan Africa retrieved from different MODIS products. They suggest that multimodal spatial structure is regionally disaggregated, and that apparent spatial structure retrieved may be dependent upon the remote sensing product and spatial scale of retrievals.…”

“…It notable that only two papers were focused on the understory grassland [19,30]. Nevertheless, there is still a role for polar orbiting medium resolution sensors [19,27,28] and a pressing need for a for a modern imaging spectrometer in orbit.…”