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

Abstract

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 of θ. 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.

Sapling growth gradients interact with homogeneous disturbance regimes to explain savanna tree cover discontinuities

“…We describe site selection, setup, and tree cover mapping in detail elsewhere (Holdo et al, 2020). Briefly, during Transect key: FS, Four Seasons; IK, Ikoma; MA, Makoma; MM, Mbuzi Mawe; SI, Simiyu; SO, Soit; TA, Tawiri; TO, Togoro three separate field campaigns (in January-February 2016, July 2016 and September-October 2016) we set up eight ~1-km transects straddling transitions from woody to open (i.e., virtually treeless in many cases) habitats across SNP (Figure 1).…”

“…Within each transect, we used a stratified randomization approach to identify 100 saplings and to deploy 10 environmental sensors (Appendix S1). We used Google Earth™ (GE from this point forwards) to map the tree cover fraction at high (~1 m) spatial resolution (Holdo et al, 2020). We used tree cover within a 30-m radius of a given sapling or sensor (TC 30 ) to characterize local tree neighborhoods (Holdo et al, 2020).…”

“…Although this gap can be partly attributable to compositional differences between the overstory and understory, our study suggested that it is at least partly driven by the failure of saplings to recruit into adult stages . In addition, the Serengeti exhibits the type of sudden transition in tree cover (Eby et al, 2017;Holdo et al, 2020) that has been observed in savannas elsewhere (e.g., Veldhuis et al, 2017). In a recent investigation conducted along the tree cover gradients we use for our present study, we showed that tree cover variation is correlated with hydrological drivers caused by edaphic properties rather than by variation in fire frequency or herbivore biomass across space, suggesting that bottom-up factors such as resource availability or moisture stress may be more important determinants of tree cover transitions than topdown factors (Holdo et al, 2020).…”

“…In addition, the Serengeti exhibits the type of sudden transition in tree cover (Eby et al, 2017;Holdo et al, 2020) that has been observed in savannas elsewhere (e.g., Veldhuis et al, 2017). In a recent investigation conducted along the tree cover gradients we use for our present study, we showed that tree cover variation is correlated with hydrological drivers caused by edaphic properties rather than by variation in fire frequency or herbivore biomass across space, suggesting that bottom-up factors such as resource availability or moisture stress may be more important determinants of tree cover transitions than topdown factors (Holdo et al, 2020). That study, however, was based on testing associations between tree cover and a number of external drivers, and did not examine tree demographic responses to those drivers.…”

Seasonal weather and climate prediction over area burned in grasslands of northeast China

“…Burned grassland extent is influenced by regional climate change through increasing temperature which reduces fuel moisture and, thereby, increases flammability [13][14][15][16] , but climate change also alters productivity, biomass abundance, and carbon emissions 17,18 , all factors which drive fuel accumulation dynamics 13,19,20 . Further, effects of changing climates on fire activity may only manifest once critical thresholds are crossed 21,22 .…”