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Multiple stable states, bifurcations and thresholds are fashionable concepts in the ecological literature, a recognition that complex ecosystems may at times exhibit the interesting dynamic behaviours predicted by relatively simple biomathematical models. Recently, several papers in Global Ecology and Biogeography, Proceedings of the National Academy of Sciences USA, Science and elsewhere have attempted to quantify the prevalence of alternate stable states in the savannas of Africa, Australia and South America, and the tundra–taiga–grassland transitions of the circum‐boreal region using satellite‐derived woody canopy cover. While we agree with the logic that basins of attraction can be inferred from the relative frequencies of ecosystem states observed in space and time, we caution that the statistical methodologies underlying the satellite product used in these studies may confound our ability to infer the presence of multiple stable states. We demonstrate this point using a uniformly distributed ‘pseudo‐tree cover’ database for Africa that we use to retrace the steps involved in creation of the satellite tree‐cover product and subsequent analysis. We show how classification and regression tree (CART)‐based products may impose discontinuities in satellite tree‐cover estimates even when such discontinuities are not present in reality. As regional and global remote sensing and geospatial data become more easily accessible for ecological studies, we recommend careful consideration of how error distributions in remote sensing products may interact with the data needs and theoretical expectations of the ecological process under study.

1. The stress-gradient hypothesis (SGH) predicts an increasing importance of facilitative mechanisms relative to competition along gradients of increasing environmental stress. Although developed across a variety of ecosystems, the SGH's relevance to the dynamic tree—grass systems of global savannas remains unclear. Here, we present a meta-analysis of empirical studies to explore emergent patterns of tree—grass relationships in global savannas in the context of the SGH. 2. We quantified the net effect of trees on understorey grass production relative to production away from tree canopies along a rainfall gradient in tropical and temperate savannas and compared these findings to the predictions of the SGH. We also analysed soil and plant nutrient concentrations in subcanopy and open-grassland areas to investigate the potential role of nutrients in determining grass production in the presence and absence of trees. 3. Our meta-analysis revealed a shift from net competitive to net facilitative effects of trees on subcanopy grass production with decreasing annual precipitation, consistent with the SGH. We also found a significant difference between sites from Africa and North America, suggesting differences in tree—grass interactions in the savannas of tropical and temperate regions. 4. Nutrient analyses indicate no change in nutrient ratios along the rainfall gradient, but consistent nutrient enrichment under tree canopies. 5. Synthesis. Our results help to resolve questions about the SGH in semi-arid systems, demonstrating that in mixed tree—grass systems, trees facilitate grass growth in drier regions and suppress grass growth in wetter regions. Relationships differ, however, between African and North American sites representing tropical and temperate bioclimates, respectively. The results of this meta-analysis advance our understanding of tree—grass interactions in savannas and contribute a valuable data set to the developing theory behind the SGH.

The majority of research on savanna vegetation dynamics has focused on the coexistence of woody and herbaceous vegetation. Interactions among woody plants in savannas are relatively poorly understood. We present data from a 10-yr longitudinal study of spatially explicit growth patterns of woody vegetation in an East African savanna following exclusion of large herbivores and in the absence of fire. We examined plant spatial patterns and quantified the degree of competition among woody individuals. Woody plants in this semiarid savanna exhibit strongly clumped spatial distributions at scales of 1–5 m. However, analysis of woody plant growth rates relative to their conspecific and heterospecific neighbors revealed evidence for strong competitive interactions at neighborhood scales of up to 5 m for most woody plant species. Thus, woody plants were aggregated in clumps despite significantly decreased growth rates in close proximity to neighbors, indicating that the spatial distribution of woody plants in this region depends on dispersal and establishment processes rather than on competitive, density-dependent mortality. However, our documentation of suppressive effects of woody plants on neighbors also suggests a potentially important role for tree-tree competition in controlling vegetation structure and indicates that the balanced-competition hypothesis may contribute to well-known patterns in maximum tree cover across rainfall gradients in Africa.

Staver & Hansen (2015, Global Ecology and Biogeography y doi: 10.1111/geb.12285) comment on our recent paper (Hanan et al., Global Ecology and Bioge ography, 2014, 23, 259-263) in which we argue that classification and regression tree methods used with remote sensing data to predict tree cover may bias inference of bifurcations in savanna vegetation communities. While we agree with several of their comments, we remain unconvinced that a remote sensing product based on an inherently discontinuous statistical approach can, or should, be used to test for discontinuities.

S taver & H ansen (2015, G lobal E cology and B iogeography , doi: 10.1111/geb.12285) comment on our recent paper ( H anan et al ., G lobal E cology and B iogeography , 2014, 23 , 259–263) in which we argue that classification and regression tree methods used with remote sensing data to predict tree cover may bias inference of bifurcations in savanna vegetation communities. While we agree with several of their comments, we remain unconvinced that a remote sensing product based on an inherently discontinuous statistical approach can, or should, be used to test for discontinuities.

Savannas, characterized by the co-dominance of herbaceous and woody vegetation, support an estimated 20% of the global human population and account for ∼30% of terrestrial net primary productivity. Interactions among savanna trees and grasses determine important ecosystem functions such as hydrological and biogeochemical cycles and production and transpiration rates, and impact the availability of resources (fuel-wood, grass for livestock) fundamental for human wellbeing. Additionally, interactions among trees may be an important driver of savanna vegetation structure, though few existing studies empirically estimate the intensity and importance of savanna tree-tree interactions. A clear understanding of the mechanisms that govern the coexistence of trees and grasses and their interactions in savanna landscapes is crucial to our ability to predict their responses to changing climatic and anthropogenic disturbance regimes. I present research aimed at advancing our understanding of emergent trends in savanna plant interactions and the underlying mechanisms responsible for observed patterns. First, I present the results of a meta-analysis of empirical studies that quantify the net effect of savanna trees on grass production under tree canopies relative to production away from trees. We found that the effect of trees on subcanopy herbaceous production varies predictably with climate, such that trees in arid savannas generally promote grass growth and trees in mesic regions suppress growth. This finding is consistent with a general theoretical model predicting the relative importance of facilitative processes for species coexistence. Termed the stress gradient hypothesis (SGH), the theory predicts an increasing importance of facilitation with increasing environmental stress, such as high water-stress typical of arid savannas. I then present results from two empirical studies designed to experimentally test the predictions of the SGH and infer mechanistic drivers by relating abiotic covariates to plant growth in the presence and absence of neighbors. In the shortgrass steppe (SGS) in northeastern Colorado, we found a net-neutral effect of shrubs and grasses on the other life form, contrary to expected facilitation. We suggest shrub morphology and interactive effects of topography and soil texture are primarily responsible for observed patterns of growth. At five savanna field sites situated along a rainfall gradient (i.e. water-stress gradient) in Mali, West Africa, we found the net effect of trees on grass growth to be consistent with SGH predictions. Light availability and distance to tree boles best explained shifts in herbaceous production along the rainfall gradient. Lastly, I present results from a longitudinal study in an East African savanna estimating tree growth as a function of the size and distance of neighboring woody competitors. In so doing, we quantified the magnitude of inter-tree competition and inferred its impact on stand spatial structure through spatial point pattern analysis. Overall, this research increases our understanding of biotic interactions among savanna plants. The effects of savanna trees on subcanopy grass production generally conform to the predictions of the SGH, and appear to be mediated by microclimate modification by tree canopies related to light availability and water balance. The effects of grasses on trees along environmental gradients are less clear, though we found net neutral effects on woody growth over one growing season in tropical and temperate shrub-grass systems, suggesting that active competitive and facilitative mechanisms largely offset, or that the effects of grasses on plant-available resources for woody species are negligible. Finally, we found that shrubs aggregate at local scales, despite significant neighbor competition. We suggest competition among woody plants influences production and relative species abundance, but dispersal and establishment bottlenecks are likely more important for landscape-scale spatial structure. These results have important implications for our theoretical understanding of coexistence between woody and herbaceous vegetation. Furthermore, we provide empirical data that can be used to refine and parameterize vegetation models predicting savanna ecosystem processes and the global distribution of mixed tree-grass systems.

Theory and empirical evidence for the impacts of fire and herbivory in savannas is well established – they are top-down disturbances that maintain savannas in disequilibrium states away from potential tree cover. In African savannas the demand for fuelwood is extremely high, so tree harvest likely also has an impact, both directly and indirectly, on tree cover, density, and biomass. Many savanna trees resprout vigorously from the base after harvest. However, harvested trees regenerate as saplings susceptible to fire and browsing, so harvest may have important demographic consequences. Here, we report the effects of tree harvest, and its interaction with fire and herbivory, on savanna dynamics by analyzing woody regrowth following a harvest in arid Sahelian and mesic Guinean savannas in Mali, West Africa. Tree harvest resulted in an overall reduction in wood production per tree compared to growth in non-harvested trees. Regrowth, either biomass or height, did not differ among fire and herbivory treatments. Our results suggest that the resprouting abilities that savanna trees have evolved to cope with frequent fire are essential for surviving tree harvest and subsequent disturbance. In these savannas, regrowth is rapid enough in the first growing season to escape the impact of dry season fires.