Jayashree Ratnam

Savanna vegetation-fire-climate relationships differ among continents.

Surveying Savannas Savannas are structurally similar across the three major continents where they occur, leading to the assumption that the factors controlling vegetation structure and function are broadly similar, too. Lehmann et al. (p. 548) report the results of an extensive analysis of ground-based tree abundance in savannas, sampled at more than 2000 sites in Africa, Australia, and South America. All savannas, independent of region, shared a common functional property in the way that moisture and fire regulated tree abundance. However, despite qualitative similarity in the moisture–fire–tree-biomass relationships among continents, key quantitative differences exist among the three regions, presumably as a result of unique evolutionary histories and climatic domains. Evolution cannot be overlooked when aiming to predict the potential global impacts on savanna dynamics in a warming world. Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.

Native ungulates of diverse body sizes collectively regulate long-term woody plant demography and structure of a semi-arid savanna

Summary 1. Large mammalian herbivores are well recognized to play important roles in regulating woody cover and biomass in savannas, but the extent to which browsing ungulates are capable of regulating woody populations in the absence of other disturbances such as fire is unclear. Moreover, the degree to which browser effects on savannas operate through effects on woody plant recruitment vs. mortality has rarely been examined. 2. We conducted a 10-year, replicated herbivore exclusion experiment in a semi-arid savanna in East Africa (mean annual rainfall = 514 mm), where fires have been actively suppressed for decades. Browsers dramatically influenced recruitment, growth and mortality of all size classes of woody vegetation. A decade of herbivore exclusion resulted in a sevenfold increase in recruitment, a 2.5-fold decrease in mortality and a threefold increase in woody biomass inside exclosures, while biomass outside exclosures remained relatively unchanged. 3. At the plant community level, extensive browsing of seedlings by small-bodied ungulates suppressed woody recruitment in this semi-arid system, generating a ‘browsing trap’ comparable to the ‘fire trap’ reported for mesic systems. Browsing by large- and medium-bodied ungulates reduced both growth and survival of individuals in larger size classes. 4. At the plant species level, browser impacts were variable. Although browsers negatively influenced recruitment of all species, they had little to no impact on the mortality of some dominant species, resulting in a longterm, browser-driven shift in woody species composition that was largely mediated via their differential effects on plant mortality rates rather than recruitment. 5. Synthesis. Our results demonstrate unequivocally that, even in the absence of fire, native browsing ungulates can exert dramatic ‘top-down’ controls in semi-arid savannas, influencing all aspects of woody plant demography. Besides suppressing woody plant recruitment, browsers can also have substantial cumulative long-term impacts on growth and mortality rates of woody plants, including adults, which can differ between species in ways that fundamentally alter the structure and function of woody vegetation. In semi-arid rangelands, intact communities of native browsing ungulates thus provide a critical ecosystem service by regulating woody cover, and their removal (or extinction) from these systems can lead to rapid woody encroachment.

Contrasting effects of defaunation on aboveground carbon storage across the global tropics

Defaunation is causing declines of large-seeded animal-dispersed trees in tropical forests worldwide, but whether and how these declines will affect carbon storage across this biome is unclear. Here we show, using a pan-tropical data set, that simulated declines of large-seeded animal-dispersed trees have contrasting effects on aboveground carbon stocks across Earths tropical forests. In our simulations, African, American and South Asian forests, which have high proportions of animal-dispersed species, consistently show carbon losses (2–12%), but Southeast Asian and Australian forests, where there are more abiotically dispersed species, show little to no carbon losses or marginal gains (±1%). These patterns result primarily from changes in wood volume, and are underlain by consistent relationships in our empirical data (∼2,100 species), wherein, large-seeded animal-dispersed species are larger as adults than small-seeded animal-dispersed species, but are smaller than abiotically dispersed species. Thus, floristic differences and distinct dispersal mode–seed size–adult size combinations can drive contrasting regional responses to defaunation.

Savannahs of Asia: antiquity, biogeography, and an uncertain future

The savannahs of Asia remain locally unrecognized as distinctive ecosystems, and continue to be viewed as degraded forests or seasonally dry tropical forests. These colonial-era legacies are problematic, because they fail to recognize the unique diversity of Asian savannahs and the critical roles of fire and herbivory in maintaining ecosystem health and diversity. In this review, we show that: the palaeo-historical evidence suggests that the savannahs of Asia have existed for at least 1 million years, long before widespread landscape modification by humans; savannah regions across Asia have levels of C4 grass endemism and diversity that are consistent with area-based expectations for non-Asian savannahs; there are at least three distinct Asian savannah communities, namely deciduous broadleaf savannahs, deciduous fine-leafed and spiny savannahs and evergreen pine savannahs, with distinct functional ecologies consistent with fire- and herbivory-driven community assembly. Via an analysis of savannah climate domains on other continents, we map the potential extent of savannahs across Asia. We find that the climates of African savannahs provide the closest analogues for those of Asian deciduous savannahs, but that Asian pine savannahs occur in climates different to any of the savannahs in the southern continents. Finally, we review major threats to the persistence of savannahs in Asia, including the mismanagement of fire and herbivory, alien woody encroachment, afforestation policies and future climate uncertainty associated with the changing Asian monsoon. Research agendas that target these issues are urgently needed to manage and conserve these ecosystems. This article is part of the themed issue ‘Tropical grassy biomes: linking ecology, human use and conservation’.

African and Asian Savannas

We review the distribution, ecology, and environmental controls of savanna structure and function, with a focus on African and Asian savannas. Savannas or mixed tree–grass systems occupy most of Africa but cover less of Asia. On both continents, they support unique faunal and floral assemblages. Rainfall, soil nutrients, fire, and herbivory are important determinants of savanna structure, their relative effects driving the observed variation in the cover of trees and grasses. Given that trees and grasses use different photosynthetic pathways, and are differently limited by water and nutrients, this biome is likely to be very sensitive to future changes in precipitation, nutrient deposition, and atmospheric CO 2 .

Comment on “The extent of forest in dryland biomes”

Bastin et al. (Reports, 12 May 2017, p. 635) infer forest as more globally extensive than previously estimated using tree cover data. However, their forest definition does not reflect ecosystem function or biotic composition. These structural and climatic definitions inflate forest estimates across the tropics and undermine conservation goals, leading to inappropriate management policies and practices in tropical grassy ecosystems.

Conservation lessons from large‐mammal manipulations in East African savannas: the KLEE, UHURU, and GLADE experiments

African savannas support an iconic fauna, but they are undergoing large‐scale population declines and extinctions of large (>5 kg) mammals. Long‐term, controlled, replicated experiments that explore the consequences of this defaunation (and its replacement with livestock) are rare. The Mpala Research Centre in Laikipia County, Kenya, hosts three such experiments, spanning two adjacent ecosystems and environmental gradients within them: the Kenya Long‐Term Exclosure Experiment (KLEE; since 1995), the Glade Legacies and Defaunation Experiment (GLADE; since 1999), and the Ungulate Herbivory Under Rainfall Uncertainty experiment (UHURU; since 2008). Common themes unifying these experiments are (1) evidence of profound effects of large mammalian herbivores on herbaceous and woody plant communities; (2) competition and compensation across herbivore guilds, including rodents; and (3) trophic cascades and other indirect effects. We synthesize findings from the past two decades to highlight generalities and idiosyncrasies among these experiments, and highlight six lessons that we believe are pertinent for conservation. The removal of large mammalian herbivores has dramatic effects on the ecology of these ecosystems; their ability to rebound from these changes (after possible refaunation) remains unexplored.

Seed dispersal of Vitex glabrata and Prunus ceylanica by Civets (Viverridae) in Pakke Tiger Reserve, north-east India: spatial patterns and post-dispersal seed fates

Civets are considered potentially important seed dispersers in tropical forests of Asia, but relatively little is known about spatial patterns of dispersal and post-dispersal fates of civet-dispersed seeds. We explored these aspects of civet seed dispersal for two tree species Vitex glabrata (Lamiaceae), also known as smooth chaste tree and Prunus ceylanica (Rosaceae), in Pakke Tiger Reserve, a tropical forest reserve in north-east India. Pakke has five known species of viverrids: small Indian civet (Viverricula indica), large Indian civet (Viverra zibetha), common palm civet (Paradoxurus hermaphroditus), masked palm civet (Paguma larvata) and the binturong (Arctictis binturong). For both tree species, civets as a group dispersed seeds (100% of scats that we found) within 50 meters from fruiting trees and deposited seeds onto multiple substrates including tree branches, forest floor, and fallen logs. However, the distribution of seeds among substrates differed for the two tree species: while most seeds of V. glabrata (> 90%) were deposited onto canopy branches and fallen logs, the majority of P. ceylanica seeds (> 70%) were deposited on the forest floor. For both tree species, seeds deposited on logs experienced higher seed predation than seeds on the forest floor, especially when local seed densities (number of seeds in 1m2 area around the scat and in the scat) were high. Further, seed viability of P. ceylanica was significantly lower on logs (~35%) than on the forest floor (~65%). For both tree species, civets neither dispersed seeds far from fruiting trees nor to sites where seeds experienced either low predation or high survival, suggesting that while civets were legitimate dispersers, they were not especially effective.

Multi-proxy evidence for an arid shift in the climate and vegetation of the Banni grasslands of western India during the mid- to late-Holocene:

Tropical semi-arid grasslands are a widespread and ecologically and economically important terrestrial biome. Here, we use paleoecology to understand woodland–grassland transitions across the mid- to late-Holocene period in the Banni grassland, western India. Multi proxy analyses involving palynology, phytoliths and elemental geochemistry were carried out on two sediment cores retrieved from wetlands (Chachi and Luna), to understand temporal fluctuations in vegetation, moisture availability and other environmental parameters. Based on the results, the Chachi core was divided into two major climatic phases. Phase 1 (4600–2500 cal. yr BP) was characterised by high precipitation and abundance of pollen types and phytolith morphotypes that indicate the presence of woody savanna, and mesic herbaceous taxa. Phase 2 (2500 cal. yr BP to the present) was characterised by lower precipitation, lower abundance of mesic taxa and an increase in grass phytolith abundance. However, the period from ~1000 cal. yr BP to the present was characterised by the increased abundance of leguminous taxa, dryland herbs/shrubs and a decline in grass phytolith abundance. The Luna core (~1000 cal. yr BP to the present) also showed results matching with the Chachi core for this latter period. Overall, moisture availability in the ecosystem appears to have declined since 4600 cal. yr BP, and the vegetation has responded to this. Although the balance between tree, shrub and grass elements has fluctuated, overall, the region has remained as an open ‘grass and shrub savanna’ with sparse woody vegetation throughout this period. Our study provides insights into the vegetation dynamics and environmental settings in a poorly understood tropical arid-grassland ecosystem from Asia during the mid-late-Holocene.

Facilitative interactions among co-flowering Primula species mediated by pollinator sharing

We investigated flowering phenology, pollinator visitation and visitor community composition in communities of self-incompatible sympatric Primula species in a high-elevation Himalayan ecosystem. Within the tight constraints imposed by short growing seasons in such ecosystems, interactions among co-occurring plants for pollinators may vary from competition to facilitation, depending on the specifics of the system. We found that pollinator community composition changed with elevation in this system: lepidopterans were the dominant visitors at lower elevations (2200–3000 masl), bees (other than bumblebees) dominated at mid-elevations (3000–3800 masl) and bumblebees dominated at higher elevations (3800–4600 masl). However, within an elevation zone, there were no significant differences in pollinators amongst co-occurring Primula species. At a focal study site where multiple Primula species co-occurred, our results showed that even while the overall flowering periods of these species broadly overlapped, the peak flowering periods of different Primula species were temporally segregated. Upon further inferring the nature of interaction amongst co-flowering Primula species, we found that plots with higher Primula diversity (≥ 2 species) and density (80–100 individuals) experienced significantly higher pollinator visitation, compared with plots with single species and low flower densities (40–50 individuals). Our results suggest that in this community of sympatric, self-incompatible Primula species, a broadly aggregated, synchronous floral display of multiple species results in pollinator facilitation by attracting a greater number of pollinator visitors. Within this broadly synchronous display, the temporal segregation of peak flowering period of individual species may reduce competition for pollinators and limit heterospecific pollen transfer.