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

Abstract

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'.

Keywords: Asian savannahs; diversity; fire; functional traits; herbivory; tropical dry forest.

Figure 1.

Predicted distributions of savannahs in Asia based on the climate envelope for (a) Africa, (b) Australia and (c) South America. The results suggest that Asia supports larger areas with climates that are similar to those occupied by African savannahs than either Australian or South American savannahs. The numbers shown are known field locations of different Asian savannahs. Images corresponding to these numbers are shown in figure 2.

Figure 2.

Images of different Asian savannahs with numbers on the images corresponding to the numbered geographical locations shown in figure 1. (1) Fine-leafed and spiny Acacia savannah in Gujarat, western India. (2) Broadleaf Anogeissus–Terminalia savannah in Mudumalai, southern India. (3) Broadleaf Anogeissus savannah in Nagarjuna-Sagar in south-eastern India. (4) Broadleaf Dipterocarp savannah in Pa Hin Ngam, Thailand and (5) Broadleaf Dipterocarp savannah underlain by the deciduous bamboo Vietnamosasa spp. in Mondulkiri, Cambodia. (6) Mixed broadleaf savannah in Yuanjiang Valley savannah, Yunnan, China. (7) Pine savannah (Pinus kesiya) on Mt Ugo, The Philippines. (8) Eucalypt savannah (Eucalyptus alba) on Mount Curi, East Timor, The Lesser Sunda islands. (9) Fine-leafed and spiny Acacia savannah on Mount Manatuto, East Timor, The Lesser Sunda islands. (10) Fine-leafed and spiny savannah in Sariska Tiger Reserve, central India and (11) pine savannah (Pinus roxburghii) in the Himalayan foothills in Dehradun, north India. Photo credits: Jayashree Ratnam, Edmund February, Mahesh Sankaran, Komsant Inroung (http://lifexdiscovery.blogspot.in), Kyle Tomlinson, Don Franklin, Anne Jimenez (Wikimedia Commons).

Figure 2.

Images of different Asian savannahs with numbers on the images corresponding to the numbered geographical locations shown in figure 1. (1) Fine-leafed and spiny Acacia savannah in Gujarat, western India. (2) Broadleaf Anogeissus–Terminalia savannah in Mudumalai, southern India. (3) Broadleaf Anogeissus savannah in Nagarjuna-Sagar in south-eastern India. (4) Broadleaf Dipterocarp savannah in Pa Hin Ngam, Thailand and (5) Broadleaf Dipterocarp savannah underlain by the deciduous bamboo Vietnamosasa spp. in Mondulkiri, Cambodia. (6) Mixed broadleaf savannah in Yuanjiang Valley savannah, Yunnan, China. (7) Pine savannah (Pinus kesiya) on Mt Ugo, The Philippines. (8) Eucalypt savannah (Eucalyptus alba) on Mount Curi, East Timor, The Lesser Sunda islands. (9) Fine-leafed and spiny Acacia savannah on Mount Manatuto, East Timor, The Lesser Sunda islands. (10) Fine-leafed and spiny savannah in Sariska Tiger Reserve, central India and (11) pine savannah (Pinus roxburghii) in the Himalayan foothills in Dehradun, north India. Photo credits: Jayashree Ratnam, Edmund February, Mahesh Sankaran, Komsant Inroung (http://lifexdiscovery.blogspot.in), Kyle Tomlinson, Don Franklin, Anne Jimenez (Wikimedia Commons).