Theme 2. Biogeography, ecology and conservation of savannas

Savannas are vegetation types that are defined by the coexistence of trees and grasses. They cover about ¼ of terrestrial land surface, and are widespread in tropical and sub-tropical regions of the world. Fire, mammalian herbivory and drought are endemic disturbances in savannas.

Savannas in Asia are poorly recognised and described, often being treated as degraded landscapes rather than an authentic vegetation, and therefore lack sufficient conservation.

Our work on savannas in Asia focuses on the following:

1. Understanding how old the savannas are, in an effort to prove that they have existed in different parts of Asia before modern humans altered the landscape.
2. Describing the different savanna subtypes in terms of their species compositions and some functional traits, and relating them to environmental parameters across Asia.
3. Evaluating the conservation status of the identified savanna communities using remote sensing methods to understand where changes are occurring and what is worth saving.
   

Key findings

• Species-rich savannas have been widely distributed across tropical Asia since the Miocene (Ratnam et al. 2016). We compiled compiled multiple lines of evidence for savannas, including fossil and phylogenetic evidence, C4 grass richness assessments, and climate space assessments. We concluded savannas have been present in all the major regions of tropical Asia (East Asia, Southeast Asia, Malesia, and South Asia) for at least 1 million years. Savannas across all four regions in tropical Asia contain endemic C4 grass taxa and high C4 richness. They occupy monsoonal climate regions either side of the equatorial belt. We described three distinct Asian savanna communities, namely deciduous broadleaf woodlands, deciduous fine-leafed and spiny savannas, and evergreen pine savannas, each with distinct functional ecologies consistent with fire- and herbivory-driven community assembly.
• Dominant C4 grasses have been widely present across Yunnan since at least the early Pleistocene (Chu et al. 2021). We reconstructed the phylogeography of two C₄ grasses, Heteropogon contortus and Themeda triandra, which are widely distributed in Old World savannas. We found that both species possess distinct haplotype clades in Yunnan that split more than 2 million years ago, and differ in their climatic preferences, indicating the presence of upland and lowland ecotypes. This indicates savanna communities were extensive across valleys and tablelands in Yunnan before human disturbance.
• Savannas are the most widely distributed vegetation type in Yunnan, but are also suffering the greatest losses of surface area (Lapuz et al. 2022). We examined how the savanna vegetation in Yunnan has changed over the period 1986-2016 due to land-use change, using remote sensing analysis. The total area of savanna in Yunnan was ~ 40%, more than the total area of forest (~30%). Savannas lost about 10% of their area over the 30 years whereas forest area remained stable. Losses were both due to direct human land-use change but also due to switches in vegetation states, possibly due to suppression of fires or atmospheric CO₂ enrichment.
• Savannas in Asia will undergo transitions towards forests as the climate warms (Scheiter et al. 2020). We used DGVM modelling to try to understand how vegetation in tropical Asia will change under climate change and atmospheric CO2 enrichment. DGVM modelling predicts large areas of south Asia and southeast Asia having a deciduous savanna woodland configuration at present. However, under different RCP scenarios, those areas will become more dense and more evergreen as they transition towards forests, indicating savanna woodlands are threatened by climate change.
• Dry deciduous woodlands in central Myanmar are a mosaic of savanna and forest communities mediated by soil fertility (Khaing et al. 2019). Dry dipterocarp woodlands are savannas with open canopies and grassy understories that occur over low soil P and K, whereas mixed deciduous woodland is a closed canopy forests with an understory dominated by non-graminoid herbs. Fire occurs across both communities, where it is sustained by grass and leaf litter respectively.
  

Publications

Chu et al. 2021. Phylogeography of two widespread C₄ grass species suggest tableland and valley grassy biome in southwestern China pre-date human modification. GECCO, e01835.

Khaing et al. 2019. Determinants of plant composition, diversity and structure in a seasonally dry forest in Myanmar. Global Ecology and Conservation 19: e00669

Lapuz et al. 2022. Greater loss and fragmentation of savannas than forests over the last three decades in Yunnan Province, China. Environmental Research Letters 17: 014003.

Ratnam et al. 2016. Savannahs in Asia: evidence for antiquity, biogeography, and an uncertain future. Philosophical Trans. of the Royal Society B 371: 20150305 (^†Joint First Author)

Scheiter et al. 2020. Climate change promotes transitions to tall evergreen vegetation in tropical Asia. Global Change Biology 26: 5106-5124.