Fire and land use effects on biodiversity in the southern Sumatran wetlands

Abstract

We studied the long-term effects of an intensified fire regime following logging and altered land use practices on the biodiversity and successional dynamics of forests on three sites (Sugihan, Mesuji, and Pampangan) of roughly 300 km² each, distributed across the continuous wetland ecosystem of southern Sumatra. Satellite image analysis and ecological and socio-economic surveys were combined to reveal the vegetation characteristics, and links to fire, land use history, and site conditions.

Since the 1970s–1980s, this ecosystem has been subject to widespread repeated fires associated with or following intensive logging, transmigration, and plantation development, sonor or swamp rice cultivation, and other local resource use. Burn traces in the soil profile suggest that earlier fire episodes were far more limited in their coverage than the severe and frequent burning of the recent past. The result has been a rapid transformation from mature, high mixed species forests to sedge grasslands, savannas, and open to dense mono-specific stands of fast-growing fire-adapted species. Current vegetation types are largely differentiated by structural rather than compositional differences, which are significantly linked to fire frequency or time since last fire. The more frequent and recent the fires, the more open the landscape, with reduced density and basal area of trees.

At present, the area is very species-poor. Most patches in Sugihan and Mesuji are dominated by a single species—Melaleuca cajuputi—in the tree, sapling, and seedling layers except for degraded mixed forests which have a mix of species in all layers. Degraded mixed forests were more recently logged and burnt just once in 1991. Pampangan has a different species composition with all patches dominated by Combretocarpus rotundatus in the tree layer, which is linked to greater organic matter depth. However, in the deep peats of Pampangan the sapling and seedling layers are also dominated by Melaleuca cajuputi in most patches suggesting a future shift in overstory composition to this more freshwater wetland-adapted species. This compositional shift is likely due to peast subsidence and increased likelihood of flooding following repeated burning.

The live fuel structure with tall, dense lower strata to mid-strata of flammable herbaceous and woody species and scattered to dense tree cover renders all the vegetation types extremely fire-prone in dry years. The flammable vegetation combined with increased development and population pressures on these last frontiers makes contiued wides pread fires highly likely. This will lead to further simplification of species composition and structure, and degradation of the landscape into treeless plains.

Resource depletion has led to falling incomes and fewer livelihood options in southern Sumatra. Fires and their negative impacts have expanded into the northern provinces of Sumatra as well with timber and oil palm plantation development and/or migrating populations in search of livelihood options. Given the large contribution of peatland fires to trans-boundary haze, carbon emissions, and global warming, reducing and controlling fires in the wetlands of Sumatra is of high priority. Fire management issues and options for the wetlands of southern Sumatra are discussed.