Determinants of tree cover in tropical floodplains

In uplands, water availability and fire limit tree cover, differentiating between tropical forest, savanna, and grassland biomes. In contrast, we know less about tropical floodplain tree cover determinants, although these habitats similarly include near-treeless grasslands, moderately treed savannas, and forests. Using GIS and remotely sensed data from 13 large tropical and sub-tropical floodplain ecosystems on five continents, we show that floodplain tree cover increases with climatic water balance (= rainfall - potential evapotranspiration) and decreases with frequent fire, as in uplands. Additionally, in floodplains, higher elevations and steeper slopes have higher tree cover, likely because these areas escape the worst inundation. Moreover, the effects of fire and topography are stronger in wetter regions. More prolonged and frequent flooding also reduce tree cover in climatically wetter floodplains but increase it in drier ones, although the effects are surprisingly weak compared to those of water balance, fire, and topography. Opposite to uplands, sandier soils have lower tree cover, likely because the benefits presented by deep tree roots in uplands are less important in floodplains with abundant water. While outright wetland conversion proceeds globally, floodplain tree cover faces other anthropogenic threats, like changing fire frequency and dam construction, especially in wet climates.