Determinants of tree cover in tropical floodplains

Abstract

Tree cover differentiates forests from savannas and grasslands. In tropical floodplains, factors differentiating these systems are poorly known, even though floodplains cover 10% of the tropical landmass. Seasonal inundation potentially presents trees with both challenges (soil anoxia) and benefits (moisture and nutrient deposition), the relative importance of which may depend on ecological context, e.g. if floods alleviate water stress more in more arid ecosystems. Here, we use remotely sensed data across 13 large tropical and sub-tropical floodplain ecosystems on five continents to show that climatic water balance (i.e. precipitation-potential evapotranspiration) strongly increases floodplain tree cover in interaction with flooding, fire and topography. As predicted, flooding increases tree cover in more arid floodplains, but decreases tree cover in climatically wetter ones. As in uplands, frequent fire reduced tree cover, particularly in wet regions, but-in contrast with uplands-lower elevation and sandier soils decreased tree cover. Our results suggest that predicting the impacts of changing climate, land use and hydrology on floodplain ecosystems depends on considering climate-disturbance interactions. While outright wetland conversion proceeds globally, additional anthropogenic activities, including alteration of fire frequencies and dam construction, will also shift floodplain tree cover, especially in wet climates.

Keywords: biome; disturbance regime; flood pulse; inundation; vegetation; wetland.

Figure 1.

Study regions. Blue shading (dark grey in print) in each region depicts the areal extent of natural vegetation floodplain pixels included in the analysis. Dark grey areas show the larger extent from which final study pixels were subset. (Online version in colour.)

Figure 2.

Variable importance plots illustrating the relative influence of predictors on floodplain tree cover across 13 ecosystems (‘global’ model) and within each ecosystem. Importance is measured as the average increase in mean squared error of predicted versus actual tree cover when using permuted predictor values. Black vertical bars indicate variable importance for hydrological variables retained in the sensitivity analysis (see electronic supplementary material, text) after removal of less important and highly collinear (r ≥ 0.70) hydrological predictors.

Figure 3.

Conditional regression plots for linear models of floodplain tree cover across 13 floodplain ecosystems (‘global’ linear model). Model-predicted tree cover is plotted for x-variable values at the 10th, 50th and 90th percentile of climatic water balance (red, purple and green lines online; top, middle, and bottom lines at left of each panel in hard-copy). Note the often-weaker effects (shallower slopes) in drier ecosystems. Predictors not plotted in each panel are held constant at their medians. Ninety-five per cent confidence intervals are all thinner than the regression lines. Plot backgrounds show the relative density of tree cover observations and plots are ordered by global variable importance.