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Floodplain vegetation responses to flood regime in the seasonal Okavango delta, Botswana

Floodplain vegetation responses to flood regime in the seasonal Okavango delta, Botswana
Michael Murray-Hudson

2009

Department of Biological Sciences, University of Florida, Gainesville, FL 32611, UNITED STATES OF AMERICA.

ABSTRACT

As the global anthropogenic footprint grows, its effect is perhaps nowhere as profound as on the water cycle. Multiple influences act to change the variability of hydrological regimes: increasing rates of change in climate, a warmer, more carbon-rich biosphere, changes in land cover, and manipulations of river systems for water and energy supply. This increased variability is manifested as change in ecosystem services. Wetlands, which provide many such services, are particularly susceptible to changes in hydrology, and if we are to continue deriving ecosystem services from them, we need to understand the potential effects of hydrological change.

This dissertation describes the development of tools for investigating the effects of hydrological change on floodplain plant communities in the Okavango Delta, a flood-pulsed wetland at the terminus of a river basin shared by three countries. A hierarchical vegetation classification system for floodplains in the seasonally-pulsed Boro and Xudum distributaries was developed based on vegetation survey data from 30 floodplains. Analysis of two time-series of satellite imagery was used to derive flooding history in terms of extent, annual frequency and monthly duration. Correlations were established between hydroperiod parameters and plant species composition; the predictive capacity of remotely-sensed hydroperiod was tested using a 25-site calibration data set and a 5 site validation set. Species assemblages defined through cluster analysis reflect the influence of hydroperiod in 2 distinct groups: i) assemblages of regularly flooded areas, in which species composition is driven by flood duration; ii) assemblages of infrequently flooded areas, in which species composition follows a typical dry-land succession from herbaceous to woody species between flood pulses. Finally, floodplain classes and species hydroperiod response relationships were used to construct a multi-species distribution models as a layer for a spatial hydrological model.

In the semi-arid environment of the Delta, hydroperiod is clearly a major determinant of floodplain plant ecology. The tools outlined here provide both a simple system for monitoring of floodplain status and trends through time, and a system for testing scenarios of future hydrological change and its potential effects on species distributions. Temporal testing of the models, to improve confidence in their ability to predict species distribution change, and extension of the classification system to include assemblages of perennially inundated floodplains are needed. Better knowledge of species autecology would improve the interpretation of species distributions and consequently the modeling.