Mike Murray-Hudson

Disaggregating Hydroperiod: Components of the Seasonal Flood Pulse as Drivers of Plant Species Distribution in Floodplains of a Tropical Wetland

Wetland hydroperiod consists of different components, including frequency, duration and depth. A significant proportion of the seasonally flood-pulsed Okavango Delta is inundated for part of each year. Variation in hydroperiod, driven by the interaction of climate and ecological factors, results in a mosaic of vegetation communities. These communities are highly dynamic over temporal and spatial scales. This study aimed to identify quantitative relationships between hydroperiod components and floodplain vegetation, to better understand the potential effects of hydrological change. A stratified random sample of 30 floodplains was surveyed for species composition and abundance. Hydroperiod components for sample quadrats were estimated from remote sensing and field measurement. Ordination demonstrated strong correlations between species composition and flood frequency, duration, years-since-last-flood and depth. Eight statistically discrete vegetation communities were derived from cluster analysis. Generalized linear models of species distributions along hydrological gradients were used to derive frequency and duration optima. Means of these parameters differed significantly between communities. Increasing hydroperiod corresponded with a progression from grasslands through sedge-dominated to aquatic communities. Species in infrequently flooded areas indicated disturbance and a succession trend towards open woodland. In the sedgelands, average depth was the strongest correlate, while in grasslands and aquatic communities, this was duration.

A vegetation-based hierarchical classification for seasonally pulsed floodplains in the Okavango Delta, Botswana

A classification scheme is presented for seasonal floodplains of the Boro-Xudum distributary of the Okavango Delta, Botswana. This distributary is subject to an annual flood-pulse, the inundated area varying from a mean low of 3 600 km2 to a mean high of 5 400 km2 between 2000 and 2006. A stratified random sample of 30 sites was surveyed for species composition and abundance in March–June 2007, using multiple quadrats along transects orthogonal to the floodplain long axis. A combination of indicator species analysis and ordination was used to derive a hierarchical classification system for floodplains, based on species assemblages. Indicator species analysis was used to identify ecologically meaningful levels of division, at four and nine classes. The four main classes of floodplain were: (1) Dry Floodplain Grassland (main indicators Urochloa mosambicensis, Ipomoea coptica, Chloris virgata and Pechuel-Loeschea leubnitziae); (2) Seasonally Flooded Grassland (Nicolasia costata, Eragrostis lappula, Cyperus sphaerospermus and Setaria sphacelata); (3) Seasonally Flooded Sedgeland (Eleocharis dulcis, Leersia hexandra, Oryza longistaminata and Cyperus articulatus); and (4) Seasonal Aquatic Communities (Sacciolepis typhura, Eleocharis variegata, Fuirena pubescens and Cycnium tubulosum). The resultant dendrogram provides an objective routine for classifying floodplains in the Boro-Xudum distributary in an ecologically meaningful way. This classification will assist in monitoring changes in vegetation resulting from hydrological change.

Post-fire succession on savanna habitats in the Okavango Delta wetland, Botswana

Drying floodplains that were not inundated for more than 10y were investigated to relate plant species and vegetationstructuretotime-since-fireforthesouthernOkavangoDeltawetlandinBotswana.Theaimsoftheresearch weretostudypost-firesuccessiononsavannahabitatsintheOkavangoDeltaandtoinvestigatethemechanismsbehind vegetation changes with time-since-fire. We hypothesize that vegetation structure rather than species occurrence is affected by the fire event and that the driving forces of post-fire succession change from fire-driven to competition- driven with increasing time-since-fire. A total number of 153 study sites were investigated, representing areas of 2-12y without a fire. Both woody and herbaceous plant species were recorded and cover/abundance values were measured. The vegetation changes with time-since-fire could be assigned to three different successional stages: (1) the post-fire conditions shortly after fire, supporting high species numbers due to open space and little competition; (2)thedominanceofcompetitiveperennialgrassspecies(e.g.Urochloamosambicensis)after6-8y;and(3)theenhanced establishment of woody species after about 10y, creating new habitats in their vicinity. High species numbers were found after short and long fire-free periods, and different species occurred shortly after the fire event and at late stages ofthepost-firesuccession.Avarietyofdifferentinter-fireintervalsisthereforeexpectedtoberesponsibleforsustaining plant species richness and habitat diversity in the study area. Post-fire succession could be described as not only being driven by the life history of the species, but also by seedbank dynamics and plant mobility in early successional stages and by competition and niche differentiation in late-successional stages.

Macrophyte Species Distribution, Indices of Biotic Integrity, and Sampling Intensity in Isolated Florida Marshes

This study examined the distribution of wetland plants used in macrophyte-based index of biotic integrity (IBI) metrics to determine the effectiveness of zone sampling in assessing wetland condition. Using sampling data from a previous study of 74 emergent isolated wetlands, macrophyte taxonomic data and resulting IBI metrics were analyzed for various wetland zones and compared against the original site metrics. Zones were defined by parsing each sampling transect (from wetland edge to center) into thirds—creating an outer, intermediate, and inner zone—and constructing two additional zones representing two-thirds of the original sampling area—an outer plus intermediate zone and an intermediate plus inner zone. While a highly-significant decrease in mean species richness was observed from the wetland exterior to interior, there were no significant differences between metric values derived from site data and those derived from zone data. Linear correlations between metrics from the outer zone and site data were highly significant (R2 > 0.94), but linear correlation strength decreased with the intermediate and inner zones, respectively. Thus, macrophyte-based indicators of biological integrity can be efficiently assessed by sampling the outer third of a wetland—a 66% reduction in the area and effort required to monitor isolated wetland condition.

Water Implications of Biofuel Development in Semi-Arid Sub-Saharan Africa: Case Studies of Four Countries

Biofuel production may have considerable impacts on water resources. To analyze the implications of biofuel development on water resources in the semi-arid parts of Botswana, Zambia, Tanzania and Mali, case studies were elaborated. In all four countries plans are under way to develop comprehensive biofuel policies. Botswana and Mali have similarities since they are water scarce countries with high dependency on imported food and energy. Whilst large areas of Mali and Botswana are semi-arid, the semi-arid parts in Zambia and Tanzania are of smaller size and have relatively suitable conditions for biofuel production. In Zambia and Tanzania, there are sugarcane and jatropha-based biofuel projects, whereas in Botswana and Mali biofuel production is mainly based on jatropha. It is shown that the expansion of biofuel projects in all four countries may adversely affect water resources. It is therefore recommended that water scarce countries such as Botswana and Mali should engage in biofuel projects which do not require much irrigation. The production model of integrating biofuel production with food crop production as practised in Mali provides useful lessons for Botswana and other countries. Whilst in Zambia and Tanzania large-scale projects based on contract farming could be expanded to reach the full potential of these countries, it is crucial to carefully monitor their impacts on water resources.

Simulating Climate Impacts on Water Resources: Experience from the Okavango River, Southern Africa

The Okavango River is one of the largest and most important rivers in Southern Africa. The river rises in Angola, a country that has just emerged from a civil war of three decades. The annual flood pulse of the river feeds the Okavango Delta: one of the most valuable environmental resources of the African continent. The Okavango River water and its ecosystem resources are critically important sources of livelihoods in the basin. Pressures from livelihoods and development are already impacting on the environment and are likely to increase. Moreover, future development will occur against the background of climate variability and change. This chapter describes research conducted under the EU-funded project‘Water and Ecosystem Resources in Regional Development’ (WERRD), whose aims included development of scenario modelling as a tool for integrated water resource management in the Okavango River basin. The impact of climate change scenarios on downstream river flow and flooding in the Okavango Delta are simulated using a suite of hydrological models. The simulated impacts of climate change are sensitive to the choice of GCM and the IPCC SRES greenhouse gas (GHG) emission scenarios. The simulated impacts are considerable larger that those of the selected development scenarios although the uncertainty in the magnitude of future changes remains high.

Interactions between flooding and upland disturbance drives species diversity in large river floodplains

Understanding and predicting vegetation patterns in floodplains are essential for conservation and/or restoration of river floodplains subject to hydrological alterations. We propose a conceptual hydroecological model to explain the disturbance mechanisms driving species diversity across large river floodplains. These ecosystems harbor a unique set of flood-tolerant species different from the surrounding upland vegetation. In elevation gradients across pristine floodplains, the greater the flooding, the fewer the number of plant species. As terrain elevation increases, flood depth and duration decrease and it is more likely that species composition is influenced by external natural or human-driven disturbances. The spatial interaction between the natural flood regime and upland factors creates patterns of disturbance gradients that influence how floodplain vegetation establishes. In regions where upland conditions are subject to strong external disturbances, species diversity peaks at intermediate stages along the disturbance gradient. We demonstrate this concept with observations from the Central Amazon and Pantanal in Brazil, the Mekong’s Tonle Sap in Cambodia, and the Okavango Delta in Botswana. We discuss how this model could be further elaborated and validated to inform management of large river basins under the impact of upstream-induced flood pulse alterations.

Keeping it simple: Monitoring flood extent in large data-poor wetlands using MODIS SWIR data

Abstract Characterising inundation conditions for flood-pulsed wetlands is a critical first step towards assessment of flood risk as well as towards understanding hydrological dynamics that underlay their ecology and functioning. In this paper, we develop a series of inundation maps for the Okavango Delta, Botswana, based on the thresholding of the SWIR band (b7) MODIS MCD43A4 product. We show that in the Okavango Delta, SWIR is superior to other spectral bands or derived indices, and illustrate an innovative way of defining the spectral threshold used to separate inundated from dry land. The threshold is determined dynamically for each scene based on reflectances of training areas capturing end-members of the inundation spectrum. The method provides a very good accuracy and is suitable for automated processing.

Mapping inundation extent, frequency and duration in the Okavango Delta from 2001 to 2012

The frequency and duration of inundation in river systems are important for a variety of applications, such as water resource management, floodplain mapping and habitat restoration. The Okavango Delta, the fifth largest Ramsar site in the world, has experienced a series of large floods since 2007, following the much lower flooding of the 1990s and early 2000s. This study aimed to establish inundation frequency and duration maps of the Okavango Delta for 2001 to 2012 using MOD09Q1 and MOD11A1. An earlier methods paper testing MODIS for flood mapping in the delta showed 99.4% overall accuracy for inundation mapping of the delta, with a kappa coefficient of 80%. A method based on a time-varying threshold derived from bimodal single-band histograms was used to classify the images into inundated and non-inundated areas. The method relies on the difference in reflectance between waterbodies and dry or vegetated soil, with the former having low reflectance and the latter displaying higher reflectance values. The method takes into account seasonal changes in reflectance. Inundation maps showed a gradual decrease in the deltas maximum annual inundation extent from 2001 to 2003, followed by a gradual increase from 2004 to 2012, with maximum inundation extent during 2010–2012.

Influence of flood variation on seasonal floodplain vegetation communities in the Okavango Delta, Botswana

This study investigated the influence of flooding variation on floodplain vegetation in the Nxaraga Lagoon seasonal floodplains by sampling community composition and soil nutrient content in 1997, when flood levels were unusually low, and again in 2010 when flood levels were unusually high. In each of the eight vegetation zones identified in 1997, five plots were randomly selected. Flooding duration in each plot was estimated as the number of weeks in which the plots were inundated. Multi-response permutation procedures (MRPP) analysis was used to compare vegetation communities under low and high floods. Soil pH, Na, Mg, K, Ca and P content were analysed. The influence of environmental variables on species distribution was investigated using non-metric multi-dimensional scaling (NMS) ordination. Total plant species richness decreased from 88 during low flood to 53 during high flood, and community co-dominant species changed during high flood. Mean percentage cover decreased for some species, but increased for others. Hydroperiod increased in all zones. During the high flood period the content of Na and K was highest in Zone 8 (tertiary floodplain); Ca, Mg and pH content was highest in Zones 6 and 8 (secondary and tertiary floodplains, respectively), while P was highest in Zone 5. Hydroperiod, Na and pH were found to be correlated with, and are therefore thought to influence, community composition and distribution.