Kevin H. Rogers

River flows and water wars: emerging science for environmental decision making

Real and apparent conflicts between ecosystem and human needs for fresh water are contributing to the emergence of an alternative model for conducting river science around the world. The core of this new paradigm emphasizes the need to forge new partnerships between scientists and other stakeholders where shared ecological goals and river visions are developed, and the need for new experimental approaches to advance scientific understanding at the scales relevant to whole-river management. We identify four key elements required to make this model succeed: existing and planned water projects represent opportunities to conduct ecosystem-scale experiments through controlled river flow manipulations; more cooperative interactions among scientists, managers, and other stakeholders are critical; experimental results must be synthesized across studies to allow broader generalization; and new, innovative funding partnerships are needed to engage scientists and to broadly involve the government, the private sector, and NGOs.

Riparian vegetation-environment relationships: complimentarity of gradients versus patch hierarchy approaches.

. Two prominent conceptual frameworks, environmental gradients and patch hierarchies, are used in combination to describe vegetation patterns along a riparian corridor in a semi-arid South African system. We adopt both approaches, since riparian corridors are characterized by both strong environmental gradients above, away from and along the river, as well as a mosaic of patches in the geomorphology at multiple hierarchical scales. Constrained and unconstrained ordinations were used to determine the variability in vegetation pattern accounted for by the gradient and the geomorphic patch hierarchy data sets. The gradient data set consisted of vertical, lateral and longitudinal dimensions of the macro-channel, while the patch hierarchy data set consisted of substratum type, morphological unit and channel type. Elevation up the macro-channel bank, of the gradient data set, explained the main variation in vegetation pattern, and alluded to overriding processes of flooding frequency and water availability as determinants of vegetation pattern. Along the fluvially dynamic macro-channel floor (lower elevation range), patchiness at the scale of the morphological unit best explained vegetation pattern. This relationship with morphological units suggests that the formation of well developed alluvial bars, and the degree of bedrock influence are important processes. The nested hierarchical framework used provided a good basis for identifying scale specific pattern in a relational manner. In systems characterized by strong environmental gradients as well as a patch mosaic at different spatial and temporal scales, the combined use of both perspectives to develop a fuller understanding of vegetation pattern is imperative and is encouraged.

Fostering Complexity Thinking in Action Research for Change in Social-Ecological Systems

Complexity thinking is increasingly being embraced by a wide range of academics and professionals as imperative for dealing with today’s pressing social–ecological challenges. In this context, action researchers partner directly with stakeholders (communities, governance institutions, and work resource managers, etc.) to embed a complexity frame of reference for decision making. In doing so, both researchers and stakeholders must strive to internalize not only “intellectual complexity” (knowing) but also “lived complexity” (being and practicing). Four common conceptualizations of learning (explicit/tacit knowledge framework; unlearning selective exposure; conscious/competence learning matrix; and model of learning loops) are integrated to provide a new framework that describes how learning takes place in complex systems. Deep reflection leading to transformational learning is required to foster the changes in mindset and behaviors needed to adopt a complexity frame of reference. We then present three broad frames of mind (openness, situational awareness, and a healthy respect for the restraint/ action paradox), which each encompass a set of habits of mind, to create a useful framework that allows one to unlearn reductionist habits while adopting and embedding those more conducive to working in complex systems. Habits of mind provide useful heuristic tools to guide researchers and stakeholders through processes of participative planning and adaptive decision making in complex social–ecological systems.

The regeneration potential of the seed bank of an ephemeral floodplain in South Africa

Abstract Germination from the seed banks of ephemeral floodplain wetlands of the Nyl River in South Africa was quantified in a glasshouse experiment to examine the potential of the seed bank for revegetation. Sediments from three sites with different wetting and drying histories (permanent, seasonal and occasional inundation) were collected in late summer after flooding and germination but before the seed bank was replenished. Samples were flooded artificially after dry, wet or wet/dry pretreatment and germination was recorded. Samples were then dried and reflooded to assess germination from the residual seed bank. All sites had a species-rich germinable seed bank. The water regime history of each site did not influence the number of species or individuals present in any trial or pretreatment. More species and individuals germinated in the first germination trial than the second. Fewer species germinated from the samples collected from above the water line than from underwater. Sixteen species (12 aquatic) and 1392 individuals germinated in the first trial. Most species from the field communities also germinated from the seed bank. Twelve species, including 2 new species, germinated from the residual seed bank. Many of the species from these wetlands have persistent seed banks with staggered germination of propagules. Species maintain themselves over space (sites), conditions (water regimes) and time (trials) by a range of life-cycle patterns. Wetland communities that depend on their seed banks for revegetation between wetting and drying events may be altered by human-induced changes to water regimes.

The effects of extreme floods on the biophysical heterogeneity of river landscapes

Studies of large infrequent disturbances, such as the Mount St Helens volcanic eruption, the 1988 Yellowstone National Park fires, and Hurricane Hugo, show that such events leave a heterogeneous imprint on a landscape, and that this imprint subsequently influences ecological response. But what imprint does a large infrequent flood disturbance leave on a river landscape, and how does the imprint influence river ecosystem response to disturbance? We used a landscape ecological framework to examine the associations between the imprint of an extreme flood and the response of woody riparian vegetation in the Sabie River (Kruger National Park, South Africa) landscape. We found that the flood left a heterogeneous imprint, consisting of remnant vegetated patches, remnant physical patches, and newly created physical patches. The structure and composition of riparian vegetation assemblages subsequently differed among these patches. Heterogeneity of the river landscape mosaic may result in multiple trajectories of e...

Regional insight into savanna hydrogeomorphology from termite mounds

Global vegetation models predict the spread of woody vegetation in African savannas and grasslands under future climate scenarios, but they operate too broadly to consider hillslope-scale variations in tree-grass distribution. Topographically linked hydrology-soil-vegetation sequences, or catenas, underpin a variety of ecological processes in savannas, including responses to climate change. In this study, we explore the three-dimensional structure of hillslopes and vegetation, using high-resolution airborne LiDAR (Light Detection And Ranging), to understand the long-term effects of mean annual precipitation (MAP) on catena pattern. Our results reveal that the presence and position of hillslope hydrological boundaries, or seeplines, vary as a function of MAP through its long-term influence on clay redistribution. We suggest that changes in climate will differentially alter the structure of savannas through hydrological changes to the seasonally saturated grasslands downslope of seeplines. The mechanisms underlying future woody encroachment are not simply physiological responses to elevated temperatures and CO(2) levels but also involve hydrogeomorphological processes at the hillslope scale.

Seasonality/aseasonality of aquatic macrophytes in Southern Hemisphere inland waters

The term aseasonality is used in this paper to describe environmental conditions which either lack annual seasonal change or have periodicities of change which are longer or shorter than the seasons. Environmental factors act on plants either as stresses or disturbances and changes in environment can signal the onset of conditions which are favourable or unfavourable to plant growth and reproduction. Plant life-histories are thus adapted to these environmental factors and respond to them with both seasonal and aseasonal periodicities, depending on their manner of occurrence and effect on the plants. A review of pertinent studies from the Southern Hemisphere shows that plants of the same life-form (submerged, floating, emergent) might differ in the types of adaptation and response to environmental conditions according to latitude but that the periodicity of response could be seasonal or aseasonal regardless of latitude. The concept of seasonality versus aseasonality is therefore misleading and an oversimplification of the variety of periodicities with which the environment acts on plant genotypes. Limnological principles of the Northern Hemisphere are applicable to aquatic macrophytes in the Southern Hemisphere but there is a particular need for research into the effects of biotic variables and water level fluctuations on aquatic plants and communities in the latter.

A framework for exploring the determinants of savanna and grassland distribution

Abstract An understanding of the factors governing grass–tree coexistence in savannas and exclusion of trees in grasslands remains elusive. We contend that progress in understanding these factors is impeded by a reliance on a falsification approach and an excessive concern over type I errors (false positives), which results in premature rejection of hypotheses, inadequate attention to scale, and a miring rather than galvanizing of ecological discussions. An additional hindrance to progress may be that investigations tend to focus on processes within either savannas or grasslands, while ignoring the boundary between the two. We propose a new scientific framework for identifying determinants of savanna and grassland distribution, which advocates (a) the recognition of ecosystems and biomes as complex adaptive systems, (b) a scientific methodology based on adaptive inference, and (c) explicit consideration of patch boundaries at various scales. Analysis of processes operating at dynamic savanna–grassland boundaries should permit better separation of ultimate from proximate factors controlling grass–tree interactions within the individual biomes. The proposed savanna–grassland framework has potential for application in other areas of ecology facing similar problems.

A description of the functional vegetation pattern of a semi-arid floodplain, South Africa

The floodplain ecosystem of the Nyl River is located in a semi-arid region of South Africa where water limits both human development and ecosystem functioning. Proposed upstream impoundments threaten the ecosystems functioning and hence its conservation value and eco-tourist potential. Articulation of these threats to the floodplain ecosystem requires a predictive understanding of the relationships between the biota and hydro-geomorphic processes. This study provides the basis for the establishment of these relationships by presenting a description of the plant assemblages of the Nyl River floodplain, identifying environmental correlates of this vegetation pattern, and by distinguishing functional plant groups. A correspondence analysis identified three major vegetation associations on the floodplain ecosystem: near-channel sites, hydromorphic sites and sodic sites. Geomorphic landform type, soil moisture and soil texture were the best correlates of the vegetation pattern. Elevation above the channel and distance from the channel were poor correlates of vegetation pattern. The distribution of functional groups, which were defined by plant life forms, showed that the near-channel sites were dominated by prostrate and decumbent grasses, hydromorphic sites by erect grasses, and sodic sites by a variety of succulent life forms. The plant-environment relationships recognized suggest that future distributions of functional plant groups may provide a sensitive index of the impacts of anticipated reductions in run-off.

Water depth and biotic insulation: Major determinants of back-swamp plant community composition

Based on phytosociological data, a polythetic divisive classification technique resulted in the delineation of eight broad vegetation types in the back-swamp areas of the Maunachira River System of the Okavango Delta, Botswana. A detrended correspondence analysis indicated that water depth was the major environmental factor influencing the distribution of submerged, floating-leaved and tall, emergent species dominated communities. The remaining communities, with relatively distinct boundaries between each of them, were of short emergent species assemblages rooted in peat deposits with a water depth of less than 0.7 m. Their species composition was not related to water depth, conductivity, pH, redox potential, water temperature or total nitrogen or phosphorus concentrations in the water. The relationship between the present day wetland plant community composition and its environment may be masked by long term, biotic, ‘insulating’ processes such as the accumulation of resources during peat formation and clonal plant growth. This insulation process does not lead necessarily to long term community stability as has been previously suggested (Mitsch and Gosselink 1986).