Academic journal article African Studies Quarterly

Using Repeat Photography to Observe Vegetation Change over Time in Gorongosa National Park

Academic journal article African Studies Quarterly

Using Repeat Photography to Observe Vegetation Change over Time in Gorongosa National Park

Article excerpt

Introduction

Dryland ecosystems, defined as water limited systems, cover more land globally than any other ecosystem type and range from desert to savanna landscapes in their structure. Globally, dryland ecosystems are projected to experience broad scale change in composition and productivity related to global environmental change, specifically climate change. Dryland expansion has been noted globally and caused by a net reduction in soil available moisture due to changes in precipitation regimes. This global expansion of drylands has resulted in savanna like composition in new and ever expanding areas. (1) Savannas, such as those that dominate Gorongosa National Park (GNP) sustain up to one-third of the world's human population and 13.6 percent of global Net Primary Productivity. (2) Savannas are a globally distributed ecosystem that can be defined as grassland with scattered trees or shrubs. Across southern Africa upwards of 54 percent of the landscape is deemed savanna and these areas are highly heterogeneous in composition. (3) The primary drivers of savanna pattern and process that produce differential responses of vegetation cover are fire, grazing, climate variability and agriculture. (4)

Monitoring land-cover changes in savanna ecosystems can be difficult because of their highly heterogeneous composition. (5) However, monitoring these system changes is important to ecosystem function and diversity. Therefore, it is critical to monitor land-cover changes across these sensitive savanna regions as events such as shrub encroachment and landscape degradation have been noted in the literature. (6) Up to 31 percent of southern Africa's savannas may be considered degraded. (7) Landscape degradation in this context is defined as a decrease in vegetation cover (or even a complete loss), a shift in species towards annual plants, shrub encroachment (vegetation densification), long-term overgrazing, weakening perennial grasses, and/or a decrease in biodiversity. (8) Shrub encroachment is an ecological process observed across Southern Africa, including South Africa and Botswana. (9) However, a literature review shows that across Mozambique, particularly in and around GNP, studies on and quantification of shrub encroachment have not taken place. The present study aims to remedy this. Shrub encroachment is of particular concern to ecologists because of the transformation of habitats, that is to say, the loss of one habitat and the gain of a fundamentally different, and often less desirable, type. (10) Potential impacts of shrub encroachment are biogeochemical and biophysical changes. Factors that catalyze shrub encroachment are the exclusive use of moisture by the encroaching shrub species, high amounts of soil nutrients, low fire frequency, and high cattle selectivity. (11)

Remote measurements of past landscapes can be used to study change over time. One way this can be done is via the use of traditional photography, specifically a process called repeat photography. Repeat photography is the practice of taking a photograph from the same physical perspective at different points in time. This is a technique that was first developed in Europe in the 1880s to evaluate landscape change with glaciers. (12) It has since expanded across all ecosystems types globally. (13) Repeat photography is a valuable tool because it gives us a much longer time-series of data than satellite remote sensing alone, which only goes back to the mid-1970s, and in terms of more useful spatial and temporal analysis, the mid-1980s. We also have a much higher spatial resolution with ground-point photography. (14)

Gorongosa National Park (GNP), where this research was undertaken, is a mid-latitude highly heterogeneous savanna landscape, which has undergone significant human influence across the last 70 years. (15) The goal of this study is to assess vegetation change, in particular related to tree and shrub covers using repeat photography with two related research questions. …

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