Rangeland Mismanagement in South Africa: Failure to Apply Ecological Knowledge

Chronic, heavy livestock grazing and concomitant fire suppression have caused the gradual replacement of palatable grass species by less palatable trees and woody shrubs in a rangeland degradation process termed bush encroachment in South Africa. G razing policymakers and cattle farmers alike have not appreciated the ecological role fire and native browsers play in preventing bush encroachment. Unpredictable droughts are common in South Africa but have deflected too much blame for bush encroachment away from grazing mismanagement. Bush encroachment is widespread on both black and white farms, although the contributing socioeconomic, cultural, and political forces differ. Managers at Madikwe Game Reserve have reintroduced fire and native game animals into a formerly overgrazed system in an attempt to remediate bush encroachment, with encouraging preliminary results. A bush control program is needed that educates cattle farmers about the ecological causes of bush encroachment and encourages the use of fire and native browsers as tools for sustainable grazing management.     

How shrub encroachment under climate change could threaten pollination services for alpine wildflowers: A case study using the alpine skypilot,Polemonium viscosum

Under climate change, shrubs encroaching into high altitude plant communities disrupt ecosystem processes. Yet effects of encroachment on pollination mutualisms are poorly understood. Here, we probe potential fitness impacts of interference from encroaching Salix (willows) on pollination quality of the alpine skypilot, Polemonium viscosum. Overlap in flowering time of Salix and Polemonium is a precondition for interference and was surveyed in four extant and 25 historic contact zones. Pollinator sharing was ascertained from observations of willow pollen on bumble bees visiting Polemonium flowers and on Polemonium pistils. We probed fitness effects of pollinator sharing by measuring the correlation between Salix pollen contamination and seed set in naturally pollinated Polemonium. To ascertain whether Salix interference occurred during or after pollination, we compared seed set under natural pollination, conspecific pollen addition, and Salix pollen addition. In current and past contact zones Polemonium and Salix overlapped in flowering time. After accounting for variance in flowering date due to latitude, Salix and Polemonium showed similar advances in flowering under warmer summers. This trend supports the idea that sensitivity to temperature promotes reproductive synchrony in both species. Salix pollen is carried by bumble bees when visiting Polemonium flowers and accounts for up to 25% of the grains on Polemonium pistils. Salix contamination correlates with reduced seed set in nature and when applied experimentally. Postpollination processes likely mediate these deleterious effects as seed set in nature was not limited by pollen delivery. Synthesis: As willows move higher with climate change, we predict that they will drive postpollination interference, reducing the fitness benefits of pollinator visitation for Polemonium and selecting for traits that reduce pollinator sharing.     

Biodiversity change in heathland and its relationships with shifting local fire regimes and native species expansion

Aims Understanding the relationships among disturbance, invasion and species change is essential for effective management of many systems. We investigated relationships among fire history, invasion by a native tree species, Allocasuarina huegeliana, and diversity change to understand the potential drivers of plant community alteration in a complex and biodiverse system.Methods We used plant species surveys from 1983 and 2011 to quantify species loss/gain and thence compositional changes. Additionally, we surveyed population densities of the invasive species and collated long-term fire history data for each site. General linear models and non-parametric models were used to assess the strength of relationships between the three variables of interest.Important findings Within the last 30 years, ~11% of the plant species richness was lost from the reserve. At an individual site level, we found only a 4% average decrease in overall plant species richness, but large species losses and gains that imply considerable compositional shifts. Though such shifts might be expected over 30 years, many of the gained species were common, potentially opportunistic species, while those lost were often locally rare woody perennials. In addition, gained species tended to be expanding their recorded range westward suggesting that they may be responding to the regional drying climate. The relationship between invasion density and species loss was strong over all spatial scales. We identified a potential state change to dominance by the native invasive particularly as high densities prevented species gain at the site scale. In these extreme cases of high invasive density and high biodiversity loss, we argue that there may be a need to directly address the expanding native population.     

Vegetation dynamics in a Quercus‐Juniperus savanna: An isotopic assessment

Abstract. Woody plants are increasing in many grassland and savanna ecosystems around the world. As a case in point, the Edwards Plateau of Texas, USA, is a vast region (93 000 km²) in which rapid woody encroachment appears to be occurring. The native vegetation (prior to the Anglo‐European settlement 150–200 yr ago) and the biogeochemical consequences of woody encroachment in this region, however, are poorly understood. To assess these matters we measured plant and soil δ¹³C, soil organic C and soil N content from grasslands and two important woody patch types (mature Quercus virginiana clusters and Juniperus ashei woodlands) in this region. Soil δ¹³C values showed that relative productivity of C₃ species has increased in grassland and both woody habitats in recent times. δ¹³C of SOC in grasslands and Q. virginiana clusters increased with depth from the litter layer to 30 cm (grasslands =?21 to ?13‰Q. virginiana clusters =?27 to ?17‰) and were significantly different between habitats at all depths, indicating that Q. virginiana has been a long‐term component of the landscape. In J. ashei woodlands, soil δ¹³C values (at 20–30 cm depth) near the woodland edge (‐13‰) converged with those of an adjacent grassland (‐13‰) while those from the woodland interior (‐15‰) remained distinct, indicating that the woodland has been present for many years but has recently expanded. Concentrations and densities of SOC and total N were generally greater in woody patches than in grasslands. However, differences in the amount of SOC and N stored beneath the two woody patch types indicates that C and N sequestration potentials are species dependent.     

Bush encroachment control demonstrations in southern Ethiopia: 1. Woody species survival strategies with implications for herder land management

We investigated six different bush encroachment control treatments on the responses of 29 woody species in southern Ethiopia. During the post-treatments (i.e. between June 2004 and 2005), we monitored tree stump total kill, partial kill and the responses of woody species to disturbances, in terms of coppicing and seedling regeneration. Protection from disturbance promoted seedling regeneration by 4.6%. The effects of tree cutting and fire greatly reduced woody species regeneration, while tree cutting and fire combined with grazing treatment was more effective in reducing coppicing. The fire and grazing methods were effective in reducing the regeneration of seedlings, although grazing alone had no effect. The effect of the tree cutting was comparable with tree cutting and fire, as well as tree cutting and fire combined with grazing on woody species regenerations. Overall, tree cutting and fire combined with grazing were more effective in suppressing the regeneration of encroaching species. We identified two major disturbance strategies. Some species invested in coppicing, while others invested in seedling recruitment. About 10% of the species were adapted relatively to either of the strategies. Only one of the 29 woody species was ranked among the top five in both of the two disturbance-adaptation strategies. The results have important implications for post-treatment management and public education.     

Extent and spatial patterns of grass bald land cover change (1948–2000), Oregon Coast Range,USA

Globally, temperate grasslands and meadows have sharply declined in spatial extent. Loss and fragmentation of grasslands and meadows may impact biodiversity, carbon storage, energy balance, and climate change. In the Pacific Northwest region of North America, grasslands and meadows have declined in extent over the past century. Largely undocumented in this regional decline are the grass balds of the Oregon Coast Range, isolated grasslands in a landscape dominated by coniferous forests. This study was conducted to quantify the spatial extent and patterns of grass bald change. Five balds in the Oregon Coast Range were evaluated using historical aerial photographs and recent digital orthophoto quadrangles (DOQ). Over the time period of study (1948/1953 to 1994/2000), bald area declined by 66%, primarily from forest encroachment. The number and average size of bald vegetation patches declined, while edge density increased. Tree encroachment into balds was inversely related to distance from nearest potential parent trees. Spatial patterns of bald loss may result from a forest to bald gradient of unfavorable environmental conditions for tree establishment and/or seed dispersal limitation. Species dependent on balds may be at risk from loss of bald area and increased fragmentation, although metrics of habitat fragmentation may not reflect species-specific habitat requirements. Tree encroachment patterns and increased bald edge densities suggest increasing rates of bald loss in the future. The remote sensing nature of this study cannot determine the fundamental causes of bald decline, although prior research suggests climate change, cessation of native burning, successional changes in response to prior wildfires, and cessation of livestock grazing all may have potential influence.     

Post-Fire Resource Redistribution in Desert Grasslands: A Possible Negative Feedback on Land Degradation

Desert grasslands, which are very sensitive to external drivers like climate change, are areas affected by rapid land degradation processes. In many regions of the world the common form of land degradation involves the rapid encroachment of woody plants into desert grasslands. This process, thought to be irreversible and sustained by biophysical feedbacks of global desertification, results in the heterogeneous distribution of vegetation and soil resources. Most of these shrub-grass transition systems at the desert margins are prone to disturbances such as fires, which affect the interactions between ecological, hydrological, and land surface processes. Here we investigate the effect of prescribed fires on the landscape heterogeneity associated with shrub encroachment. Replicated field manipulation experiments were conducted at a shrub-grass transition zone in the northern Chihuahuan desert (New Mexico, USA) using a combination of erosion monitoring techniques, microtopography measurements, infiltration experiments, and isotopic studies. The results indicate that soil erosion is more intense in burned shrub patches compared to burned grass patches and bare interspaces. This enhancement of erosion processes, mainly aeolian, is attributed to the soil–water repellency induced by the burning shrubs, which alters the physical and chemical properties of the soil surface. Further, we show that by enhancing soil erodibility fires allow erosion processes to redistribute resources accumulated by the shrub clumps, thereby leading to a more homogeneous distribution of soil resources. Thus fires counteract or diminish the heterogeneity-forming dynamics of land degradation associated with shrub encroachment by enhancing local-scale soil erodibility. Author Contributions  SR—Conceived of or designed study, performed research, analyzed data, wrote the paper; PD—Conceived of or designed study, performed research, wrote the paper; LW—Performed research, analyzed data; GO—Contributed new methods, analyzed data; SC—Conceived of or designed study; CW—Performed research, contributed new methods or models; and SM—Contributed new methods or models.     

Shrub encroachment effect on the evapotranspiration and its component—A numerical simulation study of a shrub encroachment grassland in Nei Mongol,China

Aims Shrub encroachment is a common global change phenomenon occurring in arid and semi-arid regions. Due to the difficulty of partitioning evapotranspiration into shrub plants, grass plants and soil in the field, there are few studies focusing on shrub encroachment effect on the evapotranspiration and its component in China. This study aims to illustrate shrub encroachment effect on evapotranspiration by the numerical modeling method. Methods A two-source model was applied and calibrated with the measured evapotranspiration (ET) by the Bowen ratio system to simulate evapotranspiration and its component in a shrub encroachment grassland in Nei Mongol, China. Based on the calibrated model and previous shrub encroachment investigation, we set three scenarios of shrub encroachment characterized by relative shrub coverage of 5%, 15% and 30%, respectively, and quantified their effects caused by shrub encroachment through localized and calibrated two-source model.Important findings The two-source model can well reconstruct the evapotranspiration characteristics of a shrub encroachment grassland. Sensitivity analysis of the model shows that errors for the input variables and parameters have small influence on the result of partitioning evapotranspiration. The result shows that shrub encroachment has relatively small influence on the total amount of ET, but it has clear influence on the proportion of the components of evapotranspiration (E/ET). With shrub coverage increasing from 5% to 15% and then 30%, the evapotranspiration decreased from 182.97 to 180.38 and 176.72 W·m^-2, decreasing amplitude values of 0.34% and 0.44%, respectively. On average, E/ET rises from 52.9% to 53.9% and 55.5%, increasing amplitude values to 2.04% and 3.25%. Data analysis indicates that shrub encroachment results in smaller soil moisture changes, but clear changes of ecosystem structure (decreasing ecosystem leaf area index while increasing vegetation height) which lead to the decrease of transpiration fraction through decreasing canopy conductance. The research highlights that, with the shrub encroachment, more water will be consumed as soil evaporation which is often regarded as invalid part of evapotranspiration and thus resulting in the decrease of water use efficiency.