Alter Plant Community Structure Research Articles

Carbon dioxide enrichment alters plant community structure and accelerates shrub growth in the shortgrass steppe.

A hypothesis has been advanced that the incursion of woody plants into world grasslands over the past two centuries has been driven in part by increasing carbon dioxide concentration, [CO(2)], in Earth's atmosphere. Unlike the warm season forage grasses they are displacing, woody plants have a photosynthetic metabolism and carbon allocation patterns that are responsive to CO(2), and many have tap roots that are more effective than grasses for reaching deep soil water stores that can be enhanced under elevated CO(2). However, this commonly cited hypothesis has little direct support from manipulative experimentation and competes with more traditional theories of shrub encroachment involving climate change, management, and fire. Here, we show that, although doubling [CO(2)] over the Colorado shortgrass steppe had little impact on plant species diversity, it resulted in an increasingly dissimilar plant community over the 5-year experiment compared with plots maintained at present-day [CO(2)]. Growth at the doubled [CO(2)] resulted in an approximately 40-fold increase in aboveground biomass and a 20-fold increase in plant cover of Artemisia frigida Willd, a common subshrub of some North American and Asian grasslands. This CO(2)-induced enhancement of plant growth, among the highest yet reported, provides evidence from a native grassland suggesting that rising atmospheric [CO(2)] may be contributing to the shrubland expansions of the past 200 years. Encroachment of shrubs into grasslands is an important problem facing rangeland managers and ranchers; this process replaces grasses, the preferred forage of domestic livestock, with species that are unsuitable for domestic livestock grazing.

Effects of past and current disturbance on carbon cycling in grassland mesocosms

In species rich grasslands, management factors may affect carbon storage both directly (e.g. defoliation) and indirectly, by altering plant community structure. We set up a mesocosm experiment to separate these direct and indirect effects. Monoliths were sampled from two plots of a semi-natural, species-rich pasture at Theix (France), which had been subjected to contrasted disturbance levels, high versus low grazing, for 14 years. These monoliths were placed in transparent enclosures in natural light and temperature conditions. At the start of the experiment, half of the monoliths in each disturbance treatment were shifted to the opposite disturbance regime. Above and below ground CO 2 fluxes were then measured continuously over 2 years. The net below ground carbon storage was positively correlated ( P < 0.001) with net ecosystem productivity (NEP) and was negatively correlated ( P < 0.001) with above ground net primary productivity. The net canopy photosynthesis, net ecosystem productivity and net below ground carbon storage were significantly higher for ecosystems previously adapted to a low rather than a high disturbance regime, irrespective of the disturbance level during the mesocosm experiment. In contrast, above ground net primary productivity (ANPP) was significantly enhanced by a high disturbance level during the experiment. ANPP and NEP showed a faster response to an increase rather than a decrease in disturbance level during the experiment. Grassland ecosystems adapted to frequent disturbance by grazing and cutting stored less carbon compared to ecosystems adapted to a low disturbance regime.

The effects of loess erosion on soil nutrients, plant diversity and plant quality in Negev desert wadis

Severe erosion, initiated by climatic changes during the Late Pleistocene-Early Holocene period and resultant declines in dust deposition, causes the formation of waterfalls during the winter floods in many wadi systems in the central Negev desert of Israel. In some areas, erosion of the original loess substrate has been complete, so that the underlying rock has been exposed. We examined the effects of this erosion in four wadis in the central Negev desert on soil nutrients, plant community structure and plant quality. We predicted that erosion has caused reductions in soil nutrients. Reductions in soil nutrients should result in reductions in plant cover. Furthermore, reduced soil nutrient availability should cause reductions in the nutrient status and quality of the plants growing there. In addition to the loss of biodiversity that may result, this erosion may result in economic hardship for the Bedouin peoples whose herds depend on these resources. In this study, there were significant negative effects of erosion on soil organic carbon, nitrate nitrogen and water-holding capacity, but not on soil phosphorus, conductivity or pH. Furthermore, there was a negative effect of soil erosion on an overall measure of soil quality derived from a principal components analysis in three of the four wadis we studied. Erosion resulted in an increase in plant species richness and significantly altered plant community structure in eroded areas of wadis. Increased plant species richness in eroded sites is consistent with the intermediate disturbance hypothesis of plant community structure. Plants growing in eroded areas did not differ in two quality indices (nitrogen content and digestibility), although plants typical of eroded areas had significantly lower levels of common digestion inhibitors (total polyphenols) and toxins (alkaloids) than plants from undisturbed sites. These last-mentioned results are contrary to our prediction and are consistent with the notion that plants growing in disturbed (e.g. eroded) sites maximize growth at the expense of investments in defense.

Winter Feeding of Elk in Western North America

Winter feeding of elk (Cervus elaphus) is a topic that has engendered a great deal of debate among wildlife biologists, policy makers, and the general public. The first institutional feeding of elk in North America occurred in Jackson Hole, Wyoming, where several thousand elk are still fed during most winters at the National Elk Refuge. Winter feeding of elk is employed on an annual basis by state agencies in Idaho, Oregon, Utah, Washington, and Wyoming. During 1995-99, an average 31,000 elk were fed in those 5 states at a cost of $1.6 million. Most feeding programs originated due to conflicts between elk and agricultural uses of historic elk winter range. Wildlife managers generally resorted to feeding to reduce damage by elk to crops, and to provide economic benefits of maintaining more elk than diminished winter habitat could sustain. Several negative consequences result from feeding elk. These include (1) the monetary costs of feeding, which divert dollars from other resource programs; (2) excessive herbivory that alters plant community structure and consequently affects the value of habitats near elk feedgrounds to other wildlife species; (3) changes in elk behavior that are of both spatial and philosophical siguificance; (4) diseases, which are more readily transmitted among densely concentrated animals, threaten the welfare of elk and other species, and shape resource management; and (5) public perceptions that may lead to the devaluing of habitat. These consequences argue for a shift from a production-consumption model of elk management toward management that embraces conservation of all species, maintenance of ecosystem functions, and sustainability of resources. 1 suggest proactive alternatives to winter feeding, which may avert conflict situations that precipitate public and political pressures to feed elk.

Below‐ground herbivory promotes soil nutrient transfer and root growth in grassland

Extremely little is known about the ecosystem‐level implications of below‐ground herbivory, which often represents the dominant form of consumption of primary productivity. We provide the first empirical evidence that low levels of below‐ground herbivory may promote soil nutrient flux and root growth of both host plants and companion plants. Low levels of white clover (Trifolium repens L.) root infection by clover cyst nematodes (Heterodera trifolii Goffart) increased root growth by 141% and 219% in the host plant and the uninfected neighbouring grass (Lolium perenne L.), respectively. Root infection increased the size of the soil microbial biomass in the root zone and the transfer of 15N from the host plant to soil and the neighbouring grass. These data suggest that low amounts of below‐ground herbivory may increase the transfer of plant carbon and nitrogen below‐ground, leading to increases in root growth and soil nutrient recycling in grasslands. Presumably, such interactions will influence the competitive interactions between plant species, altering plant community structure in grasslands.

Fungal endophyte symbiosis and plant diversity in successional fields

Increasing evidence suggests that microbial interactions are important determinants of plant biodiversity. The hypothesis that fungal endophyte symbiosis reduces diversity in successional fields was tested by manipulating infection of tall fescue, the most abundant perennial grass in the eastern United States. Over a 4-year period, species richness declined and tall fescue dominance increased in infected plots relative to uninfected plots without differences in total productivity. A host-specific endophyte, with negligible biomass, altered plant community structure in this long-term field experiment and may be reducing plant diversity throughout its expanding range.

Biotic and abiotic correlates of small mammal community structure in the Groendal Wilderness Area, Eastern Cape, South Africa

We investigated the relations between environmental features (biotic and abiotic) and small mammal communities in the Groendal Wilderness Area, South Africa. Habitat architecture (expressed as both horizontal and vertical foliage density) and total plant canopy cover (especially shrub canopy cover) are the most important correlates of small mammal community structure at this site. The mechanisms for these relationships are complex, probably involving physiological, social and anti-predator effects as well as reflecting nutritional resources. Furthermore, man and fire influence small mammal community structure. These effects are as a result of altered plant community composition and structure by plantation and pasture establishment and burning. In order to maintain a diversity of small mammal communities, management should ensure a mosaic of diverse vegetation communities.