Impact of human influences on the vegetation of the Western Himalaya

Summary Effects of human influences on the vegetation of the Western Himalaya have been reviewed. Impacts of forest management practices, over-grazing, surface mining, defence and development operations, and road building activities, are severe in the region. This has created an alarming situation for soil and water conservation. Increased soil loss and runoff pose a threat to various reservoirs built for various uses and have increased hazards of erosion and floods in the Indus and Ganga basin regions.Nomenclature follows Hooker (1872–1897) and Gupta (1968).     

Invasions by alien plant species of the agro-pastoral ecotone in northern China: Species-specific and environmental determinants

The establishment, reproduction, dispersal, and distribution of alien plants are affected by various factors during the transition from being newly introduced in a habitat to being invasive. In the agro-pastoral ecotone of northern China, comprising farmlands and natural grasslands, the biological characteristics of alien plant species were the key intrinsic factors (propagation characteristics and competitive ability), followed by such extrinsic factors as human interference and environmental heterogeneity. Among biological characteristics, the life form may be an important and useful indicator of the invasive ability of a species, and the risk of invasion is greater from alien species that are poisonous, inedible, and have traits that facilitate wide dispersal. Farmlands may serve as initial shelters for alien species, from which they spread into neighbouring habitats, whereas natural grassland may act as a barrier to plant invasions. Management practices detrimental to grasslands, including overgrazing, reclamation, and road construction, often facilitate the invasions; therefore, counter measures such as reseeding and a ban on grazing need special attention. Environmental factors including precipitation, nutrients, prevailing winds, fires, and topography may be other factors that promote or block the process of invasion. In studying ways of preventing or controlling such invasions, alien plants with short life cycle, prolific seed production, and strong competitiveness, deserve particular attention and so do human activities that may damage the environment and fragile habitats.     

Desertification,no‐change or alternative states: Can we trust simple models on livestock impact in dry rangelands?

Abstract. It is remarkable that after many thousands of years of pastoralism in arid and semi‐arid savannas, there is still no clear answer to the basic question: Does livestock grazing have more than a marginal effect on the dynamics of arid and semi‐arid land vegetation? A small study of semi‐arid savanna vegetation along a spatial gradient in grazing pressure, repeated three times over 19 years is used as a basis for discussing the behaviour of dry land vegetation under heavy grazing. Three basic theories are compared: (1) the theory that heavy grazing causes desertification; (2) the theory that heavy grazing causes no directed change in the vegetation of arid and semi‐arid rangelands; and (3) the theory that heavy grazing leads to a switch between alternative states of vegetation. On the basis of the current data the first two theories are rejected, but the conclusion is that there is much evidence for a more complex behaviour of dry rangelands under grazing than what is accommodated in any of the three theories. Probably, site‐specific properties including interactive and indirect effects of herbivory in the system are important for vegetation development under grazing in arid and semi‐arid rangelands.     

A fence-line contrast reveals effects of heavy grazing on plant diversity and community composition in Namaqualand,South Africa

Changes in plant species richness and community composition were investigated across a fence separating heavily grazed communal and lightly grazed commercial farming systems in Namaqualand, South Africa. No significant differences in plant species richness between communal and commercial farming systems were detected either locally within individual plots or overall across all plots. Within-plot, richness of species tolerant of grazing, such as annuals and geophytes, has increased, while the richness of large palatable shrub species has decreased on the communal rangeland. In terms of plant cover, species' responses to grazing were strongly associated with growth form. Annuals and geophytes formed the majority of grazing increasers, while large, presumably palatable, shrubs and leaf succulents were characteristic grazing decreasers. An investigation into population processes of five shrub species revealed that heavy grazing on the communal rangeland has resulted in: reduced size of palatable shrub species; reduced flower production and seedling recruitment of palatable species; increased density and recruitment of the unpalatable shrub, Galenia africana. Reductions in shrub volume, reproductive output and seedling recruitment were most marked in the palatable shrub Osteospermum sinuatum and were in the order of 90%. The results are further discussed in terms of their relevance to rangeland dynamics and the current land use practices of the region.     

The effects of reindeer grazing on the composition and species richness of vegetation in forest–tundra ecotone

We conducted an 8-year exclosure experiment (1999–2006) in a forest–tundra ecotonal area in northwestern Finnish Lapland to study the effects of reindeer grazing on vegetation in habitats of variable productivity and microhabitat structure. The experimental sites included tundra heath, frost heath and riparian habitats, and the two latter habitats were characterized by hummock-hollow ground forms. The total cover of vegetation, cover of willow (Salix spp.), dwarf birch (Betula nana), dwarf shrubs, forbs and grasses (Poaceae spp.) increased in exclosures in all habitats. The increase in the total cover of vegetation and in the covers of willow and dwarf birch tended to be greatest in the least productive tundra heath. Opposing to the increase in the dominant vascular plant groups, the cover and species number of bryophytes decreased in exclosures. We conclude that the effects of reindeer grazing on vegetation composition depend on environmental heterogeneity and the responses vary among plant groups. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Species‐specific disturbance tolerance,competition and positive interactions along an anthropogenic disturbance gradient

Question: As it has been found that stress promotes positive interactions mediated by physical amelioration of the environment, is it possible that interactions may turn positive with increasing chronic anthropogenic disturbance (CAD) intensity? Also, is it possible that species that do not tolerate disturbance may require environmental amelioration by their neighbours in disturbed areas, whereas tolerant species may not? Location: The semi‐arid grassland in Concepción Buenavista, Oaxaca, southern Mexico. Methods: We assessed interaction intensity and importance through a neighbour removal experiment along a CAD gradient for three species differing in disturbance tolerance. Water potential was monitored on vegetated and bare soil. Results: A shift from competitive effects in low CAD sites to positive interactions in degraded sites was found. The disturbance‐tolerant species did not respond to CAD, whereas the less tolerant species changed its interactions drastically in terms of growth and reproduction. The species with medium tolerance had an intermediate response. Neighbours promoted germination in all species. Vegetation removal reduced soil humidity. Conclusions: Positive interactions seemingly resulted from the amelioration of the abiotic stresses induced by vegetation removal. The dependence on neighbours to germinate, grow, or reproduce suggests that if CAD eliminates the plant cover, vegetation will hardly recover. Irreversible changes are known to occur in communities where positive interactions predominate, but CAD may set the conditions for irreversible shifts even in communities where interactions are normally competitive.     

火烧和放牧影响下羊草草地微气候的比较研究

对羊草草地火烧、未火烧、过度放牧和禁牧区植物群落温度、土壤温度、土壤湿度、大气温度、相对湿度进行了测定．结果表明,白天火烧区群落温度比未火烧区低２．５～５℃,而夜晚则高于未火烧区０．１～１℃．白天火烧区土壤温度比未火烧区高５～７,５℃,夜晚为１～２．５℃．过牧区群落温度与禁牧区差异不显著,但过牧区土壤温度基本高于禁牧区,土壤湿度小于禁牧区．同一区中,白天群落温度变化为低处大于高处,夜晚则相反,而白天土壤温度为表层大于深层,夜晚则基本为深层大于表层．     

Thornbush invasion in a savanna ecosystem in eastern Botswana

The paper describes the thornbush invasion (bush encroachment) found in the savanna at Olifants Drift in eastern Botswana. Overgrazing by cattle is responsible for the opening up of the grass sward and has enabled woody species to establish. A quantification of this process between 1950 and 1975 in an area of ca. 100 km² is given in terms of plant density as well as aboveground biomass. The loss of herbaceous biomass which generally accompanies bush encroachment and the possibilities for pasture restoration are mentioned.Species nomenclature follows the Flora of Southern Africa and the system in use at the herbarium of the Botanical Research Institute at Pretoria, South Africa.     

Socio-ecological analysis of desertification in the Mu-Us Sandy Land with satellite remote sensing

To understand the contemporary process of desertification in the Mu-Us Sandy Land, China, we assessed the current status of desertification from satellite images. We analyzed the effect of land use by comparing desertification images and GIS-based thematic data. Socioeconomic factors were studied through interviews with local people. The desertified area ratio in each village was related to the sand dune ratio. Desertification in this region was affected by the activity of sand dunes. The revegetated area ratio had a close relationship with the cropland ratio. The development of cropland and surrounding woods contributed to revegetation of desertified land. Desertified areas were distributed mainly in pastoral regions. Critical carrying capacity of grassland for sheep and goats was approximately 2 head/ha. High population density of goats was evident in the severely desertified ranges. Goats were increasingly introduced to earn money through modernization, but they overgrazed fenced pastures. Further environmental education is needed to promote sustainable land use.     

The role of the soil seed bank in vegetation recovery on an oceanic island severely damaged by introduced goats

Question: Are the seed banks of an isolated subtropical oceanic island capable of naturally regenerating vegetation either with species of the historical forest community or with the existing grassland community after severe damage to the vegetation by goats? Location: Nakoudojima Island, Bonin Archipelago (Ogasawara Shoto), Japan. Methods: Soil samples were collected at 0–5 cm and 5–10 cm depths from seven plots in forests, grasslands, artificially matted areas and bare land. Soil seed banks were assessed using the seedling emergence method followed by the hand‐sorting of ungerminated seeds. We determined the size and composition of the seed banks in upper soil layers of plots and compared the seed banks to the standing vegetation. Results: A total of 12 220 seedlings belonging to 42 species from 20 families germinated. Total mean seed density (0–5 cm depth) was low in all plots within forest, grassland, and heavily degraded vegetation types (34.7 ± 8.6 to 693.5 ± 123.6, 58.6 ± 7.8 to 107.1 ± 10.0, and 1.1 ± 0.5 to 7.2 ± 2.3 seeds/m², respectively). Forbs and graminoids dominated the seed banks of grassland and forest plots including Cyperus brevifolius, Gnaphalium pensylvanicum, Oxalis corniculata and Solanum nigrum, and these alien species comprised 90% of the density of the seed bank. There was little correlation between seed banks and standing vegetation of the island (Sørensen similarity coefficient values 0.26 to 0.45). Conclusions: If natural regeneration occurs from the seed bank of the island, future vegetation will not move toward the original forest community, because the seed bank is dominated by non‐native herbaceous grassland species. Though isolated, a few forest remnants with low species richness could be an important source for the natural re‐establishment of forest on the island; however, seed availability may be limited by either poor dispersal or pollination so that woody species will probably recover very slowly on this goat‐impacted island.