Shrub encroachment can reverse desertification in semi-arid Mediterranean grasslands

The worldwide phenomenon of shrub encroachment in grass-dominated dryland ecosystems is commonly associated with desertification. Studies of the purported desertification effects associated with shrub encroachment are often restricted to relatively few study areas, and document a narrow range of possible impacts upon biota and ecosystem processes. We conducted a study in degraded Mediterranean grasslands dominated by Stipa tenacissima to simultaneously evaluate the effects of shrub encroachment on the structure and composition of multiple biotic community components, and on various indicators of ecosystem function. Shrub encroachment enhanced vascular plant richness, biomass of fungi, actinomycetes and other bacteria, and was linked with greater soil fertility and N mineralization rates. While shrub encroachment may be a widespread phenomenon in drylands, an interpretation that this is an expression of desertification is not universal. Our results suggest that shrub establishment may be an important step in the reversal of desertification processes in the Mediterranean region.     

Shrub expansion stimulates soil C and N storage along a coastal soil chronosequence

Expansion of woody vegetation in grasslands is a worldwide phenomenon with implications for C and N cycling at local, regional and global scales. Although woody encroachment is often accompanied by increased annual net primary production (ANPP) and increased inputs of litter, mesic ecosystems may become sources for C after woody encroachment because stimulation of soil CO₂ efflux releases stored soil carbon. Our objective was to determine if young, sandy soils on a barrier island became a sink for C after encroachment of the nitrogen‐fixing shrub Morella cerifera, or if associated stimulation of soil CO₂ efflux mitigated increased litterfall. We monitored variations in litterfall in shrub thickets across a chronosequence of shrub expansion and compared those data to previous measurements of ANPP in adjacent grasslands. In the final year, we quantified standing litter C and N pools in shrub thickets and soil organic matter (SOM), soil organic carbon (SOC), soil total nitrogen (TN) and soil CO₂ efflux in shrub thickets and adjacent grasslands. Heavy litterfall resulted in a dense litter layer storing an average of 809 g C m^?2 and 36 g N m^?2. Although soil CO₂ efflux was stimulated by shrub encroachment in younger soils, soil CO₂ efflux did not vary between shrub thickets and grasslands in the oldest soils and increases in CO₂ efflux in shrub thickets did not offset contributions of increased litterfall to SOC. SOC was 3.6–9.8 times higher beneath shrub thickets than in grassland soils and soil TN was 2.5–7.7 times higher under shrub thickets. Accumulation rates of soil and litter C were highest in the youngest thicket at 101 g m^?2 yr^?1 and declined with increasing thicket age. Expansion of shrubs on barrier islands, which have low levels of soil carbon and high potential for ANPP, has the potential to significantly increase ecosystem C sequestration.     

Temporal biodiversity change in transformed landscapes: a southern African perspective

Landscape transformation by humans is virtually ubiquitous, with several suggestions being made that the world's biomes should now be classified according to the extent and nature of this transformation. Even those areas that are thought to have a relatively limited human footprint have experienced substantial biodiversity change. This is true of both marine and terrestrial systems of southern Africa, a region of high biodiversity and including several large conservation areas. Global change drivers have had substantial effects across many levels of the biological hierarchy as is demonstrated in this review, which focuses on terrestrial systems. Interactions among drivers, such as between climate change and invasion, and between changing fire regimes and invasion, are complicating attribution of change effects and management thereof. Likewise CO(2) fertilization is having a much larger impact on terrestrial systems than perhaps commonly acknowledged. Temporal changes in biodiversity, and the seeming failure of institutional attempts to address them, underline a growing polarization of world views, which is hampering efforts to address urgent conservation needs.     

Impacts of a mineral lick‐centred land use system on woody vegetation cover in an East African Savannah

Evaluation of woody vegetation changes with distance from a salt crater was conducted in the semi‐arid rangelands of southern Ethiopia. Data on live woody plants were collected over three seasons at 0, 1, 4, 6, 9 and 12 km from the salt crater. The density and diversity of woody plants differed significantly (p < .01) along the distance gradient. Six woody plant families were identified of which Fabaceae and Burseraceae were the dominant families. Acacia drepanolobium, Acacia nilotica, Commiphora africana and Acacia mellifera were among the severely encroaching woody species. There were high proportions of seedlings and saplings recorded closer to the salt crater showing a vigorous recruitment by woody plants. Woody plant encroachment along the 12‐km transect ranged from a low to severe encroachment, which could be translated into poor rangeland condition. Changes in soil characteristics increased grazing pressure and sedentary settlement around the salt crater, and the breakdown of traditional institutions seems to be major contributing factors to these vegetation changes. We suggest that severely encroached areas could be improved through a combination of methods such as bush clearing, prescribed fire, browsing animals and proper grazing management.     

Net changes in regional woody vegetation cover and carbon storage in Texas Drylands, 1937–1999

Although local increases in woody plant cover have been documented in arid and semiarid ecosystems worldwide, there have been few long‐term, large‐scale analyses of changes in woody plant cover and aboveground carbon (C) stocks. We used historical aerial photography, contemporary Landsat satellite data, field observations, and image analysis techniques to assess spatially specific changes in woody vegetation cover and aboveground C stocks between 1937 and 1999 in a 400‐km² region of northern Texas, USA. Changes in land cover were then related to topo‐edaphic setting and historical land‐use practices. Mechanical or chemical brush management occurred over much of the region in the 1940–1950s. Rangelands not targeted for brush management experienced woody cover increases of up to 500% in 63 years. Areas managed with herbicides, mechanical treatments or fire exhibited a wide range of woody cover changes relative to 1937 (?75% to + 280%), depending on soil type and time since last management action. At the integrated regional scale, there was a net 30% increase in woody plant cover over the 63‐year period. Regional increases were greatest in riparian corridors (33%) and shallow clay uplands (26%) and least on upland clay loams (15%). Allometric relationships between canopy cover and aboveground biomass were used to estimate net aboveground C storage changes in upland (nonriparian) portions of regional landscapes. Carbon stocks increased from 380 g C m^?2 in 1937 to 500 g C m^?2 in 1999, a 32% net increase across the 400 km² region over the 63‐year period. These plant C storage change estimates are highly conservative in that they did not include the substantial increases in woody plant cover observed within riparian landscape elements. Results are discussed in terms of implications for ‘carbon accounting’ and the global C cycle.     

Rainforest expansion reduces understorey plant diversity and density in open forest of eastern Australia

The expansion of rainforest pioneer trees into long‐unburnt open forests has become increasingly widespread across high rainfall regions of Australia. Increasing tree cover can limit resource availability for understorey plant communities and reduce understorey diversity. However, it remains unclear if sclerophyll and rainforest trees differ in their competitive exclusion of understory plant communities, which contain most of the floristic diversity of open forests. Here, we examine dry open forest across contrasting fire histories (burnt and unburnt) and levels of rainforest invasion (sclerophyll or rainforest midstorey) to hindcast changes in understorey plant density, richness and composition. The influence of these treatments and other site variables (midstorey structure, midstorey composition and soil parameters) on understorey plant communities were all examined. This study is the first to demonstrate significantly greater losses of understorey species richness, particularly of dry open‐forest specialists, under an invading rainforest midstorey compared to a typical sclerophyll midstorey. Rainforest pioneers displaced over half of the understorey plant species, and reduced ground cover and density of dry forest specialists by ~90%. Significant understorey declines also occurred with increased sclerophyll midstorey cover following fire exclusion, although losses were typically less than half that of rainforest‐invaded sites over the same period. Understorey declines were closely related to leaf area index and basal area of rainforest and wattle trees, suggesting competitive exclusion through shading and potentially belowground competition for water. Around 20% of displaced species lacked any capacity for population recovery, while transient seed banks or distance‐limited dispersal may hinder recovery for a further 68%. We conclude that rainforest invasion leads to significant declines in understorey plant diversity and cover in open forests. To avoid elimination of local native plant populations in open forests, fires should occur with sufficient frequency to prevent overstorey cover from reaching a level where shade‐intolerant species fail to thrive.     

Illecebrum verticillatum L. in the outer Hebrides

Summary

Scottish habitats are here reviewed in a European context. Examples are selected in order to identify those recognisable as distinctively Scottish as well as others related to Scandinavian or European counterparts. The paper begins with a resumé of the main environmental influences in Scotland, then describes examples of habitats almost unique to, or best represented in Scotland. This is followed by reference to some distinctively western versions of more widely distributed habitats, and others corresponding to related types elsewhere. The bearing of this on site selection for nature conservation is discussed. Hitherto, in the UK selection for National Nature Reserves and Sites of Special Scientific Interest has been based largely on the criteria listed in A Nature Conservation Review (Ratcliffe, 1977). It is important, however, also to review Scottish habitats in a European context, as attempted in this paper. In making proposals (now approaching completion) for Special Areas of Conservation, to be recommended by the UK Government for adoption by the European Commission, it is now our duty to include not only habitats for which we have special responsibility because they are unique to or best represented in Scotland, or have distinctively western features, but also sufficient examples of all the main European types occurring in our country.

Nomenclature of British flowering plants and ferns follows Stace (1991).     

Effects of western juniper (Juniperus occidentalis) control on ecosystem nitrogen stocks in central Oregon,USA

控制西美圆柏数量对美国俄勒冈州中部生态系统氮储量的影响 在美国俄勒冈州,控制西美圆柏(Juniperus occidentalis)的数量是一种公认的牧场管理措施,该做法 有助于恢复蒿草草原(sagebrush steppe)生境,该生境对野生动物和家畜都十分重要。然而,尽管控制木本 植物数量会对当地的流域管理和区域性氮库造成重要影响,但砍伐西美圆柏对生态系统中氮元素的影响问题尚未得到很好的解决。本文定量研究了两个相邻流域生态系统中的氮储量,其中一个流域经过处理(流域内的大部分圆柏已被清除掉),而另一个流域未处理(圆柏未被清除)。在圆柏被移除13年后,我们测定了两个流域里圆柏树林、灌木丛、草丛和枯枝落叶层的地上氮储量,以及两个流域中两个土层(0–25和25–50 cm)内的地下氮储量(根系和土壤氮储量)。研究结果表明,未处理流域的地上氮储量是处理流域 的6.9倍,因为未处理流域的地上生物量要大得多。然而,由于砍伐圆柏导致林下植被的根系生物量增加,所以处理流域的根系氮储量是未处理流域的3.1倍。0–25和25–50 cm土层氮储量没有受到圆柏砍伐的影响。总体而言,生态系统总氮储量在处理流域(9536 kg N ha⁻¹)和未处理流域(9456 kg N ha⁻¹)之间并没有显著差异。在两个流域里,生态系统中最大的氮积累量(至少95%的生态系统总氮量)均存在于地下(0–25 cm深的土壤以及根系)。这项研究证明,清除圆柏并不会显著影响生态系统的储氮能力。     

Wildlife species benefitting from a greener Arctic are most sensitive to shrub cover at leading range edges

Widespread expansion of shrubs is occurring across the Arctic. Shrub expansion will substantially alter arctic wildlife habitats. Identifying which wildlife species are most affected by shrubification is central to predicting future arctic community composition. Through meta‐analysis, we synthesized the published evidence for effects of canopy‐forming shrubs on birds and mammals in the Arctic and Subarctic. We examined variation in species behaviour, distribution and population dynamics in birds and mammals in response to shrub cover (including shrub cover indicators such as shrub occurrence, extent, density and height). We also assessed the degree of heterogeneity in wildlife responses to shrub cover and synthesized the remaining literature that did not fit the criteria for our quantitative meta‐analyses. Species from higher green vegetation biomass habitats (high Normalized Difference Vegetation Index, NDVI, across their distribution) were more likely to respond positively to shrub cover, demonstrating the potential for species to expand from boreal to arctic habitats under shrubification. Wildlife populations located in the lowest vegetation biomass (low NDVI) areas of their species’ range had the greatest proportion of positive responses to shrub cover, highlighting how increases in performance at leading edges of invaders distributions may be particularly rapid. This demonstrates the need to study species at these leading edges to accurately predict expansion potential. Arctic specialists were poorly represented across studies (limited to 5 bird and 0 mammal species), this knowledge gap potentially explains the few reported negative effects of shrub cover (3 of 29 species). Species responses to shrub cover showed substantial heterogeneity and varied among sites and years in all studies with sufficient replication to detect such variation. Our study highlights the importance of responses at species range edges in determining outcomes of shrubification for arctic birds and mammals and the need for greater examination of potential wildlife losers under shrubification.