Soil crust microfungi along a southward rainfall gradient in desert ecosystems

We examined mycobiota structure, diversity level, and density of microfungal isolates in the biological soil crusts (BSC) and non-crusted soil of the northern and central Negev desert in 10 locations along a southward rainfall gradient. A total of 87 species from 49 genera were isolated. The mycobiota of BSC (80 species) was characterized by dominance of melanin-containing fungi, the remarkable contribution of sexual ascomycete species, and low abundance of the genera Penicillium and Aspergillus. Species richness and the contribution of dark-colored fungi with large, multicellular spores negatively and significantly correlated with rainfall. The comparison with mycobiota of the non-crusted soil showed that in BSC communities, the xeric “desert” component (melanics) was significantly more pronounced, and the mesic “forest” component (Penicillium) was much less represented. In addition, the weak influence of rainfall on spatial variations of most observed mycobiotic characteristics indicated that microenvironmental factors (soil moisture, temperature, organic matter content) influenced the development of studied communities more essentially than macroenvironmental (climatic) factors.     

Diversity declines in Microstegium vimineum (Japanese stiltgrass) patches

The spread of invasive plant species and their impacts on plant communities have received international attention as global trade and global environmental change enhance the colonization and establishment of introduced species and threaten the integrity of native ecosystems. Because introduced species vary in their impact, studying the relationship between invasion and native communities is necessary to guide allocation of finite management resources. By studying adjacent pairs of invaded and uninvaded plots across an eastern United States forested landscape, we demonstrate Microstegium vimineum was associated with local declines in species richness and cover of native species. Negative impacts of M. vimineum on species richness did not emerge until August when M. vimineum cover and height were greatest, highlighting the value of following study subjects through the growing season. In contrast, native species cover was already lower in invaded plots early in the growing season. M. vimineum invasion was not the only important driver of species richness and community composition within the study region; abiotic environmental gradients, such as soil nitrate concentration and pH, across the six study sites were also important in affecting species richness and cover, but lessened in explanatory power through the growing season. We conclude that M. vimineum has effects on community structure that may have long-term consequences for biodiversity. Studies which track sites through time and consider multiple scales are required as invaders impact multiple biotic and abiotic factors operating at different spatial and temporal scales.     

黄土高原植被变化对气象干旱多尺度响应特征与机制

基于黄土高原1983—2015年间的植被状况指数(VCI)与1~48个月尺度的标准化降水蒸散发指数(SPEI)和标准化降水指数(SPI),利用Pearson相关系数法、线性回归法和Mann-Kendall趋势检验法等方法研究了黄土高原地区植被对气象干旱的多时间尺度时空响应特征。结果表明:(1)1983—2015年,黄土高原植被状况整体趋于改善,但黄土高原整体干湿状况变化趋势不大。(2)黄土高原绝大部分区域植被变化与气象干旱指数呈现显著正相关关系,表明植被活动受到水分的限制较强。但是,过去几十年,黄土高原地区植被受到水分限制的影响程度有逐渐减轻的趋势。(3)黄土高原植被对短时间尺度的水分盈亏变化相对敏感,尤其是耕地和草地对1~4个月的SPEI更为敏感,而林地对SPEI的响应时间尺度较为分散。黄土高原VCI与SPEI、SPI的最大相关系数均主要出现在生长季(4—10月),表明水分条件在生长季对植被活动的影响较为显著。     

Drought-sensitivity ranking of deciduous tree species based on thermal imaging of forest canopies

Most climate change projections for Central Europe predict higher mean summer temperatures and prolonged summer drought periods. However, in diverse mixed forest stands we expect tree species specific responses to water shortage, as tree species are highly variable in rooting depth and physiological traits related to the water balance. Here, we assessed the drought sensitivity of the water relations of six deciduous forest tree species at four sites with contrasting water availability by airborne thermal imagery of canopy foliage temperature, sap flow and soil water potential. Canopy architecture had a consistent influence on canopy foliage temperature with ‘dense canopy’ species (Acer pseudoplatanus, Fagus sylvatica and Tilia platyphyllos) being warmer (0.5-1.5 K) than ‘open canopy’ species (Fraxinus excelsior, Prunus avium and Quercus petraea). While the canopy foliage was close to air temperature at the beginning of the drought period (ΔT_C−A = −0.1 to 0.7 K) it strongly warmed up with ongoing drought, especially at the two ‘dry’ sites with a ΔT_C−A of 3.5-5 K. The pronounced canopy foliage warming at the ‘dry’ sites after 22 days of drought was associated with reduced transpiration rates as sap flow was curtailed by 20-35% in all species except F. excelsior and Q. petraea. Based on canopy foliage temperature and sap flow data, we considered A. pseudoplatanus the most drought sensitive species followed by F. sylvatica, T. platyphyllos and P. avium and the two ring-porous species F. excelsior and Q. petraea being clearly the least sensitive ones. At drier sites, increasing summer temperatures and drought might change the competitive abilities of tree species in favour of those that are able to maintain transpirational fluxes and cooler canopies such as F. excelsior and Q. petraea.     

Chromosomal regions controlling seedling drought resistance in Israeli wild barley, <Emphasis Type="Italic">Hordeum spontaneum</Emphasis> C. Koch

The objective of the present study was to map quantitative trait loci (QTL) controlling drought-resistance traits at seedling stage of Israeli wild barley (Hordeum spontaneum). Mapping was conducted using an F₄ mapping population derived from a cross between genotype WQ23-38 from xeric (Wadi Qilt, 144 mm annual rainfall) and MA10-30 from mesic (Maalot, 790 mm annual rainfall) environments. Obvious segregation was observed for traits including time to wilt, leaf relative elongation rate, recovery rate, shoot regrowth, osmolarity, relative water content, leaf length, and root length with the coefficient of variation ranging from 10 to 77%. Eighteen putative QTL effects were identified (at false discovery rate = 10%) by single-trait analyses and confirmed by multi-trait analysis and multiple interval mapping using a new MultiQTL package. The results of our study suggest that the xeric genotypes adopted survival strategies while the mesic genotypes adopted growth-sustain strategies to cope with drought stress.     

TRMM卫星降水数据在区域干旱监测中的适用性分析

为了评估高时空分辨率的卫星遥感降水产品在干旱监测中的适用性,该文基于热带降雨卫星(tropical rainfall measurement mission,TRMM)产品和气象站点的观测数据,利用标准化降水指数(standardized precipitation index,SPI)分别计算了2种数据源的不同时间尺度(1、3、6和12个月)的河南省1998-2016年干旱事件发生的年际和空间变化。结果表明:TRMM 3B43月尺度降水数据与气象站点的观测数据具有显著的相关性,相关系数均高于0.9,除少数情况下,TRMM 3B43对降水存在略微高估现象;两种数据源计算的各时间尺度(1、3、6和12个月)存在着很高的一致性,波动幅度随着时间尺度的增大而减小;1998年以来河南省春季发生干旱事件的年份是2000、2001年,夏季发生干旱事件的年份是1999、2014年,秋季发生干旱事件的年份1998、2001、2007年,冬季发生干旱事件的年份1999、2012年,另外,基于SPI-12得出发生干旱的年份是1999、2001、2012和2013年;根据站点的不同时间尺度的两种数据源的SPI时间序列进行相关性分析,其相关系数都高于0.7,两者间的一致性很高,说明TRMM数据能够替代站点观测数据进行干旱的监测与评估。     

Monitoring of meteorological drought and its impact on rice (Oryza sativa L.) productivity in Odisha using standardized precipitation index

The dynamics of meteorological drought and its relationship with block level rice productivity over the Eastern Indian state of Odisha was assessed during the wet season (June to November) using the standardized precipitation index (SPI). The time series of rainfall data (1983–2008) from 168 rain gauge stations was used to derive the 1- and 3-month SPI of different wet season months. The 1- and 3-month SPI data were interpolated to map spatial patterns of meteorological drought and its severity, and the maps of a drought (2008) and normal (2007) year were discussed in detail. Further, the time series of SPI was exploited to assess the drought risk in Odisha. Correlation analysis of 1- and 3-month SPI with rice productivity index (RPI) showed that the SPI of 1-month time scale particularly in July (r = 0.49) and October (r = 0.33) had significantly stronger relationship with RPI than any of the 3-month SPI individually during wet season. The cumulative degree of severity of drought could be better explained by 3-month SPI map when drought events are well spread in the preceding months. Regression models were developed using 1- and 3-month SPI for forecasting rice productivity of blocks with varying proportion of rainfed area in Odisha. Model developed based on 1-month SPI accounted for 27% yield variability in rice and could be used for forecasting rice productivity.     

Restoration of woodpasture on former agricultural land: The importance of safe sites and time gaps before grazing for tree seedlings

Woodpastures (open, grazed woodlands with a mosaic of grassland, shrub and tree patches) are of high biological and cultural value and have become a threatened ecosystem in Europe. Spontaneous tree regeneration in the presence of large herbivores, is an essential process for management and restoration of this structurally diverse habitat. We examined the suitability of five vegetation types (grasslands, ruderal vegetations, tall sedges, rush tussocks and bramble thickets), grazed by large herbivores, for tree regeneration. We hypothesized that bramble thickets and tall herb communities operate as safe sites for palatable tree species through the mechanism of associational resistance. We set up a field experiment with tree seedlings in grazed and ungrazed conditions and recorded mortality and growth of seedlings of two palatable tree species (Quercus robur and Fraxinus excelsior) during three growing seasons. In the same experiment, we studied the effect of a two year’s initial time gap before grazing.Bramble thickets were suitable safe sites for survival and growth of seedlings of both species. Tall sedges, soft rush tussocks and ruderal vegetations with unpalatable or spiny species provided temporal protection, allowing seedlings to survive. Tree regeneration in livestock grazed grassland was highly constrained. Rabbits may undo the nursing effects of bramble thickets. The first year’s survival is of major importance for the establishment of trees. Subsequent grazing affects growth rather than survival. A two year’s initial time gap before grazing, had positive effects on survival, but did not enhance outgrowth of unprotected trees.     

Phenotypic variation in caryopsis dormancy and seedling salt tolerance in wild barley, Hordeum spontaneum, from different habitats in Israel

The phenotypic variation in caryopsis dormancy and seedling salt tolerance was investigated in 16 wild barley ecotypes in Israel. Depth of dormancy, as reflected by the time to maximum germination percentage, ranged from 15 to 103 days under dormancy-break treatment. Lower dormancy was characteristic of the mesic ecotypes, whereas deeper dormancy was characteristic of the xeric ecotypes. Dormancy-break patterns were revealed by growth curves: the xeric ecotype showed an S-shaped curve, whereas the mesic ecotype displayed a reverse L-shaped curve. Seedling salt tolerance was assessed by the ratio of root or coleoptile length in a seedling grown in 100 or 200 mM NaCl solution to that of a seedling grown in water. The root- and coleoptile-length ratios of mesic ecotypes were much higher than that of the xeric ecotypes, except that there was no observable difference in coleoptile-length ratio at 100 mM NaCl. The mesic ecotype was more tolerant to salt than the xeric ecotype at the young seedling stage, and seedling salt tolerance was negatively correlated with caryopsis dormancy depth. Thus evolutionary background environments have had a strong effect on the intensity of caryopsis dormancy in wild barley. Through natural selection, wild barley has adapted to dry and hot environments by increasing dormancy but not young seedling salt tolerance.     

Short rotation forestry – Earthworm interactions: A field based mesocosm experiment

Short rotation forestry (SRF) which consists of planting rapidly growing native and non-native tree species has been introduced to the UK to increase woody biomass production. A largely unknown aspect of SRF species is their interaction with soil fauna, of which the earthworm community is a major component. Earthworms have a pronounced impact on litter decomposition, nutrient cycling and tree growth. Conversely, tree litter and root chemistry can impact on the associated earthworm community development. The aim of this study was to determine direct interactions between SRF species and earthworms. A field-based mesocosm experiment was conducted using Betula pendula (birch) and Eucalyptus nitens (eucalyptus) with two earthworm species Lumbricus terrestris and Allolobophora chlorotica. The one year experiment revealed that native birch and non-native eucalyptus had a similar influence on L. terrestris population development. However, birch had a positive impact on A. chlorotica population establishment compared with eucalyptus. In the presence of earthworms, total tree biomass and leaf nitrogen concentration of eucalyptus were increased respectively by 25% and 27% compared with an earthworm-free control. In the presence of earthworms, surface litter incorporation was greater for both tree species (almost 5 times for birch and 3 times for eucalyptus) compared with controls. This work showed direct SRF-earthworm interactions which differed for tree species.     

不同时间尺度下气象干旱向水文干旱传播过程

利用黔中地区1970—2016年逐月降水以及径流计算标准化降水指数（SPI）和标准化径流指数（SSI）,分别表征气象、水文干旱,采用游程理论等方法分析了该地区不同时间尺度的气象水文干旱动态变化特征,并以交叉关联函数以及干旱传播阈值等方法探求不同时间尺度上气象干旱向水文干旱传播时间。结果表明：（1）黔中地区气象水文干旱化态势以上升趋势为主,重度和极端干旱发生的频数增加,长时间尺度上,气象与水文干旱特征具有一定的同步性。（2）不同季节上干旱传播时间（PT）不同,春季PTs在1~5个月,夏季PTs在1~3个月,秋季PTs在1~4个月,冬季PTs为1~3个月。（3）在地形地貌影响下,不同时间尺度的PTs不同。3个月时间尺度下,阳长、高车和麦翁的PTs为1.36,2.00,1.58个月;6个月时间尺度下,PTs为2.75,2.55,3.00个月;12个月时间尺度下,PTs为3.33,2.56,7.17个月。     

Responses of butterfly and moth species to restored cattle grazing in semi-natural grasslands

The effects of restorative grazing on the abundance of butterfly and moth species were studied in mesic semi-natural grasslands of SW Finland differing in management history: (1) old continuously grazed, (2) restored (with ca 5 years of reinitiated grazing), and (3) abandoned former pastures.Generalized linear modelling of species abundances and indicator species analysis produced qualitatively similar results. Only three species (Polyommatus icarus, Lycaena hippothoe and Camptogramma bilineatum) were most abundant in old pastures, whereas 12 species (Polyommatus semiargus, Polyommatus amandus, Brenthis ino, Aphantopus hyperantus, Scopula immorata, Idaea serpentata, Scotopteryx chenopodiata, Epirrhoe alternata, Cybosia mesomella, Polypogon tentacularius, Hypena proboscidalis and Cryptocala chardinyi) were most abundant in abandoned pastures. None of the old-pasture species had become more abundant in restored pastures. Three species, Epirrhoe hastulata, Xanthorhoe montanata and Chiasmia clathrata, occurred equally abundantly in abandoned and in restored pastures indicating a slow progress of restoration.Species associated with old pastures differed from species associated with abandoned pastures in their recent distributional changes in Finland. The species of old pastures showed decreasing trends, whereas those of abandoned pastures showed mainly increasing trends in their distribution. In five out of 11 species, the preferred successional stage differed markedly between this study and previous studies conducted in Central Europe.We conclude that (1) ca 5 years of restorative grazing in mesic grasslands has been insufficient for the colonisation of old-pasture species in the restored sites, (2) different management intensities are needed regionally for the maintenance of grassland insect diversity and (3) application of the knowledge on successional preferences of different species in conservation management, even in climatically similar regions, should be made with caution.     

淮河流域冬小麦主要生育期旱涝时空特征及对产量的影响

旱涝灾害在淮河流域发生频繁,对农作物产量影响较大。该文基于淮河流域1961-2014年140个气象站逐日降水、气温数据,利用标准化降水指数(standardized precipitation index,SPI)和标准化降水蒸散指数(standardized precipitation evapotranspiration index,SPEI)定量分析冬小麦各生育期气候特征,结合1961-2012年20个站点冬小麦单产数据计算标准化产量残差(standardized yield residuals,SYR),探讨冬小麦主要生育期旱涝灾害对其产量的影响。结果表明:1)冬小麦各生育期气温均呈显著上升趋势(P0.05),气温呈南高北低、西高东低的格局,东部趋势高于西部且返青抽穗期大部分站点趋势值最高,而降水量沿纬度变化与气温一致,其变化趋势在冬小麦各生育期差异明显。2)冬小麦返青抽穗期干旱增加趋势最大(P0.05),旱涝灾害在全生育期、返青抽穗期和灌浆成熟期主要受气温影响,在冬前生长期和越冬期主要受降水量影响,同时冬小麦各生育期SPEI干旱化趋势大于SPI。3)除冬前生长期外,SYR与SPI和SPEI均呈负相关关系,以返青抽穗期相关性最高,在返青抽穗期达到中度干旱或中度湿润时产量减少。该研究尝试明晰淮河流域发生在冬小麦生育期的旱涝灾害对其产量的影响,为冬小麦生育期防灾减灾提供基础支撑。     

基于生态分区的1980-2015年内蒙古干旱动态

为研究内蒙古干旱时空分布特征,将内蒙古地区分为荒漠生态分区、草原生态分区和森林生态分区,选用标准化降水指数为气象干旱评价因子,对1980—2015年内蒙古降水数据进行了动态分析。结果表明:（1）1980—2015年,内蒙古草原和森林区的SPI12有轻微减小趋势,而荒漠区的SPI12有增加趋势。（2）荒漠区干旱强度主要为中旱和轻旱,草原区以轻旱为主,森林区发生不明显干旱、轻旱和中旱的频率一样。荒漠和草原区干旱影响范围主要为无明显干旱和全域性干旱,而森林区主要为无明显干旱和区域性干旱。（3）春季3个区的SPI都有明显增加趋势。夏季3个区的SPI均有减小趋势。秋季荒漠区的SPI有增加趋势,草原区变化趋势不明显,而森林区SPI有减小趋势。冬季森林和草原区的SPI有增大趋势,而荒漠区SPI无明显变化趋势。（4）春季3个区多数站点的SPI值均有增加趋势。夏季3个区多数站点的SPI值都呈减小趋势。内蒙古秋季SPI变化趋势率表现为西部地区增大,东部地区减小。冬季荒漠生态分区SPI有增加趋势的站点多数分布于西部,有减小趋势的站点多分布于东部,草原区SPI有增大趋势的站点数较多,森林区的全部站点的SPI值都有增大的趋势。研究结论可为内蒙古地区干旱的监测预警和防灾减灾提供理论依据。     

Tree species diversity,species regeneration and biological productivity of seeded Acacia catechu Willd. In rehabilitated limestone mines in the North West Indian Himalayas

Degraded sites like mine spoils on hillslopes are characterized by poor soil physico‐chemical environments and instability. Reclamation of such areas using low cost bioengineering and vegetative methods were carried out in an abandoned limestone mine in the lower western Indian Himalayas and the impact of these measures on vegetation diversity, secondary succession and growth of seeded Acacia catechu Willd. after two decades of protection are reported in this paper. Results reveal that introduction of fast growing leguminous species affected species diversity and hampered the regeneration and growth of native vegetation, due to the dense shade of the over‐storey component. Species diversity (H') levels increased with the opening of the canopy at different elevations, with the highest diversity (1·624) being recorded on the hilltop with Acacia catechu and Wedlandia exserta dominating the area. Enumeration of various species in the growth stages indicated a large population in the pole stage followed by tree and sapling stage indicating scattered regeneration mostly by 1‐2 species. Tree associations in the lower, middle and upper reaches were of Leucaena leucocephala—A. catechu, A. catechu—Boehemeria rugulosa and A. catechu‐W. exserta, respectively. Seeded A. catechu plants now in the tree stage have attained an average height of 5·5 m and dbh of 10·44 cm with an average wood volume of 0·027 m³ in a period of 16–21 years. At current prices of heartwood (US $435 per m³) of A. catechu which is used in the manufacture of ‘kattha’, the investments made two decades ago (US$ 13043) in the rehabilitation of the limestone mine are fairly justified. Copyright © 2010 John Wiley & Sons, Ltd.     

Soil properties and tree growth at medieval ridge and furrow sites in Brandenburg,northeastern Germany

Background

Ridge and furrow (RIFU) systems and associated soils are a widespread legacy of medieval agriculture, are archives of historical land use, and might affect recent ecosystems. Open questions about RIFU formation and potential legacy effects still exist, especially related to physical soil properties.

Aims

Our aims were (1) to characterize the soil properties of RIFU soils and (2) to compare the drought sensitivity and the growth resistance in extremely dry years of trees growing on ridges and furrows, respectively.

Methods

We studied soil physical (bulk density, saturated soil hydraulic conductivity, and texture) and chemical (soil pH, soil organic matter, and nitrogen content) properties and the climate sensitivity of tree growth on RIFU systems for three study sites in Prignitz, Germany.

Results

RIFU systems showed a high spatial heterogeneity of soil stratigraphy due to ridge construction and increased accumulation of soil moisture and organic matter in furrows due to post-abandonment pedogenesis. Slight spatial differences in soil physical properties were found, with increased air capacity in ridge soils and higher available water contents in furrow soils. No differences in drought sensitivity were observed for trees growing on ridges and furrows, except for a wet site, where trees in furrows showed a higher sensitivity. Resistance in dry years tended to be similar or increase from furrows to ridges.

Conclusions

The results reflect a spatial differentiation of stratigraphy and post-abandonment pedogenesis on abandoned RIFU systems and suggest an adaption to different moisture conditions through RIFU construction. Differences in drought sensitivity of tree growth with relative land surface could only be detected for one of the three sites, where trees were found to be less drought sensitive on ridges.     

Investigation of meteorological drought characteristics in Fars province,Iran

Drought is a lack of moisture in nature, consistent or unnatural. The term consistent refers to a state of shortage, and unnatural refers to the deviation from an index of the regional natural condition. The objective of this research was to examine and forecast the intensity, duration, frequency and extent of droughts in Fars province. For this purpose, and in order to survey droughts in the province, a standard precipitation index (SPI) was determined as a selective index, considering its capacity to facilitate accurate positional analysis between various regions of the drought extent. For this, precipitation data within the same statistical period of 32 years (1968 to 1999) in 5 time scales of 3, 6, 12, 24 and 48 months from some 26 stations located within and out of the province were used, and the trends were assessed by using a time series analysis. At the next stage, RUN theory and Markov Chain were combined, on this basis to create transition probability and a steady matrix, and then the conditions of long term droughts in the province in terms of number and duration were forecast for the subsequent 10 years. The characteristics of the extent of drought were determined by using the Geographic Information System (GIS) with the help of geo-statistical methods. The results of the drought station surveys for the mentioned characteristics showed that the highest drought intensity (in terms of % deviation from normal) was at the Gozoon station and the longest duration of drought in months was at the Polkhan station; droughts both of long duration and with the highest frequency during the statistical period were observed at the Jahrom and Darshahy stations. The map of the extent of droughts showed higher intensity in the south and central regions of the province, while northern parts of the province were less sensitive to the effects of droughts. Also, the results of the maps of drought duration in various periods showed that droughts in the south of the province were of higher intensity and longer duration than elsewhere.     

Controlled ectomycorrhization of an exotic legume tree species Acacia holosericea affects the structure of root nodule bacteria community and their symbiotic effectiveness on Faidherbia albida, a native Sahelian Acacia

Many fast growing tree species have been introduced to promote biodiversity rehabilitation on degraded tropical lands. Although it has been shown that plant productivity and stability are dependent on the composition and functionalities of soil microbial communities, more particularly on the abundance and diversity of soil symbiotic micro-organisms (mycorrhizal fungi and rhizobia), the impact of tree introduction on soil microbiota has been scarcely studied. This research has been carried in a field plantation of Acacia holosericea (Australian Acacia species) inoculated or not with an ectomycorrhizal fungus isolate, Pisolithus albus IR100. After 7 year's plantation, the diversity and the symbiotic properties of Bradyrhizobia isolated from the plantation soil or from the surrounding area (Faidherbia albida (Del.) a. Chev. parkland) and able to nodulate F. albida, a native Sahelian Acacia species, have been studied. Results clearly showed that A. holosericea modified the structure of Bradyrhizobia populations and their effectiveness on F. albida growth. This negative effect was counterbalanced by the introduction of an ectomycorrhizal fungus, P. albus, on A. holosericea root systems.In conclusion, this study shows that exotic plant species can drastically affect genotypic and symbiotic effectiveness of native Bradyrhizobia populations that could limit the natural regeneration of endemic plant species such as F. albida. This effect could be counterbalanced by controlled ectomycorrhization with P. albus. These results have to be considered when exotic tree species are used in afforestation programs that target preservation of native plants and soil ecosystem rehabilitation.     

Responses of soil respiration and its components to drought stress

Purpose

Climate change is likely to increase both intensity and frequency of drought stress. The responses of soil respiration (R _s) and its components (root respiration, R _r; mycorrhizal respiration, R _m; and heterotrophic respiration, R _h) to drought stress can be different. This work aims to review the recent and current literature about the variations in R _s during the period of drought stress, to explore potential coupling processes and mechanisms between R _s and driving factors in the context of global climate change.

Results and discussion

The sensitivity of soil respiration and its components to drought stress depended on the ecosystems and seasonality. Drought stress depressed R _s in mesic and xeric ecosystems, while it stimulated R _s in hydric ecosystems. The reductions in supply and availability of substrate decreased both auto- and heterotrophic respirations, leading to the temporal decoupling of root and mycorrhizal respiration from canopy photosynthesis as well as C allocation. Drought stress also reduced the diffusion of soluble C substrate, and activities of extracellular enzymes, consequently, limited microbial activity and reduced soil organic matter decomposition. Drought stress altered Q ₁₀ values and broke the coupling between temperature and soil respiration. Under drought stress conditions, R _m is generally less sensitive to temperature than R _r and R _h. Elevated CO₂ concentration alleviated the negative effect of drought stress on soil respiration, principally due to the promotion of plant C assimilation subsequently, which increased substrate supply for respiration in both roots and soil microorganisms. Additionally, rewetting stimulated soil respiration dramatically in most cases, except for soil that experienced extreme drought stress periods. The legacy of drought stress can also regulate the response of soil respiration rate to rewetting events in terrestrial ecosystems through changing abiotic drivers and microbial community structure.

Conclusions and perspectives

There is increasing evidence that drought stress can result in the decoupling of the above- and belowground processes, which are associated with soil respiration. However, studies on the variation in rates of soil respiration and its components under different intensities and frequencies of drought stress over the ecosystems should be reinforced. Meanwhile, molecular phylogenetics and functional genomics should be applied to link microbial ecology to the process of R _s. In addition, we should quantify the relationship between soil respiration and global change parameters (such as warming and elevated [CO₂]) under drought stress. Models simulating the rates of soil respiration and its components under global climate change and drought stress should also be developed.     

Oil prospecting and its impact on large rainforest mammals in Loango National Park, Gabon

Resource extraction is increasingly affecting protected areas worldwide. However, aside from studies on logging, limited information is available about the effect this has on wildlife, which may be of great consequence, especially when endangered species could be affected. Specifically, the effect of intense human-induced noise during oil exploration on wildlife is poorly understood. We explore the effect of seismic oil exploration on large mammal distribution in an 80 km² area of Loango National Park, Gabon. Following the ecological theory of habitat disturbance, we predicted that changes in habitat use in response to noise disturbance would scale with the body/home range size of each species examined. Our study was conducted over six months before, during and after low-impact seismic operations. We recorded counts along transects of indirect signs of elephants (Loxondota africana cyclotis), chimpanzees (Pan troglodytes troglodytes), gorillas (Gorilla gorilla gorilla), duikers (Cephalophus spp.), and the vocalizations of five monkey species (Cercocebus torquatus,Cercopithecus cephus, C. nictitans, C. pogonias and Lophocebusalbigena) and modeled seismic impact over different spatial scales (small, intermediate and large). We found that elephants avoided seismic activity on all three spatial scales, apes avoided on the intermediate and small scales, and there was no effect for duikers and monkeys. We conclude that low-impact seismic operations can cause considerable temporary habitat loss for species with large ranges and suggest that the impact on those endangered species can be minimized by adequately spacing seismic lines and activity in space and time to enable species to move away from the progressive noise disruption.