Adaptive spatiotemporal distribution of soil microfungi in 'Evolution Canyon' III, Nahal Shaharut, extreme southern Negev Desert, Israel

We examined adaptive spatiotemporal mycobiota structure in the soil of 'Evolution Canyon' III, Nahal Shaharut, in the extreme southern Negev, Israel. A total of 223 species representing 80 genera were isolated using the soil dilution plate method. The microfungal communities in all localities and seasons were characterized by a superdominance of dark-coloured species with large multicelled conidia:  Ulocladium atrum, U. botrytis, Alternaria alternata, and Al. chlamydospora . Species of the genus Aspergillus (mainly As. fumigatus ) and teleomorphic ascomycetes comprised a basic part of the thermotolerant mycobiota obtained at a temperature of 37 °C. Isolate density displayed high positive dependence on organic matter content. Density was subject to drastic spatiotemporal (especially spatial) fluctuations, with maximum levels found in the shady valley bottom locality. The lowest biodiversity indices were estimated in localities under shrubs and in the most stressful summer and spring. The results demonstrated a clear effect of harsh desert climatic and edaphic selection on the adaptive variation of the mycobiota studied.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 263–277.     

Soil microfungal communities of 'Evolution Canyons' in Israel – extreme differences on a regional scale

On a regional scale, we compared the local adaptive patterns of soil mycobiota revealed in four 'Evolution Canyons' located in the northern and southern parts of Israel. These microsites were chosen according to the framework of the Institute of Evolution, University of Haifa programme, focusing on the effect of interslope environmental divergence on biodiversity patterns. The comparative analysis demonstrated remarkable differences in spatiotemporal structure of the microfungal communities and their biodiversity level (species richness, heterogeneity and equitability). In the desert 'Evolution Canyon', stress-selected, slow-reproducing, dark-coloured species with large, multicelled conidia were dominant. At the same time, in the forest localities of the northern canyons, as well as in the agriculturally disturbed locality with soil degradation, ruderal-selected, fast-reproducing Penicillium species predominated. Environmental natural selection appeared to be the major factor affecting adaptive diversity patterns of soil microfungi in the studied area.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 157–163.     

Adaptive melanin response of the soil fungus Aspergillus niger to UV radiation stress at "Evolution Canyon", Mount Carmel, Israel

Background

Adaptation is an evolutionary process in which traits in a population are tailored by natural selection to better meet the challenges presented by the local environment. The major discussion relating to natural selection concerns the portraying of the cause and effect relationship between a presumably adaptive trait and selection agents generating it. Therefore, it is necessary to identify trait(s) that evolve in direct response to selection, enhancing the organism''s fitness. “Evolution Canyon” (EC) in Israel mirrors a microcosmic evolutionary system across life and is ideal to study natural selection and local adaptation under sharply, microclimatically divergent environments. The south-facing, tropical, sunny and xeric “African” slope (AS) receives 200%–800% higher solar radiation than the north-facing, temperate, shady and mesic “European” slope (ES), 200 meters apart. Thus, solar ultraviolet radiation (UVR) is a major selection agent in EC influencing the organism-environment interaction. Melanin is a trait postulated to have evolved for UV-screening in microorganisms. Here we investigate the cause and effect relationship between differential UVR on the opposing slopes of EC and the conidial melanin concentration of the filamentous soil fungus Aspergillus niger. We test the working hypothesis that the AS strains exhibit higher melanin content than strains from the ES resulting in higher UV resistance.

Methodology/Principal Findings

We measured conidial melanin concentration of 80 strains from the EC using a spectrophotometer. The results indicated that mean conidial melanin concentration of AS strains were threefold higher than ES strains and the former resisted UVA irradiation better than the latter. Comparisons of melanin in the conidia of A. niger strains from sunny and shady microniches on the predominantly sunny AS and predominantly shady ES indicated that shady conditions on the AS have no influence on the selection on melanin; in contrast, the sunny strains from the ES displayed higher melanin concentrations.

Conclusions/Significance

We conclude that melanin in A. niger is an adaptive trait against UVR generated by natural selection.     

Gastropod biodiversity at the 'Evolution Canyon' microsite, lower Nahal Oren, Mount Carmel, Israel

Twenty-six species of gastropods (terrestrial, shell-bearing snails and slugs) were recorded at the 'Evolution Canyon' microsite, lower Nahal Oren, Mount Carmel, Israel. Twenty-five species were recorded at the temperate, mesic north-facing slope (NFS) and 20 species at the xeric south-facing slope (SFS). Out of these species, six were NFS specific ( Pilorcula raymondi hebraica , Euchondrus septemdentatus , Monacha crispulata , Pyramidula rupestris hierosolymitana , Truncatellina haasi , and Vitrea contracta ) and one was SFS specific ( Prolimax eustrictus ). The interslope difference was probably partly due to missing forest species at the SFS in comparison with the NFS. Twenty-two species were Levantine endemics (84.6%) and four species were more widely distributed in the Palaearctic region (15.4%). The Levantine species are inhabitants of the mesic and mainly mountainous regions, but four species ( Granopupa granum , Calaxis hierosolymarum , Cecilioides acicula , and Helix engaddensis ) also penetrate the deserts. Seven species ( Buliminus labrosus , H. engaddensis , Levantina spiriplana caesareana , Metafruticicola fourousi , Monacha syriaca , Sphincterochila cariosa , and Xeropicta vestalis joppensis ) were significantly more abundant on the SFS than on the NFS. The local physical microclimatic sharp divergence leads to gastropod adaptive interslope biotic divergence caused by natural selection.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 147–155.     

Variation in vegetation and seed bank in a Chilean semi‐arid community affected by ENSO 1997

Abstract. We followed plant cover and soil seed density of shrubs and herbs in two markedly contrasting years with regard to annual rainfall in two opposite‐facing slopes (mesic vs xeric) and an intervening, relatively wet, ravine of a typical creek in semi‐arid Chile. During the ENSO year 1997 cover of ephemerals increased in all three sites; 43% vs 8% on the xeric slope 75% vs 26% on the mesic slope and 57% vs 32% in the ravine. The number of species was almost three times higher on the xeric slope (27 vs 10 species), increased by 47% on the mesic slope (28 vs 19) and by 14% in the ravine (24 vs 21). Cover of shrubs plus perennial grasses increased from 52% in 1996 to 59% in 1997 on the xeric slope, but no significant cover changes were found on the mesic slope or the ravine. On the xeric slope peak density of seeds was 4500/m² in 1996, while during the ENSO year it was 24000/m². On the mesic slope equivalent values were 3000 and 17000 seeds/m² while in the ravine figures were 8500 and 27000 seeds/m². The strong responses of ephemerals and seed bank to the ENSO‐driven wet year of 1997 demonstrate the importance of this phenomenon in replenishing, recovering and probably maintaining ephemeral vegetation in this region.     

Drought response of five conifer species under contrasting water availability suggests high vulnerability of Norway spruce and European larch

The ability of tree species to cope with anticipated decrease in water availability is still poorly understood. We evaluated the potential of Norway spruce, Scots pine, European larch, black pine, and Douglas‐fir to withstand drought in a drier future climate by analyzing their past growth and physiological responses at a xeric and a mesic site in Central Europe using dendroecological methods. Earlywood, latewood, and total ring width, as well as the δ¹³C and δ¹⁸O in early‐ and latewood were measured and statistically related to a multiscalar soil water deficit index from 1961 to 2009. At the xeric site, δ¹³C values of all species were strongly linked to water deficits that lasted longer than 11 months, indicating a long‐term cumulative effect on the carbon pool. Trees at the xeric site were particularly sensitive to soil water recharge in the preceding autumn and early spring. The native species European larch and Norway spruce, growing close to their dry distribution limit at the xeric site, were found to be the most vulnerable species to soil water deficits. At the mesic site, summer water availability was critical for all species, whereas water availability prior to the growing season was less important. Trees at the mesic were more vulnerable to water deficits of shorter duration than the xeric site. We conclude that if summers become drier, trees growing on mesic sites will undergo significant growth reductions, whereas at their dry distribution limit in the Alps, tree growth of the highly sensitive spruce and larch may collapse, likely inducing dieback and compromising the provision of ecosystem services. However, the magnitude of these changes will be mediated strongly by soil water recharge in winter and thus water availability at the beginning of the growing season.     

Effects of position, understorey vegetation and coarse woody debris on tree regeneration in two environmentally contrasting forests of north-western Patagonia: a manipulative approach

Aim To investigate the differential effects of position within gaps, coarse woody debris and understorey cover on tree seedling survival in canopy gaps in two old‐growth Nothofagus pumilio (Poepp. & Endl.) Krasser forests and the response of this species to gaps in two forests located at opposite extremes of a steep rainfall gradient. Location Nahuel Huapi National Park, at 41° S in north‐western Patagonia, Argentina. Methods In both study sites, seedlings were transplanted to experimental plots in gaps in three different positions, with two types of substrate (coarse woody debris or forest floor), and with and without removal of understorey vegetation. Survival of seedlings was monitored during two growing seasons. Soil moisture and direct solar radiation were measured once in mid‐summer. Seedling aerial biomass was estimated at the end of the experiment. Results Mid‐summer soil water potential was lowest in the centre of gaps, in plots where the understorey had been removed, and highest at the northern edges of gaps. Direct incoming radiation was highest in gap centres and southern edges, and lowest at northern edges. Seedling mortality was highest in gap centres, in both sites. Coarse woody debris had a positive effect on seedling survival during summer in the mesic forest and during winter in the xeric forest. The removal of understorey cover had negative effects in gap centres during summer. Seedling final aerial biomass was positively affected by understorey removal and by soil substrate in both sites. In the dry forest gaps, seedling growth was highest in northern edges, whereas it was highest in gap centres in the mesic forest. Overall growth was positively related to survival in the xeric forest, and negatively related in the mesic forest. Main conclusions Survival and growth were facilitated by the shade of gap‐surrounding trees only in the xeric forest. Understorey vegetation of both forests facilitated seedling survival in exposed microsites but competed with seedling growth. Nurse logs were an important substrate for seedling establishment in both forests; however, causes of this pattern differed between forests. Water availability positively controls seedling survival and growth in the xeric forest while in the mesic forest, survival and growth are differentially controlled by water and light availability, respectively. These two contrasting old‐growth forests, separated by a relatively short distance along a steep rainfall gradient, had different yet unexpected microenvironmental controls on N. pumilio seedling survival and growth. These results underscore the importance of defining microscale limiting factors of tree recruitment in the context of large‐scale spatial variation in resources.     

Identifying plant traits associated with topographic contrasts in a rugged and diverse region (Klamath‐Siskiyou Mts,OR, USA)

Rugged topography affects species distributions and community composition by creating contrasting mesic (cool, moist) and xeric (warm, dry) microclimates on adjacent slopes. This microclimatic heterogeneity is thought to have contributed to species survival during past climate fluctuations. Within a rugged and botanically rich region, we asked what functional, distributional, and/or biogeographic traits distinguished the species significantly associated with xeric or mesic microclimates. For each of 236 species in 4773 plots in the Klamath‐Siskiyou Mountains, we tested for significant associations with mesic or xeric topographic microclimates inferred from high‐resolution topographic variables. For the subset of species showing significant associations, we then compared their functional traits, biogeographic origins, and macroclimatic attributes to those of other species. We also tested the dependence of topographic associations on elevation, canopy cover, and soil type. Many species in the region (40%) showed significant tendencies to be found only in either mesic or xeric topography. ‘Mesic’ species tended to be of northern biogeographic origin and to have geographic ranges with higher mean precipitation; ‘xeric’ species had the opposite attributes. Species occurred more often in mesic microclimates when they occurred on low‐nutrient serpentine soils, and were more often found in xeric microclimates at high elevations. Functional traits such as specific leaf area were not significant predictors of species association with topographic microclimate. Biogeographic origins and the mean precipitation (rather than temperature) of species geographic ranges are the best indicators of species that are found in cool/moist northerly or hot/dry southerly microclimates.     

Abiotic determinants of the fynbos/succulent karoo boundary,South Africa

This study investigates the influence of texture, soil moisture and nutrient status on the growth and survival of seedlings of two typical fynbos (Leucadendron pubescens and Passerina vulgaris) and succulent karoo (Ruschia spp.) species, which grow in the boundary zone between these two vegetation types. Seedlings of each species were grown in shalederived and sandstone‐derived soils and under xeric and mesic regimes. Under the xeric regime, the shale‐derived and sandstone‐derived soils represented fine and coarse‐textured soils, respectively. Under the mesic regime, the same soils represented nutrient‐rich and nutrient‐poor soils, respectively. The seedlings of both fynbos species died rapidly under the xeric regime, irrespective of soil type. In contrast, the succulent karoo seedlings survived for over 77 days without water. Under mesic conditions, the fynbos seedlings grew faster than the succulent karoo seedlings, irrespective of soil type. Fynbos seedlings appear to be directly limited by the environment (moisture and salinity), whereas succulent karoo seedlings may be limited by interactions with other plants.     

Effect of Sowing Season,Plant Cover,and Climatic Variability on Seedling Emergence and Survival in Burned Austrocedrus chilensis Forests

Fire is the most important disturbance factor in Cypress (Austrocedrus chilensis) forests in Patagonia, Argentina. This ecosystem recovers poorly after fire, and direct sowing could be a potentially useful restoration practice. To evaluate the effect of season of sowing, post‐fire plant cover (PC), and climatic variability on seedling emergence and survival, three direct sowing studies were established in two burned cypress stands: Trevelin (xeric conditions) and El Bolsón (mesic conditions). Two studies were conducted in winter (2000 and 2001) and one in spring (2001). Precipitation was higher than the mean during the 2000–2001 growing season and lower during 2001–2002. At both sites, emergence and survival were much higher for winter‐ than for spring‐sown seedlings. In the xeric stand, emergence and survival of winter‐sown seedlings increased with medium and high PC values, after the humid and dry summers, respectively. However, most spring‐sown seedlings did not emerge, and those that did were short‐lived. Because of the more favorable growing conditions in the mesic stand, PC had no effect on emergence and only favored first year survival of winter‐sown seedlings after the dry summer. Spring‐sown seedlings showed no association with PC in the mesic site, probably because the first summer was exceptionally humid. We speculate that shading plants exert a positive effect on cypress seedling establishment, likely by reducing the stress from high temperatures and low water availability. Sowing of small patches under the protection of understory vegetation could be useful in restoring burned cypress stands.     

Phylogeny of Australasian agamid lizards based on nuclear and mitochondrial genes: implications for morphological evolution and biogeography

Recent mtDNA phylogenies of Australasian agamid lizards are highly incongruent with existing morphological views. To resolve this discrepancy we sequenced two nuclear gene regions, c- mos and brain-derived neurotrophic factor (BDNF). These were highly concordant with each other and the mtDNA phylogeny, but not the morphology. A combined molecular analysis reveals substantial hidden support (additional phylogenetic signal that emerges only when the data sets interact in a combined analysis). Bayesian posteriors, and a partitioned bootstrap procedure introduced here, indicate strong support for most nodes. The resultant tree implies extensive morphological homoplasy, with many genera emerging as non-monophyletic ( Amphibolurus , Rankinia , Ctenophorus , Physignathus , Diporiphora ). The water and forest dragons ( Physignathus and Hypsilurus ) form a paraphyletic basal assemblage to the more derived Australian forms such as Amphibolurus and Ctenophorus , which include almost all the xeric taxa. However, the thorny devil Moloch horridus is a basal lineage and not closely related to the other arid forms. Tree topology, inferred divergence dates, palaeogeographical and palaeoclimatic data are all consistent with Miocene immigration into Australia from the north by mesic forest ecomorphs, followed by initial diversification in mesic habitats before radiation into xeric habitats facilitated by increasing aridity.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 343–358.     

Metabolic rate and endurance capacity in Australian varanid lizards (Squamata: Varanidae: Varanus)

In ecomorphological and ecophysiological studies, locomotor performance is often considered to be an intermediate step between the form of an organism and its environment. We examined this premise by measuring morphology, physiology and circular track endurance in the closely related group of Australian varanid lizards. Body size, body mass and relative body proportions were poor indicators of endurance. Body mass was not correlated with endurance and size-free lower forelimb length had only a weak relationship with endurance. Instead, maximal metabolic rate was positively correlated with endurance capacity in varanids. A comparison of varanids with other groups of lizards supported this result as varanids showed both elevated maximal metabolic rate and elevated endurance scores when compared with similar sized non-varanid lizards. There was support for a strong association between endurance with foraging mode and climate. Varanid species with higher endurance tended to be widely foraging and from xeric climates, while sit-and-wait and mesic species showed reduced endurance.   © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 664–676.     

Onset of summer flowering in a 'Sky Island' is driven by monsoon moisture

Temperatures for the southwestern USA are predicted to increase in coming decades, especially during the summer season; however, little is known about how summer precipitation patterns may change. We aimed to better understand how nonsucculent plants of a water-limited gradient encompassing xeric desert to mesic mountain-top may respond to changes in summer conditions. We used a species-rich 26-yr flowering record to determine species' relationships with precipitation and temperature in months coincident with and previous to flowering. The onset of summer flowering was strongly influenced by the amount and timing of July precipitation, regardless of elevation or life form, suggesting the critical importance of soil moisture in triggering summer flowering in this region. Future changes in the timing or consistency of the early monsoon will probably impact directly on the onset of flowering for many species in this region. In addition, a key implication of predicted increasing temperatures is a decrease in available soil moisture. At all elevations, many species may be expected to flower later in the summer under the decreased soil moisture conditions associated with warmer temperatures. However, impacts on summer flowering may be greater at higher elevations, because of the greater sensitivity of mesic plants to water stress.     

The adaptive significance of insect gall distribution: survivorship of species in xeric and mesic habitats

Summary We studied the relationship between habitat moisture and gall-forming insect populations. Population sizes for most galling taxa were significantly larger in xeric habitats compared with mesic habitats. Our results indicate that the differential abundance of galling insects in these habitats is due primarily to differential mortality and survivorship. Mortality factors acting upon eight insect galling species (belonging to eight genera and four families) were measured on six species (five genera and five families) of host plants. Survival was significantly higher for galling populations inhabiting xeric habitats compared with mesic habitats. Parasitism was higher in mesic habitats in seven of eight habitats and fungus-induced diseases were higher in five of seven habitats. Mortality due to predation and other (unknown) factors showed no clear trends. Overall, there was a tendency towards lower mortality and consequently higher survival for populations inhabiting xeric habitats. We hypothesize that reduced mortality caused by natural enemies and endophytic fungi has contributed to the speciation and radiation of galling insects in apparently harsh environments.     

Soil micromycete diversity in the hypersaline Dead Sea coastal area,Israel

In the present study, a soil microfungal community was examined over a one-year period (1999–2000) at the western shore of the Dead Sea. A total of 78 species from 40 genera were isolated. The most prominent features of mycobiota of the territory studied were: (i) the prevailing number of melanin-containing micromycetes (46 species, 65.5 % of the total isolate number); (ii) a large share of teleomorphic Ascomyceta (26 species, 18.5 % of isolates); (iii) combination of true soil and plant surface inhabiting species; (iv) spatial and temporal variation of the mycobiota composition; (v) very low fungal density (nearly 500-fold lower than in the Judean Desert soil). These features are formed under the extremely stressful xeric and oligotrophic conditions in which the Dead Sea coastal micromycete community exists. Nine species (Alternaria alternata, A. raphani, Aspergillus niger, Aureobasidium pullulans, Chaetomium globosum, Ch. murorum, Cladosporium cladosporioides, Penicillium aurantiogriseum, and Stachybotrys chartarum) were considered a characteristic micromycete complex for the Dead Sea coastal habitat based on the spatial and temporal occurrence of these species. Many of the micromycetes isolated, including almost all the species listed above, are known to be distributed worldwide occurring in different soil types. This confirms the conclusion of many mycologists working in areas with saline and arid soils that there is no halo-and thermophilous mycobiota characteristic for those soils.     

Response patterns of net photosynthesis to moisture of mosses in xeric habitats

The response patterns of net photosynthesis to moisture level of mosses in xeric habitats were compared with those in mesic habitats, in order to determine whether the former species are better adapted to the xeric condition with regard to carbon gain. Moss species examined wereRhacomitrium lanuginosum andR. barbuloides in xeric open habitats andDicranum japonicum, Hypnum plicatulum, Ptilium crista-castrensis, Pleurozium schreberi andHylocomium splendens in mesic habitats on the coniferous forest floor in the upper subalpine zone of Mt. Fuji. Three additional xerophytic species collected at other localities,Ptychomitrium polyphylloides, Grimmia pilifera andHedwigia ciliata, were also examined. Five species in the xeric habitats showed an optimum range of moisture level for net photosynthesis, 2 to 3g·g⁻¹. On the other hand, species in the forest showed a wider optimum range, 3 to 8g·g⁻¹. Net photosynthetic rate at the moisture level of 0.5g·g⁻¹ was positive in xerophytic mosses, but negative in most forest mosses. Moisture levels where external capillary water disappeared and drop of water potential began was determined by blotting water-saturated shoots with membrane filters. These moisture levels were low in the xerophytic mosses and high in the forest mosses, although there were some exceptions. It was concluded that mosses in xeric habitats are better adapted for the efficient use of water for photosynthesis than those in mesic habitats.     

Heritable variation in the survival of seedlings from Patagonian cypress marginal xeric populations coping with drought and extreme cold

The rear edges of tree species have begun to be perceived as highly valuable for genetic resources conservation and management. In view of expected climatic changes, the responses of trees at their xeric limits may largely be determined by their capacity to cope with augmented environmental variance. We assess the heritability of early survival of Patagonian cypress in two common-garden field tests with contrasting summer water deficits, comprising 140 and 163 open-pollinated families from 10 marginal xeric populations. The first experiment underwent less rigorous conditions than the average mesic, Mediterranean climatic conditions, which were sufficient to reveal additive genetic effects of summer drought on seedling survival. The second trial suffered strong summer water-deficit stress and a winter extreme cold event. In this harsher environment, the heritabilities of survival under summer water-deficit stress were high in all the populations (h ²?=?0.84 on average), while the heritabilities of seasonal, extreme cold survival were moderate or even nil (h ²?=?0.28 on average). We did not find evidence of genetic differentiation among populations in their capabilities to survive droughts and cold extremes. Our results indicate that even when climatic changes were strong enough to cause the extinction of the most threatened populations, heritable variation for traits underlying drought and cold tolerances may allow the marginal xeric edge of cypress to persist under augmented environmental variance, without losing overall genetic diversity.     

Forest/environment relationships in Yosemite National Park,California, USA

Forest compositional patterns in Yosemite National Park, California, were related to environmental factors through numerical classification of forest types, arrangement of forest types along elevational and topographic gradients, and development of regression models relating basal area of common tree species to environmental variables. The eight forest types are differentiated primarily by elevation zone and secondarily by topographic setting. Lower montane forests (1200–1900 m) were divided into the Abies concolor/Calocedrus type occurring primarily on mesic sites and the Pinus ponderosa/Calocedrus type predominantly on xeric sites. Upper montane forests (1900–2500 m) included the Abies concolor/Abies magnifica type on mesic sites, the Abies magnifica/Pinus type on somewhat more xeric sites, and Juniperus occidentalis/Pinus jeffreyi woodlands on granitic domes. Subalpine forests (2500–3300 m) embraced three types: Tsuga mertensiana/Pinus forests on mesic sites, monotypic Pinus contorta forests on drier sites, and Pinus albicaulis/Pinus contorta groves at treeline. Regression models consistently included elevation and soil magnesium content as explanatory variables of species basal area totals. The two Abies spp. were negatively correlated with soil magnesium levels, whereas other montane species (e.g. Calocedrus decurrens, Pinus lambertiana, and Pinus ponderosa) exhibited positive correlation with soil magnesium. Topography and soil physical properties were only infrequently incorporated into species regression models.Abbreviations DBH= diameter at breast height (1.4 m) - DCA= detrended correspondence analysis - TWINSPAN= two-way indicator species analysis     

Composition change and vegetation degradation of riparian forests in the altai plain,NW China

Riparian forests of the Altai Plain in China were studied usingDetrended Canonical Correspondence Analysis (DCCA) and Two-way Indictor SpeciesAnalysis (TWINSPAN). The species could be divided into hydrophytes,hygrophytes,hygro-mesophytes, xero-mesophytes, xerophytes, and high xerophytes. Riverrun-off, water table, and physical components of the soil decided thedistribution of the species. The forests could be classified into wood swamp,hygro-mesic forest, mesic forest and xeric forest. As a specific habitat in thedesert of northwest China, the river valleys harbored most of thePopulus and Salix species recorded inChina. However, the forest has been gradually invaded by adjacent desertspecies. Meanwhile, the native species diversity of the forest has beendeclining as the soil has become more saline and more xeric through intensiveirrigation practice and dam construction in the upper rivers.