Ramet dynamics in a centrifugally expanding clonal sedge: a matrix analysis

Carex humilis is a clonal sedge that can form distinct rings of densely aggregated ramets. We hypothesize that rings form because both production of new ramets and ramet dispersal are positively correlated to ramet size. This would lead to an overrepresentation of fast-moving and large ramets with high ramet production at the periphery, whereas slow-moving and small ramets with low ramet production would mainly be found in the interior of rings. We use matrix models to analyse how ramet populations both at the periphery and in the interior develop in the absence of ramet dispersal. We found that the stable size class distributions of ramets predicted by the models were not different from the distributions found in the field. Also, the asymptotic ramet population growth rates (λ₁) were the same. Hence, we conclude that rings would form even in the absence of a link between ramet dispersal and ramet production. Further analysis of the matrix models showed that the ramet population increases at the periphery but decreases in the interior of rings because medium and large ramets produce fewer large ramets in the interior than at the periphery. We also found that the temporal variance in λ₁ and transitions rates during the four study years was much higher at the periphery than in the interior. Our results suggest that rings may form because C. humilis ramets use below-ground resources from a much larger area than the one covered by the shoots. As the clone grows larger, the soil volume available to the ramets in the interior decreases because their access to soil outside the ring is cut-off by the ramets at the periphery. Ramet density in the interior is therefore decreasing.     

Role of topography and soils in grassland structuring at the landscape and community scales

The relative importance of abiotic factors in community assembly is debated and thought to be dependent on the scale. I investigated the relative role of topography and soils as structuring agents at the landscape and the community scales in 126 subalpine calcareous grasslands in the Pyrenees, in terms of species composition and abundance. I wished to know: (1) the role of abiotic factors in the organization of plant communities across the landscape; (2) how much of the variation in community distribution was accounted for by abiotic factors; and (3) how well their role applied to the distribution of dominant species at the landscape and the community scales. The hypothesis was: abiotic factors play an important role in community distribution in the landscape, but species interactions are more important within communities. Multivariate methods generated four communities, organized in two contrasting groups along the main vegetation axis, which explained 13% of the variation: mesic grasslands (Nardus stricta and Festuca nigrescens communities) and xeric grasslands (Carex humilis and Festuca gautieri communities). Mesic communities were more acidic and fertile than xeric communities. Changes in the abiotic environment, accounting for up to 80% of the variation in the vegetation, were smooth, while the transition between xeric and mesic grasslands was sharp in terms of species composition. The distribution in the landscape of the first main species from each community was closely related to abiotic factors, which modeled poorly the abundance of the main species at smaller scales. At the within-community scale, the explanatory power of biotic relationships was community dependent, producing the most significant models for plants highly dominant within their communities, such as N. stricta and F. gautieri. Contrary to current hypothesis, there was a shift from mainly positive relationships among dominant species in fertile mesic communities to mainly negative in infertile xeric ones.     

Spatial patterns of an endemic Mediterranean palm recolonizing old fields

Throughout Europe, increased levels of land abandonment lead to (re)colonization of old lands by forests and shrublands. Very little is known about the spatial pattern of plants recolonizing such old fields. We mapped in two 21–22‐ha plots, located in the Doñana National Park (Spain), all adult individuals of the endozoochorous dwarf palm Chamaerops humilis L . and determined their sex and sizes. We used techniques of spatial point pattern analysis (SPPA) to precisely quantify the spatial structure of these C. humilis populations. The objective was to identify potential processes generating the patterns and their likely consequences on palm reproductive success. We used (1) Thomas point process models to describe the clustering of the populations, (2) random labeling to test the sexual spatial segregation, and (3) mark correlation functions to assess spatial structure in plant sizes. Plants in both plots showed two critical scales of clustering, with small clusters of a radius of 2.8–4 m nested within large clusters with 38–44 m radius. Additional to the clustered individuals, 11% and 27% of all C. humilis individuals belonged to a random pattern that was independently superimposed to the clustered pattern. The complex spatial pattern of C. humilis could be explained by the effect of different seed‐dispersers and predators' behavior and their relative abundances. Plant sexes had no spatial segregation. Plant sizes showed a spatial aggregation inside the clusters, with a decreasing correlation with distance. Clustering of C. humilis is strongly reliant on its seed dispersers and stressful environmental conditions. However, it seems that the spatial patterns and dispersal strategies of the dwarf palm make it a successful plant for new habitat colonization. Our results provide new information on the colonization ability of C. humilis and can help to develop management strategies to recover plant populations.     

The effect of grazing on bumblebees in the high rangelands of the eastern Tibetan Plateau of Sichuan

Many Tibetans who were formerly nomadic yak herders are settling near towns. However, yak remain a traditional symbol of wealth, which can lead to increased local stocking densities and severe over-grazing. We used belt-transect counts to study the area around Hongyuan in Sichuan. We find that intensive summer grazing significantly reduces the vegetation height and is associated with significant reductions in bumblebee-food-plant abundance and bumblebee diversity. For the significantly reduced bumblebee species, we identify the most frequently used and preferred bumblebee-food plants. For the food plants, we identify changes in absolute flower availability and changes in bumblebee visits. In particular, reductions in the bumblebees Bombus supremus, B. filchnerae, B. humilis and B. impetuosus are associated with reductions in flowers of the food plants Hedysarum and Saussurea.     

Genetic variation and habitat conditions in Betula humilis Schrk. populations in Poland,Belarus and Latvia

A rapidly growing human population and the extirpation of numerous species make modern wetlands among the most threatened of ecosystems. One of the endangered wetland plants in central and eastern Europe is Betula humilis. B. humilis forms comparably abundant populations on both well-preserved and drained fens; thus, we aimed in this paper to investigate the relationships between habitat quality parameters (e.g. concentrations of calcium and magnesium, groundwater levels, pH and conductivity) and genetic diversity in 12 populations of B. humilis from Poland, Belarus and Latvia. For the purposes of this study, alcohol dehydrogenase (ADH) gene sequences and nuclear microsatellites were chosen as molecular markers. Analyses revealed that only the Ca²⁺ concentration was significantly negative relative to the observed heterozygosity of microsatellites, which suggests that calcium access is the only factor influencing genetic variation at B. humilis localities. No genetic differentiation was detected between populations occupying wet and dry habitats in different parts of the species range. A minimum spanning tree based on ADH gene haplotypes appears to confirm the B. humilis population history previously inferred from analyses of nuclear microsatellites and chloroplast markers.     

2种嵩草属植物形态特征对生境干旱化的响应

该研究于西藏自治区东南部的东达山高山草甸沿生境干旱化梯度设置10个样地,采用线性回归方法分析优势种高山嵩草(Kobresia pygmaea)和矮生嵩草(K.humilis)叶片数量与其它构件数值(分株数量、茎基直径、根系数量和根系长度)之间的关系,并采用线性回归斜率测度分株功效、叶片萌生能力、根系分生功效和根系伸长功效,以探讨嵩草的分株能力、茎基生长和根系生长对生境干旱化过程的响应机制。结果显示:(1)2种嵩草植物的叶片数量与4种构件数值均为显著线性正相关关系。(2)随着生境干旱化程度增加,高山嵩草分株数量增加,矮生嵩草分株受干旱抑制程度高,分株数量呈下降趋势;2种嵩草分株功效下降,即单株叶片数量因干旱化程度增加而减少。(3)2种嵩草的茎基直径、叶片萌生能力随着生境干旱化程度增加而下降;高山嵩草叶片萌生能力的变化与生境干旱化梯度一致,具有连续性;矮生嵩草叶片萌生能力对生境干旱化的适应性弱,干旱到一定程度发生骤降。(4)随着生境干旱化,高山嵩草的根系数量和长度均增加,矮生嵩草根系长度增加,但数量却无规律变化;2种嵩草的根系分生功效和根系伸长功效均下降。研究表明:嵩草属植物分布的最适宜生境为表面稍有积水的沼生生境。为适应生境干旱化,高山嵩草降低叶片萌生率以减少蒸腾作用,增加分株数量以增强对空间的占有能力,并增加根系数量和长度来提高对土壤水分的吸收能力,因此对干旱有较强适应性且分布范围广;矮生嵩草只通过降低叶片萌生率,增加根系长度响应干旱化生境,其分布范围较窄。     

The role of a native tussock grass (Paspalum quadrifarium Lam.) in structuring plant communities in the Flooding Pampa grasslands, Argentina

The patches of Paspalum quadrifarium-dominated grasslands found atpresent could be remnants of the vegetation that defined the Pampas landscape inthe past. The mere presence of such physical structures should lead to systemsin which many other species are dependent upon both the autogenic creation ofsurface area for living space and the autogenic and allogenic modulation ofresources controlled by these structures. We carried out amensurative–comparative study in naturally occurring sites dominated byP. quadrifarium that occupy different positions within thelandscape in the Flooding Pampa. We found different species assemblages in thesestructurally homogeneous stands, related to the edaphic and topographicgradients. The comparison with sites from similar positions in the landscapewhere this native tussock-grass was absent showed that the dominance ofP. quadrifarium is a minor determinant of floristicdifferences, which are mainly controlled by abiotic factors. We assigned thesedominated grassland stands to previously delimited vegetation units for theFlooding Pampa grasslands and we compared them with the vegetation unitsassigned, in terms of composite properties of the community that may beindicators of ecosystem processes, such as biological invasions andproductivity. Our results show that this native tussock grass is associated withhigher floristic richness, higher representation of perennials and grasses andlower presence of alien plants. This homogeneous pattern of variation across thevegetation units would suggest that, despite the absence of dramatic changes incommunity composition, the dominance of this tussock grass plays an importantrole in structuring species diversity patterns.     

Temporal and sexual variation of leaf-produced pollinator-attracting odours in the dwarf palm

Information on intra-specific variation in pollinator-attracting floral traits provides clues to selective pressures imposed by pollinators. However, these traits also reflect constraints related to floral phenology or morphology. The specific weevil pollinator Derelomus chamaeropsis of the dioecious Mediterranean dwarf palm Chamaerops humilis is attracted by volatile compounds that leaves, and not flowers, release during anthesis. Production of these olfactory cues is thus probably not constrained by any other floral function. This provides the opportunity to study variation of a floral trait that is not produced by a floral organ. We studied volatile compounds emitted by leaves of 12 individual C. humilis over the whole flowering season. The quantity of volatile compounds emitted by leaves reached a maximum when plants required pollinator visits. The relationship between odour emission and floral phenology was slightly different between male and female plants, probably reflecting differences in the exact time at which females and males benefit from pollinator visitation. Male plants produced higher quantities of volatile compounds than females. Odour composition was highly variable among individuals but did not differ between male and female plants. In this system, female C. humilis are pollinated by deceit and pollinators should be selected to avoid visiting them. The absence of sexual difference in blend composition may thus prevent pollinators from discriminating between male and female plants.An erratum to this article can be found at     

干旱胁迫对欧李幼苗表型可塑性的影响

该研究以欧李为材料,探讨了干旱胁迫对欧李表型可塑性的影响。结果表明:(1)随着干旱胁迫的加剧,欧李根生物量、枝叶生物量、植株总生物量积累、根冠比和根冠比胁迫指数均呈现先升高后降低的趋势,在T1处理下达到最大值,并显著高于其他处理(α=0.05)。(2)随土壤含水量的降低,欧李根的生物量分配指数呈现先增加后降低的趋势,叶生物量与之相反,在T1处理下根的生物量分配指数最大,枝叶的最小(α=0.05)。(3)在水分供应为60%～80%时,欧李的株高、冠幅、基径、二级分枝数、主根长、主根直径及侧根数量均达到最大值(α=0.05),对一级分枝数的生长没有显著影响。(4)随着水分胁迫的加剧,叶片长从T2处理开始下降,叶片宽、单片叶面积及比叶面积均呈现先增加后减少的趋势(α=0.05)。综上可得,欧李通过调整形态特性和各器官生物量积累及其分配对不同干旱胁迫条件产生了较强的可塑性。     

The ‘small barbs’ Barbus humilis and B. trispilopleura of Lake Tana (Ethiopia): Are they Ecotypes of the Same Species?

Four species of small barbs (Barbus, subgenus Enteromius Cope, 1869) are known from Lake Tana, isolated in the Ethiopian highlands: B. humilis, B. trispilopleura, B. pleurogramma (all Boulenger, 1902) and B. tanapelagius de Graaf, 2000. However, only three species appear valid from cluster analysis using 32 morphometric characters and taking specimens from different locations in the southern Gulf of Lake Tana during August–October 1999. B. humilis and B. trispilopleura significantly differ from B. tanapelagius and B. pleurogramma in up to 36 characters. However, B. humilis and B. trispilopleura cannot be distinguished from each other by morphometric analysis or by gut contents. Specimens from clear, shallow rocky areas with vegetation have a darker back, will be more susceptible to birds, have significantly higher infection by cestodes, smaller size at first reproduction, lower fecundity, and correspond most to the B. trispilopleura phenotype. Specimens in turbid deeper water without vegetation are most similar to the B. humilis phenotype. We conclude that both species actually are extremes (ecotypes) of a continuum, belonging to a single biological species. The observed variation may well be induced by habitat-dependent predation pressure by birds. The high frequency (57%) of spot numbers intermediate between Boulengers number for B. trispilopleura (3) and for B. humilis (0) demonstrates the continuum best. Pigment spots and colour change in response to aquarium conditions and are in this case no valid taxonomic characters. Both characters may reduce the risk of predation. It is concluded that B. trispilopleura is a synonym of B. humilis. For future research we recommend to use the most appropriate name, B. humilis, for both types.     

Net primary productivity and spatial distribution of vegetation in an alpine wetland,Qinghai-Tibetan Plateau

To initially describe vegetation structure and spatial variation in plant biomass in a typical alpine wetland of the Qinghai-Tibetan Plateau, net primary productivity and vegetation in relationship to environmental factors were investigated. In 2002, the wetland remained flooded to an average water depth of 25 cm during the growing season, from July to mid-September. We mapped the floodline and vegetation distribution using GPS (global positioning system). Coverage of vegetation in the wetland was 100%, and the vegetation was zonally distributed along a water depth gradient, with three emergent plant zones (Hippuris vulgaris-dominated zone, Scirpus distigmaticus-dominated zone, and Carex allivescers-dominated zone) and one submerged plant zone (Potamogeton pectinatus-dominated zone). Both aboveground and belowground biomass varied temporally within and among the vegetation zones. Further, net primary productivity (NPP) as estimated by peak biomass also differed among the vegetation zones; aboveground NPP was highest in the Carex-dominated zone with shallowest water and lowest in the Potamogeton zone with deepest water. The area occupied by each zone was 73.5% for P. pectinatus, 2.6% for H. vulgaris, 20.5% for S. distigmaticus, and 3.4% for C. allivescers. Morphological features in relationship to gas-transport efficiency of the aerial part differed among the emergent plants. Of the three emergent plants, H. vulgaris, which dominated in the deeper water, showed greater morphological adaptability to deep water than the other two emergent plants.     

青藏高原高寒草甸门源草原毛虫取食偏好及其与植物C、N含量的关系

草食性昆虫对不同植物物种的取食存在偏好,这种取食偏好可能受其自身对蛋白质和碳水化合物的需求及二者平衡的调节。以青藏高原高寒矮嵩草草甸31种常见植物及门源草原毛虫为对象,通过饲喂实验,研究了草食性昆虫对不同物种和不同功能群植物的取食偏好,及其与植物叶片C、N含量和C∶N之间的关系。在31种植物中,门源草原毛虫对19种植物进行了取食,尤其对矮嵩草、红棕薹草、藏异燕麦和垂穗披碱草四种植物表现出强烈的取食偏好,而对另外12种植物未进行任何取食。在物种水平上,门源草原毛虫取食量与植物叶片N含量呈显著负相关,与叶片C∶N呈正相关。从功能群水平上看,门源草原毛虫对莎草类的取食偏好最大,而对豆科植物取食偏好最低;相应地,莎草类植物叶片N含量最低、C∶N最高,而豆科植物叶片N含量最高、C∶N最低。因此,即使在土壤有效氮匮乏、植物生长受氮素限制的高寒草甸生态系统,植物体内N含量的增加也可能不利于草食性昆虫的取食。门源草原毛虫对优势植物矮嵩草和垂穗披碱草的取食对高寒矮嵩草草甸物种共存和生物多样性维持可能具有重要的作用。     

Mechanisms of vegetation-ring formation in water-limited systems

A common patch form in dryland landscapes is the vegetation ring. Vegetation patch formation has recently been attributed to self-organization processes that act to increase the availability of water to vegetation patches under conditions of water scarcity. The view of ring formation as a water-limited process, however, has remained largely unexplored. Using laboratory experiments and model studies we identify two distinct mechanisms of ring formation. The first mechanism pertains to conditions of high infiltration contrast between vegetated and bare soil, under which overland water flow is intercepted at the patch periphery. The decreasing amount of water that the patch core receives as the patch expands, leads to central dieback and ring formation. The second mechanism pertains to plants with large lateral root zones, and involves central dieback and ring formation due to increasing water uptake by the newly recruited individuals at the patch periphery. In general the two mechanisms act in concert, but the relative importance of each mechanism depends on environmental conditions. We found that strong seasonal rainfall variability favors ring formation by the overland-flow mechanism, while a uniform rainfall regime favors ring formation by the water-uptake mechanism. Our results explain the formation of rings by fast-growing species with confined root zones in a dry-Mediterranean climate, such as Poa bulbosa. They also explain the formation of rings by slowly growing species with highly extended root zones, such as Larrea tridentata (Creosotebush).     

Annual variation and influence of climate on the ring width and wood hydrosystem of Prosopis flexuosa DC trees using image analysis

We developed chronologies based on the width of tree rings, total area of vessels, and the number of vessels per tree ring of the Prosopis flexuosa wood samples from the xerophytic woodlands of central Argentina. We evaluated the influence of climate on these tree-ring characteristics considering the period from 1940 to 2004 (65 years). The width of the rings, the number of vessels, and the total area of vessels were positively influenced by regional precipitation corresponding to the seasonalized November to December period, which reflects the importance of the water availability in the initial stage of the formation of the wood. The width of the rings and the total area of vessels were negatively influenced by temperature during the same period, while the number of vessels was not significantly correlated with temperature. The high temperatures in spring increase evapotranspiration, which reduces water availability to plants and results in an inverse growth response. This study was the first to develop chronologies based on anatomical characters of wood from the arid and semiarid regions of South America and with significant applications in ecological and climatic studies.     

Comparative evaluation of two olive fruit fly parasitoids under varying abiotic conditions

Psyttalia lounsburyi (Silvestri) and P. humilis (Silvestri) (Hymenoptera: Braconidae) were evaluated in California for their potential to control the invasive olive fruit fly, Bactrocera oleae (Rossi) (Diptera: Tephritidae). Psyttalia lounsburyi is a specialist on B. oleae while P. humilis also attacks other tephritid species. Field cage trials, conducted from 2006 to 2009, were used to compare P. lounsburyi and two populations of P. humilis (Kenya and Namibia) in California’s interior valley and coastal regions. Both parasitoid species reproduced on B. oleae in all trials. Under similar abiotic conditions, offspring production per female was higher in P. humilis than in P. lounsburyi, suggesting that host specificity by P. lounsburyi does not confer a higher efficiency on B. oleae in cultivated olives. Two abiotic factors were shown to impact parasitoid efficiency. First, adult parasitoid survival was poor during periods of high summer temperatures, common to the olive production areas in California’s interior valleys. Second, parasitism levels were lower on B. oleae larvae feeding in larger Ascolano cv. fruit than in smaller Manzanillo cv. fruit. Results are discussed relative to biological control of B. oleae in commercial olives and the usefulness of natural enemies specialized to attack fruit flies in wild olives compared with the larger cultivated olive fruit.     

Vegetative and seedling regeneration after fire in planted Sardinian pinewood compared with that in other areas of Mediterranean-type climate

Aim Fire is a key disturbance in Mediterranean‐type climates. It has effects on plant community structure and composition and on the evolution of different groups of the flora. This study aimed to quantify changes in demography and vegetative regeneration caused by fire in key species in Mediterranean vegetation and to examine the hypothesis of convergence in characteristics of species in relation to fire by comparing the occurrence of bioecological groups of plants in the Mediterranean basin with groups of plants in other areas of Mediterranean‐type climate. Methods Changes in the demography and species composition after fire were studied in a plant community developed from a pinewood (Pinus halapensis Miller plantation) in north‐western Sardinia. The demography of the three main species dominating the community after fire, Cistus monspeliensis L., Pistacia lentiscus L. and Chamaerops humilis L., was quantified by quadrat measurements in areas of the plantation which had never been burnt and in different parts 1 and 8 years after fire. Vegetative regeneration of resprouting species was also quantified. From the results of the survey and examination of published material, comparison was made between bioecological groups of species in different regions with Mediterranean‐type climate to assess the similarities and differences in the biogeographic distribution of what can be broadly considered fire‐related functional groups of plants. Results Seeder and sprouter species showed contrasting population responses to fire in an area planted to pines over 50 years ago but subsequently in part subjected to fire. Mean species richness at a 25 m² scale decreased by c. 28% over 8 years after fire. C. monspeliensis in the unburnt pinewood was sparse and sporadic but recruitment after fire was prolific and produced plant densities similar to those of similar seeder species in other Mediterranean Basin areas and in Californian chaparral. However density was very variable between burnt areas of different ages. In particular, seedling recruitment was much lower under the canopies of P. lentiscus than away from the canopy. Sprouter species (P. lentiscus and C. humilis) had recruited into the pinewood over time since planting of the pines but did not recruit after fire. Above‐ground parts of plants were killed by fire and resprout stem density per plant increased initially but later decreased slightly due to self thinning of stems on each plant. Main conclusions Plant demographic and floristic changes after fire in the Sardinian pinewood were similar to those reported for other Mediterranean plant communities. Resprouting species only recovered by vegetative means and, in contrast to seeder species, underwent thinning of stems within, rather than between, plants, so plant density did not change after fire in resprouting species. The Tertiary relic, strongly resprouting, animal‐dispersed group of species in the Mediterranean Basin has a homologous group in the North American mediterraneoid flora but not in the Southern Hemisphere. In contrast, the general group of resprouting species which recruit after and/or between fires in the Southern Hemisphere mediterraneoid areas corresponds to only a small group of taxa in the Mediterranean basin. The bioecological group of seeder species, which are killed by and establish prolifically after fire, are represented in both Northern and Southern Hemisphere Mediterranean‐type heathlands but are much more species rich in the south.     

Shrub facilitation of Quercus humilis regeneration in succession on calcareous grasslands

Abstract. In southern France, the natural invasion by Quercus humilis of calcareous grassland takes place in a mosaic of herbaceous and scrubby patches. We hypothesized that the presence of the shrubs Buxus sempervirens and Juniperus communis alter the rate and the pathway of the succession by facilitating the regeneration of Q. humilis. To infer the process of facilitation at a large scale, the spatial distribution of Q. humilis was studied in relation to acorn sources and the type of plant cover in grazed and ungrazed sites. Abundant recruitment up to 80 m from the wood margins and from isolated oak trees in grassland shows that acorns are dispersed effectively. At the three study sites, the density of Q. humilis individuals was higher under shrubs than in grassland, suggesting that facilitation may occur. This density difference was much higher in the grazed sites than in the ungrazed site. Moreover, before grazing by livestock, the distribution of first-year seedlings is independent of vegetation cover. Thus, shrubs improve Q. humilis regeneration by protecting individuals from grazing. The high density of individuals at the northern edge of shrubs suggests that a second facilitation mechanism may exist, probably related to improved germination conditions. Facilitation by shrubs appears to be very important for Q. humilis dynamics.     

Photosynthetic limitations and volatile and non‐volatile isoprenoids in the poikilochlorophyllous resurrection plant Xerophyta humilis during dehydration and rehydration

We investigated the photosynthetic limitations occurring during dehydration and rehydration of Xerophyta humilis, a poikilochlorophyllous resurrection plant, and whether volatile and non‐volatile isoprenoids might be involved in desiccation tolerance. Photosynthesis declined rapidly after dehydration below 85% relative water content (RWC). Raising intercellular CO₂ concentrations during desiccation suggest that the main photosynthetic limitation was photochemical, affecting energy‐dependent RuBP regeneration. Imaging fluorescence confirmed that both the number of photosystem II (PSII) functional reaction centres and their efficiency were impaired under progressive dehydration, and revealed the occurrence of heterogeneous photosynthesis during desiccation, being the basal leaf area more resistant to the stress. Full recovery in photosynthetic parameters occurred on rehydration, confirming that photosynthetic limitations were fully reversible and that no permanent damage occurred. During desiccation, zeaxanthin and lutein increased only when photosynthesis had ceased, implying that these isoprenoids do not directly scavenge reactive oxygen species, but rather protect photosynthetic membranes from damage and consequent denaturation. X. humilis was found to emit isoprene, a volatile isoprenoid that acts as a membrane strengthener in plants. Isoprene emission was stimulated by drought and peaked at 80% RWC. We surmise that isoprene and non‐volatile isoprenoids cooperate in reducing membrane damage in X. humilis, isoprene being effective when desiccation is moderate while non‐volatile isoprenoids operate when water deficit is more extreme.     

Spatial patterns in cushion‐dominated plant communities of the high Andes of central Chile: How frequent are positive associations?

Question: In stressful abiotic environments positive plant interaction is expected to be a frequent and an important process driving community composition and structure. In the high Andes in central Chile, the cushion plant Azorella madreporica dominates plant communities and appears to benefit the assemblage of species that grows within it. However, there are also many other species that grow outside this nurse cushion plant, which may or may not interact with this species. What is the prevailing type of spatial associations among the plant species that are not growing inside the nurse plant? What is the type of interactions between cushion plants and those species growing outside them? Location: Molina River basin (33°20'S, 70°16’ W, 3600 m a.s.l.), in the Andes of central Chile, ca. 50 km east of Santiago. Methods: Two accurate mapping plots of individual plants of different species were located at two summits (Franciscano and Tres Puntas sites). The spatial distributions and associations between species growing outside cushions and within cushions at each site were estimated by point‐pattern analyses using the univariate and bivariate transformations of Ripley's K‐functions. Results: We found both positive and, especially, negative spatial associations (8 out of 12 species in Franciscano site) between A. madreporica cushions and plants growing outside them. However, most of the species showed positive spatial associations among them. The variation in spatial association was site‐specific and also depended on the type of plants involved. Adesmia spp., the second most abundant non‐cushion species, displayed negative associations with cushions and positive associations with other species growing outside cushions. Conclusions: Our study suggests very complex interactions among species, which ranged from positive to negative, and are also affected by abiotic environmental conditions.     

The Regeneration of a Highly Disturbed Ecosystem: A Mined Peatland in Southern Québec

We studied the natural regeneration of an ombrotrophic peatland (Cacouna bog) located in southern Québec that was disturbed by peat mining and other anthropogenic activities over a 200-year period. Using an extensive collection of historical documents, as well as dendrochronological data, we reconstructed the history of the peatland. We also sampled vegetation and environmental variables, and integrated the data in a geographic information system. More than 60% of the total area of the bog was mined between 1942 and 1975, and 98 km of ditches were dug to drain the site. The peatland lost 34% of its initial peat volume between 1946 and 1998. Although the bog was severely disturbed, the spontaneous revegetation of the site by vascular plants was successful (90%–100% cover). However, only 10% of the total mined area has been recolonized by Sphagnum species, mainly because drainage ditches are still operational and contribute to drying out the bog. Water table level, peat deposit thickness, and pH are abiotic factors strongly influencing the vegetation composition in the bog. Spatial and historical factors are also important components in this study since they explain, either alone or in interaction with abiotic factors, 44% of the variation of the species data. The intensity of mining activities and the pattern of abandonment of mined sectors strongly influenced abiotic factors, which in turn affected the revegetation process. Even if the Sphagnum cover of the bog is low, the rapid “recovery” of the vegetation cover in the peatland indicates that after the reestablishment of an appropriate hydrological regime, a highly disturbed peatland has a considerable potential for regeneration. Received 24 April 2001; accepted 30 October 2001.