Spatial patterns of the germinable soil seed bank of coexisting perennial-grass species in grazed shrublands of the Patagonian Monte

We compared the size and spatial pattern of the germinable soil seed bank (GSB) of the three dominant perennial-grass species in the arid Patagonian Monte of Argentina. These species differ in plant functional traits, seed morphology, and in their preference by grazers. During three consecutive years, we sampled the soil seed bank at 1 m intervals and the intercepted diameter of perennial-grass species and shrub patches along 50-m linear transects at three sites with different grazing pressure. Soil samples were incubated at field capacity during 12 weeks and emerged seedlings counted. The largest-seeded and most xerophytic Stipa speciosa did not form a GSB. The other two grass species with more mesophytic traits and smaller seeds than S. speciosa formed different GSB. Poa ligularis formed a smaller and more heterogeneous GSB than Stipa tenuis. Grazing had not effect on GSB size of P. ligularis but increased the proportion of seeds under dense shrub canopies. In contrast, grazing reduced the size and also increased the proportion of the GSB of S. tenuis under shrub canopies. We conclude that the size and the spatial pattern of the GSB of the dominant perennial-grass species in the arid Patagonian Monte were related not only to seed and plant traits but also to the spatial distribution of grass plants and their exposure to grazers.     

Ecological traps and species distribution models: a challenge for prioritizing areas of conservation importance

Species distribution models analyse how species use different types of habitats. Their spatial predictions are often used to prioritize areas for conservation. Individuals may, however, prefer settling in habitat types of low quality compared to other available habitats. This ecological trap phenomenon is usually studied in a small number of habitat patches and consequences at the landscape level are largely unknown. It is therefore often unclear whether the spatial pattern of habitat use is aligned with the behavioural decisions made by the individuals during habitat selection or reflects actual variation in the quality of different habitat types. As species distribution models analyse the pattern of occurrence in different habitats, there is a conservation interest in examining what their predictions mean in terms of habitat quality when ecological traps are operating. Previous work in Belgium showed that red-backed shrikes Lanius collurio are more attracted to newly available clear-cut habitat in plantation forests than to the traditionally used farmland habitat. We developed models with shrike distribution data and compared their predictions with spatial variation in shrike reproductive performance used as a proxy for habitat quality. Models accurately predicted shrike distribution and identified the preferred clear-cut patches as the most frequently used habitat, but reproductive performance was lower in clear-cut areas than in farmland. With human-induced rapid environmental changes, organisms may indeed be attracted to low-quality habitats and occupy them at high densities. Consequently, the predictions of statistical models based on occurrence records may not align with variation in significant population parameters for the maintenance of the species. When species expand their range to novel habitats, such models are useful to document the spatial distribution of the organisms, but data on population growth rates are worth collecting before using model predictions to guide the spatial prioritization of conservation actions.     

The role of nurse plants in the establishment of shrub seedlings in the semi-arid subtropical Andes

In the nurse plant syndrome, or nurse association, seedlings (beneficiaries) are associated with adult shrubs/trees (benefactors). This phenomenon has been documented in several regions of the planet. Abiotic stress amelioration (one mechanism of facilitation) is one of the causes of this association. Most of the studies addressing the nurse syndrome have been conducted on spatial scales of a few hectares and have focused on only one or a few species. Moreover, there is an almost complete lack of studies addressing the incidence and characteristics of the nurse phenomenon in the arid Andes of South America. We undertook a first approximation to the study of facilitation in these ecosystems. The study was conducted at local and regional scales and involved the assessment of the spatial distribution of juveniles (seedlings and saplings) of 51 populations of 16 shrub and 12 cactus species in relation to shrub cover at 20 localities of the Prepuna (subtropical Andes of Bolivia and Argentina, 20–26°S). In terms of spatial distribution, the juveniles of most of the populations of shrubs studied were distributed both under the shrubs and in open spaces, thereby showing an apparent indifference to microhabitat. Globose and opuntioid cacti were preferentially distributed below the canopies of shrubs and were usually more associated with the dominant shrub species, which stood out as better potential nurses. The pattern was consistent throughout the region, including the more mesic and arid localities. The fact that Prepuna woody species are capable of establishing in open spaces would confer this region a greater resilience. Our findings further suggest that community dynamics in arid and semi-arid environments are more variable than previously thought.     

Spatial patterns of tree recruitment in a relict population of Pinus uncinata: forest expansion through stratified diffusion

Aim To infer future changes in the distribution of isolated relict tree populations at the limit of a species’ geographical range, a deep understanding of the regeneration niche and the spatial pattern of tree recruitment is needed. Location A relict Pinus uncinata population located at the south‐western limit of distribution of the species in the Iberian System of north‐eastern Spain. Methods Pinus uncinata individuals were mapped within a 50 × 40‐m plot, and their size, age and reproductive status were estimated. Data on seed dispersal were obtained from a seed‐release experiment. The regeneration niche of the species was assessed based on the associations of seedling density with substrate and understorey cover. The spatial pattern of seedlings was described using point‐pattern (Ripley's K) and surface‐pattern (correlograms, Moran's I) analyses. Statistical and inverse modelling were used to characterize seedling clustering. Results Pine seedlings appeared aggregated in 6‐m patches. Inverse modelling estimated a longer mean dispersal distance (27 m), which corresponded to the size of a large cluster along the north to north‐eastward direction paralleled by an eastward trend of increasing seedling age. The two spatial scales of recruitment were related to two dispersal processes. The small‐scale clustering of seedlings was due to local seed dispersal in open areas near the edge of Calluna vulgaris mats: the regeneration niche. The long‐range expansion might be caused by less frequent medium‐distance dispersal events due to the dominant north‐westerly winds. Main conclusions To understand future range shifts of marginal tree populations, data on seed dispersal, regeneration niche and spatial pattern of recruitment at local scales should be obtained. The monitoring of understorey communities should be a priority in order to predict correctly shifts in tree species range in response to global warming.     

Effects of competition and litter on a carnivorous plant, Drosera capillaris (Droseraceae)

Although carnivorous plants are widely recognized as being generally restricted to open habitats, tests of the effects of competition on individual performance are extremely rare. In this study, I examined the effects of the removal of herbaceous and shrub canopies on seedling density and growth, survival, and reproduction of phytometers of a small insectivorous plant, Drosera capillaris (pink sundew). I also examined the distribution of this species in relation to the occurrence of woody species in a frequently burned wet savanna in southeastern Mississippi. Killing plants and removing dead biomass increased seedling density in both open areas and shrub thickets. The removal of dead biomass following herbicide application was critical to increasing densities of seedlings. Killing plants with herbicide without also clearing residual litter and standing dead was not sufficient to increase seedling densities in shrub thickets. Although the removal of the groundcover canopy strongly influenced the density of seedlings, it had very little effect on survival, growth, and reproduction of small phytometers during a single growing season. Survival of phytometers was greater in open areas than in shrub thickets, regardless of whether the groundcover canopy was removed. Densities of both seedlings and adults were greater in open areas away from shrub thickets than beneath the woody canopies of thickets and were negatively correlated with the leaf area index of groundcover vegetation. Results of this study show that the establishment of this carnivorous plant species is limited in part by the effects of litter on seedling density in both open areas and shrub thickets.     

退化中的长白山西坡灌木苔原优势种分布差异

长白山苔原带植被正在发生显著变化,灌木苔原中灌木植物分布范围萎缩,重要值下降。通过样方调查数据,分析灌木苔原中优势种的变化,灌木分布格局和灌木群落结构特征沿海拔的差异,旨在揭示长白山灌木苔原退化的区域差异,为明确其退化机理提供基础数据。研究表明:(1)长白山西坡灌木苔原退化严重,多种草本植物已经侵入,并成为优势种。目前7个优势种中灌木仅占2席,草本植物占据5席,与1979年的样方调查结果相比灌木优势种的数量和地位都明显下降。7个优势种均为聚集分布,各优势种分布呈现斑块化、分离化,统一的灌木苔原面临解体;大部分灌木苔原群落中,出现了草本层,苔原带下部灌木苔原中草本层高于灌木层,物种组成和群落形态接近草木苔原。(2)灌木在各海拔均仍有广泛分布,但其空间分布格局明显不同。在海拔2300m以下,灌木的分布产生较强的聚集现象,特别是在海拔2100m以下这种聚集分布现象更为突出;在海拔2300m以上灌木的聚集程度较弱。(3)长白山西坡灌木苔原退化的区域分异明显,在海拔2100m以下灌木苔原退化严重,成为草-灌苔原;在海拔2100—2300m之间,灌木苔原退化较严重,成为灌-草苔原;在海拔2300m以上,退化较轻,仍为灌木苔原。由此推断,长白山西坡灌木苔原的退化机理应包括两个方面:草本植物入侵,种间竞争导致灌木退化,以及环境变化导致灌木退化,二者皆可能是全球气候变化的结果。     

Influence of Habitat and Intrinsic Characteristics on Survival of Neonatal Pronghorn

Increased understanding of the influence of habitat (e.g., composition, patch size) and intrinsic (e.g., age, birth mass) factors on survival of neonatal pronghorn (Antilocapra americana) is a prerequisite to successful management programs, particularly as they relate to population dynamics and the role of population models in adaptive species management. Nevertheless, few studies have presented empirical data quantifying the influence of habitat variables on survival of neonatal pronghorn. During 2002–2005, we captured and radiocollared 116 neonates across two sites in western South Dakota. We documented 31 deaths during our study, of which coyote (Canis latrans) predation (n = 15) was the leading cause of mortality. We used known fate analysis in Program MARK to investigate the influence of intrinsic and habitat variables on neonatal survival. We generated a priori models that we grouped into habitat and intrinsic effects. The highest-ranking model indicated that neonate mortality was best explained by site, percent grassland, and open water habitat; 90-day survival (0.80; 90% CI = 0.71–0.88) declined 23% when grassland and water increased from 80.1 to 92.3% and 0.36 to 0.40%, respectively, across 50% natal home ranges. Further, our results indicated that grassland patch size and shrub density were important predictors of neonate survival; neonate survival declined 17% when shrub density declined from 5.0 to 2.5 patches per 100 ha. Excluding the site covariates, intrinsic factors (i.e., sex, age, birth mass, year, parturition date) were not important predictors of survival of neonatal pronghorns. Further, neonatal survival may depend on available land cover and interspersion of habitats. We have demonstrated that maintaining minimum and maximum thresholds for habitat factors (e.g., percentages of grassland and open water patches, density of shrub patches) throughout natal home ranges will in turn, ensure relatively high (>0.50) neonatal survival rates, especially as they relate to coyote predation. Thus, landscape level variables (particularly percentages of open water, grassland habitats, and shrub density) should be incorporated into the development or implementation of pronghorn management plans across sagebrush steppe communities of the western Dakotas, and potentially elsewhere within the geographic range of pronghorn.     

干旱河谷微生境变化对乡土植物幼苗定植的影响

干旱区植被斑块状分布格局引起的微生境差异对植被更新影响显著。气候变化和人类活动扰动下, 干旱区生态系统微生境多样化, 急需揭示乡土植物定植对不同微生境斑块变化的响应及其种间差异性, 并采用微生境调控技术促进退化生态系统植被恢复。该研究选择岷江干旱河谷区自然分布的灌木、半灌木和裸地微生境斑块, 采用移栽鞍叶羊蹄甲(Bauhinia brachycarpa)幼苗的试验方法, 揭示微生境变化对幼苗定植的影响; 进一步以极端退化的道路边坡为案例, 通过6种乡土植物种子直播试验探讨微生境调控技术及其对乡土植物幼苗定植的促进作用。结果显示, 在自然生态系统中, 裸地斑块上幼苗保存率和生物量显著大于植被斑块, 表明裸地微生境有利于幼苗定植; 养分添加仅对裸地斑块中幼苗生物量积累有促进作用。在裸地斑块中, 叶片生物量所占的比例和比叶面积较小, 相反根和茎生物量所占的比例较大。道路边坡上植被恢复试验结果显示, 6种乡土植物均能较好地适应土石混杂的边坡生境, 多数物种出苗率大于60%; 灌木幼苗保存率大于75%, 并且形成镶嵌式乡土灌草群落结构。地表覆盖和养分添加提高了边坡上种子出苗率和幼苗保存率, 促进了幼苗定植和结构稳定。该研究提供了有效促进工程边坡上乡土植物定植的方法, 可为干旱、半干旱生态系统退化荒坡和工程破坏地乡土植被恢复提供理论依据和技术指导。     

Effect of grazing on plant patterns in arid ecosystems of Patagonian Monte

Our objective was to assess the relationship between the spatial patterning of perennial grasses (total, grazed, and non‐grazed) and shrub patches in rangelands under different grazing pressures of the Patagonian Monte. We selected three grazed paddocks with the usual stocking rate for the area, where previous studies showed that a piosphere formation is common. At each paddock, we analysed the grain of heterogeneity at sites located at two distances from the single watering point (near, far), using high‐resolution aerial photographs. At these sites, we also assessed in the field the density, size, cover, and spatial patterning of grazed and non‐grazed perennial grasses and shrub patches. The grain of heterogeneity of shrub patches was coarser in sites near the watering point than in those distant from it, as a consequence of the increase in size of both, bare soil and shrub patches. Field sampling showed that a coarse grain of heterogeneity relative to fine‐grained sites resulted from changes in species composition, increased bare soil areas and reduced perennial grass cover. In coarse‐grained sites, lower perennial grass cover resulted from lower density and/or smaller size of grass bunches than in fine‐grained sites. We did not find significant differences among sites in the proportion of perennial grazed grasses. Since the density and cover of perennial grasses was higher in fine‐ than in coarse‐grained sites, we suggested that fine‐grained sites are more important as feeding stations than coarse‐grained sites. The consequences of this differential use could lead to degradation of fine‐grained sites and to higher homogeneity in spatial plant structure and floristic composition within paddocks with respect to the condition observed at present, increasing the size of the highly degraded zone within the piosphere. At the patch level, we found that at about one third of the sampled transects, both total and non‐grazed perennial grasses were spatially aggregated with shrub patches. However, in most transects grazed perennial grasses were indifferently distributed in relation with shrub patches, showing that grazers display high selectivity of foraging sites at macro level (i.e. high and low grazing pressure sites at the paddock level), but random occupancy of vegetation units (randomness in the distribution of grazed perennial grasses at the patch level). The intensity of the positive association between non‐grazed grasses and shrub patches was higher in fine‐grained than in coarse‐grained sites and may be attributed to higher protection against herbivores associated to denser shrub patches in fine‐ relative to coarse‐grained sites. We concluded that a feedback exists between the spatial distribution of species preferred by grazers and the spatial patterning of use of these species.     

The impact of forest spread on a marginal population of a protected peony (<Emphasis Type="Italic">Paeonia officinalis</Emphasis> L.): the importance of conserving the habitat mosaic

In the Mediterranean region of Europe, land-use changes have allowed for rapid colonisation of open habitats by woody species. As a result, it is critical to gather information on how protected species in open habitats respond to forest spread in such areas. Our objective is to quantify whether spatial heterogeneity of the vegetation associated with recent forest closure influences demographic structure and maternal fertility in a population of the protected Paeonia officinalis L. In closed woodland, adult plants of P. officinalis are almost exclusively vegetative, in open habitats seedlings are rare and on the woodland edge there is a relative over-representation of flowering plants and seedlings. Forest closure dramatically reduces flowering frequency, but has no significant effect on maternal fertility of flowering plants. The spatial aggregation of seedlings close to the maternal plants suggests that dispersal is spatially restricted. Together, these results suggest that the viability of the population requires a transitional habitat between open garrigues or grassland with spaced trees and woodland. A management programme incorporating tree and shrub thinning and cutting of parcels in rotation to maximise the length of the forest edge could maintain a habitat mosaic that favours the persistence of this species in the study site.     

Fire-mediated effects of shrubs, lichens and herbs on the demography of Hypericum cumulicola in patchy Florida scrub

Understanding the effect of disturbance and interspecific interactions on population dynamics and availability of suitable habitats for colonization and growth is critical for conservation and management of endangered species. Hypericum cumulicola is a narrowly endemic, small perennial herb virtually restricted to open areas of well-drained white sand in Florida rosemary scrub, a naturally patchy community that burns about every 20–80 years. Over 1 year (September 1994 to September 1995) we evaluated variation in survival, growth and fecundity among 1214 individuals in 14 rosemary scrub patches of different sizes (0.09–1.85 ha) and fire histories (2, 8–10, and >20 years since the last fire). Fire kills aboveground individuals of H. cumulicola, but new individuals were present a year after fire. Recruitment decreased in patches more than a decade post-fire. Survival, annual height growth rate, and fecundity (number of flowers and fruits) were higher in recently burned patches. Scrub patch size did not affect these demographic variables. Survival was positively associated with the presence of conspecifics and negatively related to proximity to the dominant shrub Florida rosemary (Ceratiola ericoides), prior reproductive output, and ground lichen cover. Since H. cumulicola and other herbaceous species in the rosemary scrub depend on sporadic fires to decrease interference of shrubs and ground lichens, its persistence may be threatened by fire suppression. Received: 4 December 1996 / Accepted: 5 June 1997     

Proximity is not a proxy for parentage in an animal-dispersed Neotropical canopy palm

We used parentage analysis to estimate seedling recruitment distances and genetic composition of seedling patches centred around reproductive trees of the animal-dispersed Neotropical canopy palm Iriartea deltoidea in two 0.5 ha plots within second-growth forest and one 0.5 ha plot in adjacent old-growth forest at La Selva Biological Field Station in north-eastern Costa Rica. Seedlings were significantly spatially aggregated in all plots, but this pattern was not due to dispersal limitation. More than 70 per cent of seedlings were dispersed at least 50 m from parent trees. Few seedlings were offspring of the closest reproductive trees. Seedling patches observed beneath reproductive trees originate from dozens of parental trees. Observed patterns of seedling distribution and spatial genetic structure are largely determined by the behaviour of vertebrate seed dispersers rather than by spatial proximity to parental trees.     

Birds,Cattle, and Bracken Ferns: Bird Community Responses to a Neotropical Landscape Shaped by Cattle Grazing Activities

Large areas of tropical moist forests have been converted to cattle pastures, generating complex landscapes where different habitats are represented by small patches with an uneven spatial distribution. Here, we describe how bird communities respond to the different elements present in a livestock landscape that was originally dominated by tropical moist forest. We surveyed six habitats: open pastures, pastures with shrubs, early‐ and middle‐secondary forests, mature forest, and pastures invaded by bracken ferns (Pteridium aquilinum). Bird diversity was high in secondary and mature forests, and low in fern‐invaded sites and open pastures. Fern‐dominated sites had the lowest bird species richness, and trophic guild diversity of all habitats. Habitat structure affected both bird species richness and densities in similar ways. Tree species richness was the habitat attribute that had a bigger positive effect on bird species richness. Bird community structure varied among sampled habitats, separating habitats in two major groups (forests and pastures). Our data indicate that bracken fern‐invaded pastures were the worst habitat condition for avian communities. To increase bird diversity, we recommend to eliminate or manage bracken fern and to increase shrub and tree cover in open pastures to provide food resources and shelter for birds. Finally, we encourage the maintenance of secondary and mature forest remnants as a strategy to conserve resident birds within a landscape dominated by livestock activities.     

Effects of spatial patterns on the pollination success of a less attractive species

Plant individuals rely on pollinator services for their reproduction and often have to share these services with co‐occurring neighbours, creating complex indirect plant–plant interactions. Many current theoretical models focus on the effect of floral resources’ density on the net outcome of these indirect plant–plant interactions, often neglecting the identity of plant species in the communities and especially the species’ spatial pattern. To fill this gap, we created a spatially explicit model whose goal was to study the interplay between relative densities and spatial distribution patterns of two plant species differing in their attractiveness for pollinators. Since theory predicts that pollinator behaviour strongly governs the outcome of pollination, we allowed the pollinators to systematically change their plant preferences based on their foraging experience. Thus the interplay between density and spatial patterns of plants was tested over a continuum of behaviours from specialists to generalists. Our most striking finding was that reproductive success of the less attractive species was affected in an opposite way by spatial patterns depending on whether the species had relatively low or high densities. Namely, when the less attractive species was highly abundant, its survival was higher when aggregated in large monospecific patches than when uniformly distributed. On the other hand, when the attractive species was more abundant, the less attractive species survived better when uniformly distributed. These results were consistent as long as the scale of the plant spatial aggregation was similar to or larger than the pollinators’ detection range. Our results suggest that aggregated plant spatial patterns manipulate pollinator behaviour by trapping them within monospecific patches. This effect was sufficiently strong to enhance the survival of a competitively inferior species and hence to act in a way similar to the more familiar niche or temporal separation among plant species.     

Facilitation of seedling establishment by an endemic shrub in tropical coastal sand dunes

Facilitation is predicted to occur on coastal sand dunes as these ecosystems have the harsh physical conditions common during primary succession. In a tropical dune system on the Gulf of Mexico the spatial patterns of plant distribution were analyzed and the hypothesis that facilitation is the responsible mechanism was tested by monitoring changes in the micro-environment and performing a seedling planting experiment under natural conditions. Densities of seedlings and adults of late colonizer grasses were two to six times greater beneath the early colonizer shrub, Chamaecrista chamaecristoides, than in the exposed areas. Temperatures on the sand surface, wind speed and sand accretion were significantly reduced by the shade of Chamaecrista. Only phosphate contents during the dry season were significantly higher in sand in the shade than in exposed sand. In the field experiment, successful establishment of seedlings of two late colonizer grasses (Trachypogon plumosus and Schizachyrium scoparium) was low but was concentrated exclusively beneath the shrubs. The introduced plants were reproductive one year after onset of the experiment and remained vigorous after three years. The observed spatial aggregation among the target species may be a result of improved conditions in the shade of the shrub, which facilitated the survival and establishment of late colonizers. On top of the environmental amelioration, protection from recurrent disturbances such as substrate mobility is an additional beneficial effect of the shrub.     

Establishment and reproduction of Aeschynomene virginica (L.) (Fabaceae) a rare,annual, wetland species in relation to vegetation removal and water level

Aeschynomene virginica is a rare annual plant found in freshwater tidal wetlands of the eastern United States. We hypothesized that standing vegetation and water inundation were two important environmental factors in its population dynamics. To test these hypotheses, we sowed seeds into plots with undisturbed vegetation or plots with all aboveground vegetation removed in 1998 and 1999. Presence/absence of seedlings was noted and seedling survival to reproduction, final size, and seed set were measured throughout both growing seasons. Seedling establishment from germination to the first true leaf stage increased with decreasing water depth. Vegetation removal plots had greater seedling establishment, higher seedling survival, and higher seed set per plant than non-removal plots. In a greenhouse study designed to test the effects of water level on seed germination and seedling establishment, no seedlings established in submerged soils, and seed germination and seedling establishment were lower in waterlogged soil than in wet soil. Physical stress associated with deeper water likely limits the distribution of A. virginica to higher elevations, where seeds that colonize patches with low vegetative cover are more likely to produce reproductive adults that produce more seeds relative to patches with established vegetation. A. virginica appears to be a fugitive species specializing on open habitat patches in tidal wetlands. This species may be dependent on disturbances for population establishment and maintenance.     

Facilitation and interference underlying the association between the woody invaders Pyracantha angustifolia and Ligustrum lucidum

Questions: 1. Is there any post‐dispersal positive effect of the exotic shrub Pyracantha angustifolia on the success of Ligustrum lucidum seedlings, as compared to the effect of the native Condalia montana or the open herbaceous patches between shrubs? 2. Is the possible facilitation by Pyracantha and/or Condalia related to differential emergence, growth, or survival of Ligustrum seedlings under their canopies? Location: Córdoba, central Argentina. Methods: We designed three treatments, in which ten mature individuals of Pyracantha, ten of the dominant native shrub Condalia montana, and ten patches without shrub cover were involved. In each treatment we planted seeds and saplings of Ligustrum collected from nearby natural populations. Seedlings emerging from the planted seeds were harvested after one year to measure growth. Survival of the transplanted saplings was recorded every two month during a year. Half of the planted seeds and transplanted saplings were cage‐protected from rodents. Results: Ligustrum seedling emergence did not differ among treatments while growth was significantly higher in the absence of shrub cover. Sapling survival was significantly higher under the canopy of Pyracantha, intermediate under Condalia, and lowest in the absence of shrub cover. Caging did not affect growth but enhanced seedling emergence and sapling survival. Conclusion: The differential sapling survival in the shrub canopy treatments is consistent with natural sapling distribution. Pyracantha and, less so, Condalia, has a nurse‐plant effect on Ligustrum. This results from contrasting effects of the shrubs on different stages of the life cycle of Ligustrum: no effect on seedling emergence, negative on seedling growth, and positive on sapling survival. This suggests that efforts to control the expansion of Ligustrum over the landscape should tackle Pyracantha as well.     

Litter decomposition in a sandy Monte desert of western Argentina: Influences of vegetation patches and summer rainfall

Abstract: We tested the hypothesis that shrub canopies interact with monthly rain pulses to control litter decomposition in a sandy Monte desert, in Argentina. We assessed (i) the potential for litter decomposition of soils beneath the canopies of two dominant shrub species (Larrea divaricata and Bulnesia retama, Zygophyllaceae R. Br.) and from bare‐ground microsites or ‘openings’; (ii) litter decomposition at different spatial patches over the summer rainy season; and (iii) the interaction between vegetation patches and monthly rain pulses on short‐term litter decomposition, or decomposition pulses. In a greenhouse experiment, we found buried litter decomposition to be higher in soils from under the canopies of a dominant shrub species compared with soils from openings and sterilized controls. This, and higher nutrient concentration under shrub soils, suggest undercanopy soils may support a microbial community capable of decomposing litter at higher rates than soils in bare openings. However, ?eld trials showed that shrub patches did not affect leaf litter decomposition over the rainy season, at least for short periods. We found an interaction between shrub patches and incubation time at the end of the ?eld experiment, with higher litter decomposition rates under B. retama canopies. In a monthly ?eld experiment, we found monthly rain pulses signi?cantly explained decomposition pulses, irrespective of patch type. Our ?ndings support the hypothesis that shrub soils have a greater potential for litter decomposition, but this is not directly translated to the ?eld possibly due to interactions with abiotic factors. Rain pulses create conditions for decomposition pulses to occur at shorter time scales, whereas rainfall may interact with a dominant shrub undercanopy to control litter mass loss over longer time scales.     

Different cost of reproduction for the males and females of the rare dioecious shrub Corema conradii (Empetraceae)

Males and females of dioecious plant species often differ in their reproductive investment. Such differences frequently result in differential demographic costs represented by lower growth, survival, and/or frequency of reproduction, and/or by more variable reproductive effort through time for females. We present the results of a study on Corema conradii, a rare dioecious shrub of the coastal dune heathlands of northeastern North America. We estimated the reproductive investment of both males and females, determined their age structure, and compared their spatial patterns in a population at ?les-de-la-Madeleine, Quebec. We also determined the sex ratio of the four populations known to occur on the islands. Males invested more in reproduction at flowering, but when fruit production was considered, female reproductive investment was higher in terms of biomass, Mg, and Ca, but not in terms of N, P, and K. The age frequency distribution of males and females did not differ significantly from one another. The population dispersion pattern was contagious, with patches of similar-age individuals. There was no spatial segregation between males and females, although the sex ratio varied somewhat spatially. Females did not start reproducing at a later age than males and did not appear to have a shorter longevity. However, the crown and radial growth rates of females were lower than those of males. When estimated by the crown intercept method, the sex ratio of all four populations was male biased. However, because males had a higher crown growth rate, genet sex ratio was in fact balanced. Higher investment in reproduction was associated with a lower growth rate, which represents a differential cost of reproduction according to sex in this species.     

Seedling growth variation in response to sand burial in four <Emphasis Type="Italic">Artemisia</Emphasis> species from different habitats in the semi-arid dune field

We compared seedling growth of four Artemisia species dominated at different habitats to determine whether interspecific seedling growth variation of a same genus in tolerance to burial can be used to explain plant distribution in the sand dune field. Interdune lowland species, Artemisia gmelinii, stabilized dune species, A. frigida, semi-stabilized dune species, A. halodendron, and active dune species, A. wudanica were selected. Seedlings grown for 3 weeks were treated at five burial depths for three burial times in pot experiments. Species from the habitats with little burial had smaller survival rate, dry weight and stem elongation speed than those from the habitats with intensive burial when buried. Furthermore, when buried, the former tended to adjust biomass allocation between shoot and root and produce adventitious buds, while the latter tended to maintain a constant root:shoot ratio and produce adventitious roots. We conclude that (1) seedlings of species with a long evolutionary history of exposure to sand burial (from the active sand dune), show quicker stem growth when buried than do seedlings of species from the habitats with little or no sand burial; (2) seedlings of species which do not change root:shoot ratio might be more tolerant of sand burial than those do; (3) seedlings of species from the habitats with intensive sand burial is prone to produce adventitious roots and seedlings of species from the habitats with little or no sand burial tend to produce adventitious buds when buried.