Effect of shrubs and seasonal variability of rainfall on the establishment of Aspidosperma quebracho‐blanco in two edaphically contrasting environments

Abstract Three experiments were conducted to verify if an increase in environmental stress level would affect the interactions between two species of nurse shrubs and seedlings of Aspidosperma quebracho‐blanco. This is a mesic species with a generalist distribution over an extensive environments gradient. The relationship between Larrea divaricata and seedlings of A. quebracho‐blanco was studied in two contrasting soils, a silty loam soil with higher surface clay content and a sandy loam soil. The effect of seasonal variability of rainfall on the initial establishment of seedlings under the shade of L. divaricata was evaluated in three consecutive years. The effect of nurse plant shade was tested comparing two shrub species with different types of leaf life span (sclerophyllous‐evergreen and leguminous‐deciduous). The natural establishment of A. quebracho‐blanco depended on shaded microsites, but not on the type of shade provided by different nurse shrubs. Emergence and initial establishment depended on interactions of soil type and seasonal rainfall variation with nurse plants. The importance of facilitation increased with clay soil (CS). Sandy soil was ‘less humid’ than CS under shrub shade. However, establishment success depends on opportune even rainfall distribution in interaction with nurse plant presence.     

Direct versus indirect facilitation (herbivore mediated) among woody plants in a semiarid Chaco forest: A spatial association approach

In arid environments, direct facilitation (microhabitat amelioration) and indirect facilitation (‘associational resistance’ via protection from herbivory) among plants of different species may act simultaneously. Little is known about their relative effects. One way to disentangle the effects is by evaluating spatial associations. We examined the relative importance of these two mechanisms of facilitation in the semiarid Chaco vegetation of north‐central Argentina, through an eight‐way observational study in which we quantified the degree of spatial association between saplings of each of two key tree species, Schinopsis lorentzii (Anacardiaceae) and Aspidosperma quebracho‐blanco (Apocynaceae), with shrub neighbours either possessing spines or without spines and in both an ungrazed site and a site with a long history of cattle grazing. We analysed data across 400 subparcels at each site with spatial analysis by distance indices. Saplings of both tree species showed positive spatial associations with spiny shrubs in the grazed site but not in the ungrazed site, and never with non‐spiny shrubs. This result suggests that spiny shrubs may indeed provide associational resistance for saplings of key tree species in grazed habitats in these dry subtropical forests, that is, that indirect facilitation may predominate over direct facilitation. If confirmed by experimental studies, this result can have implications for the silvopastoral management of rapidly expanding ranches in the semiarid Chaco, where current practice includes the near elimination of native shrubs.     

The shrub Rhodomyrtus tomentosa acts as a nurse plant for seedlings differing in shade tolerance in degraded land of South China

Questions: What are the nurse effects of Rhodomyrtus tomentosa in degraded land of South China? Are canopy or soil factors responsible for the main nurse effect? Do facilitative effects increase with the shade tolerance of the target species? Location: Degraded shrubland in South China. Methods: Seedlings of three native climax woody species (Schima superba, Michelia macclurei, Castanopsis fissa) that differ in shade tolerance were subjected to four treatments by transplantation: (1) under the canopy of R. tomentosa shrubs; (2) in open interspaces without vegetation cover (control); (3) under the canopy of R. tomentosa from which canopies had been removed; and (4) in open interspaces without vegetation but covered by branches and leaves of R. tomentosa. Results: At low soil nutrient levels, increased canopy shade, soil porosity and soil moisture under the canopy of R. tomentosa increased seedling survival of the climax tree species S. superba, C. fissa and M. macclurei, and shoot height of S. superba. The nurse effect (a form of facilitation) of R. tomentosa depended more on canopy shade than on soil amelioration. The magnitude of facilitation or nurse effect was positively correlated with shade tolerance of the target species. Conclusions: Use of nurse plants in restoration differs from traditional reforestation (clearing and/or burning to reduce interspecific competition between target tree species and non‐target species) because it focuses on positive interactions between nurse plants and target plants that increase establishment of target species and reduce time required for restoration. Because nurse effects of R. tomentosa shrubs tended to be larger on target species with greater shade tolerance, shade‐tolerant plants are suggested as target species to accelerate restoration.     

Palm species richness,abundance and diversity in the Yucatan Peninsula,in a neotropical context

Species richness, abundance and diversity patterns in palm communities in the Yucatan Peninsula were compared at three sites with different forest types (semi‐deciduous, semi‐evergreen and evergreen), as well as different precipitation, geomorphology and soil depth. All individual palms, including seedlings, juveniles and adults, were identified and counted in forty‐five (0.25 ha) transects. A total of 46 000 individual palms belonging to 11 species from nine genera and two subfamilies were recorded. Palm richness, diversity and abundance were highest in the evergreen forest. Species from the subfamily Coryphoideae dominated the semi‐deciduous and semi‐evergreen forests while species from the subfamily Arecoideae dominated the evergreen forest. Seven species were found only in the evergreen forest. Chamaedorea seifrizii and Sabal yapa were found in all three forest types, while Thrinax radiata was found in the semi‐deciduous and semi‐ evergreen forests and Cocothrinax readii only in the semi‐evergreen forest. Compared to other neotropical palm communities, the richness and diversity in the Yucatan Peninsula are lower than in the western Amazon basin. Although palm richness and diversity on the Yucatan Peninsula were positively associated with precipitation, other variables, in particular soil depth and fertility as well as habitat heterogeneity (microtopography and canopy cover), need to be considered to better understand the observed patterns.     

Density and distance-to-adult effects of a canker disease of trees in a moist tropical forest

We compared the spatial distribution of stem cankers on the canopy tree Ocotea whitei (Lauraceae) in a 20-ha plot on Barro Colorado Island, Panama, with spatial and temporal patterns of mortality in this host over the previous decade. The cankers occur both on adult and juvenile individuals, aothough juveniles are much more likely the adults to show symptoms. Disease incidence is host-density dependent, and both the presence of the disease and host mortality are more likely close to than far from a conspecific adult, which resulted in a net spatial shift of the juvenile population away from conspecific adults through time. Disease incidence is lower than expected among juveniles of O. whitei growing near to adults of the non-susceptible canopy tree Beilschmiedia pendula. The coincidence of spatial patterns of canker incidence and host mortality suggest a role for the disease in regulating host spatial distribution, in agreement with predictions of the Janzen-Connell hypothesis.     

Composition,structure and diversity of cove forest stands in the Great Smoky Mountains: a patch dynamics perspective

Abstract. Cove forests of the Great Smoky Mountains are North American examples of old-growth temperate forest. Ecological attributes of seven stands were studied using one 0.6 - 1.0 ha plot per stand. Stand basal area (39 - 55 m²/ha) and biomass (326 - 471 Mg/ha) were high for temperate deciduous forest. Density ranged from 577 to 1075 stems/ha. All stands had a mixture of deciduous canopy species. Only rarely did a single species comprise more than half of the stand by density, basal area or biomass. Shade-intolerant species were present at low levels (1 - 5 % of total stand density). A wide range of stem diameters was characteristic of most species. However, some species lacked small stems, indicating discontinuous regeneration. Stands tended to have 10 - 20 tree species per ha and at least five species had biomass levels > 10 Mg/ha, indicating high evenness. Canopy gaps covered 10 % of the total area (2 - 21 % by stand). Gaps and conspecific patches of canopy trees > 0.05 ha in size were infrequent. Spatial analyses revealed a variety of patterns among species at inter-tree distances of 1 to 25 m. When all species were combined, juveniles showed aggregation, and adults were often hyperdispersed. Analyses for individual species confirmed that the mosaic of canopy species is influenced by non-random spatial processes. Adults of several species were aggregated at distances > 10 m. Juveniles of all major species exhibited aggregation. Several species exhibited regeneration near conspecific adults. This pattern suggested limited mobility for such species within the shifting mosaic. A diverse patchwork resulted despite the fact that many species did not exhibit segregation of adults and juveniles. Further understanding of patch dynamics and the potential for compositional steady state in cove forests requires long-term study with spatial data.     

Is the spatial pattern of a tree population in a seasonally dry tropical climate explained by density‐dependent mortality?

Spatial pattern of tropical plants is initially generated by limited seed dispersal, but the role of density‐dependent and independent mechanisms as modifiers of these patterns across ontogeny is poorly understood. We investigated whether density‐dependent mortality (DDM) and environmental heterogeneity can drive spatial pattern across the ontogeny of a tree in a seasonally dry tropical climate. We used Moran's I correlograms and spatial analysis by distance indices (SADIE) to assess the spatial patterns of the pre‐ and post‐germinative stages of Cordia oncocalyx (Boraginaceae), an abundant tree endemic in the deciduous thorny woodland in the northeastern Brazilian semiarid region. We also used RDA to analyse the effect of DDM and environmental heterogeneity (measured by microtopography and canopy openness) in the mortality and recruitment. Seeds, seedlings, juveniles and adults showed aggregated spatial patterns; infants and immatures were randomly distributed; adults, seeds and seedlings attracted each other while adult, juveniles and immatures repulsed each other. Infant and seedling mortality rates were related to DDM and the recruitment from infant to juvenile was more influenced by spatial heterogeneity. Attraction was determined by local dispersal; repulsion was related to DDM and environment heterogeneity, which allowed the return to aggregation in adult stage. Together, these results indicated that spatial pattern can change across ontogeny, in which the initial stages are responsive to DDM and the final stages are influenced by spatial heterogeneity.     

When do nurse plants stop nursing? Temporal changes in water stress levels in Austrocedrus chilensis growing within and outside shrubs

Question: Does the proximity of shrubs affect seasonal water stress of young Austrocedrus chilensis trees (a native conifer of the Austral Temperate Forest of South America) in xeric sites? Location: A. chilensis xeric forest in northwest Patagonia, Argentina. Methods: We examined the dependence of predawn twig water potential on tree development (seedling to adult) and proximity to nurse shrubs during spring and summer. We analysed spatial associations of seedlings, saplings and adult trees with nurse shrubs, and also evaluated if trees affected shrub canopy vitality. Results: Water stress in Austrocedrus trees was affected by shrub presence. Small trees (i.e.<0.5 m in height) growing in the open were most stressed, particularly in summer. Small trees growing within a shrub canopy had low water stress and little change between spring and summer. The opposite trend, however, was true for the medium‐height category (i.e. 0.5‐1.5 m in height); trees in this size category were more stressed when growing within the shrub canopy than in the open. Larger Austrocedrus trees (i.e.>2 m in height) were not affected by shrub presence. Austrocedrus trees were spatially associated with shrubs in all height classes; however, the percentage of living shrub canopy decreased with tree height. Conclusions: In xeric areas of northwest Patagonia, the strength and direction of interactions between A. chilensis and shrubs, in terms of tree water stress, are dynamic and modulated by tree size and environmental conditions. Overall, positive effects of shrubs on early developmental stages appear to be more important than subsequent negative interactions, since nursing effects could generate a spatial association of shrubs and Austrocedrus trees that persists through later successional stages. These findings shed light on mechanisms behind successional changes, and have important conservation and management implications.     

Within‐stand spatial structure and relation of boreal canopy and understorey vegetation

Question: How do spatial patterns and associations of canopy and understorey vegetation vary with spatial scale along a gradient of canopy composition in boreal mixed‐wood forests, from younger Aspen stands dominated by Populus tremuloides and P. balsamifera to older Mixed and Conifer stands dominated by Picea glauca? Do canopy evergreen conifers and broad‐leaved deciduous trees differ in their spatial relationships with understorey vegetation? Location: EMEND experimental site, Alberta, Canada. Methods: Canopy and understorey vegetation were sampled in 28 transects of 100 contiguous 0.5 m × 0.5 m quadrats in three forest stand types. Vegetation spatial patterns and relationships were analysed using wavelets. Results: Boreal mixed‐wood canopy and understorey vegetation are patchily distributed at a range of small spatial scales. The scale of canopy and understorey spatial patterns generally increased with increasing conifer presence in the canopy. Associations between canopy and understorey were highly variable among stand types, transects and spatial scales. Understorey vascular plant cover was generally positively associated with canopy deciduous tree cover and negatively associated with canopy conifer tree cover at spatial scales from 5–15 m. Understorey non‐vascular plant cover and community composition were more variable in their relationships with canopy cover, showing both positive and negative associations at a range of spatial scales. Conclusions: The spatial structure and relation of boreal mixed‐wood canopy and understorey vegetation varied with spatial scale. Differences in understorey spatial structure among stand types were consistent with a nucleation model of patch dynamics during succession in boreal mixed‐wood forests.     

Pattern and mortality in Colorado Desert plants

Summary We tested for intraspecific interference among Colorado Desert shrubs using an integrated analysis of spatial pattern and juvenile mortality. The data set included 7,000 woody perennials of 24 species in a mapped hectare of Joshua Tree National Monument, California. The site is dominated by Ambrosia dumosa (62.0% of the stems), with Larre tridentata a conspicuous secondary species (2.3% of the stems). Analyses of static pattern for common species showed: (1) aggregated adults and juveniles for Ambrosia dumosa, Erigonum fasciculatum, Mirabilis bigelovii, and Sphaeralcea ambigua, with more aggregation among juveniles than adults; (2) randomly distributed adults and juveniles for Krameria grayi, Opuntia rasmosissima, Simondsia chinensis, and Yucca schidigera. The summed volumes and distances between nearest conspecific neighbors were positively correlated for Ambrosia dumosa and Larrea tridentata, but not significantly correlated for eight remaining species with 100 individuals. Static pattern suggests only weak evidence for negative interactions in Ambrosia and Larrea, and little evidence for other species. Alternative mechanisms other than negative interaction that could give rise to these static patterns are discussed. Juvenile mortality was documented for four common species (Ambrosia dumosa, Eriogonum fasciculatum, Mirabilis bigelovii, and Sphaeralcea ambigua) that experienced substantial mortality. Analyses show: (1) the proportion of individuals that died was independent of the initial density of conspecifics; (2) distance to conspecific adults did not differ for juveniles that died versus those that survived; and (3) death was no more likely for juveniles that contacted other plants than for those that were isolated. The exception was a vine, Mirabilis bigelovii, whose juveniles survived better in contact with other plants. In sum, neither spatial pattern nor patterns of mortality showed clear evidence of negative intraspecific interference.     

Vegetation responses in Alaskan arctic tundra after 8 years of a summer warming and winter snow manipulation experiment

We used snow fences and small (1 m²) open‐topped fiberglass chambers (OTCs) to study the effects of changes in winter snow cover and summer air temperatures on arctic tundra. In 1994, two 60 m long, 2.8 m high snow fences, one in moist and the other in dry tundra, were erected at Toolik Lake, Alaska. OTCs paired with unwarmed plots, were placed along each experimental snow gradient and in control areas adjacent to the snowdrifts. After 8 years, the vegetation of the two sites, including that in control plots, had changed significantly. At both sites, the cover of shrubs, live vegetation, and litter, together with canopy height, had all increased, while lichen cover and diversity had decreased. At the moist site, bryophytes decreased in cover, while an increase in graminoids was almost entirely because of the response of the sedge Eriophorum vaginatum. These community changes were consistent with results found in studies of responses to warming and increased nutrient availability in the Arctic. However, during the time period of the experiment, summer temperature did not increase, but summer precipitation increased by 28%. The snow addition treatment affected species abundance, canopy height, and diversity, whereas the summer warming treatment had few measurable effects on vegetation. The interannual temperature fluctuation was considerably larger than the temperature increases within OTCs (<2°C), however. Snow addition also had a greater effect on microclimate by insulating vegetation from winter wind and temperature extremes, modifying winter soil temperatures, and increasing spring run‐off. Most increases in shrub cover and canopy height occurred in the medium snow‐depth zone (0.5–2 m) of the moist site, and the medium to deep snow‐depth zone (2–3 m) of the dry site. At the moist tundra site, deciduous shrubs, particularly Betula nana, increased in cover, while evergreen shrubs decreased. These differential responses were likely because of the larger production to biomass ratio in deciduous shrubs, combined with their more flexible growth response under changing environmental conditions. At the dry site, where deciduous shrubs were a minor part of the vegetation, evergreen shrubs increased in both cover and canopy height. These changes in abundance of functional groups are expected to affect most ecological processes, particularly the rate of litter decomposition, nutrient cycling, and both soil carbon and nitrogen pools. Also, changes in canopy structure, associated with increases in shrub abundance, are expected to alter the summer energy balance by increasing net radiation and evapotranspiration, thus altering soil moisture regimes.     

Seasonal and diurnal leaf movements of Rhododendron maximum L. in contrasting irradiance environments

Summary Leaf orientation (azimuth and angle) and leaf curling were measured seasonally and diurnally on Rhododendron maximum L. under an evergreen and a deciduous canopy. The microclimatic conditions under the evergreen canopy (mixed pine and hemlock) were characterized by lower irradiance but similar temperature, and vapor pressure deficit (vpd) to that under the deciduous canopy (mixed oak and maple). Under both canopies irradiance was more intense during winter months.On a seasonal basis leaf angle was closer to horizontal under the evergreen canopy but there was no difference between leaf curling in the two sites. Stomatal conductance was higher under the deciduous canopy but stomata were closed in the winter (following canopy abscission) under the evergreen and deciduous canopies even during warm winter days. Leaf water potentials were lower in the winter and Rhododendron maximum had higher leaf water potentials under the evergreen canopy.Significant association between mean leaf angle and curling index were found above a mean leaf angle of 70°. Leaf curling was highly associated with leaf temperature where 0° C was a critical value stimulating leaf curling. Leaf angle was linearly related to leaf temperatures above 0° C although this relationship was different under the two canopy types as a result of differing irradiance or differing water potential.     

SPRING GROWTH OF SHOOTS AND ROOTS IN SHRUBS OF AN ALASKAN MUSKEG

Early spring shoot and fine-root development of four evergreen and three deciduous shrub species were analyzed in a subarctic muskeg at Fairbanks, Alaska. The overwintered foliage of the evergreen shrubs regreened earlier than new leaves developed on the deciduous species. Likewise, the evergreen shrubs produced new fine roots earlier than the deciduous species. The total nonstructural carbohydrate (TNC) concentration did not decline in the evergreen shrub Ledum palustre during the spring development. This contrasted with the deciduous shrub Betula glandulosa, where a significant TNC reduction in stem tissue coincided with bud break and fine-root growth flush.     

Vegetation shift from deciduous to evergreen dwarf shrubs in response to selective herbivory offsets carbon losses: evidence from 19 years of warming and simulated herbivory in the subarctic tundra

Selective herbivory of palatable plant species provides a competitive advantage for unpalatable plant species, which often have slow growth rates and produce slowly decomposable litter. We hypothesized that through a shift in the vegetation community from palatable, deciduous dwarf shrubs to unpalatable, evergreen dwarf shrubs, selective herbivory may counteract the increased shrub abundance that is otherwise found in tundra ecosystems, in turn interacting with the responses of ecosystem carbon (C) stocks and CO₂ balance to climatic warming. We tested this hypothesis in a 19‐year field experiment with factorial treatments of warming and simulated herbivory on the dominant deciduous dwarf shrub Vaccinium myrtillus. Warming was associated with a significantly increased vegetation abundance, with the strongest effect on deciduous dwarf shrubs, resulting in greater rates of both gross ecosystem production (GEP) and ecosystem respiration (ER) as well as increased C stocks. Simulated herbivory increased the abundance of evergreen dwarf shrubs, most importantly Empetrum nigrum ssp. hermaphroditum, which led to a recent shift in the dominant vegetation from deciduous to evergreen dwarf shrubs. Simulated herbivory caused no effect on GEP and ER or the total ecosystem C stocks, indicating that the vegetation shift counteracted the herbivore‐induced C loss from the system. A larger proportion of the total ecosystem C stock was found aboveground, rather than belowground, in plots treated with simulated herbivory. We conclude that by providing a competitive advantage to unpalatable plant species with slow growth rates and long life spans, selective herbivory may promote aboveground C stocks in a warming tundra ecosystem and, through this mechanism, counteract C losses that result from plant biomass consumption.     

Disentangling facilitation and seed dispersal from environmental heterogeneity as mechanisms generating associations between savanna plants

Question: How can we disentangle facilitation and seed dispersal from environmental heterogeneity as mechanisms causing spatial associations of plant species? Location: Semi‐arid savanna in the Kimberley Thorn Bushveld, South Africa. Methods: We developed a two‐step protocol for the statistical differentiation of association‐promoting mechanisms in plants based on the Acacia erioloba–Grewia flava association. Individuals of the savanna shrub G. flava and the tree A. erioloba were mapped on four study plots. Disentangling the mechanism causing the association of G. flava and A. erioloba involved tests of three spatial and one non‐spatial null model. The spatial null models include homogeneous and heterogeneous Poisson processes for spatial randomness based on the bivariate spatial point patterns of the four plots. With the non‐spatial analysis, we determined the relationship between the canopy diameter of A. erioloba trees and presence or absence of G. flava shrubs in the tree understorey to find whether shrub presence requires a minimum tree canopy diameter. Results: We first showed a significant positive spatial association of the two species. Thereafter, the non‐spatial analysis supported an exclusion of environmental heterogeneity as the sole cause of this positive association. We found a minimum tree size under which no G. flava shrubs occurred. Conclusions: Our two‐step analysis showed that it is unlikely that heterogeneous environmental conditions caused the spatial association of A. erioloba and G. flava. Instead, this association may have been caused by seed dispersal and/or facilitation (e.g. caused by hydraulic lift and/or nitrogen fixation by the host tree).     

The small scale spatial pattern of C3 and C4 grasses depends on shrub distribution

At micro‐site scale, the spatial pattern of a plant species depends on several factors including interactions with neighbours. It has been seen that unfavourable effects generate a negative association between plants, while beneficial effects generate a positive association. In grasslands, the presence of shrubby species promotes a particular microenvironment beneath their canopy that could affect differently the spatial distribution of plants with different tolerance to abiotic conditions. We measured photosynthetic active radiation, air temperature and wind speed under shrub canopies and in adjacent open sites and analysed the spatial distribution of four grass species (two C₃ and two C₄) in relation to shrub canopy in a grazed sub‐humid natural grassland in southern Uruguay. Radiation, air temperature and wind speed were lower under shrubs than in adjacent open sites. The spatial distribution of grasses relative to the shrub canopy varied depending on the photosynthetic metabolism of grasses. C₄ grasses showed a negative association or no correlation with the shrubs, whereas C₃ grasses showed a positive association. Our results highlight the importance of the photosynthetic metabolism of the grasses in the final outcome of interactions between grasses and shrubs. Micro‐environmental conditions generated underneath shrubs create a more suitable site for the establishment of C₃ than for C₄ grasses. These results show that facilitation could be more important than previously thought in sub‐humid grasslands.     

Greater deciduous shrub abundance extends tundra peak season and increases modeled net CO2 uptake

Satellite studies of the terrestrial Arctic report increased summer greening and longer overall growing and peak seasons since the 1980s, which increases productivity and the period of carbon uptake. These trends are attributed to increasing air temperatures and reduced snow cover duration in spring and fall. Concurrently, deciduous shrubs are becoming increasingly abundant in tundra landscapes, which may also impact canopy phenology and productivity. Our aim was to determine the influence of greater deciduous shrub abundance on tundra canopy phenology and subsequent impacts on net ecosystem carbon exchange (NEE) during the growing and peak seasons in the arctic foothills region of Alaska. We compared deciduous shrub‐dominated and evergreen/graminoid‐dominated community‐level canopy phenology throughout the growing season using the normalized difference vegetation index (NDVI). We used a tundra plant‐community‐specific leaf area index (LAI) model to estimate LAI throughout the green season and a tundra‐specific NEE model to estimate the impact of greater deciduous shrub abundance and associated shifts in both leaf area and canopy phenology on tundra carbon flux. We found that deciduous shrub canopies reached the onset of peak greenness 13 days earlier and the onset of senescence 3 days earlier compared to evergreen/graminoid canopies, resulting in a 10‐day extension of the peak season. The combined effect of the longer peak season and greater leaf area of deciduous shrub canopies almost tripled the modeled net carbon uptake of deciduous shrub communities compared to evergreen/graminoid communities, while the longer peak season alone resulted in 84% greater carbon uptake in deciduous shrub communities. These results suggest that greater deciduous shrub abundance increases carbon uptake not only due to greater leaf area, but also due to an extension of the period of peak greenness, which extends the period of maximum carbon uptake.     

Frugivorous birds mediate sex‐biased facilitation in a dioecious nurse plant

Abstract. Facilitation by dispersal occurs if the nurse plant acts as a focus which is actively selected by seed dispersers and enhances the fitness of the facilitated plant. Sex‐biased facilitation may be produced if seed dispersers tend to concentrate the seeds under female, fruit‐bearing plants of dioecious species more often than under conspecific males. Juniperus sabina is a dioecious shrub with a prostrate growth form from Mediterranean high mountains that modifies many microhabitat characteristics related to seedling establishment and survival. Soil water availability, maximum soil temperature in summer, organic matter and total nitrogen content, were different on open ground as compared with beneath J. sabina shrubs, irrespective of its sex. Other studied characteristics such as soil bulk density and soil compaction after rain did not differ between the microhabitats considered. Some species, such as Juniperus communis, Pinus nigra, Helleborus foetidus and Euphorbia nicaeensis, are spatially associated to J. sabina shrubs, strongly suggesting a facilitative role. The anemochorous P. nigra and myrme‐chorous H. foetidus and E. nicaeensis did not associate preferentially to any sex of J. sabina. Only J. communis, an endozoochorous species sharing the same bird dispersers as J. sabina, presented a female‐biased spatial association with the nurse plant. Seed dispersal mediated by birds attracted by the fruit‐rewarding females of J. sabina explains the sex‐biased spatial pattern of Juniperus communis.     

Population Ecology of Hybrid Mesquite (Prosopis Species) in Western Australia: How Does it Differ from Native Range Invasions and What are the Implications for Impacts and Management?

Beneficial exotic trees and shrubs have been widely spread throughout semiarid and arid regions of the world. These trees and shrubs can however cause severe negative impacts. Mesquite (Prosopis species), native to the New World, is one example which continues to be promoted despite causing serious impacts both in its native and introduced ranges. We describe the population structure of the largest population of fire-tolerant hybrid mesquite (P. velutina × P. glandulosa var. glandulosa × P. pallida) in Australia, which was intentionally established in the 1930s. We compare it with invasive populations within its native range, and consider the implications for managing exotic mesquite invasions. We found relatively high juvenile densities at all levels of canopy cover (<30% to 90–100%), and low mortality rates for both juveniles and adults (<2%/y), which suggests that populations are still in an early phase of invasion. Exotic populations differed from native range populations in being more dense (average 4,859 adults/ha), having a sizable sapling (seedling and juvenile) bank that can remain quiescent under canopy cover (average 10,914 seedlings and juveniles/ha), failing to act as nurse plants for native shrubs (<8 native shrubs/ha), and almost totally excluding the herbaceous (grass) layer (average 0.3% cover). Our results suggest that ecosystem impacts in the introduced range are likely to be even worse, and management even more difficult, than has already been reported for invasions within its native range. The lack of feasible means for managing highly invasive, broad-scale mesquite populations need to be addressed, and needs to be considered explicitly when promoting mesquite as a beneficial plant.     

Shoot growth phenology of co-existing evergreen and deciduous species in an oak forest

Shoot growth phenology was compared for the saplings of evergreen and deciduous woody species sharing the same microsite. Growth initiation occurred earlier in evergreens (among co-stratal species) while deciduous species completed their growth earlier. Shoot growth rate was significantly greater (P<0.01) for deciduous trees than evergreen trees. The amount of shoot elongations and shoot diameter was also significantly greater (P<0.01) for deciduous trees than evergreens. On the other hand, among shrubs the amount of shoot elongation and shoot diameter was greater for evergreens but the rate of elongation and diameter was more or less similar for both. The duration of shoot elongation and shoot diameter was significantly longer in evergreens than the deciduous species. Leaf packing (number of leaves per shoot) was significantly more dense in evergreen trees (P<0.01) than in deciduous tree species. Leaf packing was more dense in evergreen than deciduous shrubs but the difference was not significant. Leaf area (per individual leaf) at full expansion was significantly greater (P<0.01) in deciduous species. Leaf dry mass and specific leaf mass in the initial stage was significantly greater for evergreen species than for deciduous species. The number of buds/10 cm of shoot was higher in evergreens. However, the per cent mortality was also higher in them.