Nurse rocks influence forest expansion over native grassland in southern Brazil

Questions: Do small rocky outcrops favor forest plant colonization and establishment in grasslands? If so, what are the potential mechanisms involved in this process? Location: Araucaria forest and Campos grassland mosaic in southern Brazilian highlands (29°29′S, 50°12′W). Methods: We collected data on the density of forest woody species in plots located on rocky outcrops and in open fields subject to different management regimes. We evaluated the influence of outcrops on the density of forest plants ≤60‐cm tall, and the effects of other environmental variables and of site on plant density; we also used information on diaspore traits of the species to discuss the way in which plants disperse to the outcrops. Results: Rocky outcrops harbored a significantly higher density of forest plants than open fields, irrespective of site effect, and rock height was the best predictor for plant density on outcrops. Diaspores of all recorded species possess characteristics associated with dispersal by birds or mammals or by both. Conclusions: Small rocky outcrops markedly influence forest expansion by acting as perches for vertebrate dispersers and as nurse objects and safe sites for plants. Forest expansion starting in small outcrops possibly occurs as follows: perching of dispersers and increase of seed rain on rocks, promotion of better conditions of establishment for forest plants by nurse rocks, protection of plants sensitive to grazing and fire, and nucleation of forest vegetation.     

Plant dispersal strategies and the colonization of Araucaria forest patches in a grassland‐forest mosaic

Questions: In a natural grassland‐forest mosaic: What is the influence of phylogeny and diaspore traits related to disperser attraction (DAT) on (1) seed size/number trade‐off (SSNT) in woody species colonizing forest patches; (2) on the frequency of the species? 3. What is the influence of forest patch area on mean seed size and number. 4. Do phylogeny and DAT expressed at the species level affect this relationship? Location: Campos grassland and Araucaria forest in São Francisco de Paula, RS, Brazil, at ca. 29°28’ S; 50° 13’ W. Methods: Forest patches of different sizes in a grassland site recovering for ten years since human disturbances were surveyed by the relative abundance of vertebrate‐dispersed woody saplings. We described colonizer species according to taxonomic phylogenetic relationships and diaspore type, size and color. We analyzed with a variation partitioning method their influence on SSNT and on species frequency in the patches. At the community level we regressed mean seed size and number on forest patch area and evaluated how these relationships were affected by phylogeny and DAT at the species level. Results: 1. Phylogeny and DAT mostly explained seed size and seed number per diaspore variation. 2. By controlling phylogeny and DAT influence the frequency of species in forest patches was positively associated with their seed number in the diaspores, and negatively associated with their seed size. 3. Mean seed size and seed number at the community level were positively associated with patch area. 4. When phylogeny and DAT influences on seed size were removed this relationship was stronger for seed size and weaker for seed number. Conclusions: 1. Energy allocation to dispersal in detriment of offspring survival increased the successful establishment of colonizer species in forest patches, despite phylogenetic relationships and DAT variation in their diaspores. 2. Although patch area exerted a selective pressure on seed size, habitat preferences of dispersers may also influence patch colonization.     

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.     

Araucaria Forest conservation: mechanisms providing resistance to invasion by exotic timber trees

Since only 12.6% of the Brazilian Araucaria Forest remains and timber tree monocultures are expanding, biological invasion is a potential threat to the conservation of natural forest remnants. Here, we test (1) the susceptibility of Araucaria Forest to invasion by Pinus taeda and Eucalyptus saligna, (2) the efficiency of different mechanisms controlling the early establishment of these two exotic timber tree species, and (3) the potential of the native timber tree Araucaria angustifolia to establish successfully in ecologically-managed monocultures of Araucaria, Pinus and Eucalyptus. In Araucaria Forest, more than a thousand Pinus seeds landed annually in a hectare; however, experimentally exposed seeds were 100% removed in only 6 days. Furthermore, all experimentally transplanted seedlings of Pinus taeda and Eucalyptus saligna died in less than a year in Araucaria Forest, but not in the monocultures. Correlative evidence suggests that this mortality was associated to plant community richness, plant abundance, and soil fertility. Araucaria angustifolia, in contrast, showed an establishment success in ecologically-managed tree monocultures as high as that exhibited in its natural habitat. The current resistance of Araucaria Forest to invasion by exotic timber trees is good news for the conservation of Araucaria Forest remnants and for its keystone species. The understanding of the mechanisms providing such resistance against invasion points towards management tools for minimizing future threats.     

Love of Nurse Plants is Not Enough for Restoring Oak Forests in a Seasonally Dry Tropical Environment

The highest concentration of oak species in the world occurs in Mexico, but human activities have strongly degraded these oak forests. Mexican oaks have high economic, social, and cultural value, and restoring these forests is of paramount importance for the people of Mexico. Here, we propose a method for restoring oak forests using native shrubs that colonize degraded areas as nurse plants for oak seedlings. To test the viability of this proposal, seedling transplant experiments were performed in a degraded area near a protected oak forest relict. Two pioneer shrubs were identified as potential nurse species: Mimosa luisana and Senecio sp. The target oak species was Quercus castanea . Oak seedlings were located beneath the canopies of both shrubs and in the surrounding area without shrub cover. Water is a limiting resource for oak establishment in seasonally dry environments; therefore, we included irrigation systems in our experimental design to determine whether the combination of nurse plants plus watering led to higher rate of survival than the presence of nurse species alone. Seedling survival without watering was less than 20% both beneath nurse species and in the surrounding habitat. When water was supplied, survival rate beneath nurse species increased up to 58% while survival in the surrounding habitat did not differ from that observed in treatments without watering. Our results indicate that survival rate of oak seedlings is increased by the presence of nurse plants only when water is supplied. This suggests that restoration of oak forests in these degraded areas requires both nurse plants and watering.     

Canopy manipulations of exotic Bitter Willow (Salix elaeagnos) forest for indigenous seedling recruitment: A pilot study

The invasive exotic tree species Bitter Willow (Salix elaeagnos; Salicaceae) has colonised areas of rank exotic grassland and has been found to contain indigenous seed, dispersed by frugivorous birds into the monospecific stands. This small pilot study examined whether indigenous seedlings that have germinated in the understorey of exotic Bitter Willow stands could be stimulated to establish through the creation of small‐scale canopy gaps. In Bitter Willow forest, four single Bitter Willow trees were poisoned to create canopy gaps. Light transmission and seedling regeneration of tree and shrub species were assessed beneath both the four manipulated and three comparable intact Bitter Willow canopies. Over 3 years, seedling height and density increased more beneath opened compared to intact Bitter Willow canopies. These results suggest that Bitter Willow can fill the roles of both a facilitative nurse and a perch tree. Larger‐scale canopy manipulation experiments of both Bitter Willow and other Salix species are needed to determine the full potential of canopy manipulations for forest restoration.     

Evaluating nurse plants for restoring native woody species to degraded subtropical woodlands

Harsh habitats dominated by invasive species are difficult to restore. Invasive grasses in arid environments slow succession toward more desired composition, yet grass removal exacerbates high light and temperature, making the use of “nurse plants” an appealing strategy. In this study of degraded subtropical woodlands dominated by alien grasses in Hawai'i, we evaluated whether individuals of two native (Dodonaea viscosa, Leptocophylla tameiameia) and one non‐native (Morella faya) woody species (1) act as natural nodes of recruitment for native woody species and (2) can be used to enhance survivorship of outplanted native woody species. To address these questions, we quantified the presence and persistence of seedlings naturally recruiting beneath adult nurse shrubs and compared survival and growth of experimentally outplanted seedlings of seven native woody species under the nurse species compared to intact and cleared alien‐grass plots. We found that the two native nurse shrubs recruit their own offspring, but do not act as establishment nodes for other species. Morella faya recruited even fewer seedlings than native shrubs. Thus, outplanting will be necessary to increase abundance and diversity of native woody species. Outplant survival was the highest under shrubs compared to away from them with few differences between nurse species. The worst habitat for native seedling survival and growth was within the unmanaged invasive grass matrix. Although the two native nurse species did not differentially affect outplant survival, D. viscosa is the most widespread and easily propagated and is thus more likely to be useful as an initial nurse species. The outplanted species showed variable responses to nurse habitats that we attribute to resource requirements resulting from their typical successional stage and nitrogen fixation capability.     

Plant Functional Types of Woody Species Related to Fire Disturbance in Forest–Grassland Ecotones

The study of plant functional types (PFTs) has been widely emphasized when analysing plant community changes in relation to variations in climate and disturbance regime. In this study, we search for PFTs of woody species near forest–grassland boundaries in South Brazil where, due to climate, forests tend to expand over grassland but are being restricted by frequent fires. We aimed at answering the questions: (i) which plant functional types of forest woody species can establish in adjacent grassland subject to fire disturbance and (ii) which plant functional types of forest and grassland woody species are related to short-term community dynamics in frequently burned grassland. Traits were assessed in woody plants in 156 plots (6.75 m²) arranged in 12 transects across forest–grassland boundaries with different fire history in their grassland part. The analysis used a recursive algorithm to search for traits and PFTs maximally associated to spatial distance from forest limit in one analysis, and elapsed time since last fire in another. As a result, nine PFTs of forest woody species were identified that best described community patterns associated to distance from forest. Resprouting ability characterized forest plants able to colonize grasslands. PFT diversity was higher in border plots than inside forest or grassland. Four PFTs of forest and grassland woody species best described woody species community patterns in the grassland associated to elapsed time since fire. Taller individuals of single-stemmed shrubs predominated in late post-fire recovery (3–4 years), while shorter multi-stemmed shrubs in recently burned areas (3 months to 1 year). PFTs of forest trees occurred in border plots or, as established adults, in grassland, remaining unaffected by fire. We conclude that easily measurable structural plant traits, such as those used in our study, are sufficient to evaluate post-fire community dynamics. Forest PFTs in burned grassland are restricted to those with resprouting ability to survive recurrent fire events. Establishment success is highest on protected sites with lesser or low-intensity fire.     

Facilitation by herbivore-mediated nurse plants in a threatened tree, Taxus baccata: local effects and landscape level consistency

We investigated the regeneration of a threatened tree, the yew Taxus baccata, in relation to the presence of fleshy‐fruited woody plants acting as seed dispersal foci as well as protecting yew recruits against ungulate herbivores. We seek to determine if local facilitative effects are consistent across landscape in the Cantabrian range (NW Spain). Yew seed rain by birds mostly concentrated under yew trees and beneath hollies Ilex aquifolium. Seedling emergence distributed similarly to seed rain, but first‐year seedling survival was higher beneath hollies. In one site where woody vegetation was structured as nucleation centres (multispecific patches of fleshy‐fruited plants acting as foci for seed rain) yew recruits mostly occurred in yew‐dominated centres, suggesting dispersers‐mediated facilitation. However, holly was the main nurse plant for most of these recruits, considering the nurse as the species whose canopy covered directly the yew recruit. Living beneath nurse plants reduced herbivore damage on saplings and enhanced seedling survival. A planting experiment with yew rooted‐cuttings beneath different spiny shrubs corroborated this effect. Additional evidence on yew recruitment limitation by herbivory emerged from one population where ungulates were fence‐excluded. Our results suggest that nurse plants mitigate the negative effect of herbivores on yew regeneration, by providing defence against browsing and trampling. Shelter ability related to nurse structure, cone‐shaped shrubs with branches at their bases acting better as a barrier. Paradoxically, this structure resulted from heavy browsing on nurse plants. The study of yew regeneration and habitat structure in seven sites provided evidence for the consistency of facilitation by holly at the landscape level, since local values of yew recruitment positively related to nurse ground cover. Range‐scale yew management must consider the local functioning of the interaction among avian seed‐dispersers, nurse fleshy‐fruited plants and ungulate herbivores, in combination with regional measures, targeting the habitats where facilitation emerges.     

Moms are better nurses than dads: gender biased self‐facilitation in a dioecious Juniperus tree

Questions: Can gender of nurse plants affect regeneration patterns and spatial population structure? Is there a seed‐seedling conflict in the regeneration process? What factors are responsible for the clumped spatial population structure observed for adult trees? Location: Mediterranean cold semi‐arid high mountains in Spain. Methods: The spatial pattern of adult Juniperus thurifera trees was studied by means of Ripley's K‐analysis. χ² analyses were used to test for natural seedling frequency in each of three main microhabitats: (1) under female and (2) male tree canopies and (3) in open interspaces. The observed pattern was explained experimentally by studying seed and seedling survival for two years. Survival probabilities were calculated across life stages for each of three main microhabitats. Results: Adult J. thurifera trees were aggregated in space. Most seedlings were found underneath female J. thurifera trees. Experimental studies demonstrated that from seed dispersal to seedling survival all life stages showed the same positive or negative trend within a given microhabitat, indicating stage coupling and no seed‐seedling conflicts. Attraction of frugivo‐rous birds by reproductive female junipers and improvement of environmental conditions beneath tree canopies were the main factors responsible for the variation in seedling density among microhabitats; highest underneath female trees and lowest in open interspaces. Conclusions: In dioecious species, the gender of nurse plants can significantly determine the spatial population structure. In J. thurifera forests, facilitation beneath female trees occurs among all life stages without any sign of seed‐seedling conflict. The most critical factors shaping the spatial population structure were directed seed dispersal and environmental amelioration beneath female conspecific trees.     

Ants as diaspore removers of non-myrmecochorous plants: a meta-analysis

Diaspore (e.g. seeds, fruits) dispersal is pivotal for plant communities and often involves several steps and different dispersing agents. Most studies focusing on diaspore dispersal by animals have highlighted the role of vertebrates, neglecting the role of ants in the diaspore dispersal of non-myrmecochorous plants. Diaspore dispersal by ants is especially relevant in the current scenario of declining of vertebrate populations and, consequently, collapse of the dispersal system of large-seeded plants. Although ants can never compensate for the dispersal service provided by vertebrates, they can mitigate the impact of vertebrate decline via removal of diaspores deposited on the ground. We have used a meta-analytical approach to investigate the contribution of ants in the removal of non-myrmecochorous diaspores (through vertebrate exclusion experiments). We considered the number of diaspore removal as effect size and factors such as plant growth forms, diaspore and ant size, habitat type as moderators. In addition, we investigated the role of such factors on the diaspore removal distance by ants. Ants played complementary role to non-myrmecochorous diaspore removal services provided by vertebrates (mean Hedges’ g of −0.30). The ant diaspore removal was 69% higher for diaspores from shrubs than that of tree diaspores and removal of small-sized diaspores were 69% and 70% higher in comparison to medium- and large-sized diaspores, respectively. Regarding the diaspore removal distance by ants, those of tree species were removed 32% farther than those of shrub species, and diaspores were removed three- times farther in the savanna than in rainforest ecosystems. Our results highlight the shrubs and small-sized diaspores. Regarding the diaspore removal distance, the ants can be crucial for the dispersal of tree diaspores and in the savanna ecosystems. Finally, considering the biodiversity crisis, the ants may play an even more important role than appreciated in diaspores dispersal.     

Shrub emergence and seedling growth in microenvironments created by Prosopis glandulosa

Abstract. Facilitation of the establishment of certain plant species by nurse plants is a common phenomenon in arid and semiarid ecosystems. The most commonly reported mechanisms of facilitation include cooler temperatures and increased soil nutrients beneath the nurse plant canopy, which favor establishment of other plant species. During conversion of upland grasslands to thorn woodland in southern Texas, Prosopis glandulosa appears to facilitate establishment of other woody plants, including Celtis pallida, whereas Acacia smallii occurs only in habitats between P. glandulosa canopies. We tested the hypothesis that light intensity and soils under P. glandulosa canopies facilitate seedling emergence and growth of C. pallida but inhibit seedling emergence and growth of A. smallii. In the field, C. pallida and A. smallii seeds were planted under P. glandulosa canopies and in adjacent interspaces. Percent emergence of C. pallida seedlings was greater under the canopy of P. glandulosa, whereas percent emergence of A. smallii seedlings was greater in interspaces. In a greenhouse experiment, seeds of each species were planted in pots filled with soil from under P. glandulosa canopies or from adjacent interspaces. Two treatments, shade and sunlight, were imposed and plants harvested seven weeks later. Seedling mass of both species was greater in canopy soil than in interspace soil in sunlight but mass of the two species did not differ between soil sources in shade. Canopy soils contained more total and available nitrogen than interspace soils. These results suggest that light is more limiting than nutrients under shaded conditions and so neither species can take advantage of the high nutrients beneath P. glandulosa. Shade and greater soil nutrients beneath P. glandulosa do not appear to be the major factors that facilitate C. pallida or inhibit A. smallii. Aggregation of C. pallida beneath P. glandulosa canopies appears to be a complex process that involves both passive facilitation (seed dispersal by birds) and active facilitation (reduction of seed dormancy by under-canopy temperatures) operating only during the seed germination stage with successional mechanisms other than facilitation operating during later stages of shrub establishment and growth.     

Post‐Establishment Seedling Success in the Brazilian Cerrado: A Comparison of Savanna and Forest Species1

Due to frequent fire, low nutrient availability, and prolonged drought, tropical savanna is a stressful environment for the survival and growth of woody plant seedlings. To understand why forest species do not succeed in this environment while savanna species are able to persist, the effects of fire and woody cover on seedlings of these two functional groups were investigated in the Brazilian Cerrado. Seedlings were established in experimental plots under three densities of woody cover, in sites protected from fire and sites to be subjected to fire. There was a clear difference in the ability of savanna and forest species to survive fire. None of the three forest species were able to survive fire during the first two years of life, whereas eight of the nine savanna species were able to resprout following fire. The small seed size of the ninth savanna species, Miconia albicans, predisposed its seedlings to be sensitive to fire, because there was a strong positive correlation between seed size and survivorship. Savanna species were less dependent on woody cover than were forest species, which exhibited higher growth and survival under tree canopies than in open grassland. The low rates of establishment and survival of forest trees in savanna, combined with high sensitivity to fire, appear sufficient to prevent the expansion of forest into savanna under current fire regimes in the Cerrado.     

Patterns of Seed Dispersal and Dispersal Failure in a Hawaiian Dry Forest Having Only Introduced Birds

Dry forests are among the most endangered natural communities in the Hawaiian Islands. Most have been reduced to isolated trees and small forest fragments in which native tree species reproduce poorly. The replacement of native birds by introduced generalists may be contributing to dry forest decline through modification of seed dispersal patterns. To document seed dispersal by introduced birds, we conducted foraging observations on fleshy-fruited trees and measured seed rain under trees and in adjacent open areas for 1 year in a dry forest dominated by native trees. Although trees covered only 15.2 percent of the study area, 96.9 percent of the bird-dispersed seeds were deposited beneath them. The Japanese white-eye (Zosterops japonicus) was the principal dispersal agent. Among bird-dispersed seeds, those of the invasive tree Bocconia frutescens accounted for 75 percent of all seeds collected beneath trees (14.8 seeds/m²/yr) and the invasive shrub Lantana camara accounted for 17 percent. Although nearly 60 percent of the reserve's native woody species possess fleshy fruits, introduced birds rarely disperse their seeds. Native trees accounted for <8 percent of all bird-dispersed seeds and are consequently experiencing dispersal failure by falling directly under parent trees. Smaller-seeded non-native plants, in contrast, may be benefiting from dispersal by introduced birds. Current dispersal patterns suggest that these readily disseminated non-native plants may eventually replace the remaining native flora.     

Dispersal traits and dispersal patterns in an oro-Mediterranean thorn cushion plant formation of the eastern High Atlas, Morocco

This study describes the dispersal traits and dispersal patterns of 51 perennial plant species belonging to 19 families in an oro-Mediterranean thorn cushion plant formation on the High Atlas, Morocco. Diaspore type, mass, number, antitelechoric mechanisms and dispersal time were studied with respect to growth forms, dispersal modes and spatial dispersal. Species spanned 10⁵ range of diaspore mass, which coincided with those found in other high mountain regions. Diaspore mass was significantly higher in trees and shrubs than in semi-shrubs and perennial herbs. Barochorous and zoochorous species are more likely to have heavy diaspores, whereas anemochorous and ballistic species have a medium diaspore mass and semachorous and ombro-hydrochorous species have low diaspore mass. Diaspore number was significantly higher in trees and shrubs than in semi-shrubs and perennial herbs. The barochorous, ombro-hydrochorous and zoochorous species tended to produce higher diaspore numbers than species with other dispersal modes. Bradyspory was well-developed by trees and large shrubs dispersed by biotic vectors. Synaptospermy was represented by its long dispersal component. Myxospermy was significantly associated with semi-shrubs and perennial herbs with restricted spatial dispersal. It seems that ombro-hydrochory combined with myxospermy and a high number of light seeds is an efficient mechanism that ensures successful establishment of the most typical and endemic thorn cushion plant species, such as Alyssum, Vella and Ormenis. In our study area, the highest dispersal availability was synchronized with the dry summer season (July–August) and the beginning of the rainy months (September). The dispersal peak for the wind-dispersed species, which is the most effective primary dispersal mode, occurs during the dry season, while dispersal for the biotic-dispersed species takes place throughout the year.     

Testing for the influence of niche and neutral factors on sapling community assembly beneath isolated woody plants in grasslands

Questions: Perching and nursing effects drive initial steps of forest expansion over grasslands. Nursing effect is obviously related to niche mechanisms, while perching effect is likely to result both from neutral and niche factors. This study assessed the effect of neutral and niche factors on species composition in sapling communities developing beneath isolated trees/shrubs (ITS) in grassland. Location: A mosaic of Campos grassland and Araucaria forest in São Francisco de Paula, southern Brazil (29°28′S, 50°13′W). Methods: We described sapling communities beneath 32 ITS using mean number of forest woody saplings of different species. We performed a stepwise canonical correspondence analysis (CCA) to select ITS traits that maximized the association with species composition. Then we evaluated the contribution of distance from seed source, ITS traits and distance‐structured ITS traits on sapling community assembly using a variation partitioning method based on CCA and partial CCA. Results: Sapling species composition was significantly explained by ITS traits (ITS dispersal mode, ITS growth form, crown area:ITS height ratio, crown area, ITS height and crown area:volume ratio). Distance from seed sources explained only a minor, non‐significant fraction of sapling species composition. Distance‐structured trait variation was negligible. Conclusions: Sapling community assembly beneath ITS was mostly explained by niche factors related to both nursing and perching effects. Dispersal limitation explained only a small fraction of variation in species composition beneath ITS, suggesting that neutral‐based perching effect had a minor role in community assembly.     

Soil type, microsite, and herbivory influence growth and survival of Schinus molle (Peruvian pepper tree) invading semi-arid African savanna

Naturalization of Schinus molle (Anacardiaceae) has been observed in semi arid savanna of the Northern Cape Province of South Africa. However, with high dispersal ability, the species is expected to achieve greater densities and invade more widely. The study involved a field manipulation experiment over 14 months using a factorial block design to examine transplanted seedlings in different savanna environments. The experiments examine the effects of soil type (sandy and clay), microsite, and herbivores on seedling performance (establishment, growth and survival). Seedlings were grown in a greenhouse and individually transplanted into four treatment groups: in open grassland, under tree canopies, and with and without cages to exclude large herbivores (cattle and game). The same experiment was repeated in two different soil types: coarse sand and fine-textured clay soil. Results suggest that protection provided by canopies of large indigenous Acacia trees facilitates S. molle invasion into semi-arid savanna. In the field, S. molle seedlings performed considerably better beneath canopies of indigenous Acacia trees than in open areas regardless of soil type. Whether exposed or protected from large herbivores, no seedlings planted in open grassland survived the first winter. Although, seedlings grew better and had higher survival rates beneath tree canopies than in the open sites, exposure to large herbivores significantly decreased heights and canopy areas of seedlings compared with those protected from large herbivores. The effect was greater on clay soil than on sandy soil. The results suggest that low temperature (frost), and possibly inter-specific competition with grasses, may limit S. molle seedling establishment, survival and growth away from tree canopies in semi arid savannas. Low soil nutrient status and browsing may also delay growth and development of this species. The invasive potential of S. molle is thus greatest on fertile soils where sub-canopy microsites are present and browsing mammals are absent.     

Facilitation by nurse shrubs of resprouting behavior in a post-fire shrubland in northern Patagonia,Argentina

Abstract. The nurse-plant syndrome is a widely recognized example of positive (facilitative) influences of plant species on the establishment and growth of other species. Most studies of the nurse-plant syndrome have been on species that reproduce mainly from seed rather than vegetatively. In this study, we experimentally compared the influences of two species of nurse shrubs, Schinus patagonicus and Berberis buxifolia, on the survival and growth of vegetatively reproducing herbaceous and woody plants in a post-fire shrubland in northern Patagonia, Argentina. The vegetation beneath shrubs was removed by clipping and, in a paired-sample design, one half of the canopy of each shrub was removed. We determined species richness, counted number of resprouts, and measured photon flux density and soil moisture beneath cut and uncut halves of each shrubs. Abundances of resprouts were several times greater beneath the uncut vs. the cut shrubs, as was the mean number of species. Thus, shrubs have a strong facilitating influence as measured by resprout densities and the number of species. Numbers of resprouts and of species were twice as high beneath Schinus as beneath Berberis, implying important differences in the facilitative effects of the two shrubs species. Microsites beneath Schinus were characterized by lower and more heterogeneous light levels but by greater soil moisture. Even though the reproductive mode in this post-fire shrubland is overwhelmingly vegetative, facilitation by nurse shrubs is important and differentially effective for different species of nurse shrubs.     

Seed rain into upland plant communities in Hong Kong, China

Hong Kong is an extreme example of tropical landscape degradation, with no substantial remnants of the original forest cover and a highly impoverished disperser fauna. Seed availability is a potential limiting factor in vegetation recovery in such landscapes. To assess the quantity and quality of the seed rain of woody taxa, seed traps were placed in the major upland vegetation types: fire-maintained grassland, shrubland, and secondary forest. Within the grassland site, traps were placed under isolated trees, isolated male and female shrubs of Eurya chinensis, and in the open. Seeds were collected every 2 weeks for 2 years. The seed rain was highest under female shrubs in grassland (6455 seeds m⁻² year⁻¹), where it was almost entirely confined to their fruiting period. Next highest were isolated trees (890 seeds), followed by male isolated shrubs (611 seeds), shrubland (558 seeds), forest (129 seeds) and open grassland (47 seeds). The number of seed taxa was highest in shrubland (59), followed by isolated trees (42), forest (42), female isolated shrubs (28), male isolated shrubs (15), and open grassland (9). The seed rain differed in species composition between the forest, shrubland, and grassland sites, while the differences within the grassland site were largely in terms of quantity. Birds (particularly bulbuls, Pycnonotus spp.) are known or inferred to be the major dispersal agents for 85% of the seed taxa trapped, 99% of the total number of seeds trapped, and 99.8% of the seeds trapped in the grassland site. Few taxa and of the total seeds were dispersed by wind and no seed taxa were definitely dispersed by fruit bats. The results suggest that even in the most degraded landscape the seed rain is adequate for the development of woody vegetation cover, but that human intervention will be needed for the restoration of plant diversity.     

Spatial relations between shrubs and Prosopis glandulosa canopies

Abstract. Prosopis glandulosa, an arborescent legume, may act as a nurse plant that facilitates the establishment of other woody species. We hypothesized that attenuation of radiant energy and increased soil nutrients beneath P. glandulosa canopies facilitate establishment of subordinate shrubs and shrub cluster development. We determined the spatial distribution pattern of shrubs under P. glandulosa at three locations in southern Texas. Density of Celtis pallida, Zanthoxylum fagara, and total woody plants were comparable among the four cardinal directions at each location, which countered the prediction that shrub density would be greater on the north side of P. glandulosa canopies if attenuation of solar energy was a factor in cluster development. Total woody plant density increased with increasing P. glandulosa basal diameter, canopy radius, and height only at one location. Total woody plant density decreased with increasing total N in the upper 15 cm of soil at two of the three locations. Late in shrub cluster development, extraction of N from the soil and incorporation of N into plant tissue in dense shrub clusters may operate to inhibit further increases in subordinate shrub density.