Recruitment Of Woody Species Research Articles

Maintaining an open landscape: Comparison of management methods for semi-natural grasslands: A Swedish multi-site study

Traditional management of semi-natural grasslands by grazing or annual mowing is not always feasible and the current study set out to evaluate alternative management methods. At eleven locations in southern Sweden, block experiments were conducted evaluating seven treatments in 5 m*20 m plots: grazing, annual mowing, annual spring burning, mowing every third year, mechanical removal of woody plants, herbicide control of woody plants, and untreated control. After approximately 13 years, trends for woody plants and species richness, and the occurrence of management-dependent plant species, low-grown species, and pollinator-attracting plant species were analysed. Overall, the annual mowing and grazing treatments resulted in fewer woody plants, the highest species richness, and more management-dependent, low-grown, and pollinator-attracting species. The untreated control plots showed the opposite effect, whereas less intense management (annual burning, mowing every third year, and mechanical and chemical treatments of woody plants) showed mixed and often intermediate effects. Compared to grazing and mowing, less intense management methods do not constitute long-term management alternatives to preserve typical features of species-rich grasslands. However, they may be short-term alternatives at sites where the recruitment of woody species is absent or rare. However, to prevent the encroachment of woody species, less intensive management might be a better long-term alternative at more productive sites with slow or inhibited woody species recruitment.

Lack of woody species recruitment in isolated deep canyon deciduous woodlands in central Texas, USA1

Lack of woody species recruitment in isolated deep canyon deciduous woodlands in central Texas, USA1

Contribution of species and functional richness to carbon storage in eucalypt woodland restoration

Biologically diverse forest and woodland restoration can mitigate climate change and biodiversity loss. Understanding the trade-off between carbon storage and biodiversity is important to maximise the value of restoration opportunities. Ecological theory suggests biological diversity contributes positively to carbon storage until the relationship plateaus (i.e., species are added with no effect on carbon storage), and yet in practice, tree monocultures are perceived to sequester more carbon than diverse plantings. Here, we measured the contribution of plant diversity to carbon storage in trees and shrubs ten years after planting for yate (Eucalyptus occidentalis) woodland restoration. We found no evidence for a trade-off between carbon storage and plant diversity. We planted yate with mixes of yate woodland species including native grasses, shrubs, and trees. Ten years after planting, grass establishment was poor, and density of trees and shrubs was patchy. There was some unassisted recruitment of woody species. Diversity was measured as observed species richness and observed functional richness, which incorporated taxonomic diversity, structural diversity, and presence/absence of nitrogen-fixing legumes. Carbon stocks measured were above and belowground tree and shrub biomass, leaf and twig litter biomass, and organic carbon (%) in the topsoil. Across all three carbon stocks diverse yate plantings sequestered as much carbon as yate monocultures. Observed species richness had a weak negative effect on tree and shrub biomass in the best linear mixed model but this effect diminished with the removal of an outlier. Tree density had a positive effect on tree and shrub biomass in models with and without the outlier. There was no effect of diversity on litter biomass and soil organic carbon. Instead, linear mixed models of these stocks showed the influence of soil properties. Litter biomass increased with soil potassium and decreased with soil pH. Soil organic carbon increased with initial soil organic carbon and soil nitrogen. There are potential additional biodiversity benefits of diverse plantings that we did not measure (e.g., soil erosion control, wildlife habitat). We call for more field experiments to explore these potential benefits and to generate the ecological knowledge needed to improve the quality of global forest and woodland restoration efforts.

Rehabilitación ecológica en el noreste patagónico: Supervivencia y reclutamiento de especies nativas en suelos con diferentes texturas

In the northeastern Patagonian region, soils show marked heterogeneity, mainly due to their texture. It is also observed strong degradation processes in this region. The objective was to estimate survival rate of native species in ecological rehabilitation trials in degraded areas, on soils of different textures, and evaluate recruitment around surviving transplanted individuals at each site. Studies were performed in three sites that had a similar land use history, in loamy, sandy loam and sandy soils. At each site, three 100 m2 plots, excluded from cattle grazing were installed. Individuals of woody (Condalia microphylla, Geoffroea decorticans, Prosopis flexuosa y Schinus johnstonii) and herbaceous (Nassella longiglumis, Nassella tenuis, Pappophorum caespitosum and Piptochaetium napostaense), native species were transplanted. Recruitment was estimated in 50x50 cm plots, leaving the surviving transplanted individual in the center. Survival rate and recruitment were estimated four years after transplantation. Recruitment was categorized into forage and non-forage woody plants and annual or perennial, forage, and non-forage herbaceous plants. Soil texture seems did not influence in survival of herbaceous transplanted species, but it did in a woody plant (S. johnstonii), that was significantly higher in sandy loam soils. Richness of recruited woody and herbaceous species was similar between sites, except for annual non-forage herbaceous plants, being richer in loam and sandy loam sites. Transplanted woody species recruitment was not detected, but recruitment of other native woody species was recorded, and the recruitment of perennial herbaceous species was only of the transplanted species. Recruitment density did not show correspondence with soil texture, except for forage and non-forage annual herbaceous species, that was higher at the loam and sandy loam sites, respectively. Most of the selected species presented survival rates that make them suitable for ecological rehabilitation projects. Seed supply of each site would have a preponderant importance to recruitment recovery.

Forest restoration by different nucleation techniques in Urochloa grassland

Abstract: The objective of this work was to evaluate the effect of brushwood, black plastic mulch, herbicide, and artificial perch on the natural regeneration of native species in Urochloa grassland. The experiment was conducted between February 2014 and February 2016 in the Dense Ombrophilous Forest, in the municipality of Morretes, in the state of Paraná, Brazil. The treatments were: herbicide, herbicide + perch, black plastic mulch, black plastic mulch + perch, brushwood + herbicide, brushwood + herbicide + perch, and a control treatment. The evaluations were carried out at 4, 8, 12, 18, and 24 months after the installation of the experiment, by counting and identifying regenerating woody species and estimating visually the percentage of herbaceous coverage. Initially, brushwood and black plastic mulch reduced the Urochloa grasses; however, this effect was lost over time due to the rapid growth of the grasses from the edges to the center of the plots. The use of perches in the treatments does not allow a significant increase of other species because of the continued inhibiting conditions for the establishment of seedlings. The herbicide is effective in removing the grasses; however, the recruitment of woody species is only satisfactory when perches are used to attract the dispersing fauna. For a successful ecological restoration of pastures, there is a need for the local elimination of Urochloa grasses.

Biomass Dynamics in a Fragment of Brazilian Tropical Forest (Caatinga) over Consecutive Dry Years

Increases in water scarcity due to climate change, especially in dry regions, can affect the dynamics of successional species. In view of the longest sequence of dry years (2010–2019) to have occurred in the Brazilian semi-arid region, with a consequent reduction in water availability, the influence of rainfall distribution on the production of above-ground plant biomass was investigated in a Dry Tropical Forest (DTF). This natural change monitoring experiment was conducted over 11 years (2009–2019) in a fragment of DTF under regeneration for 40 years, in the district of Iguatu, Ceará, Brazil. All living individuals of the woody component with a Diameter at Ground Level (DGL) ≥3 cm and a height (h) ≥100 cm were measured during 2009–2010, 2015–2016, 2018–2019. Biomass production was calculated using an allometric equation defined for DTF species. A mean mortality rate of 134 ind. ha−1 yr−1 was registered, with a recruitment of 39 ind. ha−1 yr−1, generating a mean deficit of 95 ind. ha−1 yr−1. The mean reduction in biomass was 3.26 Mg ha−1 yr−1. Climate conditions during consecutive dry years have a direct effect on the mortality and recruitment of woody species, with a recruitment/mortality ratio of 0.11. Shrubby-tree individuals of smaller diameter showed less resilience to the cumulative effect of drought.

Species-specific effects of the herbaceous layer on recruitment of woody species under different shading and precipitation conditions

Herbs may affect the recruitment of woody species. However, most studies have evaluated the impact of one particular herb species or the herbaceous layer as a whole. It is less known if different herb species have different effects on recruitment of woody species, and whether these effects depend on environmental conditions. In this paper, we evaluated the effects of different native and exotic herb species on recruitment of nine woody species in central Chile, under different conditions of shading and precipitation. The experiment was carried out in an open ruderal area in Santiago, Chile. We conducted a factorial experiment with 360 1 × 1 m pots with monospecific crops of eight herb species and a control without herbs, two levels of irrigation (245 and 534 mm) simulating two precipitation regimes, and the presence or absence of artificial shade simulating a woody canopy. Nine woody species common to the region were sown in each pot. Luminosity (PAR), soil moisture, herb volume, richness of woody species and seedling density of woody species recruited were measured. The effects of herb species on woody species richness varied from neutral in the open condition to mainly negative in the shaded condition. Differences in irrigation did not modify the effects of herbs on species richness. The effects of herb species were more variable when woody species were considered individually. Negative and neutral effects predominated, although some positive effects were observed under shaded and wetter conditions. Our results suggest that different herb species have distinct effects on recruitment of specific woody species and species richness, which mainly depend on shade conditions and secondarily on the precipitation regime, and that herb-woody species interactions are highly specific. Thus, management focused in herb controlling should be species-specific, in terms of herb and woody species, and dependent on climate and woody cover.

Reclutamiento de especies leñosas en bosques tropicales expuestos a incendios: una revisión

Ocampo-Zuleta, K., Bravo, S. 2019. Recruitment of woody species in tropical forests exposed to wildlandfires: an overview. Ecosistemas 28(1):106-117. Doi.: 10.7818/ECOS.1642 The wildfires have been perceived during the first decades of the last century as an event with negative consequences for the environment. This perception has changed since then, with the advance of studies of fire ecology in different ecosystems of the planet and the consideration of the various parameters that define the fire regime (frequency, intensity, severity, seasonality and extension). Fire is a key ecological disturbance to the natural dynamics of fire-prone ecosystems, where species have adaptations and/or strategies to resist, tolerate and/or persist in their natural fire regime. Persistence at a population level that depends on the ability to establish new individuals from seed banks (recruitment). The objective of this review was to analyze the antecedents in relation to the recruitment of woody species from tropical forests, in terms of environmental changes in the postfire environment and seminal traits that could influence the germination and establishment of seedlings in the postfire environment. The efficiency in the recruitment varies according to the intensity of the fire, the tolerance of the seeds to the thermal shock and the aptitude to germinate and establish seedlings in the postfire environment. Factors such as seed production and dispersion, and predation are key to the success of recruitment in these environments. This review will help with the planning and monitoring of ecological restoration tasks in areas of tropical forests affected by fires of different origins.

Environmental constraints to native woody species recruitment in invaded mountain woodlands of central Argentina

Nowadays forests, which were formerly considered resistant to invasion due to its shady understories, are been transformed to invaded landscapes. Thus, the aim of this study was to evaluate the regeneration of native woody species in Ligustrum lucidum invaded woodlands. In addition, we aim to identify the main environmental differences between the native and invaded woodlands and evaluate the effect of those conditions in the regeneration phase of dominant native woody species. Finally, we analyze how is the emergence and survival of the exotic species in relation to that exhibited by native species in both environmental situations. For this, we performed a seed addition experiment placing three native woody species (Celtis ehrenbergiana, Lithraea molleoides, Vachellia caven) and the dominant exotic (L. lucidum) in native and invaded woodlands of the Chaco mountains, central Argentina. Also, we performed a laboratory experiment simulating the differences in light recorded at the field (main abiotic difference between both woodland types). Our field experiment indicated that, if seeds are actively added, the percentage of seedlings emergence of native species could be similar in invaded and native woodlands. However, we observed a trend of lower seedling survival after winter within the exotic monospecific stands, which was significant only for the native species C. ehrenbergiana. We suggest that this constraint could be partly related to the shadier conditions imposed by L. lucidum, since this is the major environmental change within the invaded woodlands in comparison with native ones. Consistently, the experimental simulation showed that the increased in shade reduced the overall growth of seedlings. As was mostly observed in temperate biome, our study in a subtropical seasonally dry region indicates that invaded areas may not affect the emergence stage but limit the survival of seedlings. Among the native species, C. ehrenbergiana seems to be the most suitable species to start restoration, showing the highest performance throughout the year. Combined actions of seed addition and canopy opening might counteract the abiotic constraint and contribute to restoration success of this invaded landscape.

Seasonal patterns of gas exchange and water relations in juveniles of two Neotropical savanna tree species differing in leaf phenology

Ecophysiological studies on Neotropical savanna trees have shown that there is a resistance gradient of seasonal water stress in deciduous and evergreen species. Some species perform like drought avoiders, characterized by high transpiration rates during the wet season that can be sustained by evergreen deep rooted species during the dry season. Nevertheless, water deficit in superficial soil layers, fire damages and nutritional requirements could differentially affect juveniles of both phenological types determining their spatial patterns of distribution (evergreen species sparse in open grasslands and deciduous species gathered in groves). This study examined seasonal variations in water relations and photosynthesis in juveniles of two tree species differing in leaf phenology: Curatella americana (evergreen) and Cochlospermum vitifolium (deciduous). We wanted to test the hypothesis that the improved ability of evergreen deep rooted species like C. americana to colonize open savannas is related to drought tolerance and a conservative gas exchange pattern in juveniles that photosynthesize even during the dry season, while one might expect a water spender and avoider character in propagules of deciduous species like C. vitifolium that are active mostly during the rainy season. The studied species showed high transpiration rates during the rainy period, indicating little water availability limitations on surface soil layers. Even so, both species exhibited a distinctive decrease in stomatal conductance towards the end of the rainy season, an indication of water availability restrictions and/or a response to higher air evaporative demands in the shallow rooted juveniles. At the peak of the dry season, C. americana presented comparable transpiration rates with wet season measurements, indicating that the studied juveniles had reached more permanent (or stable) water sources at soil depths of 30 cm (approximate maximum root lengths measured). Yet, C. americana showed a significant decrease in water potentials from wet to dry seasons, and osmotic adjustment as a drought tolerance mechanism. Overall, the evergreen functional type (juvenile plants of C. americana) showed tolerance to water deficit, maintaining functional leaves in the dry season. On the other hand, in absence of a water deficit, juveniles of C. vitifolium showed higher photosynthetic rates that could be translated into high growth rates in wet habitats. However, the low water deficit tolerance of the juveniles of C. vitifolium seems to be the main handicap for its colonization potential of the open savanna environment. In conclusion, as predicted, the greater tolerance to water deficit of the evergreen studied species suggests that resistance to water stress determines the observed distribution pattern of tree species in the study area, where only the evergreen ones colonize the open savanna, acting in its adult stages as a focus for the recruitment of woody species.

Natural or assisted succession as approach of forest recovery on abandoned lands with different land use history in the Andes of Southern Ecuador

Forest recovery on disturbed areas is of special significance in the Ecuadorian Andes, where deforestation is a serious problem. Natural diachronic succession was evaluated on three large plots or sites, differing in their land use and vegetation composition, one is dominated by grass species on an abandoned pasture (Pasture site), the other two are post-fire vegetation dominated by bracken (Bracken site) and various shrubs (Shrub site). Additionally, we assessed the effectiveness of manual removal of competitive herbaceous species to accelerate forest recovery. Monitoring was done in 2003, 2005 and 2007 on 48 subplots of 116 m2 each recording species richness and woody-species density. Results showed that the Pasture site demonstrated a competitive effect of exotic grasses on woody species recruitment with much lower species recruitment and density, suggesting serious inhibition of natural forest regeneration and an unclear successional trajectory. The Bracken and Shrub sites became significantly similar floristically and there is evidence for a marked facilitation of woody plant recruitment correlated with light availability on the ground. Both sites showed characteristics of classic succession, with Shrub showing a higher species richness and density while late-successional species are poorly represented on the Bracken site. However, NMDS ordination using species density showed that the two trajectories may not be converging towards a common “final state”. Manual weeding was ineffective for accelerating forest recovery. These results suggest that the main limiting factor for the recruitment of woody species on the Pasture site is strong grass competition and must be addressed before seed availability, while seed availability seems to be the constraining factor for Bracken and Shrub site development, thus direct seeding or planting may be effective in accelerating forest recovery.

Multiscale assessment of woody species recruitment in Mediterranean shrublands: facilitation and beyond

AbstractAimForest recovery in Mediterranean environments is influenced by factors such as aridity, herbivory and facilitation by shrubs, as well as by seed limitation in the case of highly fragmented forests. How these various factors interact can determine the direction of secondary succession, yet these interactions are poorly understood. We assessed the relative importance of several factors in forest species recruitment in Retama sphaerocarpa (L.) Boiss (Retama) shrublands at different spatial scales.LocationCentre of the Iberian Peninsula.MethodsWe surveyed mid‐ and late successional woody species common in Holm oak forests in 29 Retama shrublands that are distributed along an environmental gradient of increasing aridity and herbivory (regional scale) and are located at various distances from forest patches of different sizes (landscape and local scale). In each Retama shrubland, we analysed the effects of microhabitat (under Retama canopy vs open gaps), aridity, presence of nurse shrubs, herbivory, based on the density of pellet droppings, and propagule pressure, measured as the ratio between the size of forest patches acting as seed sources and their distance to Retama shrubs.ResultsQuercus ilex, Asparagus acutifolius and Juniperus oxycedrus were the mid‐ and late successional woody species most recruited in Retama shrublands. Their frequency, which increased with precipitation, was higher under Retama canopy than in gaps. Differences in Q. ilex recruitment between Retama canopy or gaps increased with rainfall, suggesting a decrease in Retama facilitation effectiveness with increasing aridity. Frequency of recruited oaks increased with the total area of woodland remnants located at <0.5 km, yet propagule pressure did not modulate the positive effect of Retama and rainfall on recruitment. Forest patches at distances >0.5 km did not contribute to recruitment.ConclusionsPresence of the shrub R. sphaerocarpa and dispersal‐related processes at local scale are main determinants of the colonization of shrublands by late successional woody species. Preserving Retama shrublands and Q. ilex woodland remnants is therefore crucial for extensive passive restoration of Mediterranean oak forests. However, facilitation by Retama is much weaker when both aridity and herbivory are high, regardless of seed source availability.

Biotic drivers of seedling establishment in Neotropical savannas: selective granivory and seedling herbivory by leaf‐cutter ants as an ecological filter

Summary Herbivory has been shown to have prominent top‐down effects on vegetation in Paleotropical savannas, where consumers of early stages of life history act as demographic bottlenecks. Such impact has been largely ignored in Neotropical savannas, however, despite insect consumption being linked to reduced recruitment of woody species. We hypothesize that Atta leaf‐cutter ants – the prevalent herbivores in the Neotropics – alter the establishment of woody plant seedlings in the Brazilian Cerrado by reducing seed availability and seedling survival. We also hypothesize that the demographic effects will be species‐specific. To test these hypotheses, we experimentally (i) compared seed removal in areas with and without Atta foraging and (ii) compared the survivorship of seedlings exposed to or protected from Atta foraging. Both experiments were performed with the same common Cerrado species which allowed us to evaluate the potential net effects of consumers at the population and community levels. Overall seed removal rates in sites with Atta foraging were greater than those where ants were absent (59.2% and 39.2% of seeds removed, respectively). There were differences in removal for 10 of the 12 tested species, with per species removal 1.1‐ to 3.8‐fold greater in areas with Atta foraging. On transplanted plants, 45% of seedlings exposed to Atta were attacked, resulting in a survival 7.6% lower than that of protected seedlings. Seedling survival was 11.8–31.5% lower in five species, with the largest differences in survival between treatments during the dry season. Taken together, these results support the hypothesis that consumers can lead to demographic bottlenecks in Neotropical savanna plant species, with effects varying among life‐history stages and between species. Synthesis. Granivory by ants has been linked to reduced seedling recruitment of woody species in the Brazilian Cerrado. Our results show that leaf‐cutter ants may largely limit early seedling establishment of woody species by reducing seed availability and seedling survival with differential species‐specific effects. Atta ants may therefore be acting as an ecological filter, which coupled with potential selectivity in foraging on reproductive life‐history stages, may ultimately influence the relative abundance of different species and hence the structure and composition of Cerrado vegetation.

Pasture succession in the Neotropics: extending the nucleation hypothesis into a matrix discontinuity hypothesis.

The nucleation hypothesis appears to explain widespread patterns of succession in tropical pastures, specifically the tendency for isolated trees to promote woody species recruitment. Still, the nucleation hypothesis has usually been tested explicitly for only short durations and in some cases isolated trees fail to promote woody recruitment. Moreover, at times, nucleation occurs in other key habitat patches. Thus, we propose an extension, the matrix discontinuity hypothesis: woody colonization will occur in focal patches that function to mitigate the herbaceous vegetation effects, thus providing safe sites or regeneration niches. We tested predictions of the classical nucleation hypothesis, the matrix discontinuity hypothesis, and a distance from forest edge hypothesis, in five abandoned pastures in Costa Rica, across the first 11 years of succession. Our findings confirmed the matrix discontinuity hypothesis: specifically, rotting logs and steep slopes significantly enhanced woody colonization. Surprisingly, isolated trees did not consistently significantly enhance recruitment; only larger trees did so. Finally, woody recruitment consistently decreased with distance from forest. Our results as well as results from others suggest that the nucleation hypothesis needs to be broadened beyond its historical focus on isolated trees or patches; the matrix discontinuity hypothesis focuses attention on a suite of key patch types or microsites that promote woody species recruitment. We argue that any habitat discontinuities that ameliorate the inhibition by dense graminoid layers will be foci for recruitment. Such patches could easily be manipulated to speed the transition of pastures to closed canopy forests.

Deer Browsing Delays Succession by Altering Aboveground Vegetation and Belowground Seed Banks

Soil seed bank composition is important to the recovery of natural and semi-natural areas from disturbance and serves as a safeguard against environmental catastrophe. White-tailed deer (Odocoileus virginianus) populations have increased dramatically in eastern North America over the past century and can have strong impacts on aboveground vegetation, but their impacts on seed bank dynamics are less known. To document the long-term effects of deer browsing on plant successional dynamics, we studied the impacts of deer on both aboveground vegetation and seed bank composition in plant communities following agricultural abandonment. In 2005, we established six 15×15 m fenced enclosures and paired open plots in recently fallowed agricultural fields near Ithaca, NY, USA. In late October of each of six years (2005–2010), we collected soil from each plot and conducted seed germination cycles in a greenhouse to document seed bank composition. These data were compared to measurements of aboveground plant cover (2005–2008) and tree density (2005–2012). The impacts of deer browsing on aboveground vegetation were severe and immediate, resulting in significantly more bare soil, reduced plant biomass, reduced recruitment of woody species, and relatively fewer native species. These impacts persisted throughout the experiment. The impacts of browsing were even stronger on seed bank dynamics. Browsing resulted in significantly decreased overall species richness (but higher diversity), reduced seed bank abundance, relatively more short-lived species (annuals and biennials), and fewer native species. Both seed bank richness and the relative abundance of annuals/biennials were mirrored in the aboveground vegetation. Thus, deer browsing has long-term and potentially reinforcing impacts on secondary succession, slowing succession by selectively consuming native perennials and woody species and favoring the persistence of short-lived, introduced species that continually recruit from an altered seed bank.

Post-fire salvage logging alters species composition and reduces cover, richness, and diversity in Mediterranean plant communities

An intense debate exists on the effects of post-fire salvage logging on plant community regeneration, but scant data are available derived from experimental studies. We analyzed the effects of salvage logging on plant community regeneration in terms of species richness, diversity, cover, and composition by experimentally managing a burnt forest on a Mediterranean mountain (Sierra Nevada, S Spain). In each of three plots located at different elevations, three replicates of three treatments were implemented seven months after the fire, differing in the degree of intervention: “Non-Intervention” (all trees left standing), “Partial Cut plus Lopping” (felling 90% of the trees, cutting the main branches, and leaving all the biomass in situ), and “Salvage Logging” (felling and piling the logs, and masticating the woody debris). Plant composition in each treatment was monitored two years after the fire in linear point transects. Post-fire salvage logging was associated with reduced species richness, Shannon diversity, and total plant cover. Moreover, salvaged sites hosted different species assemblages and 25% lower cover of seeder species (but equal cover of resprouters) compared to the other treatments. Cover of trees and shrubs was also lowest in Salvage Logging, which could suggest a potential slow-down of forest regeneration. Most of these results were consistent among the three plots despite plots hosting different plant communities. Concluding, our study suggests that salvage logging may reduce species richness and diversity, as well as the recruitment of woody species, which could delay the natural regeneration of the ecosystem.

Woody species recruitment under monospecific plantations of pioneer trees - facilitation or inhibition?

The successional model of forest restoration is based on the facilitation process, in which the establishment of pioneer tree species favors the late successional tree species. We tested the hypothesis of facilitation through a case study, comparing tree species diversity in the understory of two Neotropical native tree species plantations, Trema micrantha (L.) Blüme (Cannabaceae) and Schinus molle L. (Anacardiaceae). Results obtained under the plantations were compared with an adjacent area under spontaneous secondary succession, analyzing the ligneous plant diversity, soil physic-chemistry and shading. Additionally, the seed rain under the two plantations was analyzed. The area of spontaneous secondary succession and the area with T. micrantha had the highest ligneous plant diversity. The understory of T. micrantha plantation had the lowest light intensity and higher late successional species abundance. In addition, T. micrantha had higher plant diversity than S. molle, which could be explained at least partially by higher diversity in the seed rain. Higher litterfall and mineral content of leaves and twigs of T. micrantha did not coincide with higher topsoil mineral content under that species. Instead, soil under S. molle had higher level of P than the soil under T. micrantha. Data suggest that a high diversity of ligneous plants could be thrived by the spontaneous succession. On the other hand, T. micrantha had promoted a higher richness and abundance of late successional species, which could be related to a more pronounced shade effect, in agreement with the facilitation conception. The lowest diversity and density of ligneous plant species under S. molle characterizes an inhibition effect. T. micrantha could be included in restoration plans assembled with other species or combined with areas without intervention, whenever the spontaneous regeneration is possible.

Sporadic rainy events are more critical than increasing of drought intensity for woody species recruitment in a Mediterranean community

The understanding of the impact of extreme climatic events under a global climate change scenario is crucial for the accurate forecast of future plant community dynamics. We have experimentally assessed the effect of drier and wetter summer conditions on the recruitment probabilities and the growth of seedlings from eight woody species representative of the most important functional groups in the community, pioneer shrubs, mid-successional shrubs and trees, across the main habitats in the study area (open habitat, shrubland, and forest). Our hypothesis proposes that wet summer conditions would represent a good opportunity for tree species regeneration, enhancing both forest maintenance and expansion. A drier summer scenario, on the other hand, would limit forest regeneration, and probably hinder the colonization of nearby habitats. We found a habitat effect on the emergence, survival, and final biomass, whereas different climate scenarios affected seedling survival and biomass. A wet summer boosted growth and survival, whereas greater drought reduced survival only in some cases. These results were modulated by the habitat type. Overall, shrub species presented higher survival and growth and were less affected by more severe drought, whereas some tree species proved to be extremely dependent on wet summer conditions. We conclude that the reduction in frequency of wet summers predicted for the coming decades in Mediterranean areas will have greater consequences for species recruitment than will increased drought. The different response of the species from the various functional groups has the potential to alter the composition and dominance of future plant communities.

Effects of resource availability on plant recruitment at the community level in a Mediterranean mountain ecosystem

Coexisting plant species usually differ in resource requirements, which may also vary within species at successive demographic stages. Such differences become extremely important during the early life stages, since these are the most critical phases in woody-species recruitment, they depend heavily on resources, and they may determine future community composition. Under a global-change scenario, where climatic conditions, nutrient availability, and habitat characteristics are expected to be altered, it is difficult to predict the way in which plant recruitment will be affected. To understand the impact of different global-change drivers on community recruitment, we sowed a set of species representative of the different successional groups of a complete Mediterranean woody community under field conditions, and studied their emergence, growth, and survival along the main resource gradients of light, water, and nutrients. The light and nutrient gradients followed the natural range of conditions in the study area, but water availability was manipulated to simulate three contrasting climatic scenarios: wetter, drier, and current conditions. Structural equation modelling was used to provide a comprehensive analysis of the factors and relations governing plant recruitment. Overall, seedling emergence was determined directly by light; growth was determined by light and summer soil moisture; and survival was determined by summer soil moisture. Light was the main factor indirectly affecting the demographic stages of all species. However, the magnitude of the direct and indirect relationships varied among species. Particularly, species differed in their response to the expected drier climatic conditions, some (e.g. Pinus sylvestris, Acer opalus) being much more vulnerable than others (e.g. Cytisus scoparius, Salvia lavandulifolia). These differential responses could translate as major shifts in the structure of the overall plant community. Our results support the idea that the analysis of complex relations among essential resources is critical for accurate forecasts of the impact of climate change on community dynamics.

The Effects of Herbivory by a Mega- and Mesoherbivore on Tree Recruitment in Sand Forest, South Africa

Herbivory by megaherbivores on woody vegetation in general is well documented; however studies focusing on the individual browsing effects of both mega- and mesoherbivore species on recruitment are scarce. We determined these effects for elephant Loxodonta africana and nyala Tragelaphus angasii in the critically endangered Sand Forest, which is restricted to east southern Africa, and is conserved mainly in small reserves with high herbivore densities. Replicated experimental treatments (400 m2) in a single forest patch were used to exclude elephant, or both elephant and nyala. In each treatment, all woody individuals were identified to species and number of stems, diameter and height were recorded. Results of changes after two years are presented. Individual tree and stem densities had increased in absence of nyala and elephant. Seedling recruitment (based on height and diameter) was inhibited by nyala, and by elephant and nyala in combination, thereby preventing recruitment into the sapling stage. Neither nyala or elephant significantly reduced sapling densities. Excluding both elephant and nyala in combination enhanced recruitment of woody species, as seedling densities increased, indicating that forest regeneration is impacted by both mega- and mesoherbivores. The Sand Forest tree community approached an inverse J-shaped curve, with the highest abundance in the smaller size classes. However, the larger characteristic tree species in particular, such as Newtonia hildebrandtii, were missing cohorts in the middle size classes. When setting management goals to conserve habitats of key importance, conservation management plans need to consider the total herbivore assemblage present and the resulting browsing effects on vegetation. Especially in Africa, where the broadest suite of megaherbivores still persists, and which is currently dealing with the ‘elephant problem’, the individual effects of different herbivore species on recruitment and dynamics of forests and woodlands are important issues which need conclusive answers.