Inter-annual variability of seed rain and seedling establishment of two woody Mediterranean species under field-induced drought and warming

We aimed to assess the impact of warmer and drier climate change conditions on the seed rain and seedling establishment of Globularia alypum L. and Erica multiflora L., two dominant species in Western coastal Mediterranean shrublands. We performed a non-intrusive field experiment in which we increased the night-time temperatures and excluded spring and autumn rainfall. We monitored the seed rain over 5 years and the seedling recruitment over 9 years on these experimental plots. Seed rain of E. multiflora was enhanced by warming treatment in relation to control, and higher annual rainfall, while seed rain of G. alypum was increased by drought treatment in relation to control, dry years and higher minimum annual temperature. Annual rainfall enhanced the seedling emergence of both species, which also positively correlated with annual mean temperatures. Drought treatment significantly decreased seedling emergence for both species, which was higher in open areas than below vegetation cover. The seedling survival of both species diminished at closer distances to competing neighbours, and in G. alypum seedling survival was higher with lower annual mean temperatures and higher annual rainfall, but also in drought treatment, which have experienced vegetation cover decline. The study confirms that the increasing aridity in Mediterranean ecosystems would constrain the early stages of development in typical co-occurring shrubs. However, there are contrasting responses to climatic conditions between species recruitment, which might favour changes in vegetation through modification of species relative abundance.     

Fire,canopy cover and seedling dynamics in Mediterranean shrubland of northeastern Spain

Abstract. Seedling establishment and survival of the main woody species of early-successional shrubland in northeastern Spain were studied from 1992 to 1995 with emphasis on the importance of vegetation cover (existence of open areas and the situation beneath the plant canopy) and microhabitat (occurrence of stones, litter and bare soil). In the absence of fire, vegetation cover (measured at a scale of 30cm × 30 cm) was not correlated with seedling emergence of most species, nor with growth and survival of seedlings up to one year old. Seedlings older than one year showed a similar pattern: their density was not significantly different in both kinds of habitats. The emergence of seedlings was mainly associated with the presence of mature plants – which can provide seeds – and with the absence of unsuitable microhabitats (large stones, deep litter and bare soil). In April 1994 a wildfire burned the study area. This enabled a study of the pattern of post-fire establishment. After the wildfire, seedling emergence of several species increased and most species produced seedlings with higher survival and growth rates than in the period before the fire. Pre-fire cover, however, did not show significant effects on post-fire seedling dynamics in most cases.     

亚热带-温带过渡区秦岭落叶阔叶林幼苗存活机制

【目的】物种幼苗的存活与各种生物和非生物因素密切相关,研究关键因素对幼苗存活的影响有助于理解群落物种共存的主要作用机制。【方法】以秦岭落叶阔叶林25 hm²固定样地的木本植物幼苗为对象,对11 408棵幼苗的生存动态开展连续5年(2015—2019年)的监测,利用广义线性混合模型(GLMMs)在群落水平上对影响不同年龄阶段幼苗存活的主要生物与非生物因素进行分析。【结果】(1)从群落水平来看,对幼苗存活影响最大的是生物因素,幼苗存活率与同种幼苗邻体密度和同种大树邻体胸高断面积呈显著负相关,与异种幼苗邻体密度呈显著正相关,表明物种在幼苗时期受到强烈的负密度制约效应;(2)从苗龄水平上来看,除了生物因素,影响幼苗存活的主要因素还包括海拔等非生物因素,但非生物因素的影响随着苗龄增大而减小。【结论】影响幼苗存活因素是多样的,其中生物因素的影响更显著,促进秦岭大样地中幼苗共存的主要机制为负密度制约效应。     

Alien seedling recruitment as a response to altitude and soil disturbance in the mountain grasslands of central Argentina

Climate and disturbance are considered key factors in explaining plant invasion, mainly by their effect on alien seedling recruitment. We tested whether soil disturbance by digging and altitude, as a sub-set of climate, affected the seedling recruitment of two alien species (Cirsium vulgare and Melilotus alba) in the mountain grasslands of central Argentina. We performed a seed-addition factorial experiment with altitude as a fixed factor co-variable (6 levels), time since sowing (6 levels) as an autoregressive co-variable, and soil disturbance by digging (two levels) and seed addition (two levels) as fixed factors. Seeds of the studied species were sown in replicated undisturbed and disturbed soil plots, in grassland stands located every 200 m from 1200 to 2200 m a.s.l. The number of emerging seedlings at each plot was recorded during a period of 8 months, at intervals of 30–60 days. Seedlings of both alien species emerged at all the studied altitudes, but only the recruitment of C. vulgare increased linearly with altitude. Moreover, the time window of seedling recruitment for this species increased with altitude. Soil disturbance produced a significant decrease in overall seedling emergence of both species; however, an increase in emergence was observed for C. vulgare at an altitude over 2000 m a.s.l. Natural emergence inside control plots without seed addition was only observed for Cirsium vulgare, which indicates that this species was not seed limited. Our results show that altitude and the absence of disturbance do not restrict the recruitment of C. vulgare and M. alba in these natural mountain grasslands. Moreover, these alien species seem to be tolerant to and even favoured by conditions occurring at higher altitudes and in absence of micro-site soil disturbances in the Córdoba mountain grasslands.     

Regional and local variation in seedling emergence,mortality and recruitment of a perennial herb in Mediterranean mountain habitats

Spatial patterns of seedling regeneration of woody Mediterranean plants are widely documented, with seedlings and juveniles growing almost exclusively under shrub and tree canopy. Neighbourhood habitat amelioration and consequent facilitative effects on recruitment have been extensively suggested as the major determinant of such spatial pattern for Mediterranean vegetation. Much less is known on recruitment patterns of perennial herbs. As herbs differ from woody plants in many relevant ecological aspects and in their life cycles, we would expect particularities in their recruitment dynamics. We analysed the spatial (regional, local and fine-scale) variation in environment (herb cover, litter depth, air temperature and irradiance) and its relationship with seedling emergence, mortality, and recruitment of the perennial herb Helleborus foetidus (Ranunculaceae) during 2 years. We conducted the study in three distant localities in south-eastern Iberian Peninsula (defining the regional scale). Local scale was defined in terms of microhabitat type based on three categories of vegetation cover (under evergreen shrubs or trees, under deciduous spiny-shrubs, and in open interspaces). Finally, fine-scale variation was defined in terms of 1 m² sampling plots. Our results showed regional, local and fine-scale variations in recruitment, with the major source of variation changing across scales. Seed input was important in determining differences in recruitment both at the largest (regional) and the finest scale. Environment had minor importance in shaping differences in recruitment at these two scales. At the local scale, variation between microhabitats was mainly related to differences in seedling survival through facilitative effects (alleviation of water and irradiance stresses) of shrub and tree cover, although such facilitative effect was not consistent in all localities or years. Our study thus points to a similarity in the spatial patterns of recruitment (and in the environmental factors shaping such patterns) between Mediterranean woody plants and perennial herbs, but much more effort is needed to assess the generality of such similarity.     

子午岭三种生境下辽东栎幼苗定居限制

辽东栎(Quercus wutaishanica)是子午岭地区的乡土乔木树种,也是该地区气候顶极群落的建群种,其幼苗的补充更新影响着森林群落的结构及物种组成。在3种典型生境(辽东栎林、人工油松林、灌草丛)中,设置3因素(种子、干扰、遮荫)两水平的野外播种实验(随机区组设计),记录辽东栎幼苗出苗量,并监测幼苗同生群在3种生境中3a间的生长状况,以确定种子及微生境在辽东栎幼苗补充过程中的限制作用。播种实验样方大小30cm×30cm,共计216个样方。结果显示,在辽东栎林及油松林内,增加种子和干扰强度(去除枯落物),能引起出苗量和幼苗补充量的显著增加,且2种处理方式间存在交互作用,表明在郁闭林冠下,辽东栎幼苗的补充受到了种子和微生境的双重限制,枯落物是导致微生境限制的主要因素之一。在灌草丛生境,各种处理方式均不能增加幼苗的补充量,表明辽东栎无法在开阔生境(强光照、干旱)中完成实生幼苗的补充更新。3种生境中的幼苗同生群生存分析表明,辽东栎幼苗在森林群落中存活率显著高于灌草丛群落。根据幼苗生长指标判断,在3种生境中,人工油松林是辽东栎幼苗定居的最佳场所。与实验预期相反,灌木对辽东栎幼苗的补充无显著影响。     

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.     

Contrasting effects of grazing on the early stages of woody encroachment in a Neotropical savanna

Woody encroachment in savannas represents an ecological process of current global interest given its negative impact on ecosystem functioning, particularly on forage production. Traditional savanna models propose competition and niche differentiation as the main mechanisms allowing tree-grass coexistence. Demographic models, instead, propose abiotic and biotic factors as bottlenecks controlling vital rates and transitions from seeds to adult trees. The role played by domestic grazing on woody encroachment is yet controversial. Here, using a multistage tree life approach, we combine both models and evaluate the role of grazing and herbaceous vegetation on woody recruitment in a Neotropical savanna dominated by Vachellia caven, a successful and widely spread encroacher tree species. We performed three experiments to evaluate seed predation, seedling emergence and survival of V. caven by manipulating cattle grazing (grazed and ungrazed areas) and herbaceous vegetation presence (vegetated and unvegetated). Finally, we combined the results of the three experiments to estimate the probability of plant recruitment across these experimental factors. Grazing decreased seed predation by half, did not modify seedling emergence and decreased seedling survival. Herbaceous vegetation did not affect seed predation nor seedling emergence rate, but increased seedling survival. Overall, the net effect of grazing on V. caven recruitment was neutral since the increase in seed availability due to the reduction in seed predation rate was compensated by the negative effect of grazing on seedling survival. Our analysis revealed that cattle grazing and herbaceous vegetation had contrasting effects on the seed and seedling life stages. We propose that in order to restrain the early stages of encroachment, cattle grazing pressure could be managed following the seasonality of demographic tree transitions. Through rotational grazing amongst paddocks, stocking rates could be relaxed during the primary dispersal stage to maximize granivory, and then increased to enhance the chance of seedling consumption and trampling.     

Artificial sowing of endangered dry grassland species into disused basalt quarries

Steppe-like dry (semi) natural grasslands are valuable and endangered habitats in central Europe. In the study area (the ?eské st?edoho?í Hills, Czech Republic), they occur in fragments on southern slopes of volcanic hills, often in the vicinity of quarries, which are numerous there. We performed sowing experiments and observed seedling recruitment of six species, typical for the dry grasslands, in 9 basalt quarries located in 3 different climatic regions over 3 yr. The sowing experiments were established in young successional stages, 5–12 yr after quarrying was stopped. The objectives of the experiments were to evaluate: (i) if seedling recruitment and survival of the target species are possible regarding different climatic regions and (ii) what is the influence of weather fluctuations among years on seedling survival. Seedlings of all studied species were able to recruit and survive in the early successional stages at least in some quarries. The species, except one, showed significant differences in recruitment among the climatic regions, with the best recruitment and survival in the driest and warmest region. Seedlings of two species did not recruit in the wettest and coldest region. All species survived in the driest and warmest region, while only one did so in the coldest and wettest region. Different weather conditions in the studied years significantly influenced recruitment of two species. The number of localities (floristic records) of particular species in the regions was the best predictor of species germination and survival. Thus a traditional floristic survey may help to predict success of species in restoration projects. It emerged that artificial sowing can be considered in restoration programs as a way of contributing to restoration of dry grasslands in disused quarries.     

Spatio‐temporal variation in the effect of herbaceous layer on woody seedling survival in a Chilean mediterranean ecosystem

Questions: What is the effect of herbaceous layer on seedling establishment of three woody pioneer species in open areas of central Chile under a semi‐arid mediterranean climate? How do inter‐annual and habitat conditions (slope aspect) modulate this effect? Under high stress conditions such as the drier year and habitat (north‐facing slope) do herbs reach low abundance and have neutral effects on woody seedlings? Under medium stress conditions for these woody species, such as the wetter year and south‐facing slope, does the herbaceous layer reach greater abundance and have positive effects on woody seedlings due to increasing soil water content? Location: A watershed on the outskirts of Santiago, Chile, subjected to clearing of woody vegetation through firewood extraction and human‐set fires. Methods: In spring 2007, we set up 20 plots (3 m × 2 m). Half of each plot had herbs removed manually and by application of herbicide. In both halves of each plot, one seedling (8 months old) of each of the three native woody species (Colliguaya odorifera, Schinus polygamus and Quillaja saponaria) was planted and survival monitored subsequently. The experiment was repeated in two consecutive growing seasons (2007–2008 and 2008–2009) that differed significantly in total precipitation (152 and 256.5 mm, respectively), and replicated in two sites that differed in aspect and abiotic conditions: a moister south‐ and a drier north‐facing slope. Results: In the first and drier year, the herbaceous layer had low cover and no significant effect on seedling survival of woody species. During the second year, herbs had greater cover and a significant positive effect on spring survival of C. odorifera in the north‐facing slope, which was lost after summer. During this wetter year on the south‐facing slope, herb cover had a positive effect on survival of S. polygamus (mainly during summer). Conclusions: The role of mostly ruderal herbs on woody seedling establishment depended on the species, rainfall of the current year and slope aspect, and may be explained by soil moisture patterns. This suggests that the effect of ruderal herbs on woody seedlings shifts from neutral under high stress conditions produced by drought to positive under moderate stress conditions. Our results contribute to understand interactions between ruderal herbs and woody species under contrasting abiotic conditions. Therefore, control of the herbaceous layer may not be needed in restoration programmes for this region. Moreover, herbs may benefit restoration of woody cover in mesic habitats.     

Grass competition is more important than seed ingestion by livestock for Acacia recruitment in South Africa

Although grazing livestock may have direct negative effects on woody species through herbivory and trampling, (heavy-)grazing is often associated with woody plant encroachment. Two main mechanisms can explain the positive effects of grazers on woody cover: (1) Grazers reduce the interspecific competition with trees and may reduce fuel load, and (2) gut passage through livestock increases seedling establishment by scarification (increased germination) and fertilization by dung (higher survival and growth of seedlings). We tested the effects of fire, grass, cattle ingestion (transit) and dung on germination, seedling height and survival as well as on recruitment of Acacia sieberiana in a sub-humid grassland of South Africa. About 8,000 seeds were planted in the field in a randomized block design. The removal of grass by grazing and/or fire had the most important effect on Acacia recruitment in savanna. Our findings highlight the hierarchy of the main factors affecting Acacia recruitment, which provides new insights to the understanding of woody plant encroachment.     

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

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

Effects of seed burial and fire on seedling and sapling recruitment, survival and growth of African savanna woody plant species

Seed predation and seedling mortality can act as strong demographic “bottlenecks” to sapling recruitment in African savanna woodlands. Fire also limits tree recruitment from saplings by suppressing their growth. I conducted field experiments with 13 woody plant species to assess the effects of seed burial on seedling emergence rates and effects of fire on seedling and sapling survival and growth rates over a period of 8 years at a savanna plot in central Zambia, southern Africa. Seed removal rates by small rodents varied among years and buried seeds had significantly higher emergence rates than seeds exposed to predators in most but not all the species. Annual burning reduced sapling growth in some species but in other species saplings experienced successive shoot die back even in the absence of fire. The findings show that for some woody species, seed predation is an important constraint to seedling recruitment but not for others and annual fires are important hindrances to demography and growth for some species but not others. Thus, demographic “bottlenecks” occur at different life history stages in different savanna woody species and these have the potential to alter woody tree competitive relationships and ultimately savanna structure.     

<Emphasis Type="Italic">Pinus nigra</Emphasis> Arn. ssp <Emphasis Type="Italic">salzmannii</Emphasis> early recruitment and initial seedling growth in warmer and drier locations: the role of seed and soil provenance

Current decreasing precipitation and increasing temperatures promote the likelihood of extreme drought events and may alter the recruitment capacities of tree species. Spanish black pine (Pinus nigra ssp. salzmannii) initial recruitment is being one of the most affected pine species by changing conditions with alterations in the future species distribution. In this context, a cross-exchange experiment was implemented using an outdoor nursery located in a warmer and drier location for testing different Spanish Black pine seeds and soil provenance combinations in relation to early recruitment and initial seedling growth. Soil and seeds were collected at a high (HA, 1641 m.a.s.l.) and low (LA, 1099 m.a.s.l.) altitude in Cuenca Mountains (Spain). Then, a cross-sown experiment using HA and LA soils and seeds was set up in an outdoor nursery, which is located in Albacete (704 m.a.s.l.). Soil quality, seedling emergence, seedling survival, initial seedling growth and total seedling dry mass were measured after one year. We found higher seed emergence and seedling survival by combining LA soil with LA seeds or HA soil with HA seeds. Seedlings from LA seeds with both soil origins and seedlings from HA seeds with LA soils allocated more biomass to roots than seedlings from HA sites growing in HA soils under drier and warmer conditions. These results support the idea that autochthonous provenances have the potential to adapt to changing climatic conditions in their habitats.     

Rodent seed predation and seedling recruitment in mesic grassland

Seedling recruitment of two grasses (Arrhenatherum elatius and Festuca rubra) and two herbs (Centaurea nigra and Rumex acetosa) was measured in areas with and without rodents to which seeds of each species were sown at three seed densities (1000, 10,000 and 50,000 seeds m⁻²) in two seasons (spring and autumn 1995). Seed removal was measured for 10-day periods and the fate of seedlings was followed for 15 months after sowing. The proportion of seed removed ranged from 6 to 85% and increased with increasing seed density for each species. Rodents had no effect on seedling emergence or survival in the spring sowing. In the autumn sowing, rodents reduced seedling emergence of all four species sown at 1000 and 10,000 seeds m⁻² but had no impact at 50,000 seeds m⁻², presumably because of microsite limitation. We suggest the difference between spring and autumn arose because emergence was seed limited in autumn but microsite limited in spring; microsite availability was higher in autumn because a summer drought killed plants, reduced plant biomass and opened up the sward. Fifteen months after the autumn sowing, fewer A. elatius and C. nigra seedlings survived on plots exposed to rodents. This result reflected not only the reduced seedling emergence but also increased seedling mortality (seedling herbivory) in sites exposed to rodents. In contrast, F. rubra and R.acteosa showed density-dependent seedling survival which compensated for initial differences in seedling emergence, so that no effect of rodents remained after 15 months. The results suggest that rodent seed predation and seedling herbivory exert strong effects on seedling recruitment of A.elatius and C. nigra when recruitment conditions are favourable (conditions that lead to high microsite availability) and may contribute to both species being maintained at low densities in the grassland. The results also demonstrate that highly significant impacts of rodent seed predation at the seedling emergence stage can disappear by the time of plant maturation. Received: 2 March 1998 / Accepted: 28 September 1998     

Limits to tree species invasion in pampean grassland and forest plant communities

Factors limiting tree invasion in the Inland Pampas of Argentina were studied by monitoring the establishment of four alien tree species in remnant grassland and cultivated forest stands. We tested whether disturbances facilitated tree seedling recruitment and survival once seeds of invaders were made available by hand sowing. Seed addition to grassland failed to produce seedlings of two study species, Ligustrum lucidum and Ulmus pumila, but did result in abundant recruitment of Gleditsia triacanthos and Prosopis caldenia. While emergence was sparse in intact grassland, seedling densities were significantly increased by canopy and soil disturbances. Longer-term surveys showed that only Gleditsia became successfully established in disturbed grassland. These results support the hypothesis that interference from herbaceous vegetation may play a significant role in slowing down tree invasion, whereas disturbances create microsites that can be exploited by invasive woody plants. Seed sowing in a Ligustrum forest promoted the emergence of all four study species in understorey and treefall gap conditions. Litter removal had species-specific effects on emergence and early seedling growth, but had little impact on survivorship. Seedlings emerging under the closed forest canopy died within a few months. In the treefall gap, recruits of Gleditsia and Prosopis survived the first year, but did not survive in the longer term after natural gap closure. The forest community thus appeared less susceptible to colonization by alien trees than the grassland. We conclude that tree invasion in this system is strongly limited by the availability of recruitment microsites and biotic interactions, as well as by dispersal from existing propagule sources.     

Effect of Sowing Season,Plant Cover,and Climatic Variability on Seedling Emergence and Survival in Burned Austrocedrus chilensis Forests

Fire is the most important disturbance factor in Cypress (Austrocedrus chilensis) forests in Patagonia, Argentina. This ecosystem recovers poorly after fire, and direct sowing could be a potentially useful restoration practice. To evaluate the effect of season of sowing, post‐fire plant cover (PC), and climatic variability on seedling emergence and survival, three direct sowing studies were established in two burned cypress stands: Trevelin (xeric conditions) and El Bolsón (mesic conditions). Two studies were conducted in winter (2000 and 2001) and one in spring (2001). Precipitation was higher than the mean during the 2000–2001 growing season and lower during 2001–2002. At both sites, emergence and survival were much higher for winter‐ than for spring‐sown seedlings. In the xeric stand, emergence and survival of winter‐sown seedlings increased with medium and high PC values, after the humid and dry summers, respectively. However, most spring‐sown seedlings did not emerge, and those that did were short‐lived. Because of the more favorable growing conditions in the mesic stand, PC had no effect on emergence and only favored first year survival of winter‐sown seedlings after the dry summer. Spring‐sown seedlings showed no association with PC in the mesic site, probably because the first summer was exceptionally humid. We speculate that shading plants exert a positive effect on cypress seedling establishment, likely by reducing the stress from high temperatures and low water availability. Sowing of small patches under the protection of understory vegetation could be useful in restoring burned cypress stands.     

A Neighborhood Analysis of the Consequences of Quercus suber Decline for Regeneration Dynamics in Mediterranean Forests

In forests, the vulnerable seedling stage is largely influenced by the canopy, which modifies the surrounding environment. Consequently, any alteration in the characteristics of the canopy, such as those promoted by forest dieback, might impact regeneration dynamics. Our work analyzes the interaction between canopy neighbors and seedlings in Mediterranean forests affected by the decline of their dominant species (Quercus suber). Our objective was to understand how the impacts of neighbor trees and shrubs on recruitment could affect future dynamics of these declining forests. Seeds of the three dominant tree species (Quercus suber, Olea europaea and Quercus canariensis) were sown in six sites during two consecutive years. Using a spatially-explicit, neighborhood approach we developed models that explained the observed spatial variation in seedling emergence, survival, growth and photochemical efficiency as a function of the size, identity, health, abundance and distribution of adult trees and shrubs in the neighborhood. We found strong neighborhood effects for all the performance estimators, particularly seedling emergence and survival. Tree neighbors positively affected emergence, independently of species identity or health. Alternatively, seedling survival was much lower in neighborhoods dominated by defoliated and dead Q. suber trees than in neighborhoods dominated by healthy trees. For the two oak species, these negative effects were consistent over the three years of the experimental seedlings. These results indicate that ongoing changes in species’ relative abundance and canopy trees’ health might alter the successional trajectories of Mediterranean oak-forests through neighbor-specific impacts on seedlings. The recruitment failure of dominant late-successional oaks in the gaps opened after Q. suber death would indirectly favor the establishment of other coexisting woody species, such as drought-tolerant shrubs. This could lead current forests to shift into open systems with lower tree cover. Adult canopy decline would therefore represent an additional factor threatening the recruitment of Quercus forests worldwide.     

Strong microsite control of seedling recruitment in tundra

The inclusion of environmental variation in studies of recruitment is a prerequisite for realistic predictions of the responses of vegetation to a changing environment. We investigated how seedling recruitment is affected by seed availability and microsite quality along a steep environmental gradient in dry tundra. A survey of natural seed rain and seedling density in vegetation was combined with observations of the establishment of 14 species after sowing into intact or disturbed vegetation. Although seed rain density was closely correlated with natural seedling establishment, the experimental seed addition showed that the microsite environment was even more important. For all species, seedling emergence peaked at the productive end of the gradient, irrespective of the adult niches realized. Disturbance promoted recruitment at all positions along the environmental gradient, not just at high productivity. Early seedling emergence constituted the main temporal bottleneck in recruitment for all species. Surprisingly, winter mortality was highest at what appeared to be the most benign end of the gradient. The results highlight that seedling recruitment patterns are largely determined by the earliest stages in seedling emergence, which again are closely linked to microsite quality. A fuller understanding of microsite effects on recruitment with implications for plant community assembly and vegetation change is provided.     

Combined effects of climate,habitat, and disturbance on seedling establishment of <Emphasis Type="Italic">Pinus pinaster</Emphasis> and <Emphasis Type="Italic">Eucalyptus globulus</Emphasis>

The natural expansion of forestry trees into habitats outside plantations is a concern for managers and conservationists. We studied seedling emergence and survival of the two main forestry species in Portugal: Eucalyptus globulus (exotic) and Pinus pinaster (native); using a seed addition experiment. Our main objective was to evaluate the combined effects of climate (mild-summer and warm-summer climate), habitat (oak forest and shrubland), and disturbance (vegetation removal and non-disturbance) on the seedling establishment of species in semi- and natural habitats. Furthermore, we tested the effect of the “sowing season” (autumn and spring) on seedling emergence and survival. Overall, seedling establishment of both species was enhanced by light and water. However, we found important interactions among climate, habitat, and disturbance on both species’ emergence and survival. The differences between habitats were more evident in the mild-summer climate than in the warm-summer climate. Our results also suggested that seedling survival may be enhanced by shrub cover in drier conditions (warm-summer climate). Eucalyptus globulus appears more sensitive to drought and disturbance changes than P. pinaster. In shrublands and mild-summer climate conditions, disturbance especially promoted E. globulus seedling establishment, while the forest canopy and the shade appeared to control it in both climatic conditions. After the first summer life, very low seedling survival was observed in both species, although the colonization of new areas appeared to be more limited for E. globulus. Our study suggests that climate conditions influence the effect (direction and intensity) of habitat and disturbance (plant–plant interactions) on seedling survival. Thus, the effect of light availability (forest canopy) and disturbance (vegetation removal) on these species establishment is climate context-dependent. This study presents very useful information to understand future shifts in these species distribution and has direct applications for the management of natural establishment outside the planted areas, and the management of the understorey to favor forest regeneration or limit forest colonization.