Recruitment and survival of Argyrodendron actinophyllum seedlings in an Australian rainforest

The capacity of seedlings to survive for extended periods beneath intact forest increases the likelihood of regeneration of many species of canopy trees in rainforests. I studied the demographics of Argyrodendron actinophyllum (F.M.Bail.) H.L.Edlin seedlings in a subtropical rainforest in northern New South Wales. A mast seeding of A. actinophyllum was observed and subsequent survival of seedlings monitored over a four year period. Densities of seedlings that emerged correlated with seedfall, while seedfall depended on the size and distance to the surrounding trees. Mortality of seedlings showed density-dependence at higher seedling densities (above about 100 seedlings m^?2), apparently in response to browsing pressure that varied with the density of seedlings. Seedlings that were protected from vertebrates by exclosure cages had lower mortality rates than unprotected seedlings and showed no density response. Glasshouse experiments showed seedling growth was reduced by defoliation, light intensity and initial seed weight, and that seedlings could not persist at light intensities below about 1% ambient, which occur in darker patches on the forest floor. Possible mechanisms whereby the observed spatial and temporal patterns of seedling recruitment could reduce the likelihood of the species becoming more common relative to other tree species in the forest are discussed.     

Plant–plant interactions change during succession on nurse logs in a northern temperate rainforest

Plant–plant interactions change through succession from facilitative to competitive. At early stages of succession, early‐colonizing plants can increase the survival and reproductive output of other plants by ameliorating disturbance and stressful conditions. At later stages of succession, plant interactions are more competitive as plants put more energy toward growth and reproduction. In northern temperate rainforests, gap dynamics result in tree falls that facilitate tree regeneration (nurse logs) and bryophyte succession. How bryophyte‐tree seedling interactions vary through log succession remains unclear. We examined the relationships of tree seedlings, bryophyte community composition, bryophyte depth, and percent canopy cover in 166 1.0 m² plots on nurse logs and the forest floor in the Hoh rainforest in Washington, USA, to test the hypothesis that bryophyte‐tree seedling interactions change from facilitative to competitive as the log decays. Tree seedling density was highest on young logs with early‐colonizing bryophyte species (e.g., Rhizomnium glabrescens) and lowest on decayed logs with Hylocomium splendens, a long‐lived moss that reaches depths >20 cm. As a result, bryophyte depth increased with nurse log decay and was negatively associated with tree seedling density. Tree seedling density was 4.6× higher on nurse logs than on the forest floor, which was likely due to competitive exclusion by forest floor plants, such as H. splendens. Nurse logs had 17 species of bryophytes while the forest floor had six, indicating that nurse logs contribute to maintaining bryophyte diversity. Nurse logs enable both tree seedlings and smaller bryophyte species to avoid competition with forest floor plants, including the dominant bryophyte, H. splendens. H. splendens is likely a widespread driver of plant community structure given its dominance in northern temperate forests. Our findings indicate that plant–plant interactions shift with succession on nurse logs from facilitative to competitive and, thus, influence forest community structure and dynamics.     

Regeneration of beech (Fagus crenata) after the simultaneous death of undergrowing dwarf bamboo (Sasa kurilensis)

Seedling recruitment and survivorship of beech (Fagus crenata) were studied with special reference to the simultaneous death of undergrowing bamboo (Sasa kurilensis). The survival rate of beech seedlings on the floor whereSasa had withered was much higher than that on the floor whereSasa survived. Damping off caused the largest mortality among beech seedlings. However, the allocation pattern of matter to different parts of the seedlings indicated that their survival was greatly affected by production economy. The dense cover of dwarf bamboo prevented the establishment of beech seedling banks on the forest floor. The interval between the times when simultaneous death ofSasa occur and the length of its recovery period are thus important factors controlling the dynamics of beech forests in Japan.     

Where do seedlings go? A spatio‐temporal analysis of seedling mortality in a semi‐arid gypsophyte

Studies of seedling population dynamics often focus on survival because it provides an integrated measure of seedling performance. However, this approach involves a substantial loss of information because survival is the net result of a wide range of mechanisms. The present study overcomes these shortcomings by investigating spatial and temporal patterns in the causes of plant mortality in a population of Helianthemum squamatum seedlings. We use new point pattern analyses based on K functions combined with a new null model (“independent labeling”). A total of 871 seedlings of H.squamatum were mapped and regularly monitored over an 18‐month period. More than 60% of seedlings died during this period. Causes of mortality were spatially structured, and these structures shifted through time. Small differences in either the time of emergence or the environment surrounding H. squamatum seedlings had profound influences on their fate. Seedlings emerging late in the season under the canopy of adult plants died from drought more often than expected, whereas those emerging earlier in the same microsite survived more than expected. The identity of neighbors also affected the spatio‐temporal dynamics of mortality causes. Our results show that seedling‐adult interactions cannot be easily predicted from simple models, and that the time of seedling emergence, its age and the identity of its neighbors determine the sign and the spatial scale of these interactions. The new methods introduced in this article open an avenue for the detailed analyses of the spatio‐temporal dynamics of plant mortality and can help to disentangle the complexity of biotic interactions along environmental severity gradients.     

Tree seed germination and seedling establishment in treefall gaps and understorey in a subtropical forest of northeast India

Abstract Seed germination, and survival and growth of seedlings of four dominant tree species, Quercus dealbata, Quercus griffithii, Quercus glauca and Schima khasiana were studied in the treefall gaps and forest understorey of an undisturbed mature-phase humid subtropical broadleaved forest in northeast India. Three important microenvironmental factors namely photosynthetically active radiation (PAR), soil moisture and litter depth, were also measured in the forest understorey and gaps and correlated with seedling mortality. Seed germination of S. khasiana was significantly higher in the treefall gaps than in the understorey; among the tree species studied, it had the highest germination. Quercus seedlings were abundant in the understorey and small gaps, while S. khasiana seedlings were more numerous in the large gaps. The survivorship curves for the seedling populations revealed that the three Quercus species survived better in the understorey, while S. khasiana did so in the gaps. PAR and soil moisture were positively correlated with tree seedling mortality, which occurred mainly during the winter months. The Quercus seedlings grew better in the forest understorey and small gaps and S. khasiana seedlings in the large gaps. The differential performance of the tree seedlings to the conditions prevailing in the understorey and gaps of two sizes indicates that different species were adapted to different light environments depending upon their optimum requirements. This could be an effective mechanism for promoting species coexistence in the forest community.     

Fungal colonization and decomposition of Castanopsis sieboldii leaves in a subtropical forest

Fungi play a crucial role in the decomposition of lignin in fallen leaves but few studies have examined the functional roles of ligninolytic fungi associated with the decomposition of fallen leaves on tropical forest soils. This study examined fungal populations responsible for lignin decomposition in Castanopsis sieboldii leaves in a subtropical evergreen broad-leaved forest in southern Japan. Fallen leaves of C. sieboldii are characterized by the occurrence of bleached portions attributable to fungal colonization of leaf tissues and decomposition of lignin. The bleached area accounted for 29.7%, on average, of the total area of C. sieboldii fallen leaves in the study site. Leaf mass per unit area (LMA) and lignin content were lower in the bleached area than in the surrounding nonbleached area of the same leaves, indicating that removal of lignin enhanced mass loss from leaf tissues and created small-scale heterogeneity of decomposition within single leaves. An unidentified species of Lachnocladiaceae (Basidiomycetes) was isolated frequently from the bleached area and caused selective decomposition of lignin in leaves under pure culture conditions, indicating that this fungus was responsible for the bleaching. The greater hyphal length of basidiomycetes in the bleached area than in the nonbleached area supported the finding that this Lachnocladiaceae sp. was associated with the bleaching. The relatively rapid decomposition of C. sieboldii leaves on the subtropical forest soil is partly attributable to colonization of the litter by this Lachnocladiaceae sp.     

Smouldering fire-induced changes in a Mediterranean soil (SE Spain): effects on germination,survival and morphological traits of 3-year-old <Emphasis Type="Italic">Pinus pinaster</Emphasis> Ait.

In the present study, a smouldering fire was reproduced in a substrate from a Pinus pinaster forest in the southeastern Iberian Peninsula. Experiments were carried out, in laboratory, using soil monoliths to assess the short-term fire-induced effects on germination, survival and morphological traits in young (3-year-old) specimens of Pinus pinaster Ait. The fire caused a severe reduction in the litter and humus layer relative to a control (unburnt) soil. A lower percentage of accumulated germination (29% in the burnt soil compared with 71% in the control soil) reduced final seedling density, and a lower seedling height was observed in burnt soil. Furthermore, the amount of biomass fixed per unit of leaf area and the concentration of foliar nutrients were lower in the seedlings grown in the burnt soil. However, the amount of biomass fixed per individual seedling was significantly higher in the burnt soil than in the control soil. The results confirm the observed lesser P. pinaster recruitment in burnt stands in southeastern Spain.     

Effect of bryophytes on survival of conifer seedlings in subalpine forests of central Japan

Bryophytes form the major seedbed for coniferous trees in the subalpine forests of central Japan. Field experiments were conducted on the floor of a closed stand dominated byTsuga on Mt Fuji to examine the seedling survival ofAbies veitchii andTsuga diversifolia in seven substrate types in relation to the morphological characteristics of the seeds and seedlings. NeitherAbies norTsuga seedlings could survive on dwarf-bamboo litter beyond 2 years after the seed rain.Abies seedlings survived not only in all types of bryophyte communities but also in larch litter. In contrast, the survival ofTsuga seedlings was restricted to communities of smaller-statured bryophytes found on logs. The seeds and 1st year seedlings ofA. veitchii were larger than those ofT. diversifolia. Abies seedlings produced hypocotyls taller than any bryophyte community and radicles which were long enough to penetrate into the moist humus layer, whereasTsuga seedling radicles presumably penetrate into humus only in bryophyte communities forming a thin mat on logs. The effect of bryophytes on survival of tree seedlings therefore depends on the relationships between the morphology of seeds and seedlings and the structure of bryophyte communities.     

Microenvironmental conditions for Japanese alder seedling establishment in a hummocky fen

In this article, we focused on hummocky microtopography as a prominent feature of mires and explored the microenvironmental conditions suitable for alder seedling establishment. Japanese alder (Alnus japonica) forest is widely distributed in wetlands in northern Japan. However, because alder seedlings are rare in mires, alder population dynamics and conditions that favor the establishment of alder seedlings are still unknown. The study was conducted in northern Japan at a site in mesotrophic mire. We surveyed the seedling density, the microenvironmental conditions (light, litter cover, and soil quality), and the density of dispersed seeds in alder forest and in the adjacent herbaceous fen. In addition, we performed a laboratory experiment to examine the germination characteristics of alder. Seedlings grew only on hummocks in alder forest. The percentage of litter cover on hummocks was lower than in hollows, and the density of dispersed seeds in alder forest was much higher than in herbaceous fen. Seeds of Japanese alder germinated under both light and dark conditions, and the germination rate were high under light and high-temperature conditions. Our results suggest that litter cover may inhibit seedling establishment and hummocks that characterized by less litter cover are suitable place for the establishment of seedlings. We conclude that hummocky microtopography and abundant seed rain in the mire enable the establishment of Japanese alder seedlings.     

Direct and indirect effects of understorey bamboo shape tree regeneration niches in a mixed temperate forest

Plant cover plays a major role in shaping the nature of recruitment microsites through direct (resource mediated) and indirect (consumer mediated) interactions. Understorey plants may differentially affect seedling establishment, thus contributing to regeneration-niche separation among canopy tree species. We examined patterns of early tree seedling survival resulting from interactive effects of understorey bamboo (Chusquea culeou) and resident consumers in a mixed temperate Patagonian forest, Argentina. Newly germinated seedlings of Nothofagus dombeyi and Austrocedrus chilensis were planted in bamboo thickets and non-bamboo patches, with or without small-vertebrate exclosures. We found species-specific patterns of seedling survival in relation to bamboo cover. Nothofagus survival was generally low but increased under bamboo, irrespective of cage treatment. Desiccation stress accounted for most Nothofagus mortality in open, non-bamboo areas. In contrast, Austrocedrus survival was highest in non-bamboo microsites, as most seedlings beneath bamboo were killed by small vertebrates through direct consumption or non-trophic physical damage. There was little evidence for a negative impact of bamboo on tree seedling survival attributable to resource competition. The balance of simultaneous positive and negative interactions implied that bamboo presence facilitated Nothofagus early establishment but inhibited Austrocedrus recruitment via apparent competition. These results illustrate the potential for dominant understorey plants to promote microsite segregation during early stages of recruitment between tree seedlings having different susceptibilities to water stress and herbivory. We recognise, however, that patterns of bamboo–seedling interactions may be conditional on moisture levels and consumer activity during establishment. Hence, both biotic and abiotic heterogeneity in understorey environments should be incorporated into conceptual models of regeneration dynamics and tree coexistence in forest communities.     

Fungal decomposition of woody debris of Castanopsis sieboldii in a subtropical old-growth forest

Fungi, especially basidiomycetes, are the primary agents of woody debris decomposition in terrestrial forest ecosystems. However, quantitative data regarding the abundance and decay activity of wood-inhabiting fungi are lacking, especially for tropical and subtropical areas. This study demonstrates the dynamics of decay columns of wood-inhabiting fungi within decaying woody debris of Castanopsis sieboldii and the wood decay activities of those fungi in a subtropical natural forest. Among six basidiomycetes and two ascomycetes observed as sporocarps on fallen boles of C. sieboldii, Microporus affinis was most abundantly observed in terms of frequency of sporocarps and as percentage area of decay columns within cross-sections of boles, especially those in the early stages of decomposition. In decay columns of M. affinis, both acid-unhydrolyzable residue (AUR) and holocellulose decayed simultaneously, and wood relative density decreased to 45.8% of that of fresh C. sieboldii wood. A pure culture decay test under laboratory conditions showed that M. affinis was a strong decomposer of AUR and holocellulose. These results suggest that M. affinis has a central role in lignocellulose decomposition of wood of C. sieboldii in the early stages of decomposition.     

Density dependence is not very prevalent in a heterogeneous subtropical forest

There is a growing body of evidence demonstrating that tree survival is influenced by negative density‐dependence, but it is still controversial how the effect may vary with life‐stage, and to what extent it plays a role in regulating tree survival in heterogeneous subtropical forests. In this study, we investigated density‐dependent effects on tree survival of six tree species in a 5‐ha subtropical forest in eastern China. The roughly 45 000 individuals in the forest were fully censused in 2003 and 2008. For each of these species, we used an inhomogeneous pair‐correlation function to quantify the change in spatial distribution for different size classes, and a case‐control design to study seedling–adult associations in 2003. Autologistic regression was used to determine the influence of neighborhood factors on individual survival from 2003 to 2008. We found that seedlings of five species were repulsed by distance to nearest conspecific adults in terms of their survival, consistent with predictions of the Janzen–Connell mechanism. By contrast, only the least shade‐tolerant Schima superba had a negative relationship with individual survival and conspecific distance‐weighted basal area. This suggests that the Janzen–Connell effect is only prevalent at the early seedling stage or seed‐to‐seedling phase. The strength of clustering significantly declined at sapling–pole and pole–adult transitions for Sycopsis sinensis and at seedling–sapling transition for Cleyera pachyphylla. Correlations between individual survival and conspecific abundance for these species were consistent with trends in the strength of clustering. These results suggest that density dependence plays a limited role in individual survival and species spatial structure beyond the early seedling stage (i.e. after true leaves growing) in this forest. In addition, this study indicates that including individuals from early life‐stages and factoring out potential confounding factors such as habitat preference are important in studies that seek evidence for density dependence in forest trees.     

亚热带中山常绿阔叶林木本植物幼苗数量动态及其与生境的相关性

以百山祖自然保护区5 hm2永久样地150个幼苗监测站木本植物幼苗为研究对象,2008—2011年定期调查样方中幼苗的种类、数量、萌发、死亡等,探究亚热带中山常绿阔叶林幼苗种类组成、数量动态及其与生境的相关性。结果表明:1)百山祖样地在2008年至2011年出现的幼苗属于26科,40属,共53个物种,不同物种萌发时段有异;2)2009年样地幼苗存活比率为7.7%,2010年为-20.8%,2011年则是-0.3%,幼苗存活比率不高,种类和数量呈减少趋势;3)存活幼苗中有明显的优势物种,分别为光亮山矾(Symplocos lucida)、尖连蕊茶(Camellia cuspidata)、浙闽新木姜子(Neolitsea aurata var.undulatula)、尖叶菝葜(Smilax arisanensis)和短尾柯(Lithocarpus brevicaudatus),5个物种之和占幼苗总数比例50%;4)种子的萌发与生境有极显著的相关性,且与生境因子中水分关联最大;5)存活幼苗数与样站坡位、水分、落叶层厚度呈现显著相关性,水和光照是影响幼苗存活的主要因素。     

Shrub–tree interactions and environmental changes drive treeline dynamics in the Subarctic

Treelines have drawn persistent research interest as they can respond markedly to climate. However, the mechanisms that determine tree seedling recruitment and the response of the forest‐tundra ecotone to environmental changes remain poorly understood. We hypothesise that treeline tree seedling performance depends on the interplay between climatic and soil nutritional changes and facilitative and competitive interactions between trees and shrubs. We conducted a seedling transplantation experiment with Betula pubescens at a subarctic treeline, in northern Sweden, which followed a full factorial design with four treatment factors relating to environmental regimes of stress and resource availability: site (forest vs treeline); temperature (+/? passive warming); shrub presence (+/?Vaccinium myrtillus removal); and nutrient availability (+/? NPK addition). During three growing seasons we assessed the establishment and performance of Betula. The experimental manipulations caused highly significant effects on seedling performance. Although Vaccinium enhanced seedling survival and reduced the effects of excessive solar radiation and insect herbivory, the seedlings growing with the shrub had a poorer performance by the end of the experimental period. Also, seedlings in the forest had a poorer performance than those at the treeline. Betula seedlings showed a very pronounced and positive response to passive warming and to nutrient addition, but such effects were more evident at the treeline site and often interacted with the presence of Vaccinium. This experiment shows that shrub–tree interactions are important drivers of subarctic treeline dynamics and that they vary with time and space. Facilitation, competition, herbivory and environmental changes at the tree seedling stage act as important filters in structuring the forest–tundra ecotone. We demonstrate that changes in this ecotone cannot be simply predicted from changing temperature patterns alone, and that complex interactions need to be considered, not only between shrubs and trees, but also with herbivores and between warming and soil nutrient availability.     

Seedling responses to decreased snow depend on canopy composition and small‐mammal herbivore presence

Winter is becoming warmer and shorter across the northern hemisphere, and reductions in snow depth can decrease tree seedling survival by exposing seedlings to harmful microclimates. Similarly, herbivory by small mammals can also limit the survival and distribution of woody plants, but it is unclear whether winter climate change will alter small‐mammal herbivory. Although small‐scale experiments show that snow removal can either increase or decrease both soil temperatures and herbivory, we currently lack snow‐removal experiments replicated across large spatial scales that are needed to understand the effect of reduced snow. To examine how winter herbivory and snow conditions influence seedling dynamics, we transplanted Acer saccharum and Tsuga canadensis seedlings across a 180 km latitudinal gradient in northern Wisconsin, where snow depth varied seven‐fold among sites. Seedlings were transplanted into one of two herbivory treatments (small‐mammal exclosure, small‐mammal access) and one of two late‐winter snow removal treatments (snow removed, snow unmanipulated). Snow removal increased soil freeze‐thaw frequency and cumulative growing degree‐days (GDD), but the magnitude of these effects depended on forest canopy composition. Acer saccharum survival decreased where snow was removed, but only at sites without conifers. Excluding small mammals increased A. saccharum survival at sites where the small‐mammal herbivore Myodes gapperi was present. Excluding small mammals also increased T. canadensis survival in plots with < 5 cm snow. Because variation in canopy composition and M. gapperi presence were important predictors of seedling survival across the snow‐depth gradient, these results reveal complexity in the ability to accurately predict patterns of winter seedling survival over large spatial scales. Global change scenarios that project future patterns of seedling recruitment may benefit from explicitly considering interactions between snow conditions and small‐mammal winter herbivory.     

Secondary succession and effects of clear-logging on diversity in the subtropical forests on Okinawa Island,southern Japan

In order to clarify the recovery process of the subtropical forest on Okinawa Island, southern Japan, biomass accumulation and the successional trend of species diversity with time were investigated by comparing plots of old-growth and clear logged secondary forests. Self-thinning was an important factor in the development of young secondary forests, and the small variance in tree size within a stand was related to the stand not being fully stratified after clear-cutting. A large variance of size structure in old secondary and old-growth forests implies re-initiation of the understorey. Additionally, the trajectory of stand development indicated that the subtropical forest quickly recovered aboveground biomass, which reached its upper limit at about 50 years after disturbance. However, there was a large distinction in species diversity between the secondary forests and old-growth forests. The diversity of forest floor plants did not recover well after being clear-cut. This indicates that management of the subtropical forest should not only take timber-oriented tree species into account, but also the biodiversity in ground flora. The secondary forests were characterized by Castanopsis sieboldii and Schima wallichii, and the monopolization of C. sieboldii through secondary succession had a negative influence on species diversity. Distylium racemosum dominated at the late development stage and was considered a long-lived competing species that reduced the dominance of C. sieboldii and facilitated the co-occurrence of understorey species. Light-demanding pioneer tree species such as S. wallichii that regenerated after logging should be replaced by competitive effects of climax species, and thus relayed floristic change might increase species diversity along secondary succession.Nomenclature: Hatushima and Amano (1994).     

Seed size,establishment sites and species coexistence in a Chilean rain forest

Abstract. Seedling densities on the forest floor and on elevated microsites (logs and stumps) were compared for eight woody species in a temperate rain forest in southern Chile. Degree of association with elevated microsites varied significantly between species, showed no systematic relationship with reported shade tolerance, but was significantly negatively correlated with seed mass. Large-seeded Podocarpus nubigena established preferentially on undisturbed forest floor sites, whereas seedlings of small-seeded species such as Nothofagus nitida and Laurelia philippiana were found mainly on fallen logs and stumps. The abundance of large seedlings and saplings of N. nitida on logs/stumps, and the growth forms of canopy trees, confirm that recruitment of this species occurs mainly on decaying wood. The relationship between seed size and microsite preferences may be caused by effects of seed size on (1) ability to establish in forest floor litter and (2) retention of seeds on logs. Seedling occupancy of logs and stumps varied with state of decay. Few seedlings of any species were present on logs in the early stages of decay. N. nitida established earlier than the other species, attaining maximum abundance on wood in the middle decay classes. Species richness and overall seedling abundance were highest on wood in advanced stages of decay. Seed size differences are suggested as a determinant of differential utilization of forest floor heterogeneity, and hence of plant species coexistence.     

Determinants of coastal treeline and the role of abiotic and biotic interactions

Recent die-off of coastal forests has been attributed primarily to the effects of sea level rise by correlation with tide-gage records. Due to the temporal and spatial scales involved, direct monitoring of sea level rise impacts is challenging and its attribution can be confounded by both land-use history and species interactions. Here we present experimental evidence for a micro-tidal, oligohaline estuarine system that the location of coastal treeline is determined by both environmental controls and positive and negative species interactions. We conducted field surveys and a transplant experiment to determine the controls on pine seedling establishment and survival along a salinity and flooding gradient. Using a two-way changepoint model we determined that sawgrass cover (Cladium jamaicense) and salinity concentrations interact to control natural pine regeneration (Pinus taeda). The long-term removal of sawgrass resulted in increased soil salinity and high rates of (planted) pine seedling mortality. In contrast, pine seedlings planted directly under sawgrass were able to survive at the same level as upland forest plots because of reduced salinity levels. This research provides evidence that sawgrass can facilitate pine seedling survival, but also suggests that either competitive exclusion by sawgrass or dispersal limitations prevent initial pine seedling regeneration. We propose that forest dynamics are closely coupled to fire, which consumes sawgrass biomass and reduces competition between pine seedlings and grass. Following fire, pine seedling establishment and the regrowth of sawgrass facilitates long-term pine seedling survival. Under this scenario, recent marsh invasion into coastal forests may not necessarily represent a permanent state change in locations where abiotic stress is not the only determinant for community composition.     

Recruitment dynamics in a rainforest seedling community: context-independent impact of a keystone consumer

The influence of keystone consumers on community structure is frequently context-dependent; the same species plays a central organising role in some situations, but not others. On Christmas Island, in the Indian Ocean, a single species of omnivorous land crab, Gecarcoidea natalis, dominates the forest floor across intact rainforest. We hypothesised that this consumer plays a key role in regulating seedling recruitment and in controlling litter dynamics on the island, independent of the type of vegetation in which it occurred. To test this hypothesis, we conducted crab exclusion experiments in two forest types on the island and followed the dynamics of seedling recruitment and litter processing for six years. To determine if these effects were likely to be general across the island, we compared land crab densities and seedling abundance and diversity at ten sites across island rainforest. Surveys across island rainforest showed that seedlings of species susceptible to predation by land crabs are consistently rare. Abundance and diversity of these species were negatively correlated to red crab abundance. Although red land crabs may be important determinants of seedling recruitment to the overstorey, differences in overstorey and seedling composition at the sites suggests that recruitment of vulnerable trees still occurs at a temporal scale exceeding that of this study. These “windows” of recruitment may be related to infrequent events that reduce the effects of land crabs. Our results suggest that unlike the context dependence of most keystone consumers in continental systems, a single consumer, the red land crab, consistently controls the dynamics of seedling recruitment across this island rainforest.     

Environmental heterogeneity and biotic interactions mediate climate impacts on tropical forest regeneration

Predicting the fate of tropical forests under a changing climate requires understanding species responses to climatic variability and extremes. Seedlings may be particularly vulnerable to climatic stress given low stored resources and undeveloped roots; they also portend the potential effects of climate change on future forest composition. Here we use data for ca. 50,000 tropical seedlings representing 25 woody species to assess (i) the effects of interannual variation in rainfall and solar radiation between 2007 and 2016 on seedling survival over 9 years in a subtropical forest; and (ii) how spatial heterogeneity in three environmental factors—soil moisture, understory light, and conspecific neighborhood density—modulate these responses. Community‐wide seedling survival was not sensitive to interannual rainfall variability but interspecific variation in these responses was large, overwhelming the average community response. In contrast, community‐wide responses to solar radiation were predominantly positive. Spatial heterogeneity in soil moisture and conspecific density were the predominant and most consistent drivers of seedling survival, with the majority of species exhibiting greater survival at low conspecific densities and positive or nonlinear responses to soil moisture. This environmental heterogeneity modulated impacts of rainfall and solar radiation. Negative conspecific effects were amplified during rainy years and at dry sites, whereas the positive effects of radiation on survival were more pronounced for seedlings existing at high understory light levels. These results demonstrate that environmental heterogeneity is not only the main driver of seedling survival in this forest but also plays a central role in buffering or exacerbating impacts of climate fluctuations on forest regeneration. Since seedlings represent a key bottleneck in the demographic cycle of trees, efforts to predict the long‐term effects of a changing climate on tropical forests must take into account this environmental heterogeneity and how its effects on regeneration dynamics play out in long‐term stand dynamics.