Seed rain of fleshy and dry propagules in different habitats in the temperate rainforests of Chiloé Island,Chile

In temperate rainforests on Chiloé Island in southern Chile (42°S), most canopy trees bear fleshy, avian‐dispersed propagules, whereas emergent tree species have dry, wind‐borne propagules. In the present study, the following hypothesis was tested: regardless of species, fleshy propagules are deposited in greater numbers in canopy gaps and in forest margins and hence have a more heterogeneous seed shadow than wind‐dispersed propagules. To test this hypothesis, the seed rains of these two types of propagules were compared in the following forest habitats: (i) tree‐fall gaps (edges and centre); (ii) forest margins with adjacent pastures; and (iii) under closed canopy (forest interior). Seed collectors (30‐cm diameter) were placed in two (15 and 100 ha) remnant forest patches (n = 60–100 seed collectors per patch) distributed in the four habitats. Seeds were retrieved monthly from each collector during two reproductive seasons (1996, 1997). In both years, the seed rain was numerically dominated by two species with dry propagules (Laureliopsis philippiana and Nothofagus nitida) and three species with fleshy fruits (Drimys winteri, Amomyrtus luma, and Amomyrtus meli). The seed shadows of the two species with dry, wind‐dispersed seeds differed markedly. Seeds of L. philippiana were deposited predominantly in canopy openings, whereas N. nitida seeds fell almost entirely in the forest interior. The fleshy‐fruited species, Drimys and Amomyrtus spp., had similar seed deposition patterns in the various habitats studied, but the between‐year differences in seed rain were greater in Drimys winteri than in Amomyrtus spp. Although no more than 10% of fleshy‐fruited propagules reached the margins of the patch, approximately 7% of these were carried there by birds. Every year, canopy gaps (pooling data from edges and centres) concentrated approximately 60% of the total seed rain of both propagule types in both forest patches. Forest margins received less than 20% of the total seed rain, which was largely dominated by fleshy‐fruited species. Seed shadows were a species‐specific attribute rather than a trait associated with propagule type and dispersal mode.     

Competition among canopy trees in indigenous forests: An analysis of the ‘additive basal area’ phenomenon

The basal area of indigenous forest plots containing large canopy individuals appears to be larger than plots without them. One explanation for this effect is the avoidance of competition for light due to these large individuals emerging above the rest of the canopy and thus casting relatively little shade. In this way the basal area of these emergent individuals becomes ‘additive’ to that of the ‘rest’ of the individuals on a plot. The ‘additive basal area’ phenomenon was tested for in the Knysna Forest, South Africa by regressing the basal area of focal species versus the total basal area of 0.04‐ha plots, as well as against the basal area of the ‘rest’. Regression analysis suggested weak competition and a stronger additive effect. However, no emergent individuals occurred in the study taxa. A strong impact of the size of the largest individual on total plot basal area was found. It is suggested that the reason for this is that large individuals overcome spatial and packing limitations in forests.     

Interactive effects of introduced herbivores and post‐flowering die‐off of bamboos in Patagonian Nothofagus forests

Question: In November 2000, Chusquea culeou, a bamboo species dominating Andean forest understories in southern Argentina and Chile, massively flowered and died over a north‐south distance of ca. 120 km. Because bamboo is the major forage for large herbivores in these forests, we examined the interactive influences of the bamboo die‐off and herbivory by introduced cattle on understory and tree regeneration. Location: Lanín National Park, Argentina. Methods: Permanent plots, in and outside livestock exclosures, were installed in a Nothofagus dombeyi forest in patches of flowered and non‐flowered C. culeou. Plots were monitored over four years for changes in understory composition and tree seedling densities and heights. Results: After the C. culeou die‐off, new establishment of N. dombeyi was low, both with and without herbivory. Livestock alone directly increased N. dombeyi seedling mortality through physical damage. However, tree seedling browse ratings and height growth were interactively affected by bamboo flowering and herbivory; unfenced plots in flowered bamboo patches had the shortest seedlings, highest browse ratings, and lowest tree seedling annual growth rates. Understory cover was higher where livestock were excluded, and this effect was intensified in the patches of flowered bamboo. Neither herbivory nor bamboo flowering resulted in major changes in species composition, with the exception of Alstroemeria aurea. Conclusion: Effects of livestock on N. dombeyi regeneration were contingent on flowering of C. culeou. Prior to introduction of livestock, N. dombeyi regeneration was probably successful beneath canopy gaps during windows of opportunity following bamboo die‐off, but now livestock impede tree regeneration. Herbivory during bamboo withering periods also produces more open understories, particularly affecting palatable heliophyllous herb species such as Alstroemeria aurea. The results underscore the importance of assessing herbivore impacts on tree regeneration during relatively short periods of potential tree regeneration immediately following rare bamboo flowering and die‐off.     

Gap-phase dynamics and coexistence of a long-lived pioneer and shade-tolerant tree species in the canopy of an old-growth coastal temperate rain forest of Chiloé Island, Chile

Aim A major question with regard to the ecology of temperate rain forests in south‐central Chile is how pioneer and shade‐tolerant tree species coexist in old‐growth forests. We explored the correspondence between tree regeneration dynamics and life‐history traits to explain the coexistence of these two functional types in stands apparently representing a non‐equilibrium mixture. Location This study was conducted in northern Chiloé Island, Chile (41.6° S, 73.9° W) in a temperate coastal rain forest with no evidence of stand disruption by human impact. Methods We assessed stand structure by sampling all stems within two 50 × 20 m and four 5 × 100 m plots. A 600‐m long transect, with 20 uniformly spaced sampling points, was used to quantify seedling and sapling densities, obtain increment cores, and randomly select 10 tree‐fall gaps. We used tree‐ring analysis to assess establishment periods and to relate the influences of disturbances to the regeneration dynamics of the main canopy species. Results Canopy emergent tree species were the long‐lived pioneer Eucryphia cordifolia and the shade‐tolerant Aextoxicon punctatum. Shade‐tolerant species such as Laureliopsis philippiana and several species of Myrtaceae occupied the main canopy. The stem diameter distribution for E. cordifolia was distinctly unimodal, while for A. punctatum it was multi‐modal, with all age classes represented. Myrtaceae accounted for most of the small trees. Most tree seedlings and saplings occurred beneath canopy gaps. Based on tree‐ring counts, the largest individuals of A. punctatum and E. cordifolia had minimum ages estimated to be > 350 years and > 286 years, respectively. Shade‐tolerant Myrtaceae species and L. philippiana had shorter life spans (< 200 years). Most growth releases, regardless of tree species, were moderate and have occurred continuously since 1750. Main conclusions We suggest that this coastal forest has remained largely free of stand‐disrupting disturbances for at least 450 years, without substantial changes in canopy composition. Release patterns are consistent with this hypothesis and suggest that the disturbance regime is dominated by individual tree‐fall gaps, with sporadic multiple tree falls. Long life spans, maximum height and differences in shade tolerance provide a basis for understanding the long‐term coexistence of pioneer and shade‐tolerant tree species in this coastal, old‐growth rain forest, despite the rarity of major disturbances.     

Competition and regeneration in quaking aspen–white fir (Populus tremuloides–Abies concolor) forests in the Northern Sierra Nevada,USA

Question: How does competition between quaking aspen (Populus tremuloides) and white fir (Abies concolor) affect growth and spatial pattern of each species? Location: The northern Sierra Nevada, California, USA. Methods: In paired plots in mixed aspen‐ (n=3) or white fir‐dominated (n=2) stands, we mapped trees and saplings and recorded DBH, height, species, and condition and took increment cores. We tallied seedlings by species. Tree ring widths were used as a measure of basal area change over the last decade, and canopy openness was identified using hemispherical photographs. Linear mixed models were used to relate neighborhood indices of competition, stand, and tree‐level variables to diameter increment. Spatial patterns of stems were identified using the Neighborhood Density Function. Results: White fir radial growth was higher in aspen‐ than white fir‐dominated plots. Individual‐level variables were more important for white fir than for aspen growth, while variables representing competitive neighborhood were important only for aspen. The forest canopy was more open in aspen‐ than white fir‐dominated stands, but ample aspen seedlings were observed in all stands. Canopy stems of aspen and white fir were randomly distributed, but saplings and small trees were clumped. Aspen saplings were repelled by canopy aspen stems. Conclusions: Variation in canopy openness explained more stand–stand variation in white fir than aspen growth, but high light levels were correlated with recruitment of aspen seedlings to the sapling class. Radial growth of aspen was predicted by indices of neighborhood competition but not radial growth of white fir, indicating that spacing and stem arrangement was more important for aspen than white fir growth. Fire suppression has removed a major disturbance mechanism that promoted aspen persistence and reduced competition from encroaching conifers, and current forests favor species that regenerate best by advance regeneration (white fir).     

Regeneration dynamics and life history differences in southern Chilean Nothofagus forests: a synthesis

Large, infrequent natural disturbances have been proposed as a key component in determining the distribution and abundance of Nothofagus in southern Andean temperate forests. In this study, a comparison of influences of small-to-intermediate-sized natural disturbances and effects of selective logging on the establishment and growth of Nothofagus forests in south-central Chile are synthesized with the results of other forest studies to develop general hypotheses on the regeneration dynamics of southern Chilean Nothofagus alpina forests. A synthesis is given for regeneration pattern in the abundance of 18 temperate rain forests in the Andean Range, Chile. The study aimed to determine the role of life history differences in promoting coexistence of the five main tree species (N. alpina, N. dombeyi, N. pumilio, N. macrocarpa and Laurelia philippiana) in N. alpina-dominated forests. Age data reported for N. alpina, a shade mid-tolerant emergent tree in the temperate rain forests of southern Chile, indicate maximum lifespans > 650 years, figures unprecedented for N. alpina. In low elevation stands, N. alpina coexists with broad-leaved evergreen tree species, such as L. philippiana, L. sempervirens, Persea lingue, and in these stands an intermittent establishment of Nothofagus occurred and appeared to be most dependent on small-to-intermediate disturbances. In high-elevation stands, in contrast, Nothofagus establishment was less dependent on disturbance, regeneration being much more continuous even in the absence of canopy openings. The forests studied provide another example of the general pattern of increasing dependence of Nothofagus on disturbance towards the more productive end of the environmental gradients. As a long-lived pioneer and despite its dependence on disturbance in lowland sites, N. alpina has been subject to selection for complementary growth, adult survivorship, and mid-tolerance to shade. Thus, interspecific differences in juvenile and adult life history characteristics of N. alpina and its competitors may be sufficient to maintain its persistence in the landscape. In conclusion, this study of population structures and replacement patterns provides a comprehensive picture on our understanding of regeneration dynamics and trait differentiation in southern temperate forests by recognizing both the influences of environmental gradients (i.e., altitude) on competing species and the disturbance regimes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tree mortality and effects of release from competition in an old‐growth Fagus‐Abies‐Picea stand

Abstract. In a montane mixed Fagus‐Abies‐Picea forest in Babia Gora National Park (southern Poland), the dynamics of an old‐growth stand were studied by combining an 8‐yr annual census of trees in a 1‐ha permanent sample plot with radial increments of Abies and Picea growing in the central part of the plot. The mortality among the canopy trees was relatively high (10% in 8 yr), but the basal area increment of surviving trees slightly exceeded the losses caused by tree death. DBH increment was positively correlated with initial diameter in Abies and Picea, but not in Fagus. For individual trees smaller than the median height, basal area increment was positively related to the basal area of old snags and the basal area of recently deceased trees in their neighbourhood, but negatively related to the basal area of live trees. Dendrochronological analysis of the past growth patterns revealed numerous periods of release and suppression, which were usually not synchronized among the trees within a 0.3 ha plot. The almost normal distribution of canopy tree DBH and the small number of young individuals in the plot indicated that stand dynamics were synchronized over a relatively large area and, hence, were consistent with the developmental phase concept. On the other hand, the lack of synchronization among periods of growth acceleration in individual mature Abies and Picea trees conforms more closely to the gap‐dynamics paradigm.     

Stand dynamics of the shade-tolerant conifers Podocarpus nubigena and Saxegothaea conspicua in Chilean temperate rain forest

Abstract. Population structures were used to infer regeneration patterns of the conifers Podocarpus nubigena and Saxegothaea conspicua and associated angiosperm tree species in six stands in the temperate rainforests of the coastal ranges of southern Chile. On poorly-drained, nutrient-poor and high altitude sites, where most of the associated species were fairly shade-intolerant and light-crowned, dense all-aged populations and the presence of numerous saplings beneath the canopy suggested continuous regeneration of both P. nubigena and S. conspicua. On more favourable sites, where several of the associated angiosperms were highly shade-tolerant and densecrowned, both conifers, particularly P. nubigena, were less abundant, and their regeneration from seed appeared to be sporadic. S. conspicua's great longevity (≥ 750 yr) and frequent vegetative reproduction are probably crucial to its persistence in competition with shade-tolerant broadleaved species in undisturbed stands on favourable sites. Results of this study, and a review of related literature on southern temperate forests are not consistent with the hypothesis that heavily-shaded, infrequently-disturbed habitats are an evolutionary refuge for conifers. Sites likely to have high leaf area indices and infrequent disturbance are better exploited by shade-tolerant angiosperms.     

Effects of individual size,local competition and canopy closure on the stem volume growth in a monoclonal Japanese cedar (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis> D. Don) plantation

We studied how the dominant factor affecting stem volume growth changes during stand development in a monoclonal stand of Cryptomeria japonica D. Don. Stem analysis was used to compare growth history of trees in an unthinned plot (closed canopy) and a thinned plot (open canopy). In the unthinned plot, the dominant factor affecting stem volume growth was basal area (BA) before canopy closure, whereas neighborhood competition index (CI) was the dominant factor after canopy closure. In contrast, the dominant factor affecting stem volume in the thinned plot was BA throughout stand development. Spearman’s rank correlation coefficient between BA and CI continued to increase after canopy closure and size rank among individuals became increasingly fixed. Our results indicated that stem volume growth shifts from size-dependent to competition-dependent growth at canopy closure. The apparent correlation between tree size and growth rate observed in many previous studies may be the result of competition-mediated positive feedback between size and growth.     

Tree growth in an African woodland savanna affected by disturbance

Questions: How does tree growth in a tropical woodland savanna vary as a function of size, and how is it affected by competition from neighbours, site attributes, and damage caused by disturbance? Location: western Zimbabwe. Methods: Trees of common species were tagged, mapped, and measured annually between 2001 and 2003 in a Kalahari sand woodland savanna. Diameter increments were analysed with mixed model regressions for the largest ramet in each genet. Stem diameter and damage, soil texture, and indices of competition at multiple spatial scales were used as covariates. Results: Stem diameter increased initially and then declined as a function of size in undamaged trees, which grew faster than damaged trees. Growth in damaged trees declined with size. No site differences were detected, and there was evidence for between‐tree competition on growth only in the fastest‐growing species, Brachystegia spiciformis. In several species the growth rate of the largest ramet increased as a function of the basal area of secondary ramets, contrary to expectations. For many species, the growth models showed poor explanatory power. Conclusions: Growth in Kalahari sand savanna trees varies as a function of size and changes in tree architecture caused by disturbance agents such as fire, frost, and elephant browsing. Disturbance may thus play an important role on vegetation dynamics through its effects on growth in the post‐disturbance phase. Growth is highly stochastic for some species in this system, and more deterministic in others. It is hypothesized that this dichotomy may be driven by differences in rooting depth among species.     

Nothofagus dombeyi regeneration in declining Austrocedrus chilensis forests: Effects of overstory mortality and climatic events

This research examines the regeneration dynamics of Nothofagus dombeyi and Austrocedrus chilensis in A. chilensis-dominated forests growing near the eastern limit of N. dombeyi where precipitation is limiting. In these forests the widespread decline and mortality of overstory A. chilensis trees, known as ‘mal del ciprés’ (cypress sickness), generates large canopy gaps in which new individuals establish. Our objective was to study the population dynamics of N. dombeyi and A. chilensis in these forests to investigate the influences of overstory tree death and climatic variation on establishment. We sampled 6 symptomatic A. chilensis stands and used dendrochronological techniques to reconstruct basal area development and regeneration establishment over time. Bivariate event analysis was performed to examine the temporal relationships between tree establishment and mortality events and climatic variation. Overstory A. chilensis trees established as post-fire cohorts, with subsequent establishment of A. chilensis and N. dombeyi during the past 50–60 years. Regeneration in the past two decades was primarily N. dombeyi. The establishment of both A. chilensis and N. dombeyi was synchronous with overstory tree mortality events, but it was more consistent among stands and prolonged for N. dombeyi. Establishment of A. chilensis was not associated with climatic events but N. dombeyi establishment was synchronous with droughts, possibly related to climate-driven mortality creating canopy gaps or reducing competition within gaps. We have demonstrated that N. dombeyi has the ability to establish in post-fire A. chilensis-dominated forests resulting in mixed-species, uneven-aged forests. The ongoing increase in the abundance of N. dombeyi relative to A. chilensis represents a shift in composition and increased complexity in stand structure driven by ‘mal del ciprés’ and climatic variation.     

Planting Seedlings in Tree Islands Versus Plantations as a Large‐Scale Tropical Forest Restoration Strategy

Planting tree seedlings in small patches (islands) has been proposed as a method to facilitate forest recovery that is less expensive than planting large areas and better simulates the nucleation process of recovery. We planted seedlings of four tree species at 12 formerly agricultural sites in southern Costa Rica in two designs: plantation (entire 50 × 50 m area planted) and island (six patches of three sizes). We monitored seedling survival, height, and canopy area over 3 years. To elucidate mechanisms influencing survival and growth, we measured soil and foliar nutrients, soil compaction, and photosynthesis. Survival of all species was similar in the two planting designs. Seedling height and canopy area were greater in plantations than islands at most sites, and more seedlings in islands decreased in height due to damage incurred during plot maintenance. Survival, height, and canopy area were both site‐ and species‐specific with the two N‐fixing species (Inga edulis and Erythrina poeppigiana) greater than the other species (Terminalia amazonia and Vochysia guatemalensis). Foliar N was higher in Terminalia and Vochysia in sites where Inga growth was greater. Soil nutrients, however, explained a small amount of the large differences in growth across sites. Leaf mass per area was higher in islands, and P use efficiency was higher in plantations. Our results show advantages (good seedling survival, cheaper) and disadvantages (more seedling damage, slightly lower growth) to the island planting design. Our study highlights the importance of replicating restoration strategies at several sites to make widespread management recommendations.     

Roles of dominant understory Sasa bamboo in carbon and nitrogen dynamics following canopy tree removal in a cool‐temperate forest in northern Japan

To clarify the role of dense understory vegetation in the stand structure, and in carbon (C) and nitrogen (N) dynamics of forest ecosystems with various conditions of overstory trees, we: (i) quantified the above‐ and below‐ground biomasses of understory dwarf bamboo (Sasa senanensis) at the old canopy‐gap area and the closed‐canopy area and compared the stand‐level biomasses of S. senanensis with that of overstory trees; (ii) determined the N leaching, soil respiration rates, fine‐root dynamics, plant area index (PAI) of S. senanensis, and soil temperature and moisture at the tree‐cut patches (cut) and the intact closed‐canopy patches (control). The biomass of S. senanensis in the canopy‐gap area was twice that at the closed‐canopy area. It equated to 12% of total biomass above ground but 41% below ground in the stand. The concentrations of NO₃^? and NH₄⁺ in the soil solution and soil respiration rates did not significantly change between cut and control plots, indicating that gap creation did not affect the C or N dynamics in the soil. Root‐length density and PAI of S. senanensis were significantly greater at the cut plots, suggesting the promotion of S. senanensis growth following tree cutting. The levels of soil temperature and soil moisture were not changed following tree cutting. These results show that S. senanensis is a key component species in this cool‐temperate forest ecosystem and plays significant roles in mitigating the loss of N and C from the soil following tree cutting by increasing its leaf and root biomass and stabilizing the soil environment.     

Tree regeneration responses in a lowland Nothofagus-dominated forest after bamboo dieback in South-Central Chile

The bamboo Chusquea quila (Poaceae:Bambuseae) is the most abundant understory species ofNothofagus-dominated forests at low elevations inthe Chilean Lake District. Species of this genus strongly inhibit theestablishment and growth of tree species, especially those of the genusNothofagus. At intervals of many years, this bambooflowerssynchronously and dies, creating large-scale disturbances. The mainobjective of this study was to determine the influence of bamboo dieback ontreeregeneration, especially of shade-intolerant species. The forest studiedis a remnant stand of old-growth forest dominated by emergent individualsof N. obliqua and Eucryphiacordifoliawhich project about 10 m above a main canopy formed byAextoxicon punctatum, Laureliaphilippiana, and Podocarpus saligna. Treeseedlings that established prior to bamboo dieback were recorded in six gapsdensely covered by bamboo. After the dieback event in the early 1990's,recruitment, height growth and survivorship were monitored in four gaps duringtwo growing seasons. After the synchronous mass flowering and death ofChusqueaquila, both reorganization of advance regeneration and new seedlingrecruitment were observed in gaps. The advance regeneration consisted mainly ofthe shade-tolerant species A. punctatum,L. philippiana, and Amomyrtus luma.These species together with E. cordifolia accounted forover 90% of the total individuals in gaps. During the following twoseasons, c. 40% of the advance regeneration either died or was damaged.Root suckers of E. cordifolia and L.philippiana were taller and grew faster than the seedlings of otherspecies. Nothofagus obliqua was unable to establishadvanceregeneration in gaps formerly dominated by bamboo. New recruitment resulted inthe synchronized establishment of treeseedlings, especially less shade-tolerant species such as N.obliqua, Aristotelia chilensis,Rhaphithamnus spinosus, and E.cordifolia. These species recruited into the gaps almost entirelyduring the first season right after the bamboo dieback. However, by the end ofthe second season, these species had a lower density due to a combination oflower recruitment rate, shortened recruitment period, and greater mortalityratecompared to the shade-tolerant species. New bamboo seedlings grew fasterand were more abundant than tree seedlings, except for root suckers (bothnew establishment and advance regeneration) of E.cordifolia which is more likely to successfully recruit into thecanopy. Seedlings of N. obliqua compete poorly with bambooseedlings; its successful recruitment may require bamboo flowering coincidentwith a mast seed year for the tree species.     

Carbohydrate storage,survival, and growth of two evergreen <Emphasis Type="Italic">Nothofagus</Emphasis> species in two contrasting light environments

A number of traits have been attributed important roles in tolerance of shade by plants. Some explanations emphasize traits enhancing net carbon gain; others emphasize energy conservation traits such as storage of non-structural carbohydrates (NSC). To date, cross-species studies have provided mixed support for the role of NSC storage in low-light survival. We examined NSC status, survival, biomass, and growth of large seedlings of two evergreen species of differing shade tolerance (Nothofagus nitida and N. dombeyi) grown in deep shade and 50% light for two growing seasons. We expected to find higher NSC concentration in the more shade-tolerant N. nitida and since allocation to storage involves sacrificing growth, higher growth rate in the shade-intolerant N. dombeyi. NSC concentration of both species was >twofold higher in 50% light than in deep shade, and in roots and stems did not differ significantly between species in either environment. NSC contents per plant were also similar between dead and living plants in deep shade. N. dombeyi outgrew N. nitida in 50% light, while this pattern was reversed in deep shade. Survival in deep shade was not correlated with NSC concentration. Leaf mass fraction was similar between species in 50% light, but lower in N. dombeyi in deep shade. Results provide little evidence of a link between carbohydrate storage and low-light survival in Nothofagus species, and support the view that understorey survival is primarily a function of net carbon gain. Patterns of variation in NSC concentration of the temperate species we studied are likely dominated by more important influences than adaptation to shade, such as limitation of growth or adaptation to cold stress.     

Resource competition and fire‐regulated nutrient demand in carnivorous plants of wet pine savannas

Question: What are the mechanisms by which fire reduces competition for both a short‐lived and a long‐lived species in old‐growth ground‐cover plant communities of wet pine savannas (originally Pinus palustris, replaced by P. elliottii)? Location: Outer coastal plain of southeastern Mississippi, USA. Methods: I reviewed previous competition experiments and proposed a new hypothesis to explain the relationship between fire, competition, and species co‐existence in wet longleaf pine savannas. The first study is about growth and seedling emergence responses of a short‐lived carnivorous plant, Drosera capillaris, to reduction in below‐ground competition and above‐ plus below‐ground competition. The second study deals with growth and survival responses of a long‐lived perennial carnivorous plant, Sarracenia alata, to neighbour removal and prey‐exclusion to determine if a reduction in nutrient supply increased the intensity of competition in this nutrient‐poor system. Results: Fire increased seedling emergence of the short‐lived species by reducing above‐ground competition through the destruction of above‐ground parts of plants and the combustion of associated litter. Prey exclusion did not increase competitive effects of neighbours on the long‐lived species. However, because the experiment was conducted in a year without fire, shade reduced nutrient demand, which may have obviated competition for soil nutrients between Sarracenia alata and its neighbours. Conclusion: Repeated fires likely interact with interspecific differences in nutrient uptake to simultaneously reduce both above‐ground competition and competition for nutrients in old‐growth ground cover communities in pine savannas. Restoration practitioners should consider the possibility that the composition of the plant community is just as important as fire in ensuring that frequent fires maintain species diversity.     

Exposure to herbivores increases seedling growth and survival of American chestnut (Castanea dentata) through decreased interspecific competition in canopy gaps

The successful development of early stages of blight‐resistant hybrid stock has increased hopes for restoration of American chestnut (Castanea dentata) to eastern North American forests. However, these forests have undergone substantial ecological change in the century since the functional extirpation of American chestnut, and it remains unknown to what extent American chestnut will be able to recolonize contemporary forests. In particular, high densities of white‐tailed deer (Odocoileus virginianus) and competition with mesophytic tree species such as maple (Acer) may impede chestnut regeneration, much as they affect oak (Quercus). We used a split‐plot analysis of variance (ANOVA) design to examine the effects of canopy gaps and herbivory on survival and growth of third generation backcrossed (BC₃) hybrid chestnut seedlings over two growing seasons in central Indiana, U.S.A. Only 4 of 588 (0.7%) seedlings in closed‐canopy plots survived to the end of the study, as opposed to 264 of 589 (45%) seedlings in gap plots. Within the gap treatment, fencing was associated with reduced chestnut survival as well as reduced herbivory and increased cover of non‐chestnut vegetation. Our results indicate that herbivory may indirectly benefit chestnut regeneration by suppressing competition. However, this beneficial effect is likely context‐dependent and additional work is needed to establish the conditions under which it occurs.     

Lack of disturbance as an explanation for the additive basal area phenomenon in a stratified Afrotemperate forest

Abstract Explanations for the additive basal area (BA) phenomenon in forests are frequently given in the context of stratification and the avoidance of competition for space or resources (e.g. light, nutrients, moisture). Thus, large individuals avoid competition for light by emerging above the canopy and, in so doing, the BA of emergent individuals is often ‘additive’ to that of the rest of the individuals in a stand. The additive BA phenomenon was evident in a stratified Afrotemperate forest and was not confined to the emergent stratum but occurred also within the canopy stratum where gymnosperm BA appeared to be additive. However, there was no evidence that stand BA was at carrying capacity (i.e. presumably neither space nor nutrient resources were limiting) and there was no statistical evidence of competitive effects. We argue that avoiding competition is an insufficient explanation for how biomass accumulates in stratified forests and suggest that local variation in disturbance regime and tree life‐history provide a more plausible and general explanation for current forest stand structure and dynamics. We suggest that the additive effect is a result of long‐lived individuals persisting in the prolonged absence of disturbance to canopy and emergent strata.     

The influence of nitrogen on rain forest dipterocarp seedlings exposed to a large increase in irradiance

Dipterocarps dominate the canopy of lowland tropical rain forest in South‐east Asia. Seedlings of these species form diverse assemblages on the forest floor where low irradiance severely limits their growth. Further growth depends largely upon the increased irradiance that can occur with the creation of canopy gaps. However, the response of dipterocarp seedlings to increased irradiance and their subsequent establishment in the canopy may be influenced by the availability of other resources, such as nutrient availability. We investigated the influence of nitrogen supply on aspects of the photosynthetic physiology and growth of seedlings of four dipterocarp species (Shorea leprosula, Shorea johorensis, Shorea oleosa and Dryobalanops lanceolata) growing under low irradiance, during transfer from low to high irradiance, and during subsequent growth at high irradiance. All four species increased growth and photosynthetic capacity in response to N‐supply at high irradiances but not at low irradiance approximating that which can be expected to occur in the forest understorey. When seedlings grown at low irradiances and varying N‐supply were exposed to a large increase in irradiance, all species showed some degree of initial photodamage (measured through chlorophyll fluorescence), the extent of which was similar between species but differed markedly depending on the pre‐exposure growth irradiance and N‐supply. Greater photodamage occurred in seedlings grown at lower compared with higher N‐supply and irradiance. Despite these initial difference in the extent of this photodamage, all seedlings demonstrated a similar capacity to recover from damage. However, the alterations in the photosynthetic physiology of leaves during this recovery differed between species and depended on N‐supply. Under high N‐supply all species apart from S. oleosa increased photosynthetic capacity per unit chlorophyll following exposure to high irradiance by increasing photosynthetic capacity per unit leaf area while, under low N‐supply, an increase in photosynthetic capacity per unit leaf only occurred in D. lanceolata. Our results suggest that variations in N‐availability may have a much greater impact on the relative competitiveness of dipterocarp seedlings during the regenerative phase following canopy gap formation than physiological differences between seedlings. Our results demonstrate a potentially significant role for N‐availability in the regeneration dynamics and distribution of canopy‐dominating dipterocarp species.     

Effects of size,neighbors, and site condition on tree growth in a subtropical evergreen and deciduous broad‐leaved mixed forest,China

Successful growth of a tree is the result of combined effects of biotic and abiotic factors. It is important to understand how biotic and abiotic factors affect changes in forest structure and dynamics under environmental fluctuations. In this study, we explored the effects of initial size [diameter at breast height (DBH)], neighborhood competition, and site condition on tree growth, based on a 3‐year monitoring of tree growth rate in a permanent plot (120 × 80 m) of montane Fagus engleriana–Cyclobalanopsis multiervis mixed forest on Mt. Shennongjia, China. We measured DBH increments every 6 months from October 2011 to October 2014 by field‐made dendrometers and calculated the mean annual growth rate over the 3 years for each individual tree. We also measured and calculated twelve soil properties and five topographic variables for 384 grids of 5 × 5 m. We defined two distance‐dependent neighborhood competition indices with and without considerations of phylogenetic relatedness between trees and tested for significant differences in growth rates among functional groups. On average, trees in this mixed montane forest grew 0.07 cm year^?1 in DBH. Deciduous, canopy, and early‐successional species grew faster than evergreen, small‐statured, and late‐successional species, respectively. Growth rates increased with initial DBH, but were not significantly related to neighborhood competition and site condition for overall trees. Phylogenetic relatedness between trees did not influence the neighborhood competition. Different factors were found to influence tree growth rates of different functional groups: Initial DBH was the dominant factor for all tree groups; neighborhood competition within 5 m radius decreased growth rates of evergreen trees; and site condition tended to be more related to growth rates of fast‐growing trees (deciduous, canopy, pioneer, and early‐successional species) than the slow‐growing trees (evergreen, understory, and late‐successional species).