Altered climate leads to positive density‐dependent feedbacks in a tropical wet forest

Climate change is predicted to result in warmer and drier Neotropical forests relative to current conditions. Negative density‐dependent feedbacks, mediated by natural enemies, are key to maintaining the high diversity of tree species found in the tropics, yet we have little understanding of how projected changes in climate are likely to affect these critical controls. Over 3 years, we evaluated the effects of a natural drought and in situ experimental warming on density‐dependent feedbacks on seedling demography in a wet tropical forest in Puerto Rico. In the +4°C warming treatment, we found that seedling survival increased with increasing density of the same species (conspecific). These positive density‐dependent feedbacks were not associated with a decrease in aboveground natural enemy pressure. If positive density‐dependent feedbacks are not transient, the diversity of tropical wet forests, which may rely on negative density dependence to drive diversity, could decline in a future warmer, drier world.     

Drought effects on seedling survival in a tropical moist forest

The amount and seasonality of rainfall varies strongly in the tropics, and plant species abundance, distribution and diversity are correlated with rainfall. Drought periods leading to plant stress occur not only in dry forests, but also in moist and even wet forests. We quantified experimentally the effect of drought on survival of first year seedlings of 28 co-occurring tropical woody plant species in the understory of a tropical moist forest. The seedlings were transplanted to plots and subjected to a drought and an irrigation treatment for 22 weeks during the dry season. Drought effects on mortality and wilting behavior varied greatly among species, so that relative survival in the dry treatment ranged from 0% to about 100% of that in the irrigated treatment. Drought stress was the main factor in mortality, causing about 90% (median) of the total mortality observed in the dry treatment. In almost half of the species, the difference in survival between treatments was not significant even after 22 weeks, implying that many of the species are well adapted to drought in this forest. Relative drought survival was significantly higher in species associated with dry habitats than in those associated with wet habitats, and in species with higher abundance on the dry side of the Isthmus of Panama, than in those more abundant on the wet side. These data show that differential species survival in response to drought, combined with variation in soil moisture availability, may be important for species distribution at the local and regional scale in many tropical forests.     

Spatial and temporal variability of canopy structure in a tropical moist forest

Tree fall gaps are widely considered to play a prominent role in the maintenance of species diversity, while the spatiotemporal variability of canopy structure within closed forest stands is largely ignored. In this study we examined the vertical and horizontal components of canopy structure and its seasonal variability in a tropical wet semideciduous rainforest in Panama. Leaf area indices (LAI) were derived from measurements of diffuse radiation and empirically-based leaf angle distribution by mathematical inversion of a light interception model. Vertical distribution of LAI was non-homogeneous with 50% of the leaf area being concentrated in the uppermost 5 m of the canopy. In the wet season, when foliage is most abundant, the horizontal distribution of LAI in a 2100 m² plot ranged widely from 3 to 8, with a mean of 5.41. Changes in mean LAI between wet and dry seasons were small but highly significant. While ca 40% of the area was not affected by local changes in LAI, sizeable small scale changes in LAI did occur between wet and dry season in some locations. Local changes in LAI ranged from –2.3 to 2.4. These changes resulted in a 50% or more increase in light reaching the forest floor at 29% of the measuring locations, and a doubling or more at 13% of the location. Our results imply that structural heterogeneity by simple tree fall gaps do not adequately describe the dynamics of forest canopies.     

Non‐additive effects of density‐dependent and density‐independent factors on brown trout vital rates

Interactions between density‐dependent and density‐independent processes can lead to variation in both growth and survival rates. Detecting such effects, however, will often require sampling on an individual level and at the appropriate spatial and temporal scale. This study documents substantial variation in survival and growth of stream‐dwelling brown trout Salmo trutta from a small Norwegian stream. The data is based on seasonal capture–recaptures of individually marked trout on fixed stations during eight years. The fish were small‐sized, rarely reaching sizes larger than 20 cm and ages older than seven years. Density varied between 0.2–0.8 fish m^?2. Variation in survival and recapture probabilities was analysed using program MARK. Apparent survival (the probability of being alive and present within the study area) generally decreased with increasing trout density and increasing drought level (measured as lowest observed water flow) during both winter and summer. Further, there was a significant interaction effect between density and water flow, indicating that density‐dependent effects on survival predominated when environmental conditions were benign (no drought), while density‐independent processes were most important under harsh environmental conditions (drought). Observed length‐at‐age during autumn indicated a more or less linear growth trajectory throughout life, and no effect of density, water flow or temperature was found. However, using the individual‐based capture–recapture data to estimated specific growth rate, significant positive effects of water flow and temperature and a negative effect of density were identified. Thus, the capture–recapture data suggest a strong potential for population regulation at the rather low densities found in this stream, and regulation may occur both through effects on survival and growth.     

Density‐dependent survival varies with species life‐history strategy in a tropical forest

Species coexistence in diverse communities likely results from multiple interacting factors. Mechanisms such as conspecific negative density dependence (CNDD) and varying life‐history strategies related to resource partitioning are known to influence plant fitness, and thereby community composition and diversity. However, we have little understanding of how these mechanisms interact and how they vary across life stages. Here, we document the interaction between CNDD and life‐history strategy, based on growth‐mortality trade‐offs, from seedling to adult tree for 47 species in a tropical forest. Species’ life‐history strategies remained consistent across stages: fast‐growing species had higher mortality than slow‐growing species at all stages. In contrast, mean CNDD was strongest at early life stages (i.e. seedling, sapling). Fast‐growing species tended to suffer greater CNDD than slow‐growing species at several, but not all life stages. Overall, our results demonstrate that coexistence mechanisms interact across multiple life stages to shape diverse tree communities.     

Relative effects of different leaf attributes on sapling growth in tropical dry forest

Aims Soil moisture content (SMC) influences establishment, survival and development of plant species and is considered as the most important limiting factor in tropical dry forest (TDF). In this study we attempt to establish the relationship between leaf attributes and of tree saplings in TDF and address the following questions: (i) how are the functional attributes of dominant tree saplings of TDF affected by seasonal changes in SMC at different habitats?, (ii) what is the relationship of functional attributes with each other?, (iii) how are the functional attributes and their plasticity affected by habitat conditions? and (iv) can the functional attributes in single or in combination predict the growth rate of tree saplings of TDF? The study was conducted on four sites (Hathinala, Gaighat, Harnakachar and Ranitali, listed in order of decreasing SMC) within the tropical dry deciduous forest in northern India.Methods We analysed eight leaf attributes, specific leaf area (SLA); leaf dry matter content (LDMC); leaf nitrogen concentration (leaf N); leaf phosphorus concentration (leaf P); chlorophyll concentration (Chl); mass-based photosynthetic rate (A mass); mass-based stomatal conductance (Gs mass); intrinsic water use efficiency (WUEi) and three growth attributes, relative diameter increment (RDI); relative height increment (RHI); relative growth rate (RGR) of the 10 dominant tree saplings (viz., Acacia catechu, Anogeissus latifolia, Boswellia serrata, Buchanania lanzan, Diospyros melanoxylon, Hardwickia binata, Lagerstroemia parviflora, Lannea coromandelica, Shorea robusta and Terminalia tomentosa) of a TDF and observed the effects of site, season and species for a period of 2 years. Saplings were selected in gradients of deciduousness. Step-wise multiple regression was performed to predict RDI, RHI and RGR from mean values of SMC and leaf attributes.Important findings All the 11 attributes were interrelated and differed significantly among the 10 saplings. Species response varied across sites and seasons. Across the SMC gradient, the attributes showed variable plasticity that differed across species. Among the 10 saplings, the highly deciduous Boswellia serrata showed the maximum plasticity in seven functional attributes. According to the step-wise multiple regressions, 65% variability in RDI and 67% variability in RGR were due to Gs mass, and for RHI, 61% variability was due to A mass. SMC and the other attributes, viz ., SLA, Chl, WUEi and LDMC in combination could contribute only for ~2–6% of the variability in RDI, RHI and RGR, which indicates that other traits/factors, not accounted in this study are also important in modulating the growth of tree saplings in TDFs. In conclusion, growth of the tree saplings in the tropical dry environment is determined by soil moisture, whereas the response of saplings of different tree species is modulated by alterations in key functional attributes such as SLA, Chl, WUEi and LDMC.     

Host-specificity of folivorous insects in a moist tropical forest

1. To assess the degree of herbivore host-specificity in the moist tropical forest on Barro Colourado Island, Panama, I conducted an extensive series of feeding trials on the common insect herbivores from 10 tree species.
2. The herbivores were offered leaves from both congeneric and confamilial plant species to their known host species, as well as leaves from the most abundant tree species in the forest.
3. The amount of damage caused by these herbivores to young, expanding leaves was also measured on nine of the tree species.
4. Of 46 herbivores species (seven Coleoptera, one Orthoptera, 38 Lepidoptera), 26% were specialized to a single plant species, 22% were limited to feeding on a single genus and 37% were able to feed on several genera within a single family. The remaining 15% were generalists, able to feed from several different plant families.
5. The causes of leaf damage varied extensively across the tree species. On average, specialist herbivores caused 58% of the damage to young leaves, generalists herbivores 8% and fungal pathogens 34%. For four of the tree species, pathogens were the most important cause of leaf damage.
6. In this forest, most chewing herbivores appear to have fairly narrow diets, and these specialists are responsible for most of the insect herbivory.     

Identifying forest‐obligate birds in tropical moist cloud forest of Andean Ecuador

ABSTRACT Large‐scale transformation of forested landscapes is a major factor in loss of biological diversity in the American tropics. Investigators examining the responses of species to deforestation rarely control for variation in the amount of forest relative to other habitats at the landscape‐level. Bellavista Reserve on the western slope of the Andes in Ecuador is located between similar‐sized areas of pristine, protected forest, and deforested landscapes. We used strip‐transect counts and mist netting to evaluate habitat use by passerine birds in a habitat mosaic consisting of abandoned pastures, forest edges, forest fragments, and large blocks of interior tropical montane cloud forest (TMCF). During 3600 net hours, we had 1476 captures, including 346 recaptures. Of 78 species captured in mist nets, 30 had sufficient counts for Poison Rate Regression (PRR) modeling (a statistical method for comparing counts). Twelve species (40%) had capture patterns indicative of an affinity for mature TMCF, and 6 species (20%) had significantly higher counts in degraded areas (forest edge, forest fragment, and regenerating pastures) than in interior TMCF. The remaining 40% showed no significant bias in detection among habitats. Combined with strip‐count data, our results suggest that about 38% of the 119 species sampled at the Bellavista Reserve occur primarily in mature TMCF, avoiding edges and early second‐growth forest. Populations of these species may be vulnerable to further loss, fragmentation, and degradation of TMCF and, as such, deserve additional study and a place on lists of species of conservation concern.     

Oxygen isotope ratio stratification in a tropical moist forest

Summary Oxygen isotope ratios were determined in leaf cellulose from two plant species at Barro Colorado (Republic of Panama) in 4 different plots, two of which were undergoing an irrigation treatment during the dry season. There is a gradient in ¹⁸O values of leaf cellulose from the understory to canopy leaves, reflecting the differences in relative humidity between these two levels of the forest. This gradient is most pronounced in irrigated plots. For irrigated plots there was a highly significant correlation between ¹⁸O and ¹³C values, which was not observed in control plots. This relationship can be explained by humidity controlling stomatal conductance. Low humidity affects ¹⁸O values of leaf water during photosynthesis, which isotopically labels cellulose during its synthesis. Low humidity also decreases stomatal conductance, which affects discrimination against carbon-13 by photosynthetic reactions, thus affecting the ¹³C values of photosynthates. WUE values calculated by using plant carbon and oxygen isotope ratios were similar to those observed with gas exchange measurements in other tropical and temperate area. Thus the concurrent analysis of carbon and oxygen isotope ratios of leaf material can potentially be useful for long term estimation of assimilation and evapotranspiration regimes of plants.     

Root biomass distribution in a moist tropical montane forest

Effects of N sources (ammonium, nitrate and ammonitrate) and VA mycorrhizae (Glomus intraradices) on rhizosphere soil characteristics (pH, exchangeable acidity, exchangeable cations, inorganic N concentrations) growth and nutrient acquisition of coffee seedlings (Coffea arabica L. cv guatemala) were investigated in a pot study with an acid soil (Red Bluff Loam) sterilized by autoclaving. Ammonium addition decreased rhizosphere pH while nitrate and ammonitrate additions both increased rhizosphere pH. Mycorrhizae induced a higher pH, a lower exchangeable acidity and higher values of exchangeable cations in the rhizosphere. Ammonium addition resulted in a lower mycorrhizal infection than the two other N sources. Mycorrhizal plants grew better and accumulated more N, Ca and Mg than non-mycorrhizal plants.     

Density and distance-to-adult effects of a canker disease of trees in a moist tropical forest

We compared the spatial distribution of stem cankers on the canopy tree Ocotea whitei (Lauraceae) in a 20-ha plot on Barro Colorado Island, Panama, with spatial and temporal patterns of mortality in this host over the previous decade. The cankers occur both on adult and juvenile individuals, aothough juveniles are much more likely the adults to show symptoms. Disease incidence is host-density dependent, and both the presence of the disease and host mortality are more likely close to than far from a conspecific adult, which resulted in a net spatial shift of the juvenile population away from conspecific adults through time. Disease incidence is lower than expected among juveniles of O. whitei growing near to adults of the non-susceptible canopy tree Beilschmiedia pendula. The coincidence of spatial patterns of canker incidence and host mortality suggest a role for the disease in regulating host spatial distribution, in agreement with predictions of the Janzen-Connell hypothesis.     

Cascading effects of belowground predators on plant communities are density‐dependent

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density‐mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density‐dependent population change in predators, that is, an increase in low‐density treatments, but a decrease in high‐density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density‐dependent cascading effects on detritivore and soil microbial communities.     

Spatio‐temporal dynamics of density‐dependent dispersal during a population colonisation

Predicting population colonisations requires understanding how spatio‐temporal changes in density affect dispersal. Density can inform on fitness prospects, acting as a cue for either habitat quality, or competition over resources. However, when escaping competition, high local density should only increase emigration if lower‐density patches are available elsewhere. Few empirical studies on dispersal have considered the effects of density at the local and landscape scale simultaneously. To explore this, we analyze 5 years of individual‐based data from an experimental introduction of wild guppies Poecilia reticulata. Natal dispersal showed a decrease in local density dependence as density at the landscape level increased. Landscape density did not affect dispersal among adults, but local density‐dependent dispersal switched from negative (conspecific attraction) to positive (conspecific avoidance), as the colonisation progressed. This study demonstrates that densities at various scales interact to determine dispersal, and suggests that dispersal trade‐offs differ across life stages.     

Facilitation among plants can accelerate density‐dependent mortality and steepen self‐thinning lines in stressful environments

The speed and slope of plant self‐thinning are all affected by plant–plant interactions across environmental gradients. Possible mechanisms driving the self‐thinning dynamics include the relative strength of root versus shoot competition, and the interplay between competition and facilitation. Although these mechanisms often act in concert, their relative importance has not yet been fully explored. We used both a one‐layer and a two‐layer zone‐of‐influence (ZOI) model to examine how competition and facilitation drive self‐thinning across stress gradients. As a development of the traditional ZOI model, the two‐layer version explicitly models shoot and root growth and neighbor interactions, and thus the overall size‐symmetry of competition is regulated by the relative strength of root versus shoot competition. One‐layer model simulations revealed that increasingly asymmetric competition accelerated thinning, and steepened (slope ranged from about –1 to –4/3) and lowered self‐thinning lines. Stress slowed down density‐dependent mortality considerably when competition was not completely symmetric. Stress significantly decreased the self‐thinning intercept, while facilitation simply counteracted stress effects. Both stress and facilitation showed little effect on the slope. In the two‐layer model, both stress and facilitation affected mortality in the same way as in the one‐layer version when competition was not completely symmetric. Different from the one‐layer model, the two‐layer version showed that the effects of stress and facilitation on the self‐thinning slope were mediated by the asymmetry of competition. As stress increased, the overall asymmetry of competition shifted from asymmetric to symmetric due to increased relative strength of root competition. High stress thus dramatically flattened self‐thinning lines, whereas the inclusion of facilitation counteracted stress and led to steeper self‐thinning lines. Our two‐layer model is based on the current knowledge of plant–plant interactions, and better represents ecological realities. It can help elaborate experiments for testing the role of competition and facilitation in driving plant population dynamics.     

Asymmetric facilitation can reduce size inequality in plant populations resulting in delayed density‐dependent mortality

Size inequality in plant populations is a ubiquitous feature that has received much attention due to ecological and evolutionary implications. The mechanisms driving size inequality were mainly attributed to different modes of competition (symmetric versus asymmetric), while the potential effects of different modes of facilitation (symmetric versus asymmetric) to this pattern have not yet been fully explored. We employed an individual‐based model to explore the relative roles of both competition and facilitation simultaneously along an environmental stress gradient. Special emphasis was given to the assessment of symmetric facilitation (plants receive benefit from each other equally or proportionally to benefactors’ sizes) and asymmetric facilitation (beneficiary plants receive benefits from benefactor plants that are higher than proportional to the benefactors’ size) in altering plant size inequality. We found that independent of the particular mode of competition, symmetric facilitation generally increased size inequality, whereas asymmetric facilitation decreased it. This pattern was consistent along the stress gradient. Because of their different effects on size inequality, symmetric facilitation accelerated self‐thinning, whereas asymmetric facilitation delayed the onset of density‐dependent mortality, promoting survival under intermediate stress conditions. We compared our model predictions with both 1) a previous modelling study focusing on the effect of (symmetric) facilitation on the size inequality, and 2) re‐analysed data from a published experiment generating asymmetric facilitation of plants against enhanced ultraviolet‐B (UV‐B). Whereas our model predictions and the results of the empirical experiment were consistent, we found that previous theoretical results that solely relied on symmetric facilitation need to be re‐adjusted. Our study showed that combinations of different modes of competition and facilitation can alter size inequality in different ways and with important consequences for the onset of density‐dependent mortality during population development. Explicitly considering different modes and mechanisms of interactions (both facilitation and competition) will improve mechanistic understanding in plant ecology.     

Facilitation can maintain clustered spatial pattern of plant populations during density‐dependent mortality: insights from a zone‐of‐influence model

Recent years have seen a growing body of evidence showing that plant competition and facilitation usually operate simultaneously to drive population dynamics, community structure and ecosystem functions. However, the potential role of facilitation in spatial patterning of plant populations has rarely been explicitly examined. We used a ‘zone‐of‐influence’ model to explore how facilitation interacts with competition and abiotic stress to determine the spatial patterning of populations during density‐dependent mortality. Model simulations revealed that started with the same clustered pattern, the final pattern of simulated populations depended strongly on the interaction among facilitation, stress level and size‐symmetry of competition. Asymmetric competition consistently led to immediate and non‐random mortality towards regularity, thus rapidly decayed the initially clustered pattern to final patterns of small‐scale regularity and large‐scale randomness. The role of symmetric competition in decaying the clustered pattern increased with abiotic stress because stress‐induced reductions in plants’ growth rates can make individuals in high‐density clusters more likely to die even from symmetric competition. Facilitation played a clear role in counteracting the effect of stress, thus tended to maintain the degree of clustering of the pattern during density‐dependent mortality. This is because the amelioration of harsh conditions by neighboring plants relieved the reductions in plant growth due to competition, thus slowed down and reduced the mortality inside clusters (relative to that outside clusters). Moreover, the effect of facilitation appeared to increase with abiotic stress. Our results indicate that facilitation among neighboring plants should partially be responsible for clustered population spatial patterns observed in stressful environments, even though its contribution relative to other factors (e.g. local dispersal and environmental heterogeneity) remains to be evaluated. In addition, the potential influence of facilitation on self‐thinning trajectory should be explicitly examined in future modeling and experimental studies considering its effects on density‐dependent mortality.     

Scale‐dependent spatial patterns in benthic communities around a tropical island seascape

Understanding and predicting patterns of spatial organization across ecological communities is central to the field of landscape ecology, and a similar line of inquiry has begun to evolve sub‐tidally among seascape ecologists. Much of our current understanding of the processes driving marine community patterns, particularly in the tropics, has come from small‐scale, spatially‐discrete data that are often not representative of the broader seascape. Here we expand the spatial extent of seascape ecology studies and combine spatially‐expansive in situ digital imagery, oceanographic measurements, spatial statistics, and predictive modeling to test whether predictable patterns emerge between coral reef benthic competitors across scales in response to intra‐island gradients in physical drivers. We do this around the entire circumference of a remote, uninhabited island in the central Pacific (Jarvis Island) that lacks the confounding effects of direct human impacts. We show, for the first time, that competing benthic groups demonstrate predictable scaling patterns of organization, with positive autocorrelation in the cover of each group at scales < ~1 km. Moreover, we show how gradients in subsurface temperature and surface wave power drive spatially‐abrupt transition points in group dominance, explaining 48–84% of the overall variation in benthic cover around the island. Along the western coast, we documented ten times more sub‐surface cooling‐hours than any other part of the coastline, with events typically resulting in a drop of 1–4°C over a period of < 5 h. These high frequency temperature fluctuations are indicative of upwelling induced by internal waves and here result in localized nitrogen enrichment (NO₂ + NO₃) that promotes hard coral dominance around 44% of the island's perimeter. Our findings show that, in the absence of confounding direct human impacts, the spatial organization of coral reef benthic competitors are predictable and somewhat bounded across the seascape by concurrent gradients in physical drivers.