Untangling positive and negative biotic interactions: views from above and below ground in a forest ecosystem

In ecological communities, the outcome of plant-plant interactions represents the net effect of positive and negative interactions occurring above and below ground. Untangling these complex relationships can provide a better understanding of mechanisms that underlie plant-plant interactions and enhance our ability to predict population, community, and ecosystem effects of biotic interactions. In forested ecosystems, tree seedlings interact with established vegetation, but the mechanisms and outcomes of these interactions are not well understood. To explore such mechanisms, we manipulated above- and belowground interactions among tree seedlings, shrubs, and trees and monitored seedling survival and growth of six species (Pinus banksiana, Betula papyrifera, P. resinosa, Quercus rubra, P. strobus, and Acer rubrum) in mature pine-dominated forest in northern Minnesota, USA. The forest had a moderately open canopy and sandy soils. Understory manipulations were implemented in the forest interior and in large gaps and included removal of shrubs (no interactions), tieback of shrubs (belowground), removal of shrubs with addition of shade (aboveground), and unmanipulated shrubs (both below- and aboveground). We found that shrubs either suppressed or facilitated seedling survival and growth depending on the seedling species, source of interaction (e.g., above- or belowground), and ecological context (e.g., gap or forest interior). In general, shrubs strongly influenced survival and growth in gaps, with more modest effects in the forest interior. In gaps, the presence of shrub roots markedly decreased seedling growth and survival, supporting the idea that belowground competition may be more important in dry, nutrient-poor sites. Shrub shade effects were neutral for three species and facilitative for the other three. Facilitation was more likely for shade-tolerant species. In the forest interior, shrub shade negatively affected seedling survival for the most shade-intolerant species. For several species the net effect of shrubs masked the existence of both positive and negative interactions above and below ground. Our results highlight the complexity of plant-plant interactions, demonstrate that outcomes of these interactions vary with the nature of resource limitation and the ecophysiology of the species involved, and suggest that ecological theory that rests on particular notions of plant-plant interactions (e.g., competition) should consider simultaneous positive and negative interactions occurring above and below ground.     

林窗大小和位置对丽江云杉自然更新幼苗存活和生长的影响

通过 1997～ 2 0 0 0年对 32个林窗的连续观测调查 ,研究了滇西北玉龙雪山自然保护区云杉坪亚高山丽江云杉林林窗大小和林窗位置对自然更新幼苗存活和生长的影响 .4个生长季节的观测结果表明 :林窗与林下非林窗内的幼苗大小和幼苗存活数量差异明显 .林窗由小到大 ,单位面积内的自然更新苗木数量逐渐减少 ,小林窗中更新苗为大林窗的 5倍左右 ,而林下的更新苗很少 ,0 .2ind .10m-2 .中等林窗和小林窗内的幼苗数量在从南到中心到北的位置上几乎没有明显的差异 ;大林窗中存在由南到北的位置差异 ,更新幼苗数量逐渐减少 .从更新幼苗的生长来看 ,中等林窗内的幼苗 ,高度最大、生长最快 ,定居阶段的平均年高生长为 (7.1± 0 .5 )cma-1,小林窗次之 ,大林窗和林下幼苗个体最小 ,生长最慢 .进一步从林窗位置来看 ,中、小林窗幼苗大小几无位置差异 ,大林窗则由南到北 ,幼苗由大变小 .从幼苗存活数量和大小来看 ,中等林窗大小是丽江云杉幼苗更新的适宜面积 ,对该类型退化亚高山针叶林恢复提供了一定的参考 .图 4表 2参 2 3     

Life history trade-offs in tropical trees and lianas

It has been hypothesized that tropical trees partition forest light environments through a life history trade-off between juvenile growth and survival; however, the generality of this trade-off across life stages and functional groups has been questioned. We quantified trade-offs between growth and survival for trees and lianas on Barro Colorado Island (BCI), Panama using first-year seedlings of 22 liana and 31 tree species and saplings (10 mm < dbh < 39 mm) of 30 tree species. Lianas showed trade-offs similar to those of trees, with both groups exhibiting broadly overlapping ranges in survival and relative growth rates as seedlings. Life history strategies at the seedling stage were highly correlated with those at the sapling stage among tree species, with all species showing an increase in survival with size. Only one of 30 tree species demonstrated a statistically significant ontogenetic shift, having a relatively lower survival rate at the sapling stage than expected. Our results indicate that similar life history trade-offs apply across two functional groups (lianas and trees), and that life history strategies are largely conserved across seedling and sapling life-stages for most tropical tree species.     

Widespread density-dependent seedling mortality promotes species coexistence in a highly diverse Amazonian rain forest

Negative density-dependent mortality can promote species coexistence through a spacing mechanism that prevents species from becoming too locally abundant. Negative density-dependent seedling mortality can be caused by interactions among seedlings or between seedlings and neighboring adults if the density of neighbors affects the strength of competition or facilitates the attack of natural enemies. We investigated the effects of seedling and adult neighborhoods on the survival of newly recruited seedlings for multiple cohorts of known age from 163 species in Yasuni National Park, Ecuador, an ever-wet, hyper-diverse lowland Amazonian rain forest. At local scales, we found a strong negative impact on first-year survival of conspecific seedling densities and adult abundance in multiple neighborhood sizes and a beneficial effect of a local tree neighborhood that is distantly related to the focal seedling. Once seedlings have survived their first year, they also benefit from a more phylogenetically dispersed seedling neighborhood. Across species, we did not find evidence that rare species have an advantage relative to more common species, or a community compensatory trend. These results suggest that the local biotic neighborhood is a strong influence on early seedling survival for species that range widely in their abundance and life history. These patterns in seedling survival demonstrate the role of density-dependent seedling dynamics in promoting and maintaining diversity in understory seedling assemblages. The assemblage-wide impacts of species abundance distributions may multiply with repeated cycles of recruitment and density-dependent seedling mortality and impact forest diversity or the abundance of individual species over longer time scales.     

Neighborhood and community interactions determine the spatial pattern of tropical tree seedling survival

Factors affecting survival and recruitment of 3531 individually mapped seedlings of Myristicaceae were examined over three years in a highly diverse neotropical rain forest, at spatial scales of 1-9 m and 25 ha. We found convincing evidence of a community compensatory trend (CCT) in seedling survival (i.e., more abundant species had higher seedling mortality at the 25-ha scale), which suggests that density-dependent mortality may contribute to the spatial dynamics of seedling recruitment. Unlike previous studies, we demonstrate that the CCT was not caused by differences in microhabitat preferences or life history strategy among the study species. In local neighborhood analyses, the spatial autocorrelation of seedling survival was important at small spatial scales (1-5 m) but decayed rapidly with increasing distance. Relative seedling height had the greatest effect on seedling survival. Conspecific seedling density had a more negative effect on survival than heterospecific seedling density and was stronger and extended farther in rare species than in common species. Taken together, the CCT and neighborhood analyses suggest that seedling mortality is coupled more strongly to the landscape-scale abundance of conspecific large trees in common species and the local density of conspecific seedlings in rare species. We conclude that negative density dependence could promote species coexistence in this rain forest community but that the scale dependence of interactions differs between rare and common species.     

Seed predation by Neotropical rain forest mammals increases diversity in seedling recruitment

Seed dispersal and seedling recruitment (the transition of seeds to seedlings) set the spatiotemporal distribution of new individuals in plant communities. Many terrestrial rain forest mammals consume post-dispersal seeds and seedlings, often inflicting density-dependent mortality. In part because of density-dependent mortality, diversity often increases during seedling recruitment, making it a critical stage for species coexistence. We determined how mammalian predators, adult tree abundance, and seed mass interact to affect seedling recruitment in a western Amazonian rain forest. We used exclosures that were selectively permeable to three size classes of mammals: mice and spiny rats (weighing <1 kg), medium-sized rodents (1-12 kg), and large mammals (20-200 kg). Into each exclosure, we placed seeds of 13 tree species and one canopy liana, which varied by an order of magnitude in adult abundance and seed mass. We followed the fates of the seeds and resulting seedlings for at least 17 months. We assessed the effect of each mammalian size class on seed survival, seedling survival and growth, and the density and diversity of the seedlings that survived to the end of the experiment. Surprisingly, large mammals had no detectable effect at any stage of seedling recruitment. In contrast, small- and medium-sized mammals significantly reduced seed survival, seedling survival, and seedling density. Furthermore, predation by small mammals increased species richness on a per-stem basis. This increase in diversity resulted from their disproportionately intense predation on common species and large-seeded species. Small mammals thereby generated a rare-species advantage in seedling recruitment, the critical ingredient for frequency dependence. Predation by small (and to a lesser extent, medium-sized) mammals on seeds and seedlings significantly increases tree species diversity in tropical forests. This is the first long-term study to dissect the effects of various mammalian predators on the recruitment of a diverse set of tree species.     

Common ectomycorrhizal networks may maintain monodominance in a tropical rain forest

Most tropical rain forests contain diverse arrays of tree species that form arbuscular mycorrhizae. In contrast, the less common monodominant rain forests, in which one tree species comprises more than 50% of the canopy, frequently contain ectomycorrhizal (ECM) associates. In this study, I explored the potential for common ECM networks, created by aggregations of ECM trees, to enhance seedling survivorship near parent trees. I determined the benefit conferred by the common ECM network on seedling growth and survivorship of an ECM monodominant species in Guyana. Seedlings with access to an ECM network had greater growth (73% greater), leaf number (55% more), and survivorship (47% greater) than seedlings without such access, suggesting that the ECM network provides a survivorship advantage. A survey of wild seedlings showed positive distance-dependent distribution and survival with respect to conspecific adults. These experimental and survey results suggest that the negative distance-dependent mechanisms at the seedling stage thought to maintain tropical rain forest diversity are reversed for ECM seedlings, which experience positive feedbacks from the ECM network. These results may in part explain the local monodominance of an ECM tree species within the matrix of high-diversity, tropical rain forest.     

Are trade-offs in plant resprouting manifested in community seed banks?

Trade-offs in allocation to resprouting vs. seedling regeneration in plants are predicted to occur along fire disturbance gradients. Increased resprouting ability should be generally favored in plant communities with a high probability of crown fire return. Hence, communities dominated by resprouters are predicted to have smaller seed banks than those dominated by species killed by fire. We tested whether there were trait shifts in resprouting ability among growth forms (short-lived herbaceous vs. ground-dwelling perennials vs. shrubs) and among communities (rocky outcrop vs. sclerophyll forest) with contrasting probabilities of crown fire return. Resprouting was more common in ground-dwelling perennials and in the sclerophyll forest community with a high probability of crown fire. Soil seed banks were sampled in rocky outcrop and sclerophyll forest communities in recently burned (18 months) and long-since-burned (12 years) locations at interspersed replicated sites. Collected seed banks were treated with orthogonal treatments of fire stimuli or no stimuli, and seedling emergence was measured in controlled conditions. Seed bank composition reflected the pattern of extant vegetation, with resprouting species being more common in the community with a higher probability of crown fire. Overall, however, resprouting species were poorly represented in the seed bank compared to those species killed by fire. Predicted shifts in allocation to seed production were strongly manifested in community seed banks across the disturbance gradient. Fewer species, seedlings, and seedlings per adult emerged from seed banks in the sclerophyll forest. This suggests that the dominance of resprouting species influences recruitment at the community scale. Community patterns in the seed bank also reflected predicted trade-offs with plant size and growth rate. Short-lived species that are killed by fire dominated the seed bank on rocky outcrops, while longer-lived resprouting species were found in low abundance. Life history trade-offs in persistence and regeneration strongly contribute to coexistence patterns between and within communities with contrasting probabilities of fire return.     

Multigenerational genetic analysis of tropical secondary regeneration in a canopy palm

Iriartea deltoidea (Arecaceae) is an abundant canopy palm with a wide geographic distribution in Neotropical wet forests. We analyzed the genetic profile across three generations of Iriartea within a 43-ha area encompassing two areas of second-growth and adjoining old-growth forest at La Selva Biological Field Station in northeastern Costa Rica. A total of 311 reproductively mature trees, 99 large saplings, 207 small saplings, and 601 seedlings were genotyped using 141 AFLP loci. Parentage analysis revealed high dispersal distances, both for seed (over 2.3 km) and pollen (over 3.8 km), indicating a large genetic neighborhood within La Selva Biological Station. In a 20-ha area of second growth, the founding palm population was dominated by a small number of parental trees located in the adjacent old-growth forest; two old-growth trees contributed 48% of the second-growth genes. The genetic diversity of reproductively mature trees in this second-growth forest was significantly reduced compared to adjacent old-growth forest. Within 400 m of the border with old-growth forest, we observed a similar reduction of genetic diversity in saplings, and an even greater loss of genetic diversity in the second generation of seedlings. Nearly half of these seedlings were offspring of local parents. In contrast, in the distant portion of second-growth forest (400-800 m from the old-growth border), parentage analysis showed that 40% of seedlings originated from outside the study area and only 10% were offspring of local parents. These high levels of gene flow maintained genetic diversity in saplings and seedlings similar to levels observed in old-growth forest. Our findings highlight the importance of gene flow from diverse seed and pollen sources for sustaining levels of genetic diversity of tree populations in second-growth forests.     

Invasibility and Effects of Amur Honeysuckle in Southwestern Ohio Forests

The Asian exotic Amur honeysuckle ( Lonicera maackii [Rupr.] Herder) has become the dominant shrub in many forests in southwestern Ohio and in some other locations in the eastern United States. Our research focused on the invasibility of forest communities and relationships of L. maackii to the abundance of tree seedlings and herbs. We surveyed 93 forest stands near Oxford, Ohio (USA) to determine L. maackii cover, time since invasion, tree canopy cover, tree basal area, and a shade tolerance index. Stepwise multiple regression indicated that greater than one-half of the variation (r ² = 0.550) in Lonicera cover was correlated to five variables (in descending order of importance): tree canopy cover, distance from Oxford, shade tolerance index, tree basal area, and time since invasion. The results suggest that high light levels and proximity to an abundant seed source increase forest invasibility. Tree seedling density, species richness of seedlings, and herb cover were all inversely related to L. maackii cover. When Lonicera becomes abundant, future structure and composition of forests could be affected and local populations of herbs threatened.     

Hotspots of Epiphytic Lichen Diversity in Two Young Managed Forests

Understanding within-stand variation in diversity of epiphytes will provide an improved basis for producing timber while conserving biological diversity. Two 80-ha, 50–year–old managed stands of conifers were surveyed to locate 0.4 ha putative "diversity" plots, the areas appearing most diverse in lichen epiphytes. These plots were generally located in areas made heterogeneous by canopy gaps, wolf trees (trees with large-diameter lower branches), and old-growth remnant trees. "Matrix" plots, in contrast, were chosen at random from the remaining, more homogenous forest. Diversity plots hosted from 25% to 40% more epiphytic lichen species than matrix plots in both stands. The strongest within-stand gradients in species composition were correlated with percentage of plot occupied by gaps and wolf trees. Percentage of the plot in gaps was correlated with species richness (r = 0.79). In the more structurally diverse stand, diversity and abundance of nitrogen-fixing "cyanolichens" were correlated with percentage of the plot occupied by gaps (0.5 < r < 0.9), and alectorioid lichens were correlated with percentage of the plot occupied by old-growth remnant trees (0.5 < r < 0.6). In the stand with more homogenous structure, percentage of the plot under gaps was correlated with regionally common species that were otherwise absent or sparse in the matrix. Protecting gaps, hardwoods, wolf trees, and old-growth remnant trees during thinning or other partial cutting is likely to promote the majority of epiphytic macrolichens in young conifer forests. Because these features are easily recognized on aerial photos and on the ground by land managers, it is practical to manage for forest structures that would promote lichen diversity.     

岷江干旱河谷灌木幼苗的土壤养分适应性研究

岷江上游干旱河谷地区的植被恢复是目前急需解决的问题,小马鞍羊蹄甲（Bauhinia faberi var. microphylla）是干旱河谷地区的优势乡土灌木,并作为当地植被恢复过程中的重要备选物种,因而进行小马鞍羊蹄甲种群在土壤贫瘠的干旱河谷的养分限制研究十分必要。针对性地进行施肥将影响到小马鞍羊蹄甲幼苗存活、生长和定殖,而如何有效提高该物种的存活和生长速率对于干旱河谷植被恢复将具有重要的意义。采用野外调查与模拟实验相结合的方法,分别研究了干旱河谷地区3个典型群落（干旱河谷的核心区——飞虹,干旱河谷灌丛与亚高山森林的过渡区——北部的石大关、干旱河谷灌丛与温性森林的过渡区——南部的蓝新镇）中小马鞍羊蹄甲幼苗叶片的化学计量特征,和施肥试验[w（有效氮）分别为100、280、460 mg·kg–1、w（有效磷）分别为12、24、48 mg·kg–1、w（氮）分别为40、70、100 mg·kg–1、w（磷）分别为12和24 mg·kg–1]中叶片化学计量特征及幼苗生长参数（叶片数、基径和株高）和各器官（根茎叶）生物量。结果表明：在野外各演替阶段的小马鞍羊蹄甲幼苗生物量和营养元素质量分数都随着磷肥的增加而增加,表明磷素是植物生长的限制因子,同时N∶P比均大于16,也暗示其受到P养分的限制;在室内施肥试验中,施加N肥没有促进小马鞍羊蹄甲生物量的积累,反而抑制了幼苗生长;施加P肥促进了幼苗生物量的积累,表明幼苗缺乏P元素,养分限制类型为P限制。     

Distinct mycorrhizal communities on new and established hosts in a transitional tropical plant community

The extent to which interspecific plants share mycorrhizal fungal communities depends on the specificity of the symbiosis. For tropical forest tree seedlings, colonization by mycorrhizal fungi associated with established vegetation could have important consequences for survival and growth. I used a novel molecular technique to assess the potential for sharing of mycorrhizas in forest and pasture in southern Costa Rica, by identifying arbuscular mycorrhizal (AM) fungi in roots of the forest canopy tree species Terminalia amazonia, pasture grasses Urochloa ruziziensis and U. decumbens, and seedlings of T. amazonia planted into experimental reforestation plots. I tested the hypotheses that experimental seedlings were colonized either by the AM fungal community of the forest T. amazonia (suggesting host specificity) or of Urochloa (suggesting absence of specificity/importance of local environment). After two years, pasture-grown T. amazonia seedlings were colonized by neither community, but rather by a species of Glomus that was rarely observed on the other plants. These results suggest that conspecific seedlings planted into existing vegetation generate a distinct mycorrhizal community that may influence competitive interactions and the relative costs and benefits of the AM fungal symbiosis at early stages in the life cycle of tropical trees.     

Plant-Mammal Interactions in Tropical Bolivian Forests with Different Hunting Pressures

Abstract: We studied plant-animal interactions and vegetation structure in two geographically close tropical Bolivian forests subjected to different hunting intensities. We hypothesized that reduction of mammals openface> 1 kg in an "intensively hunted forest," compared with an "occasionally hunted forest," should correlate with decreased seed predation and seedling trampling, increased seedling survival and density, and decreased tree-species diversity at the seedling stage in relation to the adult stage. The occasionally hunted forest held 1.7 times as many mammalian species as the intensively hunted forest. As predicted, predation of Astrocaryum murumuru seeds was 34.2% lower in the intensively hunted forest. Similarly, trampling of model seedlings was 5.4 times lower and seedling survival was 1.15 times greater in the intensively hunted forest than that in the occasionally hunted forest. But the intensively hunted forest displayed lower seedling densities and a higher ratio of seedling diversity to tree diversity than did the occasionally hunted forest. Reduction of peccaries from the intensively hunted forest may explain much of the between-site differences in seed predation, trampling, and seedling survival. Lack of consistent differences in seedling density and diversity could mean that reduced granivory and trampling may be counteracted by reduced seed dispersal. The reduction of mammalian populations may produce a complex mosaic of forest patches with distinct degrees of structural change, depending on the intensity of defaunation.     

Long-term CO2 enrichment of a forest ecosystem: implications for forest regeneration and succession.

The composition and successional status of a forest affect carbon storage and net ecosystem productivity, yet it remains unclear whether elevated atmospheric carbon dioxide (CO2) will impact rates and trajectories of forest succession. We examined how CO2 enrichment (+200 microL CO2/L air differential) affects forest succession through growth and survivorship of tree seedlings, as part of the Duke Forest free-air CO2 enrichment (FACE) experiment in North Carolina, USA. We planted 2352 seedlings of 14 species in the low light forest understory and determined effects of elevated CO2 on individual plant growth, survival, and total sample biomass accumulation, an integrator of plant growth and survivorship over time, for six years. We used a hierarchical Bayes framework to accommodate the uncertainty associated with the availability of light and the variability in growth among individual plants. We found that most species did not exhibit strong responses to CO2. Ulmus alata (+21%), Quercus alba (+9.5%), and nitrogen-fixing Robinia pseudoacacia (+230%) exhibited greater mean annual relative growth rates under elevated CO2 than under ambient conditions. The effects of CO2 were small relative to variability within populations; however, some species grew better under low light conditions when exposed to elevated CO2 than they did under ambient conditions. These species include shade-intolerant Liriodendron tulipifera and Liquidambar styraciflua, intermediate-tolerant Quercus velutina, and shade-tolerant Acer barbatum, A. rubrum, Prunus serotina, Ulmus alata, and Cercis canadensis. Contrary to our expectation, shade-intolerant trees did not survive better with CO2 enrichment, and population-scale responses to CO2 were influenced by survival probabilities in low light. CO2 enrichment did not increase rates of sample biomass accumulation for most species, but it did stimulate biomass growth of shade-tolerant taxa, particularly Acer barbatum and Ulmus alata. Our data suggest a small CO2 fertilization effect on tree productivity, and the possibility of reduced carbon accumulation rates relative to today's forests due to changes in species composition.     

Multidimensional trade-offs in species responses to disturbance: implications for diversity in a subtropical forest

Species employ diverse strategies to cope with natural disturbance, but the importance of these strategies for maintaining tree species diversity in forests has been debated. Mechanisms that have the potential to promote tree species coexistence in the context of repeated disturbance include life history trade-offs in colonization and competitive ability or in species' ability to survive at low resource conditions and exploit the temporary resource-rich conditions often generated in the wake of disturbance (successional niche). Quantifying these trade-offs requires long-term forest monitoring and modeling. We developed a hierarchical Bayes model to investigate the strategies tree species employ to withstand and recover from hurricane disturbance and the life history trade-offs that may facilitate species coexistence in forests subject to repeated hurricane disturbance. Unlike previous approaches, our model accommodates temporal variation in process error and observations from multiple sources. We parameterized the model using growth and mortality data from four censuses of a 16-ha plot taken every five years (1990-2005), together with damage data collected after two hurricanes and annual seed production data (1992-2005). Species' susceptibilities to hurricane damage as reflected by changes in diameter growth and fecundity immediately following a storm were weak, highly variable, and unpredictable using traditional life history groupings. The lower crowding conditions (e.g., high light) generated in the wake of storms, however, led to greater gains in growth and fecundity for pioneer and secondary-forest species than for shade-tolerant species, in accordance with expectation of life history. We found moderate trade-offs between survival in high crowding conditions, a metric of competitive ability, and long-distance colonization. We also uncovered a strong trade-off between mean species fecundity in low crowding conditions, a metric of recovery potential, and competitive ability. Trade-offs in competitive and colonization ability, in addition to successional niche processes, are likely to contribute to species persistence in these hurricane-impacted forests. The strategies species employ to cope with hurricane damage depend on the degree to which species rely on sprouting, repair of adult damage, changes in demographic rates in response to enhanced resource availability after storms, or long-distance dispersal as recovery mechanisms.     

Canopy Seed Banks as Time Capsules of Biodiversity in Pasture-Remnant Tree Crowns

Abstract: Tropical pastures present multiple barriers to tree regeneration and restoration. Relict trees serve as “regeneration foci” because they ameliorate the soil microclimate and serve as safe spots for dispersers. Here, we describe another mechanism by which remnant trees may facilitate pasture regeneration: the presence of seed banks in the canopy soil that accumulates from decomposing epiphytes within the crowns of mature remnant trees in tropical cloud forest pastures. We compared seed banks of canopy soils (histosols derived from fallen leaves, fruits, flower, and twigs of host trees and epiphytes, dead bryophytes, bark, detritus, dead animals, and microorganisms, and dust that accumulate on trunks and the upper surfaces of large branches) in pastures, canopy soils in primary forest trees, and soil on the forest floor in Monteverde, Costa Rica. There were 5211 epiphytic and terrestrial plant seeds in the three habitats. All habitats were dominated by seeds in a relatively small number of plant families, most of which were primarily woody, animal pollinated, and animal dispersed. The density of seeds on the forest floor was greater than seed density in either pasture‐canopy or forest‐canopy soils; the latter two did not differ. Eight species in 44 families and 61 genera from all of the habitats were tallied. There were 37 species in the pasture‐canopy soil, 33 in the forest‐canopy soil, and 57 on the forest floor. Eleven species were common to all habitats. The mean species richness in the pasture canopy was significantly higher than the forest canopy (F =83.38; p < 0.02). Nonmetric multidimensional scaling ordination revealed that the communities were distinct. Greenhouse experiments verified that many of these seeds were viable, with 29 taxa germinating (23 taxa in pruned mats [mimic of exposed conditions] and 16 taxa in control mats [intact conditions]) within 2 months of observation. Nearly half the species that germinated were characteristic of primary forests (primary forest samples, 19%; pasture samples, 29%). This supports the idea that canopy seed banks of pasture trees can function as time capsules by providing propagules that are removed in both space and time from the primary forest. Their presence may enhance the ability of pastures to regenerate more quickly, reinforcing the importance of trees in agricultural settings.     

Effects of Long-Term Ungulate Exclusion and Recent Alien Species Control on the Preservation and Restoration of a Hawaiian Tropical Dry Forest

Abstract: Although the destruction of tropical rain forests receives much attention, tropical dry forests are in general far more threatened and endangered. Eliminating grazing ungulates is often considered a key first step toward protecting these ecosystems, but few studies have investigated the long-term effects of this technique. We examined the effects of ungulate exclusion from a 2.3-ha native dry-forest preserve on the island of Hawaii by comparing its present flora to the flora of an adjacent area subjected to continuous grazing since the preserve was fenced over 40 years ago. Relative to this adjacent area, the fenced preserve contained a more diverse flora with substantially greater coverage of native overstory and understory species. Until recently, however, regeneration of native canopy trees within the preserve appears to have been thwarted by a dominant herbaceous cover of alien fountain grass (   Pennisetum setaceum ) and predation by alien rodent species. Our results indicate that although ungulate exclusion may be a necessary and critical first step, it is not sufficient to adequately preserve and maintain Hawaii's remaining tropical dry forest remnants. Our recent efforts to control the dominant alien species within the fenced preserve suggest that this practice may facilitate both the regeneration of native species and the colonization and potential invasion of new alien plants. Comparisons of seedlings of the dominant native canopy tree Diospyros sandwicensis growing in sites both dominated by and free of fountain grass suggested that fountain grass inhibits Diospyros seedling growth and photosynthesis but may increase survival if seedlings are protected from ungulates.     

Habitat-specific impacts of multiple consumers on plant population dynamics

Multiple consumers often attack seeds, seedlings, and adult plants, but their population-level consequences remain uncertain. We examined how insect and small mammal consumers influenced the demography and abundance of the perennial shrub, bush lupine (Lupinus arboreus). In grassland and dune habitats we established replicate experimental lupine populations in 81-m2 plots that were either protected from, or exposed to, herbivorous voles and granivorous mice (via fencing) and/or root feeding insects (via insecticide treatment). Populations were initiated with transplanted seedlings in 1999 and 2000. We followed the demography of these cohorts, subsequent generations, and the seed bank for 5.5 years. Voles and insects killed many seedlings in dune (1999 only) and grassland (1999 and 2000) habitats. After 2000, insects and voles had minimal effects on seedling or adult survival. Seed predation by granivorous mice, however, greatly depressed seedling recruitment, resulting in lower adult lupine abundance in control plots vs. those protected from rodents. In grasslands, initial effects of voles and insects on seedling survival produced large differences among treatments in adult plant density and the cumulative number of seeds produced throughout the experiment. Differences among grassland populations in seed rain, however, had little influence on the magnitude of seedling recruitment into this habitat. Instead, recruitment out of a preexisting seed bank compensated for the lack of seed production in populations exposed to consumers. Shading by dense adults in plots protected from consumers limited seedling establishment within these populations. Although differences among populations in cumulative seed rain did not influence adult establishment, populations protected from consumers accumulated substantially larger seed banks than controls. These results illustrate how density dependence, habitat-specific seed dynamics, and particular demographic impacts of consumers interact to shape plant population responses to consumers.     

Are functional traits good predictors of demographic rates? Evidence from five neotropical forests

A central goal of comparative plant ecology is to understand how functional traits vary among species and to what extent this variation has adaptive value. Here we evaluate relationships between four functional traits (seed volume, specific leaf area, wood density, and adult stature) and two demographic attributes (diameter growth and tree mortality) for large trees of 240 tree species from five Neotropical forests. We evaluate how these key functional traits are related to survival and growth and whether similar relationships between traits and demography hold across different tropical forests. There was a tendency for a trade-off between growth and survival across rain forest tree species. Wood density, seed volume, and adult stature were significant predictors of growth and/or mortality. Both growth and mortality rates declined with an increase in wood density. This is consistent with greater construction costs and greater resistance to stem damage for denser wood. Growth and mortality rates also declined as seed volume increased. This is consistent with an adaptive syndrome in which species tolerant of low resource availability (in this case shade-tolerant species) have large seeds to establish successfully and low inherent growth and mortality rates. Growth increased and mortality decreased with an increase in adult stature, because taller species have a greater access to light and longer life spans. Specific leaf area was, surprisingly, only modestly informative for the performance of large trees and had ambiguous relationships with growth and survival. Single traits accounted for 9-55% of the interspecific variation in growth and mortality rates at individual sites. Significant correlations with demographic rates tended to be similar across forests and for phylogenetically independent contrasts as well as for cross-species analyses that treated each species as an independent observation. In combination, the morphological traits explained 41% of the variation in growth rate and 54% of the variation in mortality rate, with wood density being the best predictor of growth and mortality. Relationships between functional traits and demographic rates were statistically similar across a wide range of Neotropical forests. The consistency of these results strongly suggests that tropical rain forest species face similar trade-offs in different sites and converge on similar sets of solutions.