Tree species control rates of free-living nitrogen fixation in a tropical rain forest

Tropical rain forests represent some of the most diverse ecosystems on earth, yet mechanistic links between tree species identity and ecosystem function in these forests remains poorly understood. Here, using free-living nitrogen (N) fixation as a model, we explore the idea that interspecies variation in canopy nutrient concentrations may drive significant local-scale variation in biogeochemical processes. Biological N fixation is the largest "natural" source of newly available N to terrestrial ecosystems, and estimates suggest the highest such inputs occur in tropical ecosystems. While patterns of and controls over N fixation in these systems remain poorly known, the data we do have suggest that chemical differences among tree species canopies could affect free-living N fixation rates. In a diverse lowland rain forest in Costa Rica, we established a series of vertical, canopy-to-soil profiles for six common canopy tree species, and we measured free-living N fixation rates and multiple aspects of chemistry of live canopy leaves, senesced canopy leaves, bulk leaf litter, and soil for eight individuals of each tree species. Free-living N fixation rates varied significantly among tree species for all four components, and independent of species identity, rates of N fixation ranged by orders of magnitude along the vertical profile. Our data suggest that variations in phosphorus (P) concentration drove a significant fraction of the observed species-specific variation in free-living N fixation rates within each layer of the vertical profile. Furthermore, our data suggest significant links between canopy and forest floor nutrient concentrations; canopy P was correlated with bulk leaf litter P below individual tree crowns. Thus, canopy chemistry may affect a suite of ecosystem processes not only within the canopy itself, but at and beneath the forest floor as well.     

The ecology of energy and nutrient fluxes in hemlock forests invaded by hemlock woolly adelgid

The hemlock woolly adelgid (HWA, Adelges tsugae Annand) is currently causing a severe decline in vitality and survival of eastern hemlock in North American forests. We analyzed the effects of light HWA infestation on vertical energy and nutrient fluxes from the canopy to the forest floor. Canopy throughfall, litter lysimeters, and laboratory litter microcosms were used to examine the effects of HWA-affected and unaffected throughfall on litter type, leachate, and litter chemistry. Early in the season adelgid infestation caused higher dissolved organic carbon (DOC; +24.6%), dissolved organic nitrogen (DON; +28.5%), and K (+39.3%) fluxes and lower inorganic nitrogen fluxes (-39.8%) in throughfall and in adjacent litter solutions collected beneath infested compared to uninfested trees. Needle litter collected beneath uninfested hemlock had significantly lower N concentrations compared to needles collected beneath infested trees, while no difference in N concentrations was found in birch litter. Bacteria were significantly more abundant on hemlock and birch litter beneath infested trees, while yeasts and filamentous fungi showed no consistent response to HWA throughfall. Litter microcosms showed that less DOC was leaching from birch than from hemlock needles when exposed to HWA throughfall. Overall, NH4-N and DON leachate concentrations were higher from birch than from hemlock litter. Thus, HWA-affected throughfall leads to qualitative and quantitative differences in nitrogen export from the litter layer. The N concentration of hemlock litter did not change with time, but the N concentration in birch litter increased significantly during the course of the experiment, especially when HWA-affected throughfall was applied. We suggest a nonlinear conceptual model for the temporal and vertical transition of energy and nutrient fluxes relative to progressing HWA infestation from a pure hemlock to a birch/maple-dominated forest. Progressive needle loss and changes in needle chemistry are likely to produce a humped-shaped DOC curve, while N fluxes initially decrease as infestation continues but rise eventually with hemlock decline and immigration of hardwood species. These findings suggest that it is necessary to understand the biology and specific physiological/trophic effects of exotic pests on their hosts and associated ecosystem processes in order to decipher the temporal dynamics, direction of change, and functional consequences.     

Nonadditive effects of leaf litter species diversity on breakdown dynamics in a detritus-based stream

Since species loss is predicted to be nonrandom, it is important to understand the manner in which those species that we anticipate losing interact with other species to affect ecosystem function. We tested whether litter species diversity, measured as richness and composition, affects breakdown dynamics in a detritus-based stream. Using full-factorial analyses of single- and mixed-species leaf packs (15 possible combinations of four dominant litter species; red maple [Acer rubrum], tulip poplar [Liriodendron tulipifera], chestnut oak [Quercus prinus], and rhododendron [Rhododendron maximum]), we tested for single-species presence/absence (additive) or species interaction (nonadditive) effects on leaf pack breakdown rates, changes in litter chemistry, and microbial and macroinvertebrate biomass. Overall, we found significant nonadditive effects of litter species diversity on leaf pack breakdown rates, which were explained both by richness and composition. Leaf packs containing higher litter species richness had faster breakdown rates, and antagonistic effects of litter species composition were observed when any two or three of the four litter species were mixed. Less-consistent results were obtained with respect to changes in litter chemistry and microbial and macroinvertebrate biomass. Our results suggest that loss of litter species diversity will decrease species interactions involved in regulating ecosystem function. To that end, loss of species such as eastern hemlock (Tsuga canadensis) accompanied by predicted changes in riparian tree species composition in the southeastern United States could have nonadditive effects on litter breakdown at the landscape scale.     

Increasing litter species richness reduces variability in a terrestrial decomposer system

Debate on the relationship between diversity and stability has been driven by the recognition that species loss may influence ecosystem properties and processes. We conducted a litterbag experiment in the Scottish Highlands, United Kingdom, to examine the effects of altering plant litter diversity on decomposition, microbial biomass, and microfaunal abundance. The design of treatments was fully factorial and included five species from an upland plant community (silver birch, Betula pendula; Scots' pine, Pinus sylvestris; heather, Calluna vulgaris; bilberry, Vaccinium myrtillus; wavy-hair grass, Deschampsia flexuosa); species richness ranged from one to five species. We tested the effects of litter species richness and composition on variable means, whether increasing litter species richness reduced variability in the decomposer system, and whether any richness-variability relationships were maintained over time (196 vs. 564 days). While litter species composition effects controlled variable means, we revealed reductions in variability with increasing litter species richness, even after accounting for differences between litter types. These findings suggest that higher plant species richness per se may result in more stable ecosystem processes (e.g., decomposition) and decomposer communities. Negative richness-variation relationships generally relaxed over time, presumably because properties of litter mixtures became more homogeneous. However, given that plant litter inputs continue to enter the belowground system over time, we conclude that variation in ecosystem properties may be buffered by greater litter species richness.     

Interactions among lignin, cellulose, and nitrogen drive litter chemistry-decay relationships

Litter decay rates often correlate with the initial ratios of lignin:nitrogen (N) or lignin:cellulose in litter. However, the chemical and microbial mechanisms that give rise to these patterns are still unclear. To identify these mechanisms, we studied the decomposition of a model plant system, Arabidopsis thaliana, in which plants were manipulated to have low levels of lignin, cellulose, or litter N. Nitrogen fertilizer often increases the loss of cellulose, but it suppresses the breakdown of lignin in plant litter. To understand the mechanisms driving these patterns, we decomposed plants in litterbags for one year in control and N-fertilized plots in an Alaskan boreal forest. We found that litter N had a positive effect on total mass loss because it increased the loss of lignin, N, and soluble C. Lignin had a negative effect on rates of total litter mass loss due to decreases in the loss of cellulose and hemicellulose. Cellulose had a positive effect on lignin loss, supporting the concept of a "priming effect" for lignin breakdown. However, the low-cellulose plants also lost more of their original cellulose compared to the other plant types, indicating that decomposers mined the litter for cellulose despite the presence of lignin. Low-lignin litter had higher fungal biomass and N-acetyl glucosaminidase (NAG, a chitinase) activity, suggesting that lignin restricted fungal growth and may have influenced competitive interactions between decomposers. Nitrogen fertilization increased NAG activity in the early stages of decay. In the later stages, N fertilization led to increased cellulase activity on the litters and tended to reduce lignin losses. The transition over time from competition among decomposers to high cellulase activity and suppressed lignin loss under N fertilization suggests that, in N-limited systems, N fertilization may alter decomposer community structure by favoring a shift toward cellulose- and mineral-N users.     

Volatile chemicals from leaf litter are associated with invasiveness of a neotropical weed in Asia

Some invasive plant species appear to strongly suppress neighbors in their nonnative ranges but much less so in their native range. We found that in the field in its native range in Mexico, the presence of Ageratina adenophora, an aggressive Neotropical invader, was correlated with higher plant species richness than found in surrounding plant communities where this species was absent, suggesting facilitation. However, in two nonnative ranges, China and India, A. adenophora canopies were correlated with much lower species richness than the surrounding communities, suggesting inhibition. Volatile organic compound (VOC) signals may contribute to this striking biogeographical difference and the invasive success of A. adenophora. In controlled experiments volatiles from A. adenophora litter caused higher mortality of species native to India and China, but not of species native to Mexico. The effects of A. adenophora VOCs on seedling germination and growth did not differ between species from the native range and species from the nonnative ranges of the invader. Litter from A. adenophora plants from nonnative populations also produced VOCs that differed quantitatively in the concentrations of some chemicals than litter from native populations, but there were no chemicals unique to one region. Biogeographic differences in the concentrations of some volatile compounds between ranges suggest that A. adenophora may be experiencing selection on biochemical composition in its nonnative ranges.     

Influences of a calcium gradient on soil inorganic nitrogen in the Adirondack Mountains, New York

Studies of the long-term impacts of acidic deposition in Europe and North America have prompted growing interest in understanding the dynamics linking the nitrogen (N) and calcium (Ca) cycles in forested watersheds. While it has been shown that increasing concentrations of nitrate (NO3-) through atmospheric deposition or through nitrification can increase Ca loss, the reciprocal effects of Ca on N transformation processes have received less attention. We studied the influence of soil Ca availability on extractable inorganic N (NO3- + NH4+) across a Ca gradient in the Adirondack Mountains, New York, USA. Our results did not show the direct Ca-N interaction that we had expected, but instead showed that exchangeable Ca coupled with soil moisture, soil organic matter, and ambient temperature accounted for 61% of the variability in extractable inorganic N across 11 sites over two growing seasons. Soil Ca concentrations were, however, positively related to sugar maple (Acer saccharum) and American basswood (Tilia americana) basal areas and negatively related to American beech (Fagus grandifolia) basal area. Based on litter chemistry differences among these tree species and reported potential N mineralization values, we suggest that the influence of Ca on soil inorganic N is through a multistep pathway: reciprocal interactions between soil Ca concentrations and species composition, which in turn affect the quality of litter available for N mineralization. If chronic soil Ca depletion continues, as reported in some forested ecosystems, potential shifts in biotic communities could result in considerable alterations of N cycling processes.     

Recovery of plant diversity following N cessation: effects of recruitment, litter, and elevated N cycling

Plant species richness has declined and composition shifted in response to elevated atmospheric deposition of biologically active nitrogen over much of the industrialized world. Litter thickness, litter nitrogen (N) content, and soil N mineralization rates often remain elevated long after inputs cease, clouding the prospects that plant community diversity and composition would recover should N inputs be reduced. Here we determined how N cycling, litter accumulation, and recruitment limitation influenced community recovery following cessation of long-term N inputs to prairie-like grasslands. We alleviated each of these potential inhibitors through a two-year full-factorial experiment involving organic carbon addition, litter removal, and seed addition. Seed addition had the largest effect on increasing seedling and species numbers and may be necessary to overcome long-term burial of seeds of target perennial grassland species. Litter removal increased light availability and bare sites for colonization, though it had little effect on reducing the biomass of competing neighbors or altering extractable soil N. Nonetheless, these positive influences were enough to lead to small increases in species richness within one year. We found that, although C addition quickly altered many factors assumed favorable for the target community (decreased N availability and biomass of nearby competitors, increased light and site availability), these changes were insufficient to positively impact species richness or seedling numbers over the experimental duration. However, only carbon addition had species-specific effects on the existing plant community, suggesting that its apparent limited utility may be more a result of slow recovery under ambient recruitment rather than from a lack of a restorative effect. There were dramatic interactions among treatments, with the positive effects of litter removal largely negated by carbon addition, and the positive effects of seed addition generally amplified by litter removal. It remains unclear whether each mechanism explored here will induce community recovery, but over different temporal scales. Long-term monitoring will help resolve these remaining questions. Regardless, our results suggest that reversal of species loss and compositional shifts from N deposition in prairies may be more inhibited by habitat fragmentation, recruitment limitation, and long-term suppression of fire than from continued effects of elevated N.     

An assessment of seaweed decomposition within a southern Strait of Georgia seaweed community

An assessment of litter and detritus decomposition and nitrogen content of decomposing litter is presented for ten important seaweeds within a southern Strait of Georgia (British Columbia, Canada) seaweed community sampled from August 1975 until October 1976. Litter decomposition rates varied among species with the time required for litter to disappear from litter bags ranging from 6 d for the lamina of Nereocystis luetkeana to about 70 d for Fucus distichus. Decomposition was characterized by an accelerating increase in the nitrogen: dry weight ratio of remnant litter as decomposition proceeded. Iridaea cordata detritus decomposed most rapidly, at 5.7% d^-1, while rates for Gigartina papillata, N. luetkeana, Laminaria saccharina and Laminaria groenlandica ranged from 1.8 to 3.6% d^-1. The remaining species decomposed more slowly. There was a tendency toward more rapid decomposition with decreasing crude fibre content and detritus particle size; however, it appears that morphology, habitat and growth rate are also correlated with relative decomposition rates. Of 43 taxa identified within quantitative litter collections, F. distichus (41%), I. cordata (26%), N. luetkeana (27%) and Laminaria spp. (4%) accounted for 98% of total deposition with mean peak accumulation occurring in August and September from a low near zero in January and February. Litter distribution was patchy, with most litter decomposing near its place of deposition. The application of litter decomposition rates to measured litter accumulation in a mathematical simulation of decomposition predicted the rate of seaweed litter decomposition to peak at about 1.1 g AFDW (ash-free dry weight) m^-2 d^-1 in early September from a mid-winter low near zero. In total, 56±4% of decomposing litter formed detritus, with the remainder being released as soluble matter. The annual contribution of seaweed litter biomass to detrital pathways from our study site was calculated to be 152 g AFDW m^-2.     

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.     

Soil effects of six different two-species litter mixtures that include Ulmus pumila

The tree species that contribute to decomposed leaf litter can have important effects on soil properties and thus nutrient cycling and interactions between tree species. We examined ground leaf litter and soil mixtures consisting of leaves from Ulmus pumila (Up) combined individually with leaves from one of six other species: Betula platyphylla (Bp), Quercus liaotungensis (Ql), Salix matsudana (Sm), Hippophae rhamnoides (Hr), Caragana microphylla (Cm), and Amorpha fruticosa (Af). The soil–litter mixtures were incubated for 120 days to analyse the effects of their decomposition on soil properties and to determine the interactions between the different types of litter within each mixture. The decomposed litter mixtures were composed of Up combined with Sm- or Hr-improved soil fertility relative to the pure Up mixture, but the decomposed litter mixtures were composed of Up combined with Cm-, Af-, or Ql-diminished soil properties. Three leaf mixture treatments, namely Up?×?Bp, Up?×?Sm, and Up?×?Hr, exhibited synergistic effects on soil properties (i.e. soil properties exceeding the predicted values); however, three other treatments, namely Up?×?Ql, Up?×?Cm, and Up?×?Af, exhibited antagonistic effects (i.e. properties below the predicted values). Therefore, litter from Bp, Sm, or Hr should be mixed with Up to improve soil fertility and production in plantations.     

Decomposition of diverse litter mixtures in streams

In view of growing interest in understanding how biodiversity affects ecosystem functioning, we investigated effects of riparian plant diversity on litter decomposition in forest streams. Leaf litter from 10 deciduous tree species was collected during natural leaf fall at two locations (Massif Central in France and Carpathians in Romania) and exposed in the field in litter bags. There were 35 species combinations, with species richness ranging 1-10. Nonadditive effects on the decomposition of mixed-species litter were minor, although a small synergistic effect was observed in the Massif Central stream where observed litter mass remaining was significantly lower overall than expected from data on single-species litter. In addition, variability in litter mass remaining decreased with litter diversity at both locations. Mean nitrogen concentration of single- and mixed-species litters (0.68-4.47% of litter ash-free dry mass) accounted for a large part of the variation in litter mass loss across species combinations. For a given species or mixture, litter mass loss was also consistently faster in the Massif Central than in the Carpathians, and the similarity in general stream characteristics, other than temperature, suggests that this effect was largely due to differences in thermal regimes. These results support the notion that decomposition of litter mixtures is primarily driven by litter quality and environmental factors, rather than by species richness per se. However, the observed consistent decrease in variability of decomposition rate with increasing plant species richness indicates that conservation of riparian tree diversity is important even when decomposition rates are not greatly influenced by litter mixing.     

Native fish diversity alters the effects of an invasive species on food webs

Aquatic communities have been altered by invasive species, with impacts on native biodiversity and ecosystem function. At the same time, native biodiversity may mitigate the effects of an invader. Common carp (Cyprinus carpio) is a ubiquitous, invasive fish species that strongly influences community and ecosystem processes. We used common carp to test whether the potential effects of an invasive species are altered across a range of species diversity in native communities. In mesocosms, treatments of zero, one, three, and six native fish species were used to represent the nested subset patterns observed in fish communities of lakes in Illinois, USA. The effect of the invader was tested across fish richness treatments by adding common carp to the native community and substituting native biomass with common carp. Native species and intraspecific effects reduced invader growth. The invader reduced native fish growth; however, the negative effect was minimized with increasing native richness. The zooplankton grazer community was modified by a top-down effect from the invader that increased the amount of phytoplankton. Neither the invader nor richness treatments influenced total phosphorus or community metabolism. Overall, the invader reduced resources for native species; and the effect scaled with how the invader was incorporated into the community. Higher native diversity mitigated the impact of the invader, confirming the need to consider biodiversity when predicting the impacts of invasive species.     

Nutrient regulation of organic matter decomposition in a tropical rain forest

Terrestrial biosphere-atmosphere CO2 exchange is dominated by tropical forests, so understanding how nutrient availability affects carbon (C) decomposition in these ecosystems is central to predicting the global C cycle's response to environmental change. In tropical rain forests, phosphorus (P) limitation of primary production and decomposition is believed to be widespread, but direct evidence is rare. We assessed the effects of nitrogen (N) and P fertilization on litter-layer organic matter decomposition in two neighboring tropical rain forests in southwest Costa Rica that are similar in most ways, but that differ in soil P availability. The sites contain 100-200 tree species per hectare and between species foliar nutrient content is variable. To control for this heterogeneity, we decomposed leaves collected from a widespread neotropical species, Brosimum utile. Mass loss during decomposition was rapid in both forests, with B. utile leaves losing >80% of their initial mass in <300 days. High organic matter solubility throughout decomposition combined with high rainfall support a model of litter-layer decomposition in these rain forests in which rapid mass loss in the litter layer is dominated by leaching of dissolved organic matter (DOM) rather than direct CO2 mineralization. While P fertilization did not significantly affect mass loss in the litter layer, it did stimulate P immobilization in decomposing material, leading to increased P content and a lower C:P ratio in soluble DOM. In turn, increased P content of leached DOM stimulated significant increases in microbial mineralization of DOM in P-fertilized soil. These results show that, while nutrients may not affect mass loss during decomposition in nutrient-poor, wet ecosystems, they may ultimately regulate CO2 losses (and hence C storage) by limiting microbial mineralization of DOM leached from the litter layer to soil.     

A simulation model to evaluate the impacts of invasive earthworms on soil carbon dynamics

Recent studies have reported that earthworm invasions alter native communities and impact nutrient cycling in terrestrial ecosystems. We developed a simulation model to evaluate the potential impacts of earthworm invasions on carbon dynamics, taking into consideration earthworm feeding strategies and priming effects on the microorganisms through their casting activities. Responses of carbon stocks (forest litter, soil organic matter, microbial biomass and earthworm populations) and carbon fluxes (litter decomposition, earthworm consumption, and microbial respiration) were used to evaluate an earthworm invasion of a forest ecosystem. Data from a northern temperate forest (Arnot Forest, New York) were adapted for model calibration and evaluation. Simulation results suggest that the impact and outcome of earthworm invasions are affected by pre-invasion resource availability (litter and soil organic matter), invasive earthworm assemblages (particularly feeding strategy), and invasion history (associated with earthworm population dynamics). The abovementioned factors may also determine invasion progress of earthworm species. The accuracy of the model could be improved by the addition of environmental modules (e.g., soil water regimes), precise parameters accounting for individual species attributes under different environmental conditions (e.g. utilization ability of different types of food resources), as well as earthworm population dynamics (size and structure) and interactions with predators and other invasive/indigenous species during the invasion progress. Such an earthworm invasion model could provide valuable evaluation of the complicated responses of carbon dynamics to earthworm invasions in a range of forest ecosystems, particularly under global change scenarios.     

森林凋落物分解研究进展

森林凋落物是指森林生态系统内由生物组分产生,然后归还到林地表面的所有有机物质的总称。森林凋落物在促进森林生态系统正常的物质循环和养分平衡,维持生态系统功能中具有重要作用,其分解受多因素影响,且各因素之间相互交错。不同情况下,各因子的重要性可能不同。温度和湿度被认为是影响凋落物分解主要的气候因子。凋落物随着温度升高分解速率加快,增加土壤湿度对凋落物分解有积极作用。凋落物的化学性质中,C、N比和木质素含量被认为是最重要的指标。凋落物分解前期的分解速率受到养分含量、水溶性碳化合物和结构碳化合物含量的强烈影响,而后期则更多地受到木质素及纤维素/木质素比值的支配。土壤动物可以粉碎凋落物,土壤微生物也是促进凋落物分解的重要因素,人为活动也影响凋落物分解。N沉降、全球变暖和臭氧层破坏等全球变化对森林凋落物分解的影响已逐渐成为研究热点。未来凋落物分解的研究方向是统一研究方法,开展长期定位监测,加强对分解过程中有机碳含量和释放量的研究,以及N沉降对凋落物分解作用机理的研究。     

Leaf herbivory and decomposability in a Malaysian tropical rain forest

There is accumulating evidence that similar suites of plant traits may affect leaf palatability and leaf litter decomposability. However, the possible association between leaf herbivory and litter decomposition rates across species in species-diverse natural ecosystems such as tropical rain forests remains unexplored, despite its importance in estimating the herbivory effects on carbon and nutrient cycling of ecosystems. We found no strong association between leaf herbivory and litter decomposition rates across 40 tree species in a Malaysian tropical rain forest, even though the leaf and litter traits were tightly correlated. This is because the leaf and litter traits related to herbivory and decomposition rates in the field were inconsistent. Leaf toughness accounted for only a small part of the variation in the herbivory rate, whereas a number of litter traits (the leaf mass per area, lignin to nitrogen ratio, and condensed tannin concentration) accurately predicted the decomposition rate across species. These results suggest that herbivory rate across species may not be strongly related to single leaf traits, probably because plant-herbivore interactions in tropical rain forests are highly diverse; on the other hand, plant-decomposer interactions are less specific and can be governed by litter chemicals. We also investigated two factors, phylogeny and tree functional types, that could affect the relationship between herbivory and decomposition across species. Phylogenetic relatedness among the species did not affect the relationship between herbivory and decomposition. In contrast, when the plants were segregated according to their leaf emergence pattern, we found a significant positive relationship between herbivory and decomposition rates for continuous-leafing species. In these species, the condensed tannin to N ratios in leaves and litter were related to herbivory and decomposition rates, respectively. However, we did not observe a similar trend for synchronous-leafing species. These results suggest that the relationship between herbivory and decomposition may be more greatly affected by functional types than by phylogenetic relatedness among species. In conclusion, our results suggest that well-defended leaves are not necessarily less decomposable litter in a tropical rain forest community, implying that herbivory may not generate positive feedback for carbon and nutrient cycling in this type of ecosystem.     

Invasion, Disturbance, and Competition: Modeling the Fate of Coastal Plant Populations

Abstract:  Wetland habitats are besieged by biotic and abiotic disturbances such as invasive species, hurricanes, habitat fragmentation, and salinization. Predicting how these factors will alter local population dynamics and community structure is a monumental challenge. By examining ecologically similar congeners, such as Iris hexagona and I. pseudacorus (which reproduce clonally and sexually and tolerate a wide range of environmental conditions), one can identify life-history traits that are most influential to population growth and viability. We combined empirical data and stage-structured matrix models to investigate the demographic responses of native ( I. hexagona ) and invasive ( I. pseudacorus ) plant populations to hurricanes and salinity stress in freshwater and brackish wetlands. In our models I. hexagona and I. pseudacorus responded differently to salinity stress, and species coexistence was rare. In 82% of computer simulations of freshwater marsh, invasive iris populations excluded the native species within 50 years, whereas native populations excluded the invasive species in 99% of the simulations in brackish marsh. The occurrence of hurricanes allowed the species to coexist, and species persistence was determined by the length of time it took the ecosystem to recover. Rapid recovery (2 years) favored the invasive species, whereas gradual recovery (30 years) favored the native species. Little is known about the effects of hurricanes on competitive interactions between native and invasive plant species in marsh ecosystems. Our models contribute new insight into the relationship between environmental disturbance and invasion and demonstrate how influential abiotic factors such as climate change will be in determining interspecific interactions.     

Invasive Non‐Native Species’ Provision of Refugia for Endangered Native Species

Abstract: The influence of non‐native species on native ecosystems is not predicted easily when interspecific interactions are complex. Species removal can result in unexpected and undesired changes to other ecosystem components. I examined whether invasive non‐native species may both harm and provide refugia for endangered native species. The invasive non‐native plant Casuarina stricta has damaged the native flora and caused decline of the snail fauna on the Ogasawara Islands, Japan. On Anijima in 2006 and 2009, I examined endemic land snails in the genus Ogasawarana. I compared the density of live specimens and frequency of predation scars (from black rats[Rattus rattus]) on empty shells in native vegetation and Casuarina forests. The density of land snails was greater in native vegetation than in Casuarina forests in 2006. Nevertheless, radical declines in the density of land snails occurred in native vegetation since 2006 in association with increasing predation by black rats. In contrast, abundance of Ogasawarana did not decline in the Casuarina forest, where shells with predation scars from rats were rare. As a result, the density of snails was greater in the Casuarina forest than in native vegetation. Removal of Casuarina was associated with an increased proportion of shells with predation scars from rats and a decrease in the density of Ogasawarana. The thick and dense litter of Casuarina appears to provide refugia for native land snails by protecting them from predation by rats; thus, eradication of rats should precede eradication of Casuarina. Adaptive strategies, particularly those that consider the removal order of non‐native species, are crucial to minimizing the unintended effects of eradication on native species. In addition, my results suggested that in some cases a given non‐native species can be used to mitigate the impacts of other non‐native species on native species.     

Biodiversity and Ecosystem Function

In at least some circumstances, biodiversity affects various ecosystem functions and the ways in which ecosystems respond to disturbance. Because these interactions occur at many spatial and temporal scales and throughout all levels of biological organization, it is difficult to decide where to focus attention on interactions between biodiversity and ecosystem function. The loci for initial attention is important for setting research priorities to understand these interactions further, for organizing known information to instruct the development of natural resource policies, and for identifying biodiversity conservation priorities. Holling (1992) argues that ecosystem behavior can be understood from a few dominating ecological processes that structure the ecosystem. In the temporal dimension, these key structuring processes dictate a few dominant temporal frequencies that drive other processes. Thus, the most effective strategy for studying interactions between biodiversity and ecosystem function is to focus on the key structuring processes at intermediate scales of space and time. Thereafter, other ecological conditions signify situations in which the interactions between biodiversity and ecosystem function are particularly strong: early to midsuccessional status, low soil fertility, intermediate levels of disturbance, biotic interactions only where there is collaborative indication of importance, invading species that differ significantly from native species in resource acquisition or utilization, and ecotones.