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.     

Invasive competitor and native seed predators contribute to rarity of the narrow endemic Astragalus sinuatus Piper

The conservation of rare plant species hinges on our ability to identify the underlying mechanisms that limit rare plant populations. Theory on rarity suggests that both predispersal seed predation and competition can be important mechanisms influencing abundance and/or distribution of rare plant populations. Yet few studies have tested these interactions, and those that have evaluated each mechanism independently. Astragalus sinuatus Piper (Whited's milkvetch) is a narrow endemic plant species restricted to eight populations within a 10-km2 area in eastern Washington. We used experimental and observational methods to test the effects of native insect predispersal seed predators and an invasive grass (Bromus tectorum L. [cheatgrass]) on seed set and population density of A. sinuatus. We quantified per capita seed production and pod predation rates across four sites and among four years. Seed predation rates were high across four sites (66-82%) and all years (65-82%). Experimental reduction of predispersal seed predators significantly increased per capita seed set of A. sinuatus (164-345%) at two experimental sites. Concurrently, two seed addition experiments demonstrated the effect of seed loss and presence of B. tectorum on seedling recruitment and establishment of A. sinuatus over four growing seasons. In the first seed addition experiment, we found no difference in recruitment and establishment between low (40) and high (120) seed addition levels. In the second addition experiment (one level of addition; 40 seeds), we found that recruitment and survivorship increased 200% in plots where B. tectorum was removed compared to plots where B. tectorum was present. Thus, seed addition had no impact in the presence of B. tectorum; conversely, in the absence of B. tectorum, seed addition was highly effective at increasing population numbers. Results suggest that, in areas where B. tectorum is present, recruitment is site limited, and it is seed limited when B. tectorum is absent. We recommend that managers reduce B. tectorum in an effort to increase population growth of A. sinuatus; in areas where B. tectorum is absent, short-term reduction of insect predators should be considered as a strategy to increase population growth of this rare species.     

Relative Importance of Reproductive Biology and Establishment Ecology for Persistence of a Rare Shrub in a Fragmented Landscape

Abstract:  Verticordia fimbrilepis (Turcz) ssp. fimbrilepis (Myrtaceae) is an endangered shrub that occurs in a number of populations varying in size and landscape context. We compared the importance of factors associated with its reproductive biology with that of factors influencing the regeneration niche in survival of small, isolated populations in contrasting habitat fragments. Small populations on road verges had equal or greater diversity of insect visitors to flowers, rates of pollination, and seed production compared with larger populations in conservation reserves. V. fimbrilepis seeds remained dormant in the soil for at least 30 months, and germination was stimulated by smoke. Plants were killed by fire, but mass recruitment from soil-stored seed reserves occurred in the first and second winters following fire. Our studies showed some seedling recruitment between fires, but this was strongly related to the availability of competition-free establishment sites. Whether this is enough to replace older plants as they die and thereby sustain stable populations is unknown and probably varies with the landscape. Environmental variation between fire episodes influenced population size. Drought increased mortality, but wetter years encouraged interfire recruitment. Most populations are declining and cannot recover without the occurrence of fire. In a fragmented agricultural landscape, fire suppression is the primary management practice. This may adversely affect rare species such as V. fimbrilepis and others with similar life histories that rely on a particular fire regime to persist. Population persistence is more likely to be related to stochastic environmental events than to factors associated with reproductive biology.     

Vertebrate herbivory impacts seedling recruitment more than niche partitioning or density-dependent mortality

In tropical forests, resource-based niches and density-dependent mortality are mutually compatible mechanisms that can act simultaneously to limit seedling populations. Differences in the strengths of these mechanisms will determine their roles in maintaining species coexistence. In the first assessment of these mechanisms in a Congo Basin forest, we quantified their relative strengths and tested the extent to which density-dependent mortality is driven by the distance-dependent behavior of seed and seedling predators predicted by the Janzen-Connell hypothesis. We conducted a large-scale seed addition experiment for five randomly selected tropical tree species, caging a subset of seed addition quadrats against vertebrate predators. We then developed models to assess the mechanisms that determine seedling emergence (three months after seed addition) and survival (two years after seed addition). As predicted, both niche differentiation and density-dependent mortality limited seedling recruitment, but predation had the strongest effects on seedling emergence and survival. Seedling species responded differently to naturally occurring environmental variation among sites, including variation in light levels and soil characteristics, supporting predictions of niche-based theories of tropical tree species coexistence. The addition of higher densities of seeds into quadrats initially led to greater seedling emergence, but survival to two years decreased with seed density. Seed and seedling predation reduced recruitment below levels maintained by density-dependent mortality, an indication that predators largely determine the population size of tree seedlings. Seedling recruitment was unrelated to the distance to or density of conspecific adult trees, suggesting that recruitment patterns are generated by generalist vertebrate herbivores rather than the specialized predators predicted by the Janzen-Connell hypothesis. If the role of seed and seedling predation in limiting seedling recruitment is a general phenomenon, then the relative abundances of tree species might largely depend on species-specific adaptations to avoid, survive, and recover from damage induced by vertebrate herbivores. Likewise, population declines of herbivorous vertebrate species (many of which are large and hunted) may trigger shifts in species composition of tropical forests.     

Dangerous liaisons disperse the Mediterranean dwarf palm: fleshy-pulp defensive role against seed predators

We chose the interaction between the Mediterranean dwarf palm (Chamaerops humilis) and its major seed disperser, the Eurasian badger (Meles meles), to evaluate the hypothesis that endozoochory is characterized by a mixture of conflicting and overlapping interests, with the capacity of being positive or negative for plant fitness. For instance, because of the potential protective role against invertebrate seed predators of C. humilis pulp, we expected that badger-ingested diaspores (i.e., seeds without pulp) would show lower survival than control seeds with the pulp attached. Conversely, due to the possible germination inhibitory function of C. humilis fruit pulp, it was also likely that badger-ingested seeds germinate in higher proportion than control seeds. We evaluated our predictions by carrying out a field sowing and monitoring it over two years. We estimated several stage-specific transition probabilities as well as the cumulative probability of seedling recruitment under different treatments. Our experimental results revealed the multifunctionality of C. humilis fruit pulp and that fruit ingestion by badgers had conflicting outcomes for the palm. As predicted, seed survival was much lower and seed predation by invertebrates much higher for badger-ingested than for control seeds, suggesting a defensive role of C. humilis ripe fruit pulp. All early-emerged seedlings came from badger-ingested seeds, suggesting an inhibitory function of fruit pulp. Though we did not find an effect of removal from the maternal environment on most components of fitness, seedling survival for badger-ingested seeds was higher away from than beneath conspecifics. Badgers imposed a sizeable short-term fitness cost to C. humilis and therefore could be categorized as a "dangerous liaison." Nonetheless, because of the high mobility of the badger, its dispersal service appears paramount given the severe fragmentation and isolation of most C. humilis populations across the highly humanized Mediterranean basin. Our study thus illustrates the necessity of assessing concurrently direct and indirect effects of plant-disperser interactions at different stages of the plant life cycle and recruitment process.     

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.     

Seed availability and insect herbivory limit recruitment and adult density of native tall thistle

Understanding spatial and temporal variation in factors influencing plant regeneration is critical to predicting plant population growth. We experimentally evaluated seed limitation, insect herbivory, and their interaction in the regeneration and density of tall thistle (Cirsium altissimum) across a topographic ecosystem productivity gradient in tallgrass prairie over two years. On ridges and in valleys, we used a factorial experiment manipulating seed availability and insect herbivory to quantify effects of: seed input on seedling density, insect herbivory on juvenile density, and cumulative impacts of both seed input and herbivory on reproductive adult density. Seed addition increased seedling densities at three of five sites in 2006 and all five sites in 2007. Insect herbivory reduced seedling survival across all sites in both years, as well as rosette survival from the previous year's seedlings. In both years, insecticide treatment of seed addition plots led to greater adult tall thistle densities in the following year, reflecting the increase in juvenile thistle densities in the experimental year. Seedling survival was not density dependent. Our analytical projection model predicts a significant long-term increase in adult densities from seed input, with a greater increase under experimentally reduced insect herbivory. While plant community biomass and water stress varied significantly between ridges and valleys, the effects of seed addition and insect herbivory did not vary with gradient position. These results support conceptual models that predict seedling and adult densities of short-lived monocarpic perennial plants should be seed limited. Further, the experiment demonstrates that even at high juvenile plant densities, at which density dependence potentially could have overridden herbivore effects on plant survival, insect herbivory strongly affected juvenile thistle performance and adult densities of this native prairie species.     

Postdispersal seed predation limits the abundance of a long-lived perennial forb (Lithospermum ruderale)

Loss of seeds to consumers is common in plant communities, but the degree to which these losses influence plant abundance or population growth is often unclear. This is particularly the case for postdispersal seed predation by rodents, as most studies of rodent seed predation have focused on the sources of spatiotemporal variation in seed loss but not quantified the population consequences of this loss. In previous work we showed that seed predation by deer mice (Peromyscus maniculatus) substantially reduced seedling recruitment and establishment of Lithospermum ruderale (Boraginaceae), a long-lived perennial forb. To shed light on how rodent seed predation and the near-term effects on plant recruitment might influence longer-term patterns of L. ruderale population growth, we combined experimental results with demographic data in stage-based population models. Model outputs revealed that rodent seed predation had a significant impact on L. ruderale population growth rate (lambda). With the removal of postdispersal seed predation, the projected population growth rates increased between 0.06 and 0.12, depending on site (mean deltalambda across sites = 0.08). Seed predation shifted the projected stable stage distribution of populations from one with a high proportion of young plants to one in which larger adult size classes dominate. Elasticities of vital rates also changed, with germination and growth of seedlings and young plants becoming more important with the removal of seed predation. Simulations varying the magnitude of seed predation pressure while holding other vital rates constant showed that seed predation could lower lambda even if only 40% of available seeds were consumed. These results demonstrate that rodent granivory can be a potent force limiting the abundance of a long-lived perennial forb.     

Linking fruit traits to variation in predispersal vertebrate seed predation, insect seed predation, and pathogen attack

The importance of vertebrates, invertebrates, and pathogens for plant communities has long been recognized, but their absolute and relative importance in early recruitment of multiple coexisting tropical plant species has not been quantified. Further, little is known about the relationship of fruit traits to seed mortality due to natural enemies in tropical plants. To investigate the influences of vertebrates, invertebrates, and pathogens on reproduction of seven canopy plant species varying in fruit traits, we quantified reductions in fruit development and seed germination due to vertebrates, invertebrates, and fungal pathogens through experimental removal of these enemies using canopy exclosures, insecticide, and fungicide, respectively. We also measured morphological fruit traits hypothesized to mediate interactions of plants with natural enemies of seeds. Vertebrates, invertebrates, and fungi differentially affected predispersal seed mortality depending on the plant species. Fruit morphology explained some variation among species; species with larger fruit and less physical protection surrounding seeds exhibited greater negative effects of fungi on fruit development and germination and experienced reduced seed survival integrated over fruit development and germination in response to vertebrates. Within species, variation in seed size also contributed to variation in natural enemy effects on seed viability. Further, seedling growth was higher for seeds that developed in vertebrate exclosures for Anacardium excelsum and under the fungicide treatment for Castilla elastica, suggesting that predispersal effects of natural enemies may carry through to the seedling stage. This is the first experimental test of the relative effects of vertebrates, invertebrates, and pathogens on seed survival in the canopy. This study motivates further investigation to determine the generality of our results for plant communities. If there is strong variation in natural enemy attack among species related to differences in fruit morphology, then quantification of fruit traits will aid in predicting the outcomes of interactions between plants and their natural enemies. This is particularly important in tropical forests, where high species diversity makes it logistically impossible to study every plant life history stage of every species.     

Secondary succession dynamics in estuarine marshes across landscape-scale salinity gradients

Secondary succession plays a critical role in driving community structure in natural communities, yet how succession dynamics vary with environmental context is generally unknown. We examined the importance of seedling and vegetative recruitment in the secondary succession of coastal marsh vegetation across a landscape-scale environmental stress gradient. Replicate bare patches were initiated in salt, brackish, and oligohaline marshes in Narragansett Bay, Rhode Island, USA, and allowed to recover unmanipulated or with colonizing seedlings or vegetative runners removed for three years. Seed dispersal and seed bank studies were conducted at the same sites. We found that rates of recovery were 3-10 times faster in brackish and oligohaline marshes than in salt marshes. The fast pace of recovery in oligohaline marshes was driven by seedling colonization, while recovery was dominated by vegetative runners in brackish marshes and by both seedlings and runners in salt marshes. Seed and seedling availability was much greater in oligohaline marshes with up to 24 times the seed bank density compared with salt marshes. In contrast to the facilitated succession generally found in salt marshes, oligohaline marshes follow the tolerance model of succession where numerous species colonize from seed and are slowly displaced by clonal grasses whose recovery is slowed by preemptive competition from seedlings, contributing to the higher species diversity of oligohaline marshes. These findings reveal fundamental differences in the dynamics and assembly of marsh plant communities along estuarine salinity gradients that are important for conceptually understanding wetlands and for guiding the management and restoration of various types of coastal marshes.     

Effects of life history and reproduction on recruitment time lags in reintroductions of rare plants

Reintroductions are important components of conservation and recovery programs for rare plant species, but their long-term success rates are poorly understood. Previous reviews of plant reintroductions focused on short-term (e.g., ≤3 years) survival and flowering of founder individuals rather than on benchmarks of intergenerational persistence, such as seedling recruitment. However, short-term metrics may obscure outcomes because the unique demographic properties of reintroductions, including small size and unstable stage structure, could create lags in population growth. We used time-to-event analysis on a database of unusually well-monitored and long-term (4–28 years) reintroductions of 27 rare plant species to test whether life-history traits and population characteristics of reintroductions create time-lagged responses in seedling recruitment (i.e., recruitment time lags [RTLs]), an important benchmark of success and indicator of persistence in reintroduced populations. Recruitment time lags were highly variable among reintroductions, ranging from <1 to 17 years after installation. Recruitment patterns matched predictions from life-history theory with short-lived species (fast species) exhibiting consistently shorter and less variable RTLs than long-lived species (slow species). Long RTLs occurred in long-lived herbs, especially in grasslands, whereas short RTLs occurred in short-lived subtropical woody plants and annual herbs. Across plant life histories, as reproductive adult abundance increased, RTLs decreased. Highly variable RTLs were observed in species with multiple reintroduction events, suggesting local processes are just as important as life-history strategy in determining reintroduction outcomes. Time lags in restoration outcomes highlight the need to scale success benchmarks in reintroduction monitoring programs with plant life-history strategies and the unique demographic properties of restored populations. Drawing conclusions on the long-term success of plant reintroduction programs is premature given that demographic processes in species with slow life-histories take decades to unfold.     

Exploring Population Dynamics Patterns in a Rare Fish, Zingel asper, through Capture-Mark-Recapture Methods

Abstract:  We performed a capture-mark-recapture study on one of the last populations of Zingel asper , an endemic percid species of the Rhône River basin in France. The distribution of Z. asper has decreased dramatically during the last century. We sampled three sites in suitable habitats in the Beaume River. No impact of individual tagging on survival was found. The demography of the population was analyzed using capture-recapture methods that allow the estimation of survival, recruitment, and demographic growth rates. Annual survival rates were low (0.35–0.50). The level of transience was high (5% to 25%), suggesting that a significant number of individuals were highly mobile or shifted to suboptimal habitats. Seniority rates suggested random highly variable recruitment between years. The three sites had similar variation patterns in all demographic parameters, indicating broad spatial covariation in population dynamics. We found some local differences in demographic parameters, which could be linked to local habitat quality. Individual tagging allowed for the estimation of demographic parameters that improved our understanding of Z. asper population dynamics and revealed mechanisms that may affect population persistence, such as stochastic recruitment, low survival, and frequent dispersal. The fragmentation of habitat through river damming inhibits dispersal and represents a threat to the persistence of Z. asper in the Rhône basin. Our results offer evidence of the importance of dispersal in nonmigratory fishes and confirm the usefulness of individual tagging methods in rare fish demography.     

Causes and consequences of unequal seedling production in forest trees: a case study in red oaks

Inequality in reproductive success has important implications for ecological and evolutionary dynamics, but lifetime reproductive success is challenging to measure in long-lived species such as forest trees. While seed production is often used as a proxy for overall reproductive success, high mortality of seeds and the potential for trade-offs between seed number and quality draw this assumption into question. Parentage analyses of established seedlings can bring us one step closer to understanding the causes and consequences of variation in reproductive success. In this paper we demonstrate a new method for estimating individual seedling production and average percentage germination, using data from two mixed-species populations of red oaks (Quercus rubra, Q. velutina, Q. falcata, and Q. coccinea). We use these estimates to examine the distribution of female reproductive success and to test the relationship between seedling number and individual seed production, age, and growth rate. We show that both seed and seedling production are highly skewed, roughly conforming to zero-inflated lognormal distributions, rather than to the Poisson or negative-binomial distributions often assumed by population genetics analyses. While the number of established offspring is positively associated with mean annual seed production, a lower proportion of seeds from highly fecund individuals become seedlings. Our red oak populations also show evidence of trade-offs between growth rate and reproductive success. The high degree of inequality in seedling production shown here for red oaks, and by previous studies in other species, suggests that many trees may be more vulnerable to genetic drift than previously thought, if immigration in limited by fragmentation or other environmental changes.     

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.     

Conservation of Species in Dynamic Landscapes: Divergent Fates of Silene tatarica Populations in Riparian Habitats

Abstract:  In transient environments, where local extinctions occur as a result of destruction or deterioration of the local habitat, the long-term persistence of a species requires successful colonizations at new, suitable sites. This kind of habitat tracking should be associated with the asynchronous dynamics of local populations, and it can be especially important for the conservation of rare plant species in riparian habitats. We determined spatiotemporal variation in the demography of the perennial Silene tatarica (L.) Pers. in 15 populations (1998–2003) located in periodically disturbed riparian habitats. The habitats differed according to their morphology (flat shores, slopes) and the amount of bare ground (open, intermediate, closed) along a successional gradient. We used elasticity and life-table response analyses and stochastic simulations to study the variation in population demography. Finite population growth rate was higher in intermediate habitats than in open and closed habitats. In stochastic simulations population size increased in most cases, but four populations were projected to become extinct within 12–70 years. The viability of local populations depended most on the survival and growth of juvenile individuals and on the fecundity of large fertile individuals. On a regional scale, the persistence of this species will require a viable network of local populations as protection against local extinctions caused by natural disturbances and succession. Accordingly, the long-term persistence of riparian species may depend on habitat changes; thus, their conservation requires maintenance of natural disturbance dynamics. Along regulated rivers, management activities such as the creation of open habitats for new colonization should be implemented. Similarly, these activities can be rather general requirements for the conservation of endangered species dependent on transient habitats along successional gradients.     

Hunting Increases Dispersal Limitation in the Tree Carapa procera, a Nontimber Forest Product

Abstract:  The sustainability of seed extraction from natural populations has been questioned recently. Increased recruitment failure under intense seed harvesting suggests that seed extraction intensifies source limitation. Nevertheless, areas where more seeds are collected tend to also have more intense hunting of seed-dispersing animals. We studied whether such hunting, by limiting disperser activity, could cause quantitative dispersal limitation, especially for large crops and for crops in years of high seed abundance. In each of four Carapa procera (Meliaceae) populations in French Guiana and Surinam, two with hunting and two without, we compared seed fate for individual trees varying in crop size in years of high and low population-level seed abundance. Carapa seeds are a nontimber forest product and depend on dispersal by scatter-hoarding rodents for survival and seedling establishment. Hunting negatively affected the proportion of seeds dispersed and caused greater numbers of seeds to germinate or be infested by moths below parent trees, where they would likely die. Hunting of seed-dispersing animals disproportionally affected large seed crops, but we found no additional effect of population-level seed abundance on dispersal rates. Consistently lower rates of seed dispersal, especially for large seed crops, may translate to lower levels of seedling recruitment under hunting. Our results therefore suggest that the subsistence hunting that usually accompanies seed collection is at the cost of seed dispersal and may contribute to recruitment failure of these nontimber forest products. Seed extraction from natural populations may affect seedling recruitment less if accompanied by measures adequately incorporating and protecting seed dispersers.     

Conserving Slow-Growing, Long-Lived Tree Species: Input from the Demography of a Rare Understory Conifer, Taxus floridana

Abstract:  Although land preservation and promotion of successful regeneration are important conservation actions, their ability to increase population growth rates of slow-growing, long-lived trees is limited. We investigated the demography of Taxus floridana Nutt., a rare understory conifer, in three populations in different ravine forests spanning its entire geographic range along the Apalachicola River Bluffs in northern Florida (U.S.A.). We examined spatial and temporal patterns in demographic parameters and projected population growth rates by using four years of data on the recruitment and survival of seedlings and established stems, and on diameter growth from cross-sections of dead stems. All populations experienced a roughly 10-fold increase in seedling recruitment in 1996 compared with other years. The fates of seedlings and stems between 8 and 16 mm differed among populations. The fates of stems in two other size classes (the 2- to 4-mm class and the 4- to 8-mm class) differed among both populations and years. Individual stems in all populations exhibited similarly slow growth rates. Stochastic matrix models projected declines in all populations. Stochastic matrix analysis revealed the high elasticity of a measure of stochastic population growth rate to perturbations in the stasis of large reproductive stems for all populations. Additional analyses also indicated that occasional episodes of high recruitment do not greatly affect population growth rates. Conservation efforts directed at long-lived, slow-growing rare plants like Taxus floridana should both protect established reproductive individuals and further enhance survival of individuals in other life-history stages, such as juveniles, that often do not appear to contribute greatly to population growth rates.     

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.     

聚丁二酸丁二醇酯（PBS）生物降解过程对植物生长的影响评价

通过苏丹草（Sorghum sudanense Stapf）、籽粒笕（Amaranthus hypochondriacus L.）、豇豆（Vigna sesquipedalis Wight）种子的发芽试验及其盆栽试验,考察了PBS在土壤浸提液中的生物降解性能,探讨了PBS高聚物、低聚物和合成单体对植物的发芽和生长影响。研究结果表明：①在土壤浸提液中微生物对PBS有一定的降解作用,且在降解过程中,降解液的pH变化不明显;②PBS高聚物的降解产物对植物生长没有影响;③P BS低聚物在降解初期抑制部分植物的幼苗生长,后期对植物的生长没有影响;④当丁二酸的质量浓度低于200 mg.L^-1时,对植物种子的发芽和幼苗生长不会产生影响,当质量浓度高于500 mg.L^-1时,将抑制幼苗生长;当1,4-丁二醇的质量浓度低于2 000 mg.L-1时,对种子的发芽和生长没有影响。     

Population Persistence and Offspring Fitness in the Rare Bellflower Campanula cervicaria in Relation to Population Size and Habitat Quality

Abstract: Data from several animal species and a few plant species indicate that small populations face an elevated risk of extinction. Plants are still underrepresented in these studies concerning the relation between population size and persistence. We studied the effect of population size on persistence among natural populations of the rare bellflower Campanula cervicaria in Finland. We monitored 52 bellflower populations for 8 years and found that the mean population size decreased from 24 to 14 during this period. Small populations with ≤5 individuals were more prone to losing all fertile plants than were larger ones. Reduction in population size was nevertheless unrelated to the degree of population isolation, measured as the distance to the nearest known population. To test the hypothesis that offspring fitness is lower in small populations, we germinated bellflower seeds from different-sized populations in a laboratory and found that seed germination ability was independent of population size. The seedlings raised from seeds of small populations grew faster than those taken from larger populations. Population size was negatively related to the amount of shade in the habitats. In conclusion, decreasing population sizes of C. cervicaria seemed not to be caused by lowered germination ability or growth rate in small populations; rather, population size reductions appeared to be due to closing of vegetation in the habitats.