THE RELATIVE ROLES OF ABIOTIC AND BIOTIC FACTORS IN SEEDLING DEMOGRAPHY OF ARROYO WILLOW (SALIX LASIOLEPIS: SALICACEAE)

The abiotic and biotic factors causing mortality of Salix lasiolepis (Salicaceae) seedlings were studied along a seasonally flowing stream in northern Arizona. Mortality of first-year seedlings approached or equaled 100% for the four cohorts observed. Water-addition experiments demonstrated that lack of soil surface moisture was the primary cause of mortality for seedlings through their first 2 years with > 75% of seedlings dying in the first month of growth in unwatered plots. Biotic factors, including herbivory and inter- and intraspecific competition were evaluated, but played a minor role in the dynamics of seedling populations during this study. Shade conditions between sites were inferred to influence growth and survival, and experiments verified the effect of shade in decreasing seedling size. Smaller seedlings were shown experimentally to have a greater likelihood of dying during the winter. The probability of survival between years increased with seedling age, and seedlings that survived three growing seasons were likely to join the population of mature, reproductive plants. Studies of seedling survivorship through the establishment phase have seldom reported mortality levels as extreme as those reported here.     

Intrinsic biotic factors and microsite conditions drive seedling survival in a species with masting reproduction

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Changing light conditions in pine rockland habitats affect the intensity and outcome of ant–plant interactions

Extrafloral nectar (EFN) mediates food‐for‐protection mutualisms between plants and ants. Such mutualisms exist within a complex web of biotic interactions, and in a framework provided by the abiotic environment. Both biotic and abiotic factors, therefore, affect the outcome of ant–plant interactions. We conducted an experiment to determine the effects of ant activity, and light intensity, on herbivory rates, growth, and reproductive fitness in Senna mexicana var. chapmanii, a perennial legume native to pine rockland habitats of south Florida. Forty plants were divided among four treatments in a factorial experimental design with two independent variables: ant activity and light intensity. Plants were divided equally between sunny and shady habitats, and ants were excluded from half of the plants in each habitat type. The presence of ants significantly reduced herbivory rates in S. chapmanii. In shaded habitats, the presence of ants had no effect on plant reproductive fitness, however, in sunny habitats plants with ants produced significantly more seeds over the duration of the 1‐yr study. Ants represent an important biotic defense against herbivores in S. chapmanii; however, their effects on plant fitness are dependent on light conditions. Pine rockland habitats in south Florida have been widely destroyed or mismanaged. In fragments that remain, suppression of fire has led to increased canopy closure and shading of the understory. These changes will likely negatively impact plants that rely on ants for defense. We highlight the importance of conservation efforts to preserve the pine rocklands and the fire regimes on which they rely.     

亚热带-温带过渡区秦岭落叶阔叶林幼苗存活机制

【目的】物种幼苗的存活与各种生物和非生物因素密切相关,研究关键因素对幼苗存活的影响有助于理解群落物种共存的主要作用机制。【方法】以秦岭落叶阔叶林25 hm²固定样地的木本植物幼苗为对象,对11 408棵幼苗的生存动态开展连续5年(2015—2019年)的监测,利用广义线性混合模型(GLMMs)在群落水平上对影响不同年龄阶段幼苗存活的主要生物与非生物因素进行分析。【结果】(1)从群落水平来看,对幼苗存活影响最大的是生物因素,幼苗存活率与同种幼苗邻体密度和同种大树邻体胸高断面积呈显著负相关,与异种幼苗邻体密度呈显著正相关,表明物种在幼苗时期受到强烈的负密度制约效应;(2)从苗龄水平上来看,除了生物因素,影响幼苗存活的主要因素还包括海拔等非生物因素,但非生物因素的影响随着苗龄增大而减小。【结论】影响幼苗存活因素是多样的,其中生物因素的影响更显著,促进秦岭大样地中幼苗共存的主要机制为负密度制约效应。     

Conditions for seedling establishment and probable function of seed in the clonal sedge Carex rugulosa Kük. in riverside marshes

Carex rugulosa Kük. forms large meadows in moist zones along estuarine riversides. These meadows are usually maintained by rhizomatous ramet production (clonal growth), and the appearance of seedlings is uncommon. We studied the conditions required for seedling establishment. In areas in which clonal ramets were dense (906–1050/m²), and the relative light intensity at ground level was low (0.8–3.8%), seed germination was entirely suppressed. In contrast, many seedlings (288–513/m²) were observed in areas in which clonal ramets had been considerably reduced (13–269/m²) by continuous inundation in the previous year. In these areas, the relative light intensity had increased (20.5–79.3%). It was inferred that seeds resisted the prolonged inundation that killed the ramets, and that germination was induced under these open conditions. These results suggest that the main functions of sexual reproduction in C. rugulosa are recovery after severe damage to vegetation using seeds from the seed bank and the dispersal of seeds to areas without dense vegetation. Most seedlings died when the marshy meadow was temporarily inundated by heavy rain. However, transplanted seedlings survived better at artificially elevated sites that were not submerged. This shows that inundation during the seedling stage impedes seedling survival. Therefore, seeds can contribute to recovery after ramet decline or colonization to open area when (i) the rainfall is not heavy to cause seedling submergence or (ii) seeds are dispersed to higher area which is not inundated.     

Positive and negative biotic interactions and invasive Triadica sebifera tolerance to salinity: a cross‐continent comparative study

Exotic plant species may exhibit abiotic niche expansions that enable them to persist in a greater variety of habitat types in their introduced ranges than in their native ranges. This may reflect variation in limitation by different abiotic niche dimensions (realized niche shift) or phenotypic effects of biotic interactions that vary among ranges (realized niche expansion). Novel abiotic and biotic environments in the introduced range may also lead to genetic changes in exotic plant traits that enhance their abiotic stress tolerance (fundamental niche expansion). Here, we investigated how biotic interactions (aboveground herbivory and soil organisms) affect plant salinity tolerance using the invasive species Triadica sebifera from China (native range) and US (introduced range) populations grown in common gardens in both ranges. Simulated herbivory significantly reduced survival in saline treatments with reductions especially large at low salinity. Soil sterilization had a negative effect on survival at low salinity in China but had a positive effect on survival at low salinity in the US. Triadica survival and biomass were higher for US populations than for China populations, particularly in China but salinity tolerance did not depend on population origin. On average, arbuscular mycorrhizal (AM) colonization was higher for US populations, US soils and low salinity. These factors had a significant, positive, non‐additive interaction so that clipped seedlings from US populations in low saline US soils had high levels of AM colonization. Overall, our results show that phenotypic biotic interactions shape Triadica's salinity tolerance. Positive and negative biotic interactions together affected plant performance at intermediate stress levels. However, only aboveground damage consistently affected salinity tolerance, suggesting an important role for enemy release in expanding stress tolerance.     

Herbivory and competition slow down invasion of a tall grass along a productivity gradient

Competition models including competition for light predict that small plant species preferred by herbivores will be outshaded by taller unpreferred plant species with increasing productivity. When the tall plant species is little grazed by the herbivores, it can easily invade and dominate short vegetation. The tall-growing grass Elymus athericus dominates the highly productive stages of a salt-marsh succession in Schiermonnikoog and is not preferred by the herbivores which occur there, hares and geese. We studied how interspecific competition and herbivory affected performance during early establishment of this species with increasing productivity. Seedlings were planted in the field in a full factorial design, manipulating both interspecific competition and herbivory. The experiment was replicated along a natural productivity gradient. Competition reduced aboveground biomass production and decreased the number of ramets that were produced but did not affect survival of seedlings. The negative effects of competition on seedling performance increased with increasing productivity. In contrast to our expectations, herbivory strongly reduced seedling survival, especially at the unproductive sites and had only small effects on seedling growth. The present study shows that unpreferred tall-growing species cannot easily invade vegetation composed of short preferred species. Grazing by (intermediate-sized) herbivores can prevent establishment at unproductive sites, and increased competition can prevent a rapid invasion of highly productive sites. Herbivores can have a long-lasting impact on vegetation succession by preventing the establishment of tall-growing species, such as E. athericus, in a window of opportunity at young unproductive successional stages.Plant nomenclature follows Van der Meijden et al. (1990)     

The relative contributions of sexual reproduction and clonal propagation in Opuntia rastrera from two habitats in the Chihuahuan Desert

1 The clonal cactus Opuntia rastrera shows predominantly sexual reproduction in grasslands (GH) and clonal propagation in nopaleras (NH). We assessed the effects of light, herbivory, water availability and the habitat an offspring came from on the survival and growth of sexual or clonal offspring (i.e. seedlings and cladodes), through 3- and 4-year common garden and short-term greenhouse experiments.
2 Shading by nurse plants increased seedling survival in the field by an order of magnitude, and a small additional advantage due to predator protection by grasses was observed. Strong herbivory transforms a facultative nurse–protégé relationship for seedlings into an obligatory one.
3 In the greenhouse seedlings grew better under shade, but in the field the production of the first cladode was delayed in seedlings in the more shaded GH. Competition for soil resources may be more intense under a dense grass tussock than under a open shrub, thus affecting the nurse–protégé relationship. Seedling survival under nurse plants was similar in GH and NH, but higher plant cover suggests that a larger number of seedlings will establish in GH in the long term.
4 Cladode survival was higher in NH. Cladodes were more successful than seedlings at establishing in intercanopy areas, possibly due to physiological differences as well as their ability to survive partial predation. Cladode survival in intercanopy areas may explain the enhanced clonal propagation in the more open NH scrubland, together with their susceptibility to the flooding which affects GH.
5 The high seedling and cladode survival in the greenhouse experiments contrasted with that observed in the field, indicating that survival is determined by the interaction between herbivores, plants and abiotic conditions rather than the physiological aptitude of the plants.     

Spatial and temporal habitat use patterns for salt marsh nekton: implications for ecological functions

We synthesized information on temporal and spatial patterns of salt marsh habitat use by nekton in order to infer the importance of five main types of marsh function: reproduction, foraging, refuge from predation, refuge from stressful environmental conditions and environmental enhancement of physiology. We then extended the concept that intertidal gradients regulate habitat use patterns of nekton to a more universal concept that applies to all salt marsh habitats. We contend that all marsh habitats are linked to each other and to adjacent estuarine habitats along a depth gradient that mediates gradients in abiotic and biotic conditions. Tidal, diel and seasonal shifts in the magnitude and direction of these gradients result in gradients in tidal, diel and seasonal variation in biotic and abiotic conditions within the salt marsh. Collectively these gradients form the `marsh gradient'. We propose that patterns of marsh use and ecological function for nekton result primarily from physiological and behavioral responses to this marsh gradient. A comparison of habitat use patterns in the context of the marsh gradient is an important – and underutilized – method to study marsh function and essential fish habitat. We note that our limited insight into the function of the marsh habitat results from a significant lack of information on the occurrence and causes of tidal, diel and ontogenetic marsh use patterns by nekton; this is particularly relevant with respect to data on the variation in environmental conditions along marsh gradients over tidal, diel and seasonal cycles and on how environmental variation on these scales influences nekton behavior, growth and survival.     

Biotic interactions mediate the expansion of black mangrove (Avicennia germinans) into salt marshes under climate change

Many species are expanding their distributions to higher latitudes due to global warming. Understanding the mechanisms underlying these distribution shifts is critical for better understanding the impacts of climate changes. The climate envelope approach is widely used to model and predict species distribution shifts with changing climates. Biotic interactions between species, however, may also influence species distributions, and a better understanding of biotic interactions could improve predictions based solely on climate envelope models. Along the northern Gulf of Mexico coast, USA, subtropical black mangrove (Avicennia germinans) at the northern limit of its distribution grows sympatrically with temperate salt marsh plants in Florida, Louisiana, and Texas. In recent decades, freeze‐free winters have led to an expansion of black mangrove into salt marshes. We examined how biotic interactions between black mangrove and salt marsh vegetation along the Texas coast varied across (i) a latitudinal gradient (associated with a winter‐temperature gradient); (ii) the elevational gradient within each marsh (which creates different marsh habitats); and (iii) different life history stages of black mangroves (seedlings vs. juvenile trees). Each of these variables affected the strength or nature of biotic interactions between black mangrove and salt marsh vegetation: (i) Salt marsh vegetation facilitated black mangrove seedlings at their high‐latitude distribution limit, but inhibited black mangrove seedlings at lower latitudes; (ii) mangroves performed well at intermediate elevations, but grew and survived poorly in high‐ and low‐marsh habitats; and (iii) the effect of salt marsh vegetation on black mangroves switched from negative to neutral as black mangroves grew from seedlings into juvenile trees. These results indicate that the expansion of black mangroves is mediated by complex biotic interactions. A better understanding of the impacts of climate change on ecological communities requires incorporating context‐dependent biotic interactions into species range models.     

Drivers of seedling survival in a temperate forest and their relative importance at three stages of succession

Negative density dependence (NDD) and niche partitioning have been perceived as important mechanisms for the maintenance of species diversity. However, little is known about their relative contributions to seedling survival. We examined the effects of biotic and abiotic neighborhoods and the variations of biotic neighborhoods among species using survival data for 7503 seedlings belonging to 22 woody species over a period of 2 years in three different forest types, a half‐mature forest (HF), a mature forest (MF), and an old‐growth forest (OGF), each of these representing a specific successional stage in a temperate forest ecosystem in northeastern China. We found a convincing evidence for the existence of NDD in temperate forest ecosystems. The biotic and abiotic variables affecting seedlings survival change with successional stage, seedling size, and age. The strength of NDD for the smaller (<20 cm in height) and younger seedlings (1–2 years) as well as all seedlings combined varies significantly among species. We found no evidence that a community compensatory trend (CCT) existed in our study area. The results of this study demonstrate that the relative importance of NDD and habitat niche partitioning in driving seedling survival varies with seedling size and age and that the biotic and abiotic factors affecting seedlings survival change with successional stage.     

SEEDLING RECRUITMENT LIMITATION IN WHITE CLOVER (TRIFOLIUM REPENS; LEGUMINOSAE)

White clover (Trifolium repens L.) is a stoloniferous clonal legume. Dynamics of white clover populations are controlled by the recruitment of both seedlings (genets) and node production (ramets). To understand the role of seedling dynamics in natural populations of white clover, the recruitment and mortality of seedlings in a lawn and pasture clover population were followed and the roles of biotic and abiotic factors in seedling recruitment and survival were assessed. Seedling recruitment was several orders of magnitude less than vegetative recruitment in both lawn and pasture populations. However, at least several new genets are added to these populations on a yearly basis. In both locations, physical disturbances may open sites for seed germination, as disturbed sites had higher seed germination rates than undisturbed sites. However, these sites were not favorable for long-term establishment due to such factors as frost upheaval. Therefore, there are trade-offs in safe site attributes for seed germination and establishment that strongly limit recruitment in natural populations of white clover.     

Herbivory affects salt marsh succession dynamics by suppressing the recovery of dominant species

Disturbance can generate heterogeneous environments and profoundly influence plant diversity by creating patches at different successional stages. Herbivores, in turn, can govern plant succession dynamics by determining the rate of species replacement, ultimately affecting plant community structure. In a south-western Atlantic salt marsh, we experimentally evaluated the role of herbivory in the recovery following disturbance of the plant community and assessed whether herbivory affects the relative importance of sexual and clonal reproduction on these dynamics. Our results show that herbivory strongly affects salt marsh secondary succession by suppressing seedlings and limiting clonal colonization of the dominant marsh grass, allowing subordinate species to dominate disturbed patches. These results demonstrate that herbivores can have an important role in salt marsh community structure and function, and can be a key force during succession dynamics.     

Nature of the Interference Mechanism of Sugarcane (Saccharum officinarum L.) Straw

Sugarcane (Saccharum officinarum L.) straw left in the field after harvest interferes with the growth of winter and summer weeds. In the last years, there was a progressive move away from burning sugarcane straw to retaining it on the soil surface after harvest to prevent soil degradation and environmental pollution. Water-soluble phenolics leachated from straw into soil may suppress weed growth. A study was carried out to investigate (1) the effect of biotic (unautoclaved) soil treated with burned and unburned sugarcane straw leachates on seedling growth and foliar proline content of beggarticks (Bidens subalternans L.) and wild mustard (Brassica campestris L.), (2) the modification of sugarcane straw phytotoxicity in abiotic (autoclaved) soil and biotic (unautoclaved) soil plus activated charcoal, and (3) changes of inorganic ions and phenolic contents in biotic soil after treatment with burned and unburned sugarcane straw leachate. Unburned straw leachate significantly inhibited root elongation of 7-d-old beggarticks and wild mustard seedlings. Burned straw leachate did not affect seedling growth of the assayed weeds suggesting that organic straw phytotoxins were involved. Experiments with activated charcoal, however, did not provide clear evidence supporting the involve of organic molecules in straw phytotoxicity. Unburned straw leachate incorporated in biotic soil was more inhibitory than in abiotic soil on root growth suggesting that microbial activity is involved in sugarcane straw interference. There was no evidence of nutrient microbial immobilization. Unburned sugarcane straw leachate increased total phenolic content in biotic soil more than in abiotic soil or biotic soil plus charcoal. Burned sugarcane straw leachate did not increase phenolic compounds levels in biotic soil. Linear regression analysis indicated a strong correlation between levels of soil phenolic contents and root growth inhibition. Soil characteristics evaluated in soil treated with burned and unburned sugarcane straw leachate suggest that straw phytotoxicity is related with organic molecules, such as phenolic compounds, rather than to variations in inorganic nutrients. Unburned straw leachate induced proline accumulation in seedling leaves of both beggarticks and wild mustard. Proline foliar content was higher in seedlings grown in biotic soil than in seedlings grown in biotic soil plus charcoal suggesting that straw organic constituents induced proline accumulation. Proline foliar content of seedlings grown in biotic soil treated with burned straw leachate was not significantly different from water control. The present study showed that sugarcane straw leachate interferes with seedling growth of beggarticks and wild mustard and that water-soluble phenolics can play a role in the seedling growth inhibition of the assayed weeds.     

Changes in rainfall pattern affect crab herbivory rates in a SW Atlantic salt marsh

Climatic fluctuations usually change the intensity of existing interactions. Thus, in the context of the global climate change, it is important to consider new potential interactions or changes that may appear. Heavy rainy periods (one of the consequences of global climate change in eastern-central Argentina) can promote flooding in some estuaries (mainly on coastal lagoons), and thus, affect interactions between species. In this work we investigate if climatic fluctuations can affect Spartina densiflora Brong. (dominant marsh plant) survival through a chain of biotic and abiotic interactions in a SW Atlantic costal lagoon (37° 40′S, 57° 23′W; Mar Chiquita, Argentina). To achieve this, the long-term rainfall behavior of this region, and the effect of rainy periods on submergence of estuarine marsh areas (using satellite images) were analyzed. Then, the effect of flooding on the activity of the dominant herbivore of this system, the burrowing crab Neohelice granulata (= Chasmagnathus granulatus), was studied using pitfall traps. Finally, the effect of flooding on crab herbivory rates and plant survival were analyzed using transplants, stem-marking and flooding experiments. Long-term rainfall behavior showed that mean annual rainfall has increased during the last century, with the occurrence of more rainy years, and increases in cumulative monthly rainfall increased the submerged area of the S. densiflora marsh. Also, crab activity in the marsh largely increased during periods of flooding, associated with more than 100% increments in herbivory rates and stem mortality. These results reveal that increments in rainfall regime can trigger a cascade of abiotic and biotic interactions leading to increased marsh mortality, and stresses the importance of considering both, biotic and abiotic factors, together to predict changes in community organization.     

Resistance of Island Rainforest to Invasion by Alien Plants: Influence of Microhabitat and Herbivory on Seedling Performance

Experiments have promise in determining mechanisms by which communities resist invasion. Growth and survivorship of transplanted seedlings of introduced tree species (Leucaena leucocephala, Muntingia calabura, Adenanthera pavonia, and Clausena excavata) were used to assess abiotic (light regime) and biotic resistance (herbivory) to invasion of rainforest on Christmas Island (Indian Ocean). At four sites, seedlings were transplanted into the forest edge along roadside verges and into adjacent intact forest; half were caged to prevent access by the dominant seedling consumer, the red land crab (Gecarcoidea natalis). Red crab densities did not differ between roadside and forest plots. Red crabs initially reduced survivorship of Leucaena in both edge and interior plots but virtually all seedlings in the forest interior were dead after 41 weeks. Survivorship of Muntingia was also initially reduced by crabs in the forest edge, but again, all but one seedling died by the end of the experiment. Seedlings of Adenanthera and Clausena fared much better overall, surviving well in both locations. Red crabs had no overall effect on Adenanthera survival but significantly reduced survival of Clausena in forest plots. For both species, seedling performance was greater in the forest edge than in the forest interior. Red crabs had no effect on height increment for either species; however, for Clausena, red crabs reduced seedling mass in the forest interior. Both Adenanthera and Clausena were able to persist in the intact forest. Clausena is now actively invading intact rainforest, but Adenanthera appears dispersal-limited. Resistance factors in intact forest appear hierarchical: biotic resistance afforded by land crabs can impede establishment of some plant invaders but seedling responses to abiotic factors (e.g., the light regime) largely overwhelm its effect. Together, these two community attributes are likely to restrict the range of plant invaders to a small suite of species that can successfully establish in intact rainforest on the island. However, increased propagule pressure from a variety of shade-tolerant species and further declines in the abundance of the dominant seedling consumer may lead to increasing invasion success in this island rainforest. This revised version was published online in July 2006 with corrections to the Cover Date.     

Establishment of Carex stricta Lam. seedlings in experimental wetlands with implications for restoration

The loss of Carex dominated meadows due to agricultural drainage in the previously glaciated midcontinent of North America has been extensive. The lack of natural Carex recruitment in wetland restorations and the failures of revegetation attempts underscore the need for information on the establishment requirements of wetland sedges. In this study, seedlings of Carex stricta Lam. were planted in three experimental wetlands in east-central Minnesota, USA to investigate the biotic and abiotic environmental limitations to establishment. Seedlings were planted along an elevational water depth gradient to assess the effects of water depth and water level fluctuation on seedling survival and growth. A different water level fluctuation regime was assigned to each of the experimental wetlands to assess seedling tolerance for seasonal water level changes. The effects of seedling planting density and the presence or absence of non-sedge colonizers on seedling survival and growth were also studied. The experiment was followed for three growing seasons. The results of this study indicate that C. stricta seedlings were sensitive to the timing and duration of inundation during the first growing season. Once established, plants tolerated a broad range of seasonal drying and flooding conditions. Seedling and juvenile growth was slowed by non-sedge colonizers during the first two growing seasons, but by the third growing season, C. stricta was able to out-grow all annual and perennial weeds, except the aggressive perennial, Phalaris arundinacea L. The rapid growth of C. stricta plants, once established, indicates that the use of seedlings is a successful method for (re) introducing this tussock sedge into wetland restorations under a variety of environmental conditions. Comparison with other studies performed under similar conditions suggests that planting of seedlings is a more appropriate method of establishing this species than the use of transplanted rhizomes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Salt stress limitation of seedling recruitment in a salt marsh plant community

Summary Seedling recruitment in salt marsh plant communities is generally precluded in dense vegetation by competition from adults, but is also relatively rare in disturbance-generated bare space. We examined the constraints on seedling recruitment in New England salt marsh bare patches. Under typical bare patch conditions seed germination is severely limited by high substrate salinities. We examined the germination requirements of common high marsh plants and found that except for one notably patch-dependent fugitive species, the germination of high marsh plants is strongly inhibited by the high soil salinities routinely encountered in natural bare patches. Watering high marsh soil in the greenhouse to alleviate salt stress resulted in the emergence of up to 600 seedlings/225 cm². The vast majority of this seed bank consisted of Juncus gerardi, the only common high marsh plant with high seed set. We tested the hypothesis that salt stress limits seedling contributions to marsh patch secondary succession in the field. Watering bare patches with fresh water partially alleviated patch soil salinities and dramatically increased both the emergence and survival of seedlings. Our results show that seedling recruitment by high marsh perennial turfs is limited by high soil salinities and that consequently their population dynamics are determined primarily by clonal growth processes. In contrast, populations of patch-dependent fugitive marsh plants which cannot colonize vegetatively are likely governed by spatially and temporally unpredictable windows of low salinities in bare patches.     

Effects of local biotic neighbors and habitat heterogeneity on seedling survival in a spruce‐fir valley forest,northeastern China

Seedlings are vulnerable to many biotic and abiotic agents, and studying seedling dynamics helps understand mechanisms of species coexistence. In this study, the relative importance of biotic neighbors and habitat heterogeneity to seedling survival was examined by generalized linear mixed models for 33 species in a spruce‐fir valley forest in northeastern China. The results showed that the relative importance of these factors varied with species and functional groups. Conspecific negative density dependence (CNDD) was important to the survival of Abies nephrolepis and Picea koraiensis seedling, whereas phylogenetic negative density dependence (PNDD) was critical to Pinus koraiensis and Betula platyphylla, as well as functional groups of tree, deciduous, and shade‐intolerant seedlings. For shrubs and Acer ukurunduense, habitat heterogeneity was significant. Despite of the significance of CNDD, PNDD, and habitat heterogeneity on seedling survival, large proportions of the total variance were not accounted for by the studied variables, suggesting the needs to examine the influences of other factors such as pests, diseases, herbivores, forest structure, species functional traits, and microclimatic conditions on seedling survival in the future.     

Interspecific competition and insect herbivory reduce bush lupine (Lupinus arboreus ) seedling survival

Seedlings suffer high mortality in most plant populations, with both competition and herbivory proposed as being important mechanisms causing seedling death. The relative strength of these factors, however, is often unknown. Here I ask how interspecific competition for light and insect herbivory jointly affect seedling survival of bush lupine (Lupinus arboreus), a native shrub common to coastal California. Bush lupine seedlings germinate in grasslands during winter, and throughout spring potentially compete for light with surrounding fast-growing annual grasses. By early summer, after grasses have died, seedlings can be defoliated by a locally abundant caterpillar, the western tussock moth (Orgyia vetusta). I examined the relative importance of competition and insect herbivory on seedling survival in two separate experiments. First, I compared seedling mortality in plots either exposed to or protected from tussock moth larvae. Plants were protected from herbivory by the judicious use of insecticide; control plants were sprayed with water. Tussock moth herbivory resulted in significantly greater (31%) seedling mortality. To determine the effects of interspecific competition for light on seedling survival, I manipulated the density of grass surrounding lupine seedlings. I removed all vegetation surrounding some individuals, and left intact vegetation surrounding others. Reducing competition resulted in a 32% increase in seedling survival from February to June, as well as changes in seedling growth. To determine whether there are interactive effects of competition and herbivory on seedling survival, I enclosed tussock moth larvae on half of all surviving seedlings within each of the two prior competition treatments, comparing growth and survival of defoliated and undefoliated seedlings. Defoliation in June led to an additional 50% mortality for individuals that had grown with competitors through spring, and a 53% additional mortality for seedlings that grew without competitors through spring. Thus, although competition and herbivory both caused substantial seedling mortality, there was no statistical interaction between these factors. Competition-free plants were not less vulnerable to herbivory than plants that previously grew with competitors. Taken together, these experiments indicate that competition and herbivory are both important sources of mortality for bush lupine seedlings. Received: 4 April 1996 / Accepted: 5 November 1996