Light gradient partitioning by tropical tree seedlings in the absence of canopy gaps

To explore the importance of light availability for seedling growth in low light environments, we examined light-dependent growth, biomass allocation and mortality of tree seedlings growing in sites with 0.2-6.5% full sun, the range of light commonly encountered in the understory of closed canopy, lowland tropical forests. We transplanted seedlings of the canopy tree species, Dipteryx panamensis, Virola koschnyii, and Brosimum alicastrum into second-growth forest and native tree plantations at La Selva Biological Station, Costa Rica. We assessed seedling survival, growth, and seedling light environments bimonthly for 14 months. Plants were harvested at the end of the study to assess leaf area, total biomass, biomass partitioning and root growth. Survivorship of all species exceeded 60% across all microsites, although both D. panamensis and B. alicastrum had lower probabilities of survival in the darkest microsites. All species showed a strong positive relationship between light availability and growth, increasing in total biomass as light increased. However, the strength of the growth response differed among species causing a change in the rank order of species growth rates as light availability increased. Although D. panamensis showed the lowest growth rates in the darkest microsites, a strong response to increasing light led to a cross-over in performance, such that D. panamensis had the highest growth rate at the highest light levels studied. These data suggest that resource gradient partitioning could occur even in low light environments (0.2-6.5%). Given the limited range of light regimes sampled (i.e., non-gap microsites), our data demonstrate that growth of tropical tree seedlings beneath closed canopies is highly sensitive to light availability and that shade-tolerant species vary in these responses. Our results show that understory light heterogeneity, in the absence of canopy gaps, can significantly affect recruitment processes for shade-tolerant tree species.     

Savanna tree–grass competition is modified by substrate type and herbivory

Question: Woody plant and grass interactions in savannas have frequently been studied from the perspective of the response of one growth form on the other but seldom evaluated as two‐way interactions. What causes woody plant encroachment in semi‐arid savannas and what are the competitive responses of tree seedlings and grasses on rocky and sandy substrates? Methods: In this greenhouse study, we investigated the influence of substrate and grazing on responses to interspecific competition by tree seedlings and grasses. We measured competitive/facilitative responses on biomass and nutrient status of tree seedlings and grasses grown together. Results: Interspecific competition suppressed growth of trees and grasses. Tree seedlings and uncut grass accumulated double the biomass when grown without competition relative to when they competed. Competitive responses varied on different substrates. Grass biomass on rocky substrate showed no response to tree competition, but appeared to be facilitated by trees on sandy substrate. Grass clipping resulted in higher tree seedling biomass on rocky substrate, but not on sandy substrate. There was a positive response of grass nutrient status to competition from tree seedlings. Conclusion: Selective grass herbivory in the absence of browsing or suppression of shade‐intolerant grasses by trees are commonly cited reasons behind bush encroachment in savannas. We show that grazing may confer a competitive advantage to tree seedlings and promote bush encroachment more readily on rocky substrates. This may be due to the imposed sharing of the soil depth niche on rocky substrates, whereas possible niche separation on sandy substrates minimizes the advantage conferred by reduced competition.     

Additive and nonadditive effects of herbivory and competition on tree seedling mortality, growth, and allocation

The interaction between simulated cotyledon herbivory and interspecific competition was studied in a greenhouse experiment using two species of trees, Acer rubrum and Quercus palustris, which commonly invade abandoned agricultural fields. Herbivory treatments were applied as a gradient of cotyledon removal for A. rubrum with 0, 25, 50, 75, and 100% of cotyledon tissue removed. Cotyledons from Q. palustris were clipped and removed (control, early, and late removal) to create a gradient of seed reserve availability. The competition treatment consisted of plugs of old-field vegetation that filled the pots with perennial cover. Mortality of seedlings was higher with competition. There was a significant interaction between herbivory and competition with the highest mortality occurring with competition at the highest intensity of herbivory in both species. Herbivory reduced biomass for Q. palustris only, while competition reduced biomass in both species. Neither species showed an interaction between herbivory and competition for growth. There was a significant interaction between herbivory and competition on allocation patterns for both species, with greater allocation to roots with competition at the highest intensity of herbivory. This study demonstrates the potential for cotyledon herbivory and competition to interact, altering the invasion of tree seedlings into abandoned agricultural land.     

Trade-off between plant growth and defense? A comparison of sagebrush populations

We used ecotypic variation in big sagebrush (Artemisia tridentata) to examine potential trade-offs between inherent growth rate and tolerance or resistance to herbivory. Seeds were obtained from seven geographic populations, and 1,120 seedlings were established in a common garden. In one set of plots, plants were subjected to five treatments: control, regular insecticide spray, moderate browsing, severe browsing, or moderate browsing plus insecticide. Plants in a second set of plots were all untreated, and were used to estimate ambient growth, flower production, and susceptibility to herbivorous insects. In the first growing season, population differences in relative growth rate produced approximately seven-fold variation in mean biomass. Two populations of basin big sagebrush (A. tridentata tridentata) and one population of mountain big sagebrush (A. tridentata vaseyana) grew fastest; those of Wyoming big sagebrush (A. tridentata wyomingensis) showed the slowest growth. Bi-weekly application of insecticide for two growing seasons had no effect on the growth of either browsed or unbrowsed plants. All populations showed compensatory growth (but not overcompensation) in response to browsing, but the degree of compensation was unrelated to inherent growth rate. Similarly, there was no consistent relationship between plant growth rate and flower production in the second growing season. Some insects colonized fast-growing populations more frequently than slow-growing ones, but patterns of insect colonization were species-specific. At the level of geographic populations and subspecies, we found little evidence of a built-in trade-off between inherent growth rate and the ability to tolerate or resist herbivory. Because population ranks for growth rate changed substantially between seasons, attempts to correlate growth and defense characters need to account for differences in the growth trajectories of perennial plants.     

Density Effects of a Dominant Understory Herb,Isoglossa woodii (Acanthaceae), on Tree Seedlings of a Subtropical Coastal Dune Forest

Suppression of tree seedlings by the understory is an important ecological filter with implications for tree diversity and dynamics. In a greenhouse competition experiment, we used seedlings of four canopy species from coastal dune forest (Diospyros natalensis, Euclea racemosa, Sideroxylon inerme and Apodytes dimidiata) to examine the relative competitive effects of the dominant understory herb Isoglossa woodii on seedling performance. We manipulated I. woodii density, light and nutrient levels and measured growth responses. Total seedling biomass decreased with density of I. woodii. The magnitude of biomass suppression with competitor density was similar among tree species. Consequently there was no discernable hierarchy of competitive ranking among tree species. The relative growth rate of seedlings decreased at higher densities of I. woodii and increased at higher nutrient levels but was unaffected by variation in light conditions. Aboveground biomass decreased at higher densities of I. woodii and at higher light levels but increased at higher nutrient levels. Size asymmetric competition for light and nutrients may be the major driver of aboveground interactions between tree seedling and I. woodii. While tree species showed no hierarchy of competitive ability their seedlings exhibited equivalent responses to competition from an understory dominant, permitting species coexistence and the maintenance of species diversity.     

An Afforestation System for Restoring Bottomland Hardwood Forests: Biomass Accumulation of Nuttall Oak Seedlings Interplanted Beneath Eastern Cottonwood

Bottomland hardwood forests of the southeastern United States have declined in extent since European settlement. Forest restoration activities over the past decade, however, have driven recent changes in land use through an intensified afforestation effort on former agricultural land. This intense afforestation effort, particularly in the Lower Mississippi Alluvial Valley, has generated a demand for alternative afforestation systems that accommodate various landowner objectives through restoration of sustainable forests. We are currently studying an afforestation system that involves initial establishment of the rapidly growing native species eastern cottonwood (Populus deltoides Bartr. ex Marsh.), followed by enrichment of the plantation understory with Nuttall oak (Quercus nuttallii Palm.). In this article, we examine the growth and biomass accumulation by Nuttall oak seedlings to determine whether this species can be established and whether it will develop beneath the cottonwood overstory. After 3 years of growth beneath cottonwood canopies, Nuttall oak seedlings were similar in height (126 cm), but were 20% smaller in root‐collar diameter than seedlings established in open fields. Seedlings established in the open accumulated more than twice the biomass of seedlings growing beneath a cottonwood canopy. However, the relative distribution of accumulated biomass in seedlings did not differ in the two environments. Ten percent of total seedling biomass was maintained in leaf tissue, 42% was maintained in stem tissue, and 48% was maintained in root tissue on open‐grown seedlings and seedlings established in the understory of cottonwood plantations. Though establishment in the more shaded understory environment reduced Nuttall oak growth, seedling function was not limited enough to induce changes in plant morphology. Our results suggest that an afforestation system involving rapid establishment of forest cover with a quick‐growing plantation species, followed by understory enrichment with species of later succession, may provide an alternative method of forest restoration on bottomland hardwood sites and perhaps other sites degraded by agriculture throughout temperate regions.     

Herbivores modify selection on plant functional traits in a temperate rainforest understory

There is limited evidence regarding the adaptive value of plant functional traits in contrasting light environments. It has been suggested that changes in these traits in response to light availability can increase herbivore susceptibility. We tested the adaptive value of plant functional traits linked with carbon gain in contrasting light environments and also evaluated whether herbivores can modify selection on these traits in each light environment. In a temperate rainforest, we examined phenotypic selection on functional traits in seedlings of the pioneer tree Aristotelia chilensis growing in sun (canopy gap) and shade (forest understory) and subjected to either natural herbivory or herbivore exclusion. We found differential selection on functional traits depending on light environment. In sun, there was positive directional selection on photosynthetic rate and relative growth rate (RGR), indicating that selection favors competitive ability in a high-resource environment. Seedlings with high specific leaf area (SLA) and intermediate RGR were selected in shade, suggesting that light capture and conservative resource use are favored in the understory. Herbivores reduced the strength of positive directional selection acting on SLA in shade. We provide the first demonstration that natural herbivory rates can change the strength of selection on plant ecophysiological traits, that is, attributes whose main function is resource uptake. Research addressing the evolution of shade tolerance should incorporate the selective role of herbivores.     

Clipping defoliation and nitrogen addition shift competition between a C3 grass (Leymus chinensis) and a C4 grass (Hemarthria altissima)

• Human‐induced disturbances, including grazing and clipping, that cause defoliation are common in natural grasslands. Plant functional type differences in the ability to compensate for this tissue loss may influence interspecific competition.
• To explore the effects of different intensities of clipping and nitrogen (N) addition on compensatory growth and interspecific competition, we measured accumulated aboveground biomass (AGB), belowground biomass (BGB), tiller number, non‐structural carbohydrates concentrations and leaf gas exchange parameters in two locally co‐occurring species (the C₃ grass Leymus chinensis and the C₄ grass Hemarthria altissima) growing in monoculture and in mixture.
• For both grasses, the clipping treatment had significant impacts on the accumulated AGB, and the 40% clipping treatment had the largest effect. BGB gradually decreased with increasing defoliation intensity. Severe defoliation caused a significant increase in tiller number. Stored carbohydrates in the belowground biomass were mobilised and transported aboveground for the growth of new leaves to compensate for clipping‐induced injury. The net CO₂ assimilation rate (A) of the remaining leaves increased with clipping intensity and peaked under clipping intensities of 20% or 40%. Nitrogen addition, at a rate of 10 g·N·m^?2·year^?1, enhanced A of the remaining leaves and non‐structural carbohydrate concentrations, which benefited plant compensatory growth, especially for the C₃ grass. Under the mixed planting conditions, the clipping and N addition treatments lowered the competitive advantage of the C₄ grass.
• The results suggest that a combination of defoliation and N deposition have the potential to benefit the coexistence of C₃ and C₄ grasses.

     

Ecophysiological responses of Fagus sylvatica seedlings to changing light conditions. II. The interaction of light environment and soil fertility on seedling physiology

The survival and growth of natural beech regeneration after canopy removal is variable and little is known about ecophysiological mechanisms of these responses. Biomass, nonstructural carbohydrate levels and nitrogen concentrations were measured in an Italian population of European beech seedlings. Seedlings were container-grown in two types of soil, organic and mineral, collected at the study site. The seedlings were grown under three light treatments: under full beech canopy (understory), exposed to full sun only during midday (gap) and under full sun (clearing). Leaf gas exchange and chlorophyll a fluorescence parameters were measured and then foliar analyses were conducted for chlorophyll, phenolic and tannin levels. Biomass and allocation were significantly affected by light and soil treatments. The clearing seedlings and those in organic soil were larger than seedlings in the other light treatments or soil type. Total nonstructural carbohydrate concentrations were lower in the understory seedlings and significant differences between soil types were present in the gap and clearing seedlings. Nitrogen concentrations were higher in the understory seedlings and those growing in the organic soil compared to the other treatments. Gas exchange rates were highest in clearing and the organic soil seedlings. Gap seedlings exhibited photosynthetic acclimation that allowed them to utilize high light of midday and any sunflecks during the morning and afternoon. Relative fluorescence was significantly influenced by both light treatment and soil type, with the highest values observed in the gap seedlings. Light response curves showed decreasing apparent maximum quantum efficiency from the understory to clearing, while maximum photosynthetic rate was highest in the gap seedlings. Chlorophyll concentration was highest in understory seedlings and those growing in organic soil and higher in seedlings growing in organic than in mineral soil. Both foliar tannin and phenolic levels were highest in clearing seedlings, and only tannin concentrations were affected by soil type. Understory seedlings had the highest mortality and insect herbivory; the latter was found to be inversely related to tannin concentration. Overall, growth and photosynthesis in beech seedlings responded positively to high light associated with small canopy gaps. Organic soil increased seedling size, particularly in the gap and clearing environments. We conclude that forest gaps are favorable for photosynthesis and growth of European beech seedlings.     

西双版纳热带季节雨林林冠种群种内、种间竞争的初步研究

利用Hegyi的竞争指数模型,分析了西双版纳热带季节雨林的主要类型绒毛番龙眼群落中林冠种群种内和种间的竞争强度及种群和个体在群落中的竞争干扰状态.结果表明,季节雨林中林冠种群的种间竞争大于种内竞争,每个林冠种群一般与多个其它林冠种群发生竞争.多数种群既被竞争种干扰,又干扰竞争种.个体在群落中所处的层次越高,受到的竞争干扰越小,下层中的个体多数处于受压迫状态.群落中没有完全的竞争优胜者.只有成年个体处于竞争优势地位,幼苗、幼树又能在受压迫中生存的种群,才能在群落中保持长期存在.     

Intra and interspecific competition among invasive and native species during early stages of plant growth

Plant competition is a primary ecological process limiting grassland restoration success. Appropriate restoration techniques require an understanding of the degree to which intra and interspecific competition control invasive and native plant growth. The objective of this study was to determine how the intensity of intra and interspecific competition changes during early stages of plant growth. Two invasive (Bromus tectorum and Taeniatherum caput-medusae) and two native (Pseudoroegneria spicata and Poa secunda) species were grown in a diallel competition experiment, either alone or in 1:1 binary combinations and exposed to two levels of N (no N or 400 mg N kg⁻¹ soil added) in a greenhouse. Total biomass for each species was quantified over four harvests and competitive effects were calculated. Our results show that the relative magnitude of intra and interspecific competition changes through time. Intraspecific competition was intense for native species at the initial harvests and therefore important in contributing to the outcome of final size of native species seedlings. Interestingly, bluebunch wheatgrass imposed interspecific competition on annual grasses at the first two harvests and appeared to be a better competitor than Sandberg’s bluegrass. We found that fast growing invasive species became more competitive compared to slow growing native species with increasing N and appear to establish a positive feedback mechanism between size and resource uptake. Opportunities to improve restoration success exist from determining the optimum combination of density, species proportion, and their spatial arrangement in various ecosystems and environments.     

Herbivory on Temperate Rainforest Seedlings in Sun and Shade: Resistance,Tolerance and Habitat Distribution

Differential herbivory and/or differential plant resistance or tolerance in sun and shade environments may influence plant distribution along the light gradient. Embothrium coccineum is one of the few light-demanding tree species in the temperate rainforest of southern South America, and seedlings are frequently attacked by insects and snails. Herbivory may contribute to the exclusion of E. coccineum from the shade if 1) herbivory pressure is greater in the shade, which in turn can result from shade plants being less resistant or from habitat preferences of herbivores, and/or 2) consequences of damage are more detrimental in the shade, i.e., shade plants are less tolerant. We tested this in a field study with naturally established seedlings in treefall gaps (sun) and forest understory (shade) in a temperate rainforest of southern Chile. Seedlings growing in the sun sustained nearly 40% more herbivore damage and displayed half of the specific leaf area than those growing in the shade. A palatability test showed that a generalist snail consumed ten times more leaf area when fed on shade leaves compared to sun leaves, i.e., plant resistance was greater in sun-grown seedlings. Herbivore abundance (total biomass) was two-fold greater in treefall gaps compared to the forest understory. Undamaged seedlings survived better and showed a slightly higher growth rate in the sun. Whereas simulated herbivory in the shade decreased seedling survival and growth by 34% and 19%, respectively, damaged and undamaged seedlings showed similar survival and growth in the sun. Leaf tissue lost to herbivores in the shade appears to be too expensive to replace under the limiting light conditions of forest understory. Following evaluations of herbivore abundance and plant resistance and tolerance in contrasting light environments, we have shown how herbivory on a light-demanding tree species may contribute to its exclusion from shade sites. Thus, in the shaded forest understory, where the seedlings of some tree species are close to their physiological tolerance limit, herbivory could play an important role in plant establishment.     

The influence of competition from herbaceous vegetation and shade on simulated browsing tolerance of coniferous and deciduous saplings

The ability of saplings to tolerate browsing (i.e. the ability to persist with reduced biomass and to compensate for biomass loss) is influenced by the level of stress and their growth strategies. Ultimately, insight into species‐specific responses of saplings to browsing, shade and competition from neighbours will help explain diversity, structure and function of grazed ecosystems such as the endangered wood‐pasture systems. We measured the survival, whole‐sapling biomass and compensatory growth responses of two coniferous (Picea abies and Abies alba) and two deciduous (Acer pseudoplatanus and Fagus sylvatica) tree species to simulated summer browsing (one single clipping event), shade (installation of a shade cloth) and neighbour removal (mowing surrounding vegetation to ground level) treatments and the interactions between them after two‐growing seasons. For all species, there were interacting effects on growth of browsing and environmental condition (shade and neighbours). Simulated browsing resulted in relatively smaller growth losses when plants were growing slowly due to competitive conditions related to herbaceous neighbours. Although none of the clipped saplings could fully compensate for their biomass losses, the saplings were closer to compensation under high competitive conditions than under low competitive conditions. Survival of the clipped saplings remained relatively high and was only significantly reduced for Picea and Acer. Picea was least tolerant of competition and was the only species for which growth was not negatively affected by strong irradiance of a mountain pasture. Surprisingly, the tolerance of saplings to herbivory as browsing tolerance was enhanced under conditions that negatively affected sapling performance (i.e. survival and growth). Apparently, the relative impact of browsing at the early sapling stage is linked to tree life history characteristics such as competition and shade tolerance and will be lower in situations with intense competitive interactions and/or strong irradiance.     

Consequences of differing competitive abilities between juvenile and adult plants

The competitive ability of perennial plants can change with life-stage, but whether these changes have fitness consequences is unknown. We present a simple model of two components of fitness, mortality and flowering rates, for two grassland species with very different patterns of competitive ability and life-stage. Achillea millefolium seedlings are poor competitors while the adults are good competitors. In contrast, Solidago missouriensis seedlings and adults have similar competitive ability. Models of the two species show that the overall effects of competition on growth are more important than interspecific differences in competitive ability in determining mortality and flowering rates, though the higher seedling competitive ability of S. missouriensis relative to A. millefolium seedlings does result in slightly lower mortality and higher flowering rates for the former species. Simulations where both average competitive ability and relative seedling and adult competitive ability are varied predict that dominant species with high overall competitive ability should experience no advantage or disadvantage from varying competitive ability through development. When overall competitive ability is moderate, the relative costs and benefits of differential competitive abilities among adults and seedlings are variable. High seedling competitive ability relative to adult competitive ability should be favored among species with low overall competitive ability. We predict that communities with high intensity of competition should have a high frequency of species with high seedling competitive ability, while communities with lower intensity of competition should have species with a wide range of relative seedling and adult competitive ability.     

Shoot herbivory on the invasive plant, Centaurea maculosa, does not reduce its competitive effects on conspecifics and natives

Herbivory can have negative, positive, or no effect on plants. However, insect biological control assumes that herbivory will negatively affect the weed and release natives from competition. Centaurea maculosa, an invader in North America, is tolerant to herbivory, and under some conditions, herbivory may increase its competitive effects on natives. Therefore, we investigated two hypotheses: 1) herbivory stimulates compensatory growth by C. maculosa, which increases its competitive effects, and 2) herbivory stimulates the allelopathic effect of C. maculosa. In the greenhouse, Trichoplusia ni shoot herbivory reduced C. maculosa biomass when shoot damage exceeded 40% of the total original leaf area. Conspecific neighbors had no effect on C. maculosa biomass, and the presence of the natives Festuca idahoensis and F. scabrella had a positive effect on C. maculosa. Neighbors did not alter the effects of shoot herbivory. More importantly, even intense shoot herbivory on C. maculosa did not benefit neighboring plants. In a field experiment, clipping 50% of C. maculosa aboveground biomass in the early summer and again in the late summer reduced final biomass by 40% at the end of the season; however, this clipping did not affect total biomass production or reproductive output. Festuca idahoensis neighbors did not increase the effects of clipping, and aboveground damage to C. maculosa did not release F. idahoensis from competition. In the greenhouse we used activated carbon to adsorb allelochemicals, which reduced the competitive effects of C. maculosa on F. idahoensis but not on F. scabrella or other C. maculosa. However, we found no increase in the allelopathic effects of C. maculosa after shoot herbivory. In summary, our results correspond with others indicating that exceptionally high intensities of herbivory are required to suppress C. maculosa growth and reproduction; however, even intense herbivory on C. maculosa does not insure that native bunchgrasses will benefit.     

Choice of voles among genotypes of birch seedlings: its relationship with seedling quality and preference of insects

Selective feeding by herbivores on establishing seedlings has been suggested to affect genotype frequencies in several plant populations. The existence of genotypes susceptible to herbivores calls for an explanation in such populations. In the present study we assessed the choice of multiple herbivores, field voles (Microtus agrestis) and insects, among genotypes of silver birch (Betula pendula) representing variation occurring in a naturally regenerated stand. We examined how food choice of voles and insects is related to each other, competitive ability among the seedling genotypes and variation in soil fertility. We set up a field experiment and randomly assigned seedling populations, composed of mixed genotypes, to fully crossed insect exposure and fertilization treatments. After the first growing season we exposed a half of the seedling populations to vole herbivory. Voles selected clearly among the genotypes: they preferred the fastest growing seedlings as well as those with a low density of resin droplets on their stems. The preference of voles and insects among the genotypes was tightly correlated. We conclude that the effects of herbivory compensate those of intraspecific competition in this system and thus favor coexistence of genotypes differing in their susceptibility to herbivores.     

Arbuscular mycorrhizal impacts on competitive interactions between Acacia etbaica and Boswellia papyrifera seedlings under drought stress

Aims Arbuscular mycorrhizal fungi can have a substantial effect on the water and nutrient uptake by plants and the competition between plants in harsh environments where resource availability comes in pulses. In this study we focus on interspecific competition between Acaia etbaica and Boswellia papyrifera that have distinctive resource acquisition strategies. We compared the extent of interspecific competition with that of intraspecific competition.Methods In a greenhouse study we examined the influence of Arbuscular Mycorrhiza (AM) and pulsed water availability on competitive interactions between seedlings of the rapidly growing species A. etbaica and the slowly growing species B. papyrifera. A factorial experimental design was used. The factors were AM, two water levels and five species combinationsImportant findings Seedlings of both species benefitted from AM when grown alone, and the positive growth response to pulsed water availability in B. papyrifera seedlings was in contrast with the negative growth response for A. etbaica seedlings. AM also affected the competitive performance of both species. B. papyrifera was not affected by intraspecific competition, whereas A. etbaica was negatively affected compared to the seedlings grown alone. This effect was stronger in the presence of AM. In interspecific competition, A. etbaica outcompeted B. papyrifera. Mycorrhiza and pulsed water availability did not affect the outcome of interspecific competition, and the aggressivity index of A. etbaica remained unchanged. The extent to which AM influences plant competition in a drought-stressed environment may depend on belowground functional traits of the species. AM and pulsed water availability could modify the balance between intraspecific and interspecific competition. By affecting the balance between intraspecific and interspecific competition, both factors could impact the establishment and survival of seedlings.     

Competition and facilitation structure plant communities under nurse tree canopies in extremely stressful environments

Nurse plant facilitation in stressful environments can produce an environment with relatively low stress under its canopy. These nurse plants may produce the conditions promoting intense competition between coexisting species under the canopy, and canopies may establish stress gradients, where stress increases toward the edge of the canopy. Competition and facilitation on these stress gradients may control species distributions in the communities under canopies. We tested the following predictions: (1) interactions between understory species shift from competition to facilitation in habitats experiencing increasing stress from the center to the edge of canopy of a nurse plant, and (2) species distributions in understory communities are controlled by competitive interactions at the center of canopy, and facilitation at the edge of the canopy. We tested these predictions using a neighbor removal experiment under nurse trees growing in arid environments. Established individuals of each of four of the most common herbaceous species in the understory were used in the experiment. Two species were more frequent in the center of the canopy, and two species were more frequent at the edge of the canopy. Established individuals of each species were subjected to neighbor removal or control treatments in both canopy center and edge habitats. We found a shift from competitive to facilitative interactions from the center to the edge of the canopy. The shift in the effect of neighbors on the target species can help to explain species distributions in these canopies. Canopy‐dominant species only perform well in the presence of neighbors in the edge microhabitat. Competition from canopy‐dominant species can also limit the performance of edge‐dominant species in the canopy microhabitat. The shift from competition to facilitation under nurse plant canopies can structure the understory communities in extremely stressful environments.     

Effects of mycorrhizae on growth and demography of tallgrass prairie forbs

The effects of arbuscular mycorrhizal (AM) symbiosis on ramet and genet densities, vegetative growth rates, and flowering of three forb species were studied in native tallgrass prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed for six growing seasons on replicate plots in an annually burned and an infrequently burned watershed with the fungicide benomyl. Benomyl reduced mycorrhizal root colonization to an average of 4.2%, approximately a two-thirds reduction relative to controls (13.7% colonization). Mycorrhizae influenced the population structure of these forbs. Although mycorrhizal suppression had no long-term effect on genet densities and no effect on ramet survivorship throughout the growing season, the number of ramets per individual was significantly increased such that ramet densities of all three species were approximately doubled in response to long-term mycorrhizal suppression. Effects of mycorrhizae on ramet growth and reproduction varied among species. Ramet growth rates, biomass, and flowering of Salvia azurea were greater in plots with active mycorrhizal symbiosis, whereas mycorrhizae reduced ramet growth rates and biomass of Artemesia ludoviciana. Aster sericeus ramet growth rates and biomass were unaffected by the fungicide applications, but its flowering was reduced.The pattern of responses of these three species to mycorrhizae differed considerably between the two sites of contrasting fire regime, indicating that the interaction of fire-induced shifts in resource availability and mycorrhizal symbiosis together modulates plant responses and the intensity and patterns of interspecific competition between and among tallgrass prairie grass and forb species. Further, the results indicate that effects of mycorrhizae on community structure are a result of interspecific differences in the balance between direct positive effects of the symbiosis on host plant performance and indirect negative effects mediated through altered competitive interactions.     

Chemical defense lowers plant competitiveness

Both plant competition and plant defense affect biodiversity and food web dynamics and are central themes in ecology research. The evolutionary pressures determining plant allocation toward defense or competition are not well understood. According to the growth–differentiation balance hypothesis (GDB), the relative importance of herbivory and competition have led to the evolution of plant allocation patterns, with herbivore pressure leading to increased differentiated tissues (defensive traits), and competition pressure leading to resource investment towards cellular division and elongation (growth-related traits). Here, we tested the GDB hypothesis by assessing the competitive response of lima bean (Phaseolus lunatus) plants with quantitatively different levels of cyanogenesis—a constitutive direct, nitrogen-based defense against herbivores. We used high (HC) and low cyanogenic (LC) genotypes in different competition treatments (intra-genotypic, inter-genotypic, interspecific), and in the presence or absence of insect herbivores (Mexican bean beetle, Epilachna varivestis) to quantify vegetative and generative plant parameters (above and belowground biomass as well as seed production). Highly defended HC-plants had significantly lower aboveground biomass and seed production than LC-plants when grown in the absence of herbivores implying significant intrinsic costs of plant cyanogenesis. However, the reduced performance of HC- compared to LC-plants was mitigated in the presence of herbivores. The two plant genotypes exhibited fundamentally different responses to various stresses (competition, herbivory). Our study supports the GDB hypothesis by demonstrating that competition and herbivory affect different plant genotypes differentially and contributes to understanding the causes of variation in defense within a single plant species.