Contribution of insectivorous avifauna to top down control of Lindera benzoin herbivores at forest edge and interior habitats

Predation of herbivorous Lepidoptera larvae by insectivorous avifauna was estimated on Lindera benzoin in edge and interior habitats at two sites in eastern Pennsylvania (USA). Clay baits modeled after Epimecis hortaria (Geometridae) larvae, the primary herbivore of L. benzoin at our study sites, were used to estimate predation by birds. In both habitat types, models were placed on uninjured L. benzoin leaves as well as on leaves that had prior insect herbivore damage. Rates of model attack were greater, and model longevity reduced, in forest edge plots compared to interiors. Naturally occurring herbivore damage on L. benzoin was greater in forest interiors. However, model attack was not significantly greater on leaves with prior herbivory damage, suggesting that birds do not effectively use this type of leaf damage as a cue in their foraging. Our findings are consistent with a contribution of bird predation towards top-down control of herbivory in this system. We further discuss these results in a broader context considering the possible effects of habitat type on leaf quality, leaf defense, and herbivore performance.     

Plant architecture and meristem dynamics as the mechanisms determining the diversity of gall-inducing insects

Plant architecture is considered to affect herbivory intensity, but it is one of the least studied factors in plant–insect interactions, especially for gall-inducing insects. This study aimed to investigate the influence of plant architecture on the speciose fauna of gall-inducing insects associated with 17 species of Baccharis. Five architectural variables were evaluated: plant height, number of fourth-level shoots, biomass, average level and number of ramifications. The number of galling species associated with each host plant species was also determined. To test the effects of plant architecture on gall richness at the individual level, we used another data set where the number of fourth-level shoots and gall richness were determined for B. concinna, B. dracunculifolia, and B. ramosissima every 3 weeks during 1 year. The average similarity between host species based on gall fauna was low (9%), but plants with the same architectural pattern tended to support similar gall communities. The most important architectural trait influencing gall richness at the species level was the number of fourth-level shoots, which is indicative of the availability of plant meristems, a fundamental tissue for gall induction and development. This variable also showed a positive correlation with gall richness at the individual level. We propose that variations in gall richness among host species are driven by interspecific differences in plant architecture via availability of young, undifferentiated tissue, which is genetically controlled by the strength of the apical dominance. Plant architecture should have evolutionary consequences for gall communities, promoting insect radiation among architecturally similar plants through host shift and sympatric speciation. We also discuss the role of plant architecture in the global biogeography of gall-inducing insects. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of overstory and understory vegetation on the understory light environment in mixed boreal forests

Abstract. The percentage of above-canopy Photosynthetic Photon Flux Density (%PPFD) was measured at 0, 50 and 100 cm above the forest floor and above the main understory vegetation in stands of (1) pure Betula papyrifera (White birch), (2) pure Populus tremuloides (Trembling aspen), (3) mixed broad-leaf-conifer, (4) shade-tolerant conifer and (5) pure Pinus banksiana (Jack pine) occurring on both clay and till soil types. %PPFD was measured instantaneously under overcast sky conditions (nine locations within each of 29 stands) and continuously for a full day under clear sky conditions (five locations within each of eight stands). The percentage cover of the understory layer was estimated at the same locations as light measurements. Mean %PPFD varied from 2% at the forest floor under Populus forests to 15% above the understory vegetation cover under Betula forests. Percent PPFD above the understory vegetation cover was significantly higher under shade intolerant tree species such as Populus, Betula and Pinus than under shade tolerant conifers. No significant differences were found in %PPFD above the understory vegetation cover under similar tree species between clay and till soil types. The coefficient of variation in %PPFD measured in the nine locations within each stand was significantly lower under deciduous dominated forests (mean of 19%) than under coniferous dominated forests (mean of 40%). %PPFD measured at the forest floor was positively correlated with %PPFD measured above the understory vegetation and negatively correlated with cumulative total percent cover of the understory vegetation (R² = 0.852). The proportion of sunflecks above 250 and 500 mmol m^–2 s^–1 was much lower and %PPFD in shade much higher under Populus and Betula forests than under the other forests. Differences in the mean, variability and nature of the light environment found among forest and soil types are discussed in relation to their possible influences on tree succession.     

Relative availability of nitrogen in host plants of invertebrate herbivores: three possible nutritional and physiological definitions

Summary It has been postulated that free amino acids in the diet of herbivores are a more readily available source of nitrogen than are amino acids in intact proteins. When plants are subjected to any of a number of stress factors the concentration of free amino acids in their tissues may increase relative to the protein content. It has been further postulated that such changes in available nitrogen can increase the survivorship of very young herbivores feeding on the tissues of stressed plants. In this paper, the concept of readily available nitrogen is given more specific definitions in the effort to uncover an underlying nutritional or physiological mechanism responsible for such changes in larval survivorship. The first hypothesis considered is that amino acids are more readily available than proteins in the sense that, as they occur in plant tissue, they are a more nutritious source of needed nitrogenous compounds. The second hypothesis considered is that amino acids are absorbed into the insect gut free from interference by proteinase inhibitors, and thus are more chemically available than are proteins. Finally, free amino acids, because of their solubility and greater mobility, may be more available to herbivores because of physical factors restricting the movement and digestion of proteins from plant tissue into the insect gut. The applicability of each hypothesis to certain types of insects feeding on particular plant tissues is discussed.The investigation reported in this paper (No. 86-7-224) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director     

芦芽山寒温性针叶林冠层下植被beta多样性格局及其成因

群落构建机制是生态学的中心议题之一。对山地植被beta多样性格局及其成因的探究有助于加深对此问题的认识。以芦芽山寒温性针叶林群落冠层下植被为研究对象,结合野外调查与室内实验获取的详细数据,运用Mantel检验、普通最小二乘回归和典范对应分析(CCA)等统计方法,探讨了林下植被的beta多样性格局及其成因,结果显示:(1)沿海拔梯度相邻群落间草本层物种周转率呈现递减格局,而灌木层变化规律不明显;(2)灌、草层beta多样性与海拔差异、地理距离呈显著正相关关系,而与局地环境异质性关系不显著。控制海拔作用后发现,灌、草层beta多样性与地理距离关系依然显著,而当消除地理距离的线性影响后,beta多样性与海拔关系也变得不显著(3)CCA模型中,环境因子共解释了物种组成变异的74.4%,其中,海拔、坡度、凋落物厚度、乔木密度与总干面积对林下灌、草植被物种组成具有显著影响,但土壤因子的作用未见显著。综上,生境筛滤与扩散限制共同主导了芦芽山寒温性针叶林冠层下群落构建过程,但扩散限制的影响强于生境筛滤作用。     

Plant community response to loss of large herbivores: comparing consequences in a South African and a North American grassland

Loss of biodiversity poses one of the greatest threats to natural ecosystems throughout the world. However, a comprehensive understanding of the impacts of species losses from upper trophic levels is still emerging. Here we compare the impacts of large mammalian herbivore species loss on grassland plant community structure and composition in a South African and North American grassland. Herbaceous plant communities were surveyed at sites without large mammalian herbivores present and at sites with a single species of herbivore present in both locations, and additionally at one site in South Africa with multiple herbivore species. At both the North American and South African locations, plant communities on sites with a single herbivore species were more diverse and species rich than on sites with no herbivores. At the multi-herbivore site in South Africa, plant diversity and richness were comparable to that of the single herbivore site early in the growing season and to the no herbivore site late in the growing season. Analyses of plant community composition, however, indicated strong differences between the multi-herbivore site and the single and no herbivore sites, which were more similar to each other. In moderate to high-productivity ecosystems with one or a few species of large herbivores, loss of herbivores can cause a significant decrease in plant diversity and richness, and can have pronounced impacts on grassland plant community composition. In ecosystems with higher herbivore richness, species loss may also significantly alter plant community structure and composition, although standard metrics of community structure may obscure these differences.     

Photosynthetic strategies of summergreen and evergreen understory herbs of the boreal mixedwood forest

Seasonal differences in photosynthesis and stomatal conductance of four herbaceous perennials from beneath a deciduous canopy was assessed at two light levels (60 and 400 μmol m⁻² s⁻¹ photosynthetic photon flux density, PPFD) and two leaf temperatures (7 and 20°C). Leaves of an evergreen, Pyrola asarifolia Michx., a wintergreen, Cornus canadensis L., and two summergreen species, Rubus pubescens Raf. and Aralia nudicaulis L., were collected at four times during the growing season. In addition, midsummer light response curves were obtained for one summergreen (A. nudicaulis) and one evergreen species (P. asarifolia) at both 7 and 20°C. Gas exchange measurements were made in the laboratory under controlled environmental conditions. For leaves collected in April, when insolation was high due to the leafless overstory, only P. asarifolia had green leaves, and there was no effect of temperature or light on this species' photosynthesis. P. asarifolia's net assimilation rate (NA) in April was about 30% of it's maximum in late summer. In early summer (June), A. nudicaulis and R. pubescens had higher NA at the higher temperature; at this time, these summergreen species also reached their maximum NA. Midsummer photosynthetic light response curves showed that the light-saturation point was higher and more responsive to leaf temperature in the summergreen A. nudicaulis than in the evergreen P. asarifolia. The summergreen species appear to have a photosystem which performs at high rates during early- and mid-summer, as well as a taller stature which allows them to intercept more light. The photosynthetic system of the ever/wintergreen species is adapted to the low ground-level light conditions in the summer and there does not appear to be an adjustment to take further advantage of the higher light in the spring and fall period. The adaptation of the evergreen and wintergreen understory species is tolerance to low temperatures, enabling them to photosynthesize into the fall till the first continuous frosts occur in the understory and also permitting the evergreen species to begin photosynthesis early in the spring. Received: 17 October 1996 / Accepted: 2 May 1997     

Long term oviposition preference and larval performance of Schizomyia macrocapillata (Diptera: Cecidomyiidae) on larger shoots of its host plant Bauhinia brevipes (Fabaceae)

The Plant Vigor Hypothesis (PVH) proposes that natural selection on female oviposition choice results from higher fitness of larvae on more vigorous and larger plant modules. For six consecutive years we tested the PVH predictions by investigating the effect of shoot size of Bauhinia brevipes (Fabaceae) on the oviposition preference and offspring survival of the gall-midge Schizomyia macrocapillata (Diptera: Cecidomyiidae). Additionally, we analyzed the effects of bottom-up and top-down mortality forces on the system. The hypersensitive reaction (bottom-up effect) accounted for more than 90% larval mortality of S. macrocapillata, making available few galls to be found and killed by natural enemies (top-down effect). Smaller shoots were always more abundant while longer shoots were rare. Nevertheless, the percent number of galls induced by S. macrocapillata was up to 10-fold greater on the largest shoots, corroborating the preference prediction of the PVH. Schizomyia macrocapillata should use over-exploit larger shoots to maximize the preference for, and consequently increase the performance on these shoots. Our results partially support the performance prediction of the PVH: (1) the observed survival was higher than expected on longer shoots, and (2) the ratio of survival per shoot was positively related with shoot length only in 2 years. Thus, we found a link between female preference and larval performance on large-sized shoots, at least in some years. The gall-midge attack pattern in this study might be an evolved response to maximize the female preference and increased larval performance on longer shoots of the host plant.     

Effect of fire intensity on understory composition and diversity in aKalmia-dominated oak forest, New England, USA

This study investigates the understory dynamics of two mixed-oak stands following fire of varying intensity. Composition and diversity of woody and herbaceous species in the understory were measured in two stands 7–8 years after a prescribed burn. On both sites, unburned areas, low-intensity fire areas, and areas where the overstory had been severely damaged were measured. Patterns of species presence and absence following fire were consistent with an initial-floristics model. Most species increased in density and frequency following fire; onlyAralia nudicaulis andQuercus alba showed statistically significant decreases in density. Overall, 29 species increased in density following fire, while 8 declined; 29 species increased in frequency, while 6 declined. However, diversity and equitability measures were depressed on the moderately burned sites, due to rapid regrowth ofKalmia latifolia. Ordination using binary discriminant analysis suggested species responded individualistically to both burning and site variation.     

Differential response of trees and shrubs to browsing and pruning: the effects on Pistacia growth and gall-inducing aphids

The effects of long term browsing on growth and morphology of P. atlantica trees and P. palaestina shrubs, and on the colonization of these hosts by their respective gall-inducing aphid guilds, were studied in natural vegetation pastures in Israel. To simulate apical dominance release by browsing, trees and shrubs were pruned and observed one year later. P. atlantica responded strongly to browsing, producing a crown of dense and sharp dry branches and thorn-like shoots, and pruning, compensating or overcompensating for the lost biomass in the studied morphological variables. Morphological responses of P. palaestina were less evident: we found no compensation in most variables. Three species of aphids produced more galls on browsed than on control shoots of P. atlantica. Two species induced more galls on pruned shoots. The five aphid species on P. palaestina colonized browsed and unbrowsed shoots equally, and produced fewer galls on pruned shoots. The role of apical dominance in architectural responses of trees and shrubs to browsing and pruning, as related to resource availability and timing, is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Diversification and phylogenetic correlation of functional traits for co-occurring understory species in the Chinese boreal forest

Functional traits impact species interactions, community composition, and ecosystem functioning. However, few studies have focused on the diversification and phylogenetic correlation of multiple functional traits over geological time. We conducted phylogenetic comparative analysis for boreal forest understory species in northeast China to examine the diversification and phylogenetic correlation in several functional traits: leaf area (LA), leaf carbon content (LCC), leaf dry matter content (LDMC), leaf nitrogen content (LNC), plant height (PH), and specific leaf area (SLA). Phylogenetic signals showed that there were very low levels of phylogenetic niche conservatism (PNC) in understory leaf-related traits and plant height, suggesting divergence of functional traits for the co-occurring understory species. The disparity through time analyses (DTT) indicated that trait disparities mainly originated during recent divergence events and there were no differences in the observed trait disparities compared with that expected under Brownian motion. Furthermore, we found both positive and negative phylogenetic correlations among the measured functional traits. The very low levels of PNC suggest that these functional traits diverged among co-occurring understory species, and that those species are distantly phylogenetically related. The phylogenetic correlations among traits may be caused by both positively and negatively correlated adaptions that correspond to resource acquisition strategies. This study provides evidence that divergence in functional traits may reflect understory adaptions to boreal conditions.     

Release from native herbivores facilitates the persistence of invasive marine algae: a biogeographical comparison of the relative contribution of nutrients and herbivory to invasion success

The effect of herbivory and nutrient enrichment on the growth of invasive and native macroalgal species was simultaneously studied in two biogeographic regions: the Caribbean and Hawaii. Herbivores suppressed growth of invasive algae in their native (Caribbean) and invaded range (Hawaii), but despite similar levels of herbivore biomass, the intensity of herbivory was lower in Hawaii. Algal species with a circumtropical distribution did not show a similar effect of herbivores on their growth. Nutrient enrichment did not enhance growth of any algal species in either region. The reduction in herbivore intensity experienced by invasive algae in Hawaii rather than an escape from (native) herbivores provided invasive macroalgae with “enemy release” sensu the Enemy Release Hypothesis (ERH). Since native, Hawaiian herbivores still feed and even prefer invasive algae over native species, invasion scenario’s that involve predation (e.g. the ERH) could be falsely dismissed when invasive species are only studied in their invasive range. We therefore argue that escape from herbivores (i.e. enemy release) can only effectively be determined with additional information on the intensity of predation experienced by an invasive species in its native range.     

Seasonal and habitat differences affect the impact of food and predation on herbivores: a comparison between gaps and understory of a tropical forest

Herbivore populations are influenced by a combination of food availability and predator pressure, the relative contribution of which is hypothesized to vary across a productivity gradient. In tropical forests, treefall gaps are pockets of high productivity in the otherwise less productive forest understory. Thus, we hypothesize that higher light availability in gaps will increase plant resources, thereby decreasing resource limitation of herbivores relative to the understory. As a result, predators should regulate herbivore populations in gaps, whereas food should limit herbivores in the understory. We quantified potential food availability and compared arthropod herbivore and predator densities in large forest light gaps and in the intact understory in Panama. Plants, young leaves, herbivores and predators were significantly more abundant per ground area in gaps than in the understory. This pattern was similar when we focused on seven gap specialist plant species and 15 shade-tolerant species growing in gaps and understory. Consistent with the hypothesis, herbivory rates were higher in gaps than the understory. Per capita predation rates on artificial caterpillars indicated higher predation pressure in gaps in both the dry and late wet seasons. These diverse lines of evidence all suggest that herbivores experience higher predator pressure in gaps and more food limitation in the understory.     

Effects of grazing,topography, and precipitation on the structure of a semiarid grassland

Structural aspects of the shortgrass steppe plant community, functional groups, and species populations were examined in response to long-term heavy grazing and exclosure from grazing, contiguous wet or dry years, and an environmental gradient of topography. Of the three factors, relatively greater differences in community similarity were observed between catena positions, particularly on the ungrazed treatments. Grazing was intermediate between catena position and short-term weather in shaping plant community structure. Grazed treatments and ridgetops had a less variable species composition through fluctuations in weather.An increase with grazing of the dominant, heavily grazed species was observed. Basal cover and density of total species was also greater on grazed sites. The more uniform grazing lawn structure of the grazed plant communities had an influence on segregation of plant populations along topographical gradients. Segregation was less on grazed catenas, but diversity and the abundance of introduced and opportunistic-colonizer species was also less.Although the shortgrass steppe community was relatively invariant, less abundant species were dynamic and interactions occurred with respect to grazing, weather, and catena position. The effects of grazing may be mitigated by favorable growing seasons but magnified in unfavorable years in populations that are adapted to favorable sites. Grazing can be considered a disturbance at the level of the individual but it may or may not be a disturbance at the level of the population, and it is not a disturbance at the level of the community in this particular grassland.     

V型H+-ATPase结构和调控机制的研究现状

V-ATPase存在于所有已知的真核生物中,有极为重要的生理作用.近几年来,对V-ATPase的结构、功能和调控机制的研究进展很快,对复合体的组装有了进一步的认识.对V-ATPase的主要亚基已经完成序列测定及分析,对各亚基生理功能也进行了较为深入的研究.生物化学及分子生物学工作揭示：生物体内以多种方式对编码V-ATPase主要亚基的基因表达和该酶的活力进行调控.     

Rapid, landscape scale responses in riparian tundra vegetation to exclusion of small and large mammalian herbivores

Productive tundra plant communities composed of a variety of fast growing herbaceous and woody plants are likely to attract mammalian herbivores. Such vegetation is likely to respond to different-sized herbivores more rapidly than currently acknowledged from the tundra. Accentuated by currently changing populations of arctic mammals there is a need to understand impacts of different-sized herbivores on the dynamics of productive tundra plant communities. Here we assess the differential effects of ungulate (reindeer) and small rodent herbivores (voles and lemmings) on high productive tundra vegetation. A spatially extensive exclosure experiment was run for three years on river sediment plains along two river catchments in low-arctic Norway. The river catchments were similar in species pools but differed in species abundance composition of both plants and vertebrate herbivores. Biomass of forbs, deciduous shrubs and silica-poor grasses increased by 40–50% in response to release from herbivory, whereas biomass of silica-rich grasses decreased by 50–75%. Hence both additive and compensatory effects of small rodents and reindeer exclusion caused these significant changes in abundance composition of the plant communities. Changes were also rapid, evident after only one growing season, and are among the fastest and strongest ever documented in Arctic vegetation. The rate of changes indicates a tight link between the dynamics of productive tundra vegetation and both small and large herbivores. Responses were however not spatially consistent, being highly different between the catchments. We conclude that despite similar species pools, variation in plant species abundance and herbivore species dynamics give different prerequisites for change.     

Diversity of the understory vascular vegetation in 40 year-old and old-growth forest stands on Vancouver Island,British Columbia

Abstract. We studied plant diversity of the understory vascular vegetation in 40 yr-old plantations (immature stands) and old-growth forest stands on southwestern Vancouver Island, British Columbia, Canada. Site-specific comparisons using several indices of species diversity were made between: (1) immature stands segregated according to the canopy cover and dominant canopy tree species; and (2) immature and old-growth stands. There were no significant differences (P < 0.05) among immature stands in species richness (S) and the Shannon-Wiener index (H′), in relation to the canopy cover or in S, H′ and evenness (E) in relation to the dominant canopy tree species. Using the same indices, the plant diversity varied with edaphic conditions (represented by five site associations) and time (represented by two developmental stages). At both stand- and site levels, plant diversity increased with increasing soil moisture, from slightly dry to moist sites, and with increasing plant-available soil nitrogen in both immature and old-growth stands; and the plant diversity of immature stands across the sites studied was considerably lower than in old-growth stands, regardless of site association. The indices of plant diversity, floristic similarity indices, and species turnover rates indicated that the immature stands had their plant diversity at a minimum, but a drastic loss of diversity expected in the stem exclusion stage had not materialized. We attributed decline in plant diversity to the absence of old-growth structural features in immature stands. Several measures to foster the stand-level diversity were proposed.