Forest Herbs Research Articles

Invasive Earthworms Ingest and Digest Garlic Mustard Seeds at Rates Equal to Native Seeds

The European and Asian earthworms introduced to eastern North American forests have great potential to facilitate plant invasions, in part through selective seed predation and dispersal. The invasive plant Alliaria petiolata (Garlic Mustard) contains secondary metabolites that may deter earthworms from eating its seeds. In 2 growth-chamber experiments, I determined whether the invasive earthworms Lumbricus terrestris (Nightcrawler) and Eisenia fetida (Red Wiggler) could aid the spread of Garlic Mustard by ingesting its seeds at lower rates than the similar-sized seeds of the native forest herb Geranium maculatum (Wild Geranium). Earthworms had similar rates of seed ingestion regardless of earthworm or plant species and digested the majority of seeds they ate (67–73%). There was no interaction between earthworm and plant species. Given a choice between Garlic Mustard and Wild Geranium, seed selectivity cannot explain positive associations between earthworm abundance and Garlic Mustard invasion.

Underground biomass accumulation of two economically important non-timber forest products is influenced by ecological settings and swiddeners’ management in the Bago Mountains, Myanmar

Diverse tree species, understory bamboos, shrubs, and herbs in mixed deciduous forests (MDFs) provide not only timber, but also non-timber forest products (NTFPs) in terms of fuel, food, and medicinal plants. Sustainability and the economic value of commercially extracted NTFPs have been studied mainly for the aboveground vegetative components of plants. However, such important knowledge of the underground parts of the plants is limited because of difficulties observing them and lesser consideration of the lower value of NTFPs, compared with timber, in forest management. Here we established allometric models to predict the root and corm biomasses of two plants, Rauvolfia serpentina and Amorphophallus bulbifer, in relation to their aboveground, vegetative parts. Estimated underground biomasses of the two species were compared among three habitat types: a swidden field, a fallow forest after cultivation, and a ridge forest that was not subjected to swidden agriculture. The length of the longest leaf of each individual plant explained the accumulation of the underground biomass of R. serpentina (R2=0.64, P<0.001) in a positive, linear relationship, and stem diameter at ground level did the same for A. bulbifer (R2=0.92, P<0.001). The largest biomass accumulation of R. serpentina was found in swidden fields because of its light demanding nature, which was supported by a significant, negative correlation between canopy cover and leaf length (Pearson’s correlation=−0.612, P<0.05). Because A. bulbifer was cut during weeding time, we could not measure its biomass accumulation in swidden fields; however, its occurrence in swidden fields was confirmed by farmers and field observations. No difference in the biomass accumulation of A. bulbifer was observed between the fallow and ridge forests. The promising growth of both species in the same year that a fire occurred during swidden cultivation suggests that the underground regeneration source provides advantages related to the harvesting of NTFPs and allows them to co-exist with slashing and burning in a swidden system, as well as surface fires in an MDF. Establishment of the two species in fallow vegetation was possibly facilitated by ecological and anthropogenic factors such as the reproductive bulbils of A. bulbifer and the broken root fragments of R. serpentina left after the harvest, as well as the dormancy period of these species during the dry season. This study revealed the cultivation potential of NTFPs that are suitable for community and agroforest plantations using a swidden system in an MDF that experiences fires.

Experimental warming alters migratory caribou forage quality.

Global declines in caribou and reindeer (Rangifer) populations have drawn attention to the myriad of stressors that these Arctic and boreal forest herbivores currently face. Arctic warming has resulted in increased tundra shrub growth and therefore Rangifer forage quantity. However, its effects on forage quality have not yet been addressed although they may be critical to Rangifer body condition and fecundity. We investigated the impact of 8yrs of summer warming on the quality of forage available to the Bathurst caribou herd using experimental greenhouses (n=5) located in mesic birch hummock tundra in the central Canadian Low Arctic. Leaf forage quality and digestibility characteristics associated with nutrients (nitrogen and phosphorus), phenolics, and fiber were measured on the deciduous shrub Betula glandulosa (an important Rangifer diet component) at six time points through the growing season, and on five other very common vascular plant and lichen species in late summer. Experimental warming reduced B.glandulosa leaf nitrogen concentrations by ~10% in both late June and mid-July, but not afterwards. It also reduced late summer forage quality of the graminoid Eriophorum vaginatum by increasing phenolic concentrations 38%. Warming had mixed effects on forage quality of the lichen Cetraria cucullata in that it increased nutrient concentrations and tended to decrease fiber contents, but it also increased phenolics. Altogether, these warming-induced changes in forage quality over the growing season, and response differences among species, highlight the importance of Rangifer adaptability in diet selection. Furthermore, the early season reduction in B.glandulosa nitrogen content is a particular concern given the importance of this time for calf growth. Overall, our demonstration of the potential for significant warming impacts on forage quality at critical times for these animals underscores the importance of effective Rangifer range conservation to ensure sufficient appropriate habitat to support adaptability in forage selection in a rapidly changing environment.

Secondary dispersal of forest herb seeds from raccoon dung: contrasting service by multiple vectors

Secondary movement of seeds deposited in dung potentially improves their chances of seedling establishment but has rarely been documented in temperate-zone deciduous forest. We followed mayapple (Podophyllum peltatum) seeds after deposition in raccoon (Procyon lotor) dung to test for secondary dispersal in a long-established deciduous forest in southeastern Ohio, USA. Marked seeds were incorporated into raccoon dung, placed in natural forest sites, and observed at regular intervals until the subsequent Spring. Abiotic seed movement was monitored during natural and simulated rain events. To identify animal vectors and describe their interaction with seeds, motion-triggered wildlife cameras were trained on dung with embedded mayapple seeds. Seed movement was initially limited by adhesion in the viscid dung matrix, but 15% of seeds eventually moved >20 cm from the point of placement, a minimal estimate of a shoot’s competitive neighborhood (maximum movement distance = 130 cm). Significantly greater movement occurred on slopes and in the absence of leaf litter. Rainfall moved seeds as far as 51 cm. White-footed mice (Peromyscus leucopus) and chipmunks (Tamias striatus) began removing seeds within 24 h of dung placement, presumably caching a portion of them outside the 1.5-m radius of observation. At least four other animal species interacted with dung in ways unrelated to seed presence, which nevertheless resulted in seed movement on a scale of centimeters. Thus, multiple secondary processes caused seed movement, each resulting in a distinctive seed distribution. The forest herb potentially benefits through reduction in seed density and distribution to a variety of microsites.

Effects of an invasive shrub, Lonicera maackii, and a generalist herbivore, white-tailed deer, on forest floor plant community composition

Both overabundant ungulates and invasive shrubs negatively impact forest floor plants, but few studies have investigated their interaction. We tested for direct effects and interactions of white-tailed deer, Odocoileus virginianus, and the invasive shrub, Lonicera maackii, on forest floor plant richness and composition in five sites in southwest Ohio. In each site a deer exclosure and an unfenced plot (each 20×20m) were established in 2010; in half of each plot all L. maackii was removed. Each spring and summer, 2011–2014, percent cover of each plant species in 50×20cm subplots was recorded.Herb layer species richness was greater where L. maackii was removed, and this effect tended to be stronger where deer were excluded. Species composition in the spring was not affected by treatments, whereas summer species composition (in one year) was affected by L. maackii treatment. Cover of graminoids, spring herbs, and vines were lower where L. maackii was intact, whereas cover of shrubs and tree seedlings were lower, and cover of bare ground higher, where deer had access.Our findings reveal that deer and L. maackii negatively impact different components of the forest herb layer. However, the combined effects on species richness tended to be antagonistic (less than additive), possibly due to deer browse reducing the cover of this invasive shrub. This suggests that the diversity of the herb layer will be enhanced more by both removal of invasive shrubs and reduction of the deer population than by either management action alone.

Ecology of forest insect invasions

Forests in virtually all regions of the world are being affected by invasions of non-native insects. We conducted an in-depth review of the traits of successful invasive forest insects and the ecological processes involved in insect invasions across the universal invasion phases (transport and arrival, establishment, spread and impacts). Most forest insect invasions are accidental consequences of international trade. The dominant invasion ‘pathways’ are live plant imports, shipment of solid wood packaging material, “hitchhiking” on inanimate objects, and intentional introductions of biological control agents. Invading insects exhibit a variety of life histories and include herbivores, detritivores, predators and parasitoids. Herbivores are considered the most damaging and include wood-borers, sap-feeders, foliage-feeders and seed eaters. Most non-native herbivorous forest insects apparently cause little noticeable damage but some species have profoundly altered the composition and ecological functioning of forests. In some cases, non-native herbivorous insects have virtually eliminated their hosts, resulting in major changes in forest composition and ecosystem processes. Invasive predators (e.g., wasps and ants) can have major effects on forest communities. Some parasitoids have caused the decline of native hosts. Key ecological factors during the successive invasion phases are illustrated. Escape from natural enemies explains some of the extreme impacts of forest herbivores but in other cases, severe impacts result from a lack of host defenses due to a lack of evolutionary exposure. Many aspects of forest insect invasions remain poorly understood including indirect impacts via apparent competition and facilitation of other invaders, which are often cryptic and not well studied.

Syntaxonomic adaptability of lower layer plants of the forest communities in the North-Eastern Ukraine

&lt;p&gt;In the North-Eastern part of Ukraine the forest communities occupy considerable area and have great ecological and economic value. In natural forests herbs and subshrubs maintain the canopy cover through the recovery mechanism when the undergrowth of woody plants by height constitutes a part of the herbaceous and subshrub layer. Cenoflora within the lower layers of mixed coniferous-broad leaved forests of the southern part of the Eurasian forest zone is a fairly complete structural unit, especially in the central parts of the moisture and soil fertility gradients. Syntaxonomic adaptability of the lower layer plants of the forest communities in the North-East of Ukraine is established, and their ecological and biological diversity is determined as a result of the researches conducted. A group of species of forest herbs and subshrubs by the ecological status takes an intermediate position because of their habitat versatility. The species of this group are characterized by the vegetative propagation along with the effective seed reproduction. The habitat versatility and combination of the two types of reproduction are the main reasons for a wide representation of the species of this group in different syntaxons. A high floristic biodiversity of the forest communities of the North-Eastern part of Ukraine and their flora heterogeneity are determined based on the comparative floristic analysis. The floristic difference between the two major classes of vegetation in this region is shown, and it is found that Querco-Fagetea class is characterized by a high nutrient status and good warming-up under slightly reduced soil moisture, whereas the main feature of Vaccinio-Piceetea class is low soil fertility.&lt;/p&gt;

Changes in the nature of environmental limitation in two forest herbs during two decades of forest succession

AbstractAimsThe environmental legacies of past land use generally have an important impact on the establishment and persistence of forest understorey plant species in post‐agricultural forests. Here we studied how the nature of such environmental limitation changes during the first decades of forest development.LocationDeciduous forest ‘Muizenbos’, northern Belgium.MethodsTwo forest herbs with comparable life‐history traits, but contrasting rates of post‐agricultural forest colonization (Primula elatior, Geum urbanum) were introduced 18 yr ago. Introduction sites were ancient or post‐agricultural forest; seeds or adults were added to permanent plots where the understorey vegetation was initially cleared or left undisturbed. We measured chemical soil conditions and recorded the understorey plant community to characterize the sites. The effects of the treatments (land use, vegetation clearing) were tested on the present‐day occurrence and abundance of the introduced species across the plots. We also quantified whether the temporal changes in the total number of individuals per site was different between the two land‐use types and consistent within land‐use types.ResultsLand use had an important effect on soil and vegetation. In post‐agricultural forest, the P availabilities were many times higher, the plant community diversity was lower and the competitive species were significantly more abundant compared with ancient forest. The present‐day abundance of P. elatior was significantly higher in the ancient forest sites, both for planted and seeded plots, while for G. urbanum land use had no effect. The initial vegetation clearing had a persistent effect on the establishment success of both species. The 18‐yr‐long time series showed consistent changes in the total population sizes in ancient forest: P. elatior has been expanding and G. urbanum initially increased and then declined over the past decade. In post‐agricultural forest, temporal patterns were more divergent and partly related to differences in dominant tree species: differences in acidification rates and changes in insolation caused by canopy dieback have put the populations on new and unforeseen trajectories.ConclusionsThe longer the experiment is running, the more the individual sites show separate dynamics. In general, we showed that environmental limitation affects plant colonization at least during the first two decades of secondary forest succession, but also that as the post‐agricultural forests develop, the nature of environmental limitation changes.

Impacts of root herbivory on seedlings of three species of boreal forest trees

Current meta-analysis has identified the shortage of information on responses of woody plants to root damage as the most critical gap in studies of belowground insect herbivory. We explored the impacts of natural and simulated root herbivory on seedlings of three boreal forest-forming trees, Pinus sylvestris, Picea abies and Betula pendula. We compared the effects of natural herbivory and mechanical damage by placing one larva of the common cockchafer, Melolontha melolontha, of different instars (to manipulate root loss) into a pot with a one-year-old seedling or we excised a portion of the roots by inserting a 13-mm diameter cork borer into the soil. During the experiment, which lasted from 2 to 8 weeks, the larvae consumed 53–73% of root biomass, whereas the mechanical damage treatment removed 30–40% of roots. Seedling mortality was low (5%) across all treatments, but the aboveground biomass decreased by 3–43% due to natural herbivory and by 6–30% due to mechanical damage. The M. melolontha larvae consumed a larger amount of roots from B. pendula than from either P. sylvestris or P. abies, but the larvae showed similar relative growth rates and mass gains, indicating a different efficiency of conversion of ingested food (ECI). The values of ECI calculated for larvae of M. melolontha (2.3–3.5%) appeared much lower than the 17% reported earlier for larvae of foliage-feeding beetles, presumably due to the high energetic cost of locomotion in the soil. Consequently, root-feeding insects consume much more plant biomass than leaf-feeding insects do to gain similar body mass, and therefore they inflict relatively more damage to plants per unit of mass gained. Based on the biomass of insect herbivores in European forests, we estimate that they consume 0.5% of the available fine root biomass. The effects of low levels of root damage on growth and reproduction of woody plants remain virtually unknown, emphasising the need for studies of plant- and ecosystem-level effects of minor but chronic root herbivory.

Contrasting patterns of fine-scale herb layer species composition in temperate forests

Although being well described at the landscape level, patterns in species composition of forest herb layer are rarely studied at smaller scales. Here, we examined fine-scale environmental determinants and spatial structures of herb layer communities in thermophilous oak- and hornbeam dominated forests of the south-eastern part of the Czech Republic.Species composition of herb layer vegetation and environmental variables were recorded within a fixed grid of 2 × 2 m subplots regularly distributed within 1-ha quadrate plots in three forest stands. For each site, environmental models best explaining species composition were constructed using constrained ordination analysis. Spatial eigenvector mapping was used to model and account for spatial structures in community variation. Mean Ellenberg indicator values calculated for each subplot were used for ecological interpretation of spatially structured residual variation.The amount of variation explained by environmental and spatial models as well as the selection of variables with the best explanatory power differed among sites. As an important environmental factor, relative elevation was common to all three sites, while pH and canopy openness were shared by two sites. Both environmental and community variation was mostly coarse-scaled, as was the spatially structured portion of residual variation. When corrected for bias due to spatial autocorrelation, those environmental factors with already weak explanatory power lost their significance. Only a weak evidence of possibly omitted environmental predictor was found for autocorrelated residuals of site models using mean Ellenberg indicator values.Community structure was determined by different factors at different sites. The relative importance of environmental filtering vs. spatial processes was also site specific, implying that results of fine-scale studies tend to be shaped by local conditions. Contrary to expectations based on other studies, overall dominance of spatial processes at fine scale has not been detected. Ecologists should keep this in mind when making generalizations about community dynamics.

Effects of deer on the photosynthetic performance of invasive and native forest herbs.

Overabundant generalist herbivores can facilitate non-native plant invasions, presumably through direct and indirect modifications to the environment that affect plant performance. However, ecophysiological mechanisms behind ungulate-mediated plant invasions have not been well-studied. At a long-term Odocoileus virginianus (white-tailed deer) exclusion site in a temperate deciduous forest, we quantified deer-mediated ecophysiological impacts on an invasive biennial Alliaria petiolata (garlic mustard) and two palatable native herbaceous perennials, Maianthemum racemosum and Trillium grandiflorum. In mid-summer, we found that leaf-level light availability was higher in unfenced areas compared with areas fenced to exclude deer. Alliaria in unfenced areas exhibited 50 % higher mean maximum photosynthetic rates compared with fenced areas. Further, specific leaf area decreased by 48 % on average in unfenced areas, suggesting leaf structural responses to higher light levels. Similarly, Maianthemum had 42 % higher mean photosynthetic rates and 33 % decreased mean specific leaf area in unfenced areas, but these functional advantages were likely countered by high rates of deer herbivory. By contrast, Trillium exhibited significantly lower (26 %) maximum photosynthetic rates in unfenced areas, but SLA did not differ. Deer-mediated differences in light saturated photosynthetic rates for all three species were only significant during months with overstory tree canopy cover, when light availability in the herb layer was significantly lower in fenced areas. Alliaria's enhanced photosynthetic rates implicate overabundant deer, a situation that is nearly ubiquitous across its invaded range. Collectively, our results provide empirical evidence that generalist herbivores can alter non-native plant physiology to facilitate invasion.

Exotic Invasive Shrub Glossy Buckthorn Reduces Restoration Potential for Native Forest Herbs

Invasive glossy buckthorn could reduce restoration potential for understory native forest herbs by compromising their growth and biodiversity. Few studies of glossy buckthorn’s effects on forest herbs exist, and none were done in early-successional, partially open hardwood forests. This study was conducted in a mature hybrid poplar plantation invaded by buckthorn, located in southeastern Québec. We tested the effect of buckthorn removal on the growth of three forest herb species, whether this effect varied among species, and if canopy type (two poplar clones) influenced this effect. Forest herbs were planted in herbicide (buckthorn removed) and control treatments in the plantation understory, an environment similar to that of early-successional hardwood forests. Over the first two growing seasons, species showed specific reactions to buckthorn cover. Mean relative growth rate (RGR) for Asarum canadense and Polygonatum pubescens was increased in the herbicide treatment (48% and 33%, respectively) and decreased in the control treatment (−35% and −33%, respectively). Sanguinaria canadensis growth was the highest among species, with no difference between treatments. No effects of canopy type were detected. Results suggest that planting forest herbs for restoration purposes may be unsuccessful if buckthorn is present. Important changes in understory flora biodiversity are likely to occur over the long term in forests invaded by buckthorn.

Where does the community start, and where does it end? Including the seed bank to reassess forest herb layer responses to the environment

AbstractQuestionBelow‐ground processes are key determinants of above‐ground plant population and community dynamics. Still, our understanding of how environmental drivers shape plant communities is mostly based on above‐ground diversity patterns, bypassing below‐ground plant diversity stored in seed banks. As seed banks may shape above‐ground plant communities, we question whether concurrently analysing the above‐ and below‐ground species assemblages may potentially enhance our understanding of community responses to environmental variation.LocationTemperate deciduous forests along a 2000 km latitudinal gradient in NW Europe.MethodsHerb layer, seed bank and local environmental data including soil pH, canopy cover, forest cover continuity and time since last canopy disturbance were collected in 129 temperate deciduous forest plots. We quantified herb layer and seed bank diversity per plot and evaluated how environmental variation structured community diversity in the herb layer, seed bank and the combined herb layer–seed bank community.ResultsSeed banks consistently held more plant species than the herb layer. How local plot diversity was partitioned across the herb layer and seed bank was mediated by environmental variation in drivers serving as proxies of light availability. The herb layer and seed bank contained an ever smaller and ever larger share of local diversity, respectively, as both canopy cover and time since last canopy disturbance decreased. Species richness and β‐diversity of the combined herb layer–seed bank community responded distinctly differently compared to the separate assemblages in response to environmental variation in, e.g. forest cover continuity and canopy cover.ConclusionsThe seed bank is a below‐ground diversity reservoir of the herbaceous forest community, which interacts with the herb layer, although constrained by environmental variation in e.g. light availability. The herb layer and seed bank co‐exist as a single community by means of the so‐called storage effect, resulting in distinct responses to environmental variation not necessarily recorded in the individual herb layer or seed bank assemblages. Thus, concurrently analysing above‐ and below‐ground diversity will improve our ecological understanding of how understorey plant communities respond to environmental variation.

Do deer and raccoons defecate in the right place? Fitness consequences of vertebrate seed dispersal for a deciduous forest herb.

Precision of seed placement in a heterogeneous environment is often assumed to select for the evolution of animal-mediated dispersal systems, but this hypothesis has rarely been tested in a multivariate sense. We quantify the microsite fitness benefits of dispersal by white-tailed deer (Odocoileus virginianus) and raccoons (Procyon lotor) for mayapple (Podophyllum peltatum), a shade-tolerant perennial herb, in deciduous forests of southeastern Ohio, USA. Micro-environmental variables were recorded at dung-deposition microsites, at rooting points of mayapple shoots, and at random (control) points in the forest. Fitness was assessed as the degree of overlap in ordinations of microsites by environmental variables. Mayapple occupied a broad sector (56%) of environment space corresponding to low and mid-slope positions, ravines, and proximity to trees. Deer and raccoon defecation placed dung in 71-74 and 86-95% of environment space, respectively, reaching mayapple microsites in 57-60 and 53-54% of cases. Deer placed dung in mayapple environment space 7-9% more often than predicted by random distribution, and raccoons placed dung in mayapple space 0-5% more often, consistent with only a modest degree of directed dispersal. Thus, the precision hypothesis is only weakly supported. The greatest fitness benefit of vertebrate dispersal appears to be the broad distribution of seeds, thereby increasing the probability of randomly reaching a suitable microsite. Imprecise dispersal suggests that secondary mechanisms of seed movement need to be explored in deciduous forest communities.

Predicting North American Scolytinae invasions in the Southern Hemisphere.

Scolytinae species are recognized as one of the most important tree mortality agents in coniferous forests worldwide, and many are known invaders because they are easily transported in wood products. Nonnative trees planted in novel habitats often exhibit exceptional growth, in part because they escape herbivore (such as Scolytinae) pressure from their native range. Increasing accidental introductions of forest pest species as a consequence of international trade, however, is expected to diminish enemy release of nonnative forest trees. In this context, there is need to characterize patterns of forest herbivore species invasion risks at global scales. In this study, we analyze the establishment potential of 64 North American Scolytinae species in the Southern Hemisphere. We use climate-based ecological niche models (MaxEnt) to spatially define the potential distribution of these Scolytinae species in regions of the Southern Hemisphere were pines are planted. Our model predicts that all of the pine-growing regions of the Southern Hemisphere are capable of supporting some species of North American Scolytinae, but there are certain "hotspot" regions, southeastern Argentina, Bolivia, Chile, Peru and southwestern Australia, that appear to be suitable for a particularly large number of species. The species with the highest predicted risk of establishment were Dendroctonus valens, Xyleborus intrusus, Hylastes tenuis, Ips grandicollis, Gnathotrichus sulcatus, and Ips calligraphus. Given that global commerce is anticipated to continue to increase, we can expect that more Scolytinae species will continue to establish outside their range. Our results provide information useful for identifying a global list of potential invasive species in pine plantations, and may assist in the design of comprehensive strategies aimed at reducing pest establishment in Southern Hemisphere forest plantations.

Response of spring flora to nearly two decades of deer exclusion and resurgent woody understories within exclosures

Abstract Exclosure studies have illustrated the profound effects overabundant populations of white-tailed deer can have on forest plant communities. However, complete exclusion of deer creates an artificial condition in most forests, and examining the long-term response of plant species composition and diversity to exclusion can provide valuable insight into the historic role of deer herbivory in forests. In 2014, we resampled spring flora and woody regeneration in a series of exclosure and control plots established in Great Smoky Mountains National Park in 1996. By comparing our data to regeneration data from 2002 and spring flora data from 2004, we were able to examine differences in vegetation at 6–8 and 18 yrs after deer exclusion. We observed a significant (P < 0.001) increase in the density of woody stems between 2002 and 2014 across all sites, with significantly more stems found within exclosures (P < 0.001). This increase in woody stem abundance within exclosures was associated with a decline in t...

The Impact of Tree Diversity on Different Aspects of Insect Herbivory along a Global Temperature Gradient - A Meta-Analysis.

Forests with higher tree diversity are often assumed to be more resistant to insect herbivores but whether this effect depends on climatic conditions is so far poorly understood. In particular, a forest’s resistance to herbivory may depend on mean annual temperature (MAT) as a key driver of plant and insect phenology. We carried out a global meta-analysis on regression coefficients between tree diversity and four aspects of insect herbivory, namely herbivore damage, abundance, incidence rate and species richness. To test for a potential shift of tree diversity effects along a global gradient of MAT we applied mixed-effects models and estimated grand mean effect sizes and the influence of MAT, experimental vs. observational studies and herbivores diet breadth. There was no overall effect of tree diversity on the pooled effect sizes of insect herbivore damage, abundance and incidence rate. However, when analysed separately, we found positive grand mean effect sizes for herbivore abundance and species richness. For herbivore damage and incidence rate we found a significant but opposing shift along a gradient of MAT indicating that with increasing MAT diversity effects on herbivore damage tend towards associational resistance whereas diversity effects on incidence rates tend towards associational susceptibility. Our results contradict previous meta-analyses reporting overall associational resistance to insect herbivores in mixed forests. Instead, we report that tree diversity effects on insect herbivores can follow a biogeographic pattern calling for further in-depth studies in this field.

How Biotic Differentiation of Human Impacted Nutrient Poor Deciduous Forests Can Affect the Preservation Status of Mountain Forest Vegetation

A significant loss of biodiversity resulting from human activity has caused biotic homogenisation to become the dominant process shaping forest communities. In this paper, we present a rare case of biotic differentiation in European temperate deciduous forest herb layer vegetation. The process is occurring in nutrient poor oak-hornbeam forests in mountain areas (Polish Eastern Carpathians, Central Europe) where non-timber use was converted into conventional forest management practice. This change contributed to increases in the nitrogen content and pH reaction of the soil that, contrary to predominant beliefs on the negative impact of habitat eutrophication on diversity, did not result in a decrease in the latter. We discuss possible reasons for this phenomenon that indicate the important role of tree stand composition (an increasing admixture of beech worsening the trophic properties of the soil). The second issue considered involves the effect of the changes in herb species composition of oak-hornbeam forest on its distinctiveness from the beech forest predominating in the Polish Eastern Carpathians. Unfortunately, despite the increase in the species compositional dissimilarity of oak-hornbeam forest, a reduction in their distinctiveness in relation to the herb species composition of beech forest was found. Such a phenomenon is an effect of the major fragmentation of oak-hornbeam forests, a spread of beech forest-type species, and forest management that gives preference to beech trees. Consequently, it can be expected that changes occurring in oak-hornbeam forest vegetation will contribute to a decrease in the forest vegetation variability at the regional scale.

Congruence of the spatial pattern of light and understory vegetation in an old-growth, temperate mixed forest

Light is one of the most important drivers of understory vegetation in forests, influencing the patterns of total cover as well as the abundance of individual species.Based on a multi-scale approach, the relationships between the amount and pattern of relative diffuse light and forest understory were studied in an old-growth, temperate mixed forest (Hungary). The recorded vegetation variables were the cover of the vascular understory (herbs, woody seedlings), the bryophyte layer, and some selected vascular understory species.The pattern of light showed aggregations at two scales: 10×10 and 25×25m. Both vascular understory and bryophyte cover had significant positive correlations with light availability, and their spatial pattern was related to it. The pattern of seedlings displayed the strongest relationships with that of light at a coarser scale (25×25m) than herbs and bryophytes (10×10m). At the species level, Festuca heterophylla, Fragaria vesca and Poa nemoralis were characterized as light-demanding herbaceous species (their spatial pattern was congruent with light), Brachypodium sylvaticum and Carex pallescens were transitional, while some species proved to be shade-tolerant (e.g. Ajuga reptans, Dryopteris carthusiana, Viola reichenbachiana). Regarding seedlings, the patterns of Betula pendula, Carpinus betulus, Pinus sylvestris and Quercus petraea were related to the pattern of light.According to our observations, diversity and composition of vascular forest understory and bryophytes were related to heterogeneous light conditions. Forest management should maintain continuous shelter on the stand level; however, smaller gaps are necessary for the survival of light-demanding forest herbs and bryophytes, and larger gaps for tree seedlings.

Invasive stink bug favors naïve plants: Testing the role of plant geographic origin in diverse, managed environments.

With the introduction and establishment of exotic species, most ecosystems now contain both native and exotic plants and herbivores. Recent research identifies several factors that govern how specialist herbivores switch host plants upon introduction. Predicting the feeding ecology and impacts of introduced generalist species, however, remains difficult. Here, we examine how plant geographic origin, an indicator of shared co-evolutionary history, influences patterns of host use by a generalist, invasive herbivore, while accounting for variation in plant availability. The brown marmorated stink bug, Halyomorpha halys, is a highly polyphagous Asian herbivore and an economically important invasive pest in North America and Europe. In visual surveys of 220 plant taxa in commercial nurseries in Maryland, USA, H. halys was more abundant on non-Asian plants and selected these over Asian plants. The relationship between the relative use of plants and their availability was strongly positive but depended also on plant origin at two of our three sites, where the higher relative use of non-Asian plants was greatest for highly abundant taxa. These results highlight the importance of considering both plant origin and relative abundance in understanding the selection of host plants by invasive generalist herbivores in diverse, natural and urban forests.