Plant Community Heterogeneity Research Articles

EVALUATION OF RGB-BASED VEGETATION INDICES FROM UAV IMAGERY TO ESTIMATE FORAGE YIELD IN GRASSLAND

Abstract. Monitoring forage yield throughout the growing season is of key importance to support management decisions on grasslands/pastures. Especially on intensely managed grasslands, where nitrogen fertilizer and/or manure are applied regularly, precision agriculture applications are beneficial to support sustainable, site-specific management decisions on fertilizer treatment, grazing management and yield forecasting to mitigate potential negative impacts. To support these management decisions, timely and accurate information is needed on plant parameters (e.g. forage yield) with a high spatial and temporal resolution. However, in highly heterogeneous plant communities such as grasslands, assessing their in-field variability non-destructively to determine e.g. adequate fertilizer application still remains challenging. Especially biomass/yield estimation, as an important parameter in assessing grassland quality and quantity, is rather laborious. Forage yield (dry or fresh matter) is mostly measured manually with rising plate meters (RPM) or ultrasonic sensors (handheld or mounted on vehicles). Thus the in-field variability cannot be assessed for the entire field or only with potential disturbances. Using unmanned aerial vehicles (UAV) equipped with consumer grade RGB cameras in-field variability can be assessed by computing RGB-based vegetation indices. In this contribution we want to test and evaluate the robustness of RGB-based vegetation indices to estimate dry matter forage yield on a recently established experimental grassland site in Germany. Furthermore, the RGB-based VIs are compared to indices computed from the Yara N-Sensor. The results show a good correlation of forage yield with RGB-based VIs such as the NGRDI with R2 values of 0.62.

Grassland plant community spatial patterns driven by herbivory intensity

Heterogeneity is a critical driver in rangeland ecosystems and can be used to direct and quantify management success across landscapes. Plant community dynamics and patterns are frequently examined in relation to herbivory, but most patterns are discussed either in a single dimension, or across species, but at small spatial scales within individual communities. Therefore, we designed a study to measure plant community spatial patterns across similar topographic position and soil type in pastures with three different herbivory intensities. We assessed plant community spatial patterns in the northern mixed-grass prairie near Streeter, ND, USA (46°45′N, 99°28′W) in pastures moderately or intensively grazed or non-grazed exclosures established for 25 years. Plant species composition and abundance were recorded along 50-m transects in three replicate pastures for each grazing intensity (n = 3) at mid-summer twice separated by 10 years. Fine and coarse-scale diversity were similar between the moderately and intensively grazed pastures and greater than the non-grazed exclosures in both years (p ≤ .05). Plant species composition was similar among the three grazing intensities and years. Plant community heterogeneity was highest for moderately grazed pastures followed by intensively and non-grazed exclosures respectively. Similarly, patch size was smallest for moderate pastures followed by intensive pastures and non-grazed exclosures for both years. However, while patch size in both moderate and intensive pastures was similar across years respectively, patch size in non-grazed exclosures increased over time. Land management activities varying the disturbance intensities or timing of disturbance can influence specific species composition and relative abundance among species groups. Grazing intensity influenced both fine- and coarse-scale plant community patterns and created different, but stable, plant community patch sizes over time. Based on these results, the ecological cost of inactivity in managing grasslands may have profound consequences on plant community stability and the wildlife that depend on them.

Small-Scale Woodland Reduction Practices Have Neutral or Negative Short-Term Effects on Birds and Small Mammals

Woodland reduction has been under way for decades to improve habitat for certain wildlife species, increase forage for livestock, improve watershed function and reduce soil erosion, and increase plant community heterogeneity. Land managers have implemented a variety of techniques to reduce woodlands. Yet most studies on outcomes are observational and focus on plant communities; fewer studies experimentally compare the relative effects of woodland reduction methods on wildlife. We conducted an experiment to evaluate the effects of three mechanical tree removal methods on habitat use by native birds and abundance of small mammals in the first 2 yr after treatment. Located in the Piceance Basin, Colorado, United States, this study consisted of seven replicated 1-ha stands of pinyon-juniper woodland treated with chaining, roller-chop, hydro-ax, as well as untreated plots (n = 28 plots). We found no differences in initial bird habitat use or small mammal abundance among the woodland reduction treatment methods. However, we found evidence that habitat use was significantly lower in all woodland reduction treatment plots than in control plots for birds of dense woodland and open woodland habitats, and that use was positively associated with tree cover. Furthermore, no grassland or shrubland obligate birds used the treatment plots, suggesting that small-scale woodland reduction treatments may not provide attractive habitat for shrubland or grassland birds, at least within 2 yr following treatment. Because some bird species responded negatively to all methods of woodland reduction treatments, and no bird or small mammal species responded positively, the initial effects of small-scale chaining, roller-chop, and hydro-ax treatments on wildlife should not be overlooked.

Flooding effects on grassland species composition in the Azul creek basin, Argentina

From a hydrological point of view, the characteristic of the water behaviour in catchments so depressed as the Azul creek basin (centre of Buenos Aires province, Argentina) is water accumulation above the land surface. Thus, water on the ground does not have a single runoff direction, but moves in a disorderly, indefinite and unpredictable way. Considering that periodic floods are a typical disturbance of the region, the objective of this study is to analyse, under field conditions, the transformative effect of prolonged flooding on floristic composition, taking into account the different vegetation patches and their relative position over the relief, the chemical characteristics and the groundwater fluctuation, and some edaphic properties in each site. Vegetation samplings were performed during three consecutive springs, when the grassland was on different hydrological conditions due to very different rainfall precedent histories. A digital terrain model of the study area was built and a flow accumulation map was created from it. Pits were dug to describe edaphic variables and shallow wells were drilled for monitoring the groundwater characteristics. Flooding, in relation with surface and groundwater dynamics and soil characteristics, is the factor that determines and promotes the differentiation among sites that are relatively close, contiguous and even topographically in almost identical positions. So, some patches of vegetation get their differentiation through the limiting conditions of their soils, while others receive greater influence from the hydrodynamics to which they are subject. Thus, in this study it becomes evident how certain stands are floristically homogenised or differentiated over time according to their flooding conditions and, hence, according to the area from which they receive surface and groundwater flow. Also, results corroborate the way the water regime determines the structure and heterogeneity of plant communities in such environments.

Substantial long‐term effects of carcass addition on soil and plants in a grassy eucalypt woodland

AbstractThe decomposition of large vertebrate carcasses generates small‐scale disturbances characterized by changes in soil chemistry and new opportunities for plant establishment. Yet few studies have examined whether this effect is still evident several years after death, or has consequences for landscape‐scale heterogeneity. We examined soil chemistry and plant species richness and composition at 12 kangaroo carcasses (~30 kg initial mass) five years after their initial placement. Each carcass was paired with a nearby “control” site for comparison. We found that soil phosphorus was eight times higher at carcasses than at control sites, but that nitrogen concentration was similar. We also found that plant composition was substantially different between each carcass and control pair, with 80% of carcasses dominated by exotic species (mostly weedy annuals). Notably, overall variability in plant species composition across carcass sites was not different from the variability at control sites, indicating that the colonization of carcasses by weedy species did not have a homogenizing effect on plant assemblages across our study landscape. Our study demonstrates that a localized effect of large vertebrate carcasses on soil and plants was still evident after five years, indicating a state shift in the soil–plant dynamics at a carcass site. However, the effect of carcasses on landscape‐scale plant community heterogeneity was minimal because colonization was by weedy plants already present in the landscape.

Effects of plant diversity and structural complexity on parasitoid behaviour in a field experiment

1. In natural ecosystems, plants containing hosts for parasitoids are often embedded within heterogeneous plant communities. These plant communities surrounding host‐infested plants may influence the host‐finding ability of parasitoids.2. A release‐recapture‐approach was used to examine whether the diversity and structural complexity of the community surrounding a host‐infested plant influences the aggregation behaviour of the leaf‐miner parasitoidDacnusa sibiricaTelenga and naturally occurring local leaf‐miner parasitoids. Released and locally present parasitoids were collected on pottedJacobaea vulgarisGaertn.plants infested with the generalist leaf‐minerChromatomyia syngenesiaeHardy. The plants were placed in experimentally established plant communities differing in plant diversity (1–9 species) and habitat complexity (bare ground, mown vegetation, and tall vegetation). Additionally, parasitoids were reared out from host mines on the trap plants.3. Plant diversity did not influence the mean number of recapturedD. sibiricaor captures of other locally present parasitoids but the number of recaptured parasitoids was influenced by habitat complexity. NoD. sibiricaparasitoids were recaptured in the bare ground plots or plots with mown vegetation. The mean number of recapturedD. sibiricagenerally increased with increasing complexity of the plant community, whereas locally present parasitoids were captured more frequently in communities with more bare ground. There was a unimodal relationship between the number of reared out parasitoids and diversity of the surrounding vegetation with the highest density of emerged parasitoids at intermediate diversity levels.4. The present study adds to the thus far limited body of literature examining the aggregation behaviour of parasitoids in the field and suggests that the preference of parasitoids to aggregate in complex versus simple vegetation is association specific and thus depends on the parasitoid species as well as the identity of the plant community.

Can leaf area index and biomass be estimated from Braun‐Blanquet cover scores in tropical forests?

AbstractQuestionsThe loss and degradation of tropical forests is having severe impacts on the dynamics of understorey plant communities. Understanding these impacts requires efficient ways to measure vegetation change over broad spatial and temporal scales. Leaf area index (LAI) and above‐ground biomass are preferred quantitative measures of variation in plant community structure. However, their accurate measurement requires destructive sampling, which can be impractical or inappropriate. Here we test whether semi‐quantitative assessment of Braun‐Blanquet vegetation cover scores is a reliable proxy for direct quantitative measures of LAI and above‐ground biomass of differing plant growth forms (PGF) within tropical forests.LocationSix hundred square kilometre area of tropical lowland rain forest in Sabah, Malaysia.MethodsWe sampled understorey rain forest plant communities across a disturbance gradient in 2 × 2 m plots at 301 locations. We used a modified Braun‐Blanquet scale to estimate plant cover, destructively harvested all live above‐ground biomass up to a height of 2 m, calculated the above‐ground biomass of each species from separately processed stem and leaf fractions in each plot, and then calculated LAI using reference measures of specific leaf area for each species. For each of nine PGFs, we regressed LAI and biomass against the nine‐point Braun‐Blanquet ordinal transform scale (OTS) using linear mixed effects models.ResultsWe found a simple, uniform logarithmic scaling of LAI with increasing Braun‐Blanquet cover classes that was consistent across most PGFs, and with slope estimates close to 1.0. By contrast, no simple scaling relationship was found for above‐ground biomass, with most PGFs exhibiting an asymptotic relationship in which the Braun‐Blanquet estimates across high cover scores provided almost no resolution of observed variation in empirical biomass measures.ConclusionsWe found that the Braun‐Blanquet OTS provides a remarkably simple and accurate logarithmic scaling of LAI, but care should be taken in applying scaling rules uniformly across PGFs. In contrast, the Braun‐Blanquet OTS shows a more complex relationship with plant above‐ground biomass and we caution against its unconditional use for biomass estimation. The findings of this study should be broadly applicable to other ecosystems due to the heterogeneity of plant communities included in this work.

Effect of open water pools on ecosystem scale surface-atmosphere carbon dioxide exchange in a boreal peatland

Peatland carbon dioxide (CO2) exchange can vary spatially over a few meters because of the heterogeneity in plant communities, differing responses to environmental conditions, and the presence of pools in patterned peatlands. In contrast to the plant communities comprising a peatland’s vegetated surface, permanent pools that are characteristic of peatlands in temperate to subarctic regions are net sources of CO2 to the atmosphere. Measurements of net ecosystem CO2 exchange using the eddy covariance (EC) technique over peatlands without permanent pools do not show the smaller plant scale spatial heterogeneity in fluxes because the atmosphere mixes the variations in fluxes over the EC tower source area. However, if different vegetation communities and pools approach the spatial scale that they form a significant proportion of an EC tower’s source area, such as might be the case in peatlands with pools, they should be able to be discriminated if the surface fluxes by cover type are significantly different. In the present study, we evaluate if the observed variability in peatland surface CO2 exchange can be identified from 30-min net ecosystem CO2 exchange measurements using the proportion of the different plant communities or pools within the eddy covariance tower source area. Our results show that the variability in CO2 exchange at the local scale across the peatland surface has a measureable impact on the ecosystem level measurement, primarily when open water pools are present within the tower source area. Our results also suggest that large CO2 exchange rates measured above Sphagnum spp. hummocks with Picea mariana, combined with their large fractional contribution to the source area, counterbalanced the CO2 loss from the open water pools, explaining why the ecosystem as a whole was a net CO2 sink during the measurement period.

Determining spatial heterogeneity in species richness of plant community

Species richness (number of species) per small quadrat is an important characteristic of plant communities that, along with cover and biomass, can be readily measured in field surveys. We show that variance in the number of species among quadrats can be used to determine the spatial heterogeneity of plant communities. Comparisons between variance estimated based on the observed number of species among quadrats, and variance calculated based on an assumption that all species are arranged randomly among quadrats, can describe the spatial structure of species richness. We show field examples with different degrees of spatial heterogeneity in species richness to demonstrate how to survey, analyze and interpret: three natural grasslands with low heterogeneity in China, and a weedy grassland community with high heterogeneity in Japan.

Impact of ungulate exclusion on understorey succession in relation to forest management in the Intermountain Western United States

AbstractQuestionsDo successional trajectories in plant diversity, heterogeneity and dominance respond differently to ungulate exclusion in unmanaged forests vs managed forests that are thinned and burned? Is vegetation in recently thinned and burned stands more sensitive to changes in the grazing regime?LocationNortheast OR, USA.MethodsWe evaluated changes in plant community composition, diversity, heterogeneity and dominance under herbivory by multiple ungulates (cattle, elk, deer) vs ungulate exclusion at sites where trees were recently thinned and a prescribed burn was applied (managed), and in sites that were not thinned or burned in over 40 yr (unmanaged). Plant species diversity was calculated with the Simpsons index and richness as the total number of plant species. We estimated changes in plant community heterogeneity using a measure of taxonomic dissimilarity. Plant dominance was measured as the relative evenness among different plant functional groups (annual and perennial forbs and graminoids, and shrubs, subshrubs and trees).ResultsAs expected, managed sites displayed more early succession species, such as annual forbs and annual graminoids, while unmanaged sites were dominated by late‐succession species such as shrubs, subshrubs and trees. Species richness, particularly of annuals, was strongly reduced when ungulates were excluded from managed sites, and to a lesser extent from unmanaged sites for some perennial plant species. Species diversity decreased to a slightly greater extent with ungulate exclusion at managed sites. Species dominance was not influenced by ungulate exclusion. The effect of ungulate exclusion on plant heterogeneity also depended on forest management. Heterogeneity increased at managed sites and decreased in unmanaged sites with ungulate exclusion. Overall, the change in vegetation composition over time increased with the exclusion of ungulates, particularly at managed sites.ConclusionsThe strength and direction of specific vegetation and diversity responses to ungulate exclusion vary with forest management, and the influence of ungulate exclusion on plant succession is more pronounced in recently thinned and burned sites. Management of wild and domestic ungulates thus needs to account for forest management activities that alter vegetation seral stage and increase the sensitivity of vegetation to the ungulate grazing regime.

Soil Properties and Spatial Processes Influence Bacterial Metacommunities within a Grassland Restoration Experiment

AbstractMetacommunity theory proposes that a collection of local communities are linked by dispersal and the resulting compositions are a product of both niche‐based (species sorting) and spatial processes. Determining which of these factors is most important in different habitats can provide insight into the regulation of community assembly. To date, the metacommunity organization of heterotrophic soil bacteria is largely unknown. Spatial variation of soil bacterial communities could arise from (1) the resource heterogeneity produced by plant communities through root exudation and/or litter inputs; (2) the heterogeneity of soil environmental properties; and (3) pure spatial processes, including dispersal limitation and stochastic assembly. Understanding the relative importance of these factors for soil bacterial community structure and function could increase our ability to restore soil communities. We utilized an ongoing tallgrass prairie restoration experiment in northeastern Kansas to assess if restoring native plant communities produced changes in bacterial communities 6 years after restoration. We further examined the relative importance of the spatial heterogeneity of plant communities, soil properties, and pure spatial effects for bacterial community structure in the old‐field restoration site. We found that soil bacterial communities were not influenced by plant restoration, but rather, by the local heterogeneity of soil environmental properties (16.9% of bacterial community variation) and pure spatial effects (11.1%). This work also stresses the idea that restoring bacterial communities can take many years to accomplish due to the inherent changes that occur to the soil after cultivation and the time it takes for the re‐establishment of soil quality.

Thermal Adaptation and Diversity in Tropical Ecosystems: Evidence from Cicadas (Hemiptera, Cicadidae)

The latitudinal gradient in species diversity is a central problem in ecology. Expeditions covering approximately 16°54′ of longitude and 21°4′ of latitude and eight Argentine phytogeographic regions provided thermal adaptation data for 64 species of cicadas. We test whether species diversity relates to the diversity of thermal environments within a habitat. There are general patterns of the thermal response values decreasing in cooler floristic provinces and decreasing maximum potential temperature within a habitat except in tropical forest ecosystems. Vertical stratification of the plant communities leads to stratification in species using specific layers of the habitat. There is a decrease in thermal tolerances in species from the understory communities in comparison to middle level or canopy fauna. The understory Herrera umbraphila Sanborn & Heath is the first diurnally active cicada identified as a thermoconforming species. The body temperature for activity in H. umbraphila is less than and significantly different from active body temperatures of all other studied species regardless of habitat affiliation. These data suggest that variability in thermal niches within the heterogeneous plant community of the tropical forest environments permits species diversification as species adapt their physiology to function more efficiently at temperatures different from their potential competitors.

Soil nutrient patchiness and plant genotypes interact on the production potential and decomposition of root and shoot litter: evidence from short-term laboratory experiments with Triticum aestivum

Aims The purpose of this study was to test the hypotheses that soil nutrient patchiness can differentially benefit the decomposition of root and shoot litters and that this facilitation depends on plant genotypes. Methods We grew 15 cultivars (i.e. genotypes) of winter wheat (Triticum aestivum L.) under uniform and patchy soil nutrients, and contrasted their biomass and the subsequent mass, carbon (C) and nitrogen (N) dynamics of their root and shoot litters. Results Under equal amounts of nutrients, patchy distribution increased root biomass and had no effects on shoot biomass and C:N ratios of roots and shoots. Roots and shoots decomposed more rapidly in patchy nutrients than in uniform nutrients, and reductions in root and shoot C:N ratios with decomposition were greater in patchy nutrients than uniform nutrients. Soil nutrient patchiness facilitated shoot decomposition more than root decomposition. The changes in C:N ratios with decomposition were correlated with initial C:N ratios of litter, regardless of roots or shoots. Litter potential yield, quality and decomposition were also affected by T. aestivum cultivars and their interactions with nutrient patchiness. Conclusions Soil nutrient patchiness can enhance C and N cycling and this effect depends strongly on genotypes of T. aestivum. Soil nutrient heterogeneity in plant communities also can enhance diversity in litter decomposition and associated biochemical and biological dynamics in the soil.

Factors affecting mammalian carnivore use of riparian ecosystems in Mediterranean climates

Riparian ecosystems serve as movement and dispersal corridors; however, the factors that determine their use by multiple species of carnivores remain unknown. Two hypotheses can explain carnivore presence in riparian ecosystems. Higher riparian plant richness, diversity, and structure provide the resources used by carnivores (resting sites, cover, and food). Alternatively, areas with higher water availability allow species to withstand water loss and thermal gradients in the high summer temperatures. In southern Portugal we surveyed 70 transects 2 km long along riparian ecosystems during the wet winter months and again in the dry summer months, recording signs of carnivore species and the environmental context in which they occurred (vegetation descriptors, surrounding landscape, and waterway type and condition). Five carnivore species used riparian ecosystems (stone marten [Martes foina], Eurasian badger [Meles meles], common genet [Genetta genetta], Egyptian mongoose [Herpestes ichneumon], and red fox [Vulpes vulpes]). Riparian ecosystems had a diverse and heterogeneous plant community with a mix of obligate riparian, Mediterranean sclerophyllous, and exotic species. Most carnivore species responded to water channel type and standing water availability in both seasons, except for the stone marten in the wet season, which uses areas with rich riparian vegetation. Our results suggest that the use of riparian ecosystems is linked to water availability that provides water, prey, and external temperature regulation.

Large grazers modify effects of aboveground-belowground interactions on small-scale plant community composition.

Aboveground and belowground organisms influence plant community composition by local interactions, and their scale of impact may vary from millimeters belowground to kilometers aboveground. However, it still poorly understood how large grazers that select their forage on large spatial scales interact with small-scale aboveground–belowground interactions on plant community heterogeneity. Here, we investigate how cattle (Bos taurus) modify the effects of interactions between yellow meadow ants (Lasius flavus) and European brown hares (Lepus europaeus) on the formation of small-scale heterogeneity in vegetation composition. In the absence of cattle, hares selectively foraged on ant mounds, while under combined grazing by hares and cattle, vertebrate grazing pressure was similar on and off mounds. Ant mounds that were grazed by only hares had a different plant community composition compared to their surroundings: the cover of the grazing-intolerant grass Elytrigia atherica was reduced on ant mounds, whereas the relative cover of the more grazing-tolerant and palatable grass Festuca rubra was enhanced. Combined grazing by hares and cattle, resulted in homogenization of plant community composition on and off ant mounds, with high overall cover of F. rubra. We conclude that hares can respond to local ant–soil–vegetation interactions, because they are small, selective herbivores that make their foraging decisions on a local scale. This results in small-scale plant patches on mounds of yellow meadow ants. In the presence of cattle, which are less selective aboveground herbivores, local plant community patterns triggered by small-scale aboveground–belowground interactions can disappear. Therefore, cattle modify the consequences of aboveground–belowground interactions for small-scale plant community composition.

Community succession analysis and environmental biological processes of naturally colonized vegetation on abandoned hilly lands and implications for vegetation restoration strategy in Shanxi, China

Data were collected simultaneously at different succession stages using a space-for-time substitution, and were analyzed using the quantitative classification method (TWINSPAN) and the ordination technique (DCA). The community succession of natural colonized plants on abandoned hilly lands in Shanxi are below: Assoc. Potentilla chinensis + Setaria viridis -> Assoc. Artemisia sacrorum + S. viridis + Oxytropis caoraloa -> Assoc. A. sacrorum + Artemisia capillaries -> Assoc. A. capillaries + Pedivularis shansiensis + Echinops pseudosetifer -> Assoc. Hippophae rhamnoides- A. sacrorum + Cleistogenes squarrosa -> Assoc. H. rhamnoides + Ostryopsis davidiana-A. sacrorum -> Assoc. O. davidiana-A. sacrorum + Dendranthema chanetii -> Assoc. Populus davidiana-Caragana korshinskii-A. sacrorum -> Assoc. Larix principis-rupprechtii-H. rhamnoides-A. sacrorum. This established a recovery model of natural vegetation on abandoned hilly lands in Shanxi. The structure, composition and life-forms changed significantly during succession. Four indices of species diversity were used to analyze changes in the heterogeneity, dominance, richness and evenness of species during the succession process. The species richness and heterogeneity of plant communities increased significantly, the dominance decreased obviously and the evenness decreased slightly. The analysis of variance (ANOVA) also proved the significance of these five indices. Pioneer species of S. viridis, A. sacrorum, O. caoraloa, A. capillaries, P. shansiensis, E. pseudosetifer, C. squarrosa, H. rhamnoides, O. davidia, C. korshinskii, P. davidiana and L. principis-rupprechtii, etc. colonize successfully and play important roles on the vegetation restoration of abandoned hilly lands.

Beaver assisted river valley formation

AbstractWe examined how beaver dams affect key ecosystem processes, including pattern and process of sediment deposition, the composition and spatial pattern of vegetation, and nutrient loading and processing. We provide new evidence for the formation of heterogeneous beaver meadows on riverine system floodplains and terraces where dynamic flows are capable of breaching in‐channel beaver dams. Our data show a 1.7‐m high beaver dam triggered overbank flooding that drowned vegetation in areas deeply flooded, deposited nutrient‐rich sediment in a spatially heterogeneous pattern on the floodplain and terrace, and scoured soils in other areas. The site quickly de‐watered following the dam breach by high stream flows, protecting the deposited sediment from future re‐mobilization by overbank floods. Bare sediment either exposed by scouring or deposited by the beaver flood was quickly colonized by a spatially heterogeneous plant community, forming a beaver meadow. Many willow and some aspen seedlings established in the more heavily disturbed areas, suggesting the site may succeed to a willow carr plant community suitable for future beaver re‐occupation. We expand existing theory beyond the beaver pond to include terraces within valleys. This more fully explains how beavers can help drive the formation of alluvial valleys and their complex vegetation patterns as was first postulated by Ruedemann and Schoonmaker in 1938. Copyright © 2010 John Wiley & Sons, Ltd.

How much does grazing-induced heterogeneity impact plant diversity in wet grasslands?

The positive impact of grazing herbivores on plant diversity has been frequently reported in fertile grasslands. Grazing-induced heterogeneity is often evoked as an explanation for this influence. However, the relative importance of plant community heterogeneity induced by herbivores compared to other mechanisms linked to grazing remains unclear. We investigated this question by looking at 3 plant communities found in wet grasslands situated along the French Atlantic coast and traditionally grazed by horses and cattle. An experimental design set up in 1995 allowed us to compare the consequences of cattle-grazing, horse-grazing, and grazing abandonment on plant community diversity and heterogeneity. Floristic measurements made in 2007 showed that cattle and horses both had a positive impact on species richness and Shannon diversity index and that patchiness only occurred in grazed situations. The relative importance of grazing-induced patchiness within the overall positive effect of grazing on plant diversity was assessed by partitioning the diversity gain due to grazing into its additive within- and among-patch components. Grazing-induced patchiness entirely explained the increase in plant richness, whereas it accounted for only a small part of Shannon diversity. Grazing-related processes operating at the patch scale explained the main part of the increase in Shannon diversity. These processes make only a limited contribution to species recruitment, but they result in a more even species abundance distribution.

Influence of wildfire severity on riparian plant community heterogeneity in an Idaho, USA wilderness

Despite the increasing recognition of riparian zones as important ecotones that link terrestrial and aquatic ecosystems and of fire as a critical natural disturbance, much remains unknown regarding the influence of fire on stream-riparian ecosystems. To further this understanding, we evaluated the effects of mixed severity wildfire on riparian plant community structure and composition in headwater streams of the Big Creek Watershed of the Frank Church ‘River of No Return’ Wilderness of central Idaho. Five years after a large stand-replacing fire, we conducted riparian vegetation surveys at sixteen reaches across a range of burn types. Non-metric Multidimensional Scaling (NMS) and Multi-Response Permutation Procedure (MRPP) analyses showed an overall shift in community composition and structure between vegetation at unburned and severely burned reaches. Although total plant cover was significantly less at severely burned areas, recovery of the deciduous understory was apparent. Severely burned reaches were characterized by a marked increase in cheatgrass ( Bromus tectorum). Reaches that were exposed to low-severity fire were indistinguishable from unburned reaches relative to vegetation community composition and structure, pointing to a possible disturbance threshold that may need to be crossed in order to alter riparian plant communities.

Microsite and herbaceous vegetation heterogeneity after burning Artemisia tridentata steppe

Woody vegetation can create distinct subcanopy and interspace microsites, which often result in resource islands in subcanopies compared to interspaces. This heterogeneity in soil resources contributes to herbaceous vegetation heterogeneity in plant communities. However, information detailing the impact of disturbance, such as fire, that removes the woody vegetation on microsites and herbaceous vegetation heterogeneity is limited. The purpose of this study was to determine the influence of burning on microsites and herbaceous vegetation in subcanopies and interspaces. Six study sites (blocks) were located at the Northern Great Basin Experimental Range in shrub (Artemisia tridentata ssp. wyomingensis (Beetle & A. Young) S.L. Welsh)-bunchgrass plant communities and one half of each block was burned to remove A. tridentata. Herbaceous vegetation and microsite characteristics were measured 2 years post-fire in intact and burned subcanopies and interspaces. Burning resulted in microsite and herbaceous vegetation differences between intact and burned subcanopies and intact and burned interspaces. However, burned subcanopies and burned interspaces appeared to be relatively similar. The similarity in microsite characteristics probably explains the lack of differences in herbaceous vegetation cover and biomass production between burned subcanopies and burned interspaces (P > 0.05). However, some microsite and herbaceous vegetation characteristics differed between burned subcanopies and burned interspaces. Our results suggest that disturbances that remove woody vegetation reduced microsite and herbaceous vegetation heterogeneity within plant communities, but do not completely remove the resource island effect. This suggests soil resource heterogeneity may influence post-fire community assembly and contribute to diversity maintenance.