High Disturbance Intensity Research Articles

Disturbance history, neighborhood crowding and soil conditions jointly shape tree growth in temperate forests.

Understanding how different mechanisms act and interact in shaping communities and ecosystems is essential to better predict their future with global change. Disturbance legacy, abiotic conditions, and biotic interactions can simultaneously influence tree growth, but it remains unclear what are their relative contributions and whether they have additive or interactive effects. We examined the separate and joint effects of disturbance intensity, soil conditions, and neighborhood crowding on tree growth in 10 temperate forests in northeast China. We found that disturbance was the strongest driver of tree growth, followed by neighbors and soil. Specifically, trees grew slower with decreasing initial disturbance intensity, but with increasing neighborhood crowding, soil pH and soil total phosphorus. Interestingly, the decrease in tree growth with increasing soil pH and soil phosphorus was steeper with high initial disturbance intensity. Testing the role of species traits, we showed that fast-growing species exhibited greater maximum tree size, but lower wood density and specific leaf area. Species with lower wood density grew faster with increasing initial disturbance intensity, while species with higher specific leaf area suffered less from neighbors in areas with high initial disturbance intensity. Our study suggests that accounting for both individual and interactive effects of multiple drivers is crucial to better predict forest dynamics.

The interacting effect of climate change and herbivory can trigger large-scale transformations of European temperate forests.

In many regions of Europe, large wild herbivores alter forest community composition through their foraging preferences, hinder the forest's natural adaptive responses to climate change, and reduce ecosystem resilience. We investigated a widespread European forest type, a mixed forest dominated by Picea abies, which has recently experienced an unprecedented level of disturbance across the continent. Using the forest landscape model iLand, we investigated the combined effect of climate change and herbivory on forest structure, composition, and carbon and identified conditions leading to ecosystem transitions on a 300-year timescale. Eight climate change scenarios, driven by Representative Concentration Pathways 4.5 and 8.5, combined with three levels of regeneration browsing, were tested. We found that the persistence of the current level of browsing pressure impedes adaptive changes in community composition and sustains the presence of the vulnerable yet less palatable P. abies. These development trajectories were tortuous, characterized by a high disturbance intensity. On the contrary, reduced herbivory initiated a transformation towards the naturally dominant broadleaved species that was associated with an increased forest carbon and a considerably reduced disturbance. The conditions of RCP4.5 combined with high and moderate browsing levels preserved the forest within its reference range of variability, defining the actual boundaries of resilience. The remaining combinations of browsing and climate change led to ecosystem transitions. Under RCP4.5 with browsing effects excluded, the new equilibrium conditions were achieved within 120 years, whereas the stabilization was delayed by 50-100 years under RCP8.5 with higher browsing intensities. We conclude that forests dominated by P. abies are prone to transitions driven by climate change. However, reducing herbivory can set the forest on a stable and predictable trajectory, whereas sustaining the current browsing levels can lead to heightened disturbance activity, extended transition times, and high variability in the target conditions.

Biomorphological traits and leaf dry matter content are important to plant persistence in a highly unstable volcanic ground

The colonisation of newly formed territories by plants during primary succession is a crucial stage in the formation of ecosystems. Adaptations of species to withstand harsh environment allow them to survive and form pioneer communities. We analysed ten categorical and quantitative plant traits to examine their role in plant resistance to substrate instability in primary volcanic habitats. The research questions were: 1) which plant traits enable plants to persist under the most unstable ground conditions?; 2) what are the ecological strategies of species that grow under the condition of stress, such as low mineral nutrition, coupled with intense disturbance, such as unstable ground? The research was conducted on the Tolbachinsky Dol plateau in Kamchatka, Russia, on loose volcanic sediments of the 1975 eruption. At altitudes ranging from 700 m a.s.l. to 1000 m a.s.l., 40 sample plots were established along the gradients of altitude and degrees of ground instability. The species composition of vascular plants and the percentage cover of species, as well as other habitat characteristics, were assessed. Linear models were used to investigate the relationship between traits and ground instability. The principal component analysis was used to identify groups of characteristic traits. Leaf dry matter content (LDMC) had the largest explained variance. The LDMC community weighted means decreased with increasing disturbance (ground instability). A whole set of categorical traits, i.e. plant functional types, characterised the adaptation of plants to unstable ground. These characteristics include life-form (according to Serebryakov classification), density of shoots arrangement, morphology of underground organs, and, to a lesser extent, dispersal mode. The study analysed 45 species, and it was found that only four of them were adapted to perform under conditions of high substrate instability. The study showed that the mutual interaction of a suite of biomorphological traits and leaf density characteristics formed a syndrome of pioneer species adapted to unstable ground. The ecological strategies of the majority of species in the studied communities were characterised by the predominance of the stress-tolerant strategy, explained by the extremely poor mineral nutrition in the primary habitats. Under conditions of both low nutrient availability and high disturbance intensity, species with a mixed stress-tolerant-ruderal strategy are favoured. In the gradient of increasing disturbance, the role of the ruderal axis increased while stress tolerance decreased. However, the role of ecological strategy may be weakened by the presence of other important adaptive traits.

Carbon balance shift in mountain peatlands along a gradient of grazing disturbance in the tropical Andes (Colombia)

High-elevation cushion peatlands are typical ecosystems of the Andes above 4000 m of altitude, with an important role in hydrology and global carbon sequestration. In Tropical Andean context, grazing livestock is one of the main threats to cushion peatlands, altering the vegetation and the storage carbon function. The aim of this research is to understand how grazing influences cushion peatland functioning by identifying ecological thresholds for carbon balance process. The study was carried out in four Andean peatlands in the northern part of Colombian Andes during 2019–2020. We established 30 plots of 1 m2 where water table level, vegetation cover, and grazing disturbance were monitored. We also measured CO2 fluxes using an infrared gas analyzer connected to a closed static chamber, which registered net ecosystem exchange and respiration data. Considerable variation in the conservation status of Distichia muscoides cushions was found within the sampled peatlands, reflecting an heterogeneous signal of grazing disturbance that is evident at the plot-specific scale. Decreasing water table level was related with changes in dominant vegetation, from compact cushion species to grasses proliferation, exacerbating disturbance effects and carbon emissions. Mixed-effects logistic regression models showed a carbon balance shift, from CO2 sink to net emitter, in plots with high disturbance intensity and low D. muscoides cover. This study provides information for a better understanding of mountain peatlands functioning in the Tropical Andes and underlines the key role of D. muscoides cushions and the water table in carbon balance shift.

Annual forest disturbance intensity mapped using Landsat time series and field inventory data for the conterminous United States (1986–2015)

Forest disturbances can have broad impact on the climate, local environment, and the regeneration of the forest ecosystem. The nature and magnitude of such impact is largely driven by disturbance intensity. In this study, by integrating field plot measurements collected by the Forest Inventory and Analysis program with time series Landsat observations, we produced the first set of annual forest disturbance intensity map products quantifying the percentage of basal area removal (PBAR) at the 30-m resolution for the conterminous United States (CONUS) from 1986 to 2015. The derived map products revealed that during the 30-year study period, the annual average PBAR values of all disturbed pixels across CONUS ranged from 66% to 70%, and the proportion of those pixels having stand-clearing disturbances ranged from 40% to 58%. High disturbance intensities were concentrated in the Southeastern states from Texas to Virginia and along the Pacific coast and the Cascades in the West. At the national scale, the annual mean PBAR and proportion of stand clearing area (PSCA) values both appeared to follow second order trajectories, with increasing trends at the beginning, decreasing trends towards the end, and turning points around 2003. Overall, there is a net increase of 2% in PBAR and 3% in PSCA from 1986 to 2015. The temporal trends of PBAR and PSCA were also investigated at state and ecoregion levels, with substantial differences found among many states and ecoregions. While states and ecoregions generally follow second order trajectories, the majority had increasing trends throughout much of the study period, reflecting higher disturbance intensities during the later years compared to earlier years. Large increase (>10%) in PBAR was seen in several states (e.g., Virginia, Arkansas, and Minnesota) and ecoregions (e.g., Northern Minnesota Wetlands); however, large decreases (>10%) in PBAR were not observed in any states, and were seen in only one ecoregion, the Blue Mountains in the southeast. The disturbance intensity maps are available from a web portal of the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL-DAAC) at https://doi.org/10.3334/ORNLDAAC/2059.

Plant-plant co-occurrences under a complex land-use gradient in a temperate forest.

Land-use generates multiple stress factors, and we need to understand their effects on plant-plant interactions to predict the consequences of land-use intensification. The stress-gradient hypothesis predicts that the relative strength of positive and negative interactions changes inversely under increasing environmental stress. However, the outcome of interactions also depends on stress factor's complexity, the scale of analysis, and the role of functional traits in structuring the community. We evaluated plant-plant co-occurrences in a temperate forest, aiming to identify changes in pairwise and network metrics under increasing silvopastoral use intensity. Proportionally, positive co-occurrences were more frequent under high than low use, while negative co-occurrences were more frequent under low than high. Networks of negative co-occurrences showed higher centralization under low use, while networks of positive co-occurrences showed lower modularity and higher centralization under high use. We found a partial relationship between co-occurrences and key functional traits expected to mediate facilitation and competition processes. Our results shows that the stress-gradient hypothesis predicts changes in spatial co-occurrences even when two stress factors interact in a complex way. Networks of negative co-occurrences showed a hierarchical effect of dominant species under low use intensity. But positive co-occurrence network structure partially presented the characteristics expected if the facilitation was an important mechanism characterizing the community under high disturbance intensity. The partial relationship between functional traits and co-occurrences may indicate that other factors besides biotic interactions may be structuring the observed negative spatial associations in temperate Patagonian forests.

Interaction of traffic intensity and habitat features shape invasion dynamics of an invasive alien species (Ambrosia artemisiifolia) in a regional road network

Road corridors are important conduits for plant invasions, and an understanding of the underlying mechanisms is necessary for efficient management of invasive alien species in road networks. Previous studies identified road type with different traffic volumes as a key driver of seed dispersal and abundance of alien plants along roads. However, how the intensity of traffic interacts with the habitat features of roadsides in shaping invasion processes is not sufficiently understood. To elucidate these interactions, we analyzed the population dynamics of common ragweed (Ambrosia artemisiifolia L.), a common non-indigenous annual species in Europe and other continents, in a regional road network in Germany. Over a period of five years, we recorded plant densities at roadsides along four types of road corridors, subject to different intensities of traffic, and with a total length of about 300 km. We also classified roadsides in regard to habitat features (disturbance, shade). This allowed us to determine corridor- and habitat-specific mean population growth rates and spatial-temporal shifts in roadside plant abundances at the regional scale. Our results show that both traffic intensity and roadside habitat features significantly affect the population dynamics of ragweed. The combination of high traffic intensity and high disturbance intensity led to the highest mean population growth whereas population growth in less suitable habitats (e.g. shaded roadsides) declined with decreasing traffic intensity. We conclude that high traffic facilitates ragweed invasion along roads, likely due to continued seed dispersal, and can compensate partly for less suitable habitat features (i.e. shade) that decrease population growth along less trafficked roads. As a practical implication, management efforts to decline ragweed invasions within road networks (e.g. by repeated mowing) should be prioritized along high trafficked roads, and roadside with disturbed, open habitats should be reduced as far as possible, e.g. by establishing grassland from the regional species pool.

Basal area increment models accounting for climate and mixture for Austrian tree species

Forest trees exhibit a large variation in basal area increment, which depends on tree specific factors, inter-tree relations and the environment. Studying the effect of the above factors on tree growth is essential for forest ecology and forest management. In this study, a basal area increment model for 22 tree species of the Austrian National Forest Inventory was parameterized. The model included effects of tree size, competition, harvesting and disturbances, climate, soil, and species mixtures. The models were fitted using generalized additive models. Basal area increment increased with tree size for all species, and competition and density reduced tree growth. Shade-tolerant tree species showed a more pronounced decrease with competition, than light-demanding tree species. Tree growth increased with disturbance intensity, but confidence intervals for high disturbance intensities became larger. Many variants of the model that included climatic effects were tested, but the best variants used long-term-mean temperature and precipitation during the growing season and the positive and negative deviations thereof, which separated climatic site effects from temperature deviations due to yearly weather conditions. Several of the tested dryness indices were not able to explain tree growth from inventory data. Soil type and soil moisture were significant for many tree species, each of them accounting for a maximum of 10% of difference in tree growth. Little growth was observed on calcareous Leptosols, whereas trees grew best on Planosols, on flysch or calcareous Cambisols. Growth was decreased on dry sites, but very moist and wet sites could result in either a decrease or an increase in growth, depending on the tree species. Mixture effects were significant for almost all mixtures with a sufficient number of observations in the data set. Notable exceptions were the mixtures of Pinus sylvestris L. with Picea abies (L.) Karst. and Abies alba Mill. and the mixtures of Quercus spp. with Pinus sylvestris L. and Fagus sylvatica L.. Mixture effects were both favorable and adverse at individual tree level. Picea abies and Fagus sylvatica were two tree species, which were mainly favored by mixture effects. Interactions of mixture with age or site could not be modelled from the data because they were confounded with other effects of the inventory data. The new basal area increment model provided a comprehensive formulation, which can be use part of individual tree growth simulators and is easily transferable to other regions and environments and can be extended in the future.

Windows of opportunity for smooth cordgrass landward invasion to tidal channel margins: The importance of hydrodynamic disturbance to seedling establishment

Despite increasing concerns about the global threat of cordgrass (S. alterniflora) expansion and the interest in its invasion mechanisms, there is not yet a general understanding of the mechanistic processes underlying the interaction between cordgrass invasion and geomorphic structures such as tidal channels. This study elucidated the effects of the hydrodynamic disturbance of tidal channels on initial seedling establishment of cordgrass in the margins of two different types of tidal channels (i.e., main tidal channels and secondary tributaries). We performed field experiments that transplanted cordgrass seedlings to above-mentioned tidal channel margins with on-site controlled hydrodynamic conditions. The results showed that high hydrodynamic disturbance intensity (i.e., HDI) on the margin of main tidal channels (i.e., MMC) was not beneficial to cordgrass invasion, whereas low HDI created windows of opportunity for cordgrass invasion to the margin of secondary tributaries (i.e., MST) by facilitating the survival, growth, and stability of cordgrass seedlings. The presence of high HDI predominantly reduced the seedling survival and total biomass of cordgrass, whereas root biomass allocation of cordgrass increased significantly to resist dislodgment and toppling. Moreover, field investigations showed that soil salinity and moisture in the margin of tidal channels were not the limiting factors affecting the establishment of cordgrass seedlings. However, higher propagule pressure combined with suitable soil salinity-moisture conditions (i.e., low salinity and high moisture) laid a firm foundation favoring seedling establishment. Our results highlight the importance of hydrodynamic disturbance as a dominating driver regulating seedling establishment of cordgrass in tidal channel margins and the potential implications for controlling cordgrass landward invasions.

Effect of Plateau Zokor (Myospalax fontanierii) Disturbance on Plant Community Structure and Soil Properties in the Eastern Qinghai-Tibet Plateau, China

Plateau zokor (Myospalax fontanierii) is a native subterranean rodent in alpine rangeland on Qinghai-Tibet Plateau (QTP) in China, and its foraging, digging, and mounds building cause the unique disturbance pattern to alpine rangeland ecosystem. Over the past decades, the zokors have been regarded as major pests who result in the degradation of the rangeland ecosystem in the QTP and have been simply eliminated by rodenticides or traps. Understanding the function of zokors and evaluating the zokors’ impacts on alpine rangeland systems should be the solid scientific basis for zokor control. In this study, we considered the average nearest neighbor indices of zokor mounds to represent the disturbance intensity of zokor to alpine rangeland and surveyed the plant species richness, diversity indices, biomass, and soil physicochemical properties in intermound areas under different zokor disturbance intensities in alpine rangeland in Tianzhu Tibet Autonomous County, located in eastern QTP. Our results indicated that 1) the plant species diversity indices were positively correlated with zokor disturbance intensity and aboveground and belowground biomasses were not significantly different under the different disturbance intensities; 2) the importance value of the forb functional group increased as the disturbance intensities increased, and the sedge functional group showed the opposite trend. The proportions of aboveground and belowground biomasses of forbs increased as the disturbance level increased; 3) there were no changes in the dominant plant species among the different disturbance intensities, but new plant species (Polygonum viviparum and Equisetum arvense) occurred in the plots with high disturbance intensities; 4) a significant positive correlation was observed between soil moisture in the 0-20 cm layer and disturbance intensity, while soil temperature exhibited a significantly negative relationship with the disturbance intensity; however, no differences in soil chemical properties were observed; and 5) redundancy analysis identified that mound building changed soil physical properties, especially soil moisture, in intermound areas, which influenced the plant community structure. In conclusion, the plateau zokor in our study area increased plant species diversity and did not decrease plant biomass, which is beneficial for alpine rangeland systems. We suggest that rangeland managers should consider the multiple functions of zokors to an alpine rangeland ecosystem instead of simply eliminating them.

Allocating species in Grime’s strategy space: an alternative to trait-based approaches

Background: The three primary-strategy theory proposed by Grime identifies stress and disturbance as key environmental factors leading to the emergence of distinct plant strategies. These are defined by a combination of stress and disturbance tolerance. Plant strategies are usually inferred from sets of traits, but this may lead to circular reasoning and artificial restrictions to species’ distribution in strategy space. 
 Question: Can measurements of stress and disturbance tolerance be used to estimate the position of different species relative to each other in Grime’s strategy space?
 Data description: Stress, disturbance, and abundances for 50 species at 25 0.5 ha sites.
 Study site and dates: Semiarid grassland, Oaxaca, Mexico, 2014.
 Methods: Species’ tolerance to stress and disturbance were inferred from abundances, and used to allocate species in Grime’s space. We tested if some attributes of our study species changed over the strategy space according to theoretical expectations.
 Results: Most species were allocated towards high disturbance and low stress intensities. Species attributes were in line with the trends expected from their position in the strategy space. 
 Discussion: Perhaps because of a long grazing history, most species were tolerant to disturbance. The allocation of species in the strategy space using stress and disturbance measurements seemed correct based on their attributes. Thus, our measurements seem to reflect the basic principles proposed by Grime. Our method provides relative positions in the strategy space, and (as previous work) requires defining somewhat arbitrary limits to such space if we wish to label species as ruderals, competitors or stress-tolerant.

Genetic diversity and genetic structure of Decalobanthus boisianus in Hainan Island, China.

Decalobanthus boisianus is a native plant of Hainan Island, China, which has caused considerable damage to tropical forest ecosystems in recent decades. Understanding the genetic diversity and structure of this species can facilitate uncovering the molecular mechanism of its invasive ability. Here, we collected 77 individuals of D. boisianus spanning 8 distribution areas with a gradient of human disturbance intensity (i.e., low, moderate, and high disturbance intensity groups) to assess patterns of genetic diversity and structure using inter simple sequence repeat (ISSR) markers. We found that a total of 220 loci were scored with 13 primers using ISSR methods, and that 198 loci were polymorphic. The genetic diversity of D. boisianus among these eight forests decreased with increasing human disturbance intensity. Over 70% of the total genetic variation was present within populations, while less than 30% of variation was found among populations. There was a high gene flow (1.27) among them due to a lack of effective geographic barriers. The mean Nei's genetic distance of D. boisianus populations was found to be relatively small (i.e., 0.07), and the average genetic similarity of the eight populations was high (i.e., 0.93). Our findings indicate that the genetic diversity of D. boisianus correlated to human disturbance density, and that D. boisianus populations in Hainan Island have frequent gene exchange. We suggest that reduce deforestation to decrease human disturbance may be a good way to prevent the invasion of D. boisianus.

English

Seed bank (henceforth referred to as SB) refers to viable seed which is present on or in soil and associated with litter/humus. Under different land use/land cover systems and climatic regions, density of seeds in the soil is variable both in space and time. SB density decreases with increasing depth and age of seeds in the soil. Smaller seeds are more easily incorporated, moved to deeper soil layers and persist longer in the soil, whereas large sized seeds without hard seed coats lack persistence in the soil. Moreover, small and elongated seeds are more persistent than large and round seeds. In most vegetation types, grass and herbs have denser seeds in the soil than woody species. Due to shade intolerant properties, density of grasses and forbs are also higher in forest gaps and farmlands than under shade of trees. In most of vegetation types and under medium disturbances e.g. under grazing, density and diversity of species are higher than that of lower and higher disturbance intensities. Tillage practices also have negative impacts on density, composition and abundance of SB. Hence, seeds decline under conventional tillage than under conservational tillage practices. In several studies, seeds in the SB are not similar to above ground vegetation. Similarly, density and diversity of seeds in a SB exceeds that of standing vegetation. Persistence seeds in soil are essential in maintaining individual species and the plant community, conservation of genetic biodiversity and restoration of plant communities of degraded lands after disturbances under harsh environmental conditions, especially in arid regions. As a result, SB characteristics are used to elucidate seed dynamics in various vegetation types. Overall, scientific knowledge of SB is used for land use planning, to make recommendations for future cost effective measures and to establish policies for conservation or restoration programmes. Keywords: Seed bank (SB), dispersal, persistence, restoration, vegetation type.

Effects of selective cutting disturbance on soil phosphorus adsorption and desorption in a Korean pine and broad-leaved mixed forest in the Xiaoxing'an Mountains, China.

This study examined the characteristics of phosphorus (P) adsorption and desorption in surface soil (0-10 cm) of a secondary forest after selective cutting disturbance at three levels of intensity (low, medium, high) in order to reveal the effects of different disturbance intensities on soil P adsorption and desorption. Maximum adsorption amount (Qm), adsorption intensity factor, maximum buffer capacity, maximum desorption amount, average desorption rate and readily desorptable phosphorus were measured. Qm in the focal forests was 1383.93-1833.34 mg·kg-1, and Qmin forests with middle and high disturbance intensities was significantly higher than that in forests with low disturbance intensity and in primary forests. P adsorption intensity was 0.024-0.059 L·mg-1, and forests with high and low disturbance intensities increased the P adsorption intensity significantly. The maximum buffer capacity varied from 35.68 to 97.97 L·kg-1, with the highest value found in the forest with the highest disturbance intensity. Selective cutting significantly reduced the potential for phosphorus supply in the forest soils. The maximum desorption amount, average desorption rate and readily desorptable phosphorus content in the focal forests were 526.32-797.54 mg·kg-1, 14.7%-25.5% and 1.79-5.41 mg·kg-1, respectively, indicating that the ability of soil to release phosphorus significantly decreased with increasing disturbance intensity. Selective cutting changed the phosphorus adsorption and desorption characteristics by reducing the supply and release of soil phosphorus.

Response of leaf traits of common plants in alpine meadow to plateau pika disturbance

Leaf traits have been proven to reflect the adaptation of individual plants to disturbance environments in a grassland ecosystem. A field survey was conducted to investigate the effects of the disturbance intensity of plateau pika on the leaf traits of a dominant (Kobresia pygmaea) and two common plants (Elymus nutans and Anemone rivularis var. flore-minore) in an alpine meadow. This study indicated that the plateau pika disturbance enables the individuals of three plants to exhibit respective plasticity because the three plants had different leaf indices (LI) as the disturbance intensity increased. K. pygmaea, E. nutans and A. rivularis var. flore-minore had high specific leaf area (SLA), leaf dry mass content (LDMC), and leaf nitrogen content (LNC) at relatively low, moderate, and high disturbance intensities of plateau pika, respectively. K. pygmaea, E. nutans and A. rivularis var. flore-minore suffered low nutrient stress at low, moderate and high disturbance intensities due to high N : P at corresponding disturbance intensities. These results indicated that K. pygmaea, E. nutans and A. rivularis var. flore-minore grew well at relatively low, moderate, and high disturbance intensity conditions, respectively, which contributed to the improvement of alpine meadows with a higher proportion of E. nutans at a moderate disturbance intensity or the deterioration of alpine meadows with a higher proportion of A. rivularis var. flore-minore at a high disturbance intensity. Our findings suggest that leaf traits are effective tools to explain how small burrowing herbivore disturbances often lead to the improvement or deterioration of alpine meadows under different disturbance intensities.

The positive relationships between plant coverage, species richness, and aboveground biomass are ubiquitous across plant growth forms in semi-steppe rangelands

The positive relationships between biodiversity and aboveground biomass are important for biodiversity conservation and greater ecosystem functioning and services that humans depend on. However, the interaction effects of plant coverage and biodiversity on aboveground biomass across plant growth forms (shrubs, forbs and grasses) in natural rangelands are poorly studied. Here, we hypothesized that, while accounting for environmental factors and disturbance intensities, the positive relationships between plant coverage, biodiversity, and aboveground biomass are ubiquitous across plant growth forms in natural rangelands. We applied structural equation models (SEMs) using data from 735 quadrats across 35 study sites in semi-steppe rangelands in Iran. The combination of plant coverage and species richness rather than Shannon's diversity or species diversity (a latent variable of species richness and evenness) substantially enhance aboveground biomass across plant growth forms. In all selected SEMs, plant coverage had a strong positive direct effect on aboveground biomass (β = 0.72 for shrubs, 0.84 for forbs and 0.80 for grasses), followed by a positive effect of species richness (β = 0.26 for shrubs, 0.05 for forbs and 0.09 for grasses), and topographic factors. Disturbance intensity had a negative effect on plant coverage, whereas it had a variable effect on species richness across plant growth forms. Plant coverage had a strong positive total effect on aboveground biomass (β = 0.84 for shrubs, 0.88 for forbs, and 0.85 for grasses), followed by a positive effect of species richness, and a negative effect of disturbance intensity across plant growth forms. Our results shed light on the management of rangelands that is high plant coverage can significantly improve species richness and aboveground biomass across plant growth forms. We also found that high disturbance intensity due to heavy grazing has a strong negative effect on plant coverage rather than species richness in semi-steppe rangelands. This study suggests that proper grazing systems (e.g. rotational system) based on carrying capacity and stocking rate of a rangeland may be helpful for biodiversity conservation, better grazing of livestock, improvement of plant coverage and enhancement of aboveground biomass.

Plateau pika disturbances alter plant productivity and soil nutrients in alpine meadows of the Qinghai-Tibetan Plateau, China

Plateau pika (Ochotona curzoniae) is an endemic mammal in the Qinghai-Tibetan Plateau, and its activities create extensive disturbances on vegetation and soil of alpine meadow. Field surveys at two sites were conducted to determine the effects of plateau pika disturbances on important soil factors and plant biomass of vegetated land, and their relationships of the same alpine meadow type. Our study showed that plateau pika disturbances significantly increased soil organic carbon, soil total nitrogen, graminoid biomass and the number of plant species, and significantly decreased soil moisture and forb biomass, although they had no significant impacts on soil total phosphorus, soil total potassium and total biomass on vegetated land. Our study further showed that soil organic carbon, soil total nitrogen, graminoid biomass and the number of plant species were much higher at intermediate disturbance intensities than those at low and high disturbance intensities in the disturbed areas, and soil moisture showed a decreasing trend with the increase of disturbance intensity. Plateau pika disturbances altered the contribution of some important soil nutrients and moisture to plant biomass, and had different impact on the best models between plant biomass (total biomass, graminoid biomass and forb biomass) and predominant soil factors. Our results demonstrated that the optimal disturbance intensities of plateau pika were beneficial to alpine meadow. These results highlighted the influence of the presence of plateau pika and its disturbance intensity on key soil nutrients and plant productivity on vegetated land of the same alpine meadow type, which will help us better understand the role of plateau pika in the alpine meadow ecosystem.

Diversity–disturbance relationship in forest landscapes

Despite decades of research, there is an intense debate about the consistency of the hump-shaped pattern describing the relationship between diversity and disturbance as predicted by the intermediate disturbance hypothesis (IDH). Previous meta-analyses have not explicitly considered interactive effects of disturbance frequency and intensity of disturbance on plant species diversity in terrestrial landscapes. We conducted meta-analyses to test the applicability of IDH by simultaneously examining the relationship between species richness, disturbance frequency (quantified as time since last disturbance as originally proposed) and intensity of disturbance in forest landscapes. The effects of disturbance frequency, intensity, and their interaction on species richness was evaluated using a mixed-effects model. We found that species richness peaks at intermediate frequency after both high and intermediate disturbance intensities, but the richness-frequency relationship differed between intensity classes. Our study highlights the need to measure multiple disturbance components that could help reconcile conflicting empirical results on the effect of disturbance on plant species diversity.

Plant trait–digestibility relationships across management and climate gradients in permanent grasslands

Summary Dry matter digestibility is a critical component of herbage nutritive value, a major service delivered by grasslands. The aim of this study was to test whether the dominance hypothesis applies to assess the impacts of environmental gradients and management regimes on this component of herbage nutritive value in permanent grasslands. At the plant level, digestibility has been related to a number of functional traits, but whether this can be scaled up to the community level in species‐rich grasslands and how such relationships are modulated by environmental conditions and management regimes remain unknown. Our primary objective was to test whether community‐weighted means – species trait values weighted by the species abundance – of morphological, phenological and chemical traits could be used to explain variations in digestibility over a large range of climatic contexts, soil resource levels and management regimes. Our second objective was to explain variations in community digestibility within and among nine contrasting sites along large natural and man‐induced environmental gradients. Over the whole data set, digestibility and most community‐weighted means of traits responded to climatic factors and management regimes, but relations were not always significant when each site was considered separately. Community digestibility was significantly related to one or more plant traits within each site and to all of the measured traits when considering all the sites. Leaf dry matter content (LDMC) had the most consistent effects on digestibility, with a strikingly similar negative effect within each site. Potential evapotranspiration was negatively related to digestibility and contributed to explain a large part of the among‐site variance. In addition, a low return interval of disturbance and a high disturbance intensity (biomass removal) were both associated with a high digestibility. Synthesis and applications. Disturbance regime, plant traits and local climate impacted dry matter digestibility roughly equally in grasslands. The effects of community composition on digestibility and its response to abiotic factors could be successfully captured by community‐weighted means of leaf dry matter content. This functional marker can be used to develop indicators and grassland management rules to support farmers in the refinement of their practices towards specific needs, such as target production outputs.

The role of disturbance in promoting the spread of the invasive seaweed Caulerpa racemosa in seagrass meadows

Human disturbances, such as anchoring and dredging, can cause physical removal of seagrass rhizomes and shoots, leading to the fragmentation of meadows. The introduced green alga, Caulerpa racemosa, is widely spread in the North-West Mediterranean and, although it can establish in both degraded and pristine environments, its ability to become a dominant component of macroalgal assemblages seems greater in the former. The aim of this study was to estimate whether the spread of C. racemosa depends on the intensity of disturbance to the canopy structure of Posidonia oceanica. A field experiment was started in July 2010 when habitat complexity of a P. oceanica meadow was manipulated to simulate mechanical disturbances of different intensity: rhizome damage (High disturbance intensity = H), leaf removal (Low disturbance intensity = L), and undisturbed (Control = C). Disturbance was applied within plots of different size (40 × 40 cm and 80 × 80 cm), both inside and at the edge of the P. oceanica meadow, according to an orthogonal multifactorial design. In November 2011 (16 months after the start of the experiment), no C. racemosa was found inside the seagrass meadow, while, at the edge, the cover of the seaweed was dependent on disturbance intensity, being greater where the rhizomes had been damaged (H) than in leaf removal (L) or undisturbed (C) plots. The results of this study indicate that physical disturbance at the margin of seagrass meadows can promote the spread of C. racemosa.