Unproductive Sites Research Articles

Relict stands of Central European oaks: Unravelling autochthony and genetic structure based on a multi-population study

Central European white oaks expanded rapidly after the last glacial period and reached their current distribution range during the early Holocene. They have been an important resource of timber, fuelwood and animal feed for humans, who actively promoted their presence in forests and other landscape types at least since the early historical times. Besides stands with intensive management, putatively relict populations of three native oak species can be found on unproductive sites with restricted accessibility. Here, we apply chloroplast and nuclear microsatellite markers in order to address the autochthony of relict and managed stands and compare the spatial distribution of genetic variation between them. Based on data from more than 150 populations, we demonstrate that oak autochthony was preserved throughout historical times which is likely the result of traditional silvicultural treatment. This is supported by the fact that the spatial pattern of chloroplast haplotype distribution still reflects the post-glacial recolonization in both relict and old managed stands. We observed significant admixture of haplotypes only in stands established after the Second World War, which is attributable to the transfer of reproductive material used for afforestation. In terms of nuclear genetic variation, we observed marked differences among species. Quercus pubescens exhibited a pronounced genetic structure. Genetic drift and limited gene flow among its small and isolated populations in our study area might have contributed to this pattern. Varying extent of genetic introgression with other sympatric oak species could offer an additional explanation. On the contrary, the gene pools of Q. petraea and Q. robur are highly homogenous, displaying only weak isolation-by-distance. We found no significant differences of genetic diversity and differentiation between relict and managed stands. This suggests that seed transfer mostly occurred within our study area, even in those stands established in post-war times, verifying previous findings which point out limited human interference. We recommend consideration of population genetic structure for gene conservation, with a finer resolution of gene conservation units needed for Q. pubescens due to its spatial genetic structure. Both relict and old managed stands, species-pure or mixed, are suitable for conservation, as they host autochthonous gene pools. Coppice-with-standard management could contribute to preservation of autochthony. In the face of climate change, it is also important to maintain the evolutionary potential of the stands, by facilitating generative reproduction and allowing for hybridization in mixed stands.

Relocating Nest Boxes from Poor Quality Sites Can Bias Inference into Population Dynamics

ABSTRACT Nest boxes are often used to monitor animals, and it is common practice to relocate nest boxes from unproductive sites into presumably better habitat. This relocation of nest boxes means that a given nest box program progressively monitors better sites over time. Ecological theory holds that occupancy and reproduction should generally increase with habitat quality. Thus, relocating nest boxes from poor quality sites might positively bias trends in occupancy and reproduction. These biased trends might cause researchers to be overly optimistic about the status of their focal populations. To demonstrate this potential pitfall, I built a stochastic model to simulate a nest box program that relocates the least productive 25% of nest boxes every 5 yr over a 25-yr study. The model assumed occupancy and reproduction levels for the entire population were stable throughout the study, so changes in occupancy and reproduction observed in nest boxes could only be due to relocation. I implemented this model under three settings: (1) Stable, where the same sites are monitored over the entire study; (2) Random, where the unproductive nest boxes are relocated to random sites; and (3) Learning, where the unproductive nest boxes are relocated to sites of better quality. For each of the 1000 simulations per setting, I performed logistic and Poisson regressions to determine whether there were statistically identifiable (P < 0.05) temporal trends in occupancy and number of young fledged from nest boxes. As expected, occupancy and number of offspring fledged from nest boxes were stable during the Stable simulations, and increased over the 25 yr during the Random and Learning simulations. Trends in occupancy were rarely identifiable during Stable simulations, were identifiable in 46% of simulations under Random settings, and in 97% of simulations under Learning settings. Trends in number of young fledged were identifiable in 18% of simulations under Stable settings, 91% of simulations under Random settings, and 100% of simulations under Learning settings. Such statistically significant trends, induced solely by relocating poorly performing nest boxes, represent a potential pitfall when interpreting vital rates measured using nest boxes. Potential solutions might include calculating occupancy using a subset of boxes that are never considered for relocation, or statistical models that account for preferential sampling.

The effects of livestock grazing on vegetation in a semiarid grassland: a test of three hypotheses

AbstractQuestionsLivestock grazing is an important driver of plant diversity in grasslands. Different mechanisms summarized in three main hypotheses have been proposed to explain its varying effects depending on productivity. The insufficient compensation hypothesis (ICH) states that livestock causes the extinction of species that cannot compensate for biomass removal in unproductive sites, reducing species richness. The competitive release hypothesis (CRH) states that livestock increases plant diversity by attenuating competition, especially in productive environments. In the convergent evolution hypothesis (CEH), livestock has minor effects on diversity in unproductive sites because plants that endure stress are pre‐adapted to grazing. These hypotheses have rarely been compared formally, and have mostly been tested on large spatial scales. Here we simultaneously test these hypotheses on a local scale.LocationNatural semi‐arid grassland, Oaxaca, Mexico.MethodsWe performed a long‐term exclosure experiment, recording plant diversity annually over 17 years. In 2017 we measured species richness over a grazing intensity gradient. We used linear mixed‐effects models, nestedness and beta‐diversity analyses to evaluate diversity and composition in relation to grazing and productivity.ResultsPlant diversity generally increased with grazing intensity. In line with CRH, species richness increased as grazing became more intensive due to a progressive incorporation of increasingly poor competitors to the community, with a seemingly stronger effect under high productivity. This suggests that herbivores limit competitive species and prevent competitive exclusion. ICH may play a secondary role because the effect was reversed in low productivity inside exclosures.ConclusionsOur finding implies that in productive sites, the exclusion of livestock can be detrimental to plant diversity as a lack of grazing accelerates competitive exclusion. By contrast, in limiting environments, grazing can preserve diversity if livestock are kept at low stocking densities. Our results show that small‐scale variations in productivity need to be considered when designing livestock management.

Wind project performance with age: Policy, technology, markets, and the maturing wind power industry

U.S. wind generation capacity has grown by a factor of nearly 50 times in the past 20 years; now nearly a third of wind capacity is 10 years old or older. This ageing of this fleet affects the productivity of existing investments and raises the stakes for replacing these assets through repowering. Focusing on the Texas wind power fleet to examine declining productivity over time, and how those changes factor into replacement and retirement decisions, we corroborate earlier decline findings but observe a slower rate of repowering than in other settings. We then explore repowering decisions and find evidence that technological change has favored relatively unproductive sites, in contrast to the previous literature on this topic. We explore the policy implications of our findings, in particular the desirability and feasibility of using policy tools to promote more repowering.

Competition components along productivity gradients – revisiting a classic dispute in ecology

Competition is ubiquitous in plant communities with various effects on plant fitness and community structure. A long‐standing debate about different approaches to explain competition is the controversy between David Tilman and Philip Grime. Grime stated that the importance of competition relative to the impact of the environment increases along a productivity gradient, while Tilman argued that the intensity of competition is independent of productivity. To revisit this controversy, we assumed that the effects of plant–plant interactions are additive and applied the new competition indices by Díaz‐Sierra et al. (2017) in a field experiment along a productivity gradient in S‐Germany, using the rare arable plant Arnoseris minima as a study species. The ‘target technique' was applied, to separate the effects of root and shoot competition. The study plants were exposed to five competition treatments with three replicates in 18 sites, respectively. We investigated the expectation that root competition is more intense in unproductive sites than shoot competition. Additionally, we predicted survival to be less affected by competition than growth‐related plant parameters. Using the biomass of individuals without competition as a proxy for site productivity there was a positive relationship with competition importance but no relationship with competition intensity when plants experienced full competition. Survival of the target plants was unaffected by competition. Root competition was the main mechanism determining the performance of the target plants, whereas the effect of shoot competition was relatively low albeit increasing with productivity. We conclude that when considering plant–plant interactions additive both Grime's and Tilman's theories can be supported.

Human disease-associated single nucleotide polymorphism changes the orientation of DROSHA on pri-mir-146a.

The Microprocessor complex of DROSHA and DGCR8 initiates the biosynthesis of microRNAs (miRNAs) by processing primary miRNAs (pri-miRNAs). The Microprocessor can be oriented on pri-miRNAs in opposite directions to generate productive and unproductive cleavages at their basal and apical junctions, respectively. However, only the productive cleavage gives rise to miRNAs. A single nucleotide polymorphism (SNP, rs2910164) in pri-mir-146a is associated with various human diseases. Although this SNP was found to reduce the expression of miRNA, it is still not known if it affects the activity of the Microprocessor directly, and how it functions. In this study, we revealed that the SNP creates an unexpected mGHG motif at the apical junction of pri-mir-146a. This mGHG motif interacts with the double-stranded RNA-binding domain (dsRBD) of DROSHA, switching its orientation on pri-mir-146a from the basal to the apical junction. As a result, the SNP facilitates Microprocessor to cleave SNP-pri-mir-146a at its unproductive sites. Our findings help to elucidate the molecular mechanism that explains how the disease-associated SNP modulates the biogenesis of pri-mir-146a and thereby affects its cellular functions.

Assessment of radiological safety of abandoned tantalite mining sites in Oke-Ogun, Oyo State, Nigeria

Introduction: Tantalite and other solid mineral deposits in Oke-Ogun occupy a very large landscape, and the possibilities of further exploration in new landscape and abandoning unproductive sites cannot be ruled out. These landscapes when abandoned become hideout for criminals and ritualistic activities which pose a great threat to the wellness of the people in the area and Nigerians at large. Objectives: This study attempts to investigate the radiological safety of these abandoned mines so as to convert them to economical valuable sites that can serve as a means of employment for population of the unemployed youth in the area and Nigeria at large. Materials and Methods: A total of seventeen soil samples were randomly selected. These samples were processed and analyzed for activity concentrations of 226Ra, 232Th, and 40K using Hyper Pure Germanium Gamma Spectrometer. RESidual RADioactivity software version (6.5), a computer program developed by the Environmental Assessment Division of Argonne National Laboratory, was used for dose prediction for 70 years using the activity concentrations of soil samples from all the selected sites as its input parameter for the software. Predicted doses were estimated with respect to the three exposure pathways, namely external gamma irradiation from radionuclides in the soil samples, inhalation of dust particles, and inadvertent ingestion of soil. Results: The measured activity concentrations of radionuclides: 226Ra for all the sites is in the range of (17–177) Bq/kg, 232Th in the range of (2–107) Bq/kg, and 40K in the range of (650–1667) Bq/kg. The predicted dose for all the sites is within the range of 0.05–0.35 mSv/y with Eluku mining site having values greater than the permissible limit of 0.25 mSv/y. The health implication of these values is that three of the sites (Gbedu, Sepenteri, and Komu) are safe for other usages. Conclusion: Any of these three mining sites may be used for re-creational purposes, fishing and building of estates to mention but a few.

Violent Erasures: Atrocity, Photographic Archives and the Algerian War of Independence (1954–62)

This paper examines the role of photographic archives from the Algerian War of Independence (1954-62) in the formation of the conflict’s legacy in France. It argues that while acts of extreme violence enacted by the French military upon Algerian insurgents were under-represented, these blind spots should not be discarded as unproductive sites for writing histories of visual culture. Instead, they can be seen as indicative of the restrictions placed on photographic production during the war, as well as on its afterlives in national archives and collections. Following the transportation of archives from Algeria to France after the Evian accords in 1962, which ended the 132-years-long colonial rule, access to these documents became restricted. While the majority of archives were made public 50 years after the conflict had ended, some of the most sensitive documents still remain outside of the public’s view due to longer delays. These occlusions have structured the conflict’s (in)visibility in the French public sphere following the end of the war. By taking the 2012 issue of Maniere de voir-Le Monde Diplomatique, which revisits the conflict through historical imagery and contemporary photography by Bruno Boudjelal, this paper analyses the cultural, social and political processes which control vision itself.

Abiotic stress tolerance of kikuyu (Cenchrus clandestinus) and some related grasses and potential of kikuyu for agricultural and urban environments

The introduction of kikuyu (Cenchrus clandestinus (Hochst. ex Chiov.) Morrone) into Australia in 1918 has seen it become established and adapted to several geographic regions in a wide range of ecologies and environmental situations. After it naturalised to local conditions, researchers and farmers recognised the value of kikuyu in marginal and previously unproductive sites, where forage quality and quantity made this species popular with dairy farmers and pastoralists. Its versatility and prostrate, mat-forming characteristics also led to the adoption of kikuyu by local governments, homeowners and sporting organisations in urban environments as turf. Kikuyu has the ability to alleviate soil contamination and remediate soils, thus enhancing the use of previously unproductive land. However, the aggressive growth habit of the species, considered a problem in certain regions of the world, has led to a noxious weed classification in some states of the USA. This review includes information on expected changes to world agricultural and urban environments and the potential expanded role of kikuyu. The origin of kikuyu grass, genetic variability, tolerances to soil salinity and drought, and potential for genetic improvement are also discussed.

The Distribution of Cool Spots as Microrefugia in a Mountainous Area.

It has recently been proposed that microrefugia played an important role in species survival during past climate change events. However, the current distributions of microrefugia remain largely unknown. Wind-hole sites are areas affected by preferential flows of cool air generated in interstitial spaces created by rock fragments or colluvia. Alpine plant species occurring in lowland wind-hole sites isolated from alpine zones may be relicts of the last glacial period. Hokkaido, northern Japan, is known to contain many wind-hole sites in which alpine plant species can occur. Here we surveyed 55 wind-hole sites in the Kitami region, eastern Hokkaido, and observed two alpine plant species (lingonberry, Vaccinium vitis-idaea, and Labrador tea, Rhododendron groenlandicum ssp. diversipilosum var. diversipilosum) in 14 wind-hole sites. Statistical modeling showed that wind-hole sites are likely to occur in areas with high maximum slope angles and volcanic rock cover, and concave surfaces. Our predictions of wind-hole site distributions suggest that such topographic conditions are common in our study area, and that many undiscovered wind-hole sites exist. Ignoring microhabitats may greatly underestimate species distributions in topographically complex regions, and dispersed cool spots may also function as stepping stones and temporal habitats for cold-adapted species. Because these localized unique habitats usually occur in economically unproductive sites, identifying and protecting potential microrefugia (cool spots) would be a robust and cost-effective mitigation of climate change impacts.

Ecosystems supporting Tuber magnatum Pico production in Serbia experience specific soil environment seasonality that may facilitate truffle lifecycle completion

The production of Tuber magnatum Pico, the most prized and ecologically the most complex, constantly declines in natural habitats, while the success in plantation lacks. Contrary to the data from its habitats in Mediterranean sites, in Serbia this truffle colonizes typical developed alluvial forests, implying that dynamics of soil parameters and nutrient availability, rather than presence of specific ectomycorrhizal (EMC) plant host species or climate, might be crucial for supporting truffle life cycle. In order to reveal specific relationship generating soil microenvironments in productive and unproductive sites, soil water content (SWC) and temperature (T) were measured periodically in two depths, in a typical T. magnatum habitat in Western Serbia and compared to the atmospheric parameters (precipitation, air temperature). In three key time-points (spring soil water saturation, summer drought and autumn ascocarp production) soil was sampled through the profile and analyzed for nutrients that might be important for plant/fungal growth and ectomycorrhiza establishment. Results revealed that specific dynamics of soil water, aeration, available P and N, and possibly vegetation phenology, might be determining productivity of the microsites within the productive area. The hypothesis on soil water and nutrient availability seasonal dynamics, which might be controlling truffle establishment and life cycle completion in investigated area, was proposed.

Creating novel urban grasslands by reintroducing native species in wasteland vegetation

In many areas of the world, grassland species have declined due to intensified agriculture and abandonment of unproductive sites. In rural areas, restoration approaches seek to counteract this loss. Additional potential for grassland restoration arises in urban settings, especially on large-scale wastelands isolated from suitable species pools.We explored the potential of urban wastelands for the development of low-maintenance meadows. We expanded on restoration approaches of rural landscapes by working with altered urban soils and existing wasteland vegetation. We tested if grassland species of regional provenances can be successfully reintroduced by comparing different restoration treatments: (1) transfer of a threshed seed mixture (Heudrusch) from local hay meadows, (2) seeding of species of regional provenances, (3) the same seeding combined with mycorrhizal inoculation.We revealed higher total species richness in all treated plots compared to the control, with similar proportions of grassland species. Both seeding treatments showed increased proportions of target species. The proportion of target species was stable in the seeding/mycorrhiza treatment and increased in the seeding only treatment from the second to the third year of the study to the same level. Urban soil features including stone content were negatively related to target species richness. In contrast, human-mediated impacts (e.g., dog intrusion) showed no significant effect on target species richness.Our experiment demonstrates that urban wastelands are suitable habitats for grassland species. Including extant soils and resident vegetation results in novel urban meadows with a considerable share of ruderal species but where rare grassland species are able to persist.

Water and nitrogen use efficiency of common ragweed (Ambrosia artemisiifolia L.) at different nitrogen and water levels

Common ragweed (Ambrosia artemisiifolia L.) spread across Europe and other regions is becoming a serious health and economic threat. A pot experiment was conducted in 2011 to determine effect of various nitrogen (N) (10, 100 kg/ha) and water supply regime on resource use efficiency of ragweed. Ragweed plants increased their dry matter production with increased water and N availability. Nitrogen use efficiency (NUE) was decreased with N addition and was not influenced by water availability. Mean nitrogen residence time (MRT) was longer at low N and water levels. In contrast, nitrogen productivity (NP), NUE and water use efficiency (WUE) were all increased with enhanced water supply. A trade-off between parameters of NUE was attributed to differential response of NP and MRT to soil fertility and water supply. Our results confirmed that ragweed displayed high adaptation to unproductive sites. However, ragweed’s greater plasticity in response to water availability compared to N availability suggest, that water supply plays important role in its invasion success and in combination with disturbance ragweed might further spread into more productive environments.

Water and nitrogen use efficiency of common ragweed (Ambrosia artemisiifolia L.) at different nitrogen and water levels

Common ragweed (Ambrosia artemisiifolia L.) spread across Europe and other regions is becoming a serious health and economic threat. A pot experiment was conducted in 2011 to determine effect of various nitrogen (N) (10, 100 kg/ha) and water supply regime on resource use efficiency of ragweed. Ragweed plants increased their dry matter production with increased water and N availability. Nitrogen use efficiency (NUE) was decreased with N addition and was not influenced by water availability. Mean nitrogen residence time (MRT) was longer at low N and water levels. In contrast, nitrogen productivity (NP), NUE and water use efficiency (WUE) were all increased with enhanced water supply. A trade-off between parameters of NUE was attributed to differential response of NP and MRT to soil fertility and water supply. Our results confirmed that ragweed displayed high adaptation to unproductive sites. However, ragweed’s greater plasticity in response to water availability compared to N availability suggest, that water supply plays important role in its invasion success and in combination with disturbance ragweed might further spread into more productive environments.

Impact of simulated moose densities on abundance and richness of vegetation, herbivorous and predatory arthropods along a productivity gradient

Large herbivores can affect vegetation structure and species composition as well as material and energy flows in the ecosystem through their selective feeding, defecation, urination and trampling. These changes have a large potential to indirectly affect other trophic levels, but the mechanisms are poorly known. We studied the impacts of moose Alces alces browsing along a gradient of site productivity by experimentally simulating four different moose densities. Here we show that moose can affect the richness and abundance of three trophic levels in Swedish boreal forests through complex direct and indirect impacts, but in qualitatively different ways depending on how the physical habitat or food resources of a trophic level are affected. Vegetation richness had a hump‐shaped (unimodal) response to increased moose density. Leaf litter production decreased when browsing increased, which in turn depressed the abundance of flying prey for spiders. Consequently, spider abundance and richness declined monotonically. The responses of spider richness to moose density were further conditioned by site productivity: the response was positive at productive and negative at unproductive sites. In contrast, herbivorous Hemiptera were not affected by moose, most likely because the abundance of their food plants was not affected. The highest simulated moose density had an impact on all variables responding to moose even after a few years of treatment and can be considered as overabundance. We also show that the impacts of low or moderate moose density can be positive to some of the organisms negatively affected by high density. The level of herbivore population density that leads to substantial community impacts also depends on site factors, such as productivity.

Site-dependent population dynamics: the influence of spatial habitat heterogeneity on individual fitness in the sedge warbler Acrocephalus schoenobaenus

During nine years of study, we detected a mechanism corresponding to site dependence operating in a population of the sedge warbler Acrocephalus schoenobaenus inhabiting a natural wetland of high within-habitat heterogeneity. Sites with a larger share of cover by tall wetland vegetation were preempted during spring settlement; they were occupied in more breeding seasons and by more experienced males. The fitness of males occupying these sites was higher in terms of local recruit production. The total area occupied by the population expanded or contracted depending on its population size. This mechanism increased or decreased the mean site quality, influencing local recruitment of young, although mainly by changing the share of unproductive sites in the total pool of occupied sites. The results indicate that population demographic rates may depend on the spatial heterogeneity of resources at the level of individuals.

Draining an unproductive black spruce peatland stand: 18-year post-treatment tree growth and stand productivity estimation

In the eastern Canadian boreal forest, drainage of unproductive black spruce peatlands is an infrequently used silvicultural treatment. The sparse scientific literature concerning its influence on tree growth is, however, relatively positive about the possibility of using drainage to convert unproductive stands into productive ones. This study aimed to address this issue by assessing the individual tree growth and by estimating the future stand productivity on a drained black spruce peatland stand. Results showed larger mean annual post-drainage stump diameter increment with increasing ditch proximity but they also showed the lack of effect of the treatment at distances greater than 15 m from the ditch. Small-sized trees reacted more strongly to the treatment than larger ones. Estimated site productivity was also influenced by the ditch proximity. The presence of excavating mounds on only one side of the ditch greatly influenced tree growth and site productivity. Considering the obtained results, intensive drainage operations, utilizing narrow ditch spacing, would be necessary in order to transform the type of unproductive sites studied into productive ones. Key words: diameter growth, site index, forested peatland, forest drainage, Picea mariana

Sexual reproduction, clonal diversity and genetic differentiation in patchily distributed populations of the temperate forest herb Paris quadrifolia (Trilliaceae)

Clonal plant species have been shown to adopt different strategies to persist in heterogeneous environments by changing relative investments in sexual reproduction and clonal propagation. As a result, clonal diversity and genetic variation may be different along environmental gradients. We examined the regional and local population structure of the clonal rhizomatous forest herb Paris quadrifolia in a complex of forest fragments in Voeren (Belgium). Relationships between population size (the number of shoots), shoot density (the number of shoots per m2) and local growth conditions were investigated for 47 populations. Clonal diversity and genetic variation within and among 19 populations were investigated using amplified fragment length polymorphism markers. To assess the importance of sexual reproduction, seed set, seed weight and germination success were determined in 18 populations. As predicted, local growth conditions largely affected population distribution, size and density of P. quadrifolia. Populations occurring in moist and relatively productive sites contained significantly more shoots. Here, shoots were also much more sparsely distributed compared to populations occurring in dry and relatively unproductive sites, where shoots showed a strongly aggregated distribution pattern. Clonal diversity was relatively high, compared with other clonal species (G/N ratio=0.43 and Simpson's D=0.81). Clonal diversity significantly (P<0.01) decreased with increasing shoot density while molecular genetic variation was significantly (P<0.01) affected by population size and local environmental conditions. Lack of recruitment and out-competition of less-adapted genotypes may explain the decreased genetic variation in dry sites. Analysis of molecular variance revealed significant genetic variation among populations (PhiST=0.42, P<0.001), whereas pairwise genetic distances were not correlated to geographic distances, suggesting that gene flow among populations is limited. Finally, the number of generative shoots, the number of seeds per fruit and seed weight were significantly and positively related to population size and local growth conditions. We conclude that under stressful conditions populations of clonal forest plant species can slowly evolve into remnant populations characterized by low levels of genetic variation and limited sexual reproduction. Conservation of suitable habitat conditions is therefore a prerequisite for effective long-term conservation of clonal forest plant species.

Herbivory and competition slow down invasion of a tall grass along a productivity gradient.

Competition models including competition for light predict that small plant species preferred by herbivores will be outshaded by taller unpreferred plant species with increasing productivity. When the tall plant species is little grazed by the herbivores, it can easily invade and dominate short vegetation. The tall-growing grass Elymus athericus dominates the highly productive stages of a salt-marsh succession in Schiermonnikoog and is not preferred by the herbivores which occur there, hares and geese. We studied how interspecific competition and herbivory affected performance during early establishment of this species with increasing productivity. Seedlings were planted in the field in a full factorial design, manipulating both interspecific competition and herbivory. The experiment was replicated along a natural productivity gradient. Competition reduced above-ground biomass production and decreased the number of ramets that were produced but did not affect survival of seedlings. The negative effects of competition on seedling performance increased with increasing productivity. In contrast to our expectations, herbivory strongly reduced seedling survival, especially at the unproductive sites and had only small effects on seedling growth. The present study shows that unpreferred tall-growing species cannot easily invade vegetation composed of short preferred species. Grazing by (intermediate-sized) herbivores can prevent establishment at unproductive sites, and increased competition can prevent a rapid invasion of highly productive sites. Herbivores can have a long-lasting impact on vegetation succession by preventing the establishment of tall-growing species, such as E. athericus, in a window of opportunity at young unproductive successional stages.

Propagule pools mediate community assembly and diversity‐ecosystem regulation along a grassland productivity gradient

SummaryThe relative importance of regional species pools and local ecological processes in governing landscape variation in plant species diversity and productivity was evaluated in a Kansas grassland.We examined the impact of multispecies sowing treatments and experimental canopy disturbances on plant species diversity and ecosystem processes along a complex natural gradient of plant standing crop.Data collected 4 years after sowing showed that plant invasion and diversity were seed limited in unproductive sites, but microsite limited in productive sites. Effects of sowing on plant diversity along the natural landscape gradient were paralleled by significant effects of sowing on measures of local plant production and community resilience to disturbance.These results support the shifting limitations hypothesis (SLH) that landscape gradients in local plant diversity should reflect shifts in the major regulating factor, from species pools to local ecological processes, as one moves from sites of inherently low to inherently high productivity.Our findings also indicate that diversity at the level of the available propagule pool acts to constrain ecosystem productivity and stability by mediating local community assembly, by determining the availability of key species, and by governing opportunities for functional compensation within the community.In total, our results support an emerging view that community processes and ecosystem functions are dynamically linked and act reciprocally to constrain each other.