Exotic Grassland Research Articles

Spatially explicit capture-recapture estimate of hedgehog population density in exotic grassland, New Zealand

Spatially explicit capture-recapture estimate of hedgehog population density in exotic grassland, New Zealand

Extreme wildfire supersedes long-term fuel treatment influences on fuel and vegetation in chaparral ecosystems of northern California, USA

BackgroundWithin California’s chaparral ecosystems, fuel reduction treatments are commonly used to reduce the negative impacts of wildfire but the durability of fuel treatment changes to fuels and vegetation when exposed to wildfire is less well understood. This study examined the interactive effects of 15-year-old fuel treatments and an extreme wildfire on burn severity, fuel loading, and vegetation in chaparral and oak vegetation types in Whiskeytown National Recreation Area in northern California, USA. Fuel treatment types included hand thinned, mechanical mastication, mechanical mastication + prescribed burning, and prescribed burning only.ResultsVegetation and substrate burn severity was characterized as moderate across the study site and did not differ among treatments. Contrasting with higher pre-fire shrub density in the mastication + burning treatment, 2-year post-fire live shrub density did not differ among treatments. Higher pre-fire fine woody fuel loading in the mastication treatment did not correspond to post-fire fuel loading among treatments, while the hand thinned treatment was the only treatment where fine fuel loading was not significantly reduced post-fire. Total plant species richness increased in all treatment types following wildfire, largely driven by an increase in exotic species. Native cover decreased, and exotic cover increased in oak and chaparral types, but greater exotic species cover in the mastication + burning treatment in chaparral was maintained following wildfire.ConclusionsPre-fire differences in fuel and vegetation responses among treatments largely did not persist or were not detectible 1 to 2 years following wildfire. These findings suggest that the extreme wildfire conditions superseded long-term treatment differences in many fuel and vegetation metrics observed prior to wildfire. Despite subtle treatment differences, the hand thinned treatment resulted in the lowest change in fuel loading relative to all other treatments. Lastly, pre-fire differences in exotic species among fuel treatments were retained following wildfire, suggesting some treatments may have greater potential for exotic species expansion or type conversion to exotic grasslands.

Effects of competition and habitat heterogeneity on native‐exotic plant richness relationships across spatial scales

AbstractAimThe biotic resistance hypothesis posits that greater native species richness limits invasions of exotic species. However, negative native‐exotic richness relationships (NERRs) may reverse with increasing spatial scale, seemingly refuting the hypothesis. Here, we explore the effects of species competitive interactions, environmental factors, habitat heterogeneity and vertical vegetation tiers on the NERRs across spatial scales in native forests.LocationNew Zealand.MethodsWe combined vegetation, land cover and climate data to predict exotic richness from native richness at different vertical tiers (ground to canopy), land cover, plant competition (tree basal area, native ground cover), mean annual temperature and total rainfall. We considered four spatial scales, from single 20 × 20 m plots on an 8‐km grid to groups of plots across grids up to 128 km. Habitat heterogeneity was measured using the variance of climatic conditions among plots within a group.ResultsA negative native tree‐exotic richness relationship (NTERR) was observed at plot level but reversed with increasing spatial scale. Species competitive interactions showed a negative relationship with exotic richness at small/intermediate scales (≤32 km). Rainfall and temperature heterogeneity contributed to the positive NTERR at the largest scale. Adjacent exotic grassland cover had a positive relationship with exotic richness across all scales but did not prevent the NTERR from reversing.Main conclusionsOur analysis shows the importance of considering vegetation structure and adjacent land covers, along with spatial heterogeneity and climatic factors, when testing the biotic resistance hypothesis in forest ecosystems. There is a clear indication that biotic resistance is primarily driven by native trees in the overstorey, probably by limiting resources available to ground tier plants. The results support the notion that the NERR is driven by competitive interspecific interactions at small spatial scales and by habitat heterogeneity at larger scales.

Not only exotic grasslands: The scattered trees in cultivated pastures of the Brazilian Cerrado

Cultivated pastures occupy 30% of the Brazilian Cerrado. Most pastures have low productivity, demanding intensification. Pasture intensification, as conventionally proposed, promises conservation by land sparing. However, it does not embrace silvopastoral systems, a type of intensification that enhances carbon balance and in situ biodiversity conservation. In the Cerrado, silvopastoral systems would be easily established because pastures with native trees are commonly found in the landscape. In this study, we estimated the density of scattered trees using RapidEye satellite data in 200 cultivated pastures (48,519 ha). We also identified tree species by establishing 93 field plots distributed along 15 ecoregions in the Cerrado biome. We expected that the density of trees would vary in function of farm size, age of the pasture, intensification of pasture management and biophysical variables. Mean scattered tree density was 3.52 tree ha−1 (± 2.93 SD), and it was higher in ecoregions with lower and more concentrated rainfall. The most frequent tree species were those with high commercial values, such as the Myracrodruon urundeuva timber tree (aroeira) and the Caryocar brasiliense fruit tree (pequi). The average above-ground carbon stock of native Cerrado trees in pastures was estimated in 1.2 Mg ha−1 (± 1.2 SD). Our study shows that there are productive pastures with high tree density and species richness, which could be used as models for sustainable intensification strategies in different ecoregions of the Cerrado.

Repeat burning affects species composition in degraded Cumberland Plain Woodland

Cleared areas in Scheyville National Park, New South Wales, that formerly supported Cumberland Plain Woodland (CPW) now have a long unburnt mixed native and exotic grassland. To test whether re-introducing fire to these sites would benefit native species composition, and drive it towards that of neighbouring remnant woodland, an experiment was established in three grassy areas (grassy blocks) with prior agricultural land use, and one block in a remnant woodland. Replicate plots were established within blocks, and assigned to a repeat fire treatment, or unburnt controls. Plots in the fire treatment were burnt on all blocks in 2001, and in the grassy blocks only in 2005. Although the two fires did not result in recruitment of new native species on the grassy blocks, the successive fires had other beneficial effects. More native species were apparent in the above-ground vegetation on burnt than unburnt plots after both fires, and more summer-growing native grasses were apparent above ground and increased in abundance after the second fire. The large flush of exotic forbs that appeared after the first fire was not repeated after the second fire; many of these forbs had an obligate seeder fire response, and the short inter-fire interval may not have allowed replenishment of their seed bank. Burning did not result in recruitment of new native species in the grassy areas, and did not make the grassy areas more similar to the woodland in species composition. In the woodland, the first fire did not significantly affect species composition. Re-instating fire to degraded grassy sites will not by itself return the site to good quality CPW in the short term, but may contribute positively in the longer term through beneficial effects on natives and adverse effects on exotics.

Natural enemies do not contribute to negative frequency-dependence in native and exotic grassland plants

Species’ responses to antagonists, such as fungal pathogens and insect herbivores, are part of a species’ niche and are thought to contribute to species coexistence. In theory, coexistence is supposed to be maintained by the effects of equalizing mechanisms that reduce fitness differences among species, and stabilizing mechanisms that are the result of niche differences. The presence of stabilizing niche differences is hallmarked by negative frequency-dependent per capita rates of increase. However, rigorous experimental testing of how insect herbivores and fungal pathogens contribute to negative frequency dependence among co-occurring plant species, in particular in the context of biological invasions, is scarce. We present an experimental study in a grassland in central Germany, in which we established native and exotic-dominated communities on 1 m²-plots, and reduced above- and belowground fungal pathogens and insect herbivores by applying pesticides. We manipulated the frequencies of six native and six exotic focal species, in order to assemble one dominant and five rare species per community. We quantified productivity and diversity for each community, and recorded data on per capita rates of increase and other fitness components for two native and two exotic focal species over three years. We found that native and exotic-dominated communities did not differ in their productivity and diversity. Reduction of fungi and insects increased productivity but the effect was similar for both species origins, suggesting that effects of enemy release are transitory or not existent. Negative frequency-dependence was common in our species, but could not be linked to interactions with antagonists. Although focal species responded to the reduction of antagonists, these effects were only present at low frequency, suggesting a greater importance of intraspecific competition, e.g. for resources, in explaining the observed stabilizing niche differences. Offsetting responses of the two exotic focal species to both antagonist groups highlight the significance of equalizing mechanisms among native and exotic species at the community level. Our study demonstrates the importance of both equalizing and stabilizing mechanisms in the context of biological invasions and differential contributions of antagonistic interactions to these mechanisms.

Soil depth and grassland origin cooperatively shape microbial community co‐occurrence and function

AbstractMany soils are deep, yet soil below 20 cm remains largely unexplored. Exotic plants can have shallower roots than native species, so their impact on microorganisms is anticipated to change with depth. Using environmental DNA and extracellular enzymatic activities, we studied fungal and bacterial community composition, diversity, function, and co‐occurrence networks between native and exotic grasslands at soil depths up to 1 m. We hypothesized (1) the composition and network structure of both fungal and bacterial communities will change with increasing depth, and diversity and enzymatic function will decrease; (2) microbial enzymatic function and network connectedness will be lower in exotic grasslands; and (3) irrigation will alter microbial networks, increasing the overall connectedness. Microbial diversity decreased with depth, and community composition was distinctly different between shallow and deeper soil depths with higher numbers of unknown taxa in lower soil depths. Fungal communities differed between native and exotic plant communities. Microbial community networks were strongly shaped by biotic and abiotic factors concurrently and were the only microbial measurement affected by irrigation. In general, fungal communities were more connected in native plant communities than exotic, especially below 10 cm. Fungal networks were also more connected at lower soil depths albeit with fewer nodes. Bacterial communities demonstrated higher complexity, and greater connectedness and nodes, at lower soil depths for native plant communities. Exotic plant communities’ bacterial network connectedness altered at lower soil depths dependent on irrigation treatments. Microbial extracellular enzyme activity for carbon cycling enzymes significantly declined with soil depth, but enzymes associated with nitrogen and phosphorus cycling continued to have similar activities up to 1 m deep. Our results indicate that native and exotic grasslands have significantly different fungal communities in depths up to 1 m and that both fungal and bacterial networks are strongly shaped jointly by plant communities and abiotic factors. Soil depth is independently a major determinant of both fungal and bacterial community structures, functions, and co‐occurrence networks and demonstrates further the importance of including soil itself when investigating plant–microbe interactions.

An Emergy-Based Approach to Assess and Valuate Ecosystem Services of Tropical Wetland Pastures in Brazil

Wetland grasslands are important ecosystems for raising beef cattle, because they are highly productive and present forages with high quality. Most of these ecosystems are threatened by overgrazing or by being replaced by exotic pastures. Emergy synthesis approach was used to assess and value the services provided by native pastures wetland under three conservation status and also to compare them to exotic pastures on wetlands. The ecosystem service that was assessed included forage provision for calves production estimated from grazing capacity of cow with calf at the foot. Habitat maintenance to plant diversity and wild herbivores were also evaluated. The results showed that natural wetland pastures with better conservation status provided valuable ecosystem services and are highly renewable. Data envelopment analysis (DEA) showed that regardless of the conservation state of the natural pastures, wet native grasslands were more efficient than wet exotic grasslands. Replacing native pastures by exotic ones may reduce plant diversity and the renewability of the system. The proposed method has a holistic approach to pasture ecosystems and is able to help decision-makers to define sustainable management practices and to subsidise public policies when it comes to payments regarding ecosystem services.

Effects of land cover type on carabid beetles (Coleoptera: Carabidae) of the Canterbury foothills, New Zealand

Background: Land cover changes during the recent history of New Zealand have had a major impact on its largely endemic and iconic biodiversity. As in many other countries, large areas of native forest have been replaced by other land cover and are now in exotic pasture grassland or plantation forest. Ground beetles (Carabidae) are often used as ecological indicators, they provide ecosystem services such as pest control, and some species are endangered. However, few studies in New Zealand have assessed the habitat value for carabid beetles of natural forest, managed regenerating natural forest, pine plantation forest and pasture.
 Methods: We compared the carabid beetle assemblages of natural forest of Nothofagus solandri var solandri (also known as Fuscospora solandri or black beech), regenerating N. solandri forest managed for timber production, exotic pine plantation forest and exotic pasture, using pitfall traps. The study was conducted at Woodside Forest in the foothills of the Southern Alps, North Canterbury, New Zealand, close to an area where the critically endangered carabid Holcaspis brevicula was found.
 Results: A total of 1192 carabid individuals from 23 species were caught during the study. All but two species were native to New Zealand, with the exotic species present only in low numbers and one of these only in the pasture habitat. Carabid relative abundance and the number of species was highest in the pine plantation, where a total of 15 species were caught; however, rarefied species richness did not differ significantly between habitats. The sampled carabid beetle assemblages were similar across the three forested habitat types but differed significantly from the pasture assemblages based on unconstrained and canonical analyses of principal coordinates. Holcaspis brevicula was not detected in this area.
 Conclusions: Our results show that managed or exotic habitats may provide habitat to species-rich carabid assemblages although some native species occur only in natural, undisturbed vegetation. Nevertheless, it is important to acknowledge the potential contribution of these land uses and land cover types to the conservation of native biodiversity and to consider how these can be managed to maximise conservation opportunities.

Socio-ecological mechanisms for persistence of native Australian grasses under pressure from nitrogen runoff and invasive species

Nitrogen runoff in certain southeastern Australian grasslands promotes the invasion of exotic grassland species at the expense of native species. Mitigation programs can reduce runoff and thus support native species, but their success may require the awareness and support of local populations. This situation represents a coupled socio-ecological system, since nitrogen runoff caused by local populations can enable the invasion of exotic grassland species, which can in turn stimulate a social response to restore the native species. Our objective is to use a mathematical model to identify potential socio-ecological mechanisms for the persistence of native grassland species, and the parameter regimes for which these mechanisms operate. We couple a model of southeastern Australian grassland dynamics with a model of human social dynamics concerning runoff mitigation. Nitrogen runoff can enter the ecosystem either through local sources under control of a human population, or through global sources not under their control. Humans learn mitigating behaviour socially, and respond to the prevalence of native and exotic grassland species. We find that socio-ecological dynamics introduce broad parameter regimes that are not present in the ecological system in isolation from the human system. We identify two mechanisms for native grassland persistence: one is associated with significant reductions in runoff rates and/or cost of runoff mitigation programs, resulting in a stable state where the native grassland species exists or dominates, with or without the support of socio-ecological feedback. A second mechanism associated with higher rates of nitrogen input supports persistence of the native species through oscillations in species abundance and mitigation behaviour in the human population. However, this state is less favourable to the native species because the oscillations may become extreme in amplitude. Finally, we find that increasing the cost of mitigation programs not only reduces mitigating behaviour in the population but also (more surprisingly) increases the tendency for the system to destabilize into a regime of oscillations in native species biomass. We conclude that multiple socio-ecological mechanisms could potentially support native species in grassland ecosystems under stress from nitrogen runoff and invasive species. Further research can refine such models to inform policy in the face of nonlinear socio-ecological responses.

Plant species natural abundances are determined by their growth and modification of soil resources in monoculture

The abundance of a plant species in a diverse community may depend on two aspects of a plant’s resource niche: its ability to garner limiting resources for its own growth, and its ability to reduce resources available for other species. Given that these two aspects of niche can be quantified in monoculture, we tested whether plant growth or plant modification of soil resources in monoculture relate to plant abundance in naturally-assembled California grassland communities. We grew 18 native and exotic grassland plant species in replicated monocultures, measured plant biomass and soil resources in these monocultures, and then assessed how well the measured variables for each species in monoculture correlated with natural abundances of the species across the local landscape. Both aboveground and belowground plant biomass in monoculture were positively correlated with species abundances in naturally-assembled communities; aboveground monoculture biomass alone accounted for 63% of the natural variability in species abundances, whereas shallow and subsurface root biomass accounted for 28 and 38%, respectively. Nitrogen concentrations in shallow monoculture soils were also positively correlated with a species’ natural abundance in the field, accounting for 43% of the natural variability in species abundances. Our results suggest that the performance of species when grown alone—e.g., biomass production and impacts on soil resources—can inform their performance in diverse communities and across heterogeneous landscapes.

Causes and feedbacks to widespread grass invasion into chaparral shrub dominated landscapes

This study provides a unified, holistic framework for predicting the dynamics of shrub-grass conversion throughout Mediterranean-climate shrublands. This work focuses specifically on the California chaparral, which until recently has been considered resistant to invasion by exotic grasses, but in recent years appears to have undergone substantial type conversion. To synthesize current understanding of the feedbacks and anthropogenic impacts that both enhance and reduce the susceptibility of southern California chaparral to invasion and its corresponding ability to recolonize invaded areas. We review the existing literature pertaining to the factors that enhance or reduce the susceptibility of chaparral to invasion, and organize these factors and their interactions into a single unified framework of environmental drivers, ecological interactions, and historical legacies associated with the distribution and rate of such invasion. A myriad of processes interact to mediate the invasion of exotic grasses into intact chaparral. In addition, we demonstrate that feedbacks exist within both chaparral shrublands and exotic grasslands that modify the landscape in ways that can enhance their own survival, act as barriers to conversion into alternate cover types, and in some cases weaken the resistance of adjacent vegetation to invasion. We posit a methodological framework from which the many climatic, anthropogenic, edaphic, and biotic feedbacks that determine the mosaic of invasion can be modeled. This study demonstrates that substantial conversion of chaparral into deciduous grasslands has recently occurred in southern California and presents a unified framework for forecasting the dynamics of shrub-grass conversion throughout Mediterranean-climate shrublands.

Long-Term Impacts of Exotic Grazer Removal on Native Shrub Recovery, Santa Cruz Island, California

A combination of overgrazing and exotic species introduction has led to the degradation of habitats worldwide. It is often unclear whether removal of exotic ungulates will lead to the natural reestablishment of native plant communities without further management inputs. I describe here my return to sites on Santa Cruz Island, California, 12 years after initial sampling in order to gain a long-term view on native shrub reestablishment into exotic grasslands after exotic grazer removal. Santa Cruz Island was grazed by feral sheep and cattle for over a century; these exotic grazers were removed in the late 1980s and feral pigs were removed in 2005–2006. I resampled 5 sites on south-facing slopes in the Central Valley of the island to quantify native shrub cover, density, and size. Previous data suggested that one species, Eriogonum arborescens, would be able to naturally recruit in exotic grass–dominated areas. Native shrubs have shown a modest increase in cover over time, although more striking was a sharp increase in the amount of dead shrub cover and density. Recruitment events during high rainfall years probably led to the slight increase in Eriogonum cover between sampling periods. Recent drought periods, however, have probably increased mortality and otherwise slowed shrub reestablishment in these arid sites.

Priority effects are affected by precipitation variability and are stronger in exotic than native grassland species

Exotic perennial grassland species often green up earlier than their native counterparts, allowing them to gain an advantage by dominating resources early (priority effects). Precipitation variability is expected to increase with climate change, and may alter the strength of priority effects. We hypothesized that exotics will have stronger priority effects than natives, precipitation variability will impact the strength of priority effects, and precipitation variability will impact the priority effects of native species more than those of exotics. We seeded one of five native or five exotic grassland species from the Central U.S. spanning multiple functional groups 28 days prior to a native seed mix. Priority effect strength was determined by how much establishment and diversity was reduced in the mix compared to controls (no species seeded before mix). We crossed these priority effect treatments with three water variability treatments, one low variability, and two high variability with alternate timing. Exotic species had stronger priority effects than natives, and decreased diversity and establishment from the seed mix. High variability precipitation when the growing season began dry significantly increased priority effects compared to low variability and high variability beginning wet. We found no significant evidence for a more pronounced impact of precipitation on native species, but trends suggest future studies may reveal significant interactions. Although future research in the field over multiple growing seasons is needed, our results suggest priority effects of exotics in Central U.S. grasslands are independent of precipitation timing and therefore likely to persist under changing climates.

The Coyote Brush Invasion of Southern California Grasslands and the Legacy of Mechanical Disturbance

This study investigates the recent invasion of exotic grasslands by coyote brush in La Jolla Valley, California. We test the “event dependent” hypothesis that mechanical disturbances during the past century were a key cause. To examine the relationship between past mechanical disturbances and vegetation dynamics we first conducted a review of the historical literature on practices of shrub removal and documented disturbance history using historical imagery. We next analyzed vegetation‐cover change over time using remotely sensed imagery and a vegetation map to document the history of native shrub advancement into exotic grassland by species association. Finally, we determined the topographic characteristics associated with different phases of shrub advancement. We found that mechanical disturbances historically varied by topography with upper and steeper slopes being least intensively disturbed. We found that shrub advancement rates, following release from grazing, varied by slope, elevation, and time period, and that Baccharis pilularis (coyote brush) was the main species to invade the more intensively disturbed sites at lower elevations. Our results indicate that mechanical disturbances played an important role in modifying the original vegetation cover with long‐lasting consequences, including a facilitating role for the subsequent Baccharis pilularis invasion. We concluded that the practice of grazing often included exotic mechanical disturbances that had long‐lasting impacts on native plants.

Heavy metal and metalloid concentrations in soils under pasture of southern New Zealand

Heavy metal and metalloid (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) levels in 284 topsoil (0–30cm) samples were measured from 20,000km2 of southern New Zealand in a systematic survey and the concentrations and patterns between samples sites were studied. Samples were analysed by the inductively coupled mass spectrometry method on an aqua regia digest. Except for Hg, median concentrations from land classed as high producing exotic grassland (pasture; 241 samples) were consistently elevated relative to median natural background levels under native vegetation. The major control of heavy metal enrichment in pasture is P-based fertiliser application (Cd, Cu, Zn), the soil parent material (Ni, Hg) or a combination of both (As, Cr, Pb), with Pb also affected by proximity to urban centres and major roads. There may be a trend of increasing heavy metal concentration with increasing intensity of land use. Comparison shows there is strong, regional variation in median heavy metal concentrations in topsoil around New Zealand and highlight a need for systematic and consistent study of heavy metals in soil nationally.

Water balance in paired watersheds with eucalyptus and degraded grassland in Pampa biome

Rangelands of the Pampa biome, which cover regions of Argentina, Uruguay and Brazil (176,496km2 – 2.07% of Brazilian territory and 63% of Rio Grande do Sul State territory, southern region of Brazil) in South America (total area of 750,000km2), are being substituted by crops and commercial eucalyptus, with potential impacts on ecological and hydrological response of watersheds and river basins. We evaluated the influence of vegetation cover on hydrological processes by describing the water balance and its components (rainfall – P, interception, throughfall, actual evapotranspiration – ETa, groundwater recharge – G, and streamflow – Q) in two paired watersheds. One watershed being cropped to 7-year-old Eucalyptus saligna stands (forest watershed – FW; 0.83km2) and another consisted of degraded grassland with native and exotic grasslands (grassland watershed – GW; 1.10km2) used for livestock production in the Rio Grande do Sul State. The study was conducted from October 2012 to September 2014, during two hydrological years: a normal year from October 2012 to September 2013, and a wetter year from October 2013 to September 2014. Pluviometers were installed to study the partition of rain, along with watershed gauges and equipment for monitoring hydrological variables. Meteorological data (maximum and minimum temperature, relative humidity, wind speed, and solar radiation) used to calculate potential evapotranspiration were collected from a tower installed in the FW, whereas hydrological data (P and Q) were collected by sensors installed in each of the watershed spillways. During the normal year, P was 19% above the annual historical average for the region, which is 1314mm, whereas in wetter year, rainfall was 98% above the same average. Total rainfall interception was similar between years in GW (9 and 10%), but different between years in FW that was higher in wetter (24%) then in normal year (16%). In the normal year, streamflow were 64% lower in the FW compared to the GW, while ETa and G were respectively 37% and 25% greater in the FW compared to the GW. In the wetter year, streamflow was 66% lower in the FW than in the GW, while ETa and G in soil were respectively 27% and 46% greater in the FW compared to the GW. Flow with 5% time streamflow (Q5) was greater in the GW compared to FW in both normal and wetter years. Streamflow in the GW and FW were equal at Q80 and Q82 in the normal and in the wetter years, respectively, and exceedance probability curves crossed over at Q81 and Q82, where the exceedance probability curves become greater in FW than in the GW. Even if the forest watershed had greater ETa compared to the grassland watershed, benefits such as greater interception and lesser surface runoff can be highlighted, for a condition where grassland was degraded and provided low aboveground biomass. Thus, the cultivation of eucalyptus stands may provide better structural conditions and ground cover, greater infiltration and soil water retention, and increased groundwater recharge, with consequent reduction of soil degradation by erosion and increased water availability during dry periods. Long-term use of forest systems, especially when compared to degraded grassland, may provide improvement on soil physical quality. However, these comparative results may not be valid for conditions during harvesting and tillage operations, neither when soil under grasslands has improved physico-hydraulic properties.

Canopy manipulations of exotic Bitter Willow (Salix elaeagnos) forest for indigenous seedling recruitment: A pilot study

SummaryThe invasive exotic tree species Bitter Willow (Salix elaeagnos; Salicaceae) has colonised areas of rank exotic grassland and has been found to contain indigenous seed, dispersed by frugivorous birds into the monospecific stands. This small pilot study examined whether indigenous seedlings that have germinated in the understorey of exotic Bitter Willow stands could be stimulated to establish through the creation of small‐scale canopy gaps. In Bitter Willow forest, four single Bitter Willow trees were poisoned to create canopy gaps. Light transmission and seedling regeneration of tree and shrub species were assessed beneath both the four manipulated and three comparable intact Bitter Willow canopies. Over 3 years, seedling height and density increased more beneath opened compared to intact Bitter Willow canopies. These results suggest that Bitter Willow can fill the roles of both a facilitative nurse and a perch tree. Larger‐scale canopy manipulation experiments of both Bitter Willow and other Salix species are needed to determine the full potential of canopy manipulations for forest restoration.

Analyzing Landscape Trends on Agriculture, Introduced Exotic Grasslands and Riparian Ecosystems in Arid Regions of Mexico

Riparian Zones are considered biodiversity and ecosystem services hotspots. In arid environments, these ecosystems represent key habitats, since water availability makes them unique in terms of fauna, flora and ecological processes. Simple yet powerful remote sensing techniques were used to assess how spatial and temporal land cover dynamics, and water depth reflect distribution of key land cover types in riparian areas. Our study area includes the San Miguel and Zanjon rivers in Northwest Mexico. We used a supervised classification and regression tree (CART) algorithm to produce thematic classifications (with accuracies higher than 78%) for 1993, 2002 and 2011 using Landsat TM scenes. Our results suggest a decline in agriculture (32.5% area decrease) and cultivated grasslands (21.1% area decrease) from 1993 to 2011 in the study area. We found constant fluctuation between adjacent land cover classes and riparian habitat. We also found that water depth restricts Riparian Vegetation distribution but not agricultural lands or induced grasslands. Using remote sensing combined with spatial analysis, we were able to reach a better understanding of how riparian habitats are being modified in arid environments and how they have changed through time.

Potential of grass invasions in desert shrublands to create novel ecosystem states under variable climate

AbstractThe invasion of exotic grasses into shrublands is a major disturbance to dryland ecosystems. The presence of exotic grasses enhances the occurrence of wildfire in landscapes that had not evolved in the presence of fire, leading to high rates of mortality of the native vegetation. Exotic grasses could be more prone to water stress and mortality than the shrubs they replaced and may not establish during drought, facts that are crucial in ecosystems undergoing increased climatic variability. Here, we develop a process‐based modelling framework to investigate the complex dynamics resulting from the introduction of exotic grasses under variable climate. We find that the system converges towards different steady states, depending on the magnitude of climatic variability. While in the absence of climate fluctuations the shrubland state is replaced by an exotic grassland, interannual climate variability may inhibit grass invasion and stabilize the shrubland state. However, climatic variability also gives rise to a novel third, unvegetated state, with grass invasion being followed by drought, grass mortality and intense soil erosion. Most of the research on climate change effects on ecosystems has historically concentrated on the ecological impact of shifts in mean climate conditions. This study shows that changes in the variance are also important when shifts in vegetation composition (e.g. species invasions) result in different susceptibility to climatic variability. In the presence of random climate fluctuations, ecosystems can display steady states that differ from those that would exist under a constant climate or with a climate trend. Copyright © 2016 John Wiley & Sons, Ltd.