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While the influence of canopy trees on soils in natural and restored forest environments is well studied, the influence of understory species is not. Here, we evaluate the effects of outplanted native woody understory on invasive grass biomass and soil nutrient properties in heavily grass-invaded 30 + year-old plantations of a native N-fixing tree Acacia koa in Hawai'i. We analyze soils from under A. koa trees with versus without planted woody understory and compare these to soils from under remnant pasture trees of the pre-deforestation dominant, Metrosideros polymorpha where passive recruitment of native woody understory has occurred since the cessation of grazing. Simultaneously, we experimentally planted understory species at three times the density used by managers to see if this could quickly decrease grass biomass and change soil nutrient dynamics. We found that invasive grass biomass declined with understory planting in surveyed and experimental sites. Yet, woody understory abundance had no effect on N cycling. Short-term N availability and nitrification potential were higher under A. koa than M. polymorpha trees regardless of understory. Net N mineralization either did not differ (~ 1 mo) between canopy species or was higher (171 day incubations) under remnant M. polymorpha where organic matter was also higher. The only influence of understory on soil was a positive correlation with loss-on-ignition (organic matter) under M. polymorpha. We also demonstrate differential controls over N cycling under the two canopy tree species. Overall, understory restoration has not changed soil characteristics even as invasive grass biomass declines. [ABSTRACT FROM AUTHOR]

Restricting seed collecting to local populations is a common practice in restoration because it is assumed that most plants are adapted to local environmental conditions. However, there is still considerable debate about whether local seed collection should be the default seed‐provenancing strategy as the effects of climate change are increasingly considered in restoration planning. It is especially important to explore whether local seed provenance is necessary for dominant species used in restoration because the success of these projects often rests on these species. Stipa pulchra is one such species that is commonly used in grassland restoration projects in California. To explore how different seed‐provenancing strategies affected the establishment and growth of S. pulchra, we established three common garden experiments distributed across a latitudinal gradient in California. We collected seeds from seven wild populations, germinated seeds in a common greenhouse, then planted all populations at the three common gardens. We assessed growth and reproduction for 2 years. We found limited evidence that restricting seed sourcing to local populations increased the establishment of S. pulchra compared to seeds from more distant populations. Instead, we found evidence to support the use of regional admixture seed sourcing to increase resiliency to environmental variation. In particular, we found being sourced from a dry location during the dry 2018 season was a benefit, highlighting the importance of including dry‐adapted populations to increase climate resilience. Our experiment highlights the importance of exploring multiple seed‐provenancing methods when designing a project to improve long‐term success. [ABSTRACT FROM AUTHOR]

The genus Carpobrotus N.E.Br. comprises between 12 and 25 species, most of which are native to South Africa. Some Carpobrotus species are considered among the most damaging invasive species in coastal dune systems worldwide. In their introduced areas, these species represent a serious threat to native species and significantly impact soil conditions and geochemical processes. Despite being well studied, the taxonomy of Carpobrotus remains problematic, as the genus comprises a complex of species that hybridize easily and are difficult to distinguish from each other. To explore the population genetic structure of invasive Carpobrotus species (i.e., C. acinaciformis and C. edulis) across a significant part of their native and non-native ranges, we sampled 40 populations across Argentina, Italy, New Zealand, Portugal, South Africa, Spain, and the USA. We developed taxon-specific microsatellite markers using a Next Generation Sequencing approach to analyze the population genetic structure and incidence of hybridization in native and non-native regions. We identified three genetically distinct clusters, which are present in both the native and non-native regions. Based on a set of selected morphological characteristics, we found no clear features to identify taxa morphologically. Our results suggest that the most probable sources of global introductions of Carpobrotus species are the Western Cape region of South Africa and the coastline of California. We suggest that management actions targeting Carpobrotus invasions globally should focus on preventing additional introductions from the east coast of South Africa, and on searching for prospective biocontrol agents in the Western Cape region of South Africa. [ABSTRACT FROM AUTHOR]

Deforestation and subsequent land-use conversion has altered ecosystems and led to negative effects on biodiversity. To ameliorate these effects, nitrogen-fixing (N2-fixing) trees are frequently used in the reforestation of degraded landscapes, especially in the tropics; however, their influence on ecosystem properties such as nitrogen (N) availability and carbon (C) stocks are understudied. Here, we use a 30-y old reforestation site of outplanted native N2-fixing trees (Acacia koa) dominated by exotic grass understory, and a neighboring remnant forest dominated by A. koa canopy trees and native understory, to assess whether restoration is leading to similar N and C biogeochemical landscapes and soil and plant properties as a target remnant forest ecosystem. We measured nutrient contents and isotope values (δ15N, δ13C) in soils, A. koa, and non-N2-fixing understory plants (Rubus spp.) and generated δ15N and δ13C isoscapes of the two forests to test for (1) different levels of biological nitrogen fixation (BNF) and its contribution to non-N2-fixing understory plants, and (2) the influence of historic land conversion and more recent afforestation on plant and soil δ13C. In the plantation, A. koa densities were higher and foliar δ15N values for A. koa and Rubus spp. were lower than in the remnant forest. Foliar and soil isoscapes also showed a more homogeneous distribution of low δ15N values in the plantation and greater influence of A. koa on neighboring plants and soil, suggesting greater BNF. Foliar δ13C also indicated higher water use efficiency (WUEi) in the plantation, indicative of differences in plant-water relations or soil water status between the two forest types. Plantation soil δ13C was higher than the remnant forest, consistent with greater contributions of exotic C4-pasture grasses to soil C pools, possibly due to facilitation of non-native grasses by the dense A. koa canopy. These findings are consequential for forest restoration, as they contribute to the mounting evidence that outplanting N2-fixing trees produces different biogeochemical landscapes than those observed in reference ecosystems, thereby influencing plant-soil interactions which can influence restoration outcomes. [ABSTRACT FROM AUTHOR]

The increased occurrences of drought and fire may be contributing to the loss of biodiverse ecosystems in Mediterranean regions. Specifically, the conversion of diverse native shrublands, such as chaparral, to non‐native annual grassland by fire is of great conservation concern in California. To avoid or slow the loss of chaparral, it is important to understand the underlying causes of landscape conversion. Studies investigating the interaction of multiple potential drivers are particularly crucial to identification of vulnerable areas of the landscape. Here we used aerial imagery to evaluate vegetation transitions between chaparral, sage scrub, grassland, and tree domination and their potential drivers within Ventura County, CA, a strongly Mediterranean climate region. We used random forest algorithms and conditional inference trees to determine the climatic, topographic, and fire‐related variables contributing most to vegetation change. Our results support that chaparral conversion to grass (27% of chaparral plots) is a result of landscape position, fire, and drought acting in tandem. In particular, lower elevation, southwest‐facing slopes that experience a postfire drought are at very high likelihood of conversion to non‐native annual grass. Additionally, our results show that these grasslands, once formed, rarely convert to other community types. Therefore, protecting shrub‐dominated areas that are most likely to convert (low elevation, more southwest‐facing slopes, and less annual precipitation) is crucial to preserving native vegetation diversity. [ABSTRACT FROM AUTHOR]

To evaluate how increased anthropogenic nutrient inputs alter carbon cycling in grasslands, we conducted a litter decomposition study across 20 temperate grasslands on three continents within the Nutrient Network, a globally distributed nutrient enrichment experimentWe determined the effects of addition of experimental nitrogen (N), phosphorus (P) and potassium plus micronutrient (Kμ) on decomposition of a common tree leaf litter in a long‐term study (maximum of 7 years; exact deployment period varied across sites). The use of higher order decomposition models allowed us to distinguish between the effects of nutrients on early‐ versus late‐stage decomposition.Across continents, the addition of N (but not other nutrients) accelerated early‐stage decomposition and slowed late‐stage decomposition, increasing the slowly decomposing fraction by 28% and the overall litter mean residence time by 58%.Synthesis. Using a novel, long‐term cross‐site experiment, we found widespread evidence that N enhances the early stages of above‐ground plant litter decomposition across diverse and widespread temperate grassland sites but slows late‐stage decomposition. These findings were corroborated by fitting the data to multiple decomposition models and have implications for N effects on soil organic matter formation. For example, following N enrichment, increased microbial processing of litter substrates early in decomposition could promote the production and transfer of low molecular weight compounds to soils and potentially enhance the stabilization of mineral‐associated organic matter. By contrast, by slowing late‐stage decomposition, N enrichment could promote particulate organic matter (POM) accumulation. Such hypotheses deserve further testing. [ABSTRACT FROM AUTHOR]

Trees can have large effects on soil nutrients in ways that alter succession, particularly in the case of nitrogen‐(N)‐fixing trees. In Hawaiʻi, forest restoration relies heavily on use of a native N‐fixing tree, Acacia koa (koa), but this species increases soil‐available N and likely facilitates competitive dominance of exotic pasture grasses. In contrast, Metrosideros polymorpha ('ōhi'a), the dominant native tree in Hawaiʻi, is less often planted because it is slow growing; yet it is typically associated with lower soil N and grass biomass, and greater native understory recruitment. We experimentally tested whether it is possible to reverse high soil N under koa by adding 'ōhi'a litter, using additions of koa litter or no litter as controls, over 2.5 yr. We then quantified natural litterfall and decomposition rates of 'ōhi'a and koa litter to place litter additions in perspective. Finally, we quantified whether litter additions altered grass biomass and if this had effects on native outplants. Adding 'ōhi'a litter increased soil carbon, but increased rather than decreased inorganic soil N pools. Contrary to expectations, koa litter decomposed more slowly than 'ōhi'a, although it released more N per unit of litter. We saw no reduction in grass biomass due to 'ōhi'a litter addition, and no change in native outplanted understory survival or growth. We conclude that the high N soil conditions under koa are difficult to reverse. However, we also found that outplanted native woody species were able to decrease exotic grass biomass over time, regardless of the litter environment, making this a better strategy for lowering exotic species impacts. [ABSTRACT FROM AUTHOR]

1. The form and function of terrestrial plants is largely governed by the availability of water, with plants in water-limited environments expressing traits minimizing water loss and tissue damage during drought. Areas with high salinity are analogous to those with low water availability, even where water is abundant. We evaluated variation in salinity tolerance and water use strategies in an introduced halophytic shrub, Tamarix spp by measuring gas exchange rates, biomass accumulation and turgor loss points. We hypothesized that salinity tolerance is not uniformly expressed, with high salinity ecotypes expressing more conservative water use traits that maximize hydraulic safety over high conductance rates. 2. We studied two populations of Tamarix spp. along the lower Colorado River, where groundwater salinity differed by nearly sixfold. Cuttings collected from the sites were grown in a greenhouse with five salinity levels from 0 to 32 parts per thousand (ppt), imposing increasing water limitation. 3. In situ measurements of leaf water potentials (Ψ) and branch xylem anatomy were taken on ecotypes sourced from high- and low-salinity sites. In the greenhouse, biomass and gas exchange rates were evaluated over the 0-32 ppt salinity gradient, and a separate dry down experiment was performed to determine turgor loss points and stomatal responses to drying soils. 4. In the greenhouse, the low salinity population accumulated 72% more biomass when grown at 4 ppt compared to 16 ppt, while the high salinity population produced 50% more biomass when grown at 16 ppt. Net carbon assimilation was greater at lower salinities in the low salinity population but independent of salinity in the high salinity population. The high salinity population had a lower turgor loss point and exhibited greater stomatal control relative to the low salinity population. 5. Synthesis. Results provide evidence for divergence of traits related to plant water use across salinity gradients in this recently introduced halophyte. Local adaptation to increased salinity has implications in aridland riparian ecosystems, where water management or drought may lead to altered soil salinities. The interaction of trait variation within Tamarix spp. and increasing salinity is likely to favour its continued dominance. [ABSTRACT FROM AUTHOR]

Abiotic conditions can influence the effect that herbivores have on plant growth. Such biotic and abiotic interactions are of special interest in plant biological control programs because the goal of herbivore suppression of the target weed may not be reached in some abiotic settings. Additionally, target invasive plants typically occur across diverse landscapes raising the possibility that local adaptation to site-specific conditions leads to phenotypic variation that can affect herbivore responses. Here, we used Tamarix, an invasive plant, and its associated biological control agent, Diorhabda carinulata, to investigate how local variation in soil salinity and host plant origin influence interactions between the two taxa. To test if Tamarix was adapted to local conditions, we collected plants from sites with either low or high groundwater salinity, asexually propagated them through multiple generations, and then treated them with their home or reciprocal salinity levels. We found that plants accumulated the most biomass when grown at the salinities of their origin site. The biological control agent, D. carinulata preferred plants grown at source site salinity when given a choice against plants grown in the reciprocal salinity treatment. Although plants compensated for herbivory by regrowing foliage over three defoliation events and maintained similar leaf biomass through regrowth, they ultimately had a reduced basal area and 62% lower root biomass compared to the controls. Thus, herbivory caused a shift in plant allocation of resources from overall growth to compensation, reducing root and stem investment. Overall, D. carinulata caused a significantly greater reduction in total biomass in the high salinity plants than the low salinity ones when grown at their source salinity (averages of 63% and 32% respectively). Thus, the Tamarix biological control program may experience its greatest efficacy in high salinity areas where the impact of the agent is the greatest, likely due to increased water stress and reduced resources to enable regrowth. [ABSTRACT FROM AUTHOR]

Non‐structural carbohydrate (NSC) storage may be under strong selection in woody plant species that occur across broad environmental gradients. We therefore investigated carbon (C) allocation strategies in a widespread non‐native woody plant, Tamarix. We predicted that genotypes with exposure to episodic freeze events would show elevated NSC concentrations compared to warm‐adapted genotypes with the trade‐off of reduced growth and reproduction relative to warm‐adapted populations.We established an experimental common garden using genotypes of Tamarix, sourced across a strong thermal gradient within the introduced range. We measured seasonal NSC storage in coarse roots and stems, above‐ground growth and flower production.Autumn NSC concentrations were 50% higher in genotypes from sites with episodic spring freeze events compared to genotypes from warmer sites. These cold‐adapted genotypes also had a 2.3‐fold higher starch to soluble sugar ratio than warm‐adapted genotypes. Across all genotypes and seasons, NSC storage was inversely correlated with growth and reproduction.Results suggest that Tamarix from colder locations cope with freeze events by maintaining large storage pools to support tissue regrowth, but with the trade‐off of overall reduced growth and reproduction. Our results are consistent with rapid selection in C allocation strategies in response to climate in introduced woody species. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Abandoned tropical pastures offer opportunities for passive and active restoration of native forest communities. Tree architecture of remnant canopy trees may be one important factor that can facilitate native plant recruitment in abandoned pastures but has largely been overlooked. Here, we evaluated patterns of native woody plant recruitment under remnant trees in abandoned pastures on Hawai'i Island and how these might be related to both tree architectural features and landscape variables. We measured native woody stems (excluding sprouts of the tree itself) in a 5 m radius around the base of each tree and modeled total basal area of native stems as a function of tree architectural characteristics. Recruitment was positively correlated with tree height as well as horizontal woody area below 1 m (tree structure that occurred below 1 m and was < 45° angle from the ground) around the base of trees. Tall trees likely attract more avian seed dispersers due to their higher visibility on the landscape and increased crown volume. Horizontal woody area likely provides establishment microsites that are above the pasture grass layer, similar to how dead or decaying logs act as nurse substrates. Unlike previous studies, we found little evidence that landscape variables such as distance to the intact forest or nearest canopy neighbor influenced understory recruitment. Tree architectural characteristics can be important predictors of native plant recruitment in abandoned tropical pastures and should be considered in addition to local and landscape-level variables. [ABSTRACT FROM AUTHOR]

Premise: Mortality events involving drought and pathogens in natural plant systems are on the rise due to global climate change. In Santa Barbara, California, United States, big berry manzanita (Arctostaphylos glauca) has experienced canopy dieback related to a multi‐year drought and infection from fungal pathogens in the Botryosphaeriaceae family. A greenhouse experiment was conducted using Neofusicoccum australe to test the specific influences of drought and fungal infection on A. glauca. Methods: A full factorial design was used to compare four treatment groups (drought + inoculation; drought – inoculation; watering + inoculation; and control: watering – inoculation). Data were collected for 10 weeks on stress symptoms, changes in leaf fluorescence and photosynthesis, and mortality. Results: Results indicated significant effects of watering and inoculation treatments on net photosynthesis, dark‐adapted fluorescence, and disease symptom severity (P < 0.05), and a strong correlation was found between physiological decline and visible stress (P < 0.0001). Mortality differed between treatments, with all groups except for the control experiencing mortality (43% mortality in drought – inoculation, 83% in watering – inoculation, and 100% in drought + inoculation). A Kaplan–Meier survival analysis showed drought + inoculation to have the least estimated survivorship compared to all other treatment groups. Conclusions: In addition to a possible synergistic interaction between drought and fungal infection in disease onset and mortality rates in A. glauca, these results indicate that young, non‐drought‐stressed plants are susceptible to mortality from N. australe infection, with important implications for the future of wildland shrub communities. [ABSTRACT FROM AUTHOR]

Invasion ecology has grown to include scientists with diverse skill sets who focus on a range of taxa and biomes. These researchers have the capacity to contribute to practical management solutions while also answering fundamental biological questions; however, scientific endeavors often fail to meet the perceived needs of practitioners involved in on-the-ground invasive plant management. One way that researchers have sought to bridge the gap between research and practice is by surveying managers to identify areas of study that are underexplored in invasion ecology. In this paper, we build on these efforts by reviewing the current state of knowledge and suggesting new directions for research in seven areas of plant invasion ecology that are highly relevant to management: seedbanks, dispersal and spread, life history, impacts, climate change, distribution, and succession. These topics were previously identified as urgent research priorities by land managers and are underrepresented in the invasion ecology literature. In addition to highlighting key knowledge gaps for these seven areas of research, we propose steps that academics can take to cultivate academic–practitioner relationships and remove barriers to conducting management-focused research, such as co-producing research questions with managers, addressing issues of working at management-appropriate spatial and temporal scales, and considering non-traditional funding and labor sources for long-term monitoring. Greater communication and collaborative selection of basic research questions will ensure that the goals of management and invasive species research remain aligned. [ABSTRACT FROM AUTHOR]

Invasion by exotic plant species can profoundly affect native plant species performance and the inferred proximate cause is typically competition. We used invaded grasslands in the semi-arid Western USA to separate resource competition from structural interference of an exotic grass on native forb performance, specifically evaluating the role of competition from living vegetation versus litter accumulation (hereafter, thatch). We simultaneously tested whether a positive thatch-feedback exists for the dominant exotic grass species, Bromus diandrus. Thatch and B. diandrus density were manipulated and coupled with native seed addition to separate the effect of B. diandrus competition relative to thatch accumulation. To determine the response of native forb species and B. diandrus to varying thatch abundance, we created a gradient of thatch densities and measured species response, soil moisture and light availability. The thatch of B. diandrus greatly reduced native forb performance (number of individuals and biomass) and resulted in near complete exclusion of many species. The effect of living B. diandrus density on native forb performance was minimal and inconsistent. Forb performance and light availability both exhibited exponential declines with thatch build-up suggesting that light reduction is a primary mechanism through which thatch affects forbs. Simultaneous with forb suppression, B. diandrus performance was positively affected by thatch, consistent with a positive feedback initiated by invasion of this species. Our results demonstrate that thatch accumulation, rather than competition for resources, is the primary cause of native species decline in grasslands invaded by this exotic annual grass. In addition, the dominance of B. diandrus in many invaded grasslands may be reinforced through a positive thatch-feedback and require active management, such as grazing or fire, to be broken. [ABSTRACT FROM AUTHOR]

Wetting of dry soil triggers a pulse of microbial respiration that has been attributed to two broad mechanisms: (1) recycling of microbial cellular carbon (C), and (2) consumption of extracellular organic C made available to microbes by wetting. We evaluated these two mechanisms by measuring cumulative CO2 release, changes in the size and chemical composition of microbial biomass, and water-extractable organic carbon (WEOC) concentrations following artificial wetting of soil sampled from two depths at each of seven sites across California spanning a range of geologic parent materials. In samples collected from surface soil (0–10 cm depth), we found that cumulative CO2 release after wetting in the laboratory was most strongly correlated with microbial biomass. In these samples, the relative abundance of trehalose—a putative microbial osmolyte—decreased from 25% (SD = 12) to 16% (SD = 7) of the chloroform-labile fraction of the microbial biomass after wetting. This suggested a role for osmolyte consumption in generating the respiration pulse. In subsoil (40–50 cm depth, or sampled at contact with rock), however, the cumulative CO2 release after wetting was unrelated to microbial biomass and more strongly related to WEOC. The concentrations of selected microbial biomass constituents (e.g. trehalose and amino acids) in WEOC were negligible (< 1%), suggesting that cell lysis was not important in generating WEOC in this study. The amount of WEOC relative to total organic C was greatest in subsoil, and negatively related to ammonium oxalate-extractable Fe (Pearson's R = 0.42, p < 0.01), suggesting a role for soil mineralogical properties in controlling WEOC release. Together, these findings suggest that microbial cellular C and extracellular C jointly contribute to the respiration pulse, and that their relative contribution depends on depth. [ABSTRACT FROM AUTHOR]

Elevated nutrient deposition often increases primary productivity in terrestrial ecosystems and thus has the potential to increase the flux of carbon (C) into soils. An important step toward greater understanding of nutrient effects on C storage involves assessing effects on different fractions of the soil C pool across a range of soil types. We quantified the combined effects of 8 years of nitrogen (N), phosphorus (P), potassium (K), and micronutrient fertilization on the C storage in bulk soil and in density fractions at four grassland sites in California. When averaged across sites, fertilization increased soil light fraction C by 64% relative to the control in the 0–10 cm depth. The increase in light fraction C likely resulted from the fertilization-induced increase in plant C input to soil, as aboveground net primary productivity also consistently increased with fertilization across sites. Effects of fertilization on heavy fraction C were highly site specific, having positive, negative, or no effect at individual sites. The response of heavy fraction C to fertilization appeared to be related to mean annual precipitation and soil bulk density. Overall, bulk soil C concentration showed a marginally significant increase of 6% with fertilization when averaged across sites (P = 0.07). Our results indicate that biomass production and soil light fraction are generally sensitive to fertilization across grasslands in California, likely contributing to increases in soil C storage. Responses of heavy fraction C, on the other hand, vary greatly among sites and may depend on climate and soil characteristics. [ABSTRACT FROM AUTHOR]

Wildfires alter nitrogen (N) cycling in Mediterranean-type ecosystems, resetting plant and soil microbial growth, combusting plant biomass to ash, and enhancing N availability in the upper soil layer. This ash and soil N pool (that is, wildfire N) is susceptible to loss from watersheds via runoff and leaching during post-fire rains. Plant and soil microbial recovery may mitigate these losses by sequestering N compounds in new biomass, thereby promoting landscape N retention in N-limited chaparral ecosystems. We investigated the relative balance between wildfire N loss, and plant and soil microbial N uptake and stream N export for an upland chaparral watershed in southern California that burned (61%) in a high-intensity wildfire in 2009 by using a combination of stream, vegetation, soil microbial, and remote sensing analyses. Soil N in the burn scar was 440% higher than unburned soil N in the beginning of the first post-fire wet season and returned within 66 days to pre-fire levels. Stream N export was 1480% higher than pre-fire export during the first post-fire rain and returned within 106 days over the course of the following three rainstorms to pre-fire levels. A watershed-scale N mass balance revealed that 52% of wildfire N could be accounted for in plant and soil microbial growth, whereas 1% could be accounted for in stream export of dissolved nitrogen. [ABSTRACT FROM AUTHOR]

In grasslands, litter has been recognized as an important factor promoting grass persistence and the suppression of forbs. The invasive European annual grass Bromus diandrus (ripgut brome) is widespread throughout California, where it produces a persistent and thick litter layer. The native grass, Stipa pulchra, is also common in some grassland settings and can also produce persistent litter, yet it is typically associated with more forbs. Very little is known about the mechanisms through which these two common grass species influence seedling establishment of both exotic invasive and native herbs. Here, we evaluated the effect of B. diandrus and S. pulchra litter on seedling establishment of two invasive (the grass B. diandrus and the forb Centaurea melitensis) and two native (the grass S. pulchra, and the forb Clarkia purpurea) herbaceous plants in a greenhouse setting. Our results showed that B. diandrus litter cover hindered seedling establishment of the four species tested, but that the degree and mechanism of inhibition was dependent on which species was tested, life form (e.g. monocot/dicot) and seed size. Seedling emergence of the two forb species was more vulnerable to litter cover than either grass species and both forbs had smaller seed size. After germination, only seedling biomass of B. diandrus itself was reduced by litter (both B. diandrus and S. pulchra). We found no significant effects of leachate of either grass species on seedling emergence of any species, while a high concentration of B. diandrus leachates inhibited root growth of all species including B. diandrus seedlings. Stipa pulchra litter leachates did not affect S. pulchra or C. melitensis seedlings although it did suppress B. diandrus and C. purpurea seedling growth. Our findings provide direct experimental evidence for the mechanism of effect of litter on these coexisting invasive and native species. Such evidence helps advance our understanding of role of B. diandrus and S. pulchra litter in California grassland. [ABSTRACT FROM AUTHOR]

Despite a large body of research documenting invasive plant impacts, few studies have followed individual invaded sites over decades to observe how they change, and none have contrasted how compositional impacts from invasion compare to ecosystem-process impacts over a multi-decadal time-scale., Using direct measurements of plant density and composition and of ecosystems processes, we evaluate how ecosystem structure, above-ground net primary production ( ANPP), and above-ground and soil nutrient pools compare over 25 years since fire and C4 grass invasions disrupted seasonally dry Hawaiian woodlands. We compare structure and function between primary woodland that has never burned and is largely native species-dominated, with sites that had been the same woodland type but burned in alien-grass-fuelled fires in the 1970s and 1980s. The sites have not experienced fires since 1987., We report here that woody plant composition and structure continue to be dramatically changed by the initial invasions and fires that occurred 25 years ago and invaders continue to dominate in burned sites. This is reflected in continued low plant carbon pools in burned compared to unburned sites. Yet ANPP and N storage, which were dramatically lower in the initial decade after invasive-grass fuelled fires, have increased and are now indistinguishable from values measured in intact woodlands. Soil carbon pools were resilient to both invasion and fire initially and over time., Above-ground net primary production has recovered because of invasion of burned sites by a non-native N-fixing tree rather than because of recovery of native species. This invasive N-fixing tree is unlikely to return C storage of the invaded sites to those of unburned woodland because of its tissue and growth characteristics and its interactions with invasive grasses. It does not facilitate native species but rather promotes a persistent invasive grass/N-fixer savanna., Synthesis. We conclude that fire, an unusual disturbance in this system, has perpetuated the dominance of these sites by invasive species and that despite the dramatic recovery of above-ground net primary production and N pools, the ecosystem continues to be in a distinctly different state than the pre-fire, pre- Melinis community. Thus, despite the absence of further disturbance (fire), there is no evidence that succession towards the original ecosystem is occurring. The fact that N pools and above-ground net primary production recover because of a new invader ( Morella faya), highlights the unpredictability of ecosystem trajectories in the face of altered regional species pools. [ABSTRACT FROM AUTHOR]

Most ecosystems have multiple-plant invaders rather than single-plant invaders, yet ecological studies and management actions focus largely on single invader species. There is a need for general principles regarding invader interactions across varying environmental conditions, so that secondary invasions can be anticipated and managers can allocate resources toward pretreatment or postremoval actions. By reviewing removal experiments conducted in three Hawaiian ecosystems (a dry tropical forest, a seasonally dry mesic forest, and a lowland wet forest), we evaluate the roles environmental harshness, priority effects, productivity potential, and species interactions have in influencing secondary invasions, defined here as invasions that are influenced either positively (facilitation) or negatively (inhibition/priority effects) by existing invaders. We generate a conceptual model with a surprise index to describe whether long-term plant invader composition and dominance is predictable or stochastic after a system perturbation such as a removal experiment. Under extremely low resource availability, the surprise index is low, whereas under intermediate-level resource environments, invader dominance is more stochastic and the surprise index is high. At high resource levels, the surprise index is intermediate: Invaders are likely abundant in the environment but their response to a perturbation is more predictable than at intermediate resource levels. We suggest further testing across environmental gradients to determine key variables that dictate the predictability of postremoval invader composition. [ABSTRACT FROM AUTHOR]

Boom-bust dynamics - the rise of a population to outbreak levels, followed by a dramatic decline - have been associated with biological invasions and offered as a reason not to manage troublesome invaders. However, boom-bust dynamics rarely have been critically defined, analyzed, or interpreted. Here, we define boom-bust dynamics and provide specific suggestions for improving the application of the boom-bust concept. Boom-bust dynamics can arise from many causes, some closely associated with invasions, but others occurring across a wide range of ecological settings, especially when environmental conditions are changing rapidly. As a result, it is difficult to infer cause or predict future trajectories merely by observing the dynamic. We use tests with simulated data to show that a common metric for detecting and describing boom-bust dynamics, decline from an observed peak to a subsequent trough, tends to severely overestimate the frequency and severity of busts, and should be used cautiously if at all. We review and test other metrics that are better suited to describe boom-bust dynamics. Understanding the frequency and importance of boom-bust dynamics requires empirical studies of large, representative, long-term data sets that use clear definitions of boom-bust, appropriate analytical methods, and careful interpretations. [ABSTRACT FROM AUTHOR]

Aims: Biotic homogenization results in novel communities containing multiple exotic species. Many attributes that contribute to the dominance of invaders and increase invasibility of communities have been identified, but rarely have factors governing the dominance of co-occurring invaders been considered. Here we assess the presence and performance of five exotic and one native C4 grass species across local (microsite) and more broad-scale variation in fire history, soil age, altitude and precipitation. We ask whether species show preferences for the same conditions, whether all are constrained by poor soil development, and how their performance changes when alone versus co-occurring. In addition to providing information useful to managers who want to control some of these species, such information can help to predict how distributions might change with climate change. Methods: We measured percent cover and plant height of each grass species in 64 plots in each of 15 sites, located in young volcanic substrates across environmental gradients in Hawai'i Volcanoes National Park, Hawai'i, USA. Sites varied in fire history and soil age, across altitudinal and precipitation gradients. We estimated microsite heterogeneity by measuring rock cover and soil depth, where each species was present and in plots as a whole. We analyzed effects of possible controlling variables using chi-square tests and Generalized Additive Mixed Models. Important Findings: Two species (Schizachyrium condensatum and Hyparrhenia rufa) were absent in shallow soil, restricted to more mesic sites and showed increasing performance with soil depth. By contrast, two other species (Andropogon virginicus and Melinis repens) tended to occur in shallow soil with high rock cover across a wide range of sites. One additional species (Melinis minutiflora) was weakly affected by soil depth and was the dominant at higher elevation (more mesic) and on burned sites, especially in old soil. This species was largely absent from lower elevation sites, where the more widespread and opportunistic congener, M. repens, and a drought tolerant native grass, Heteropogon contortus, were more abundant. The latter was confined to this low hot zone. Introduced C4 grasses in this region of Hawai'i are distributed according to an interaction between soil requirements, climate (correlated with elevation), competitive ability and fire response. No one strategy can explain the distributions, which will likely persist unless climate changes toward warmer and drier conditions or new fires occur. The latter would promote dominance of M. minutiflora in more mesic sites, while the former would promote either native (H. contortus) or other exotic grasses (M. repens, A. virginicus). [ABSTRACT FROM AUTHOR]

Fog-drip to the soil is the most obvious contribution of fog to the water budget of an ecosystem, but several studies provide convincing evidence that foliar absorption of fog water through leaf wetting events is also possible. The focus of our research was to assess the relative importance of fog drip and fog immersion (foliar wetting) on leaf gas-exchange rates and photosynthetic capacity of a coastal pine species, Bishop pine (Pinus muricata D.Don), a drought-sensitive species restricted to the fog belt of coastal California and offshore islands. In a controlled experiment, we manipulated fog water inputs to potted Bishop pine saplings during a 3 week dry-down period. Ten saplings were randomly assigned one of two fog treatments: (1) fog drip to the soil and canopy fog immersion, or (2) fog immersion alone. Five saplings were assigned the 'control' group and received no fog water inputs. We found that fog immersion alone significantly increased carbon assimilation rates and photosynthetic capacity of saplings as soil moisture declined compared with those that received no fog at all. The highest carbon assimilation rates were observed in saplings that also received fog drip. Soil moisture was 40% higher in the fog immersion compared with the control group during the dry-down, indicating a reduced demand for soil water in saplings that had only leaves wetted by canopy interception of fog. Leaf-level physiology is more strongly enhanced by fog drip compared with fog immersion, although the results of this study provide evidence that foliar absorption is a viable mechanism by which Bishop pines use fog water and that it can enhance instantaneous plant carbon gain and potentially whole plant productivity. [ABSTRACT FROM AUTHOR]

For introduced species that have spread across a wide distributional range, phenotypic plasticity (PLA) has often been proposed as an important contributor to invasion success, because it increases the survival rate during initial colonization. In contrast, local genetic variation (LOC) has also been proposed to be important, because it could allow invaders to evolve high performance in a new habitat. While evolutionary ecologists have long been interested in understanding genetic mechanisms that allow rapid colonization and spread of species, until recently experimental tests of these concepts have been limited. As a step towards generalization in our understanding of the importance of PLA and LOC, we review the current state of the literature on this topic using meta-analysis. Here, we focused on three fundamental questions: 1) which strategy, PLA or LOC, better explains the phenotypic divergences during invader range expansion across different environmental gradients? 2) Which species characteristics correlate with the occurrence of these different phenomena? And 3) does the detection of PLA versus LOC depend on the trait studied? Using meta-analysis we found that plasticity explained a higher proportion of phenotypic variation regardless of the environmental gradients studied or plant growth forms. PLA predominated in clonal, self-compatible and perennial species, while LOC predominated in annual species. The patterns were trait-dependent: LOC was significantly more important than PLA in phenology, while opposite patterns were found in fecundity and biomass allocation. The frequent simultaneous detection of PLA and genotypic variation in PLA among invasive populations suggested that PLA might benefit from LOC to some extent. Our results also indicate that the contribution of plasticity to the competitive advantages of invasive plants may be more informative than the level of plasticity itself. [ABSTRACT FROM AUTHOR]

Ecosystem eutrophication often increases domination by non-natives and causes displacement of native taxa. However, variation in environmental conditions may affect the outcome of interactions between native and non-native taxa in environments where nutrient supply is elevated. We examined the interactive effects of eutrophication, climate variability and climate average conditions on the success of native and non-native plant species using experimental nutrient manipulations replicated at 32 grassland sites on four continents. We hypothesized that effects of nutrient addition would be greatest where climate was stable and benign, owing to reduced niche partitioning. We found that the abundance of non-native species increased with nutrient addition independent of climate; however, nutrient addition increased non-native species richness and decreased native species richness, with these effects dampened in warmer or wetter sites. Eutrophication also altered the time scale in which grassland invasion responded to climate, decreasing the importance of long-term climate and increasing that of annual climate. Thus, climatic conditions mediate the responses of native and non-native flora to nutrient enrichment. Our results suggest that the negative effect of nutrient addition on native abundance is decoupled from its effect on richness, and reduces the time scale of the links between climate and compositional change. [ABSTRACT FROM AUTHOR]

Isolated trees in savannas worldwide are known to modify their local environment and interact directly with neighboring plants. Less is known about how related tree species differ in their impacts on surrounding communities, how the effects of trees vary between years, and how composition might change following loss of the tree. To address these knowledge gaps, we explored the following questions: How do savanna trees influence the surrounding composition of herbaceous plants? Is the influence of trees consistent across different species and years? How does this change following the death of the tree? We surveyed herbaceous species composition and environmental attributes surrounding living and dead evergreen and deciduous Quercus trees in California (USA) savannas across several years that differed in their total precipitation. Oak trees of all species created distinct, homogenous understory communities dominated by exotic grasses across several sites. The composition of the low-diversity understory communities showed less interannual variation than open grassland, despite a two-fold difference in precipitation between the driest and wettest year. Vegetation composition was correlated with variation in soil properties, which were strongly affected by trees. Oaks also influenced the communities beyond the edge of the crown, but this depended on site and oak species. Low-diversity understory communities persisted up to 43 years following the death of the tree. A gradual decline in the effect of trees on the physical environment following death did not result in vegetation becoming more similar to open grassland over time. The presence of long-lasting legacies of past tree crowns highlights the difficulty of assigning control of the current distribution of herbaceous species in grassland to their contemporary environment. [ABSTRACT FROM AUTHOR]

Harsh habitats dominated by invasive species are difficult to restore. Invasive grasses in arid environments slow succession toward more desired composition, yet grass removal exacerbates high light and temperature, making the use of 'nurse plants' an appealing strategy. In this study of degraded subtropical woodlands dominated by alien grasses in Hawai'i, we evaluated whether individuals of two native ( Dodonaea viscosa, Leptocophylla tameiameia) and one non-native ( Morella faya) woody species (1) act as natural nodes of recruitment for native woody species and (2) can be used to enhance survivorship of outplanted native woody species. To address these questions, we quantified the presence and persistence of seedlings naturally recruiting beneath adult nurse shrubs and compared survival and growth of experimentally outplanted seedlings of seven native woody species under the nurse species compared to intact and cleared alien-grass plots. We found that the two native nurse shrubs recruit their own offspring, but do not act as establishment nodes for other species. Morella faya recruited even fewer seedlings than native shrubs. Thus, outplanting will be necessary to increase abundance and diversity of native woody species. Outplant survival was the highest under shrubs compared to away from them with few differences between nurse species. The worst habitat for native seedling survival and growth was within the unmanaged invasive grass matrix. Although the two native nurse species did not differentially affect outplant survival, D. viscosa is the most widespread and easily propagated and is thus more likely to be useful as an initial nurse species. The outplanted species showed variable responses to nurse habitats that we attribute to resource requirements resulting from their typical successional stage and nitrogen fixation capability. [ABSTRACT FROM AUTHOR]

Increased fire frequency has been shown to promote alien plant invasions in the western United States, resulting in persistent vegetation type change. Short interval fires are widely considered to be detrimental to reestablishment of shrub species in southern California chaparral, facilitating the invasion of exotic annuals and producing “type conversion”. However, supporting evidence for type conversion has largely been at local, site scales and over short post-fire time scales. Type conversion has not been shown to be persistent or widespread in chaparral, and past range improvement studies present evidence that chaparral type conversion may be difficult and a relatively rare phenomenon across the landscape. With the aid of remote sensing data covering coastal southern California and a historical wildfire dataset, the effects of short interval fires (<8 years) on chaparral recovery were evaluated by comparing areas that burned twice to adjacent areas burned only once. Twelve pairs of once- and twice-burned areas were compared using normalized burn ratio (NBR) distributions. Correlations between measures of recovery and explanatory factors (fire history, climate and elevation) were analyzed by linear regression. Reduced vegetation cover was found in some lower elevation areas that were burned twice in short interval fires, where non-sprouting species are more common. However, extensive type conversion of chaparral to grassland was not evident in this study. Most variables, with the exception of elevation, were moderately or poorly correlated with differences in vegetation recovery. [ABSTRACT FROM AUTHOR]

Scattered trees in grass-dominated ecosystems often act as islands of fertility with important influences on community structure. Despite the potential for these islands to be useful in restoring degraded rangelands, they can also serve as sites for the establishment of fast growing non-native species. In California oak savannas, native perennial grasses are rare beneath isolated oaks and non-native annual grasses dominate. To understand the mechanisms generating this pattern, and the potential for restoration of native grasses under oaks, we asked: what are the effects of the tree understory environment, the abundance of a dominant non-native annual grass ( Bromus diandrus), and soils beneath the trees on survival, growth, and reproduction of native perennial grass seedlings? We found oak canopies had a strong positive effect on survival of Stipa pulchra and Poa secunda. Growth and reproduction was enhanced by the canopy for Poa but negatively impacted for Stipa. We also found that Bromus suppressed growth and reproduction in Stipa and Poa, although less so for Stipa. These results suggest the oak understory may enhance survival of restored native perennial grass seedlings. The presence of exotic grasses can also suppress growth of native grasses, although only weakly for Stipa. The current limitation of native grasses to outside the canopy edge is potentially the result of interference from annual grasses under oaks, especially for short-statured grasses like Poa. Therefore, control of non-native annual grasses under tree canopies will enhance the establishment of S. pulchra and P. secunda when planted in California oak savannas. [ABSTRACT FROM AUTHOR]

Non-native species with growth forms that are different from the native flora may alter the physical structure of the area they invade, thereby changing the resources available to resident species. This in turn can select for species with traits suited for the new growing environment., We used adjacent uninvaded and invaded grassland patches to evaluate whether the shift in dominance from a native perennial bunchgrass, Nassella pulchra, to the early season, non-native annual grass, Bromus diandrus, affects the physical structure, available light, plant community composition and community-weighted trait means., Our field surveys revealed that the exotic grass B. diandrus alters both the vertical and horizontal structure creating more dense continuous vegetative growth and dead plant biomass than patches dominated by N. pulchra. These differences in physical structure are responsible for a threefold reduction in available light and likely contribute to the lower diversity, especially of native forbs in B. diandrus-dominated patches. Further, flowering time began earlier and seed size and plant height were higher in B. diandrus patches relative to N. pulchra patches., Our results suggest that species that are better suited (earlier phenology, larger seed size and taller) for low light availability are those that coexist with B. diandrus, and this is consistent with our hypothesis that change in physical structure with B. diandrus invasion is an important driver of community and trait composition., The traits of species able to coexist with invaders are rarely considered when assessing community change following invasion; however, this may be a powerful approach for predicting community change in environments with high anthropogenic pressures, such as disturbance and nutrient enrichment. It also provides a means for selecting species to introduce when trying to enhance native diversity in an otherwise invaded community. [ABSTRACT FROM AUTHOR]

In our 2011 synthesis (Bowman et al., Journal of Biogeography, 2011, 38, 2223-2236), we argued for a holistic approach to human issues in fire science that we term 'pyrogeography'. Coughlan & Petty ( Journal of Biogeography, 2013, 40, 1010-1012) critiqued our paper on the grounds that our 'pyric phase' model was built on outdated views of cultural development, claiming we developed it to be the unifying explanatory framework for all human-fire sciences. Rather, they suggest that 'historical ecology' could provide such a framework. We used the 'pyric transition' for multiple purposes but did not offer it as an exclusive explanatory framework for pyrogeography. Although 'historical ecology' is one of many useful approaches to studying human-fire relationships, scholars should also look to political and evolutionary ecology, ecosystems and complexity theories, as well as empirical generalizations to build an interdisciplinary fire science that incorporates human, ecological and biophysical dimensions of fire regimes. [ABSTRACT FROM AUTHOR]

As the main witnesses of the ecological and economic impacts of invasions on ecosystems around the world, ecologists seek to provide the relevant science that informs managers about the potential for invasion of specific organisms in their region(s) of interest. Yet, the assorted literature that could inform such forecasts is rarely integrated to do so, and further, the diverse nature of the data available complicates synthesis and quantitative prediction. Here we present a set of analytical tools for synthesizing different levels of distributional and/or demographic data to produce meaningful assessments of invasion potential that can guide management at multiple phases of ongoing invasions, from dispersal to colonization to proliferation. We illustrate the utility of data-synthesis and data-model assimilation approaches with case studies of three well-known invasive species--a vine, a marine mussel, and a freshwater crayfish--under current and projected future climatic conditions. Results from the integrated assessments reflect the complexity of the invasion process and show that the most relevant climatic variables can have contrasting effects or operate at different intensities across habitat types. As a consequence, for two of the study species climate trends will increase the likelihood of invasion in some habitats and decrease it in others. Our results identified and quantified both bottlenecks and windows of opportunity for invasion, mainly related to the role of human uses of the landscape or to disruption of the flow of resources. The approach we describe has a high potential to enhance model realism, explanatory insight, and predictive capability, generating information that can inform management decisions and optimize phase-specific prevention and control efforts for a wide range of biological invasions. [ABSTRACT FROM AUTHOR]

Returning native species to habitats degraded by biological invasions is a critical conservation goal. A leading hypothesis poses that exotic plant dominance is self-reinforced by impacts on ecosystem processes, leading to persistent stable states. Invaders have been documented to modify fire regimes, alter soil nutrients or shift microbial communities in ways that feed back to benefit themselves over competitors. However, few studies have followed invasions through time to ask whether ecosystem impacts and feedbacks persist. Here we return to woodland sites in Hawai′i Volcanoes National Park that were invaded by exotic C4 grasses in the 1960s, the ecosystem impacts of which were studied intensively in the 1990s. We show that positive feedbacks between exotic grasses and soil nitrogen cycling have broken down, but rather than facilitating native vegetation, the weakening feedbacks facilitate new exotic species. Data from the 1990s showed that exotic grasses increased nitrogen-mineralization rates by two- to fourfold, but were nitrogen-limited. Thus, the impacts of the invader created a positive feedback early in the invasion. We now show that annual net soil nitrogen mineralization has since dropped to pre-invasion levels. In addition, a seedling outplanting experiment that varied soil nitrogen and grass competition demonstrates that the changing impacts of grasses do not favour native species re-establishment. Instead, decreased nitrogen availability most benefits another aggressive invader, the nitrogen-fixing tree Morella faya. Long-term studies of invasions may reveal that ecosystem impacts and feedbacks shift over time, but that this may not benefit native species recovery. [ABSTRACT FROM AUTHOR]

A common hypothesis to explain the effect of litter mixing is based on the difference in litter N content between mixed species. Although many studies have shown that litter of invasive non-native plants typically has higher N content than that of native plants in the communities they invade, there has been surprisingly little study of mixing effects during plant invasions. We address this question in south China where Mikania micrantha H.B.K., a non-native vine, with high litter N content, has invaded many forested ecosystems. We were specifically interested in whether this invader accelerated decomposition and how the strength of the litter mixing effect changes with the degree of invasion and over time during litter decomposition. Using litterbags, we evaluated the effect of mixing litter of M. micrantha with the litter of 7 native resident plants, at 3 ratios: M1 (1∶4, = exotic:native litter), M2 (1∶1) and M3 (4∶1, = exotic:native litter) over three incubation periods. We compared mixed litter with unmixed litter of the native species to identify if a non-additive effect of mixing litter existed. We found that there were positive significant non-additive effects of litter mixing on both mass loss and nutrient release. These effects changed with native species identity, mixture ratio and decay times. Overall the greatest accelerations of mixture decay and N release tended to be in the highest degree of invasion (mix ratio M3) and during the middle and final measured stages of decomposition. Contrary to expectations, the initial difference in litter N did not explain species differences in the effect of mixing but overall it appears that invasion by M. micrantha is accelerating the decomposition of native species litter. This effect on a fundamental ecosystem process could contribute to higher rates of nutrient turnover in invaded ecosystems. [ABSTRACT FROM AUTHOR]

Alterations in natural fire patterns have negatively affected fire-prone ecosystems in many ways. The historical range of variability (HRV) concept evolved as a management target for natural vegetation composition and fire regimes in fire-prone ecosystems. HRV-based management inherently assumes that ecosystem resilience is reflected in observed ranges of past vegetation and fire dynamics, typically without knowledge of where thresholds exist beyond these dynamics. Given uncertainty in future conditions, some have argued that HRV may not adequately reflect ecosystem resilience to future fire activity. We suggest a refinement that includes concepts from the thresholds of potential concern (TPC) framework, which emphasizes tipping points at the extremes of ecosystem dynamics and other socially unacceptable outcomes. We propose bounded ranges of variation (BRV), an approach focused on building resilience by using historical information, but also by identifying socio-ecological thresholds to avoid and associated management action triggers. Here, we examine nonnative species and carbon sequestration as examples of how the BRV framework could be used in the context of conservation and fire management. [ABSTRACT FROM AUTHOR]

Climate change and biological invasions are primary threats to global biodiversity that may interact in the future. To date, the hypothesis that climate change will favour non-native species has been examined exclusively through local comparisons of single or few species. Here, we take a meta-analytical approach to broadly evaluate whether non-native species are poised to respond more positively than native species to future climatic conditions. We compiled a database of studies in aquatic and terrestrial ecosystems that reported performance measures of non-native (157 species) and co-occurring native species (204 species) under different temperature, CO2 and precipitation conditions. Our analyses revealed that in terrestrial (primarily plant) systems, native and non-native species responded similarly to environmental changes. By contrast, in aquatic (primarily animal) systems, increases in temperature and CO2 largely inhibited native species. There was a general trend towards stronger responses among non-native species, including enhanced positive responses to more favourable conditions and stronger negative responses to less favourable conditions. As climate change proceeds, aquatic systems may be particularly vulnerable to invasion. Across systems, there could be a higher risk of invasion at sites becoming more climatically hospitable, whereas sites shifting towards harsher conditions may become more resistant to invasions. [ABSTRACT FROM AUTHOR]

Non-native, invasive grasses have been linked to altered grass-fire cycles worldwide. Although a few studies have quantified resulting changes in fire activity at local scales, and many have speculated about larger scales, regional alterations to fire regimes remain poorly documented. We assessed the influence of large-scale Bromus tectorum (hereafter cheatgrass) invasion on fire size, duration, spread rate, and interannual variability in comparison to other prominent land cover classes across the Great Basin, USA. We compared regional land cover maps to burned area measured using the Moderate Resolution Imaging Spectroradiometer ( MODIS) for 2000-2009 and to fire extents recorded by the USGS registry of fires from 1980 to 2009. Cheatgrass dominates at least 6% of the central Great Basin (650 000 km2). MODIS records show that 13% of these cheatgrass-dominated lands burned, resulting in a fire return interval of 78 years for any given location within cheatgrass. This proportion was more than double the amount burned across all other vegetation types (range: 0.5-6% burned). During the 1990s, this difference was even more extreme, with cheatgrass burning nearly four times more frequently than any native vegetation type (16% of cheatgrass burned compared to 1-5% of native vegetation). Cheatgrass was also disproportionately represented in the largest fires, comprising 24% of the land area of the 50 largest fires recorded by MODIS during the 2000s. Furthermore, multi-date fires that burned across multiple vegetation types were significantly more likely to have started in cheatgrass. Finally, cheatgrass fires showed a strong interannual response to wet years, a trend only weakly observed in native vegetation types. These results demonstrate that cheatgrass invasion has substantially altered the regional fire regime. Although this result has been suspected by managers for decades, this study is the first to document recent cheatgrass-driven fire regimes at a regional scale. [ABSTRACT FROM AUTHOR]

Background and Aims Global environmental change will affect non-native plant invasions, with profound potential impacts on native plant populations, communities and ecosystems. In this context, we review plant functional traits, particularly those that drive invader abundance (invasiveness) and impacts, as well as the integration of these traits across multiple ecological scales, and as a basis for restoration and management. Scope We review the concepts and terminology surrounding functional traits and how functional traits influence processes at the individual level. We explore how phenotypic plasticity may lead to rapid evolution of novel traits facilitating invasiveness in changing environments and then ‘scale up’ to evaluate the relative importance of demographic traits and their links to invasion rates. We then suggest a functional trait framework for assessing per capita effects and, ultimately, impacts of invasive plants on plant communities and ecosystems. Lastly, we focus on the role of functional trait-based approaches in invasive species management and restoration in the context of rapid, global environmental change. Conclusions To understand how the abundance and impacts of invasive plants will respond to rapid environmental changes it is essential to link trait-based responses of invaders to changes in community and ecosystem properties. To do so requires a comprehensive effort that considers dynamic environmental controls and a targeted approach to understand key functional traits driving both invader abundance and impacts. If we are to predict future invasions, manage those at hand and use restoration technology to mitigate invasive species impacts, future research must focus on functional traits that promote invasiveness and invader impacts under changing conditions, and integrate major factors driving invasions from individual to ecosystem levels. [ABSTRACT FROM PUBLISHER]

Extreme climatic events (ECEs) - such as unusual heat waves, hurricanes, floods, and droughts - can dramatically affect ecological and evolutionary processes, and these events are projected to become more frequent and more intense with ongoing climate change. However, the implications of ECEs for biological invasions remain poorly understood. Using concepts and empirical evidence from invasion ecology, we identify mechanisms by which ECEs may influence the invasion process, from initial introduction through establishment and spread. We summarize how ECEs can enhance invasions by promoting the transport of propagules into new regions, by decreasing the resistance of native communities to establishment, and also sometimes by putting existing non-native species at a competitive disadvantage. Finally, we outline priority research areas and management approaches for anticipating future risks of unwanted invasions following ECEs. Given predicted increases in both ECE occurrence and rates of species introductions around the globe during the coming decades, there is an urgent need to understand how these two processes interact to affect ecosystem composition and functioning. [ABSTRACT FROM AUTHOR]

Many non-native plants in the US have become problematic invaders of native and managed ecosystems, but a new generation of invasive species may be at our doorstep. Here, we review trends in the horticultural trade and invasion patterns of previously introduced species and show that novel species introductions from emerging horticultural trade partners are likely to rapidly increase invasion risk. At the same time, climate change and water restrictions are increasing demand for new types of species adapted to warm and dry environments. This confluence of forces could expose the US to a range of new invasive species, including many from tropical and semiarid Africa as well as the Middle East. Risk assessment strategies have proven successful elsewhere at identifying and preventing invasions, although some modifications are needed to address emerging threats. Now is the time to implement horticulture import screening measures to prevent this new wave of plant invasions. [ABSTRACT FROM AUTHOR]

Humans and their ancestors are unique in being a fire-making species, but 'natural' (i.e. independent of humans) fires have an ancient, geological history on Earth. Natural fires have influenced biological evolution and global biogeochemical cycles, making fire integral to the functioning of some biomes. Globally, debate rages about the impact on ecosystems of prehistoric human-set fires, with views ranging from catastrophic to negligible. Understanding of the diversity of human fire regimes on Earth in the past, present and future remains rudimentary. It remains uncertain how humans have caused a departure from 'natural' background levels that vary with climate change. Available evidence shows that modern humans can increase or decrease background levels of natural fire activity by clearing forests, promoting grazing, dispersing plants, altering ignition patterns and actively suppressing fires, thereby causing substantial ecosystem changes and loss of biodiversity. Some of these contemporary fire regimes cause substantial economic disruptions owing to the destruction of infrastructure, degradation of ecosystem services, loss of life, and smoke-related health effects. These episodic disasters help frame negative public attitudes towards landscape fires, despite the need for burning to sustain some ecosystems. Greenhouse gas-induced warming and changes in the hydrological cycle may increase the occurrence of large, severe fires, with potentially significant feedbacks to the Earth system. Improved understanding of human fire regimes demands: (1) better data on past and current human influences on fire regimes to enable global comparative analyses, (2) a greater understanding of different cultural traditions of landscape burning and their positive and negative social, economic and ecological effects, and (3) more realistic representations of anthropogenic fire in global vegetation and climate change models. We provide an historical framework to promote understanding of the development and diversification of fire regimes, covering the pre-human period, human domestication of fire, and the subsequent transition from subsistence agriculture to industrial economies. All of these phases still occur on Earth, providing opportunities for comparative research. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the long-term effect of grass invasions and subsequent fire in dry submontane habitats in Hawaii in 2011. It revealed that sites that had changed to exotic grassland after a 1970 fire stayed prevailed by exotic grasses with no increase in native cover in spite 37 years of fire suppression. It showed that native species reveal little recovery in burned woodlands in Hawaii in spite decades of fire suppression.

Whether a novel trait of an invader directly contributes to increased establishment of that invader is a relatively unstudied question in plant ecology. Nitrogen (N)-fixing shrubs comprise a significant subset of grassland invaders worldwide, which suggests the potential importance of the novel trait of N-fixation in the invasion process. We indirectly tested the importance of N-fixation in the invasion of Genista monspessulana (French broom) in a California grassland by alleviating N and phosphorus (P) limitation to the grassland matrix. Grassland productivity was co-limited by N and P; N alone did not release the resident vegetation, and did not affect Genista performance. Genista was strongly limited by P: seedlings had more nodules, greater leaf N concentration, and higher growth and survival with P additions. When N was added with P, however, growth of the resident vegetation was 50-70% greater than with N or P alone, accompanied by decreases in Genista performance. This suggests that the advantage conferred to Genista by N-fixation was dampened when the resident vegetation was released from nutrient limitation. [ABSTRACT FROM AUTHOR]

The success of biological control programs is rarely assessed beyond population level impacts on the target organism. The question of whether a biological control agent can either partially or completely restore ecosystem services independent of population level control is therefore still open to discussion. Using observational and experimental approaches, we investigated the ability of the saltcedar leaf beetle [ Diorhabda carinulata (Brullé) (Coleoptera: Chrysomelidae)] to reduce the water use of saltcedar trees ( Tamarix ramosissima Ledeb.) in two sites (Humboldt and Walker Rivers) in Nevada, USA. At these sites D. carinulata defoliated the majority of trees within 25 and 9 km, respectively, of the release location within 3 years. At the Humboldt site, D. carinulata reduced the canopy cover of trees adjacent to the release location by >90%. At a location 4 km away during the first year of defoliation, D. carinulata reduced peak (August) stem water use by 50−70% and stand transpiration (July to late September) by 75% ( P = 0.052). There was, however, no reduction in stem water use and stand transpiration during the second year of defoliation due to reduced beetle abundances at that location. At the Walker site, we measured stand evapotranspiration (ET) in the center of a large saltcedar stand and found that ET was highest immediately prior to D. carinulata arrival, dropped dramatically with defoliation, and remained low through the subsequent 2 years of the study. In contrast, near the perimeter of the stand, D. carinulata did not reduce sap flow, partly because of low rates of defoliation but also because of increased water use per unit leaf area in response to defoliation. Taken together, our results provide evidence that in the early stages of population expansion D. carinulata can lead to substantial declines in saltcedar water use. The extent of these declines varies spatially and temporally and is dependent on saltcedar compensatory responses along with D. carinulata population dynamics and patterns of dispersal. [ABSTRACT FROM AUTHOR]

Invasive plants have often been shown to possess novel traits such as the ability to fix nitrogen, access unused resource pools, or the ability to exude allelopathic chemicals. We describe a case of a successful invasion where the native and non-native species are very similar in most life-history characteristics including their growth forms, lifespan, and degree of summertime activity. Data from permanent transects suggest that exotic perennial grass invaders can establish into intact native-dominated grasslands, achieving cover values from 6 to 71% over several years. We also established a 4-year competition experiment to test the effect of each group—the native and non-native perennial grasses—on the other. Competitive interactions were found to consistently favor the non-native grasses: native perennial grass productivity was significantly lower in plots with exotic perennial grasses as compared to plots without exotic perennial grasses. By contrast, productivity of the exotic perennial grasses was not reduced by the presence of the native perennial grasses. These results suggest that competitive ability, rather than a unique trait, has contributed to the success of the exotic perennial grasses in our system. Management tools to control exotic perennial grass invasions are likely to negatively influence native perennial grass populations, as strategies that succeed against the invasive species may kill or reduce the native species as well. [ABSTRACT FROM AUTHOR]