Invasive annual grass relationships to environment and grazing vary with species identity

Contrasting Responses of Intermountain West Grasses to Soil Nitrogen

Contrasting responses of Intermountain West grasses to soil nitrogen

A high relative growth rate (RGR) is thought to be an important trait allowing invasive annual grasses to exploit brief increases in nitrogen (N) supply following disturbance in the Intermountain West. Managing soils for low N availability has been suggested as a strategy that may reduce this growth advantage of annual grasses and facilitate establishment of desirable perennials grasses. The objective of this study was to examine the degree to which soil N availability affects RGR and RGR components of invasive annual and desirable perennial grasses. It was hypothesized that (1) invasive annual grasses would demonstrate a proportionately greater reduction in RGR than perennial grasses as soil N stress increased, and (2) the mechanism by which low N availability decreases RGR of annual and perennial grasses would depend on the severity of N stress, with moderate N stress primarily affecting leaf mass ratio (LMR) and severe N stress primarily affecting net assimilation rate (NAR). Three annual and three perennial grasses were exposed to three levels of N availability. RGR and components of RGR were quantified over four harvests. Moderate N stress reduced RGR by decreasing LMR and severe N stress lowered RGR further by decreasing NAR. However, reduction in RGR components was similar between invasive and natives, and as a consequence, annual grasses did not demonstrate a proportionately greater reduction in RGR than perennials under low N conditions. These results suggest managing soil N will do little to reduce the initial growth advantage of annual grasses. Once perennials establish, traits not captured in this short-term study, such as high tissue longevity and efficient nutrient recycling, may allow them to compete effectively with annuals under low N availability. Nevertheless, if soil N management does not facilitate the initial establishment of perennials in annual grass infested communities, then there is little likelihood that such techniques will provide a long-term benefit to restoration projects in these systems.

Recent environmental change associated with human activities has given rise to ecological communities that have no historical counterpart. In particular, introductions of non-native plant species have in many cases altered the structure and functioning of resident plant communities through changes in species composition and the surrounding environment within which species interact. As a result, new combinations of species are forming “novel” communities across an increasingly large portion of the earth’s land surface. Because novel plant communities differ in configuration from original native-dominated communities, they present unique challenges to management, restoration, and conservation efforts. Thus, there is a growing need to understand how novel communities function differently from the original communities they replace. In this thesis, I investigate a variety of interactions in original and novel plant communities. Using a diverse annual plant system that persists within a fragmented agricultural landscape in Western Australia, I focus on the role of local-scale species interactions, an important biotic component of plant community assembly. I explore the complexities of local-scale interactions between native and non-native invasive species in light of coexistence theory, community assembly, and conservation of native floral diversity. This thesis comprises seven chapters. The first chapter serves as a general introduction which places the thesis within the larger context of multispecies coexistence in novel plant communities. The second chapter serves as a description of the York gum (Eucalyptus loxophleba subsp. loxophleba) – jam (Acacia acuminata) woodland annual flora, the study system for the data chapters (3 – 6) which are based on laboratory and field experiments. Chapters 3 and 4 are experimental evaluations of frequency-dependent and density-dependent performance of native and invasive species that co-occur in York gum-jam annual plant communities. Chapter 5 reports on a field experiment, which investigates the performance of common native and non-native invasive annuals experiencing inter- and intraspecific competition in natural York gum-jam annual assemblages. Chapter 5 also assesses changes in community-level functioning due to compositional differences by evaluating diversity effects in novel and original annual communities. The final data chapter, Chapter 6, experimentally investigates how local-scale environmental gradients and a non-native invasive annual grass impacts annual plant community structure in the field. Finally, I conclude with a discussion of my results in Chapter 7, which unites the previous chapters, addresses limitations of the thesis, and presents suggestions for future research. Overall, my results suggest that the local-scale impacts of non-native invasive species on native species may be more variable than those often reported in the literature on plant invasions (i.e. competitive exclusion). Consistent with previous studies, I did indeed observe negative interactions among invasive non-native and native annual plant species in field and laboratory settings. Specifically, I found that some species of invasive annual grass have the potential to negatively impact native populations over very short timescales through direct competition as well as interference from litter. These negative interactions, however, were not representative of interactions in novel communities as a whole. Notably, I found interactions between native annual forbs and an invasive non-native annual grass that ranged from neutral to positive. The direction and magnitude of invader impacts were highly dependent on species identity and the composition of the community, but were generally consistent across community densities and natural environmental gradients. Species interactions play a potentially complex role in the assembly of annual plant communities post-invasion. My results lend empirical support to the notion that species in these novel communities should be considered according to their impacts rather than their origins. These studies serve as some of the first investigations into the processes that stabilize interactions among native and invasive non-native species and contribute to novel community formation and maintenance. When considered alongside large-scale patterns from observational studies, my findings demonstrate that interactions that occur over small spatial and temporal scales have the potential to influence large-scale plant community dynamics. In total, this thesis represents a valuable contribution to the community ecology and biological invasions literature, and has the potential to inform future restoration and conservation efforts in this threatened woodland ecosystem and beyond.

Invasion of annual grasses following wildfire corresponds to maladaptive habitat selection by a sagebrush ecosystem indicator species

ABSTRACTBackground: Annual grass invasion and dominance creates a self-sustaining cycle that promotes wildfires and reduces forage abundance. Restoring native plant species to annual grass-invaded ecosystems is fundamental to fostering self-sustaining native plant communities.Aims: We sought to clarify varying restoration strategies on invasive annual and native perennial grass assembly, including strategically modified seeding times, seeding rates, and added water.Methods: We tested the effects of seeding perennial grasses in autumn, spring, or seeding half in autumn and the remaining half in spring, adding water, and varying annual and perennial grass seeding rates on annual and perennial grass life history.Results: While varying perennial grass seeding times did not affect perennial grass germination rates, annual grass germination rates were highest when perennial grasses were seeded in autumn. Seeding perennial grasses in spring produced the highest adult perennial grass density in the first-growing season, but adult perennial grass density in the second-growing season was greatest when seeding occurred in autumn. Second-growing season perennial grass density was highest where annual grass seeding rates were lowest and perennial grass seeding rates were highest. Adding water in the first-growing season produced almost two-times more second-year adult perennial grasses compared to where water was not added.Conclusions: High water availability during the seeding year appears to be the most important factor for retaining perennial grasses in annual grass-invaded ecosystems.

Theoretical and empirical work has established a positive relationship between resource availability and habitat invasibility. For nonnative invasive annual grasses, similar to other invasive species, invader success has been tied most often to increased nitrogen (N) availability. These observations have led to the logical assumption that managing soils for low N availability will facilitate restoration of invasive plant-dominated systems. Although invasive annual grasses pose a serious threat to a number of perennial-dominated ecosystems worldwide, there has been no quantitative synthesis evaluating the degree to which soil N management may facilitate restoration efforts. We used meta-analysis to evaluate the degree to which soil N management impacts growth and competitive ability of annual and perennial grass seedlings. We then link our analysis to current theories of plant ecological strategies and community assembly to improve our ability to understand how soil N management may be used to restore annual grass-dominated communities. Across studies, annual grasses maintained higher growth rates and greater biomass and tiller production than perennials under low and high N availability. We found no evidence that lowering N availability fundamentally alters competitive interactions between annual and perennial grass seedlings. Competitive effects of annual neighbors on perennial targets were similar under low and high N availability. Moreover, in most cases perennials grown under competition in high-N soils produced more biomass than perennials grown under competition in low-N soils. While these findings counter current restoration and soil N management assumptions, these results are consistent with current plant ecological strategy and community assembly theory. Based on our results and these theories we argue that, in restoration scenarios in which the native plant community is being reassembled from seed, soil N management will have no direct positive effect on native plant establishment unless invasive plant propagule pools and priority effects are controlled the first growing season.

Sagebrush ecosystems of western North America are experiencing widespread loss and degradation by invasive annual grasses. Positive feedbacks between fire and annual grasses are often invoked to explain the rapid pace of these changes, yet annual grasses also appear capable of achieving dominance among vegetation communities that have not burned for many decades. Using a dynamic, remotely-sensed vegetation dataset in tandem with remotely-sensed fire perimeter and burn severity datasets, we examine the role of fire in transitions to and persistence of annual grass dominance in the U.S. Great Basin over the past 3 decades. Although annual grasses and wildfire are so tightly associated that one is rarely mentioned without the other, our findings reveal surprisingly widespread transformation of sagebrush ecosystems by invasive annual grasses in the absence of fire. These findings are discussed in the context of strategic management; we argue a pivot from predominantly reactive management (e.g., aggressive fire suppression and post-fire restoration in heavily-infested areas) to more proactive management (e.g., enhancing resistance and managing propagule pressure in minimally-invaded areas) is urgently needed to halt the loss of Great Basin sagebrush ecosystems.

For nearly a century, invasive annual grasses have increasingly impacted terrestrial ecosystems across the western United States. Weather variability associated with climate change and increased atmospheric carbon dioxide (CO₂) are making even more difficult the challenges of managing invasive annual grasses. As part of a special issue on climate change impacts on soil and water conservation, the topic of invasive annual grasses is being addressed by scientists at the USDA Agricultural Research Service to emphasize the need for additional research and future studies that build on current knowledge and account for (extreme) changes in abiotic and biotic conditions. Much research has focused on understanding the mechanisms underlying annual grass invasion, as well as assessing patterns and responses from a wide range of disturbances and management approaches. Weather extremes and the increasing occurrences of wildfire are contributing to the complexity of the problem. In broad terms, invasive annual grass management, including restoration, must be proactive to consider human values and ecosystem resiliency. Models capable of synthesizing vast amounts of diverse information are necessary for creating trajectories that could result in the establishment of perennial systems. Organization and collaboration are needed across the research community and with land managers to strategically develop and implement practices that limit invasive annual grasses. In the future, research will need to address invasive annual grasses in an adaptive integrated weed management (AIWM) framework that utilizes models and accounts for climate change that is resulting in altered/new approaches to management and restoration.

Can biological invasions induce desertification?

Fire and fuel management is a high priority in North American sagebrush ecosystems where the expansion of piñon and juniper trees and the invasion of nonnative annual grasses are altering fire regimes and resulting in loss of sagebrush species and habitat. We evaluated 10‐yr effects of woody fuel treatments on sagebrush recruitment and plant functional group interactions using Sagebrush Steppe Treatment Evaluation Project data. We used mixed‐effects ANOVAs to examine treatment effects on sagebrush density and cover and perennial and annual grass cover in expansion woodlands (prescribed fire and cut‐and‐leave) and annual grass invasion areas (prescribed fire, mowing, tebuthiuron herbicide application). We used piecewise structural equation models to evaluate interactions among sagebrush seedling density, juvenile and adult density, and cover and perennial and annual grass cover. Fuel treatments were equated to pulse or press disturbances varying in resource release and subsequent intra‐ and interspecific interactions. Prescribed fire, a high magnitude pulse disturbance with more severe effects in warm and dry sites, reduced sagebrush cover and decoupled associations among sagebrush seedlings, juvenile and adult density, and cover indicating changed population structure. Cutting and leaving trees, a low magnitude pulse disturbance in cooler and moister woodlands, increased sagebrush density and cover and generally had lesser effects on sagebrush intraspecific associations. Mowing, a moderate magnitude pulse disturbance, and tebuthiuron herbicide application, a multiyear press disturbance, reduced sagebrush cover and disrupted intraspecific relationships. Competitive release increased cover of perennial grass in all treatments but tebuthiuron. Annual grass increased in all treatments, especially prescribed fire and tebuthiuron. Annual and perennial grass interactions with sagebrush were generally rare, but in woodland treatments perennial grass suppressed annual grass through year 6. Treatments in cooler and moister woodland sites had more positive effects on sagebrush recruitment and perennial grass cover, less negative effects on sagebrush intraspecific interactions, and smaller increases in annual grass cover indicating potential increases in resilience to fire. In warmer and drier invasion sites, reductions in woody fuels resulted in lack of sagebrush recruitment, disruption of sagebrush intraspecific interactions, and progressive increases in annual grass indicating reduced resilience to fire and resistance to invaders.

Modeling Invasive Annual Grass Abundance in the Cold Desert Ecoregions of the Interior Western United States

Theoretical and empirical work has established a positive relationship between resource availability and habitat invasibility. For nonnative invasive annual grasses, similar to other invasive species, invader success has been tied most often to increased nitrogen (N) availability. These observations have led to the logical assumption that managing soils for low N availability will facilitate restoration of invasive plant-dominated systems. Although invasive annual grasses pose a serious threat to a number of perennial-dominated ecosystems worldwide, there has been no quantitative synthesis evaluating the degree to which soil N management may facilitate restoration efforts. We used meta-analysis to evaluate the degree to which soil N management impacts growth and competitive ability of annual and perennial grass seedlings. We then link our analysis to current theories of plant ecological strategies and community assembly to improve our ability to understand how soil N management may be used to restore annual grass-dominated communities. Across studies, annual grasses maintained higher growth rates and greater biomass and tiller production than perennials under low and high N availability. We found no evidence that lowering N availability fundamentally alters competitive interactions between annual and perennial grass seedlings. Competitive effects of annual neighbors on perennial targets were similar under low and high N availability. Moreover, in most cases perennials grown under competition in high-N soils produced more biomass than perennials grown under competition in low-N soils. While these findings counter current restoration and soil N management assumptions, these results are consistent with current plant ecological strategy and community assembly theory. Based on our results and these theories we argue that, in restoration scenarios in which the native plant community is being reassembled from seed, soil N management will have no direct positive effect on native plant establishment unless invasive plant propagule pools and priority effects are controlled the first growing season.

Estimates of fine fuel litter biomass in the northern Great Basin reveal increases during short fire-free intervals associated with invasive annual grasses

Effects of Agricultural Terraces on the Reestablishment of Bluestem Grasslands