Managing soil nitrogen to restore annual grass-infested plant communities: effective strategy or incomplete framework?

Vascular plant communities and biocrusts act as controlling factors in mitigating soil erosion on the Great Wall in a semi-humid area of Northwestern China

The Great Wall, a World Heritage Site and a vertical wall habitat, is under threat of soil erosion. The role of vascular plants and biocrust in controlling soil erosion has attracted attention, yet our knowledge of the underlying mechanism is limited, and there is a lack of systematic strategies for erosion prevention and control. In this study, we quantified the vascular plant community functional composition (including species diversity, functional diversity, and community-weighted mean), biocrust coverage, and soil erosion levels associated with seven different zones (lower, middle, and upper zones on East and West faces, plus wall crest) of the Great Wall. We then employed a combination of linear regression analysis, random forest model, and structural equation model to evaluate the individual and combined effects, as well as the direction and relative importance of these factors in reducing soil erosion. The results indicated that the vascular plant species richness, species diversity, functional richness, community-weighted mean, and moss crust coverage decreased significantly from the crest to the lower zone of the Great Wall (P < 0.05), and were negatively correlated with the soil erosion area and depth on both sides of the Great Wall (P < 0.05). This suggests that higher zones on the wall favored the colonization and growth of biocrusts and vascular plants and that biocrusts and vascular plants reduced soil erosion on the wall. Based on these findings, we propose a "restoration framework" for managing soil erosion on walls, based on biocrust and vascular plant communities (namely target species selection, plant community construction, biocrust inoculation, and maintenance of community stability), which aims to address the urgent need for more effective soil erosion prevention and control strategies on the Great Wall and provide practical methods that practitioners can utilize.

The Legacy of Plant Invasion: Impacts on Soil Nitrification and Management Implications

Plant invasions can have long-lasting impacts on soil nitrification, which plays a critical role in nutrient cycling and plant growth. This review examines the legacy effects of plant invasion on soil nitrification, focusing on the underlying mechanisms, context dependence, and implications for management. We synthesize literature on the positive, negative and neutral legacy effects of plant invasion on soil nitrification, highlighting the complexity of these effects and the need for further research to fully understand them. Positive legacy effects include increased soil microbial biomass or activity, potentially enhancing nutrient availability for plants. However, negative legacy effects, like reduced nitrifier abundance, can result in decreased soil nitrification rates and nutrient availability. In some cases, changes to nitrification during active invasion appear transitory after the removal of invasive plants, indicating neutral short-term legacies. We discuss the context dependence of legacy effects considering factors, including location, specific invasive plant species, and other environmental conditions. Furthermore, we discuss the implications of these legacy effects for management and restoration strategies, such as the removal or control of invasive plants, and potential approaches for restoring ecosystems with legacy effects on soil nitrification. Finally, we highlight future research directions, including further investigation into the mechanisms and context dependence of legacy effects, and the role of plant-microbe interactions. Overall, this review provides insights into the legacy effects of plant invasion on soil nitrification and their implications for ecosystems.

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

Exotic annual grasses invasion across northern Great Basin rangelands has promoted a grass-fire cycle that threatens the sagebrush (Artemisia spp.) steppe ecosystem. In this sense, high accumulation rates and persistence of litter from annual species largely increase the amount and continuity of fine fuels. Here, we highlight the potential use and transferability of remote sensing-derived products to estimate litter biomass on sagebrush rangelands in southeastern Oregon, and link fire regime attributes (fire-free period) with litter biomass spatial patterns at the landscape scale. Every June, from 2018 to 2021, we measured litter biomass in 24 field plots (60 m × 60 m). Two remote sensing-derived datasets were used to predict litter biomass measured in the field plots. The first dataset used was the 30-m annual net primary production (NPP) product partitioned into plant functional traits (annual grass, perennial grass, shrub, and tree) from the Rangeland Analysis Platform (RAP). The second dataset included topographic variables (heat load index -HLI- and site exposure index -SEI-) computed from the USGS 30-m National Elevation Dataset. Through a frequentist model averaging approach (FMA), we determined that the NPP of annual and perennial grasses, as well as HLI and SEI, were important predictors of field-measured litter biomass in 2018, with the model featuring a high overall fit (R² = 0.61). Model transferability based on extrapolating the FMA predictive relationships from 2018 to the following years provided similar overall fits (R² ≈ 0.5). The fire-free period had a significant effect on the litter biomass accumulation on rangelands within the study site, with greater litter biomass in areas where the fire-free period was <10 years. Our findings suggest that the proposed remote sensing-derived products could be a key instrument to equip rangeland managers with additional information towards fuel management, fire management, and restoration efforts.

Toward more robust plant-soil feedback research

Understanding if and how plant-soil biota feedbacks (PSFs) shape plant communities has become a major research priority. In this paper, we draw on a recent, high-profile PSF study to illustrate that certain widely used experimental methods cannot reliably determine if PSFs occur. One problem involves gathering soil samples adjacent to multiple conditioning plants, mixing the samples and then growing phytometers in the mixtures to test for PSFs. This mixed soil approach does not establish that the conditioning plant being present caused the soil biota to be present, the first step of a PSF. Also, soil mixing approximates replacing raw data with averages prior to analysis, a move certain to generate falsely precise statistical estimates. False precision also results from sample sizes being artificially inflated when phytometers are misinterpreted as experimental units. Plant biomass ratios become another source of false precision when individual plant values contribute to multiple ratio observations. Any one of these common missteps can cause still living null hypotheses to be pronounced dead, and risks of this increase with numbers of missteps. If soil organisms truly structure plant communities, then null hypotheses indicating otherwise will not survive proper testing. We discuss conceptual, experimental and analytical refinements to facilitate accurate testing.

Plant-driven changes in soil microbial communities influence seed germination through negative feedbacks

Plant-soil feedbacks (PSFs) drive plant community diversity via interactions between plants and soil microbes. However, we know little about how frequently PSFs affect plants at the seed stage, and the compositional shifts in fungi that accompany PSFs on germination.We conducted a pairwise PSF experiment to test whether seed germination was differentially impacted by conspecific versus heterospecific soils for seven grassland species. We used metagenomics to characterize shifts in fungal community composition in soils conditioned by each plant species. To investigate whether changes in the abundance of certain fungal taxa were associated with multiple PSFs, we assigned taxonomy to soil fungi and identified putative pathogens that were significantly more abundant in soils conditioned by plant species that experienced negative or positive PSFs.We observed negative, positive, and neutral PSFs on seed germination. Although conspecific and heterospecific soils for pairs with significant PSFs contained host-specialized soil fungal communities, soils with specialized microbial communities did not always lead to PSFs. The identity of host-specialized pathogens, that is, taxa uniquely present or significantly more abundant in soils conditioned by plant species experiencing negative PSFs, overlapped among plant species, while putative pathogens within a single host plant species differed depending on the identity of the heterospecific plant partner. Finally, the magnitude of feedback on germination was not related to the degree of fungal community differentiation between species pairs involved in negative PSFs. Synthesis. Our findings reveal the potential importance of PSFs at the seed stage. Although plant species developed specialized fungal communities in rhizosphere soil, pathogens were not strictly host-specific and varied not just between plant species, but according to the identity of plant partner. These results illustrate the complexity of microbe-mediated interactions between plants at different life stages that next-generation sequencing can begin to unravel.

Characterization of Adult Functional Traits of Local Populations and Cultivars of Sandberg Bluegrass and Bottlebrush Squirreltail Perennial Bunchgrasses

Plant functional traits offer an understanding of the plant's ability to cope with varying environmental impositions. The objective of this study was to evaluate the above and belowground adult morphological and chemical composition traits of local populations of Sandberg bluegrass (Poa secunda J. Presl) and Bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey) collected in Nevada and their cultivated varieties. A total of six replications (one seedling each) from each population and cultivar of the two native perennial bunchgrasses were used in a randomized complete block design experiment. Each of the six seedlings from each sourced population was transplanted into individual tree pots (28 cm diameter × 61 cm height) containing 20.4 kg of air-dried Orr gravelly sandy loam soil in mid-November, 2015 and remained in the pots for the duration of the study (23 June, 2016). Traits evaluated were, plant height, leaf length, inflorescence length, shoot biomass, forage nutritive value, root morphological traits, and root carbon and nitrogen content. Traits means were considered different at P < 0.05. For Sandberg bluegrass, the cultivar 'Mountain Home' and the population from Panther Valley tended to have greater biomass than the population from Button Point but overall, the average of the two cultivars (10.8 g/plant) did not differ in shoot biomass relative to the local populations (7.6 g/plant). For squirreltail, plant height for the George St. Sonoma and Grass Valley populations (71.3 cm) was greater than the cultivars 'Toe Jam Creek' and 'Vale' (40.5 cm) but cultivars had greater biomass (12.6 g/plant) than the local populations (5.8 g/plant). Total root length and root diameter were not different among the Sanberg bluegrass and squirreltail populations. The results from traits expounded on in this study indicate the closeness of these populations for both species at their adult stage and provide insights for building a unified framework approach among the different agencies and restoration practitioners to aid in plant assemblages for restoration success in the Great Basin and beyond.

Growth and competitive interaction between seedlings of an invasive Rumex confertus and of co-occurring two native Rumex species in relation to nutrient availability

Rumex confertus is an alien invasive perennial plant that has increased its range rapidly within central Europe in the last 100 years. This study examined the effects of a commercial fertilizer on the competition between the invasive Rumex confertus and two non-invasive native species R. acetosa or R. conglomeratus in terms of morphological and physiological traits and relative yield. All three Rumex species were grown in the open field with two levels of nutrient availability in field plots. Competition and fertilizer had significant effects on height, relative growth rate (RGR), specific leaf area (SLA) as well as shoot and root biomass of all three species. The fertilized plants had high macronutrient and nitrate contents in leaf tissue. Relative yield of R. confertus was <1, indicating that for this species the effects of interspecific competition were greater than those of intraspecific competition. The results of this experiment indicate that there is interaction between the nutrient status of the soil and the competition between species. Competitive superiority of R. confertus could explain its dominance in grasslands and in disturbed areas, and might explain its great influence on the occurrence of native species because competition intensity was high in fertilized plots.

Refining the cheatgrass-fire cycle in the Great Basin: Precipitation timing and fine fuel composition predict wildfire trends

Larger, more frequent wildfires in arid and semi-arid ecosystems have been associated with invasion by non-native annual grasses, yet a complete understanding of fine fuel development and subsequent wildfire trends is lacking. We investigated the complex relationships among weather, fine fuels, and fire in the Great Basin, USA. We first modeled the annual and time-lagged effects of precipitation and temperature on herbaceous vegetation cover and litter accumulation over a 26-year period in the northern Great Basin. We then modeled how these fine fuels and weather patterns influence subsequent wildfires. We found that cheatgrass cover increased in years with higher precipitation and especially when one of the previous 3 years also was particularly wet. Cover of non-native forbs and native herbs also increased in wet years, but only after several dry years. The area burned by wildfire in a given year was mostly associated with native herb and non-native forb cover, whereas cheatgrass mainly influenced area burned in the form of litter derived from previous years' growth. Consequently, multiyear weather patterns, including precipitation in the previous 1-3 years, was a strong predictor of wildfire in a given year because of the time needed to develop these fine fuel loads. The strong relationship between precipitation and wildfire allowed us to expand our inference to 10,162 wildfires across the entire Great Basin over a 35-year period from 1980 to 2014. Our results suggest that the region's precipitation pattern of consecutive wet years followed by consecutive dry years results in a cycle of fuel accumulation followed by weather conditions that increase the probability of wildfire events in the year when the cycle transitions from wet to dry. These patterns varied regionally but were strong enough to allow us to model annual wildfire risk across the Great Basin based on precipitation alone.

Managing competitive interactions to promote regeneration of native perennial grasses in semi-arid south-eastern Australia

Treatments to reduce available soil nitrogen and achieve specified levels of weed control were evaluated for their capacity to promote regeneration of native perennial grasses in a degraded semi-arid woodland in central-western New South Wales. Treatments were factorial combinations of nitrogen-reduction levels and weed-control levels. The four levels of nitrogen reduction were no intervention, and oversowing of an unfertilised summer crop, an unfertilised winter crop or an unfertilised perennial grass. The three weed-control levels were defined by the outcome sought rather than the chemical applied and were nil, control of annual legumes and control of all annual species (AA). Regeneration of perennial grasses, predominantly Enteropogon acicularis, was promoted most rapidly by the AA level of weed control with no introduction of sown species. Sown species negated the benefits of weed control and limited but did not prevent the regeneration of native perennials. Sown species also contributed substantially to biomass production, which was otherwise severely limited under the AA level of weed control, and they were effective in reducing soil nitrogen availability. Sown species in combination with appropriate herbicide use can therefore maintain or increase available forage in the short–medium term, permit a low rate of native perennial grass recruitment, and condition the system (by reducing soil mineral nitrogen) for more rapid regeneration of native perennials should annual sowings be discontinued or a sown grass fail to persist. Soil nitrate was reduced roughly in proportion to biomass production. High levels of soil nitrate did not inhibit native perennial grass regeneration when biomass was suppressed by AA weed control, and may be beneficial for pastoral production, but could also render sites more susceptible to future invasion of exotic annuals. The need for astute grazing management of the restored grassland is thus emphasised. This study was conducted on a site that supported a remnant population of perennial grasses. Use of the nitrogen-reduction techniques described may not be appropriate on sites where very few perennial grass plants remain.

Invasive non-native plants have a greater effect on neighbouring natives than other non-natives

Human activity is creating a global footprint by changing the climate, altering habitats and reshuffling the distribution of species. The movement of species around the globe has led to the naturalization and accumulation of multiple non-native species within ecosystems, which is frequently associated with habitat disturbance and changing environmental conditions. However, interactions among species will also influence community composition, but little is known about the full range of direct and indirect interactions among native and non-native species. Here, we show through a meta-analysis of 1,215 pairwise plant interactions between 274 vascular plant species in 21 major habitat types that interactions between non-native plants are asymmetrical with interactions between non-native and native plants. Non-native plants were always bad neighbours, but the negative effect of non-natives on natives was around two times greater than the effect of non-natives on other non-natives. In contrast, the performance of non-native plants was five times higher in the presence of a neighbouring native plant species than in the presence of a neighbouring non-native plant species. Together, these results demonstrate that invaded plant communities may accumulate additional non-native species even if direct interactions between non-natives species are negative. Put another way, invasions may be more likely to lead to more invasions, requiring more active management of ecosystems by promoting native species restoration to undermine invasive positive feedback and to assist native species recovery in invaded ecosystems.

Cheatgrass is favored by warming but not CO2 enrichment in a semi-arid grassland

Elevated CO2 and warming may alter terrestrial ecosystems by promoting invasive plants with strong community and ecosystem impacts. Invasive plant responses to elevated CO2 and warming are difficult to predict, however, because of the many mechanisms involved, including modification of phenology, physiology, and cycling of nitrogen and water. Understanding the relative and interactive importance of these processes requires multifactor experiments under realistic field conditions. Here, we test how free-air CO2 enrichment (to 600 ppmv) and infrared warming (+1.5 °C day/3 °C night) influence a functionally and phenologically distinct invasive plant in semi-arid mixed-grass prairie. Bromus tectorum (cheatgrass), a fast-growing Eurasian winter annual grass, increases fire frequency and reduces biological diversity across millions of hectares in western North America. Across 2 years, we found that warming more than tripled B. tectorum biomass and seed production, due to a combination of increased recruitment and increased growth. These results were observed with and without competition from native species, under wet and dry conditions (corresponding with tenfold differences in B. tectorum biomass), and despite the fact that warming reduced soil water. In contrast, elevated CO2 had little effect on B. tectorum invasion or soil water, while reducing soil and plant nitrogen (N). We conclude that (1) warming may expand B. tectorum's phenological niche, allowing it to more successfully colonize the extensive, invasion-resistant northern mixed-grass prairie, and (2) in ecosystems where elevated CO2 decreases N availability, CO2 may have limited effects on B. tectorum and other nitrophilic invasive species.

The Effect of Nitrogen Deposition on Plant Performance and Community Structure: Is It Life Stage Specific?

Nitrogen (N) deposition is a key global change factor that is increasing and affecting the structure and function of many ecosystems. To determine the influence of N deposition on specific systems, however, it is crucial to understand the temporal and spatial patterns of deposition as well as the response to that deposition. Response of the receiving plant communities may depend on the life stage-specific performance of individual species. We focus on the California oak savanna because N deposition to this system is complex—characterized by hotspots on the landscape and seasonal pulses. In a greenhouse experiment, we investigated the relative influence of N deposition on plant performance during early growth, peak biomass, and senescent life stages across different soil types, light, and community compositions. To represent the community we used three grass species—a native, naturalized exotic, and invasive exotic. At early growth and peak biomass stages performance was measured as height, and shoot and root biomass, and at the senescent stage as seed production. Simulated N deposition 1) increased shoot biomass and height of the native and, even more so, the naturalized exotic during early growth, 2) positively affected root biomass in all species during peak biomass, and 3) had no influence on seed production at the senescent stage. Alone, N deposition was not a strong driver of plant performance; however, small differences in performance among species in response to N deposition could affect community composition in future years. In particular, if there is a pulse of N deposition during the early growth stage, the naturalized exotic may have a competitive advantage that could result in its spread. Including spatial and temporal heterogeneity in a complex, manipulative experiment provides a clearer picture of not only where N management efforts should be targeted on the landscape, but also when.

Plant communities in harsh sites are less invaded: a summary of observations and proposed explanations

Plant communities in abiotically stressful, or 'harsh', habitats have been reported to be less invaded by non-native species than those in more moderate habitats. Here, we synthesize descriptive and experimental evidence for low levels of invasion in habitats characterized by a variety of environmental stressors: low nitrogen; low phosphorus; saline, sodic or alkaline soils; serpentine soils; low soil moisture; shallow/rocky soils; temporary inundation; high shade; high elevation; and high latitude. We then discuss major categories of hypotheses to explain this pattern: the propagule limitation mechanism suggests invasion of harsh sites is limited by relatively low arrival rates of propagules compared with more moderate habitats, while invasion resistance mechanisms suggest that harsh habitats are inherently less invasible due to stressful abiotic conditions and/or increased effects of biotic resistance from resident organisms. Both propagule limitation and invasion resistance may simultaneously contribute to low invadedness of harsh sites, but the management implications of these mechanisms differ. If propagule limitation is more important, managers should focus on reducing the likelihood of propagule introductions. If invasion resistance mechanisms are in play, managers should focus on restoring or maintaining harsh conditions at a site to reduce invasibility.

Differing effects of biosolids on native plants in grasslands of southern british columbia

The objective of this study was to determine if application of biosolids is beneficial for restoring semiarid grasslands. The effects of a one-time surface application of biosolids at a rate of 20 Mg ha on individual plant species and plant community composition were examined at three degraded semiarid grassland sites located in the southern interior of British Columbia, Canada. Biosolids application did not result in desirable changes in plant species composition at the two drier sites (with annual precipitation <400 mm) yet led to overall positive plant species changes at the least dry site (with annual precipitation of 400 mm). An important late-seral species of semiarid grasslands, bluebunch wheatgrass [ (Pursh) Á. Löve], did not respond or decreased at the two drier sites but increased at the least dry site. Exotic invasive plant species increased on all sites. The total vegetative cover of native perennial grasses increased from 41 to 99% at the least dry site, with important decreases in low-growing, early-seral forb species. Although cheatgrass ( L.) was not significantly increased by the biosolids treatment, the dominance of cheatgrass and lower growing season precipitation at the two drier sites were likely key reasons for the poor biosolids restoration success at these sites. Despite some concerns, there is potential to use biosolids to restore grassland plant communities successfully within 4 or 5 yr on more mesic grassland sites; however, biosolids use on drier sites where exotic invasives are present cannot be recommended.

The physiology of invasive plants in low-resource environments

While invasive plant species primarily occur in disturbed, high-resource environments, many species have invaded ecosystems characterized by low nutrient, water, and light availability. Species adapted to low-resource systems often display traits associated with resource conservation, such as slow growth, high tissue longevity, and resource-use efficiency. This contrasts with our general understanding of invasive species physiology derived primarily from studies in high-resource environments. These studies suggest that invasive species succeed through high resource acquisition. This review examines physiological and morphological traits of native and invasive species in low-resource environments. Existing data support the idea that species invading low-resource environments possess traits associated with resource acquisition, resource conservation or both. Disturbance and climate change are affecting resource availability in many ecosystems, and understanding physiological differences between native and invasive species may suggest ways to restore invaded ecosystems.

A functional trait perspective on plant invasion

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.

Trait convergence and plasticity among native and invasive species in resource-poor environments

PREMISE OF STUDY

Functional trait comparisons provide a framework with which to assess invasion and invasion resistance. However, recent studies have found evidence for both trait convergence and divergence among coexisting dominant native and invasive species. Few studies have assessed how multiple stresses constrain trait values and plasticity, and no study has included direct measurements of nutrient conservation traits, which are critical to plants growing in low-resource environments.

METHODS

We evaluated how nutrient and water stresses affect growth and allocation, water potential and gas exchange, and nitrogen (N) allocation and use traits among a suite of six codominant species from the Intermountain West to determine trait values and plasticity. In the greenhouse, we grew our species under a full factorial combination of high and low N and water availability. We measured relative growth rate (RGR) and its components, total biomass, biomass allocation, midday water potential, photosynthetic rate, water-use efficiency (WUE), green leaf N, senesced leaf N, total N pools, N productivity, and photosynthetic N use efficiency.

KEY RESULTS

Overall, soil water availability constrained plant responses to N availability and was the major driver of plant trait variation in our analysis. Drought decreased plant biomass and RGR, limited N conservation, and led to increased WUE. For most traits, native and nonnative species were similarly plastic.

CONCLUSIONS

Our data suggest native and invasive biomass dominants may converge on functionally similar traits and demonstrate comparable ability to respond to changes in resource availability.