Jeremy J. James

Linking nitrogen partitioning and species abundance to invasion resistance in the Great Basin

Resource partitioning has been suggested as an important mechanism of invasion resistance. The relative importance of resource partitioning for invasion resistance, however, may depend on how species abundance is distributed in the plant community. This study had two objectives. First, we quantified the degree to which one resource, nitrogen (N), is partitioned by time, depth and chemical form among coexisting species from different functional groups by injecting 15N into soils around the study species three times during the growing season, at two soil depths and as two chemical forms. A watering treatment also was applied to evaluate the impact of soil water content on N partitioning. Second, we examined the degree to which native functional groups contributed to invasion resistance by seeding a non-native annual grass into plots where bunchgrasses, perennial forbs or annual forbs had been removed. Bunchgrasses and forbs differed in timing, depth and chemical form of N capture, and these patterns of N partitioning were not affected by soil water content. However, when we incorporated abundance (biomass) with these relative measures of N capture to determine N sequestration by the community there was no evidence suggesting that functional groups partitioned different soil N pools. Instead, dominant bunchgrasses acquired the most N from all soil N pools. Consistent with these findings we also found that bunchgrasses were the only functional group that inhibited annual grass establishment. At natural levels of species abundance, N partitioning may facilitate coexistence but may not necessarily contribute to N sequestration and invasion resistance by the plant community. This suggests that a general mechanism of invasion resistance may not be expected across systems. Instead, the key mechanism of invasion resistance within a system may depend on trait variation among coexisting species and on how species abundance is distributed in the system.

Extensive summer water pulses do not necessarily lead to canopy growth of Great Basin and northern Mojave Desert shrubs

Plant species and functionally related species groups from arid and semi-arid habitats vary in their capacity to take up summer precipitation, acquire nitrogen quickly after summer precipitation, and subsequently respond with ecophysiological changes (e.g. water and nitrogen relations, gas exchange). For species that respond ecophysiologically, the use of summer precipitation is generally assumed to affect long-term plant growth and thus alter competitive interactions that structure plant communities and determine potential responses to climate change. We assessed ecophysiological and growth responses to large short-term irrigation pulses over one to three growing seasons for several widespread Great Basin and northern Mojave Desert shrub species: Chrysothamnus nauseosus, Sarcobatus vermiculatus, Atriplex confertifolia, and A. parryi. We compared control and watered plants in nine case studies that encompassed adults of all four species, juveniles for three of the species, and two sites for two of the species. In every comparison, plants used summer water pulses to improve plant water status or increase rates of functioning as indicated by other ecophysiological characters. Species and life history stage responses of ecophysiological parameters (leaf N, δ15N, δ13C, gas exchange, sap flow) were consistent with several previous short-term studies. However, use of summer water pulses did not affect canopy growth in eight out of nine comparisons, despite the range of species, growth stages, and site conditions. Summer water pulses affected canopy growth only for C. nauseosus adults. The general lack of growth effects for these species might be due to close proximity of groundwater at these sites, co-limitation by nutrients, or inability to respond due to phenological canalization. An understanding of the connections between short-term ecophysiological responses and growth, for different habitats and species, is critical for determining the significance of summer precipitation for desert community dynamics.

A Basis for Relative Growth Rate Differences Between Native and Invasive Forb Seedlings

Abstract The ability of invasive plants to achieve higher relative growth rates (RGR) than their native counterparts has been widely documented. However, the mechanisms allowing invasives to achieve higher RGR are poorly understood. The objective of this study was to determine the basis for RGR differences between native and invasive forbs that have widely invaded nutrient-poor soils of the Intermountain West. Six native and 6 invasive forbs were seeded in pots in a greenhouse, and 4 harvests were conducted over a 2-month period. These 4 harvests were used to calculate RGR and the components of RGR, net assimilation rate (rate of dry matter production per unit leaf area), leaf area ratio (LAR, leaf area per unit total plant mass), leaf mass ratio (the proportion of biomass allocated to leaves), and specific leaf area (SLA, leaf area per unit leaf biomass). Mean RGR of the 12 study species ranged between 0.04 and 0.15 g · g−1 · d−1 but was significantly higher for invasive forbs compared to native forbs (P  =  0.036). The higher RGR achieved by invasive forbs was due mainly to a greater SLA and LAR. This indicates that invasive forbs achieved higher RGR than natives primarily by creating more leaf area per unit leaf mass, not by allocating more biomass to leaf tissue or by having a higher net rate of dry matter production. A high degree of variation in RGR, SLA, and LAR was observed in native forbs, suggesting that the ability to design weed-resistant plant communities may be improved by managing for specific functional traits as opposed to functional groups.

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

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.

Congeneric serpentine and nonserpentine shrubs differ more in leaf Ca:Mg than in tolerance of low N, low P, or heavy metals

Serpentine soils limit plant growth by NPK deficiencies, low Ca availability, excess Mg, and high heavy metal levels. In this study, three congeneric serpentine and nonserpentine evergreen shrub species pairs were grown in metalliferous serpentine soil with or without NPKCa fertilizer to test which soil factors most limit biomass production and mineral nutrition responses. Fertilization increased biomass production and allocation to leaves while decreasing allocation to roots in both serpentine and nonserpentine species. Simultaneous increases in biomass and leaf N:P ratios in fertilized plants of all six species suggest that N is more limiting than P in this serpentine soil. Neither N nor P concentrations, however, nor root to shoot translocation of these nutrients, differed significantly between serpentine and nonserpentine congeners. All six species growing in unfertilized serpentine soil translocated proportionately more P to leaves compared to fertilized plants, thus maintaining foliar P. Leaf Ca:Mg molar ratios of the nonserpentine species were generally equal to that of the soil. The serpentine species, however, maintained significantly higher leaf Ca:Mg than both their nonserpentine counterparts and the soil. Elevated leaf Ca:Mg in the serpentine species was achieved by selective Ca transport and/or Mg exclusion operating at the root-to-shoot translocation level, as root Ca and Mg concentrations did not differ between serpentine and nonserpentine congeners. All six species avoided shoot toxicity of heavy metals by root sequestration. The comparative data on nutrient deficiencies, leaf Ca:Mg, and heavy metal sequestration suggest that the ability to maintain high leaf Ca:Mg is a key evolutionary change needed for survival on serpentine soil and represents the physiological feature distinguishing the serpentine shrub species from their nonserpentine congeners. The results also suggest that high leaf Ca:Mg is achieved in these serpentine species by selective translocation of Ca and/or inhibited transport of Mg from roots, rather than by uptake/exclusion at root surfaces.

A systems approach to restoring degraded drylands

Summary 1. Drylands support over 2 billion people and are major providers of critical ecosystem goods and services across the globe. Drylands, however, are one of the most susceptible biomes to degradation. International programmes widely recognize dryland restoration as key to combating global dryland degradation and ensuring future global sustainability. While the need to restore drylands is widely recognized and large amounts of resources are allocated to these activities, rates of restoration success remain overwhelmingly low. 2. Advances in understanding the ecology of dryland systems have not yielded proportional advances in our ability to restore these systems. To accelerate progress in dryland restoration, we argue for moving the field of restoration ecology beyond conceptual frameworks of ecosystem dynamics and towards quantitative, predictive systems models that capture the probabilistic nature of ecosystem response to management. 3. To do this, we first provide an overview of conceptual dryland restoration frameworks. We then describe how quantitative systems framework can advance and improve conceptual restoration frameworks, resulting in a greater ability to forecast restoration outcomes and evaluate economic efficiency and decision-making. Lastly, using a case study from the western United States, we show how a systems approach can be integrated with and used to advance current conceptual frameworks of dryland restoration. 4. Synthesis and applications. Systems models for restoration do not replace conceptual models but complement and extend these modelling approaches by enhancing our ability to solve restoration problems and forecast outcomes under changing conditions. Such forecasting of future outcomes is necessary to monetize restoration benefits and cost and to maximize economic benefit of limited restoration dollars.

Applying Ecologically Based Invasive-Plant Management

Abstract The need for a unified mechanistic ecological framework that improves our ability to make decisions, predicts vegetation change, guides the implementation of restoration, and fosters learning is substantial and unmet. It is becoming increasingly clear that integrating various types of ecological models into an overall framework has great promise for assisting decision making in invasive-plant management and restoration. Overcoming barriers to adoption of ecologically based invasive-plant management will require developing principles and integrating them into a useful format so land managers can easily understand the linkages among ecological processes, vegetation dynamics, management practices, and assessment. We have amended a generally accepted and well-tested successional management framework into a comprehensive decision tool for ecologically based invasive-plant management (EBIPM) by 1) using the Rangeland Health Assessment to identify ecological processes in need of repair, 2) amending our framework to include principles for repairing ecological processes that direct vegetation dynamics, and 3) incorporating adaptive management procedures to foster the acquisition of new information during management. This decision tool provides a step-by-step planning process that integrates assessment and adaptive management with process-based principles to provide management guidance. In our case-study example, EBIPM increased the chance of restoration success by 66% over traditionally applied integrated weed management in an invasive-plant–dominated ephemeral wetland ecosystem. We believe that this framework provides the basis for EBIPM and will enhance our ability to design and implement sustainable invasive-plant management and restoration programs.

Principles for Ecologically Based Invasive Plant Management

Abstract Land managers have long identified a critical need for a practical and effective framework for designing restoration strategies, especially where invasive plants dominate. A holistic, ecologically based, invasive plant management (EBIPM) framework that integrates ecosystem health assessment, knowledge of ecological processes, and adaptive management into a successional management model has recently been proposed. However, well-defined principles that link ecological processes that need to be repaired to tools and strategies available to managers have been slow to emerge, thus greatly limiting the ability of managers to easily apply EBIPM across a range of restoration scenarios. The broad objective of this article is to synthesize current knowledge of the mechanisms and processes that drive plant community succession into ecological principles for EBIPM. Using the core concepts of successional management that identify site availability, species availability, and species performance as three general drivers of plant community change, we detail key principles that link management tools used in EBIPM to the ecological processes predicted to influence the three general causes of succession. Although we acknowledge that identification of principles in ecology has greatly lagged behind other fields and recognize that identification of ecological principles and the conditions in which they hold are still being developed, we demonstrate how current knowledge and future advances can be used to structure a holistic EBIPM framework that can be applied across a range of restoration scenarios.

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.

Plant N capture from pulses: effects of pulse size, growth rate, and other soil resources

In arid ecosystems, the ability to rapidly capture nitrogen (N) from brief pulses is expected to influence plant growth, survival, and competitive ability. Theory and data suggest that N capture from pulses should depend on plant growth rate and availability of other limiting resources. Theory also predicts trade-offs in plant stress tolerance and ability to capture N from different size pulses. We injected K15NO3, to simulate small and large N pulses at three different times during the growing season into soil around the co-dominant Great Basin species Sarcobatus vermiculatus, Chrysothamnus nauseosus ssp. consimilis, and Distichlis spicata. Soils were amended with water and P in a partial factorial design. As predicted, all study species showed a comparable decline in N capture from large pulses through the season as growth rates slowed. Surprisingly, however, water and P availability differentially influenced the ability of these species to capture N from pulses. Distichlis N capture increased up to tenfold with water addition while Chrysothamnus N capture increased up to threefold with P addition. Sarcobatus N capture was not affected by water or P availability. Opposite to our prediction, Sarcobatus, the most stress tolerant species, captured less N from small pulses but more N from large pulses relative to the other species. These observations suggest that variation in N pulse timing and size can interact with variable soil water and P supply to determine how N is partitioned among co-existing Great Basin species.