Rebecca E. Drenovsky

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.

Invasion by Aegilops triuncialis (Barb Goatgrass) Slows Carbon and Nutrient Cycling in a Serpentine Grassland

Invasive plant species alter plant community composition and ecosystem function. In the United States, California native grasslands have been displaced almost completely by invasive annual grasses, with serpentine grasslands being one of the few remaining refugia for California grasslands. This study examined how the invasive annual grass, Aegilops triuncialis, has altered decomposition processes in a serpentine annual grassland. Our objectives were to (1) assess howA. triuncialis alters primary productivity and litter tissue chemistry, (2) determine whether A. triuncialis litter is more recalcitrant to decomposition than native litter, and (3) evaluate whether differences in the soil microbial community in A. triuncialis-invaded and native-dominated areas result in different decomposition rates of invasive and/or native plant litter. In invaded plant patches, A. triuncialis was approximately 50% of the total plant cover, in contrast to native plant patches in which A. triuncialis was not detected and native plants comprised over 90% of the total plant cover. End-of-season aboveground biomass was 2-fold higher in A. triuncialis dominated plots compared to native plots; however, there was no significant difference in belowground biomass. Both above- and below-ground plant litter from A. triuncialis plots had significantly higher lignin:N and C:N ratios and lower total N, P, and K than litter from native plant plots. Aboveground litter from native plots decomposed more rapidly than litter from A. triuncialis plots, although there was no difference in decomposition of belowground tissues. Soil microbial community composition associated with different soil patch types had no effect on decomposition rates. These data suggest that plant invasion impacts decomposition and nutrient cycling through changes in plant community tissue chemistry and biomass production.

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.

Fame, glory and neglect in meta-analyses

Ecology increasingly relies on data synthesis and integration [1,2]. We fear, however, that the academic culture and merit system in ecology has not evolved in pace with the emergent need for increased collaboration. In particular, meta-analyses, which are often based on a large number of independent data sets (e.g. [3]), are reliant on the collection of primary data, and the willingness of field and experimental researchers to share these data. Therefore, it is vital that this empirical work and expertise should be adequately valued.

Plant community and tissue chemistry responses to fertilizer and litter nutrient manipulations in a temperate grassland

Human-mediated nutrient amendments have widespread effects on plant communities. One of the major consequences has been the loss of species diversity under increased nutrient inputs. The loss of species can be functional group dependent with certain functional groups being more prone to decline than others. We present results from the sixth year of a long-term fertilization and litter manipulation study in an old-field grassland. We measured plant tissue chemistry (C:N ratio) to understand the role of plant physiological responses in the increase or decline of functional groups under nutrient manipulations. Fertilized plots had significantly more total aboveground biomass and live biomass than unfertilized plots, which was largely due to greater productivity by exotic C3 grasses. We found that both fertilization and litter treatments affected plant species richness. Species richness was lower on plots that were fertilized or had litter intact; species losses were primarily from forbs and non-Poaceae graminoids. C3 grasses and forbs had lower C:N ratios under fertilization with forbs having marginally greater %N responses to fertilization than grasses. Tissue chemistry in the C3 grasses also varied depending on tissue type with reproductive tillers having higher C:N ratios than vegetative tillers. Although forbs had greater tissue chemistry responses to fertilization, they did not have a similar positive response in productivity and the number of forb species is decreasing on our experimental plots. Overall, differential nutrient uptake and use among functional groups influenced biomass production and species interactions, favoring exotic C3 grasses and leading to their dominance. These data suggest functional groups may differ in their responses to anthropogenic nutrient amendments, ultimately influencing plant community composition.

Fame, glory and neglect in meta-analyses (Letter)

Ecology increasingly relies on data synthesis and integration [1,2]. We fear, however, that the academic culture and merit system in ecology has not evolved in pace with the emergent need for increased collaboration. In particular, meta-analyses, which are often based on a large number of independent data sets (e.g. [3]), are reliant on the collection of primary data, and the willingness of field and experimental researchers to share these data. Therefore, it is vital that this empirical work and expertise should be adequately valued.

Low leaf N and P resorption contributes to nutrient limitation in two desert shrubs

Both water and nutrients are limiting in arid environments, and desert plants have adapted to these limitations through numerous developmental and physiological mechanisms. In the Mono Basin, California, USA, co-dominant Sarcobatus vermiculatus and Chrysothamnus nauseosus ssp. consimilis are differentially N and P limited. We hypothesized that low leaf N resorption contributes to N-limitation in Sarcobatus and that low leaf P resorption contributes to P-limitation in Chrysothamnus. As predicted, Sarcobatus resorbed proportionally 1.7-fold less N than Chrysothamnus, but reduced leaf P in senescent leaves to lower levels than Chrysothamnus (8.0–10.8-fold lower based on leaf area or mass, respectively), consistent with N, but not P limitations in Sarcobatus. Again, as predicted, Chrysothamnus resorbed proportionally 2.0-fold less P than Sarcobatus yet reduced leaf N in senescent leaves to lower levels than Sarcobatus (1.8–1.3-fold lower based on leaf area or mass, respectively), consistent with P, but not N limitations in Chrysothamnus. Leaf N and P pools were approximately 50% of aboveground pools in both species during the growing season, suggesting leaf resorption can contribute significantly to whole plant nutrient retention. This was consistent with changes in leaf N vs. P concentration as plants grew from seedlings to adults. Our results support the conclusion that N-limitation in Sarcobatus and P-limitation in Chrysothamnus are in part caused by physiological (or other) constraints that prevent more efficient resorption of N or P, respectively. For these species, differential nutrient resorption may be a key physiological component contributing to their coexistence in this saline, low resource habitat.

Environmental stress and genetics influence night-time leaf conductance in the C4 grass Distichlis spicata

Growing awareness of night-time leaf conductance (gnight) in many species, as well as genetic variation in gnight within several species, has raised questions about how genetic variation and environmental stress interact to influence the magnitude of gnight. The objective of this study was to investigate how genotype salt tolerance and salinity stress affect gnight for saltgrass [Distichlis spicata (L.) Greene]. Across genotypes and treatments, night-time water loss rates were 5-20% of daytime rates. Despite growth declining 37-87% in the high salinity treatments (300 mm and 600 mm NaCl), neither treatment had any effect on gnight in four of the six genotypes compared with the control treatment (7 mm NaCl). Daytime leaf conductance (gday) also was not affected by salinity treatment in three of the six genotypes. There was no evidence that more salt tolerant genotypes (assessed as ability to maintain growth with increasing salinity) had a greater capacity to maintain gnight or gday at high salinity. In addition, gnight as a percentage of gday was unaffected by treatment in the three most salt tolerant genotypes. Although gnight in the 7 mm treatment was always highest or not different compared with the 300 mm and 600 mm treatments, gday was generally highest in the 300 mm treatment, indicating separate regulation of gnight and gday in response to an environmental stress. Thus, it is clear that genetics and environment both influence the magnitude of gnight for this species. Combined effects of genetic and environmental factors are likely to impact our interpretation of variation of gnight in natural populations.

New and Current Microbiological Tools for Ecosystem Ecologists: Towards a Goal of Linking Structure and Function

ABSTRACT Interest in the relationships between soil microbial communities and ecosystem functions is growing with increasing recognition of the key roles microorganisms play in a variety of ecosystems. With a wealth of microbial methods now available, selecting the most appropriate method can be daunting, especially to those new to the field of microbial ecology. In this review, we highlight those methods currently used and most applicable to ecological studies, including assays to study various aspects of the carbon and nitrogen cycles (e.g., pool dilution, acetylene reduction, enzyme analyses, among others), methods to assess microbial community composition (e.g., phospholipid fatty acid analysis (PLFA), denaturing gradient gel electrophoresis (DGGE), terminal restriction fragment length polymorphism analysis (TRFLP), quantitative polymerase chain reaction (qPCR)) and methods to directly link community structure to function (e.g., stable isotope probing (SIP)). In our discussion of these methods, we describe the information each method provides, as well as some of their strengths and weaknesses. Using a case study, we illustrate how these methods can be applied to investigate relationships between microbial communities and the processes they perform in wetland ecosystems. We end our discussion with a series of questions to consider prior to designing experiments, in the hope that these questions will help guide ecologists in selecting the most appropriate method(s) for their research.