Lara Souza

The natives are restless, but not often and mostly when disturbed

The argument that the threat posed by introduced species is overblown is often buttressed by the observation that native species sometimes also become invasive. An examination of the literature on plant invasions in the United States shows that six times more nonnative species have been termed invasive than native species, and that a member of the naturalized nonnative pool is 40 times more likely than a native species to be perceived as invasive. In the great majority of instances in which a native plant species is seen as invasive, the invasion is associated with an anthropogenic disturbance, especially changed fire or hydrological regime, livestock grazing, and changes wrought by an introduced species. These results suggest that natives are significantly less likely than nonnatives to be problematic for local ecosystems.

Long-term exposure to elevated CO2 enhances plant community stability by suppressing dominant plant species in a mixed-grass prairie

Significance Evaluating ecological responses to climate change is essential to predict ecosystem function under future climate scenarios. In a mixed-grass prairie, we use a multifactor field experiment to show that the effects of elevated CO2 and warming on plant community structure and productivity depend on interannual variation in precipitation. We also show that shifts in plant dominance patterns driven by elevated CO2 in a mixed-grass prairie ecosystem promoted biomass and compositional stability and resistance to interannual variation in precipitation. The economic value of grasslands is largely dependent on the relative abundance of key forage species. Thus, our results have implications for how we manage native grasslands in the face of changing climate. Climate controls vegetation distribution across the globe, and some vegetation types are more vulnerable to climate change, whereas others are more resistant. Because resistance and resilience can influence ecosystem stability and determine how communities and ecosystems respond to climate change, we need to evaluate the potential for resistance as we predict future ecosystem function. In a mixed-grass prairie in the northern Great Plains, we used a large field experiment to test the effects of elevated CO2, warming, and summer irrigation on plant community structure and productivity, linking changes in both to stability in plant community composition and biomass production. We show that the independent effects of CO2 and warming on community composition and productivity depend on interannual variation in precipitation and that the effects of elevated CO2 are not limited to water saving because they differ from those of irrigation. We also show that production in this mixed-grass prairie ecosystem is not only relatively resistant to interannual variation in precipitation, but also rendered more stable under elevated CO2 conditions. This increase in production stability is the result of altered community dominance patterns: Community evenness increases as dominant species decrease in biomass under elevated CO2. In many grasslands that serve as rangelands, the economic value of the ecosystem is largely dependent on plant community composition and the relative abundance of key forage species. Thus, our results have implications for how we manage native grasslands in the face of changing climate.

Within and between population variation in plant traits predicts ecosystem functions associated with a dominant plant species

Linking intraspecific variation in plant traits to ecosystem carbon uptake may allow us to better predict how shift in populations shape ecosystem function. We investigated whether plant populations of a dominant old-field plant species (Solidago altissima) differed in carbon dynamics and if variation in plant traits among genotypes and between populations predicted carbon dynamics. We established a common garden experiment with 35 genotypes from three populations of S. altissima from either Tennessee (southern populations) or Connecticut (northern populations) to ask whether: (1) southern and northern Solidago populations will differ in aboveground productivity, leaf area, flowering time and duration, and whole ecosystem carbon uptake, (2) intraspecific trait variation (growth and reproduction) will be related to intraspecific variation in gross ecosystem CO2 exchange (GEE) and net ecosystem CO2 exchange (NEE) within and between northern and southern populations. GEE and NEE were 4.8× and 2× greater in southern relative to northern populations. Moreover, southern populations produced 13× more aboveground biomass and 1.4× more inflorescence mass than did northern populations. Flowering dynamics (first- and last-day flowering and flowering duration) varied significantly among genotypes in both the southern and northern populations, but plant performance and ecosystem function did not. Both productivity and inflorescence mass predicted NEE and GEE between S. altissima southern and northern populations. Taken together, our data demonstrate that variation between S. altissima populations in performance and flowering traits are strong predictors of ecosystem function in a dominant old-field species and suggest that populations of the same species might differ substantially in their response to environmental perturbations.

Dual mechanisms regulate ecosystem stability under decade-long warming and hay harvest

Past global change studies have identified changes in species diversity as a major mechanism regulating temporal stability of production, measured as the ratio of the mean to the standard deviation of community biomass. However, the dominant plant functional group can also strongly determine the temporal stability. Here, in a grassland ecosystem subject to 15 years of experimental warming and hay harvest, we reveal that warming increases while hay harvest decreases temporal stability. This corresponds with the biomass of the dominant C4 functional group being higher under warming and lower under hay harvest. As a secondary mechanism, biodiversity also explains part of the variation in temporal stability of production. Structural equation modelling further shows that warming and hay harvest regulate temporal stability through influencing both temporal mean and variation of production. Our findings demonstrate the joint roles that dominant plant functional group and biodiversity play in regulating the temporal stability of an ecosystem under global change.

Plant genotype, nutrients, and G × E interactions structure floral visitor communities

Intraspecific variation in plants is driven by both genetic and environmental factors and has been shown to play an important role in determining assemblages of herbivores, predators, and pathogens. Yet, the consequences of genetic (G) and environmental (E) factors, as well as potential (G × E) interactions, for floral visitor communities remains poorly explored. In a common garden experiment, we compared the relative effects of host-plant genotype and genotypic diversity as well as soil nutrient enrichment on floral resource abundance and insect floral visitors associated with tall goldenrod, Solidago altissima. We found that the floral visitor community varied considerably among genotypes, driven predominantly by variation in floral phenology among S. altissima clones. Floral visitors also varied among nutrient treatments, though this response was much weaker than to different plant genotypes, and was likely driven by effects of floral rewards rather than of floral phenology. Importantly, we also detected several G × E interactions for both flowering and floral visitors. Taken together, our results suggest that the effects of host-plant genetic variation, and to a lesser extent G × E interactions, are key agents in structuring the diversity and composition of floral visitors.

Asynchrony among local communities stabilises ecosystem function of metacommunities

Abstract Temporal stability of ecosystem functioning increases the predictability and reliability of ecosystem services, and understanding the drivers of stability across spatial scales is important for land management and policy decisions. We used species‐level abundance data from 62 plant communities across five continents to assess mechanisms of temporal stability across spatial scales. We assessed how asynchrony (i.e. different units responding dissimilarly through time) of species and local communities stabilised metacommunity ecosystem function. Asynchrony of species increased stability of local communities, and asynchrony among local communities enhanced metacommunity stability by a wide range of magnitudes (1–315%); this range was positively correlated with the size of the metacommunity. Additionally, asynchronous responses among local communities were linked with species’ populations fluctuating asynchronously across space, perhaps stemming from physical and/or competitive differences among local communities. Accordingly, we suggest spatial heterogeneity should be a major focus for maintaining the stability of ecosystem services at larger spatial scales.

Intraspecific variation in response to warming across levels of organization: a test with Solidago altissima

Plant species, and the traits associated with them, can help buffer ecosystems to environmental perturbations. Few studies have examined whether within species variation, both among and within populations, can similarly buffer ecosystems to environmental perturbations, such as climatic warming, across levels of organization. Using a dominant plant species in the eastern US, Solidago altissima, we examined whether genotypes of the same species from both southern and northern latitude populations exhibited differential short-term responses to temperature at the cell, leaf, and plant level. At the cell level we quantified the production of reactive oxygen species (by-product of temperature stress) and total oxygen radical antioxidant capacity (which ameliorates temperature stress by-products). At the leaf and plant levels, we measured CO2 assimilation. Increasing temperatures had strong negative impacts on plant-level carbon gain, but weak impacts on cell-level antioxidant capacity. Southern latitude genotyp...

The effects of insects, nutrients, and plant invasion on community structure and function above-and belowground.

Soil nutrient availability, invasive plants, and insect presence can directly alter ecosystem structure and function, but less is known about how these factors may interact. In this 6-year study in an old-field ecosystem, we manipulated insect abundance (reduced and control), the propagule pressure of an invasive nitrogen-fixing plant (propagules added and control), and soil nutrient availability (nitrogen added, nitrogen reduced and control) in a fully crossed, completely randomized plot design. We found that nutrient amendment and, occasionally, insect abundance interacted with the propagule pressure of an invasive plant to alter above-and belowground structure and function at our site. Not surprisingly, nutrient amendment had a direct effect on aboveground biomass and soil nutrient mineralization. The introduction of invasive nitrogen-fixing plant propagules interacted with nutrient amendment and insect presence to alter soil bacterial abundance and the activity of the microbial community. While the larger-scale, longer-term bulk measurements such as biomass production and nutrient mineralization responded to the direct effects of our treatments, the shorter-term and dynamic microbial communities tended to respond to interactions among our treatments. Our results indicate that soil nutrients, invasive plants, and insect herbivores determine both above-and belowground responses, but whether such effects are independent versus interdependent varies with scale.

Plant dominance in a subalpine montane meadow: biotic vs. abiotic controls of subordinate diversity within and across sites

Background Understanding the underlying factors that determine the relative abundance of plant species is critical to predict both biodiversity and ecosystem function. Biotic and abiotic factors can shape the distribution and the relative abundance of species across natural communities, greatly influencing local biodiversity. Methods Using a combination of an observational study and a five-year plant removal experiment we: (1) documented how plant diversity and composition of montane meadow assemblages vary along a plant dominance gradient using an observational study; (2) tracked above- and belowground functional traits of co-dominant plant species Potentilla and Festuca along a plant dominance gradient in an observational study; (3) determined whether plant species diversity and composition was directly influenced by commonly occurring species Potentilla and Festuca with the use of a randomized plot design, 5-year plant removal experiment (no removal control, Potentilla removed, Festuca removed, n = 10). Results We found that subordinate species diversity and compositional dissimilarity were greatest in Potentilla and Festuca co-dominated sites, where neither Potentilla nor Festuca dominated, rather than at sites where either species became dominant. Further, while above- and belowground plant functional traits varied along a dominance gradient, they did so in a way that inconsistently predicted plant species relative abundance. Also, neither variation in plant functional traits of Festuca and Potentilla nor variation in resources and conditions (such as soil nitrogen and temperature) explained our subordinate diversity patterns. Finally, neither Potentilla nor Festuca influenced subordinate diversity or composition when we directly tested for their impacts in a plant removal experiment. Discussion Taken together, patterns of subordinate diversity and composition were likely driven by abiotic factors rather than biotic interactions. As a result, the role of abiotic factors influencing local-level species interactions can be just as important as biotic interactions themselves in structuring plant communities.

Data and code from: Asynchrony among local communities stabilizes ecosystem function of metacommunities

R code, derived metrics, and limited metadata associated with Wilcox et al. (2017). Asynchrony among local communities stabilizes ecosystem function of metacommunities. Ecology Letters. When using this data or code, please cite the original publication: Wilcox, K.R., A.T. Tredennick, S. Koerner, E. Grman, L. Hallett, M. Avolio, K. La Pierre, G. Houseman, F. Isbell, D. Johnson, J. Alatalo, A. Baldwin, E. Bork, E. Boughton, W. Bowman, A. Britton, J. Cahill, S. Collins, G-Z. Du, A. Eskelinen, L. Gough, A. Jentsch, C. Kern, K. Klanderud, A. Knapp, J. Kreyling, Y. Luo, J. McLaren, P. Megonigal, V. Onipchenko, J. Prevey, J. Price, C. Robinson, O. Sala, M. Smith, N. Soudzilovskaia, L. Souza, D. Tilman, S. White, Z. Xu, L. Yahdjian, Q. Yu, P. Zhang, Y, Zhang. (2017). Asynchrony among local communities stabilizes ecosystem function of metacommunities. Ecology Letters vol(iss):xx-xx. Additionally, please cite the Figshare file set: Wilcox, K.R., A.T. Tredennick, S. Koerner, E. Grman, L. Hallett, M. Avolio, K. La Pierre. (2017). Data and code from: Asynchrony among local communities stabilizes ecosystem function of metacommunities. Figshare. https://dx.doi.org/10.6084/m9.figshare.5384167. R Code The analysis proceeds in several steps, which can be viewed most easily by examining the ~/Wilcox_etal_DerivedData_and_Code/analysis_scripts/main_text_scripts/patches_source_all_scripts.R file. Questions about the code or analysis should be directed to Kevin Wilcox (wilcoxkr@gmail.com) or Andrew Tredennick (atredenn@gmail.com). Derived Data We provide the full set of metrics (e.g., alpha, beta, and gamma stability and diversity) for each of our study sites. The main analysis and all figures in the paper can be reproduced using these metrics. Metrics were calculated from time series of abundance data from 62 grassland sites around the globe, although primarily from North America and Europe. The data is part of the CoRRE Data Base (http://corredata.weebly.com/), and those interested in using proprietary data not included in this fileset are encouraged to contact the CoRRE data base maintainers (http://corredata.weebly.com/contact.html).