Isabel W. Ashton

Niche complementarity due to plasticity in resource use: plant partitioning of chemical N forms

Niche complementarity, in which coexisting species use different forms of a resource, has been widely invoked to explain some of the most debated patterns in ecology, including maintenance of diversity and relationships between diversity and ecosystem function. However, classical models assume resource specialization in the form of distinct niches, which does not obviously apply to the broadly overlapping resource use in plant communities. Here we utilize an experimental framework based on competition theory to test whether plants partition resources via classical niche differentiation or via plasticity in resource use. We explore two alternatives: niche preemption, in which individuals respond to a superior competitor by switching to an alternative, less-used resource, and dominant plasticity, in which superior competitors exhibit high resource use plasticity and shift resource use depending on the competitive environment. We determined competitive ability by measuring growth responses with and without neighbors over a growing season and then used 15N tracer techniques to measure uptake of different nitrogen (N) forms in a field setting. We show that four alpine plant species of differing competitive abilities have statistically indistinguishable uptake patterns (nitrate > ammonium > glycine) in their fundamental niche (without competitors) but differ in whether they shift these uptake patterns in their realized niche (with competitors). Competitively superior species increased their uptake of the most available N form, ammonium, when in competition with the rarer, competitively inferior species. In contrast, the competitively inferior species did not alter its N uptake pattern in competition. The existence of plasticity in resource use among the dominant species provides a mechanism that helps to explain the manner by which plant species with broadly overlapping resource use might coexist.

PLANT AND MICROBE CONTRIBUTION TO COMMUNITY RESILIENCE IN A DIRECTIONALLY CHANGING ENVIRONMENT

To understand the role biota play in resilience or vulnerability to environmental change, we investigated soil, plant, and microbial responses to a widespread environmental change, increased nitrogen (N). Our aim was to test the plant-soil threshold hypothesis: that changed biotic structure influences resilience to accumulated changes in N. For six years, we removed one of two codominant species, Geum rossii and Deschampsia caespitosa, in moist- meadow alpine tundra in Colorado, USA. We also manipulated nutrient availability by adding carbon (C) or N, separately and in combination with the species removals. Consistent with our hypothesis, Geum was associated with soil feedbacks that slowed rates of N cycling and Deschampsia with feedbacks that increased rates of N cycling. After a four- year initial resilience period, Geum dramatically declined (by almost 70%) due to increasing N availability. In contrast, Deschampsia abundance did not respond to changes in N supply; it only responded to the removal of Geum. Forbs and graminoids responded more positively to Deschampsia removal than to Geum removal, indicating stronger competitive effects by Deschampsia. The changed biotic interactions appear to have community-level consequences: after six years of Geum (but not Deschampsia) removal, evenness of the community declined by over 35%. Increased N affected the soil-microbial feedbacks, particularly in association with Geum. Microbial biomass N declined at higher N, as did the activities of two C-acquiring and one N- acquiring extracellular microbial enzymes. In the presence of Geum, N fertilization slowed the activity of phenol oxidase, a tannin-degrading enzyme, suggesting that microbes shift from degrading Geum-derived compounds. In the absence of Geum, acid phosphatase activity increased, suggesting increased phosphorus limitation in association with Deschampsia. With continued N deposition forecast for this system, these results suggest that initial resilience of Geum to increased N will be overwhelmed through elimination of microbial feedbacks. Once Geum declines, the loss will indirectly facilitate Deschampsia via competitive release. Because Deschampsia exerts strong competitive effects on subordinate species, increased Deschampsia abundance may be accompanied by a community-wide drop in diversity. We conclude that plant-soil feedbacks through the microbial community can influence vulnerability to exogenous changes in N and contribute to threshold dynamics.

Nitrogen preferences and plant-soil feedbacks as influenced by neighbors in the alpine tundra

Plant resource partitioning of chemical forms of nitrogen (N) may be an important factor promoting species coexistence in N-limited ecosystems. Since the microbial community regulates N-form transformations, plant partitioning of N may be related to plant–soil feedbacks. We conducted a 15N tracer addition experiment to study the ability of two alpine plant species, Acomastylis rossii and Deschampsia caespitosa, to partition organic and inorganic forms of N. The species are codominant and associated with strong plant–soil feedbacks that affect N cycling. We manipulated interspecific interactions by removing Acomastylis or Deschampsia from areas where the species were codominant to test if N uptake patterns varied in the presence of the other species. We found that Deschampsia acquired organic and inorganic N more rapidly than Acomastylis, regardless of neighbor treatment. Plant N uptake—specifically ammonium uptake—increased with plant density and the presence of an interspecific neighbor. Interestingly, this change in N uptake was not in the expected direction to reduce niche overlap and instead suggested facilitation of ammonium use. To test if N acquisition patterns were consistent with plant–soil feedbacks, we also compared microbial rhizosphere extracellular enzyme activity in patches dominated by one or the other species and in areas where they grew together. The presence of both species was generally associated with increased rhizosphere extracellular enzyme activity (five of ten enzymes) and a trend towards increased foliar N concentrations. Taken together, these results suggest that feedbacks through the microbial community, either in response to increased plant density or specific plant neighbors, could facilitate coexistence. However, coexistence is promoted via enhanced resource uptake rather than reduced niche overlap. The importance of resource partitioning to reduce the intensity of competitive interactions might vary across systems, particularly as a function of plant-soil feedbacks.

Effects of experimental manipulation of light and nutrients on establishment of seedlings of native and invasive woody species in Long Island, NY forests

While earlier studies on the process of invasion often focused on single factors or on the general explanation of ‘disturbance,’ recent work has attempted to move towards a more mechanistic understanding of the factors that promote plant community invasion. Manipulative experiments provide a means for discerning causal relationships and interactive effects of environmental factors in promoting invasion; such experiments have been conducted in a number of grassland and shrub ecosystems. This study extends multifactor manipulative experiments into forest communities to compare factors influencing early seedling establishment for native and invasive woody plants. In Long Island, NY, invasion patterns are correlated with forest community type (pine barrens or hardwood), light availability, and soil N and Ca. We conducted manipulative field experiments in two different years to determine the relative importance and interaction of experimental gaps and N and Ca addition in pine barrens and hardwood forests in promoting invasion. We used seedlings of seven common native and invasive species in the first experiment, and 16 native and invasive species paired phylogenetically in the second experiment. Light had the strongest effect on plant growth; all plants grew more in gaps. We found no difference in the average growth rates of native and invasive species. Invasives responded more to high resources than did natives, with highest relative growth rates in gaps in the more fertile soils of the hardwood forests. Opportunities for invasion may differ from year to year, with differential success of invaders only in some years and under some environmental conditions. Clearly, to understand the complex interactions between resources and invasion in forests will require many manipulative experiments across a range of environments and using suites of invasive and native species.

Phenological Changes in Alpine Plants in Response to Increased Snowpack, Temperature, and Nitrogen

Abstract Modified environmental conditions are driving phenological changes in ecosystems around the world. Many plants have already responded to warmer temperatures by flowering earlier and sustaining longer periods of growth. Changes in other environmental factors, like precipitation and atmospheric nitrogen (N) deposition, may also influence phenology but have been less studied. Alpine plants may be good predictors of phenological response patterns because environmental changes are amplified in mountain ecosystems and extreme conditions may make alpine plants particularly sensitive to changes in limiting factors like precipitation, temperature, and N. We tested the effects of increased snowpack, temperature, and N on alpine tundra plant phenology, using snow fence, open-top warming chamber, and N fertilization treatments at the Niwot Ridge Long Term Ecological Research (LTER) site. Flowering phenology of three abundant species was recorded during two growing seasons. Treatment responses varied among species and functional types. Forbs responded to warming by flowering earlier and responded to snowpack and N by flowering later; however, when both snow and N were increased simultaneously, phenology was unchanged. Graminoids flowered earlier in response to N addition. Our results demonstrate that changing environmental conditions influence plant phenology, and specifically highlight that N and multiple factor interactions can yield stronger responses than warming alone.

Shrub Expansion Over the Past 62 Years in Rocky Mountain Alpine Tundra: Possible Causes and Consequences

Abstract Woody plants are encroaching into many herbaceous-dominated communities across the globe, including arctic and alpine tundra. Quantifying the encroachment rate, testing which factors contribute to encroachment, and determining how encroachment is taking place and in which community types encroachment is occurring are essential for predicting shifts in tundra vegetation and carbon (C) storage. We examined willow cover changes from 1946 to 2008 in 18 ha of alpine tundra in Colorado using aerial photographs. We linked this pattern of change with experimental assessment of the effects of increasing summer temperatures, winter precipitation, and nitrogen (N) deposition—factors that this region has experienced over this period—on willow growth and survival. Shrub cover expanded by 441% over 62 years and is increasing at an exponential rate, corresponding to increases in C storage of 137 kg ha-1. Nitrogen and temperature facilitate willow growth and snow increases survival, although N and the combination of N plus snow decrease survival. We find clonal growth (78%) accounts for more expansion than seed dispersal (22%), and that shrubs have expanded into wet, moist, and dry meadow. In addition to a release from grazing, we suggest that global change could be driving shrub expansion.

Indirect effects of global change accumulate to alter plant diversity but not ecosystem function in alpine tundra

© 2014 British Ecological Society. Summary: Environmental change can affect species directly by altering their physical environment and indirectly by altering the abundance of interacting species. A key challenge at the interface of community ecology and conservation biology is to predict how direct and indirect effects combine to influence response in a changing environment. In particular, little is known about how direct and indirect effects on biodiversity develop over time or their potential to influence ecosystem function. We studied how nitrogen (N), winter precipitation (snow) and warming influenced diversity and ecosystem function over 6 years in alpine tundra. We used path analyses to partition direct effects of environmental manipulations from indirect effects due to changes in the abundance of two dominant plants. We hypothesize that (i) indirect effects will develop more slowly but will become stronger than direct effects over time and (ii) after 6 years, indirect effects will more strongly influence diversity while direct effects will influence ecosystem function. Indirect effects of N on diversity were consistently stronger than direct effects and actually developed quickly, prior to direct effects. Direct effects of snow on diversity were detected in year 2 but then subsequently were reversed, while indirect effects were detected in year 4 and grew stronger over time. Overall in year 6, indirect effects were much stronger than direct effects on diversity. Direct effects predominated for three of four ecosystem functions we measured (productivity, N mineralization, winter N availability). The only indirect effects we found were that N and snow indirectly affected microbial biomass N by influencing Geum abundance. Across all four ecosystem measures, indirect effects were infrequent and weaker than direct effects. Synthesis. Increasing indirect effects on diversity over time indicate that short-term experiments or monitoring of natural systems may underestimate the full magnitude of global change effects on plant communities. Moreover, explicitly accounting for changes in dominant plant abundance may be necessary for forecasting plant community response to environmental change. Conversely, weak indirect effects for ecosystem processes suggest that predicting ecosystem function without knowledge of plant responses to global change may be possible. Increasing indirect effects on diversity over time indicate that short-term experiments or monitoring of natural systems may underestimate the full magnitude of global change effects on plant communities. Explicitly accounting for changes in dominant plant abundance may be necessary for forecasting plant community response to environmental change. Conversely, weak indirect effects for ecosystem processes suggest that predicting ecosystem function without knowledge of plant responses to global change may be possible.