Altitudinal gradients do not predict plant-symbiont responses to experimental warming

Fungal symbionts, ubiquitous inhabitants of above- and belowground plant tissues, can play important roles in increasing plant tolerance to abiotic and biotic stress. Disruption of plant-fungal interactions may therefore have important consequences for plant responses to climate change. Both altitudinal gradients and warming experiments can be useful tools for understanding responses of symbioses to climate shifts, but the degree to which altitudinal patterns will predict species responses to warming has received little attention for plant-symbiont interactions. This study combined surveys along replicated altitudinal gradients with a long-term warming experiment at the Rocky Mountain Biological Laboratory (RMBL) in Colorado, USA to test the potential for disruption of plant-fungal symbioses under future climate conditions. Because multiple symbioses within the same host individual may result in complex plant-fungal responses to climate change, we examined the full mycobiome in leaves and roots. Altitudinal patterns in fungal symbioses largely did not correspond to fungal responses to experimental warming, suggesting limited utility of these frequently used methods for predicting fungal responses to climate warming. Variation in temperature influenced fungal colonization, composition, or diversity for some fungal groups and host species. However, our work indicates that effects of climate change on plant-fungal symbioses will depend on host plant identity and fungal functional group, with some associations weakened or disrupted, others affected weakly, and yet others enhanced under climate warming. Predicting how climate change will alter ecologically important symbioses should therefore involve attention to the identity and ecology of both hosts and symbionts. Our approach suggests the strength of comparing environmental gradients to warming experiments in order to gain a more nuanced perspective on how climate change may alter communities and ecosystems

KING OF THE HILL? HOW BIOTIC INTERACTIONS AFFECT BIOGEOGRAPHICAL PATTERN AND SPECIES RESPONSES TO CLIMATE CHANGE

As climate has warmed, many species have moved up mountains as physiological limits to their distributions have ameliorated. These distribution shifts are creating novel communities, begging the question: What happens to species at the tops of mountains as potential antagonists encroach upwards? Theory predicts that upward migrations will cause range contractions for high-elevation species because of novel interactions with encroaching antagonists. My dissertation work is one of the most comprehensive tests of this question to date, using a combination of ecological niche modeling (ENM), experiments, and demographic and trait-based modeling approaches. I created novel ENMs that suggest context-dependency of biotic interactions, where predictions of biotic interactions change from positive to negative over environmental gradients, is common over elevation gradients. Additionally, ENMs suggested the current focus on plant-plant interactions in niche modeling targets the most important biotic interaction for many species. I then constructed space-for-time experiments that transplanted alpine species into novel low elevation plant and mammal communities expected to encroach upwards, as well as into their native high elevation communities. Plant competition was manipulated by vegetation removals and mammals were excluded in a separate factorial experiment using below- and aboveground fencing. In both experiments, low elevation plant and mammal communities suppressed growth of alpine species to a greater extent than those antagonists found in their home range. However, demographic models suggested that environmental factors (e.g. temperature) other than novel plant and mammal communities are more consequential for determining population fate. The experiments validated a novel trait-based model of competitive interactions that can be broadly applied to other systems and conservation needs. My dissertation work found that alpine plants are unlikely to remain “king of the hill” under climate change, in part due to the upward encroachment of novel competitors and intensification of herbivore pressure

Predicting Changes in Bee Assemblages Following State Transitions at North American Dryland Ecotones

Drylands worldwide are experiencing ecosystem state transitions: the expansion of some ecosystem types at the expense of others. Bees in drylands are particularly abundant and diverse, with potential for large compositional differences and seasonal turnover across ecotones. To better understand how future ecosystem state transitions may influence bees, we compared bee assemblages and their seasonality among sites at the Sevilleta National Wildlife Refuge (NM, USA) that represent three dryland ecosystem types (and two ecotones) of the southwestern U.S. (Plains grassland, Chihuahuan Desert grassland, and Chihuahuan Desert shrubland). Using passive traps, we caught bees during two-week intervals from March–October, 2002–2014. The resulting dataset included 302 bee species and 56 genera. Bee abundance, composition, and diversity differed among ecosystems, indicating that future state transitions could alter bee assemblage composition in our system. We found strong seasonal bee species turnover, suggesting that bee phenological shifts may accompany state transitions. Common species drove the observed trends, and both specialist and generalist bee species were indicators of ecosystem types or months; these species could be sentinels of community-wide responses to future shifts. Our work suggests that predicting the consequences of global change for bee assemblages requires accounting for both within-year and among-ecosystem variation