Many semi‐arid plant communities in western North America are dominated by big sagebrush. These ecosystems are being reduced in extent and quality due to economic development, invasive species, and climate change. These pervasive modifications have generated concern about the long‐term viability of sagebrush habitat and sagebrush‐obligate wildlife species (notably Greater Sage‐Grouse), highlighting the need for better understanding of the future big sagebrush distribution, particularly at the species' range margins. The leading and trailing edges of potential climate‐driven distribution shifts are likely to be areas most sensitive to climate change. Although several processes contribute to distribution shifts, regeneration is a fundamental requirement, especially for species with episodic regeneration patterns, such as big sagebrush. We used a process‐based regeneration model for big sagebrush to simulate potential germination and seedling survival in response to climatic and edaphic conditions. We estimated current and future regeneration under 2070–2099 CMIP5 climate conditions at trailing and leading edges that were previously identified using traditional species distribution models. Our results supported expectations of increased probability of regeneration at the leading edge and decreased probability at the trailing edge compared to current levels. Our simulations indicated that soil water dynamics at the leading edge will become more similar to the typical seasonal ecohydrological conditions observed within the current range of big sagebrush. At the trailing edge, increased winter and spring dryness represented a departure from conditions typically supportive of big sagebrush. Our results highlighted that minimum and maximum daily temperatures as well as soil water recharge and summer dry periods are important constraints for big sagebrush regeneration. We observed reliable changes in areas identified as trailing and leading edges, consistent with previous predictions. However, we also identified potential local refugia within the trailing edge, mostly at higher elevation sites. Decreasing regeneration probability at the trailing edge suggests that it will be difficult to preserve and/or restore big sagebrush in these areas. Conversely, increasing regeneration probability at the leading edge suggests a growing potential for conflicts in management goals between maintaining existing grasslands and croplands by preventing sagebrush expansion versus accepting a shift in plant community composition to sagebrush dominance.

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2018. STEPWAT2: an individual-based model for exploring the impact of climate and disturbance on dryland plant communities. Ecosphere 9(8):Abstract. The combination of climate change and altered disturbance regimes is directly and indirectly affecting plant communities by mediating competitive interactions, resulting in shifts in species composition and abundance. Dryland plant communities, defined by low soil water availability and highly variable climatic regimes, are particularly vulnerable to climatic changes that exceed their historical range of variability. Individual-based simulation models can be important tools to quantify the impacts of climate change, altered disturbance regimes, and their interaction on demographic and community-level responses because they represent competitive interactions between individuals and individual responses to fluctuating environmental conditions. Here, we introduce STEPWAT2, an individual plant-based simulation model for exploring the joint influence of climate change and disturbance regimes on dryland ecohydrology and plant community composition. STEPWAT2 utilizes a process-based soil water model (SOILWAT2) to simulate available soil water in multiple soil layers, which plant individuals compete for based on the temporal matching of water and active root distributions with depth. This representation of resource utilization makes STEPWAT2 particularly useful for understanding how changes in soil moisture and altered disturbance regimes will concurrently impact demographic and community-level responses in drylands. Our goals are threefold: (1) to describe the core modules and functions within STEPWAT2 (model description), (2) to validate STEPWAT2 model output using field data from big sagebrush plant communities (model validation), and (3) to highlight the usefulness of STEPWAT2 as a modeling framework for examining the impacts of climate change and disturbance regimes on dryland plant communities under future conditions (model application). To address goals 2 and 3, we focus on 15 sites that span the spatial extent of big sagebrush plant communities in the western United States. For goal 3, we quantify how climate change, fire, and grazing can interact to influence plant functional type biomass and composition. We use big sagebrush-dominated plant communities to demonstrate the functionality of STEPWAT2, as these communities are among the most widespread dryland ecosystems in North America.

. 2016. Seed bank and big sagebrush plant community composition in a range margin for big sagebrush. Ecosphere 7(10):e01453. 10.1002/ecs2.1453Abstract. The potential influence of seed bank composition on range shifts of species due to climate change is unclear. Seed banks can provide a means of both species persistence in an area and local range expansion in the case of increasing habitat suitability, as may occur under future climate change. However, a mismatch between the seed bank and the established plant community may represent an obstacle to persistence and expansion. In big sagebrush (Artemisia tridentata) plant communities in Montana, USA, we compared the seed bank to the established plant community. There was less than a 20% similarity in the relative abundance of species between the established plant community and the seed bank. This difference was primarily driven by an overrepresentation of native annual forbs and an underrepresentation of big sagebrush in the seed bank compared to the established plant community. Even though we expect an increase in habitat suitability for big sagebrush under future climate conditions at our sites, the current mismatch between the plant community and the seed bank could impede big sagebrush range expansion into increasingly suitable habitat in the future.

Network theory allows us to understand complex systems by evaluating how their constituent elements interact with one another. Such networks are built from matrices which describe the effect of each element on all others. Quantifying the strength of these interactions from empirical data can be difficult, however, because the number of potential interactions increases nonlinearly as more elements are included in the system, and not all interactions may be empirically observable when some elements are rare. We present a novel modelling framework which uses measures of species performance in the presence of varying densities of their potential interaction partners to estimate the strength of pairwise interactions in diverse horizontal systems. Our method allows us to directly estimate pairwise effects when they are statistically identifiable and to approximate pairwise effects when they would otherwise be statistically unidentifiable. The resulting interaction matrices can include positive and negative effects, the effect of a species on itself, and allows for non‐symmetrical interactions. We show how to link the parameters inferred by our framework to a population dynamics model to make inferences about the effect of interactions on community dynamics and diversity. The advantages of these features are illustrated with a case study on an annual wildflower community of 22 focal and 52 neighbouring species, and a discussion of potential applications of this framework extending well beyond plant community ecology.