Miranda D. Redmond

Species diversity reduces parasite infection through cross‐generational effects on host abundance

With growing interest in the effects of biodiversity on disease, there is a critical need for studies that empirically identify the mechanisms underlying the diversity-disease relationship. Here, we combined wetland surveys of host community structure with mechanistic experiments involving a multi-host parasite to evaluate competing explanations for the dilution effect. Sampling of 320 wetlands in California indicated that snail host communities were strongly nested, with competent hosts for the trematode Ribeiroia ondatrae predominating in low-richness assemblages and unsuitable hosts increasingly present in more diverse communities. Moreover, competent host density was negatively associated with increases in snail species richness. These patterns in host community assembly support a key prerequisite underlying the dilution effect. Results of multigenerational mesocosm experiments designed to mimic field-observed community assemblages allowed us to evaluate the relative importance of host density and diversity in influencing parasite infection success. Increases in snail species richness (from one to four species) had sharply negative effects on the density of infected hosts (-90% reduction). However, this effect was indirect; competition associated with non-host species led to a 95% reduction in host density (susceptible host regulation), owing primarily to a reduction in host reproduction. Among susceptible hosts, there were no differences in infection prevalence as a function of community structure, indicating a lack of support for a direct effect of diversity on infection (encounter reduction). In monospecific conditions, higher initial host densities increased infection among adult hosts; however, compensatory reproduction in the low-density treatments equalized the final number of infected hosts by the next generation, underscoring the relevance of multigenerational studies in understanding the dilution effect. These findings highlight the role of interspecific competition in mediating the relationship between species richness and parasite infection and emphasize the importance of field-informed experimental research in understanding mechanisms underlying the diversity-disease relationship.

Toward accounting for ecoclimate teleconnections: intra- and inter-continental consequences of altered energy balance after vegetation change

ContextVegetation is projected to continue to undergo major structural changes in coming decades due to land conversion and climate change, including widespread forest die-offs. These vegetation changes are important not only for their local or regional climatic effects, but also because they can affect climate and subsequently vegetation in other regions or continents through “ecoclimate teleconnections”.ObjectivesWe propose that ecoclimate teleconnections are a fundamental link among regions within and across continents, and are central to advancing large-scale macrosystems ecology.Methods and resultsWe illustrate potential ecoclimate teleconnections in a bounding simulation that assumes complete tree cover loss in western North America due to tree die-off, and which predicts subsequent drying and reduced net primary productivity in other areas of North America, the Amazon and elsewhere. Central to accurately modeling such ecoclimate teleconnections is characterizing how vegetation change alters albedo and other components of the land-surface energy balance and then scales up to impact the climate system. We introduce a framework for rapid field-based characterization of vegetation structure and energy balance to help address this challenge.ConclusionsEcoclimate teleconnections are likely a fundamental aspect of macrosystems ecology needed to account for alterations to large-scale atmospheric-ecological couplings in response to vegetation change, including deforestation, afforestation and die-off.

Tree regeneration following drought‐ and insect‐induced mortality in piñon–juniper woodlands

Widespread piñon (Pinus edulis) mortality occurred across the southwestern USA during 2002-2003 in response to drought and bark beetle infestations. Given the recent mortality and changes in regional climate over the past several decades, there is a keen interest in post-mortality regeneration dynamics in piñon-juniper woodlands. Here, we examined piñon and Utah juniper (Juniperus osteosperma) recruitment at 30 sites across southwestern Colorado, USA that spanned a gradient of adult piñon mortality levels (10-100%) to understand current regeneration dynamics. Piñon and juniper recruitment was greater at sites with more tree and shrub cover. Piñon recruitment was more strongly facilitated than juniper recruitment by trees and shrubs. New (post-mortality) piñon recruitment was negatively affected by recent mortality. However, mortality had no effect on piñon advanced regeneration (juveniles established pre-mortality) and did not shift juvenile piñon dominance. Our results highlight the importance of shrubs and juniper trees for the facilitation of piñon establishment and survival. Regardless of adult piñon mortality levels, areas with low tree and shrub cover may become increasingly juniper dominated as a result of the few suitable microsites for piñon establishment and survival. In areas with high piñon mortality and high tree and shrub cover, our results suggest that piñon is regenerating via advanced regeneration.

Woodland recovery following drought-induced tree mortality across an environmental stress gradient

Recent droughts and increasing temperatures have resulted in extensive tree mortality across the globe. Understanding the environmental controls on tree regeneration following these drought events will allow for better predictions of how these ecosystems may shift under a warmer, drier climate. Within the widely distributed piñon-juniper woodlands of the southwestern USA, a multiyear drought in 2002-2004 resulted in extensive adult piñon mortality and shifted adult woodland composition to a juniper-dominated, more savannah-type ecosystem. Here, we used pre- (1998-2001) and 10-year post- (2014) drought stand structure data of individually mapped trees at 42 sites to assess the effects of this drought on tree regeneration across a gradient of environmental stress. We found declines in piñon juvenile densities since the multiyear drought due to limited new recruitment and high (>50%) juvenile mortality. This is in contrast to juniper juvenile densities, which increased over this time period. Across the landscape, piñon recruitment was positively associated with live adult piñon densities and soil available water capacity, likely due to their respective effects on seed and water availability. Juvenile piñon survival was strongly facilitated by certain types of nurse trees and shrubs. These nurse plants also moderated the effects of environmental stress on piñon survival: Survival of interspace piñon juveniles was positively associated with soil available water capacity, whereas survival of nursed piñon juveniles was negatively associated with perennial grass cover. Thus, nurse plants had a greater facilitative effect on survival at sites with higher soil available water capacity and perennial grass cover. Notably, mean annual climatic water deficit and elevation were not associated with piñon recruitment or survival across the landscape. Our findings reveal a clear shift in successional trajectories toward a more juniper-dominated woodland and highlight the importance of incorporating biotic interactions and soil properties into species distribution modeling approaches.

Vegetation Management Across Colorado Plateau BLM Lands: 1950–2003

Abstract Large tracts of land across the western United States have been managed over the last century in an effort to increase forage production, reduce the risk of wildland fires, and/or restore ecosystem structure and function. Yet documentation of this land-treatment history is lacking. With the use of data collected from Bureau of Land Management (BLM) field offices across the Colorado Plateau, we quantified the number, spatial extent, and implementation cost of tree-reduction and seeding treatments done in piñon (Pinus edulis)–juniper (Juniperus osteosperma, Juniperus monophylla, Juniperus scopulorum) woodlands between 1950 and 2003. Over 247 000 hectares of land were treated, corresponding to 6.6% of the piñon–juniper vegetation type within BLM-owned lands. Tree-reduction treatments involving chaining, bulldozing, or cabling were most prevalent between the 1950s and 1970s, with over 163 000 ha of land treated with these methods. Prescribed burning became increasingly prevalent in the 1980s, with over 43 000 ha burned. In more recent years, hydroaxe treatments have become common (4 400 ha treated), but to a much lesser extent than prescribed burns. Over 60% of these tree-reduction treatments were done in conjunction with revegetation or seeding treatments. Implementation costs of these tree-reduction treatments were over

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26.7 million, with the hydroaxe treatment having nearly three times the cost of implementation than all other tree-reduction treatments. The spatial extent of these tree-reduction treatments and associated implementation costs highlight the importance of research examining the efficacy of these treatments and the potential legacy effects. The land-use history reported in this study and the accompanying freely accessible on-line database is a useful tool to guide research and management objectives and methodology.

Woodland resilience to regional drought: Dominant controls on tree regeneration following overstorey mortality

1Department of Ecology and Evolutionary Biology, University of Colorado, UCB 334, Boulder, Colorado 80309 2 Environmental Studies Program, University of Colorado, UCB 397, Boulder, Colorado, 80309 3CU Science Education Initiative, University of Colorado, UCB 334, Boulder, Colorado, 80309 4Institute of Arctic and Alpine Research, University of Colorado, UCB 450, Boulder, Colorado, 80309 5Author to whom correspondence should be addressed: lovestowell@gmail.com

Species, Climate and Landscape Physiography Drive Variable Growth Trends in Subalpine Forests

Drought events occurring under warmer temperatures (i.e. “hotter droughts”) have resulted in widespread tree mortality across the globe, and may result in biome-level vegetation shifts to alternate vegetation types if there is a failure of trees to regenerate. We investigated how overstorey trees, understorey vegetation, and local climatic and edaphic conditions interact to influence tree regeneration, a key prerequisite for resilience, in a region that has experienced severe overstorey tree mortality due to hotter droughts and beetle infestations. We used detailed field observations from 142 sites that spanned a broad range of environmental conditions to evaluate the effects of climate and recent tree mortality on tree regeneration dynamics in the spatially extensive pinon (Pinus edulis)-juniper (Juniperus osteosperma, Juniperus monosperma) woodland vegetation type of the southwestern USA. We used a structural equation modelling framework to identify how tree mortality and local climatic and edaphic conditions affect pinon and juniper regeneration and electivity analyses to quantify the species-specific associations of tree juveniles with overstorey trees and understorey shrubs. Pinon regeneration appears to be strongly dependent upon advanced regeneration, (i.e. the survival of juvenile trees that established prior to the mortality event), the survival of adult seed-bearing trees (inferred from basal area of surviving trees) and the facilitative effects of overstorey trees for providing favourable microsites for seedling establishment. Model results suggest that local edaphoclimatic conditions directly affected pinon and juniper regeneration, such that stands with hotter, drier local climatic conditions and lower soil available water capacity had limited tree regeneration following large-scale dieback. Synthesis. We identify four indicators of resilience to hotter drought conditions: (1) abundant advance regeneration of tree seedlings; (2) sufficient canopy cover for survival of emergent seedlings and existing regeneration; (3) sufficient seed source from surviving trees with high reproductive output; (4) areas with cooler and wetter local climates and greater soil available water capacity. In the absence of these conditions, there is greater likelihood of woodlands transitioning to more xeric vegetation types following dieback.

Experimental Exposure of Helisoma trivolvis and Biomphalaria glabrata (Gastropoda) to Ribeiroia ondatrae (Trematoda)

Forests around the world are undergoing rapid changes due to changing climate and increasing physiological stress, but forest response to climate at the ecosystem scale can be highly variable due to the mixed responses of different trees across heterogeneous landscapes. To determine the response of ecosystems in the Rocky Mountains to climate stress, we investigated the response of subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii), two widely distributed subalpine forest species of Rocky Mountains, to climate warming across a region characterized by gradients of elevation, aspect and soil type. We investigated the growth trend of individual trees through time, determined the climate variables most important for driving growth and quantified the interactions between climate and topography that influence long-term growth trends and potential ecological changes across the study region. Growth trends of these two species are similar through the first part of the century, but diverge during the last several decades. Since 1975, subalpine fir growth decreased through time, while Engelmann spruce growth increased. We find that aspect and warm summer temperatures are the most important factors determining growth in subalpine fir, and subalpine fir growth declines are greatest on east- and south-facing aspects. In contrast, Engelmann spruce growth is uniformly unresponsive to climate. In addition to highlighting the importance of species-level differences in growth response to climate, our results also identify interactions between climate and local physiography as controls on long-term growth trends and suggest that the local landscape physiography can mediate climate-related stress in forested ecosystems. This work advances our understanding of how forest stress is mitigated by landscape factors at the ecosystem scale, and how interactions of species, landscape and climate will control future ecosystem composition and forest growth dynamics.

A Dirty Dozen Ways to Die: Metrics and Modifiers of Mortality Driven by Drought and Warming for a Tree Species

Abstract Experimental infections provide an important foundation for understanding host responses to parasites. While infections with Ribeiroia ondatrae cause mortality and malformations in a wide range of amphibian second intermediate host species, little is known about how the parasite affects its snail first intermediate hosts or even what species can support infection. In this study, we experimentally exposed Helisoma trivolvis, a commonly reported host of R. ondatrae, and Biomphalaria glabrata, a confamilial snail known to host Ribeiroia marini, to increasing concentrations of embryonated eggs of R. ondatrae obtained from surrogate definitive hosts. Over the course of 8 wk, we examined the effect of parasite exposure on infection status, time-to-cercariae release, host size, and mortality of both snail species. Helisoma trivolvis was a highly competent host for R. ondatrae infection, with over 93% infection in all exposed snails, regardless of egg exposure level. However, no infections were detected among exposed B. glabrata, despite previous accounts of this snail hosting a congener parasite. Among exposed H. trivolvis, high parasite exposure reduced growth, decreased time-to-cercariae release, and caused marginally significant increases in mortality. Interestingly, while B. glabrata snails did not become infected with R. ondatrae, individuals exposed to 650 R. ondatrae eggs grew less rapidly than unexposed snails, suggesting a sub-lethal energetic cost associated with parasite exposure. Our results highlight the importance of using experimental infections to understand the effects of parasite exposure on host- and non-host species, each of which can be affected by exposure.