North American tree migration paced by climate in the West, lagging in the East.

Tree fecundity and recruitment have not yet been quantified at scales needed to anticipate biogeographic shifts in response to climate change. By separating their responses, this study shows coherence across species and communities, offering the strongest support to date that migration is in progress with regional limitations on rates. The southeastern continent emerges as a fecundity hotspot, but it is situated south of population centers where high seed production could contribute to poleward population spread. By contrast, seedling success is highest in the West and North, serving to partially offset limited seed production near poleward frontiers. The evidence of fecundity and recruitment control on tree migration can inform conservation planning for the expected long-term disequilibrium between climate and forest distribution.

Is there tree senescence? The fecundity evidence.

Despite its importance for forest regeneration, food webs, and human economies, changes in tree fecundity with tree size and age remain largely unknown. The allometric increase with tree diameter assumed in ecological models would substantially overestimate seed contributions from large trees if fecundity eventually declines with size. Current estimates are dominated by overrepresentation of small trees in regression models. We combined global fecundity data, including a substantial representation of large trees. We compared size-fecundity relationships against traditional allometric scaling with diameter and two models based on crown architecture. All allometric models fail to describe the declining rate of increase in fecundity with diameter found for 80% of 597 species in our analysis. The strong evidence of declining fecundity, beyond what can be explained by crown architectural change, is consistent with physiological decline. A downward revision of projected fecundity of large trees can improve the next generation of forest dynamic models.

Forest and woodland replacement patterns following drought-related mortality.

Forest vulnerability to drought is expected to increase under anthropogenic climate change, and drought-induced mortality and community dynamics following drought have major ecological and societal impacts. Here, we show that tree mortality concomitant with drought has led to short-term (mean 5 y, range 1 to 23 y after mortality) vegetation-type conversion in multiple biomes across the world (131 sites). Self-replacement of the dominant tree species was only prevalent in 21% of the examined cases and forests and woodlands shifted to nonwoody vegetation in 10% of them. The ultimate temporal persistence of such changes remains unknown but, given the key role of biological legacies in long-term ecological succession, this emerging picture of postdrought ecological trajectories highlights the potential for major ecosystem reorganization in the coming decades. Community changes were less pronounced under wetter postmortality conditions. Replacement was also influenced by management intensity, and postdrought shrub dominance was higher when pathogens acted as codrivers of tree mortality. Early change in community composition indicates that forests dominated by mesic species generally shifted toward more xeric communities, with replacing tree and shrub species exhibiting drier bioclimatic optima and distribution ranges. However, shifts toward more mesic communities also occurred and multiple pathways of forest replacement were observed for some species. Drought characteristics, species-specific environmental preferences, plant traits, and ecosystem legacies govern postdrought species turnover and subsequent ecological trajectories, with potential far-reaching implications for forest biodiversity and ecosystem services.

Limitations to recovery following wildfire in dry forests of southern Colorado and northern New Mexico, USA.

Climate warming is contributing to increases in wildfire activity throughout the western United States, leading to potentially long-lasting shifts in vegetation. The response of forest ecosystems to wildfire is thus a crucial indicator of future vegetation trajectories, and these responses are contingent upon factors such as seed availability, interannual climate variability, average climate, and other components of the physical environment. To better understand variation in resilience to wildfire across vulnerable dry forests, we surveyed conifer seedling densities in 15 recent (1988-2010) wildfires and characterized temporal variation in seed cone production and seedling establishment. We then predicted postfire seedling densities at a 30-m resolution within each fire perimeter using downscaled climate data, monthly water balance models, and maps of surviving forest cover. Widespread ponderosa pine (Pinus ponderosa) seed cone production occurred at least twice following each fire surveyed, and pulses of conifer seedling establishment coincided with years of above-average moisture availability. Ponderosa pine and Douglas-fir (Pseudotsuga menziesii) seedling densities were higher on more mesic sites and adjacent to surviving trees, though there were also important interspecific differences, likely attributable to drought and shade tolerance. We estimated that postfire seedling densities in 42% (for ponderosa pine) and 69% (for Douglas-fir) of the total burned area were below the lowest reported historical tree densities in these forests. Spatial models demonstrated that an absence of mature conifers (particularly in the interior of large, high-severity patches) limited seedling densities in many areas, but 30-yr average actual evapotranspiration and climatic water deficit limited densities on marginal sites. A better understanding of the limitations to postfire forest recovery will refine models of vegetation dynamics and will help to improve strategies of adaptation to a warming climate and shifting fire activity.

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.

Limits to understory plant restoration following fuel-reduction treatments in a piñon-juniper woodland.

National fuel-reduction programs aim to reduce the risk of wildland fires to human communities and to restore forest and rangeland ecosystems to resemble their historical structure, function, and diversity. There are a number of factors, such as seed bank dynamics, post-treatment climate, and herbivory, which determine whether this latter goal may be achieved. Here, we examine the short-term (2 years) vegetation response to fuel-reduction treatments (mechanical mastication, broadcast burn, and pile burn) and seeding of native grasses on understory vegetation in an upland piñon-juniper woodland in southeast Utah. We also examine how wildlife herbivory affects the success of fuel-reduction treatments. Herbaceous cover increased in response to fuel-reduction treatments in all seeded treatments, with the broadcast burn and mastication having greater increases (234 and 160 %, respectively) in herbaceous cover than the pile burn (32 %). In the absence of seeding, herbaceous cover only increased in the broadcast burn (32 %). Notably, fuel-reduction treatments, but not seeding, strongly affected herbaceous plant composition. All fuel-reduction treatments increased the relative density of invasive species, especially in the broadcast burn, which shifted the plant community composition from one dominated by perennial graminoids to one dominated by annual forbs. Herbivory by wildlife reduced understory plant cover by over 40 % and altered plant community composition. If the primary management goal is to enhance understory cover while promoting native species abundance, our study suggests that mastication may be the most effective treatment strategy in these upland piñon-juniper woodlands. Seed applications and wildlife exclosures further enhanced herbaceous cover following fuel-reduction treatments.

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.

Increased aridity is associated with stronger tradeoffs in ponderosa pine vital functions.

Trees must allocate resources to core functions like growth, defense, and reproduction. These allocation patterns have profound effects on forest health, yet little is known about how core functions trade off over time, and even less is known about how a changing climate will impact tradeoffs. We conducted a 21-year survey of growth, defense, and reproduction in 80 ponderosa pine individuals spanning eight populations across environmental gradients along the Colorado Front Range, USA. We used linear mixed models to describe tradeoffs among these functions and to characterize variability among and within individuals over time. Growth and defense were lower in years of high cone production, and local drought conditions amplified year-to-year tradeoffs between reproduction and growth, where trees located at sites with hotter and drier climates showed stronger tradeoffs between reproduction and growth. Our results support the environmental stress hypothesis of masting, which predicts that greater interannual variation in tree functions will be associated with more marginal environments, such as those that are prone to drought. With warming temperatures and increased exposure to drought stress, trees will be faced with stronger interannual tradeoffs, which could lead to further decreases in growth and defensive efforts, ultimately increasing risks of mortality.

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.

The effects of ENSO and the North American monsoon on mast seeding in two Rocky Mountain conifer species.

We aimed to disentangle the patterns of synchronous and variable cone production (i.e. masting) and its relationship to climate in two conifer species native to dry forests of western North America. We used cone abscission scars to reconstruct ca 15 years of recent cone production in Pinus edulis and Pinus ponderosa, and used redundancy analysis to relate time series of annual cone production to climate indices describing the North American monsoon and the El Niño Southern Oscillation (ENSO). We show that the sensitivity to climate and resulting synchrony in cone production varies substantially between species. Cone production among populations of P. edulis was much more spatially synchronous and more closely related to large-scale modes of climate variability than among populations of P. ponderosa. Large-scale synchrony in P. edulis cone production was associated with the North American monsoon and we identified a dipole pattern of regional cone production associated with ENSO phase. In P. ponderosa, these climate indices were not strongly associated with cone production, resulting in asynchronous masting patterns among populations. This study helps frame our understanding of mast seeding as a life-history strategy and has implications for our ability to forecast mast years in these species. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

An assessment of temporal variability in mast seeding of North American Pinaceae.

Our overall objective is to synthesize mast-seeding data on North American Pinaceae to detect characteristic features of reproduction (i.e. development cycle length, serotiny, dispersal agents), and test for patterns in temporal variation based on weather variables. We use a large dataset (n = 286 time series; mean length = 18.9 years) on crop sizes in four conifer genera (Abies, Picea, Pinus, Tsuga) collected between 1960 and 2014. Temporal variability in mast seeding (CVp) for 2 year genera (Abies, Picea, Tsuga) was higher than for Pinus (3 year), and serotinous species had lower CVp than non-serotinous species; there were no relationships of CVp with elevation or latitude. There was no difference in family-wide CVp across four tree regions of North America. Across all genera, July temperature differences between bud initiation and the prior year (ΔT) was more strongly associated with reproduction than absolute temperature. Both CVp and ΔT remained steady over time, while absolute temperature increased by 0.09°C per decade. Our use of the ΔT model included a modification for Pinus, which initiates cone primordia 2 years before seedfall, as opposed to 1 year. These findings have implications for how mast-seeding patterns may change with future increases in temperature, and the adaptive benefits of mast seeding. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

Continent-wide tree fecundity driven by indirect climate effects.

Indirect climate effects on tree fecundity that come through variation in size and growth (climate-condition interactions) are not currently part of models used to predict future forests. Trends in species abundances predicted from meta-analyses and species distribution models will be misleading if they depend on the conditions of individuals. Here we find from a synthesis of tree species in North America that climate-condition interactions dominate responses through two pathways, i) effects of growth that depend on climate, and ii) effects of climate that depend on tree size. Because tree fecundity first increases and then declines with size, climate change that stimulates growth promotes a shift of small trees to more fecund sizes, but the opposite can be true for large sizes. Change the depresses growth also affects fecundity. We find a biogeographic divide, with these interactions reducing fecundity in the West and increasing it in the East. Continental-scale responses of these forests are thus driven largely by indirect effects, recommending management for climate change that considers multiple demographic rates.

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

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.