Interactions among moths, crossbills, squirrels, and lodgepole pine in a geographic selection mosaic

Repeated patterns among biological communities suggest similar evolutionary and ecological forces are acting on the communities. Conversely, the lack of such patterns suggests that similar forces are absent or additional ones are present. Coevolution between a seed predator, the red crossbill (Loxia curvirostra complex), and lodgepole pine (Pinus contorta var. latifolia) exemplifies the ecological and evolutionary predictions for coevolving systems. In the absence of another seed predator and preemptive competitor (pine squirrels Tamiasciurus hudsonicus), natural selection by crossbills results in the evolution of larger cones with thicker distal scales, while relaxation of selection by squirrels results in the evolution of cones with more seeds and a greater ratio of seed mass to cone mass. However, in one range, the Little Rocky Mountains, distal scale thickness has diverged as expected but cone size has not. In these mountains seed predation by lodgepole pine cone borer moths (Eucosma recissoriana) was about 10 times greater than in other ranges lacking squirrels. We quantified moth predation and cone traits and found that moths select for smaller cones with fewer seeds. Thus, selection by moths in the Little Rocky Mountains counters both selection by crossbills for large cone size and relaxation of selection by squirrels favoring more seeds per cone and accounts for the relatively small and few-seeded cones in these mountains. It is also apparent that selection by crossbills changes seed defenses in a manner that favors seed predation by moths, whereas selection by squirrels likely reduces such predation. These results demonstrate the importance of considering the evolutionary consequences of community context in locally evolved (coevolved) traits and interactions.     

Cones structure and seed traits of four species of large‐seeded pines: Adaptation to animal‐mediated dispersal

Seed dispersal selection pressures may cause morphological differences in cone structure and seed traits of large‐seeded pine trees. We investigated the cone, seed, and scale traits of four species of animal‐dispersed pine trees to explore the adaptations of morphological structures to different dispersers. The four focal pines analyzed in this study were Chinese white pine (Pinus armandi), Korean pine (P. koraiensis), Siberian dwarf pine (P. pumila), and Dabieshan white pine (P. dabeshanensis). There are significant differences in the traits of the cones and seeds of these four animal‐dispersed pines. The scales of Korean pine and Siberian dwarf pine are somewhat opened after cone maturity, the seeds are closely combined with scales, and the seed coat and scales are thick. The cones of Chinese white pine and Dabieshan white pine are open after ripening, the seeds fall easily from the cones, and the seed coat and seed scales are relatively thin. The results showed that the cone structure of Chinese white pine is similar to that of Dabieshan white pine, whereas Korean pine and Siberian dwarf pine are significantly different from the other two pines and vary significantly from each other. This suggests that species with similar seed dispersal strategies exhibit similar morphological adaptions. Accordingly, we predicted three possible seed dispersal paradigms for animal‐dispersed pines: the first, as represented by Chinese white pine and Dabieshan white pine, relies upon small forest rodents for seed dispersal; the second, represented by Korean pine, relies primarily on birds and squirrels to disperse the seeds; and the third, represented by Siberian dwarf pine, relies primarily on birds for seed dispersal. Our study highlights the significance of animal seed dispersal in shaping cone morphology, and our predictions provide a theoretical framework for research investigating the coevolution of large‐seeded pines and their seed dispersers.     

The influence of a competitor on the geographic mosaic of coevolution between crossbills and lodgepole pine

The geographic mosaic theory of coevolution posits that the form of selection between interacting species varies across a landscape with coevolution important and active in some locations (i.e., coevolutionary hotspots) but not in others (i.e., coevolutionary coldspots). We tested the hypothesis that the presence of red squirrels (Tamiasciurus hudsonicus) affects the occurrence of coevolution between red crossbills (Loxia curvirostra complex) and Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia) and thereby provides a mechanism giving rise to a geographic mosaic of selection. Red squirrels are the predominant predispersal seed predator and selective agent on lodgepole pine cones. However, in four isolated mountain ranges east and west of the Rocky Mountains, red squirrels are absent and red crossbills are the main predispersal seed predator. These isolated populations of pine have apparently evolved without Tamiasciurus for about 10,000 to 12,000 years. Based on published morphological, genetic, and paleobotanical studies, we infer that cone traits in these isolated populations that show parallel differences from cones in the Rocky Mountains have changed in parallel. We used data on crossbill and conifer cone morphology and feeding preferences and efficiency to detect whether red crossbills and lodgepole pine exhibit reciprocal adaptations, which would imply coevolution. Cone traits that act to deter Tamiasciurus and result in high ratios of cone mass to seed mass were less developed in the isolated populations. Cone traits that act to deter crossbills include larger and thicker scales and perhaps increased overlap between successive scales and were enhanced in the isolated populations. In the larger, isolated mountain ranges crossbills have evolved deeper, shorter, and therefore more decurved bills to exploit these cones. This provides crossbills with higher feeding rates, and the change in bill shape has improved efficiency by reducing the concomitant increases in body mass and daily energy expenditures that would have resulted if only bill size had increased. These parallel adaptations and counter adaptations in red crossbills and lodgepole pine are interpreted as reciprocal adaptations and imply that these crossbills and pine are in coevolutionary arms races where red squirrels are absent (i.e., coevolutionary hotspots) but not where red squirrels are present (i.e., coevolutionary cold-spots).     

CONFLICTING SELECTION FROM AN ANTAGONIST AND A MUTUALIST ENHANCES PHENOTYPIC VARIATION IN A PLANT

The raw material for evolution is variation. Consequently, identifying the factors that generate, maintain, and erode phenotypic and genetic variation in ecologically important traits within and among populations is important. Although persistent directional or stabilizing selection can deplete variation, spatial variation in conflicting directional selection can enhance variation. Here, we present evidence that phenotypic variation in limber pine (Pinus flexilis) cone structure is enhanced by conflicting selection pressures exerted by its mutualistic seed disperser (Clark's nutcracker Nucifraga columbiana) and an antagonistic seed predator (pine squirrel Tamiasciurus spp.). Phenotypic variation in cone structure was bimodal and about two times greater where both agents of selection co‐occurred than where one (the seed predator) was absent. Within the region where both agents of selection co‐occurred, bimodality in cone structure was pronounced where there appears to be a mosaic of habitats with some persistent habitats supporting only the seed disperser. These results indicate that conflicting selection stemming from spatial variation in community diversity can enhance phenotypic variation in ecologically important traits.     

Discrimination among pinyon pine trees by Clark's Nutcrackers: effects of cone crop size and cone characters

Summary The influences of Colorado pinyon pine (Pinus edulis) cone crop size, cone and seed weight, cone length, number of seeds per cone, number of viable seeds, and percent viable seeds on the foraging behavior of avian seed dispersal agents were examined in field and laboratory settings. In the field, there was a significant positive relationship between cone number per tree and both the absolute number of cones and the percentage of the cone crop from which seeds were harvested. Cone weight and the number of viable seeds were also significantly related to seed harvest intensity. Laboratory experiments examined the relationship between crop size and cone characters on seed harvest by 18 Clark's Nutcrackers (Nucifraga columbiana). Nutcrackers were offered a choice of two tree types: one with 20 cones attached, and another with 10 cones attached. Significantly more birds chose to remove seeds first from the tree with 20 cones than the tree with 10 cones. In timed trials, they also harvested seeds from significantly more cones on the tree with the higher cone density. In the laboratory, cones chosen for seed removal by the nutcrackers had significantly more viable seeds, more seeds, and were longer compared to cones that were not chosen. Such discriminatory foraging behavior may increase avian foraging efficiency and result in differential reproductive success of pinyon pines. This behavior may therefore influence the evolution of pinyon pine reproductive traits.     

Enhanced seed defenses potentially relax selection by seed predators against serotiny in lodgepole pine

Serotiny, the retention of seeds in a canopy seed bank until high temperatures cause seeds to be released, is an important life history trait for many woody plants in fire‐prone habitats. Serotiny provides a competitive advantage after fire but increases vulnerability to predispersal seed predation, due to the seeds being retained in clusters in predictable locations for extended periods. This creates opposing selection pressures. Serotiny is favored in areas of high fire frequency, but is selected against by predispersal seed predators. However, predation also selects for cone traits associated with seed defense that could reduce predation on serotinous cones and thereby relax selection against serotiny. This helps explain the elevated defenses in highly serotinous species. However, whether such interactions drive variation in seed defenses within variably serotinous populations has been studied rarely. We investigated the effects of phenotypic selection exerted by red squirrel (Tamiasciurus hudsonicus) predation on Rocky Mountain lodgepole pine (Pinus contorta latifolia) seeds. Squirrels preferentially harvested cones with more and larger seeds, indicating a preference for a higher food reward. We found evidence for stronger selection on trees with serotinous cones, which presumably accounts for the elevated defenses of and lower predation on serotinous compared to non‐serotinous cones. Lower levels of predation on serotinous cones in turn lessen selection against serotiny by squirrels. This has important implications because the frequency of serotiny in lodgepole pine has profound consequences for post‐fire communities and ecosystems widespread in the Rocky Mountains.     

Seed dispersal of the Korean pine,Pinus koraiensis,by the red squirrel,Sciurus vulgaris

Seed hoarding behavior of the red squirrel,Sciurus vulgaris, was studied in relation to the amount of dispersed seeds of the Korean pine,Pinus koraiensis, and the distribution of its seedlings. After removing a cone from a tree, squirrels sat on the ground and ripped off its cone scales before transporting it. A mean of 3.2 seeds were scatter-hoarded per hole. Of 7.7×10⁴ mature seeds produced in a 0.21 ha planted Korean pine forest, 22% were estimated to be directly eaten by four squirrels, 9% were hoarded by them in the pine forest and 65% were cached outside the forest. Squirrels rediscovered hoarded seeds frequently, until the ground was covered with snow, during the period from snow fall until seed germination the next spring, few hoarded seeds were utilized. Korean pine seedlings were found up to 600 m from their mother trees. Scatter-hoarding by squirrels extensively contributes to seed dispersal to places suitable for the regeneration of the Korean pine. The large size of the cone, the absciss-layer at the cone penduncle, the infrequent dehiscence of cone scales, the large and wingless seeds, and the thick seed-coats have probably all been specialized to facilitate utilization by GenusSciurus.     

CONSISTENCY AND VARIATION IN PHENOTYPIC SELECTION EXERTED BY A COMMUNITY OF SEED PREDATORS

Phenotypic selection that is sustained over time underlies both anagenesis and cladogenesis, but the conditions that lead to such selection and what causes variation in selection are not well known. We measured the selection exerted by three species of predispersal seed predators of lodgepole pine (Pinus contorta latifolia) in the South Hills, Idaho, and found that net selection on different cone and seed traits exerted by red crossbills (Loxia curvirostra) and cone borer moths (Eucosma recissoriana) over 10 years of seed crops was similar to that measured in another mountain range. We also found that the strength of selection increased as seed predation increased, which provides a mechanism for the correlation between the escalation of seed defenses and the density of seed predators. Red crossbills consume the most seeds and selection they exert accounts for much of the selection experienced by lodgepole pine, providing additional support for a coevolutionary arms race between crossbills and lodgepole pine in the South Hills. The third seed predator, hairy woodpeckers (Picoides villosus), consumed less than one‐sixth as many seeds as crossbills. Across the northern Rocky Mountains, woodpecker abundance and therefore selective impact appears limited by the elevated seed defenses of lodgepole pine.     

Mountain Pine Beetles Use Volatile Cues to Locate Host Limber Pine and Avoid Non-Host Great Basin Bristlecone Pine

The tree-killing mountain pine beetle (Dendroctonus ponderosae Hopkins) is an important disturbance agent of western North American forests and recent outbreaks have affected tens of millions of hectares of trees. Most western North American pines (Pinus spp.) are hosts and are successfully attacked by mountain pine beetles whereas a handful of pine species are not suitable hosts and are rarely attacked. How pioneering females locate host trees is not well understood, with prevailing theory involving random landings and/or visual cues. Here we show that female mountain pine beetles orient toward volatile organic compounds (VOCs) from host limber pine (Pinus flexilis James) and away from VOCs of non-host Great Basin bristlecone pine (Pinus longaeva Bailey) in a Y-tube olfactometer. When presented with VOCs of both trees, females overwhelmingly choose limber pine over Great Basin bristlecone pine. Analysis of VOCs collected from co-occurring limber and Great Basin bristlecone pine trees revealed only a few quantitative differences. Noticeable differences included the monoterpenes 3-carene and D-limonene which were produced in greater amounts by host limber pine. We found no evidence that 3-carene is important for beetles when selecting trees, it was not attractive alone and its addition to Great Basin bristlecone pine VOCs did not alter female selection. However, addition of D-limonene to Great Basin bristlecone pine VOCs disrupted the ability of beetles to distinguish between tree species. When presented alone, D-limonene did not affect behavior, suggesting that the response is mediated by multiple compounds. A better understanding of host selection by mountain pine beetles could improve strategies for managing this important forest insect. Moreover, elucidating how Great Basin bristlecone pine escapes attack by mountain pine beetles could provide insight into mechanisms underlying the incredible longevity of this tree species.     

Regional patterns of genetic diversity in Pinus flexilis (Pinaceae) reveal complex species history

Pinus flexilis (limber pine) is patchily distributed within its large geographic range; it is mainly restricted to high elevations in the Rocky Mountains and the Basin and Range region of western North America. We examined patterns of allozyme diversity in 30 populations from throughout the species' range. Overall genetic diversity (H(e) = 0.186) was high compared with that of most other pine species but was similar to that of other pines widespread in western North America. The proportion of genetic diversity occurring among populations (G(ST) = 0.101) was also high relative to that for other pines. Observed heterozygosity was less than expected in 28 of the 30 populations. When populations were grouped by region, there were notable differences. Those in the Basin and Range region had more genetic diversity within populations, a higher proportion of genetic diversity among populations, and higher levels of inbreeding within populations than populations from either the Northern or Utah Rocky Mountain regions. Patterns of genetic diversity in P. flexilis have likely resulted from a complex distribution of Pleistocene populations and subsequent gene flow via pollen and seed dispersal.     

A comparative analysis of seed and cone characteristics and seed-dispersal strategies of three pines in the subsection Sabinianae

The seed-dispersal systems of Coulter pine (Pinus coulteri), gray pine (P. sabiniana), and Torrey pine (P. torreyana), all of the subsection Sabinianae, are not well understood. These pines occur in arid and semi-arid foothills and mountains of California that are subjected to frequent fires. Cone and seed traits of these three California pines are compared to those of four species of pines (sugar pine, P. lambertiana; Jeffrey pine, P. jeffreyi; ponderosa pine, P. ponderosa; and lodgepole pine, P. contorta) that occur in more mesic environments in the nearby Sierra Nevada mountains. The cones of the Sabinianae pines are large with thick, dense scales, and the scales of gray and Coulter pines are armed with sharp, recurved spines. The seeds of all three species are large, and those of gray and Torrey pines are nearly wingless. In contrast, the Sierra Nevada pines have small to medium-sized seeds with large wings that are initially dispersed by the wind. Heavy wing loading of the Sabinianae pine seeds causes them to fall rapidly, and they are not dispersed far by wind. However, animals remove the fallen seeds rapidly, and rodents and jays scatter hoarded many seeds in the soil. This caching activity results in seedling establishment. The unusual morphology of the cones and seeds of the Sabinianae pines is interpreted as a combination of traits that attract animal dispersers, thwart the foraging activities of seed predators, and promote the survival of seeds in an environment subject to frequent fires.     

Empirical assessment of the reproductive fitness components of the hybrid pine Pinus densata on the Tibetan Plateau

Pinus densata is distributed on the Tibetan Plateau, where it forms extensive forests at high elevations. Genetic studies have provided evidence that P. densata originated through hybridization between P. yunnanensis and P. tabuliformis. To clarify the relationships among these pines, and assess their reproductive fitness in their respective habitats, we conducted a comparative analysis of eight cone and seed morphometric traits and six reproductive traits in them. Among the eight morphometric traits examined, six appeared to be intermediate in P. densata between those of P. yunnanensis and P. tabuliformis. There were significant differences among the three pines in all of the morphometric traits, and P. densata showed greater variability in these traits than the other two pines. In contrast to the morphometric traits, the reproductive traits (including the proportions of filled and empty seeds, ovule abortion rate, seed efficiency, meiotic abnormalities during microsporogenesis and pollen viability) differed little among the three pines, indicating that they have similar overall rates of effective pollination and fertilization in their respective natural environments. Despite their location on the high plateau, natural populations of P. densata appeared to have normal levels of reproductive success, comparable to those of the two parental species in their natural habitats. This study provides empirical data characterizing the reproductive success and adaptation of a stabilized homoploid hybrid in a novel habitat that is ecologically and spatially inaccessible to its parental species. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Seed and cone morphometric indicators: A new tool for the discrimination between the common Mediterranean pines Pinus halepensis Mill. and P. brutia Ten

Abstract

The two typical Mediterranean pines of Greece (Pinus halepensis Mill. and P. brutia Ten.) show a distinct, natural geographical distribution. However, in sites where they grow in sympatry (mainly due to extended reforestations), their morphological discrimination is not always easy. In the present study, three new morphometric indicators are introduced (at decreasing rank of reliability): (a) seed coat mass fraction, expressed as a percentage of the total seed mass (SM) (overall mean values 27.9 ± 0.5 (n = 160), and 58.0 ± 0.3 (n = 320) from 18 and 16 populations of Aleppo and East Mediterranean pine, respectively), (b) mean SM (16.70 ± 0.26 mg, n = 651, and 42.79 ± 0.61 mg, n = 425 from 18 and 16 populations of each of the two species, respectively) and (c) cone length/width ratio (2.20 ± 0.01, n = 651, and 1.77 ± 0.01, n = 606 from the same 18 and 16 populations, respectively). These three parameters are easily calculated, and constitute a reliable tool for the discrimination of the two pine species on the basis of their reproductive units (cones and seeds), provided that a minimal number of randomly collected samples is used for each indicator: 20 seeds for the measurement of seed coat mass fraction, about 1000 seeds for the estimation of mean SM, and 20 cones for the calculation of the cone length/width ratio. The functional implications of these adaptive traits are discussed.     

Coevolution between crossbills and black pine: the importance of competitors,forest area and resource stability

Studies of predator‐prey interactions have found that geographically structured coevolution has played an important role in the adaptive diversification of crossbills (Loxia spp.). We extend those studies by considering common crossbills (L. curvirostra) in the Mediterranean where they rely on seeds in the cones of black pine (Pinus nigra). On the continent, where tree squirrels (Sciurus vulgaris) are present, enhanced defenses against crossbills were most evident in larger areas of pine forest. On islands in the absence of tree squirrels, crossbills and black pine have coevolved in a predator‐prey arms race on Cyprus but not Corsica. In contrast to other conifers that island endemic crossbills rely upon, black pine does not hold seeds in its cones year round. Consequently, key to the strong crossbill–pine interaction on Cyprus is likely the presence of an alternative conifer that provides seeds during early summer when black pine seeds are scarce.     

Sporocarp δ15N and use of inorganic and organic nitrogen in vitro differ among host-specific suilloid fungi associated with high elevation five-needle pines

Widespread decline of whitebark and limber pines in the northern Rocky Mountains (USA) has created an imperative to understand functional diversity in their ectomycorrhizal associates. Because suilloid fungi are likely important in successful reestablishment of pines the nitrogen-related functional traits of 28 high-elevation suilloid isolates were examined. Radial growth, mass accumulation and mycelial density were measured for isolates on six different nitrogen sources. The δ¹⁵N values of suilloid sporocarps used as sources for pure cultures were compared against growth parameters to investigate a possible link between these N-related functional traits. Isolates grew poorly on nitrate and BSA and grew well on glutamine, alanyl-glutamine and ammonium phosphate, with somewhat slower growth on alanine. Isolates and species varied considerably in their growth response to different nitrogen sources. Effective use of nitrate and BSA was uncommon and associated with isolates with high inherent growth rates. Sporocarp δ¹⁵N was negatively correlated with relative growth on alanine of the corresponding isolates. Our results suggest strong similarities in N source use patterns of suilloid fungi of whitebark pine origin and those of another high-elevation five-needle stone pine, the Swiss stone pine.     

Genetic variation at allozyme and RAPD markers in Pinus longaeva (Pinaceae) of the White Mountains, California

We compared genetic diversity estimated from allozymes and from random amplified polymorphic DNA (RAPDs) in a sample of 210 Great Basin bristlecone pines (Pinus longaeva Bailey) from three groves in the White Mountains, California, USA. The White Mountains are the most westerly extension of bristlecone pine and home to the oldest known living trees. We assayed two forks of each tree to determine whether they originated from multiple seed caches of the Clark's nutcracker. Despite the limited and fragmented distribution of bristlecone pine, its level of genetic diversity was comparable to that of other pines, but lower than that reported for eastern populations of Great Basin bristlecone pine. Twenty-six of 36 allozymes were polymorphic (p(95) = 38.9%; p = 63.0%), with observed heterozygosity (H(o)) of 0.122 and expected heterozygosity (H(e)) of 0.134. The proportion of the total variation among populations (G(ST)) was only 0.011. The high proportion of trees with multiple stems was not due to germination in seed caches; only six of 210 forked trees had multiple allozyme genotypes. Of the 42 RAPD loci scored, 27 were monomorphic. Genetic diversity for RAPDs was nearly the same as that for allozymes (p(95) = 34.1%, H(e) = 0.130). However, the estimates of diversity and differentiation were much higher (H(e) = 0.321, G(ST) = 0.039) after excluding monomorphic loci.     

Growth form distribution and genetic relationships in tree clusters of Pinus flexilis,a bird-dispersed pine

Seed dispersal by the Clark's nutcracker (Nucifraga columbiana Wilson) may markedly influence the growth form and genetic population structure of limber pine (Pinus flexilis James). The nutcracker buries clusters of seeds in subterranean caches; germination of clustered seeds often results in a growth form characterized by two or more genetically distinct trees with fused or contiguous trunks (tree clusters). The occurrence of a morphologically similar form, the multi-trunk tree (a single genet branched near the base), as well as the typical single-trunked tree, complicates the study of limber pine populations. We examined growth form distribution and genetic relationships in tree clusters in limber pine populations at four elevations (from 2585 m to 3460 m) in the Colorado Front Range. At three study areas, relative occurrence of limber pine growth forms, as well as that of associated pines, was examined by a point-centered quarter survey. From the four study areas, we collected foliage from each trunk from a total of 74 clumps (combined tree clusters and multi-trunk trees) in order to differentiate the two growth forms using starch gel protein electrophoresis. Tree clumps were significantly more common in limber pine than in ponderosa or lodgepole pine (P<0.010). Although single-trunk limber pine was the most common growth form, except at the highest elevation, both multi-trunk trees and tree clusters were present in each stand. Tree clusters were estimated to comprise about 20% of the tree sites in each limber pine stand; the estimated proportion of multi-trunk trees varied by site from 5% to 77%. Trees in clusters were related, on average, as half to full siblings (mean r=0.43), but were unrelated to trees in other clusters (mean r=0.01). Electrophoretic analysis suggests possible genetic differentiation in limber pine that may be the result of different selection pressures on the growth forms.     

A seed predator drives the evolution of a seed dispersal mutualism

Although antagonists are hypothesized to impede the evolution of mutualisms, they may simultaneously exert selection favouring the evolution of alternative mutualistic interactions. We found that increases in limber pine (Pinus flexilis) seed defences arising from selection exerted by a pre-dispersal seed predator (red squirrel Tamiasciurus hudsonicus) reduced the efficacy of limber pine's primary seed disperser (Clark's nutcracker Nucifraga columbiana) while enhancing seed dispersal by ground-foraging scatter-hoarding rodents (Peromyscus). Thus, there is a shift from relying on primary seed dispersal by birds in areas without red squirrels, to an increasing reliance on secondary seed dispersal by scatter-hoarding rodents in areas with red squirrels. Seed predators can therefore drive the evolution of seed defences, which in turn favour alternative seed dispersal mutualisms that lead to major changes in the mode of seed dispersal. Given that adaptive evolution in response to antagonists frequently impedes one kind of mutualistic interaction, the evolution of alternative mutualistic interactions may be a common by-product.     

From the ground up: biotic and abiotic features that set the course from genes to ecosystems

Spatial variation in cone serotiny in Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia) across Yellowstone National Park influences initial pine recruitment after stand‐replacing fire with tremendous population, community, and ecosystem consequences. A previous study showed that much of the spatial variation in serotiny results from the balance of selection arising from high frequencies of fire favoring serotiny countered by opposing selection exerted by American red squirrels (Tamiasciurus hudsonicus) as seed predators. This earlier study, however, assumed stable local red squirrel densities over multiple generations of pines. Here, we examine environmental properties that might contribute to long‐term stability in the densities of red squirrels among sites. We found that the amount of clay in the soil, an indicator of plant and fungal growth—the latter an important food resource for red squirrels—and the coefficient of variation (CV) in diameter at breast height (DBH) of forest trees together account for a substantial amount of variation in red squirrel density. Soil development occurs over very long time scales, and thus, intersite variation in the amount of clay is unlikely to shift across pine generations. However, CV of DBH and squirrel density increase with stand age, which acts to amplify selection against serotiny with increasing interfire interval. Regardless, much of the variation in the CV of DBH is accounted for by soil bulk density, mean annual temperature, and surface curvature, which are unlikely to vary in their relative differences among sites over time. Consequently, these soil and abiotic attributes could contribute to consistent spatial patterns of red squirrel densities from one pine generation to the next, resulting in consistent local and spatial variation in selection exerted by red squirrels against serotiny.     

Leap frog in slow motion: Divergent responses of tree species and life stages to climatic warming in Great Basin subalpine forests

In response to climate warming, subalpine treelines are expected to move up in elevation since treelines are generally controlled by growing season temperature. Where treeline is advancing, dispersal differences and early life stage environmental tolerances are likely to affect how species expand their ranges. Species with an establishment advantage will colonize newly available habitat first, potentially excluding species that have slower establishment rates. Using a network of plots across five mountain ranges, we described patterns of upslope elevational range shift for the two dominant Great Basin subalpine species, limber pine and Great Basin bristlecone pine. We found that the Great Basin treeline for these species is expanding upslope with a mean vertical elevation shift of 19.1 m since 1950, which is lower than what we might expect based on temperature increases alone. The largest advances were on limber pine‐dominated granitic soils, on west aspects, and at lower latitudes. Bristlecone pine juveniles establishing above treeline share some environmental associations with bristlecone adults. Limber pine above‐treeline juveniles, in contrast, are prevalent across environmental conditions and share few environmental associations with limber pine adults. Strikingly, limber pine is establishing above treeline throughout the region without regard to site characteristic such as soil type, slope, aspect, or soil texture. Although limber pine is often rare at treeline where it coexists with bristlecone pine, limber pine juveniles dominate above treeline even on calcareous soils that are core bristlecone pine habitat. Limber pine is successfully “leap‐frogging” over bristlecone pine, probably because of its strong dispersal advantage and broader tolerances for establishment. This early‐stage dominance indicates the potential for the species composition of treeline to change in response to climate change. More broadly, it shows how species differences in dispersal and establishment may result in future communities with very different specific composition.