Article Community Structure and Functional Role of Limber Pine (Pinus flexilis) in Treeline Communities in Rocky Mountain National Park Laurel A. Sindewald 1, Diana F. Tomback 1,* and Eric R. Neumeyer 2 1 Department of Integrative Biology, University of Colorado Denver, Denver, CO 80217-3362, USA; [email protected] 2 Department of Geography and Environmental Sciences, University of Colorado Denver, Denver, CO 80217-3364, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-303-548-4400 Received: 3 July 2020; Accepted: 28 July 2020; Published: 1 August 2020 Abstract: Research Highlights: Limber pine (Pinus flexilis) is abundant in some alpine treeline ecotone (ATE) communities east of the Continental Divide in Rocky Mountain National Park (RMNP) and the Colorado Front Range. Limber pine may be able to colonize the ATE under changing climate aided by directed seed dispersal by Clark’s nutcrackers (Nucifraga columbiana). Cronartium ribicola, white pine blister rust, is a growing threat to limber pine and may affect its functional role within the ATE. Background and Objectives: The ATE is sensitive, worldwide, to increasing temperature. However, the predicted advance of treeline under a changing climate may be modified by tree species composition and interactions. We aimed to (1) examine the conifer species composition and relative abundances in treeline communities with limber pine; (2) assess which functional roles limber pine assumes in these communities—tree island initiator, tree island component, and/or solitary tree; and (3) determine whether limber pine’s occurrence as a tree island initiator can be predicted by its relative abundance as a solitary tree. Materials and Methods: We selected four study sites in RMNP above subalpine forest limber pine stands. We sampled the nearest tree island to each of forty random points in each study site as well as solitary tree plots. Results: Across study sites, limber pine comprised, on average, 76% of solitary trees and was significantly more abundant as a solitary tree than Engelmann spruce (Picea engelmannii) or subalpine fir (Abies lasiocarpa). Limber pine was a frequent component of multi-tree islands in three study sites, the major component in one study site, and dominated single-tree islands at two study sites. At three of four study sites, no species had significantly greater odds of being a tree island initiator. Limber pine was found less often as a tree island initiator than predicted from its relative abundance as a solitary tree, given the likely role of solitary trees in tree island formation. Keywords: treeline; limber pine; Pinus flexilis; alpine treeline ecotone; Rocky Mountain National Park; climate change 1. Introduction The alpine treeline ecotone (ATE), the transitional zone between subalpine forest and alpine tundra, is a mosaic of alpine tundra vegetation such as cushion plants and graminoids, bare substrate, rocks, and semi-upright or krummholz trees growing individually or together in tree islands on the landscape [1–6]. The term “krummholz” is German for “crooked wood” and refers to trees that have a stunted, twisted, or mat-like growth form due to harsh conditions at high elevations [7]. The ATE, extending from the timberline (the upper limit of subalpine forest) to treeline (the upper limit of tree growth) [8], is considered highly sensitive to climate change given its close correlation Forests 2020, 11, 838; doi:10.3390/f11080838 www.mdpi.com/journal/forests Forests 2020, 11, 838 2 of 24 with isotherms, especially mean air and root zone temperatures [4,9,10]. Fluctuations in treeline position also correlate with past climate records, reflecting temperature increases and decreases over time [2,11–14]. Treelines worldwide have been predicted to advance upward as global average temperatures increase [14]. However, only 52% of treelines—examined between 1900 and 2006—had advanced in response to increases in average temperature [15], indicating that the temperature–treeline relationship is complex and impacted by other variables. The limits and patterns of tree occurrence in the ATE are controlled by processes at the local as well as regional scales [6,12,16]. At the landscape scale, slope aspect influences solar radiation. For example, in the Northern Hemisphere, southerly aspects receive more solar radiation than northerly aspects and are therefore associated with higher temperatures and lower soil moisture [12,16,17]. The ATE is also a highly heterogeneous system at the local scale, with variations in topography, soil, moisture, and temperature [12]. Nurse objects at treeline, such as plants, rocks, deadwood, and topographic depressions, provide protective microsites that enable seed germination and establishment of new trees [5,13,18–21], particularly by providing wind shelter and shade [13], but also by reducing convective heat loss to the sky [22,23] and by re-radiating solar energy via the black-body effect [24]. Trees that are well-established in the ATE also modify microclimatic conditions downwind and build soil [25], facilitating the survival of trees and ultimately forming tree islands—that is, patches of trees on the landscape [5,21,23]. In fact, facilitation interactions between plant species or with nurse objects are now considered an important mechanism for shaping communities in harsh environments [26–28]. Climatically controlled treelines may be limited by the availability of favorable microsites for seedlings [13,21,23,29,30]. The taxonomic (family-level) composition of ATE communities is also known to limit treeline position, in some cases causing the treeline to be 200–300 m lower than if a different tree taxon were dominant [9], and subtle differences in physiology among tree species may influence the sensitivity of the treeline to changes in climate. Community-level studies of species composition, seedling niche requirements, site conditions, and interactions between species are thus necessary to understand or predict idiosyncratic treeline responses to changing climate. Given that many mountain ranges of the Rocky Mountains in western Colorado, USA, have peaks above 3200–4000 m elevation, ATE communities are widely represented in the state. Treeline communities in very few of these ranges have been studied; research on the ATE in Colorado has been primarily limited to the Front Range [1,3,7,8,31–38]. The history of research at high elevations in the Front Range provides a valuable context for ongoing work, but studies of ATE communities in the Front Range and elsewhere in Colorado have so far been limited to those dominated by Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). ATE communities including limber pine (Pinus flexilis James) as a community component, while noted to exist [1,32,38], have not been studied. Limber pine is a five-needle white pine in the subgenus Strobus, section Quinquefoliae, subsection Strobus [39]. It is found from northern New Mexico at the southern limit of its range to Alberta and southeastern British Columbia, and from North Dakota at its eastern extent through eastern California [40]. It has a notably broad elevational distribution, with the lowest recorded tree at around 870 m in North Dakota and the highest at 3810 m in Colorado [40]. Limber pine opportunistically colonizes areas that have been burned or otherwise disturbed [41,42]. However, it is slow-growing and moderately shade intolerant, and thus a poor competitor. Limber pine is seral to faster-growing, shade tolerant conifers [40,43] except on xeric, often steep sites where its tolerance of arid conditions and poorly developed soils provides a survival advantage [43–46]. Limber pine can establish under arid, windy conditions above timberline where less hardy conifers may not survive without protection [41,45,46]. Its colonization of post-burn areas and of the ATE is due in large part to the seed-caching behavior of Clark’s nutcrackers (Nucifraga columbiana), which is limber pine’s primary mode of seed dispersal [47–50]. Clark’s nutcrackers harvest ripe limber pine seeds beginning in late August and bury caches of one to fifteen seeds under a few centimeters of soil in both forested and open communities across the landscape [51–53]. The seeds are retrieved from late winter through the following summer, Forests 2020, 11, 838 3 of 24 and unretrieved seeds may germinate and produce seedlings [51,54]. Nutcrackers provide directed seed dispersal, selecting cache sites near objects that tend to facilitate seed germination and seedling survival [54–56]. Nutcrackers often choose cache sites in areas where snow is likely to blow free or melt early (e.g., open areas or near tree boles), allowing for seed retrieval in the winter and early spring. These same sites may be ecologically suitable for seed germination, with extended growing seasons due to earlier snowmelt, gaps in canopy where seedlings are released from light competition, or near objects that may provide shelter from high winds [56,57]. Clark’s nutcrackers will often cache seeds in recently disturbed areas, especially after fire, leading to rapid regeneration of these sites [48,58]. Nutcrackers have also been observed to cache in the ATE [51,52,57,59]. Directed dispersal by Clark’s nutcrackers may explain the close association of limber pine regeneration with a substrate of larger particle sizes and nurse rocks in the ATE, likely providing limber pine with an advantage over wind-dispersed species such as Great Basin bristlecone pine (Pinus longaeva)[60]. In the Great Basin region, limber pine is advancing upslope, with greater densities of limber pine regeneration than Great Basin bristlecone pine regeneration above the timberline, particularly on dolomite soils, which retain water better than other soil types [60–62]. Although limber pine seedlings are drought-tolerant [44,60], limber pine seedlings survive better with higher moisture, as demonstrated by simulated climate warming experiments [63–65]. Empirical evidence suggests overall that limber pine will be able to advance into the ATE with climate warming, especially if precipitation also increases, which is in agreement with bioclimatic envelope model predictions for the species [66].