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

Abstract

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, whichis 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.

Soil Preferences in Germination and Survival of Limber Pine in the Great Basin White Mountains

“…None of the seedlings survived beyond the first year, so the results of this study are also limited to the earliest life stages. However, since germination and the seedling stage are likely the most limiting life stages [20,21], focusing on the limitations of first-year seedlings can still be informative about this important bottleneck. It is also possible that the negative effects of dolomite soil on limber pine are not apparent until later life stages in which limber pine may ultimately be excluded.…”

“…I collected all soil on the same day in late October and sowed the seeds the day after soil was collected. bristlecone pine treeline on dolomite, an alternative hypothesis is that limber pine prefers soil types on which it is rarely found as an established adult [19,20]. This study first examines the relative abilities of different soil types to retain water over shorter (days) and longer (months) time scales.…”

“…Recent studies of Great Basin sub-alpine forests have shown that treeline is advancing through upslope establishment of young trees, but that this establishment has occurred only very recently and in infrequent pulses [19]. In addition, it appears that even though bristlecone pine is usually the dominant adult tree at treeline where it is found, typically downslope limber pine is responsible for the majority of treeline advance in these stands, with young trees "leap-frogging" over treeline adult bristlecone pine [19,20]. This mismatch is especially acute on certain soil types where bristlecone pine adults dominate the sub-alpine forests, but the vast majority of upslope establishment is limber pine [20].…”

“…In addition, it appears that even though bristlecone pine is usually the dominant adult tree at treeline where it is found, typically downslope limber pine is responsible for the majority of treeline advance in these stands, with young trees "leap-frogging" over treeline adult bristlecone pine [19,20]. This mismatch is especially acute on certain soil types where bristlecone pine adults dominate the sub-alpine forests, but the vast majority of upslope establishment is limber pine [20]. Upslope establishment by young trees is important in predicting future forests because limber pine and bristlecone pine seedlings have very high survival, even in early life stages [21].…”

“…In the western Great Basin both bristlecone pine and limber pine are found across all three Great Basin soil types, allowing for an explicit comparison of soil preference at the leading edge of treeline species. Where bristlecone pine is present, it is typically the dominant tree at treeline with the exception being on granitic soils where limber pine typically dominates [19,20]. Bristlecone pine is especially dominant on dolomite soils where nearly pure stands are often found to treeline [23].…”