Symbiotic interactions above treeline of long-lived pines: Mycorrhizal advantage of limber pine (Pinus flexilis) over Great Basin bristlecone pine (Pinus longaeva) at the seedling stage

In response to contemporary changes in climate, many tree species are shifting upslope to find favorable habitat. In the case of obligate ectomycorrhizal species, seedling growth above upper treeline depends on fungal spore availability. In the mountain ranges of the Great Basin, a recent shift in tree species stratification has been recorded, with limber pine (LP, Pinus flexilis) leapfrogging above the ancient bristlecone pine (BCP, Pinus longaeva) forest and establishing above current treeline. We compared the ability of LP and BCP to interact with soil spore banks collected at different microhabitats (next to dead trees, young live trees or in a treeless control) above current treeline in the White Mountains of California. We found an ectomycorrhizal fungal spore bank community composed of 15 species that was dominated by an undescribed and a hitherto unsequenced species of Geopora and Rhizopogon, respectively. This represents a much richer community than was found previously in this system. While both LP and BCP were able to establish ectomycorrhiza, LP was twice as likely to do so, and when comparing only seedlings that were colonized, its root system was colonized to a three-fold greater extent. BCP seedlings grown on soils collected under young live trees were much more likely to be colonized compared to soils from the other two microhabitats. Synthesis. These differences in ectomycorrhizal receptivity might help to explain why LP is currently establishing at higher rates above the BCP treeline. Furthermore, it is possible that LP saplings above treeline can provide ectomycorrhizal facilitation for BCP seedlings, enabling the subsequent shift of BCP above treeline.