The role of Pinus albicaulis as a tree island initiator in the alpine-treeline ecotone: Comparing microclimates among microsites

Background/Question/Methods

In the alpine-treeline ecotone on the eastern front of the Montana Rocky Mountains, whitebark pine (Pinus albicaulis) initiates tree island formation more often than any other conifer. Whitebark pine acts as a “nurse” object for the establishment of other conifers, especially on northeast-facing aspects. To explain these observations, we hypothesize: 1) nurse objects in general will show a more a moderate leeward microclimate compared to unprotected sites, 2) conifer nurse microsites will have a more moderate microclimate than rock and unprotected microsites, and 3) whitebark pine microsites will be more moderate than Engelmann spruce microsites. We measured soil temperature, wind speed, and soil moisture from mid-July to mid-September 2010 and 2011 for blocks of microsites on northeast facing aspects at two treeline study areas in Montana: Divide Mountain, Glacier National Park and Blackfeet Reservation, and Line Creek Research Natural Area, Custer National Forest. We compared maximum, minimum and range values for different microsites.  Moderate climate was defined as having a narrower range of soil temperatures, a tendency towards moister soils, and lower wind speeds.  All microsites were compared using a combination of ANOVA, post hoc, and ANCOVA analyses.

Results/Conclusions

Line Creek had higher maximum soil temperatures than Divide Mountain. At both study areas, Engelmann spruce showed significantly smaller soil temperature ranges than whitebark pine and therefore had less fluctuation between high and low temperatures.  In general, conifers showed lower daily maximum temperatures compared to rock and unprotected sites. Whitebark pine microsites had significantly higher soil moisture than all other microsites at Line Creek; and, for two of three blocks at Divide Mountain, conifer microsites had greater soil moisture levels than rock and unprotected sites. Nurse objects significantly reduced microsite wind speeds in the Line Creek study area, but differed in which provided better protection. In general, nurse objects moderated microclimate more than unprotected sites while Engelmann spruce and whitebark pine moderated soil temperature and soil moisture more than rock and unprotected sites. Whitebark pine did moderate wind and soil moisture, but results were inconsistent. Results support the hypothesis that nurse objects, and especially conifers are important for facilitating the establishment of other conifer species in the alpine treeline ecotone.