Whitebark Pine (Pinus albicaulis) vigor and functional roles within the alpine treeline ecotone

Background/Question/Methods

At treeline on the Eastern Front of the Rocky Mountains, whitebark pine (Pinus albicaulis) appears to facilitate tree island development. Whitebark pine is currently declining at treeline from infection by white pine blister rust, caused by Cronartium ribicola. We are studying how whitebark pine facilitates tree island formation and how blister rust mortality may affect these processes in two treeline study areas in Montana: Divide Mountain, Glacier NP and Blackfeet Reservation, and Line Creek Research Natural Area, Custer National Forest.

We are testing three hypotheses: 1) in exposed sites whitebark pine is hardier than other treeline conifers, 2) whitebark pine provides a more favorable leeward microsite for tree island recruitment than other conifers or microsites, and 3) death of windward whitebark pine in established tree islands leads to loss of vigor in leeward conifers. Here we address the first hypothesis, using general condition, shoot lengths, and shoot growth rates as measures of vigor. Branch shoots of 100 combined whitebark pine, Engelmann spruce (Picea engelmannii), and subalpine fir (Abies lasiocarpa) were measured in July and September 2011. Differences in mean shoot lengths were divided by the number of days between measurements to calculate growth rates. Shoots from 30 upright upper subalpine conifers of the same three species were measured in September 2011 for comparison.

Results/Conclusions  

General condition comparisons did not prove to be significant. We compared fully elongated September shoot lengths by species and tree growth form using ANOVA and post hoc tests. Whitebark pine had significantly longer shoots at each study area, whether compared among krummholz or upright growth forms.  Subalpine fir and Engelmann spruce shoot lengths did not differ significantly in either growth form.  We found that krummholz whitebark pine had a significantly faster July-September shoot growth rate than subalpine fir on Divide Mountain and Engelmann spruce at both study areas. Subalpine fir and Engelmann spruce growth rates did not significantly differ at either study area. Upright tree shoots were compared to krummholz shoots in t-tests and found to be significantly longer for all species, perhaps reflecting the favorable growing conditions below treeline. Data generally demonstrated that whitebark pine exhibited greater vigor, possibly indicating a growth advantage in harsh treeline environments.  Vigorous growth under harsh conditions and potentially better survival may explain why whitebark pine frequently initiates tree islands as a nurse object.