COS 115-9 - Altered disturbance patterns promote facilitated competition in aspen-conifer forests

Thursday, August 11, 2011: 4:20 PM
12B, Austin Convention Center
Joshua R. Buck, Department of Plant and Wildlife Sciences, Brigham Young University, Provo, UT and Samuel B. St Clair, Plant and Wildlife Sciences, Brigham Young University, Provo, UT

Recent studies have identified a strong facilitative association in mixed aspen-conifer forests in which microsite conditions at the base of aspen trees, an early successional species, significantly increases germination and establishment success of subalpine fir, a late successional species.  As conifers establish within aspen stands susceptibility to fire increases resulting in fire cycles that historically occurred in 70-90 year intervals.  Recent analysis suggests that climate conditions and fire suppression has drastically lengthened fire return intervals in aspen-conifer forests.   We hypothesize that longer fire cycles creates competitive interactions between aspen and subalpine fir trees that establish at their base. We identified three stand types representative of early (aspen dominant), intermediate (mixed), and late (conifer dominant) successional stages at seven field locations on the Fish Lake National Forest, UT.  Using the point quarter and nearest neighbor methods we characterized the influence of mature subalpine fir proximity on aspen mortality.


Aspen mortality increased from 4% in aspen stands to 38% in conifer dominated stands. Additionally aspen mortality approximately doubled when subalpine fir trees were located <1m from the base of an aspen tree (17% in aspen stands and 66% in conifer stands). The results suggest that facilitative associations between aspen and conifer can strongly increase aspen mortality rates under longer fire cycles. These changes in mortality rates may be a driving force behind aspen to conifer succession and recent patterns of aspen dieback in subalpine forest systems.

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