COS 168-1 - Predictions of treeline movement depend on species-specific seedling niche requirements: A case study of regeneration in limber pine communities at treeline in Rocky Mountain National Park
Alpine treelines worldwide are primarily limited by root zone temperatures, and so are expected to advance with changing climate. However, only about 60% of treelines show evidence of any advance with increases in average temperatures to date. Moisture is known to modulate the effects of temperature on treeline position, but the alpine treeline ecotone is also highly heterogeneous. Treeline response to changing climate, on a landscape level, may therefore be dependent on the availability of suitable microsites for seedling establishment, and these requirements vary by species. We surveyed 16 treeline study areas in Rocky Mountain National Park (RMNP), east and west of the Continental Divide, across aspects, and with varying species composition. We were particularly interested in including communities with limber pine (Pinus flexilis), a drought-tolerant species found primarily in subalpine forest in RMNP. Our aim was to determine whether regeneration density and species composition was related to the species composition of larger trees at treeline. We hypothesized that regeneration density would be higher in communities where limber pine is the dominant species. We surveyed two 25 m x 25 m quadrats at each study area and recorded species ID, heights, and GPS locations of every tree in the quadrat.
Results/Conclusions
Preliminary results show that regeneration is denser in communities west of the Continental Divide, where there is greater moisture availability year-round. These communities are dominated by conifers that require greater moisture, such as subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii). Limber pine treeline communities in RMNP are found almost exclusively east of the Continental Divide where they are tolerant of drier conditions in the rain shadow. Where limber pine is dominant east of the Divide, regeneration is greater than in communities where limber pine is absent or a minor component. Our preliminary results suggest that interactions between species dispersal, species-specific niche requirements of seedlings, and microsite availability are important to consider for predictions of treeline movement.
