Elevation and Temperature Effects on Carbon Balance Near Alpine-treeline: Comparison of a Treeline and Non-treeline Tree Species

Changes in carbon balance of trees may help explain temperature and range limits of conifers near alpine- treeline and the climate sensitivity of forest boundaries. Our objective was to determine which component of carbon balance most limit tree seedling growth at high elevation, and how growth processes vary in their response to temperature. We assessed relationships of temperature, carbon flux, and growth in whole seedlings of a treeline and non-treeline species (Abies lasiocarpa and Pseudotsuga menziesii, respectively) at two elevations near alpine-treeline in the Teton Range of Wyoming, USA. Seedlings were outplanted as they germinated in potting soil substrate in sites having sparse overhead canopy cover at 2450 m (high forest) and 3000 m (near treeline) elevations. Gas exchange and growth measurements were performed every 2-3 weeks when treeline was snow-free, in 2005 and 2006. Growth was less at the higher elevation in both species, and was associated with less needle area, root mass, and photosynthetic carbon assimilation (A) and respiratory efflux (R). However, R decreased more than A with cooling and at the higher elevation, causing an increase in A:R with elevation. The primary difference between species was greater growth, A, and R in the treeline species. Reductions in A per unit leaf area were expected at the higher elevation from previous studies, but were not observed. Notable differences in our experiment were the elimination of tree canopy (i.e. shading) differences that normally occur among elevations and are known to affect seedlings. Additionally, we did not detect any frost during our study periods, which resulted in part from local topography and snow pack patterns that distinguish the climate regime of the subject treeline. Moreover, microclimate measured beyond our study periods indicates that seedlings experience more frost at the lower compared to upper sites we evaluated, with the frosts typically occurring when the upper elevation site is snow-covered. The results of this study, along with consideration of differences in climate among treeline studies, have important implications for transferability of tree-climate information among treelines, and thus monitoring or predicting treeline change.