Sapling performance and size distribution across a montane deciduous/coniferous ecotone: Toward a demographic understanding of range limits

Accurate prediction of forest response to climate change requires a detailed understanding of tree species’ demography and interspecific interactions at range limits.  Low latitude/altitude range limits of cold-tolerant species are thought to be set by competition with faster growing, but less cold-tolerant species.  Alternatively, low limits may be set by climate factors such as water balance, or other biotic factors such as herbivory.  To distinguish these alternatives, I examined the performance and size distribution of saplings of six tree species in the Green Mountains of Vermont, USA:  the high elevation conifers Abies balsamea, Picea rubens, and the lower elevation deciduous species Betula allegheniensis, Acer pensylvanicum, Fagus grandifolia, and Acer saccharum.  At five elevations (680 – 890 m) spanning the deciduous/coniferous ecotone at 790 m, I measured height growth (terminal increment) on healthy high-light saplings between 75 cm and 350 cm tall (n per species ranged from 44 to 124).  I also measured sapling size distributions (up to 300 cm tall) at randomly located plots at these elevations.  Growth within each species varied little with elevation, but all species, including Abies and Picea, tended to have greatest growth at the lowest elevations, suggesting that they are limited by climate factors there.  Across all elevations, deciduous species grew twice as fast as the conifers (except A. saccharum, which did not differ from conifers).  These data are consistent with a strong role of competition in setting the lower limits of conifers.  Size distribution data were also consistent with this, and suggested that A. saccharum and F. grandifolia tend to be limited at upper elevations mainly by reduced seedling recruitment, whereas A. balsamea and P. rubens are limited at lower elevations by reduced sapling survival.