Contrasting responses to climate change at Himalayan treelines revealed by population demographics of two dominant species

<ol> <li><span>Alpine treelines are expected to shift upward due to recent climate change.  However, interpretation of changes in montane systems has been problematic because effects of climate change are frequently confounded with those of land use changes. The eastern Himalaya, particularly Langtang National Park, Central Nepal, has been relatively undisturbed for centuries and thus presents an opportunity for studying climate change impacts on alpine treeline uncontaminated by potential confounding factors.</span></li> <li><span>We studied two dominant species, <i>Abies spectabilis </i>and <i>Rhododendron campanulatum</i>, above and below the treeline on two mountains.  We constructed 13 transects, each spanning up to 400m in elevation, in which we recorded height and state (dead or alive) of all trees, as well as slope, aspect, canopy density, and measures of anthropogenic and animal disturbance. In a subset of individuals, we determined the age of the plants with tree rings.</span></li> <li>All size classes of <i>R. campanulatum</i> plants had lower mortality above treeline than below it, and young <i>R. campanulatum</i> plants (&lt;2m tall) were at higher density above treeline than below.  <i>A. spectabilis</i> shows little evidence of a position change from the historic treeline, with a sudden extreme drop in density above treeline compared to below.   Recruitment, as measured by size class distribution, was greater above treeline than below for both species but <i>A. spectabilis</i> is confined to ~25m above treeline whereas <i>R. campanulatum</i> is luxuriantly growing up to 200m above treeline. </li> <li> <i>Synthesis.</i> Evidence suggests that the elevational limits of <i>R. campanulatum</i> have shifted upward both because (1) young <i>R. campanulatum</i> plants above treeline benefited from facilitation of recruitment by surrounding vegetation, allowing upward expansion of recruitment, and (2) increased adult survival due to a general temperature amelioration to mature plants. We predict that the current pure stand of <i>R. campanulatum</i> growing above treeline will be colonized by <i>A. spectabilis</i> that will, in turn, outshade and eventually relegate <i>R. campanulatum </i>to be a minor component of the community, as is the current situation below the treeline. </li> </ol>