The Treeline Ecotone in Rolwaling Himal, Nepal: Pattern-Process Relationships and Treeline Shift Potential

Abstract

Using a landscape approach, the sensitivity, and response of a treeline ecotone to climate warming were addressed in a multiyear study in Rolwaling Himal, Nepal. The Rolwaling treeline ecotone still has a vegetation zonation unaffected by human land use, providing ideal conditions for capturing a climate signal. Along the elevational gradient, upper subalpine forests of tall, upright-growing Abies spectabilis and Betula utilis give way at the treeline to Rhododendron campanulatum krummholz at 3900-4000 m. Warming trends across Nepal have increased to more than 0.2 °C per decade. The treeline growing season mean soil temperature at 10 cm soil depth is 7.5 ± 0.6 °C, considerably higher than the limit of the life form tree. Thus, the treeline position in Rolwaling is lagging behind climatic changes. Lag factors, non-thermal drivers and feedback processes are largely related to the dense krummholz zone, which act as an effective barrier for upslope migration of other tree species. Feedback processes in the self-sustaining Rhododendron campanulatum krummholz create site conditions such as nutrient deficiency, reduced light availability and lower soil temperatures that severely restrict the competitiveness of other tree species, further constrained by allelopathic effects of Rhododendron. Tree growth-climate relationships show that climate warming-induced moisture deficits during pre-monsoon seasons have adversely affected radial tree growth of Abies spectabilis and Betula utilis over recent decades, also playing a role in retarded treeline dynamics. Regardless of stand densification and high levels of recruitment, even above the krummholz zone, a treeline shift to higher elevation is only to be expected in the medium to long term (decades to centuries).