Do Himalayan treelines respond to recent climate change? An evaluation of sensitivity indicators

Abstract

Abstract. Climate warming is expected to induce treelines to advance to higher elevations. Empirical studies in diverse mountain ranges, however, give evidence of both advancing alpine treelines as well as rather insignificant responses. The inconsistency of findings suggests distinct differences in the sensitivity of global treelines to recent climate change. It is still unclear where Himalayan treeline ecotones are located along the response gradient from rapid dynamics to apparently complete inertia. This paper reviews the current state of knowledge regarding sensitivity and response of Himalayan treelines to climate warming, based on extensive field observations, published results in the widely scattered literature, and novel data from ongoing research of the present authors. Several sensitivity indicators such as treeline type, treeline form, seed-based regeneration, and growth patterns are evaluated. Since most Himalayan treelines are anthropogenically depressed, observed advances are largely the result of land use change. Near-natural treelines are usually krummholz treelines which are relatively unresponsive. Nevertheless, intense recruitment of treeline trees suggests a great potential for future treeline advance. Competitive abilities of seedlings within krummholz thickets and dwarf scrub heaths will be a major source of variation in treeline dynamics. Tree growth-climate relationships show mature treeline trees to be responsive to temperature change, in particular in winter and pre-monsoon seasons. High pre-monsoon temperature trends will most likely drive tree growth performance in W and central Himalaya. Ecological niche modelling suggests that bioclimatic conditions for a range expansion of treeline trees will be created during coming decades.

How do soil properties affect alpine treelines? General principles in a global perspective and novel findings from Rolwaling Himal, Nepal

“…This result makes us question the paramount role of soil temperature since it indicates that such a distinct higher temperature does not necessarily correspond with treeline shift. Currently, the treeline position is rather stable, and a considerable treeline advance is not to be expected in the medium-term (several years to a few decades; Schickhoff et al, 2015). Table 2 indicates both latitude-and local scale-dependent variations in growing season mean soil temperatures at alpine treelines, suggesting that it makes sense to differentiate between tropical and extratropical (subtropical, cool/warm temperate, boreoarctic, Mediterranean), and oceanic treelines, respectively.…”

Land-use change under a warming climate facilitated upslope expansion of Himalayan silver fir (Abies spectabilis (D. Don) Spach)

“…In both our study areas, transhumance is still practiced, but in the GCA it does not affect the sampling area (Schickhoff et al 2014). During summer, cattle herds are taken to the highland, while during autumn they are brought back to the lowland.…”

“…In the GCA, the practice of collecting dead tree branches and lopping signs were observed mainly in the dense forest at lower elevations. It is a near-natural or climatic treeline in this area (Müller et al 2015;Schickhoff et al 2014). …”

Treelines—Approaches at Different Scales

“…According to a model presented by Paulsen and Körner [82], for example, treeline would be located at an elevation where the minimum length of the growing season is 94 days, with a daily minimum temperature just above zero (0.9 • C) and a mean of 6.4 • C during all these days. The extent of treeline advancement to greater elevations and more northerly latitudes, however, varies regionally, locally, and temporally as well as by tree species (e.g., [8,29,43,63,[83][84][85][86][87][88][89]). The direct influence of heat deficiency on tree establishment is often overruled by other abiotic and biotic factors (e.g., permanent strong winds and recurrent periods of drought).…”

“…Figure 6) that responds differently to climate change. Modern change in economic structure, particularly decreases or cessations of grazing alpine pastures, probably supported by warming climate, has resulted in the invasion of many former alpine pastures by trees (e.g., [5,27,83,89,96,[120][121][122][123][124]). …”