Forest management: Trees marching uphill

Ultimately, the worry of treeline scientists is focused on the trend shown by high elevation treelines in response to recent climate change, which can be a serious threat to the region’s rich biodiversity

It was summer 2008 and we were in Cholangpati at an elevation of 3500 m a.s.l., a few hours of steep and difficult climbing from Dhunche on the way to Gosaikunda. At an elevation of 3910 m above sea level, close to Lauribinayak, we found a single tree of Himalayan fir standing about 2.5 metres tall. Above this elevation high-stature trees disappeared, and is therefore termed the ‘treeline’ elevation. From here upwards a landscape with grasses and shrubs begins.

Why are there no trees above the treeline? Scientists blame harsh climate conditions for the disappearance of tree stature in the landscape. However, one may observe dwarfed individual of the tree species, termed krummholz.

The famous Swiss treeline scientist Christian Körner, the author of ‘Alpine Plant Life’, argues that tree stature (     > 2m) cannot be sustained at high elevations because it does not permit trees to decouple from the cool atmosphere during the night. Eventually, the trees lose their body heat and canopy temperature, which impairs apical meristem activity and limits tree growth.

Rising temperatures and climate change are global issues in the world today. It has been proposed that trees should be moving upwards and encroaching into the meadows as a response to global warming across the world. Melanie Harsch, a treeline ecologist from Lincoln University in New Zealand, reported that about half of the 152 treelines she has considered are indeed moving towards higher elevations. This implies that trees are marching into the meadows as a response to temperature warming. In this case, the tree species of the region, such as birch, pines and firs, will eventually take over the scenic green landscape.

In the Himalayas, these subalpine meadows are the home of several dwarf flowering plants such as asters, buttercups, and bistorts which provide a livelihood for hundreds of butterflies and bees, as well as other wild animals such as deer and blueships. Thus, the disappearance of such mountain meadows in any scale would be ‘tragic’ to the people who have enjoyed the brilliant view of floral displays framed against a background of snowfields. Eventually, many meadow species will be compelled to leave their original habitat or even become extinct.

How are the treelines in the Himalayan landscapes responding to these changes? A few studies from the region have claimed that young tree seedlings are rapidly migrating upwards as a response to warming. However, Udo Schickhoff, a German scientist and pioneer of Himalayan treeline studies, suspects that this claim is not synonymous with an actual treeline advance.

We (Prof. Annika Hofgaard, Prof. Vigdis Vandvik and I) considered tree-sized individuals (> 2 m height) for the assessment of current treeline behaviour in dry (Ngawal, Manang) and wet areas (Langtang, Rasuwa) of the Nepalese Himalayas. The trees were cored at 0 m and 2 m above ground and their age was estimated at both heights. The difference of ages at two heights is calculated as the total time span the individual has needed to become a tree. Our analysis shows that the trees at the current treeline were established during the 1990s in both areas and had remained stationary since that time.

However, the main driving forces in two areas were observed to be different. Smaller tree individuals as one goes to higher elevations indicate that the pine treelines in Ngawal were influenced by the climate. At the exposed higher elevations, the growing season may be very short during especially cold years, and severe droughts may occur during low-precipitation anomalies which reduce the growth and survival potential of tree recruits. Decreasing summer temperatures with high interannual climatic variability observed over the past decades might be one potential explanation for the recessive treeline development in this area. However, in wet Lauribinayak, despite a significant warming trend in summer temperatures over the past decades, the shaping of the fir treeline position was observed to be more influenced by grazing due to ungulate herbivores such as blue sheep.

Our report of stationary or recessive treelines in two areas of the Nepalese Himalayas despite climatic warming coincides with the result reported by Eryuan Liang, an ecologist at the ‘Chinese Academy of Sciences’, Beijing. His team reported that the treeline position at several sampling sites in Nepal and southeastern Tibet has

not changed in the past 200 years. In contrast to the treelines in other mountains, such as the Alps, Himalayan treelines are a lot higher, drier and largely controlled by precipitation. The annual rings extracted from the trees

in the region indicate that the past climate history of the area was most influenced by dry pre-monsoon seasons, which are crucial for tree growth and recruitment.

Ultimately, the worry of treeline scientists is focused on the trend shown by high elevation treelines in response to recent climate change, which can be a serious threat to the region’s rich biodiversity. Our results recommend that the managers and policy makers should be aware of the situation and start to take immediate action on high elevation forest management in the Nepalese Himalayas in order to protect endemic and endangered species.

Shrestha is a PhD, University of Bergen, Norway