Plant species distribution along an elevational gradient in the Himalayas, Nepal : On drivers, mechanisms and the effects of climate change

Samenvatting

The Himalayas are amongst the global hotspots of biodiversity. Climate warming is forcing species to shift upslope at an alarming rate in the Himalayas, posing a serious threat to its biodiversity. To ensure present-day conservation and to accurately predict how montane plant species are likely to respond to future climate change, it is urgently needed to understand how environmental conditions and species properties (i.e. functional traits) shape species’ distributions along an elevational gradient (i.e. species’ elevational distributions). In this thesis, I analysed the elevational distribution patterns of 277 plant species along an elevational gradient in the Himalayas in Nepal, and showed which environmental factors best predict species’ elevational distributions and whether highland species occupy wider elevational ranges than lowland species; how functional traits shape species’ elevational positions and lead to species partitioning of the elevational gradient; and how future global warming will affect species’ elevational ranges and distribution areas.

In Chapter 2, I asked which environmental factors best predict species’ elevational distributions and whether highland species occupy wider elevational ranges than lowland species because species living at harsher, more stressful and variable highland conditions would have wider physiological tolerances to environmental conditions and therefore occupy wider elevational ranges. For the 277 selected Himalayan plant species, I modelled their environmental niches using MaxEnt, by combining species occurrence data with 19 climatic, topographic and edaphic factors. I found that mean annual temperature, followed by soil clay content and slope are the key environmental factors that best predict species’ elevational distributions in the Himalayas. Apart from obvious changes in temperature and its temperature (e.g. temperature extremes) and non-temperature (such as irradiance and PET) covariates, both soil conditions and topography thus also play a major role in shaping species distributions in the Himalayas. Mid-elevation species (2,000-3,000 m a.s.l.) had relatively wider elevational ranges than lowland and highland species. This suggests that mid-elevation species might be less affected by climate change since their wider elevational ranges may partially buffer against climate change induced range shifts.

In Chapter 3, I asked how functional traits shape species’ elevational positions and lead to species partitioning of the elevational gradient. For a subset of 31 tree species that occupied different positions along the elevational gradient, I measured 39 traits related to carbon, water and nutrient use. I found that the highland species have a small size, small and thick leaves with low nutrient concentrations, and narrow conduits that allow them to conserve their slowly acquired carbon and nutrients and survive harsh highland conditions. In contrast, the lowland species have a large size, large and thick leaves with high nutrient concentrations, and wide conduits that allow them to be competitive and acquisitive under benign lowland conditions. I also found that stem traits (i.e. basal area) and branch traits (i.e. conduit diameter, leaf area per xylem area and specific branch length) that integrate multiple plant organs and functions are the best predictors of species’ elevational distributions. Stem and branch traits are therefore more important for species elevational distribution than commonly measured leaf traits.

In Chapter 4, I asked how future global warming will affect species’ elevational ranges and distribution areas. For 137 tree species, I used the MaxEnt models obtained in Chapter 2 to project their spatial distributions using ‘near current’ (1985) and future (2050) predicted environmental conditions. Highland species show larger predicted elevational shifts than lowland species, probably because of higher rates of warming predicted for high elevations. The upslope shifts that species have to realize to track their climatic niche is predicted to be, on average, more than 2 times faster (8 m yr-1) than currently observed shifts in the Himalayas (3 m yr-1). This implies that species may not be able to track future environmental change. The majority (71%) of the tree species are predicted to enlarge their distribution area: these are particularly species from the lowlands that may expand their upper limit, or species from the highlands that may have access to new potentially suitable highland intermountain valleys and plateaus, whereas species from mid-elevations are predicted to contract their distribution area because less area is available upslope. Hence, the actual change in distribution area depends on the peculiarities of the Himalayan topography and the area available in each elevational belt. I found that the predicted elevational shift in response to climate change varies strongly across species and depends not only on the magnitude of temperature and precipitation change at that specific elevation, but also on local soil conditions. I also found that species with conservative trait values have the largest predicted upslope shifts suggesting that an easily measurable trait, such as conduit diameter, can be used as a proxy for predicting species elevational range shifts in the Himalayas, and possibly in other montane regions.

I conclude that the plant species distribution in the Himalayas are best predicted by temperature and its temperature (e.g. temperature extremes) and non-temperature (such as irradiance and PET) covariates followed by soil clay content and slope, but that future distributions will also be affected by changes in precipitation. Multiple trade-offs in plant size, hydraulic efficiency and light competition determine species’ positions along this elevational gradient. The Himalayan plant species face an uncertain future, because they are projected to shift upslope at much faster rates in response to the climate warming than currently observed rates of shift. Based on these findings, here, I call for the establishment of migration corridors, for assisted migration such as direct seeding or planting in new potentially suitable future habitats, and for urgent and informed actions to conserve and improve biodiversity along the slopes of the Himalayas for the benefit of not only the montane communities but also the global community.