Impact of Climate Change on the Adaptation of Plant Species in Western Himalaya
By: Dr. Sanjay Kumar*
The Himalaya Mountain is among the most prominent bio-geographical entities that separates Indian subcontinent from Tibetan Plateau. Evolved during the Cenozoic era, Himalaya covers a total area of 750,000 km2 in an arc of about 2400 kilometers in length to wrap northern Pakistan, Nepal, Bhutan, and the northern and eastern parts of India. The Himalaya harbors 33,050 km2 of glaciers that give rise to at least eight largest river systems including Ganga, Yamuna, and Brahmaputra and hence this mountain is also known as “water tower of Asia”. This mountain system is home to about 25,000 species of plants (~10% of the world's total) and acts as ‘sink’ for carbon dioxide through its green and the forest cover. Himalaya in India covers an area of 0.537 million km2 with a width of 250-300 km. Importantly, Himalaya houses 13 of the 825 ecoregions of the world. The Himalayan ecosystem in India supports about 50% of the total flowering plants of which 30% flora is endemic to the region. There are about 816 tree species, 675 edibles and nearly 1743 species of medicinal value found in the Indian Himalayan region. Within Indian region, Himalaya is classified into three major zones: western Himalaya (encompasses administrative boundaries of Jammu and Kashmir, Himachal Pradesh and part of Uttarakhand), central Himalaya (comprises of hills of Uttarakhand) and eastern Himalaya (represented in Arunachal Pradesh, Sikkim and Darjeeling). Western Himalaya has two distinct regions. One region has typical mountainous zones consisting of valleys, mid and high mountainous zone, whereas the other region is trans Himalayan zone that houses cold deserts (in Lahaul and Spiti district of Himachal Pradesh and Ladakh region of Jammu and Kashmir).
Climate change is impacting the mountain ecosystems including Himalaya, by affecting water resources and vegetation. One of the most evident consequences of climate change is warming that is a major driver ecosystem change. For example, global warming of 1°C to 2 °C might shift southern boundary in Siberia northward and shrink the areas occupied by tundra and forest/tundra in Eurasia from 20 to 4%. Warming of Himalaya was estimated to be @ 0.04ºC–0.09ºC/year wherein Regional Climate Model did suggest the largest warming at highest altitudes in Himalaya. Meteorological data showed a rise of about 1.6°C in air temperature during the century wherein minimum temperature increased at a slower pace as compared to the maximum temperature. Precipitation showed a significant decreasing trend in monsoon precipitation in northwestern Himalaya though winter precipitation indicated an increasing but statistically insignificant trend. Increase in air temperature was possibly a reason for decreasing winter snowfall in some portions of Pir Panjal Range. Plant adaptation studies assume central importance in Himalaya, since vegetation in the region has limited migratory zones; any adverse change in climate might lead to extinction of species, more so since some of the species are at the edge of their spatial distribution.
A change in climate affects plant performance directly and also indirectly by affecting the other associated abiotic and biotic factors. For example, increased air and soil temperature would reduce plant duration, increase the rate of respiration, modulate the pest population dynamics, affect nutrient mineralization in soils, alter nutrient-use efficiencies, increase evapo-transpiration, and affect organic matter transformations in soil and so on. Some interesting questions under the climate change scenario are: which group of plants C3 or C4, will perform better? How the nitrogen fixers versus nitrogen fixers would behave? Will tree species be benefit more and affect the performance of under-story species due to restriction in radiations? Several studies suggested alteration in genetic diversity and species richness towards desirable biospheric properties that would lead to increase in the niche security. A few studies showed exudation of organic matter into the soil leading to stimulation of useful microbes. Such studies in Himalayan zone lead to important conclusions.
High altitude environment is often considered akin to that of preindustrial era and hence, though not in very strict sense, studies along altitudinal gradient would serve interesting site to study the impact of climate change on plant performance and response.
Enhancing CO2 uptake the nature’s way: a solution under climate change scenario
One of the major concerns under the climate change scenario is on how to sequester more CO2 in the high CO2 environment and what role the plants could play and how? Low partial pressure of CO2 at high altitude offer clues. Photosynthesis is one of the major components of carbon sequestration pathway and hence, enhancing photosynthetic efficiency is at least one of the major routes for enhancing carbon sequestration. Interestingly, photosynthesis rate does not exhibit significant alteration with change in altitude in spite of changes in partial pressure of gases. This suggested modulation in photosynthetic metabolism at different altitudes. Radiotracer studies coupled to biochemical and physiological analyses showed that C3 plants recruited phosphoenolpyruvate carboxylase (PEPCase) and a few more enzymes along with ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to capture more CO2 at low partial pressure of high altitude. Also, such an efficient carbon fixation mechanism would contribute to compensate for the relatively short growing period of the plants at high altitudes. Using the tools of molecular biology, it is possible to transfer the mechanism in crop plants for enhancing CO2 and it might lead to higher carbon sequestration.
Alleviating the oxidative stress
Global warming and regional cooling are inevitable under the climate change scenarios. Therefore, intense efforts are directed to dissect the temperature responsive plant processes with the possibility to manipulate the processes in desired plant species. Series of publications showed a negative correlation between the level of oxidative stress and plant growth; the species which experienced lesser oxidative stress, exhibited better growth under the harsh climatic conditions. Particularly, the enzymes such as glutathione reductase and superoxide dismutase (SOD) were identified to be important. Further studies using plants growing at higher elevations (~4500m amsl) yielded a highly efficient SOD that tolerated very high temperature (~121°C) and functioned from sub-zero temperature to >40°C. Crystal structure of the enzyme showed it to be the most compact amongst the reported SODs. The said SOD improved the performance of arabidopsis and potato under drought and salt-stressed conditions, at least by enhancing lignifications of vascular tissues.
Rise in leaf temperature during drought is usual when the leaf temperature can be as high as 45-50°C in the extreme cases. Under such situations production of superoxide radical is to be expected. Since SOD scavenges superoxide radical, there is a need to have the enzyme that would be stable at these temperatures for reasonable periods. Therefore, a SOD was engineered by replacing one amino acid at targeted position to obtain a highly thermostable protein. The engineered SOD in transgenic plants will confer tolerance to abiotic stresses including high temperature and drought, which are the most prevalent cues during climate change.
A multi-pronged approach for tolerance to environmental cues
There are efforts to develop plants tolerant to environmental cues or insensitive to climate change. This requires knowledge on transferable genetic machinery. The preceding discussion offered targeted approach, which at times, offers limited tolerance to plants against stresses. Hence efforts are being made for a holistic approach to address the problem. Since plants growing in high altitude are exposed to very “harsh” environment, these provided insight into the adaptive mechanisms for tolerance to abiotic stresses. Such plants have evolved strategies to express: (i) a battery of genes such as those encoding chaperons to protect the metabolic machinery, and (ii) modulate the genes that permit growth and development under stress conditions. Therefore, either suitable transcription factor(s) regulating the expression of target genes or co-expression of multiple genes would be desirable for such manipulations under the control of a vector with suitable regulatory elements involving promoters.
A comprehensive knowledge on the responses of Himalayan flora to climate change parameters is crucial not only to strategize conservation policies, but also for bio prospecting activities. There is a need to establish appropriate infrastructure such as artificial rain plots, and series of meterological stations in the region. Efforts on monitoring changes in the past and future will be rewarding. An integrated approach encompassing the fields of ecological genomics, chemical ecology and ecological proteomics will provide fine insight into the plant adaptation mechanisms, particularly when the experiments are carried out in the long term permanent monitoring plots.
*Head, Biodiversity Division, CSIR-Institute of Himalayan Bioresource Technology, Palampur-176062 (HP).