|
Shift in plant species distribution and regeneration potential
due to global warming and climate change in Himalayan region
By:Ashish Kr. Mishra and Rana Pratap Singh*
The geographic ranges of most plant and animal species are limited by climatic
factors, including temperature, precipitation, soil moisture, humidity, and
wind. Any shift in the magnitude or variability of these factors in a given
location will impact the organisms living there. Species sensitive to
temperature may respond to a warmer climate by moving to cooler locations at
higher latitudes or elevations. Himalayan forests having unique climatic
conditions support unique vegetation types. The plants are strongly adapted to
the narrow range of altitude for the specific habitats as a marginal shift in
altitude may change the climatic condition significantly. Himalaya generates
different conditions, to support unique and isolated ecosystems along the
altitudinal rise. The Himalayan forests are considered as a biodiversity hub
which provides an excellent opportunity to investigate the climatic effects on
plant distribution. Though the response of global warming has been widely
studied on the Himalayan vegetation shift, it is difficult to predict how
concurrent changes in climatic factors affect the species distribution. Despite
the uncertainties, ecological models predict that the distribution of world
biomes will shift as a result of the climate changes associated with increased
greenhouse gases in multiple ways. The distribution and size of the populations
of plants and animals within those biomes will also change, with potential
consequences for the functioning of ecosystems and for humans who are dependent
on many ecosystem goods and services.
Shift in plant species distribution in relation to global warming
A change in the agro-climatic conditions of an area can affect the population
dynamics of plants, animals and microbes which are living there and as a
consequence,.the structure and function of the entire ecosystem can also get
modified. It is evident that the high altitudinal vegetation is directly
influenced by global warming; these habitats are considered very sensitive and
vulnerable to the climatic change phenomena emerging as a key factor to regulate
the future ecological relations in the different ecosystems and biomes. The
climate-induced species range shifts have been reported along altitudinal and
latitudinal gradients. The spread of tree species involves several factors,
including dispersal, regeneration on a suitable site, maturation, and seed
production. If climate changes faster than trees can disperse to new and more
suitable areas, the composition of the forest may change and the survival of
some species could be at risk. Global-scale models are also inadequate to
evaluate the indirect effects of climate, such as disturbances from pests,
disease, fire, flooding, and wind damage. It is expected that changing climate
may shift the vegetation towards the upside or lower elevation, which may change
the composition of the forests in near future.
Species capability to shift along the altitudinal range depends on species
tolerance efficiency of warming effects, their seed dispersal capability and
their competitive ability. Only the species which have ability to compete
effectively with species occurring at higher altitudes would be able to shift
upward. There are three types of upward shifting that occurs in Himalayan
ranges: (1) upward shift in entire altitudinal range with little or no change in
range width, (2) upward altitudinal shift on higher side of the range but no
change on the lower side of the range and, (3) altitudinal range squeezed at
lower ends but no upward shift (Figure 1) (Singh & Singh,
2013). In general, altitudinal range in several species is squeezed because they
are intolerant to warmer temperatures, and are unable to expand their range
towards the upper side.
The recent climate change is mainly comprehended as a result of modification of
natural climatic conditions and the chief factors which favour it are natural as
well as anthropogenic. For meeting the ever increasing energy demands, human
activities such as burning of fossil fuels and deforestation have resulted in
the increased concentration of green house gases in the natural environment.
Among the various green house gases, carbon dioxide is considered as the most
important and its level has increased from 270 parts per million (ppm) prior to
industrialization to the value of 410 ppm in year 2017 (Kahn 2017). The impacts
of climate change are seen to be more pronounced in the sensitive high altitude
regions especially in the mountainous environments.
The retreat of Himalayan glaciers during the last two decades and the formation
of several glacial lakes evidenced the role of global warming. During the past
century, the average global air temperature near the earth’s surface has been
estimated to increase at the rate of 0.74 ± 0.18ºC (1.33 ±0.32ºf). The global
mean land surface has warmed by 0.27º C per decade since 1979 (Guisan et al.
2005). The last decade of the 20th Century and the beginning of the
21st have been the warmest periods in the entire global instrumental
temperature records. Certain naturally occurring gases, such as carbon dioxide
(CO2) and water vapour (H2O), trap heat in the atmosphere
causing a greenhouse effect. Burning of fossil fuels, like oil, coal, and
natural gas is adding CO2 to the atmosphere. The current level did
not reach this level during the past few million years.
Although plants have moved across the landscape in response to changing climate
for millennia, projections of contemporary climate change suggest that forest
tree species and their populations will need to migrate faster than their
natural ability in search of more suitable environment. Therefore, climate
change and adaptation strategies, such as assisted migration, have gained
attention since 20th Century (Williams, 2013). During the past 30 years, a few
climate changes and adaptation strategies have been proposed. One strategy is
assisted migration (Peters and Darling 1985), defined as the movement of species
and populations to facilitate natural range expansion in direct management
response to climate change. Climate has played a major role in shaping
vegetative growth, composition, and genetic variation across the landscapes.
Long-lived species, such as trees, will lag behind short-lived species in their
ability to adapt and track suitable climatic conditions. It may take several
generations (centuries to millennia) for a tree population to become adapted
through evolution to a new climate. Evidence from the fossil record and from
recently observed trends proved that changing climate has a profound influence
on species range expansion both upward and downward. Ecological ‘fingerprints’
of climate change appear across a wide range of taxonomic groups and geographic
regions and are being identified with increasing frequency.
Chen et al. (2011) have reported that terrestrial organisms are currently
shifting their distribution range along the latitude or elevation in response to
changing climate. Using a meta-analysis, they estimated that the distribution of
species has recently shifted to higher elevations at a median rate of 11.0
meters per decade, and to higher latitudes at a median rate of 16.9 kilometers
per decade. These rates are approximately two and three times faster than
previously reported migration rates. Parmesan and Yohe (2003) compiled studies
on many species including alpine herbs, birds, and butterflies, and found an
average poleward shift of 6.1 km per decade. The timber lines in European
mountains have been recorded to have moved upward by nearly 200 m since the
early twentieth century (Grabherr, 2010). Furthermore, because initiation and
cessation of growth are influenced by temperature, precipitation and light, the
forest ecosystems are likely to experience phenological imbalances with longer
growing seasons such as the bud break may occur earlier in the spring, making
those individual trees susceptible to weather events (e.g., freezing
temperatures and snowfall). In the assisted migration of plants, forest tree
species are highlighted most often because of their economic value and focus on
climate change research; however, assisted migration conducted for economic
rather than conservation reasons is also cited as a major barrier to
implementation.
Shift in plant species distribution in Indian Himalaya
In different district of central Indian Himalayas, Panigrahy et al. (2010)
reported upward shifting of alpine tree line and reported 360-430 m upward shift
in Uttarkashi (360 m), Tehri Garhwal (400 m), Rudraprayag(390 m), Bageshwar (430
m), Chamoli (360 m) and Pithoragarh (390 m). A study conducted by Telwala,
(2011), in the north-east India (Sikkim Himalaya) suggests that many of the 124
endemic plant species investigated, demonstrated a prolonged upward shift over
the past 150 years (from 1850 - 2010) with a mean species shift in altitudinal
range of 237.9±219.8 masl. The upward shift in species’ range was between 100m
to 400m in 70% of the species but, in extreme cases, the range shift was
600-800m. About half of the endemic species, showed an upward shift on upper
side of their range, leading to an altitudinal expansion on the range. In
Kashmir Himalaya the rate of upward shifting of the regional plants depends upon
the extent of changes in the local climatic conditions and by other factors
including anthropogenic pressures that operate differently in each mountain
systems of the world (Agnihotri et al., 2017). As per Kumar et al.,
2010, flowering in Rhododendron arboretum, and its other associate like
banj oak has shifted by 25 to 30 days whereas time of leafing by around 2 weeks.
Himalayan blue pines (Pinus wallichiana), Bhojpatra (Betula utelis)
and Abies spp. are on the move too in western Himalaya. From past few
decades, nuisance weed Parthenium hysterophorus and the water fern
Azolla cristata have shifted in the formerly temperate climate of the
Kashmir Himalayas. Historical range shift of plant species like Rhodiola
bupleuroide, Rheum nobile, Saussurea stella , Saussurea
uniflora and Ponerorchis chusua has been recorded in Northeast
Himalaya (Telwala et al. 2013). Migration of tropical species, including some
pines (e.g. Pinus merkusii), from lowland areas of South Asia to the
Himalaya is indicated to have occurred in the geological past during warmer
climate phase whereas Bombax ceiba, and Butea frondosa have
penetrated into the Himalayan ranges through migration along rivers that have
cut deeply into mountains.
Changes in regeneration behaviour of plants in Indian Himalayas
It is important to understand how evolution and ecological potential of
different life forms helps them to adapt climate change (Woodward and Kelly
2008), because Himalayan forests are greatly affected by climate change, water
availability and temperature. The regeneration potential of any forest directly
depends on biotic and abiotic characteristics and its geographic distribution (Gairola
et al. 2012). Regeneration patterns of species population can address climate
change by adaptive evolution or by migrating association to survive in their
favourable climate. Forest ecosystems depend on adequate regeneration potential
of tree species to be healthy and sustainable. Himalayan forest composition and
ecosystem services depend on forest structure and regeneration potential which
are considered to be changing in the alignment of tree population over time.
Economically important plant species have repeatedly been reduced from Himalayan
forests due to climate change, species shift and other natural or anthropogenic
disturbances. These phenomena have ultimately affected forest regeneration
(Sharma 2016). Emergence and establishment of seedlings in the oak dominated
forests under their natural habitat conditions in Garhwal Himalaya is reported
to be unsatisfactory (Bisht & Kuniyal 2013). In Indian lower Himalayan parts,
the regeneration in Sal (Shorea. robusta) is known to be a problem,
because its seeds are ready to germinate by mid-June when the commencement of
monsoon is uncertain. Availability of seeds, which are often limited for
Himalayan temperate species and competition among species for space, light and
water, may be the reasons for non-regeneration of certain tree species.
*School of Environmental Science, Babasaheb Bhimrao Ambedkar University,
Lucknow. E-mail: [email protected]
|
