Global Change Ecology (BIO347)

Climate-driven shifts in plant species along the elevation gradient: Are we close enough to the facts?

Upsetting climate trends

The 21^st century has witnessed the alarming signals of anthropogenic global climate change particularly in the high altitude/latitude regions of the world. This is indicated by the trend showing a rise in global temperatures by almost 0.85 °C over the past 130 years [1]. Regional trends of climate warming are rather distressing. The Himalaya for instance has experienced an increase of 1.5^oC during the past 25 years (1982-2006) [2].

 

Observed impacts

Evidence so far reveals the rapid changes in geographical distribution of several plant species and habitat types across the globe, especially in the mountain regions, as the fingerprints of anthropogenic climate – and/or land-use – change [3]. However, precise quantification of these changes is a big challenge due to complex interactions of their drivers [3], methodological inconsistencies in analysing the species’ responses [4], and unavailability of detailed data [5].

Conceptually, the warming climate has left only three options for the earth’s biota: to shift towards newly suitable geographic locations , to adapt to a new environment , or to die , or some combination of these responses [6]. Elevation is considered a proxy for temperature because atmospheric temperature decreases with an increase in elevation and the realized ecological niche (environmental space within which a species maintains itself in presence of other species) of plant species mostly remain unchanged at least for thousands of years [7]. Therefore, in response to a warmer climate, plant species are expected to shift upslope to the cooler areas to compensate for the increased temperature; whereas those living at extremes of the elevation gradient may undergo extinction. Many authors [8-13] have attributed the upslope or poleward shifts in the range or abundance of plant species to the increase in atmospheric temperature during recent decades. However, there is enormous debate regarding the severity and frequency of these changes at different scales.

 

Lack of consensus

Methodological inconsistencies

To date, assessments of climate-driven distributional changes of species have largely been based on niche-based models. Since these models ignore species’ demography, adaptive potential and biotic interactions, they often tend to predict higher rates of extinction or distributional changes, leading to higher uncertainties [14, 15]. However, the assumption of niche-based models that the intact plant community responds in a similar fashion to the warming climate is unlikely because realized niche and biotic interactions vary greatly among the species of a community [16]. Therefore, a consistent widespread pattern of upward shifts of the species and their assemblages in response to a warmer climate cannot be expected.

The process-based models additionally include the factors responsible for range dynamics such as seedling dispersal, population dynamics, biotic interactions, eco-physiological factors, and the potential persistence of species under deteriorating conditions [14]. However, such models are scarce at the continental scale because these are applicable only to well-known species for which demography or physiology has been studied for a long time.

Other factors: land-use matters !

Several factors other than the changing climate, such as increased deposition of atmospheric nitrogen, dispersal lag in species colonization, changes in moisture regime, highly responsive species pool, and biotic interactions, have also been proposed for the observed upward shift of the plant species along the elevation gradient [see [5]].

Species’ responses observed in several regions are not straightforward as estimated earlier by the niche-based models. In the European mountain summits, the amount of change in species assemblages is found to be unrelated to the amount of climate warming [5]; whereas no shifts or unexpected downward shifts in species’ ranges have been  observed in other regions [17]. Similarly, the downslope retreat of the treeline in Himalaya and Tibetan Plateau is reported as due to the changed precipitation regime rather than the warmer temperature [18]. For several plant species and their assemblages, climatic responses lag behind the degree of temperature increase [16]. For instance, ‘thermophilization’ of plant species in temperate forests of some regions often lags behind warming. Densified forest canopy cover of the forests due to a change in the land-use regime has buffered the effect of regional climate warming via microclimate cooling. The forecasts of climate-related range shifts are thus modified by change in forest cover [19]. Analysing the distributional shifts with a focus on temperature alone might be potentially misleading due to the confounding role of other factors such as precipitation and land-use change [5].

Data-gaps enhancing uncertainties

Most of the low-elevation mountain areas are densely populated; where land-use change often has robust effects on plant communities. However, observations of climate and land-use driven plant community change in lower mountain forests are scarce in comparison to those in high-elevation ecosystems [5, 17]. Studies on climate driven vegetation changes from the biologically more diverse Asian region also remain very scarce; and ecophysiological and cytogenetic data on populations of species are in addition seriously limited [20].

 

Way forward

Analysis of climatic responses of biotic communities is critical for assessing species’ extinction risks and to formulate effective conservation measures. Realistic assessment of climate-driven geographical shifts urgently requires improved monitoring at different scales, and then attributing the observed changes to climatic variables integrated with the non-climatic, ecophysiological, and genetic factors of the species. It is critically important to develop several niche-based as well as process-based models for the same species under the same scenarios and scales, and compare their predictions in order to identify robust results.

 

References

1. IPCC, Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, T.F. Stocker, et al., Editors. 2013, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp.
2. Shrestha, U.B., et al., Widespread Climate Change in the Himalayas and Associated Changes in Local Ecosystems. PLoS ONE 2012. 7(5): p. e36741. doi:10.1371/journal.pone.0036741.
3. MEA, Ecosystems and Human Well-being: Biodiversity Synthesis.World Resources Institute, Washington, DC. 2005.
4. Dawson, T.P., et al., Beyond Predictions: Biodiversity Conservation in a Changing Climate. Science, 2011. 332(6025): p. 53-58.
5. Grytnes, J.-A., et al., Identifying the driving factors behind observed elevational range shifts on European mountains. Global Ecology and Biogeography. DOI: 10.1111/geb.12170, 2014.
6. Lenoir, J. and J.-C. Svenning, Latitudinal and Elevational Range Shifts under Contemporary Climate Change. Encyclopedia of Biodiversity, 2013. 4: p. 599-611.
7. Peterson, A.T., Ecological niche conservatism: a time-structured review of evidence. Journal of Biogeography, 2011. 38(5): p. 817-827.
8. Thuiller, W., et al., Climate change threats to plant diversity in Europe. Proceedings of the National Academy of Sciences of the United States of America, 2005. 102(23): p. 8245-8250.
9. Pauli, H., et al., Signals of range expansions and contractions of vascular plants in the high Alps: observations (1994–2004) at the GLORIA*master site Schrankogel, Tyrol, Austria. Global Change Biology, 2007. 13: p. 147–156.
10. Lenoir, J., et al., A Significant Upward Shift in Plant Species Optimum Elevation During the 20th Century. Science, 2008. 320(5884): p. 1768-1771.
11. Chen, I.-C., et al., Rapid Range Shifts of Species Associated with High Levels of Climate Warming. Science, 2011. 333(6045): p. 1024-1026.
12. Pauli, H., et al., Recent plant diversity changes on Europe’s mountain summits. Science, 2012. 336(6079): p. 353-5.
13. Gottfried, M., et al., Continent-wide response of mountain vegetation to climate change. Nature Climate Change, 2012. 2(2): p. 111-115.
14. Morin, X. and W. Thuiller, Comparing niche- and process-based models to reduce prediction uncertainty in species range shifts under climate change. Ecology, 2009. 90(5): p. 1301-1313.
15. Hampe, A., Bioclimate envelope models: what they detect and what they hide. Global Ecology and Biogeography, 2004. 13: p. 469–476.
16. Bertrand, R., et al., Changes in plant community composition lag behind climate warming in lowland forests. Nature, 2011. 479(7374): p. 517-520.
17. Lenoir, J., et al., Going against the flow: potential mechanisms for unexpected downslope range shifts in a warming climate. Ecography, 2010. 33(2): p. 295-303.
18. www.scientificamerican.com/article/high-altitude-forests-in-the-himalayas-harder-hit-by-droughts/?print=true.
19. De Frenne, P., et al., Microclimate moderates plant responses to macroclimate warming. Proceedings of the National Academy of Sciences, 2013.
20. Parmesan, C., Ecological and Evolutionary Responses to Recent Climate Change. Annual Review of Ecology, Evolution, and Systematics, 2006. 37(1): p. 637-669.

 

Kuber P. Bhatta