Global Change Ecology (BIO347)

Treelines shift

An overview
In Alpine Mountain Environments, when we look from a distance, a kind of boundary line between trees and dwarf shrubs is visible, the fuzzy line or boundary where the trees start to disappear and shrubs and grass become dominant is what ecologists and biogeographers define as a treeline. Ecologists have defined trees as an individual plant which has a height above 2m (in the case of alpine regions) from the ground surface and treeline as the highest elevation of trees in a patch comprising at least three individuals (1). The treeline, in general, is an upper elevational limit of tree growth in mountains. These treelines are physiognomic boundaries along the elevation gradient (2) as a result of heat deficiency, i.e. these are temperature dependent (1). Elevation and latitude are the surrogate of temperature, which indirectly govern the position of treeline (figure 1). Hence, the treelines are sensitive to climate change (3). However, the extent and limit of the treeline ecotone are expected to be governed by several other confounding factors, such as the presence of herbivores, forms of treeline, land-use dynamics, geomorphology, moisture, and species interactions.

Figure 1: Treeline position around the globe by latitude for continental (filled points) and oceanic island (open points) (4).

In recent decades, there have been many studies focusing on treelines. A “treeline shift” has gained widespread attention, especially amongst vegetation ecologists, biogeographers, and environmentalists over the last few decades. The spatial and temporal span of those studies ranges from local to regional and few years to century. The majority of the small geographic studies have reported the upslope shift of treelines (5, 6); whereas some others found contrasting results (7). Researches have tried to connect the shifts with causative factors. Most of the studies found that the treeline shifts are related to landuse changes, climatic warming and moisture stress, as well as geomorphology, and species interaction.
Theoretically, a warming in climate causes the treeline to move/migrate up-slope in the case of mountains and pole-wards in latitude. This applies directly only in the absence of other interacting factor or their insignificant effect. For instance, the advance of the treeline was found to be a response to the warming in post-little ice age in the Swedish Scandes (8) and there have been several reports from various parts of the world about the treeline shift in response to the recent warming (5, 6). However, the treelines’ response to climatic warming is altered by several factors such as landuse change and disturbance in treeline ecotone.
A global meta-analysis (5) on treeline shifts found that only 52% of treelines have shifted up-slope while about 1.5% of treelines receded which shows that the trend of treeline advance is not universal. In the case of the up-slope shift, a rise in winter temperature the last century is found to be a major driver. The treeline shift is a result of the dispersal and establishment in treeline ecotone, which are highly sensitive to short extreme climatic events such as drought and extreme long clod periods. (9). Extreme events such as drought or severe frosts during growing season affect the recruitment and survival of seedlings, which ultimately affects treeline dynamics (10).
Landuse is another interacting factor that cause treeline shift. It contributes in retreat the treeline (5) or can mask or moderate the upwards shift of the treeline with climatic warming. For instance the treeline in Norway has been controlled by the presence of herbivores (11). Herbivores may consume tree seedlings or destroy them by uprooting, ultimately inhibiting the growth of treeline species in the ecotone. In such cases, once the disturbance is reduced or eliminated from the treeline ecotone, the treelines move towards its climatic limit by shifting the treeline up-slope (12).
To conclude
The response of treelines to global changes is rather complicated when both the climatic warming and landuse acts together. The climatic warming favours treelines advance up-slope while landuse causes the treelines to retreat or stabilize at the same elevation. In such cases, a balanced counteracts of the climatic warming and disturbances conceal the dynamic nature of treelines. However, land abandonment or a reduction in land use intensity may favour treeline up-slope shift. Regardless of the causes of the treeline shift, the shift is site specific and species specific. The response of treeline to climatic warming varies across the phytogeography regions (temperate to the tropical region) (7), and also depends on the form of treeline (5, 10, 13).
In the premise of continued climate change which is projected to continue throughout the 21st century, the treeline is expected to shift further upwards in the altitude and polewards in latitude. The shift may also be influenced by the landuse changes. In the mountains, the up-slope shift of treeline may increase competition and alpine species will suffer from more shade by a new shrub or forest canopy. Similarly, species which are unable to keep pace with warming will lose their suitable habitat affecting their population.

References
1. C. Körner, Alpine plant life: functional plant ecology of high mountain ecosystems. (Springer, Berlin, 2003), pp. XI, 344 s. : ill.
2. U. Schickhoff, in Mountain Ecosystems. Studies in Treeline Ecology, G. Broll, B. Keplin, Eds. (Springer Verlag, Berlin-Heidelberg-New York, 2005), pp. 275-354.
3. C. Körner, J. Paulsen, A world-wide study of high altitude treeline temperatures. J Biogeogr 31, 713-732 (2004).
4. A. Berdanier, Global treeline position. Nature education knowledge 3, 10-11 (2010).
5. M. A. Harsch, P. E. Hulme, M. S. McGlone, R. P. Duncan, Are treelines advancing? A global meta‐analysis of treeline response to climate warming. Ecology letters 12, 1040-1049 (2009).
6. A. Tiwari, Z.-X. Fan, A. S. Jump, S.-F. Li, Z.-K. Zhou, Gradual expansion of moisture sensitive Abies spectabilis forest in the Trans-Himalayan zone of central Nepal associated with climate change. Dendrochronologia, (2016).
7. E. M. Rehm, K. J. Feeley, The inability of tropical cloud forest species to invade grasslands above treeline during climate change: potential explanations and consequences. Ecography 38, 1167-1175 (2015).
8. L. Kullman, L. Öberg, Post‐Little Ice Age tree line rise and climate warming in the Swedish Scandes: a landscape ecological perspective. Journal of Ecology 97, 415-429 (2009).
9. S. T. Gray, J. L. Betancourt, S. T. Jackson, R. G. Eddy, Role of multidecadal climate variability in a range extension of pinyon pine. Ecology 87, 1124-1130 (2006).
10. F.-K. Holtmeier, G. Broll, Treeline advance–driving processes and adverse factors. Landscape Online 1, 1-33 (2007).
11. J. D. Speed, G. Austrheim, A. J. Hester, A. Mysterud, Experimental evidence for herbivore limitation of the treeline. Ecology 91, 3414-3420 (2010).
12. J. D. M. Speed, G. Austrheim, A. J. Hester, A. Mysterud, Elevational advance of alpine plant communities is buffered by herbivory. J Veg Sci 23, 617-625 (2012).
13. U. Schickhoff et al., Do Himalayan treelines respond to recent climate change? An evaluation of sensitivity indicators. Earth System Dynamics 6, 245 (2015).