Background
The authors write that "while climate is known to play a role in species distributions, biotic interactions such as competition also affect range limits," while further noting that both climatic and biotic controls - such as dispersal limitations and evolutionary constraints (Grinnell, 1917; Connell, 1961; Stott and Loehle, 1998; Emery et al., 2001; Sexton et al., 2011) - "may vary in strength across life stages, implying complex range shift dynamics with climate change" that at times may "have the potential to lead to unexpected range shift dynamics as temperatures warm," citing further in this regard the work of Doak and Morris (2010).

What was done
To further explore this important subject, Ettinger and Lambers quantified both climatic and competitive influences on the growth of juvenile and adult trees of three different conifer species growing on Mt. Rainier, Washington (USA), based on annual growth data of the trees that they collected and compared to the competitive environment and annual climate (100 years of data) experienced by each individual.

What was learned
The two researchers report that "growth was sensitive to heavy snowpack and cold temperatures at high elevation upper limits (treeline)," but that it "was poorly explained by climate in low elevation closed-canopy forests." And they found that "competitive effects on growth were more important for saplings than adults."

What it means
Ettinger and Lambers ultimately concluded, all things considered, that "range shifts under climate change will differ at leading vs. trailing edges." At the treeline, for example, they say that "warmer temperatures will lead to increased growth and likely to range expansion." But they suggest that "climate change will have less dramatic effects in low elevation closed-canopy forest communities, where growth is less strongly limited by climate, especially at young life stages." And merging these two observations, one can envision the very real possibility that global warming could lead, not just to a geographical shifting of ranges, but to the actual expansion of the sizes of the ranges of certain tree species.

References
Connell, J.H. 1961. Influence of interspecific competition and other factors on distribution of barnacle Chthamalus stellatus. Ecology 42: 710-723.

Doak, D.F. and Morris, W.F. 2010. Demographic compensation and tipping points in climate-induced range shifts. Nature 467: 959-962.

Emery, N.C., Ewanchuk, P.J. and Bertness, M.D. 2001. Competition and salt-marsh plant zonation: Stress tolerators may be dominant competitors. Ecology 82: 2471-2485.

Grinnell, J. 1917. Field tests of theories concerning distributional control. American Naturalist 51: 115-128.

Sexton, J.P., Srauss, S.Y. and Rice, K.J. 2011. Gene flow increases fitness at the warm edge of a species' range. Proceedings of the National Academy of Sciences, USA 108: 11,704-11,709.

Stott, P. and Loehle, C. 1998. Height growth rate tradeoffs determine northern and southern range limits for trees. Journal of Biogeography 25: 735-742.