Paper Reviewed
Harsch, M.A. and Lambers, J.H.R. 2014. Species distributions shift downward across western North America. Global Change Biology 10.1111/gcb.12697.

In the introduction to their report on climate warming effects on Earth's terrestrial plants, Harsch and Lambers (2014) write that "in general, documented and projected distribution shifts are toward the poles or higher elevations." But they say that "focusing solely on upward shifts (toward poles and higher elevations) at the distribution limit misses the fact that many species distributions are either not shifting (Harsch et al., 2009) or are shifting downward," citing the work of Lenoir et al. (2008), Moritz et al. (2008), Crimmins et al. (2011) and Tingley et al. (2012).

To better understand why species distributional means are shifting downward in many cases, the two researchers explored the relationship that exists between the direction of movement and the nature of the climate change that promoted it. This they did based on an extensive "network of occurrence records for 296 plant species collected over the past 40 years across a climatically diverse geographic section of western North America." And despite finding consistent warming across the study area, they found that "downward distribution shifts were consistent across plant lifeforms and occur in all regions across the study area."

In their efforts to identify the driving forces behind this initially unexpected behavior, Harsch and Lambers found that species shifted downward at the upper elevation limits of their ranges "when maximum temperatures increased and snowfall declined at faster rates," while species shifted downward at their lower elevation limits "when precipitation increased."

These findings, as they continue, are "consistent with downward distribution shifts being mediated by moisture stress, with declining snow at upper distribution limits increasing moisture stress (leading to a distribution expansion downward) while increasing precipitation decreases moisture stress at lower distribution limits," also "leading to a distribution expansion downward." And they thus end their paper by stating that their results "highlight the likelihood that continued winter warming and snowfall decline will limit further elevational distribution shifts both in North America and at mid- to high-latitude mountain distributions globally."

References
Crimmins, S.M., Dobrowski, S.Z., Greenberg, J.A., Abatzoglou, J.T. and Mynsberge, A.R. 2011. Changes in climatic water balance drive downhill shifts in plant species' optimum elevations. Science 331: 324-327.

Harsch, M.A., Hulme, P.E., McGlone, M.S. and Duncan, R.P. 2009. Are treelines advancing? A global meta-analysis of treeline response to climate warming. Ecology Letters 12: 1040-1049.

Lenoir, J., Gregout, J.C., Marquet, P.A., de Ruffray, P. and Brisse, H. 2008. A significant upward shift in plant species optimum elevation during the 20th century. Science 320: 1768-1771.

Moritz, C., Patton, J.L., Conroy, C.J., Parra, J.L., White, G.C. and Beissinger, S.R. 2008. Impact of a century of climate change on small-mammal communities in Yosemite National Park, USA. Science 322: 261-264.

Tingley, M.W., Koo, M.S., Moritz, C., Rush, A.C. and Beissinger, S.R. 2012. The push and pull of climate change causes heterogeneous shifts in avian elevational ranges. Global Change Biology 18: 3279-3290.