[PCA] Fwd: Species distributions shift downward across western North America

De Angelis, Patricia patricia_deangelis at fws.gov
Mon Dec 15 15:38:32 CST 2014

Species distributions shift downward across western North America

   1. Melanie A. Harsch* and
   2. Janneke Hille Ris Lambers

Global Change Biology (2014)

Using 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, we find that 63% of distributions are
shifting downward (i.e., toward lower elevations) – despite consistent
warming across the study area. Downward distribution shifts were consistent
across plant lifeforms and occur in all regions across the study area. The
proportion of species shifting downward, however, differed between regions.
To understand why species distributional means are shifting downward, we
explored the relationship between the direction of change in distribution
limits and the nature of recent climate change. We found that the
interaction between local summer temperatures and seasonal precipitation
determined the direction of distribution limit shifts. Specifically,
species shifted downward at their upper elevational limit when maximum
temperatures increased and snowfall declined at faster rates. By contrast,
species shifted downward at low their lower elevation limit when
precipitation increased. This is consistent with downward distribution
shifts being mediated by moisture stress, with declining snow at upper
distribution limits increasing moisture stress (leading to a range
contraction downward) while increasing precipitation decreases moisture
stress at lower distribution limits (leading to a distribution expansion
downward). Although we believe direct physiological impacts of altered
water availability primarily drive downward distribution shifts, altered
competitive interactions due to climate change may also be responsible –
this is an area that requires more research. Our results suggest that
future species' elevational distribution shifts will be complex, depending
on the interaction between seasonal temperature and precipitation change.

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