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Comment on “Changes in Climatic Water Balance Drive Downhill Shifts in Plant Species’ Optimum Elevations”

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Science  14 Oct 2011:
Vol. 334, Issue 6053, pp. 177
DOI: 10.1126/science.1205740


  • Fig. 1

    Changes in annual climatic water balance parameters for the 33 weather stations between the historical (1920 to 1949) and contemporary (1976 to 2005) periods compared by Crimmins et al. (3). We changed the sign on Crimmins et al.’s index (PET-P) to facilitate comparisons. Boxes encompass the 25th through 75th percentiles; the other horizontal lines indicate the 10th, 50th (median), and 90th percentiles. A majority of the extra precipitation in the contemporary period became biologically unusable surplus, and the change in Crimmins et al.’s index is mostly a reflection of this.

  • Fig. 2

    Geographic bias and its consequences. (A) The median location in California of vegetation plots in Crimmins et al.’s historical (blue dot) and contemporary (red dot) data sets. The blue and red lines through the points represent, respectively, the southern and northern transects. (B) Contemporary vegetation types are found at significantly lower elevation on the northern than the southern transect, and the difference increases with elevation. Points are based on mean elevations of the 15 dominant native vegetation types on the two transects (5); the solid line is the linear regression, with its 95% confidence interval shown as dashed lines [compare with figure 4 in (3)].

  • Fig. 3

    Schematic illustration of the differing effects of changing evaporative demand and water availability on the climatic water balance and species’ distributions [see (2) for background and supporting information]. The blue and orange circles represent two species’ climatic niches, which are fixed relative to AET and deficit. (A) The black rectangle delimits a mountain’s climatic space. Because AET + deficit = PET, the diagonal lines of slope –1 represent constant PET and thus constant elevation (2); for simplicity, we assume that all sites share similar aspect [emphasizing different aspects would shift the location of the rectangle (2) but would not affect any of the results or conclusions that follow]. PET declines and elevation increases from upper right to lower left. At a given elevation (constant PET), local water availability (as determined by soil water-holding capacity, distance from water, and the like) increases from lower right to upper left (2). (B) With regional warming (but unchanging precipitation), PET will increase at all elevations, which must increase AET, deficit, or both (2). The mountain’s new climatic space (red rectangle) therefore has shifted relative to the species’ fixed climatic niches, and the species’ niches are now found at higher elevations. (C) Increasing water availability (assuming unchanging temperature and thus unchanging PET) drives climatic changes that are nearly orthogonal to those of (B). Because AET + deficit = PET, and PET remains unchanged, increasing water availability can only cause declines in deficit offset by identical increases in AET (2). The new climatic space has shifted along the water availability gradient; for example, the blue species is no longer limited to the wettest sites (such as riparian zones or deep soils) at a given elevation. The potential elevational range of the blue species has expanded while that of the orange species has contracted. Notably, however, mean species’ elevations are unchanged. (D) With increasing evaporative demand and water availability, AET increases but deficit can either increase or decrease, depending on the magnitude of increase in demand relative to availability. Regardless of the direction of deficit changes (slightly decreasing is shown), species’ niches will be found at higher, not lower, elevations. Certain other hypothetical niche shapes and orientations (not shown) can sometimes cause individual species to behave in unexpected ways (such as shifting downward despite increasing evaporative demand, or shifting upward or downward in response to precipitation changes). However, observed distributions of vegetation types along climatic gradients confirm that, averaged over many species, the patterns described above are expected to dominate (2, 10).

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