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Increases in Summer Precipitation Would Enable Oak Woodlands to Spread into Semi-Desert Grasslands in the Southwestern United States and Adjacent Mexico
Weltzin, J.F. and McPherson, G.R.  2000.  Implications of precipitation redistribution for shifts in temperate savanna ecotones.  Ecology 81: 1902-1913.

Climate models generally predict that the world of the future will be warmer and wetter than it is currently.  The earth's present position within the historic non-CO2-forced millennial-scale oscillation of climate also suggests that warmer and wetter conditions may well be the norm in years to come.  In addition, the direct biological effects of atmospheric CO2 enrichment tend to mimic a warming and moistening of the environment (plant optimum temperatures and water use efficiencies both rise).  Hence, it is important to know what such phenomena portend for the future of the planet's many ecosystems; and the study of Weltzin and McPherson is one attempt to obtain such knowledge for a specific portion of the globe.

What was done
At an oak savanna site in southeastern Arizona, the authors established a number of experimental plots, which were isolated from ambient precipitation and soil moisture, that they hand-watered between July 1994 and October 1996 in accordance with the long-term (30-year mean) characteristics of natural precipitation there and 50% additions and subtractions from the winter and summer precipitation regimes.  In mid-July of 1994 and 1995, they also planted 49 Quercus emoryi acorns in each plot, after which they monitored seedling emergence and survival.

What was learned
In the words of the authors, "summer precipitation was most important for Q. emoryi seedling emergence and early establishment," with differences in recruitment ranging from 1.5- to 3-fold greater in the wet summer regime than in the dry summer regime.  They noted that this result was in harmony with the results of several other studies, where "establishment of other woody plants within arid and semi-arid grasslands has been attributed to intra to interannual periods of above average precipitation or soil moisture."

What it means
With so many theoretical predictions and observational trends suggesting that the world of the future will be warmer and wetter - or effectively so, due to the direct biological effects of atmospheric CO2 enrichment - the results of this study and the others it cites suggest that woody species, such as oak trees, will gradually expand onto arid and semi-arid grasslands where they presently do not grow and successfully reproduce.  As further support for this conclusion, the authors cite the work of McClaran and McPherson (1995), "who concluded that the last downslope shift in this ecotone, which occurred 700-1700 yr BP, coincided with a period of particularly high summer precipitation in the region (i.e., the Medieval Warm Period, 645-1295 yr BP)."  Perhaps this is why we are already seeing this transformation of the planet on every continent of the globe on which woody plants grow [see Trees (Range Expansions) in our Subject Index].

McClaran, M.P. and McPherson, G.R.  1995.  Can soil organic carbon isotopes be used to describe grass-tree dynamics at a savanna-grassland ecotone and within the savanna?  Journal of Vegetation Science 6: 857-862.