EPOW - Ecology Picture of the Week

Southern Beech (Nothofagus spp.)
Andes Mountains, Patagonia, southern Argentina

Explanation:   This is a story of the "realized niche" of a southern beech tree.

We are in the Andes Mountains south of the town of Bariloche in southern Argentina, South America.  This is steep country carved by alpine glaciers and then invaded long ago by several species of southern beech trees (Nothofagus).  A forest fire had recently occurred here, but notice how it burned and killed only particular swaths of trees (the southern beech trees called lenga and coihue) and left others unburned (the southern beech tree called nire), particularly higher up on the ridgeline.  What is going on here?

The answer is that various species of southern beech -- ancient trees of Gondwana heritage -- have different habitat associations and different levels of susceptibility to fire, and thus react differently to the presence of each other and to fire disturbances.  

In the Andean forests around Bariloche, nire (Nothofagus antarctica) is found in wet or damp sites within forests otherwise dominated by lenga (N. pumilio) and coihue (N. dombeyi).  Nire is also found at treeline or higher elevations above the physiological range of lenga and coihue.  Nire sometimes persists at treeline, perhaps in wetter soil sites, where lenga and coihue forests have burned completely, as shown in this week's EPOW photo.  

Canadian botanist Andy MacKinnon standing at treeline
just above a belt of nire (Nothofagus antartica) that
had resisted burning, in the Andes of southern Argentina.
(Andy resisted burning, too.)

Nire also might be able to occur in the environments and elevations where lenga and coihue dominate.  This would be nire's "fundamental" or potential niche.  But apparently nire gets out-competed by these other species.  Thus, its distribution is reduced to its "realized" or actual niche.  

How will these patterns change under scenarios of regional climate change, on top of the effects of fire disturbances and the competitive presence of other species?  Results of studies in other alpine ecosystems suggest that treeline could migrate upslope under warmer temperatures, or downslope under periods of drought or cooler temperatures.  But lenga might not be able to be as flexible, so nire might come to dominate and spread more widely.  Also, fire, which may be more frequent under regional warming and drying, might further reduce lenga's foothold and favor nire.  

And other studies (Suarez et al. 2004) found that coihue has died back under drought conditions as well.  However, nire needs wetter sites and regional drying and warming might reduce its habitat.  

So the future of these high-elevation forests may indeed see a shift in species dominance and distribution, but what it will be is uncertain.  

Foliage -- the tiny leaves -- of nire at treeline.

Information:
     Daniels, L. D., and T. T. Veblen.  2004.  Spatiotemporal influences of climate on altitudinal treeline in northern Patagonia.  Ecology 85(5):1284-1296.
     Litton, C. M., and R. Santelices. 2002. Early post-fire succession in a Nothofagus glauca forest in the Coastal Cordillera of south-central Chile. International Journal of Wildland Fire 11(2):115-125.
     Schmelter, A. 2003. Climatic response and growth-trends of Nothofagus pumilio along altitudinal gradients from arid to humid sites in northern Patagonia. Dissertationes Botanicae, Band 368, VIII. Gebr. Borntraeger Verlagsbuchhandlung, Science Publishers, Stuttgart. 114 pp.
     Suarez, M. L., L. Ghermandi, and T. Kitzberger. 2004. Factors predisposing episodic drought-induced tree mortality in Nothofagus - site, climatic sensitivity and growth trends. Journal of Ecology 92(6):954-966.