PARENT SESSION
Oral Session 8: Forest Ecology I: Dynamics and Succession.
Presiding: A Fiala and D Kashian
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room B 117.
Conditional plant interactions: Switch from facilitation to competition across an elevational gradient.
Stulz, Christopher*,1, Gehring, Catherine1, Whitham, Thomas1, 1 Northern Arizona University, Flagstaff, AZ
ABSTRACT- Recent studies demonstrate that positive plant-plant interactions (facilitation) can have important impacts on plant community structure. Models based on observational field studies have shown that the importance of facilitation increases as abiotic stress increases. However, few studies have experimentally examined the relative importance of facilitation vs. competition across a stress gradient. We examined the interactions between juvenile pinyon pine (Pinus edulis)and the shrub, apache plum (Fallugia paradoxa), at low and high elevation in semi-arid woodlands in northern Arizona. Observations at four replicate sites and a field experiment in which shrubs were completely removed or tied back resulted in three major results: 1) juvenile pinyon pines were found in association with apache plum 3 x more than expected at both high and low elevation sites, yet pinyons had 2.5 x higher mortality rates and 2 x lower growth rates in association with shrubs at high elevation while opposite patterns were observed at low elevation. 2) Experimental shrub removals confirmed these patterns with facilitation dominating at low elevation and competition at high elevation despite a record drought in the region. Complete shrub removal and tieback treatments yielded similar responses. 3) Facilitative effects remained constant for all sizes of juvenile pinyon at low elevations while competitive effects at high elevation increased as pinyon size increased. These results demonstrate that plant-plant interactions can have varying impacts on pinyon growth and survival, and show the relative importance of competition and facilitation may change dramatically if droughts in the region persist as predicted by global climate change models. The change in the sign of an interaction (i.e., facilitation to competition) across a relatively short distance argues that major interactions can switch abruptly and are strongly context dependent.
Key words: competition, facilitation, stress gradient