PARENT SESSION
Poster Session # 17: Aquatic Systems.

Thursday, August 7 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

ENSO event effects on Southern California riparian plant communities.

Lahti, Megan^*,1, Swift, Cheryl^*,1, Ross, Katie², ¹ Whittier College, Whittiier, CA² University of Virginia, Charlottesville, VA

ABSTRACT- Highly variable rainfall typical of Mediterranean climates results in large year to year fluctuations in stream discharge. In the 1998 ENSO event Los Angeles County received almost three times the normal annual precipitation. The majority of this rainfall occurred in a three month period extending from January to March, and stream discharges increased significantly during this period. We sampled the plant community along two reaches of the San Gabriel River in 2002 in order to examine the effects of this increased discharge on plant community structure. Each of the two reaches had been sampled previously in 1995. Reaches were sampled by setting out a series of 100 m ² plots within 5 m of the thalwag, 5-20 m from the thalwag and greater than 20 m from the thalwag. The community changed significantly as a result of the ENSO event. The number of stems/ha was significantly higher in 2002, but the stems were smaller since a significantly larger proportion of stems was less than 3 cm in diameter as compared to 1998 where a larger proportion of stems were greater than 8 cm in diameter. There were also significantly more stems per individual in 2002 suggesting that the increase in stems may be the result of resprouting from fallen stems. These changes in community structure were most apparent closer to the stream. The results of this study suggest that the 1998 ENSO event changed the structure of the riparian plant community significantly and that Southern California riparian plant communities respond to disturbance by resprouting from existing fallen individuals and then undergoing a period of self thinning as light becomes limiting until a more open forest composed of larger trees establishes.

Key words: enso, riparian, Southern California