RESPONSE OF A SONORAN RIPARIAN FOREST TO A 10-YEAR RETURN FLOOD

In March 1991 a 10-year return flood (368 m(3)s(-1)) occurred in the H assayampa River, a perennial stream (0.1 m(3)s(-1) base flow) within t he Sonoran Desert. Depth of the floodwater ranged from 2.64 +/- 0.20 m (mean +/- SD) near the stream to 0.47 +/- 0.31 m in the highest flood plain zone (Proso,is forest). Flow velocity was 1.7 +/- 0.6 In s(-1) a nd 0.9 +/- 0.4 m s(-1) in these same zones. An average of S cm of sedi ment was deposited on the floodplain, with maximum deposition (to 0.5 m) on densely vegetated surfaces 1-2 m above the water table. Native r iparian vegetation showed resistance and resilience to the flood distu rbance. Plants on high floodplains (e.g., Prosopis velutina trees and saplings, and Populus fremontii and Salix gooddingii trees) had low mo rtality. Populus fremontii and S. gooddingii ''pole'' trees and saplin gs were on less aggraded floodplains and sustained varying mortality d epending on floodplain elevation and depth of flood waters. For exampl e, P. fremontii pole trees on 1-2-m-high floodplains averaged 6% morta lity, compared to 40% for those on low floodplains (<1 m above the wat er table) where standing water was >2 m. Seedlings of Populus fremonti i and Salix gooddingii established abundantly after the flood along ov erflow channels and main channel sediment bars, contributing to age-cl ass diversity for these episodically recruiting species. The exotic sp ecies Tamarix pentandra had greater mortality of pole trees (62%) and low post-flood recruitment compared to P. fremontii and S. gooddingii. Survivorship of shrub species also corresponded to floodplain elevati on. Zizyphus obtusifolia grew on high-elevation floodplains and had no mortality. Shrub species of lower-elevation floodplains underwent mor tality but revegetated after the flood via asexual reproduction. For e xample, stem density of the dominant shrub (Baccharis salicifolia) dec lined by half but recovered to pre-flood levels by late summer primari ly via stem sprouting. Dominant herbaceous plants on stream banks and low floodplains (i.e., the rhizomatous perennial grasses Paspalum dist ichum and Cynodon dactylon) similarly compensated. for a 50% decline i n cover by vegetative spread. The post-flood herbaceous understory veg etation in high-elevation floodplain zones (i.e., Prosopis velutina fo rests) remained sparse throughout tile summer and shifted in compositi on from nearly monotypic stands of exotic annual species to more diver se mixtures of native and exotic annual grasses and forbs.