INSTREAM FLOW AND COTTONWOOD GROWTH IN THE EASTERN SIERRA-NEVADA OF CALIFORNIA, USA

Dendro-ecological studies indicated that radial growth of Populus tric hocarpa was significantly related to annual streamflow at 20 riparian sites in the eastern Sierra Nevada of California. The strength of the relationship varied among sites, depending on geomorphology and tree c over. The strongest correlation between streamflow and tree growth occ urred at sites in wide, unconfined valleys, where alluvial groundwater typically fluctuates directly with surface water. In such areas, tree s on streambanks and in the floodplain showed equally strong relations hips between flow and growth. In narrow mountain canyons, relationship s between tree growth and streamflow were weaker and showed more withi n-site variability. Streambank trees in the canyon settings generally showed stronger relationships with streamflow than did floodplain tree s. These data suggest that P. trichocarpa trees in confined canyons, i n comparison with those in wide alluvial valleys, may rely to a greate r extent on water sources that are not in direct hydraulic connection with surface water. Flow-growth models were also stronger at sites whe re tree basal area and density were low, including sites where flow di version has caused tree mortality. Sparse tree cover may allow for a g reater expression of flow-growth relationships by minimizing the effec ts of competition for light and other resources, and allowing for grea ter control of growth by abiotic rather than biotic factors.