Phytosphere Research

Effects of stand thinning on water relations, growth, and condition of three tree species in a riparian restoration planting

Paper presented at the Fourth Sudden Oak Death Science Symposium, Santa Cruz (Scotts Valley), CA, 15-18 June 2009 - Ecological Impacts session

T. J. Swiecki and E. A. Bernhardt
Phytosphere Research, Vacaville CA

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Swiecki, T. J.; Bernhardt, E. A. 2003. Effects of stand thinning on water relations, growth, and condition of three tree species in a riparian restoration planting. Pages 280-289 in: Faber, P. M. (ed.) California Riparian Systems: Processes and Floodplain Management, Ecology, and Restoration. 2001 Riparian Habitat and Floodplains Conference Proceedings. Sacramento, CA: Riparian Habitat Joint Venture.

Additional key words: Acer negundo, Populus fremontii, Salix laevigata, stem water potential, competition, stomatal conductance


A dense riparian planting was installed on the reconstructed floodplain of the Guadalupe River
in San Jose in 1994. Soil investigations conducted 3 years after planting revealed that trees did not root to the depth of groundwater at the site, which was confined beneath a thick clay layer present at a depth of about 3 ft. Many trees in the planting developed water stressrelated health problems. We initiated a study to determine whether reducing tree density would reduce water stress and improve tree condition in the planting. In February 1999, established trees were removed from two plots within the planting to reduce stand densities from about 250 to 124 woody plants/acre. We monitored soil moisture and tree water status, growth, and condition for three species in thinned and nonthinned plots between July 1999 and September 2000. The species showed different responses to site conditions and the thinning treatment. For cottonwood (Populus fremontii), reducing intertree competition improved water status and shoot growth, and reduced attacks by wood-boring beetles. Nevertheless, thinning did not entirely eliminate water stress in cottonwood and tree condition continued to deteriorate in both thinned and nonthinned plots. Thinning did not substantially improve growth or water status of box elder (Acer negundo), which showed the highest levels of water stress overall. Thinning had only minor effects on red willow (Salix laevigata), which maintained the best water status overall. Red willow may not
have responded to thinning because these trees were meeting their water needs by extracting moisture from the clay subsoil.

Funding for this project was provided by the Santa Clara Valley Water District through contract A2236T.