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