EFFECTS OF SITE PREPARATION ON ROOT-ZONE SOIL-WATER REGIMES IN HIGH-ELEVATION FOREST CLEARCUTS

Soil water deficits often reduce seedling growth and survival in the drier forested regions of southern British Columbia. This study investigated growing season soil water regimes on three clearcut, grass-dominated sites at different elevations in southern British Columbia to determine whether site preparation treatments could increase seedling root zone water supply. The same treatments were applied at each site and included scalping, scalping followed by ripping and herbicide application. In the untreated plots, root zone soil water supply was most limited at the lowest-elevation site and least limited at the highest-elevation site. Over the four growing seasons studied, soil water potentials at 15 cm fell as low as -900 kPa at the lowest-elevation site and as low as -700 kPa at the mid-elevation site, but remained greater than -150 kPa at the highest-elevation site. All three site preparation treatments effectively increased root zone soil water content and profile water storage, particularly at lower elevations. The three treatments were usually equally effective in increasing soil water supply at a given site. Ripping had little effect on root zone available water capacity, and creation of a surface organic mulch with herbicide did not substantially increase soil water supply in comparison with bare mineral soil surfaces. Treatments reduced evapotranspiration but also increased drainage losses at all sites.