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 d rier forested regions of southern British Columbia. This study investi gated growing season soil water regimes on three clearcut, grass-domin ated sites at different elevations in southern British Columbia to det ermine whether site preparation treatments could increase seedling roo t zone water supply. The same treatments were applied at each site and included scalping, scalping followed by ripping and herbicide applica tion. In the untreated plots, root zone soil water supply was most lim ited at the lowest-elevation site and least limited at the highest-ele vation site. Over the four growing seasons studied, soil water potenti als 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 tha n -150 kPa at the highest-elevation site. All three site preparation t reatments effectively increased root zone soil water content and profi le water storage, particularly at lower elevations. The three treatmen ts were usually equally effective in increasing soil water supply at a given site. Ripping had little effect on root zone available water ca pacity, and creation of a surface organic mulch with herbicide did not substantially increase soil water supply in comparison with bare mine ral soil surfaces. Treatments reduced evapotranspiration but also incr eased drainage losses at all sites.