Effect of alternative site preparation treatments on soil chemistry, physical properties, climate and seedling growth on a mesic site in the northern interior of British Columbia
Creators: name: Yole, D.
Publisher: name: University of British Columbia
Year: 2009

description: The influence of four commonly used site preparation methods (disc trenching, pile-and-burn, broadcast burn, no treatment) on soil microclimate, soil nutrients, bulk density, and early survival and growth of hybrid interior spruce (Picea glauca [Moench] Voss x engelmannii Parry ex. Engelm.) and lodgepole pine (Pinus contorta Dougl. ex. Loud.) seedlings were examined in plots established in the Engelmann Spruce - Sub alpine Fir (ESSF) zone in the northwest interior of British Columbia. Soil moisture was monitored in 13 potentially plantable microsite types at weekly intervals over the first two growing seasons after site preparation. Although growing season soil moisture in 1994 was significantly effected by the microsite types under study, soil moisture was not believed limiting to seedling growth in any microsite studied and soils did not reach slightly to moderately dry conditions (&gt; 2 bars tension) until late July, and only in three microsites, disc trench berm, disc trench hinge, and pile-and-burn-scalp (mineral soil exposed). Soil and air temperature were strongly affected by the four site preparation treatments and nine microsites studied, particularly in early spring periods between mid-May and mid-June. Soils (10-cm depth) warmed sooner, and for longer duration, in treatments which promoted soil drainage. Treatments which removed or incorporated insulating forest floor layers resulted in greater cumulative hours of soil temperature greater than 8°C in early spring periods, giving a distinct growth advantage over planting sites having intact forest floors. Untreated soils (slash left) were consistently colder for longer periods during the growing season. Broadcast burning decreased the frequency of frost events 20 cm above the ground surface during the 1994 growing season by 43 and 121%, relative to pile-and-burn and no treatment plots, respectively. Site preparation had strong effects on coarse fragment free bulk density of near surface (2-7 cm) and rooting zone (0-20 cm) layers one season after treatment. The coarse fragment free bulk density of the disc trench and hinge microsites 0-20 cm layer were 50 and 24% less, respectively, as compared to pre-treatment density. The disc trench berm and hinge microsites had significantly (p&lt;0.05) smaller 0-20 bulk density one season after treatment relative to other microsites studied. Both the disc trench trench-bottom and pile-and-burn under-excavator-track microsites had greatest near surface (2-7 cm) soil density. Several soil nutrients in decayed wood, forest floor and 0-20 cm mineral layers were significantly affected by site preparation treatments. With the exception of total S, exchangeable K, mineralizable N in most treatments and total N in the pile-and-burn burned-pile microsite, soil nutrients and pH in decayed wood materials increased from pre-treatment to one season post-treatment. Nutrients of forest floor materials revealed greater microsite effects than decayed wood materials and may reflect greater biological activity in this uppermost layer. Nine and 12 months after pile and broadcast burning, respectively, there were significant increases in forest floor pH and exchangable and total Ca and Mg relative to untreated soils. Site preparation treatments which promote mixing (i.e. at the berrh and hinge) resulted in significant increases in many soil nutrients in the 0-20 cm layer, most notably total C and N . Greatest one-season losses of total N , S, and C from all three soil layers studied occurred in the pile-and-burn burned-pile microsite. Superior second-year root collar diameter and height increment growth of both planted species occurred on the broadcast burn and disc trench hinge microsites. Poorest growth of both species occurred on the pile-and-burn and untreated plots and was believed at least partially associated with higher soil moisture content and slow soil warming in early spring periods in this moist, cold biogeoclimatic subzone (ESSFmc).