Soil water regime, root water extraction and the growth of fine roots of Sitka Spruce
The effect of removing soil water stress on fine root growth and the pattern of water use was studied by irrigating a small plot (20 m x 20 m) of 40 year old Sitka spruce. A control plot was established close by. Soil water content and soil matric potential were measured independently on both plots at several depths. Root growth was measured using monthly coring for the surface horizons (c. 10 cm depth) and ingrowth cores to study rooting at depth. In the absence of irrigation the 2 plots had similar water regimes. The irrigation kept matric potential > 20 kPa and water content > 20%. Areas between trees were wetter than areas close to stems and wetted up more during periods of rainfall. This is in contrast to young plantations where stemflow accounts for a much larger proportion of precipitation reaching the forest floor. At field capacity most of the water uptake was from near the surface but as the profile dries the proportion from depth increases, at the end of a dry period 50% of uptake was from the bottom half of the rooted zone. Water uptake from below the rooted zone was small. Irrigation had a significant effect on root tip density and on small root mass but not on fine and dead root masses. Root tip density stayed high through the summer with irrigation but fell on the control plot, the number of tips began to increase earlier on the control plot. This is interpreted as irrigation increasing the longevity of tips from 2-3 months (control) to 5 months. Rewetting of the soil in autumn appears to act as a cue for an increase in root growth. Irrigation had no effect on root growth at depth and there was no difference in growth at different depths on either plot.