Understory light predicts stump sprout growth in mixed multiaged stands in north coastal California

Abstract

Regeneration of commercial species is central to long-term success of multiaged management for wood production. We examined relationships between understory light, varying overstory tree retention, and growth of coast redwood (Sequoia sempervirens; commercial species) and tanoak (Notholithocarpus densiflorus) stump sprouts initiated by group selection and single-tree selection harvesting in 80–100 year old mixed stands at four sites. Treatments included a complete harvest in 1-ha group selection openings, low-density dispersed retention, and either aggregated or dispersed high-density retention. Post-harvest stand density index and basal area were useful predictors of understory light. Mean and maximum understory light did not differ significantly between treatments with the same density where residual trees were retained in aggregated versus dispersed spatial patterns. However, the dispersed retention had lower minimum light levels when compared to the aggregated retention treatment. Aspect appeared to influence understory light more in dispersed treatments. At all light levels, the dominant sprout within clumps of redwood stump sprouts generally grew faster than dominant tanoak sprouts within tanoak sprout clumps. Differences in sprout height growth between aggregated and dispersed treatments were minimal. Stump size had a significant effect on redwood stump sprout height growth, with sprouts on the largest stumps growing approximately twice as quickly as sprouts on the smallest stumps. In the low density dispersed treatment, redwood sprouts outperformed tanoak sprouts by the greatest margin. Regeneration of redwood and tanoak was most rapid within group selection openings.