Abstract:
Residual roots from previous stand components are often cited as a major benefit to stump-sprouts of tanoak in southwest Oregon and northern California. Established patterns of belowground carbon allocation and root/shoot maintenance suggest that residual root systems of stumps
will be reduced by carbohydrate depletion and root mortality and by shifts in allocation priority. A chronosequence approach was taken to relate changes in
tanoak fine-root biomass to aboveground development during the first ten years after cutting of mature tanoak stands. Tanoak stump sprouts did not maintain preexisting
fine roots. Proportional to values in the mature forest, root surface area for 3-year sprouts was 15, 52, and 82% in root diameter classes <.25, .25-1, and 1-2 mm respectively. A strong correlation between elevated soil temperature and dead root proportions suggests that increased respiratory depletion of carbohydrate supplies is the major cause of root mortality. Root dieback increased (in relative and absolute terms) with decreasing root size, which emphasizes the importance of
high resolution in belowground studies. As a result of increased root mortality and reduced root growth relative to shoot growth, the new sprout stands regained the root/shoot equilibrium found in mature forests within 4 years. This supports a general theory of functional equilibrium between root and shoot. Rapid initial rates of leaf area growth declined to a lower, stable level of relative growth rate by age 4, coinciding with the time of minimum tanoak root density and root/shoot recovery. This may be the time of minimum dominance potential for tanoak stump-sprouts. The link between aboveground and belowground growth
suggests that more extensive studies of tanoak dominance potential may be focused on aboveground characteristics. In combination with the overall decrease in tanoak roots, a pattern of increased dieback in openings between sprout clumps (until age 4) suggests that competition from residual tanoak roots will not entirely preclude the benefits of increased light for other species in
openings. However, invading herbs (primarily bracken fern) may account for substantial soil moisture depletion during tanoak sprout recovery. Compared to the fully occupied mature forest, soil moisture depletion in sprout stands was substantially reduced only in the first summer after burning. To meet conifer management objectives, tanoak control treatments may be most effective if applied when sprout stands reach minimum root occupancy (age 4 in this study). In terms of competition, compensatory effects of
invading herbs and shrubs are very important in moderating interactions between long-term dominants. Douglas-fir is the major potential crop tree on tanoak dominated sites. To effectively shift dominance to
Douglas-fir in tanoak sprout stands, planting of large seedlings and herbaceous weed control are recommended in addition to tanoak control measures.
