Redwood in California: An overview of silvicultural systems
New Zealand Tree Grower February 2007
John-Pascal Berrill and Kevin L O’Hara
|Redwood logs stockpiled for winter when harvesting is prohibited|
This can be a blessing, providing regeneration in recently cutover stands where sprout regeneration occurs quickly and is virtually guaranteed. Unfortunately, redwood regenerates very poorly from seed. Although a disturbed or cutover area will often support natural seedling regeneration, it is rare in an established stand to find redwood seedlings.
Dealing with regeneration from stumpsOne of the great conundrums of redwood silviculture is that even though the species regenerates well from stump sprouts, these are limited to the locations of previous stumps. In harvested old growth stands with well under 100 stems per hectare, this typically leaves large gaps between stumps and very clumpy distributions of trees. Sprout clumps may contain over 100 sprouts from a single stump after harvest. These sprouts self thin to more manageable numbers, but it is common to have 10 to 20 sprouts from a single stump 40 years after harvest.
Managers of even-aged stands will often supplement the natural sprout regeneration with planted seedlings. These seedlings do not grow as fast as the stump sprouts initially, but once established can grow quickly, and they form a valuable regeneration source for future rotations. Often these plantings are mixtures of redwoods and Douglas fir. Pre-commercial thinning is frequently necessary because of variable spacings of trees in young even-aged stands.
Thinning to waste prescriptions usually favour redwood by thinning out competing species, and reducing crowding in clumps of redwood stump sprouts, such as thinning to keep the three best redwood sprouts per stump, and a spacing of four metres between other trees. Sprouts originating from lower on the stump are favoured over sprouts high on stumps whose stability is compromised as the stump decays.
|Individual tree selection thinned at age 10 years Unthinned redwood regeneration 20 years after harvest|
Focus on volume productionPruning for clearwood production is uncommon, and can result in prolific epicormic sprouting if pruning is severe. There is limited chemical usage, mainly ‘hack and squirt’ or foliar treatments reducing competition from hardwoods. Commercial thinning is common.
Harvesting is severely restricted adjacent to streams where forestry practice regulations protect water quality.
The main focus of redwood management is on volume production. Little consideration is given to wood quality, or the influence of silviculture on clearwood or heartwood production.
The maximum size of log processed by sawmills has decreased since harvesting shifted from old growth to second-growth stands. However some smaller mills can still process large logs from remnant old growth trees.
|Core showing residual tree growth response from partial harvest at age approximately 65 years|
California forest practice regulations limit even-aged rotations to no less than 50 years.
|Diagram of two-aged and three-aged, multi-aged stands|
Group selection involves clearing small areas while keeping the residual stand, the matrix, intact and sometimes thinned. The residual trees mainly expand their crowns into the group areas leaving relatively more growing space provided that the cut area is large enough. Smaller groups with widths of half a tree height or less appear to get filled with expanding residual tree crowns, suppressing regeneration below.
Variable retention systemVariable retention is a flexible management system. Residual trees are retained in groups or dispersed throughout the stand, for habitat preservation, soil conservation and wood production. Once the new trees have reached merchantable size, residual trees may be harvested or retained through successive partial harvests. A new growth model – redwood MASAM – was used to compare variable retention regimes in California.
|Widely spaced residual trees|
Models of some multi-age regimesAn example regime designed using the redwood MASAM model involves a two-aged selection system producing trees with a diameter at breast height of approximately 60 centimetres. When the older group of 200 stems per hectare was removed entirely during partial harvesting, extra trees in the younger group were thinned to leave the best 200 stems per hectare.
A three-aged regime was also designed to produce trees with a DBH of 60 cm. When the oldest at 125 stems per hectare were removed, they were replaced by the middle cohort thinned from 175 stems per hectare to 125 stems, and with a DBH of approximately 41 cm. The middle cohort was replaced by the youngest, with a DBH of approximately 22 cm. In these examples, the three-aged system has a shorter cutting cycle. Each harvest reduces stand basal area by around 70% in the three-aged system, and 80% in the two- aged system. This free growing space is needed to maintain growth until the next harvest.
Careful allocation of growing spaceExtra trees are kept in younger cohorts to insure against damage and mortality, and cut at the time of partial harvesting leaving the target number of trees. Keeping too many trees in the understory wastes valuable growing space on trees that will probably never become merchantable trees. Generally, having more cohorts means greater forest cover over time, However there are probably fewer trees in each cohort, with less growing space available, and shorter cutting cycles with greater potential for site and residual stand damage from more frequent harvests.
Management regimes can be designed to meet specific owner objectives such as target tree size or timing of cash flows from partial harvesting. The key is careful allocation of growing space to trees. The most common pitfall in California is not cutting hard enough. Residual tree crowns expand to occupy the growing space, suppressing and in some cases killing stump sprouts needed to sustain the multi-aged structure.
Pascal Berrill is currently studying under Dr Kevin O’Hara, Professor of Silviculture, at the University of California.