John C. Tappeiner

Effects of thinning on structural development in 40- to 100-year-old Douglas-fir stands in western Oregon

Abstract We studied the composition and structure of the understory in thinned and unthinned Douglas-fir/western hemlock stands on 32 sites in western Oregon. These stands had regenerated naturally after timber was harvested between 1880 and 1940; they were thinned between 1969 and 1984. Commercially thinned stands had 8–60% of their volume removed 10–24 years before the study. Undisturbed old-growth Douglas-fir stands were present for comparison on 20 of these paired sites. Conifer regeneration density and frequency were consistently greater in thinned than unthinned stands. For example, average seedling density in thinned stands (1433/ha) was significantly (p≤0.001) greater than in unthinned stands (233/ha), but very similar to that in old-growth stands (1010/ha). Seedling density and frequency were strongly related to the volume removed and to stand density index (and other measures of overstory density) just after thinning. In thinned stands, the density of small trees (intermediate crown class overstory trees and advanced regeneration) was 159/ha, significantly (p≤0.001) greater than in unthinned stands (90/ha), but not significantly different from that of old-growth (204/ha). The live crown ratio of these trees in thinned stands (66%) was greater than in unthinned (44%) and old-growth (48%) stands. Cover and stem density of shrubs was variable in all three stand types. There was significantly less tall shrub cover in unthinned stands than in either thinned or old-growth stands, which did not differ. Thinned stands had the most low shrub cover. Salal and bracken fern cover was greater in thinned stands than in the other stand types, but there was no difference in sword fern and Oregongrape cover. Leaf area index in thinned stands (6.6) was not significantly different from that in unthinned (6.8) and old-growth stands (7.1); however, there was more leaf area in shrubs in the thinned stands. Thinning young Douglas-fir stands will hasten the development of multistory stands by recruitment of conifer regeneration in the understory as well as by enabling the survival of small overstory trees and growth of advanced understory regeneration. Thinning will also help develop the shrub layer by increasing tall shrub stem density and cover of some low shrubs.

SALMONBERRY CLONAL AND POPULATION STRUCTURE: THE BASIS FOR A PERSISTENT COVER'

To understand how populations of salmonberry (Rubus spectabilis) persist in common forest stand types, we studied the above- and belowground structure of salm- onberry clones and populations in upland alder and conifer stands, in riparian stands, and in 2-yr-old and 13- to 18-yr-old clearcuts in the central Oregon Coast Range. On undisturbed sites individual clones and populations replace aerial stems by rhizome extension, by production of new genets, and by sprouts from buds on old aerial stems. It appears that frequent initiation of aerial stems from these three sources enables salmonberry populations to persist and maintain a dense cover. The size distribution of aerial stems on these sites resembled that of an uneven-aged stand of trees, with stem numbers decreasing from small to large size classes. Within the first two growing seasons after disturbance to overstory trees and the understory, salmonberry populations maintained themselves by a rapid initia- tion of new rhizomes (1.0-2.5 r/i2 annually) and aerial stems (25-50 stems/i2). These populations apparently continue to grow well, since the greatest rhizome and aerial stem biomass occurred in 13- to 18-yr-old clearcuts with no overstory trees. Rhizome density of salmonberry populations, as well as rhizome and aerial stem bio- mass, was significantly and negatively related (r2 = 0.71-0.83) to the basal area of overstory trees. It appears that population structure, rhizome length and aerial stem, rhizome, and total biomass, can be predicted from measures of preharvest overstory trees, and also from salmonberry stem number and basal area. Clonal architecture varied with stand type. Salmonberry clones in alder stands were larger (18 m total rhizome) than those in conifer stands and on riparian sites (5-6 m rhizome). Clones in alder stands also produced more ramets and more aerial stems than those in conifer and riparian stands.

Understory vegetation in old and young Douglas-fir forests of western Oregon

Abstract We studied understory composition in thinned and unthinned Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco)/western hemlock ( Tsuga heterophylla (Raf.) Sarg.) stands on 28 sites in western Oregon. These stands had regenerated naturally after timber harvest, 40–70 years before thinning. Commercial thinning had occurred 10–24 years previously, with 8–60% of the volume removed from below with the intent to homogenize spacing among trees. Undisturbed old-growth Douglas-fir stands were present for comparison on 18 of these sites. Total herbaceous cover was greater in thinned (25% cover) stands than in unthinned (13% cover) or old-growth (15% cover) stands. Species richness was also greater in thinned (137) than in unthinned (114) and old-growth (91) stands ( P =0.05). Part of the increased richness was caused by the presence of exotic species in thinned stands, but there were also more native grass and nitrogen-fixing species in thinned stands than in unthinned or old-growth stands. Groups of species differed among stand-types. For example, the frequency of tall cordate-leaved species was greater in old-growth stands ( P =0.009), but their relative cover was different only between old-growth and unthinned stands ( P =0.08). Both the cover and frequency of grasses and sedges in thinned stands were greater than in unthinned or old-growth stands ( P ≤0.002). Ordination of shrub cover showed differences among old-growth and unthinned stands compared to thinned stands, mainly because of the amount of Gaultheria shallon Pursh and Polystichum munitum (Kaulf.) Presl in heavily thinned stands. Ordination of herbaceous community data showed that there were much stronger differences among sites than among stand-types. The lack of difference among stand-types demonstrates the resiliency of herbaceous communities to disturbance associated with past and current forest management.

BREEDING BIRD RESPONSES TO THREE SILVICULTURAL TREATMENTS IN THE OREGON COAST RANGE

Silvicultural alternatives to clear-cutting have been suggested to promote development, retention, or creation of late-successional features such as large trees, mul- tilayered canopies, snags, and logs. We assessed bird response to three silvicultural alter- natives to clear-cutting that retained structural features found in old Douglas-fir (Pseudo- tsuga menziesii) forests and that imitated natural disturbance regimes more closely than did traditional clear-cutting: (1) small-patch group selection treatment representing a low- intensity disturbance; (2) two-story treatment, representing a moderate to high-intensity disturbance; and (3) modified clear-cut treatment, representing a high-intensity disturbance. We counted diurnal breeding birds 1 yr prior to and 2 yr after harvest to estimate effects of the silvicultural treatments on bird communities compared with uncut controls. The small-patch group selection treatment was most similar in species composition to control stands. The two-story treatment was more similar to the modified clear-cut treatment. Ten bird species remained abundant following the small-patch group selection treatment. They declined in abundance in modified clearcuts and two-story stands. These species included four neotropical migratory species and five species with restricted geographic ranges and habitat associations. Nine species increased in response to moderate and/or high-intensity disturbances. This group included a larger proportion of species that were habitat generalists. Silvicultural treatments imitating low-intensity disturbances were most effective in retaining bird communities associated with mature forest; high-intensity disturbances such as the two-story and modified clear-cut treatments greatly altered bird community composition. Bird responses to the silvicultural treatments that we studied indicate that a variety of stand types is needed to meet needs of all species.

The effects of partial cutting on stand structure and growth of western hemlock-Sitka spruce stands in southeast Alaska

Abstract The effects of partial cutting on species composition, new and residual-tree cohorts, tree size distribution, and tree growth was evaluated on 73 plots in 18 stands throughout southeast Alaska. These partially cut stands were harvested 12–96 years ago, when 16–96% of the former stand basal area was removed. Partial cutting maintained stand structures similar to uncut old-growth stands, and the cutting had no significant effects on tree species composition. The establishment of new-tree cohorts was positively related to the proportion of basal-area cut. The current stand basal area, tree species composition, and stand growth were significantly related to trees left after harvest ( p Concerns about changing tree species composition, lack of spruce regeneration, and greatly reduced stand growth and vigor with partial cuts were largely unsubstantiated. Silvicultural systems based on partial cutting can provide rapidly growing trees for timber production while maintaining complex stand structures with mixtures of spruce and hemlock trees similar to old-growth stands.

Vine Maple Clone Growth and Reproduction in Managed and Unmanaged Coastal Oregon Douglas‐Fir Forests

Vine maple (Acer circinatum Pursh.) clone development, expansion, and regeneration by seedling establishment were studied in 5-240 yr old managed and unman- aged Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands in coastal Oregon. Stem length, number of stems, and crown area were all significantly (P ? 0.01) related to stand age, and clone development was most rapid during the first 50 yr of stand development. Following clear-cutting, clones were clumps of 1-3 m long upright stems. Fifty years after disturbance, clones consisted of 5-15 drooping aerial stems >10 m long and basal sprouts 1-2 m long; some stems had been pinned to the forest floor by fallen trees or branches and had layered. In stands >120 yr in age, clones were often quite complex, composed of several decumbent stems each of which connected the ramets of 1-10 new aerial stems. Vine maple clone expansion occurs by the layering of long aerial stems. Over 95% of the layered stems we observed had been pinned to the forest floor by fallen debris. Unsevered stems that we artificially pinned to the forest floor initiated roots within 1 yr. Thinning may favor clonal expansion because fallen slash from thinning often causes entire clones to layer, not just individual stems. Clonal vine maple seed production and seedling establishment occurred in all stages of stand development except dense, young stands following crown closure. There were more seedlings in thinned stands than in unthinned stands and in unburned clearcuts than in burned clearcuts.

Influences of climate, fire, and topography on contemporary age structure patterns of Douglas-fir at 205 old forest sites in western Oregon.

Knowledge of forest development is basic to understanding the ecology, dynamics, and management of forest ecosystems. We hypothesized that the age structure patterns of Douglas-fir at 205 old forest sites in western Oregon are extremely variable with long and (or) multiple establishment periods common, and that these patterns reflect variation in regional-scale climate, landscape-scale topography, and landscape-scale fire history. We used establishment dates for 5892 individual Douglas-firs from these sites to test these hypotheses. We identified four groups of old forest sites with fundamentally different Douglas-fir age structure patterns. Long and (or) multiple establishment periods were common to all groups. One group described old forests characterized by substantial establishment from the early 1500s to the mid-1600s, with decreasing establishment thereafter. Another group was characterized by peaks of establishment in the middle to late 1600s and in the late 1800s and early 1900s. A third group was...

Growth responses of young Douglas-fir and tanoak 11 years after various levels of hardwood removal and understory suppression in southwestern Oregon, USA

Abstract At two sites in southwestern Oregon, height, diameter, and crown width of young Douglas-fir ( Pseudotsuga menziesii ) and sprout-origin tanoak ( Lithocarpus densiflorus ) were measured 1–11 years after reducing the density of a 2-year-old tanoak stand to 0%, 25%, 50%, and 100% of its initial cover. Some plots also included suppression of understory vegetation. Tanoak cover developed linearly with time, with steepness of the growth trajectory increasing at a diminishing rate with increasing percentage of initial tanoak cover. Fifth-year cover of understory vegetation declined linearly with increasing percentage of initial tanoak cover ( R 2 = 0.29). Survival of Douglas-fir (96–100%) differed little among initial abundances of tanoak, while growth trajectories for its size became increasingly exponential with decreasing percentage of initial tanoak cover. Eleventh-year heights of Douglas-fir were similar for 0%, 25%, and 50% of initial tanoak cover; however, diameter increased linearly with decreasing percentage of initial tanoak cover ( R 2 = 0.73), and the slope of the relationship steepened with understory suppression. Our results indicate that young stands exhibiting a wide range of stand compositions and productivities can be established by early manipulations of tanoak and understory abundance. Complete removal of tanoak plus understory suppression are necessary to maximize Douglas-fir growth, while productive, mixed stands can be achieved by removing 50% or more of tanoak cover.

Predicting average growth and size distributions of Douglas-fir saplings competing with sprout clumps of tanoak or Pacific madrone

The average growth and size distributions of Douglas-fir (Pseudotsuga menziesii) saplings in three plantations were studied for 7 years (1983–1989) after thinning of associated sprout clumps of tanoak (Lithocarpus densiflorus) or Pacific madrone (Arbutus menziesii); in some cases understory vegetation (shrubs and herbs) was experimentally suppressed. Biologically based nonlinear equations explained 66, 90, and 53% of the variation in the average annual increment of Douglas-fir height, diameter-squared, and cover, respectively. Equations for annual increment of cover of hardwood and understory vegetation explained only 10 to 12% of the variation, because these parameters exhibited a high degree of variability. Model simulations demonstrated that, for the same initial levels of cover, tanoak had faster rates of cover growth than madrone and also caused greater limitations in growth of Douglas-fir. Suppression of understory vegetation increased growth of Douglas-fir only when hardwood cover was absent. Weibull functions adequately described size distributions for Douglas-fir in 92% of the individual-tree data sets. Regression functions of hardwood cover and average Douglas-fir size explained 51, 93, and 24% of the variation in the Weibull A, B, and C parameters, respectively. Model simulations with predicted Weibull parameters demonstrated that hardwood competition caused a positive skewing in size distributions for height and stem diameter of Douglas-fir.

Varying densities of Pacific madrone in a young stand in Oregon alter soil water potential, plant moisture stress, and growth of Douglas fir.

Abstract Soil water-potential and Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] moisture stress and growth were studied on a droughty, 2-ha site in southwest Oregon where Pacific madrone (Arbutus menziesii Pursh) and associated shrub and herbaceous vegetation were thinned to represent the following range of conditions: high-density madrone (H), with associated shrubs and herbs controlled; medium-density madrone (M), shrubs and herbs controlled; low-density madrone (L), shrubs and herbs controlled; no madrone (N), shrubs and herbs controlled; and no madrone (U), shrubs and herbs predominate. Soil water-potential (Ψs) at a depth of 0–30 cm was consistently higher in treatment N than in all other treatments; in 1987 this difference was significant (P Predawn plant moisture stress ( pms ) of Douglas fir was significantly (P=0.0001) less in treatment N than in all other treatments. Seasonal moisture-stress relief ( smsr ) of Douglas fir was significantly related to madrone leaf area index (L) (r2=0.75, P=0.003), and was greatest in treatment N. Seasonal moisture-stress relief of madrone was also significantly correlated with L (r2=0.66, P=0.03). There were highly significant linear relationships between both predawn and midday pms and soil water-potential for Douglas fir (r2=0.76 and 0.75) and madrone (r2=0.69 and 0.79). Results clearly show that conditions for maximum Douglas-fir growth occurred in treatment N, in which all madrone and the associated vegetation were controlled. Average diameter growth of Douglas fir was greatest in treatment N, although not significantly different from that in treatment U, and least in treatment H. In 1987, Douglas-fir growth in diameter, stem basal-area, and stem volume was strongly related to smsr (r2=0.62–0.81) and madrone L (r2=0.66–0.87), and to a lesser extent, seasonal soil tension relief ( sstr ) (r2=0.51–0.62).