Silviculture chaired by Scott Roberts

Overstory and Understory Vegetation Dynamics in Response to Thinning in Coniferous Stands in Western Oregon

Adrian Ares et al. — Mon, 22 Jun 2009 14:10:00 PDT

Thinning even-aged coniferous stands in the Pacific Northwest is aimed at accelerating development of late-successional features, while maintaining long-term forest productivity. We examined effects of thinning on overstory and understory vegetation 11 years after harvest in 40- to 60-year old forests dominated by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) on three sites in western Oregon. Each site contained an unthinned control (238-1446 tpha) and three thinning treatments selected to enhance overstory structural diversity by decreasing densities, and enhance spatial variability within stands (high density = 120 tpha; moderate density = 80 tpha; variable thin with 120, 80 and 40 tpha with circular 0.2-ha gaps comprising 20% of the stand). Stand basal area and overstory cover was greater in controls than in thinning treatments in two of the three study sites, despite large increases in thinned stands. Seedling regeneration varied greatly among sites, but was generally more abundant in variable thins with 80 tpha than in high density thins and controls. Cover of forbs, grasses, low shrubs, hardwood, and understory conifers tended to be significantly greater in the variable density treatments than in controls. On the other hand, cover of moss and litter, bare soil and logs was generally greater in controls than in thinning treatments. Cover of grasses, ferns and low shrubs was greater in variable density treatments with 40 tpha than in all other treatments; while tall shrub cover was greater in high density thin treatments. The wide range of responses suggests that applying combinations of the thinning treatments may be beneficial to encourage a high diversity of understory conditions.

Factors Affecting Understory Composition and Height Growth 13 Years Following Variable-Density Thinning of a Conifer Forest

E. Comfort et al. — Mon, 22 Jun 2009 14:30:00 PDT

Variable-density thinning (VDT) is a management option aimed at accelerating the development of late-successional structure in second-growth conifer stands. The response of the understory is paramount to the success of the treatment. This study examined the species composition, density, and height growth of post-VDT regeneration 13 years following thinning in 50- to 70-year old conifer stands at Ft. Lewis Military Reservation near Tacoma, WA. Thinning intensity had a significant effect on the density of post-VDT regeneration, but that effect varied in intensity throughout stem-mapped plots. Height growth from 2002 to 2005 was assessed for 2,535 Douglas-fir, 43 grand fir, and 39 western white pine in the post- variable-density thinning cohort of second-growth Douglas-fir stands. Thinning intensity was not significantly related to height growth for grand fir or for western white pine. Thinning intensity was significantly related to height growth for Douglas-fir, but the effect varied with initial height. Factors correlated with Douglas-fir height growth in gap-thinned subplots were measures of crown size, measures of intra-cohort crowding, strata and type of overtopping vegetation, and overstory crown coverage. Smaller regeneration (less than 1.3m tall) was also examined. Stocking of smaller regeneration was significantly greater in the light thinning intensity than in the heavier thinning. The results of this study indicate that variable-density thinning can induce variation in the abundance and growth of Douglas-fir in post-VDT cohorts.

Spatial Patterns in Forest Understories: Relationships to Overstory Thinning Intensity and Understory Plant Diversity

Paul Satterthwaite et al. — Mon, 22 Jun 2009 14:50:00 PDT

Amount, spatial distribution, and species composition of understory plant communities have been shown to respond to changes in overstory structure. While response of the amount and composition of understory vegetation to thinning has been investigated in several ecosystems, spatial distributions have received less attention. We investigated spatial statistical techniques to examine associations of patch size of clonal shrubs and annual ruderals as they relate to overstory conditions after thinnings. We assessed the interpretation of empirical semivariograms in describing spatial pattern and whether semivariogram parameters can be useful when comparing impacts of different thinning regimes. We simulated vegetation patterns to test the ability of empirical semivariograms to describe patch sizes and suggest a nonparametric semivariogram range parameter as a metric of patch size. We applied results from the simulations to data from a long-term thinning study, in which intensity and spatial patterns of thinnings varied. We used range parameters from semivariograms of percent cover to compare response of patch sizes among thinning treatments and life forms. Initial results indicated that empirical semivariograms quantified both patch sizes and distance between patches. Nonparametric semivariogram estimates of patch size showed differences among thinning treatments, suggesting that spatial patterns of overstory conditions are influencing spatial distributions of understory vegetation. Patches of selected clonal shrubs were smallest in the treatment with spatially variable thinnings. Overall patch size of clonal shrubs was less strongly associated with thinning treatments than patch size of annual ruderals, likely reflecting differences in mobility between species that mainly regenerate by sprouting versus seeds. We conclude that spatial pattern of understory vegetation is responsive to thinning treatments and empirical semivariograms can provide useful information for developing silvicultural prescriptions.

First Decadal Response to Treatment in the Acadian Forest Ecosystem Research Program

M. R. Saunders et al. — Mon, 22 Jun 2009 15:40:00 PDT

Maintenance of late-successional structures to enhance biodiversity and sequester carbon has been a major focus of forest research over the past two decades. Several long-term, disturbance-based silvicultural trials have been installed to try to balance the maintenance of complex forest conditions yet allow for economical extraction of timber. One of the oldest of these contemporary efforts is the Acadian Forest Ecosystem Research Program (AFERP) located in central Maine. AFERP is now nearly 15 years old and, having received its second harvest entry, allows for a decadal review of its long-term sustainability in terms of forest growth, regeneration response, and maintenance of complex forest structure. AFERP includes two versions of ‘expanding gap’ treatments on a 100-year rotation: a ‘large-gap’ system that encourages regeneration of species with intermediate shade tolerance by using 0.2 ha gaps expanded every 10 years; and a ‘small-gap’ system that encourages shade-tolerant species with smaller 0.1 ha gaps expanded every 20 years. AFERP also includes an unharvested control and is replicated three times across nine 8.7-11.3 ha units. Prior to treatment, there was no significant difference among units in either total volume or basal area (275±11 m3 ha-1 and 37.7±1.1 m2 ha-1, respectively [mean± 1 S.E.]). Following treatment, there were significant differences in both volume and growth, often due to differences in regeneration response and differential mortality patterns among the treatments. For example, basal area growth averaged 0.22±0.04 m2 ha-1 yr-1, -0.08±0.13 m2 ha-1 yr-1 and -0.27±0.15 m2 ha-1 yr-1 for the large gap, small gap and control treatments, respectively; this resulted from higher levels of balsam fir and paper birch mortality in the small gap and control treatments. Further, there were strong treatment effects on regeneration composition, with both gap systems having much denser Pinus strobus and Picea spp. regeneration than controls.

How Best Should We Manage Hybrid Poplar Plantations? Interactions of Site Preparation, Vegetation Control and Fertilization

Simon Bilodeau-Gauthier et al. — Mon, 22 Jun 2009 16:00:00 PDT

The silviculture of hybrid poplars is a promising solution to reduce the pressure on natural forests while maintaining the wood supply to industries. However, hybrid poplars are sensitive to competing vegetation and to inadequate soil conditions and fertility. Possible management tools include mechanical site preparation (MSP), vegetation control, and fertilization. We present here the results after five years of growth for eight formerly forested sites (40 hectares total) on Podzols in the province of Quebec, Canada. The experimental design combines four MSP treatments (harrowing, scarifying, mounding, and no preparation) with four frequencies of plant competition control by brushing (from never to once a year) and N or N+P fertilization. The best growth was found in the following MSP treatments: mounding > harrowing > scarifying > none. The effect of competition control is more evident on the more productive sites and in the less severe preparation treatments, which brings insight into the Grime-Tilman debate over the importance and intensity of competition. Fertilization can provide substantial improvement in immediate growth, although it does not seem to compensate for deficient root systems. We will discuss the causes and implications of our results for future management strategies of hybrid poplar plantations.

Effects of Prescribed Burning on Advanced Regeneration in Upland Oak Forests of Missouri

Z. Ma et al. — Mon, 22 Jun 2009 16:20:00 PDT

Seventy years of fire-suppression and inappropriate management practices have resulted in numerous forest health problems in the current oak-hickory forests of Missouri, and have often resulted in the failure to adequately attain oak regeneration. The Chilton Creek Prescribed Burning Project, initiated by The Nature Conservancy in 1996, was designed to study the effects of prescribed fire on oak regeneration. Three different approaches to the use of prescribed fire were tested: annual burning (sites burned each year), random burning (sites burned randomly with a mean return interval of 3.6 yrs), and high-intensity random burning (same as random burn, but with higher fire intensities due to south-facing steep slopes and higher fuel loads) were implemented to monitor the dynamics of advanced regeneration. The presence and condition of regeneration, in terms of tree density and aggregate height, was recorded in 26 permanent plots in 1997 (pre-fire) and again in 2007 (post-fire). Post-treatment red oak densities were reduced by 53, 49, and 26% with the annual burn, the random burn and the high-intensity random burn, respectively. The random burn treatment significantly reduced the aggregate height of hickory compared with the other two treatments – the random burn reduced aggregate height by 83% while the annual burn and high-intensity random burn reduced the height by 40% and 65%, respectively. Preliminary results from this study suggest that increasing fire intensity favors red oak regeneration, although it also favors the growth of hickory. The time since the last burns under each of these treatments likely influenced these results.

Effects of Timber Harvesting on the Understory Oak Regeneration in Upland Oak Forests in the Missouri Ozarks

Q. Yao et al. — Mon, 22 Jun 2009 16:40:00 PDT

In the Missouri Ozarks, widespread oak decline in the overstory, along with failure to attain adequate regeneration have plagued oak-hickory forests. The reason for this has largely been attributed to long-term fire suppression and low-intensity timber management that has little impact on the main canopy. The Missouri Ozark Forest Ecosystem Project (MOFEP) was initiated in 1989 by Missouri Department of Conservation as a long-term, landscape-level study designed to evaluate the effects of timber harvest activities, both even-aged and uneven-aged, on oak forest ecosystem attributes. We used sixteen-year (1990-2006) MOFEP data to evaluate oak regeneration under various stand treatments. Pre-harvest data (1990-1995) indicated that oak regeneration in both the white oak and red oak groups had an inverse relationship with overstory density. Changes in understory density of the two groups were examined following treatments that included clearcutting, intermediate harvest (e.g., thinning), single-tree selection, group selection, and a no harvest control. The post-harvest data (1997-2006) indicated that in 2001, seedling (<1.5 inches dbh) density of white oak and red oak group species were 17 times and 13 times greater respectively than in 1995 on the clearcut sites. All of the other treatments had slight, but non-significant increases in oak seedling density. In 2006, sapling (1.5 inch

Long-Term Effects of Alternative Group Selection Harvesting Designs on Stand Production

C. Halpin et al. — Tue, 23 Jun 2009 13:30:00 PDT

Interest in group selection harvesting has increased in recent years because of limitations associated with both clearcutting and single-tree selection. Field data have suggested that group selection openings can have higher production rates than single-tree gaps, but whether this translates into higher production rates at the stand level is not clear. We used CANOPY, a crown-based northern hardwoods model calibrated with data from uneven-aged and even-aged stands, to simulate sustainable harvest volumes of a number of different group selection approaches over 300 years, and also compared results with those from single-tree selection and clearcutting. When a combination of single-tree and group selection was used with groups making up 3% of the stand area per cutting cycle, net harvestable production rates were similar to those of single-tree selection, and opening size (100-4000m2) had little effect on production rates. As the percentage of the matrix in groups increased from 1 to 9% per cutting cycle, production actually showed a small but consistent decline of about 6 to 7%. When group selection was used alone with no cutting between the groups, production rates varied considerably depending on opening size and rotation age. Small group selection (200 m2) had production rates similar to or slightly higher than single-tree selection, whereas 2000 m2 openings resulted in a production declines of 30 to 35%. Large patch sizes appear to have relatively low net production because of unsalvaged mortality. Similar trends were observed in unthinned even-aged stands compared to those thinned at 15-yr intervals. Although our results confirmed that trees in even-aged stands are more efficient producers than those in uneven-aged stands, there appear to be countervailing tendencies that reduce production rates in large single-cohort patches, including a lag time during the first few decades when production rates of merchantable volume in large openings are very low.

Ecological Forestry Methods in Northern Hardwoods: Predicted Effects on Forest Structure And Production

C. Lorimer et al. — Tue, 23 Jun 2009 13:50:00 PDT

Ecological forestry methods that include growing trees to very large diameter, retention of permanent legacy trees, and maintenance of large coarse woody debris (CWD) can have significant beneficial effects on biological diversity and ecosystem processes, but little is known about the potential impacts on timber production rates. To address this need, we used the CANOPY model, a crown-based individual-tree model calibrated with data from mature and old-growth stands, to simulate 7 treatments over a span of 150 years in a second-growth northern hardwood stand. Treatments included an untreated control, standard single-tree selection cutting with maximum retained dbh of 60, 70, or 80 cm, and retention of permanent legacy trees at densities of 7, 15, or 22 per ha. Raising the maximum dbh to 70 or 80 cm reduced net harvestable production by 15-24% but resulted in higher residual stand volumes. Retention of permanent legacy trees in the 60 cm maximum dbh trial reduced production by 9-33% depending on reserve tree density. Reserve trees generally did not increase CWD levels above those obtainable under a no-reserve treatment with a 70 or 80 cm maximum dbh. The different treatments had highly variable effects on the degree to which the size distribution of live trees approached old-growth conditions. Some treatments merely maintained mature forest structure, and two of the reserve-tree treatments had structures that vacillated between mature and borderline old-growth conditions. The 80 cm maximum diameter treatment was the only one that promoted negative exponential size distributions with large trees similar to those in the later stages of old-growth development.

Patterns of Structural Response to Simulated Partial Harvesting of Boreal Mixedwood Stands

Mark Vanderwel et al. — Tue, 23 Jun 2009 14:10:00 PDT

Partial harvesting has been proposed as an approach for maintaining late-successional structure within managed boreal mixedwood stands. Although little long-term data is available to evaluate its effects in this stand type, recent advances in individual tree-based stand modeling provide an opportunity to simulate post-harvest stand development following different retention harvests. Using the stand dynamics model SORTIE-ND, we examined 40-year patterns of structural change in response to different intensities (30%, 50%, and 70% removal) and spatial patterns (uniform, small patch, large patch) of harvesting in aspen-dominated mixedwood stands. We assessed structural dynamics through a suite of variables representing the distribution of tree sizes, understory development, regeneration, standing and fallen dead wood characteristics, and within-stand heterogeneity. Partial harvesting induced a reciprocal increase in understory and downed woody debris development and decrease in overstory structure over the first 20 years after harvest, with this effect reversing after 25 years as harvest-induced regeneration reached the canopy. Densities of large trees and snags were reduced by harvesting, and did not recover to pre-harvest levels within 40 years. Harvesting promoted within-stand heterogeneity in the short and long term, and also produced transient increases in early-decay downed woody debris and ground exposure. These effects largely increased in proportion to harvest intensity. Although spatial pattern was of lesser importance than intensity, aggregated harvests induced somewhat less pronounced impacts on structure (with the exception of heterogeneity) than dispersed harvesting. These simulation results can form a basis for more detailed hypotheses regarding maintenance of late-successional stand structure and function through partial harvesting. Such hypotheses may in turn be translated into real-world silvicultural experiments to be evaluated, refined, and either accepted or rejected within an adaptive management framework.