The size-density relationship in pure and mixed red alder/Douglas-fir stands and its use in the development of a growth model

The increased interest in red alder (Alnus rubra Bong.) management instigated research in growth and yield and stand development of red alder in pure and mixed red alder/Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands. This study had the goa1s of evaluating the accuracy of the currently existing growth and yield tools for red alder and investigating the size-density relationship for pure red alder and Douglas-fir stands and mixed red alder/Douglas-fir stands. The database consisted of long-term remeasured plots in western Oregon and Washington and southwestern British Columbia. The comparison of the data with the red alder yield tables suggested lower accuracy for the Normal Yield Table for Red Alder than the Empirical Yield Table for Predominantly Alder Stands in Western Washington for prediction of both pure and mixed stand yield. Growth projections of the Stand Projection System for pure and mixed red alder/Douglas-fir stands were not accurately enough to ensure a reliable estimate of future stands conditions. The intercept and shape of the size-density trajectory and the relative density at which mortality starts (0.44) for red alder was independent of initial density. The Douglas-fir self-thinning line was above the red alder line but had a shallower slope (-0.525 for red alder vs. -0.638 for Douglas-fir). The size-density surface for pure and mixed stands exhibited a near-linear region for stands with a high proportion of red alder and a curvilinear portion for stands with a higher proportion of Douglas-fir. Stand density, relative dominance and species proportion were found to be the major determinate of stand development. The information on the size-density relationships and stand development were incorporated into a growth model

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

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

Management of Riparian Buffers: Upslope Thinning with Downslope Impacts

We examined the potential of using upslope density management to influence growth and drought tolerance of trees in untreated downslope riparian forests. Increment cores from Douglas-fir trees in three mature stands in western Oregon, USA, were collected and measured. Trees responded to an apparent edge effect up to 15 m downslope of thinned areas but not downslope of gaps. Growth responses in riparian trees were not affected by slope or potential solar radiation (as a function of location and topography). In addition, in a retrospective analysis of tree growth and allocation patterns (represented by the ratio of earlywood to latewood) and climate after treatment over a 12-year period, trees in our study area did not appear to be water limited and did not show a strong correlation with regional drought metrics. We hypothesize that vegetation layers in these riparian forest stands responded differentially to additional resources becoming available as a result of thinning, with overstory trees in riparian areas responding downslope of thinned uplands and subdominant canopy layers responding downslope of gaps. Our study demonstrates that managers can affect riparian forests with upland treatments to a limited spatial extent, which may be the only option in areas where direct riparian management is restricted due to concerns for other ecosystem services.Keywords: riparian management, western Oregon, thinning, climate, drought, riparian zon

Multiscale Controls on Natural Regeneration Dynamics after Partial Overstory Removal in Douglas-Fir Forests in Western Oregon, USA

We examined natural regeneration following operational-scale variable density retention treatments in 40-60 year old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests at seven sites for a decade following treatment. Treatments included residual overstory densities of 300, 200, and 100 trees/ha, with leave islands and gaps of three sizes (0.1, 0.2, and 0.4 ha) and an untreated control (~600 trees/ha). Natural regeneration was influenced by factors from multiple spatial scales including broad-scale differences in overstory composition among sites, mesoscale variability in topographic position, and fine-scale variability in overstory and understory competition. High local basal area (BA) decreased the probability of seedling establishment, though some seedlings established even under high BA, particularly shade-tolerant western hemlock. In contrast, recruitment of saplings (> 1.37 m height) required lower residual overstory density (i.e., 100 trees/ha), especially for shade-intolerant Douglas-fir. Understory vegetation had little effect on saplings, but was negatively related to seedling densities, particularly when overstory density was low. Variable density prescriptions can take advantage of the importance of fine-scale variability to promote regeneration of desired species mixtures, though other factors such as site overstory species composition and variation in topographic position will also influence regeneration dynamics. Including heavy overstory removal or gap creation could facilitate rapid recruitment of saplings, especially for shade-intolerant Douglas-fir.Keywords: Seedling bank, Western hemlock, Thinning, Douglas-fir, Density managementKeywords: Seedling bank, Western hemlock, Thinning, Douglas-fir, Density managemen

Multi-scale spatial controls of understory vegetation in Douglas-fir–western hemlock forests of western Oregon, USA

Forest understory vegetation is influenced by broad-scale variation in climate, intermediate-scale variation in topography, disturbance and neighborhood interactions. However, little is known about how these multi-scale controls interact to influence observed spatial patterns. We examined relationships between the aggregated cover of understory plant species (%C[subscript U]) and multi-scale controls using a large-scale experiment including treatments of low (LS), moderate (MS) and variable (VS) disturbance severity replicated in second-growth Douglas-fir (Pseudotsuga meziesii)–western hemlock (Tsuga heterophylla) forests spanning climate and topographic gradients. We developed hierarchical models using a multi-step selection process, assessing changes residual spatial autocorrelation associated with progressively broader spatial scales of influence and interaction. To examine the role of plant traits in mediating multi-scale controls, we contrasted effects for early- (%C[subscript ES]) and late-seral (%C[subscript LS]) species cover. At neighborhood scales, decreases in %C[subscript U] with overstory density were accelerated with increases in the relative importance of hemlock in the overstory in the in all but the LS treatment. At intermediate scales, %C[subscript U] was lower in areas with higher potential radiation in undisturbed control treatments but that trend was reversed in the harvested, disturbed areas. When separated, effects of multi-scale controls differed between %C[subscript ES] and %C[subscript LS]. Rates of increases in %C[subscript ES] with reductions in density increased with disturbance severity and decreased with increases in %C[subscript LS]. At broader scales, %C[subscript ES] increased with climatic moisture deficit where potential radiation was high, and %C[subscript LS] low. Similarly to %C[subscript U], %C[subscript LS] was related to a three-way interaction between overstory density, disturbance and hemlock abundance. %C[subscript LS] declined with increases in climatic moisture deficit where overstory density and the relative abundance of hemlock was high, and decreased with local increases in %C[subscript ES]. Multi-scale controls explained a portion of the observed spatial autocorrelation for %C[subscript ES] but not %C[subscript LS], suggesting the spatial patterning of %C[subscript LS] is related primarily to unmeasured processes. Results show how understory responses to overstory density and disturbance severity vary across the landscape with moisture and potential radiation, at fine scales with neighborhood structure, and with species traits. Hence, understory responses to climate change likely depend on overstory composition and structure, disturbance and species traits

Forest Restoration Using Variable Density Thinning: Lessons from Douglas-Fir Stands in Western Oregon

A large research effort was initiated in the 1990s in western United States and Canada to investigate how the development of old-growth structures can be accelerated in young even-aged stands that regenerated following clearcut harvests, while also providing income and ecosystem services. Large-scale experiments were established to compare effects of thinning arrangements (e.g., spatial variability) and residual densities (including leave islands and gaps of various sizes). Treatment effects were context dependent, varying with initial conditions and spatial and temporal scales of measurement. The general trends were highly predictable, but most responses were spatially variable. Thus, accounting for initial conditions at neighborhood scales appears to be critical for efficient restoration. Different components of stand structure and composition responded uniquely to restoration thinnings. Achieving a wide range of structures and composition therefore requires the full suite of silvicultural treatments, from leave islands to variable density thinnings and creation of large gaps. Trade-offs among ecosystem services occurred as result of these contrasting responses, suggesting that foresters set priorities where and when different vegetation structures are most desirable within a stand or landscape. Finally, the results suggested that foresters should develop restoration approaches that include multiple treatments

Bird-vegetation associations in thinned and unthinned young Douglas-fir forests 10 years after thinning

Quantitative associations between animals and vegetation have long been used as a basis for conservation and management, as well as in formulating predictions about the influence of resource management and climate change on populations. A fundamental assumption embedded in the use of such correlations is that they remain relatively consistent over time. However, this assumption of stationarity has been rarely tested – even for forest birds, which are frequently considered to be ‘indicator species’ in management operations. We investigated the temporal dynamics of bird-vegetation relationships in young Douglas-fir (Pseudotsuga menziesii) forests over more than a decade following initial anthropogenic disturbance (commercial thinning). We modeled bird occurrence or abundance as a function of vegetation characteristics for eight common bird species for each of six breeding seasons following forest thinning. Generally, vegetation relationships were highly inconsistent in magnitude across years, but remained positive or negative within species. For 3 species, relationships that were initially strong dampened over time. For other species, strength of vegetation association was apparently stochastic. These findings indicate that caution should be used when interpreting weak bird-vegetation relationships found in short-term studies and parameterizing predictive models with data collected over the short term.Keywords: Disturbance, Long-term experiment, Response variability, Young Douglas-fir forests, Bird-vegetation associations, Forest thinnin

Density management guide for red alder

Our diagram is intended to provide general guidance in density management of red alder. The management zone defines a reasonable compromise between individual-tree and stand growth and mortality. Factors considered in its formation include desired tree size at harvest, growth rate, and yield per acre. Social and legal constraints, as well as wildlife habitat and other non-timber-related considerations, may affect how the guide is applied in developing a management plan. It must be remembered, however, that the density management diagram is only a statistical construct and does not account for events and processes that are not density-related. Developing complete management plans requires that these other factors also be considered. D.E. Hibbs and D.S. DeBell are now preparing a review of the various factors to be considered throughout a full rotation of red alder

Silvicultural alternatives to conventional even-aged forest management - what limits global adoption?

Background The development of forestry as a scientific and management discipline over the last two centuries has mainly emphasized intensive management operations focused on increased commodity production, mostly wood. This “conventional” forest management approach has typically favored production of even-aged, single-species stands. While alternative management regimes have generally received less attention, this has been changing over the last three decades, especially in countries with developed economies. Reasons for this change include a combination of new information and concerns about the ecological consequences of intensive forestry practices and a willingness on the part of many forest owners and society to embrace a wider set of management objectives. Alternative silvicultural approaches are characterized by a set of fundamental principles, including avoidance of clearcutting, an emphasis on structural diversity and small-scale variability, deployment of mixed species with natural regeneration, and avoidance of intensive site-preparation methods. Methods Our compilation of the authors’ experiences and perspectives from various parts of the world aims to initiate a larger discussion concerning the constraints to and the potential of adopting alternative silvicultural practices. Results The results suggest that a wider adoption of alternative silvicultural practices is currently hindered by a suite of ecological, economic, logistical, informational, cultural, and historical constraints. Individual contexts display their own unique combinations and relative significance of these constraints, and accordingly, targeted efforts, such as regulations and incentives, may help to overcome specific challenges. Conclusions In a broader context, we propose that less emphases on strict applications of principles and on stand structures might provide additional flexibility and facilitate the adoption of alternative silvicultural regimes in a broader set of circumstances. At the same time, the acceptance of alternative silvicultural systems as the “preferred or default mode of management” will necessitate and benefit from the continued development of the scientific basis and valuation of a variety of ecosystem goods and services. This publication is aimed to further the discussion in this context