Robert S. Seymour

Natural disturbance regimes in northeastern North America—evaluating silvicultural systems using natural scales and frequencies

Many scientists and foresters have begun to embrace an ecological, natural disturbance paradigm for management, but lack specific guidance on how to design systems in ways that are in harmony with natural patterns. To provide such guidance, we conducted a comprehensive literature survey of northeastern disturbances, emphasizing papers that studied late-successional, undisturbed, or presettlement forests. Evidence demonstrates convincingly that such forests were dominated by relatively frequent, partial disturbances that produced a finely patterned, diverse mosaic dominated by late-successional species and structures. In contrast, large-scale, catastrophic stand-replacing disturbances were rare, returning at intervals of at least one order of magnitude longer than gap-producing events. Graphing the contiguous areas disturbed against their corresponding return intervals shows that these important disturbance parameters are positively related; area disturbed increases exponentially as the return interval lengthens. This graph provides a convenient metric, termed the natural disturbance comparability index, against which to evaluate both single and multi-cohort silvicultural systems based on their rotations or cutting-cycles and stand or gap sizes. We review implications of these findings for silvicultural practice in the region, and offer recommendations for emulating natural disturbance regimes.

The red spruce-balsam fir forest of Maine: Evolution of silvicultural practice in response to stand development patterns and disturbances

Red spruce (Picea rubens) and balsam fir (Abies balsamea) are so similar that “spruce-fir” is often used as if it were a single species. The early monographs of Zon (1914) and Murphy (1917) accurately characterize both species as occupying a similar ecological niche: late-successional, very tolerant of shade, shallow rooted, and widely adapted to a variety of site and stand conditions. These species differ in important ways that influence silvicultural treatment (Fowells 1965). Fir produces abundant seeds, but is so susceptible to various heart-rot fungi that its potential life span is limited by the high risk of wind breakage or uprooting. Balsam fir is often cited as the classic example of a species ruled by a pathological rotation, effectively limited to ages 40–70, depending on site quality. Fir is also the preferred host and suffers extensive mortality from defoliation by the spruce budworm (Choristoneura fumiferana). The introduced balsam wooly adelgid (Adelges piceae) also is a serious pest of fir in coastal regions but does not cause serious damage inland.

Alternative measures of stem growth efficiency applied to Abies balsamea from four canopy positions in central Maine, USA

Abstract Stem analysis data collected from 39 Abies balsamea (L.) Miller from four distinct canopy positions (open-grown, codominant, intermediate, suppressed) were used to construct a model to predict stem volume increment (VINC) from projected leaf area (PLA), and test the hypotheses that the physiological-based measure for growth efficiency ( GE , stem volume increment per leaf area) and the empirical surrogates for GE (e.g. surrogates substituting other measures of stem volume increment or PLA into the GE equation) were equal among canopy positions. VINC was best predicted from PLA with an asymptotic model which suggests that once individual trees reach a critical leaf area, very little stem growth occurs relative to leaf area. Analyses of variance revealed that the suppressed crown class was the least efficient (in terms of GE or any of its surrogates) relative to the superior canopy positions. Observed and predicted patterns of growth efficiency were found to depend on the manner in which it was calculated or predicted. Suppressed trees, however, were consistently observed, and predicted to be the least efficient in terms of stem volume increment relative to the intermediate, codominant, and open-grown crown classes.

An evaluation of the uniform stress hypothesis based on stem geometry in selected North American conifers

Abstract. The uniform stress hypothesis of stem formation was evaluated by comparing stem taper of Abies balsamea, Abies lasiocarpa, Picea rubens, Pinus contorta, Pinus elliottii, Pinus palustris, Pinus ponderosa, Pinus taeda, and Pseudotsuga menziesii to the taper expected if stems develop to uniformly distribute bending stress. The comparison was conducted by regressing stem diameter at height h (Dh) against bending moment at h (Mh) using the model Dh=φ (Mh)δ where φ and δ are fitted coefficients, and testing for δ=0.333, the hypothesized value. Twelve curves were fitted with the model. Seven of the fitted values of δ were significantly different from 0.333, but eight of the values were within ±10% of 0.333 and eleven values were within ±15% of 0.333. Where the fitted value of δ was >15% of 0.333, residuals were biased with height. Fit by relative height, values of δ were within ±10% of 0.333 for large portions of these stems. While most of the fitted values of δ support the uniform-stress hypothesis, the values of δ for Pseudotsuga menziesii trees clearly did not. Many of the fitted values of φ were inversely related to the modulus of elasticity (E) of green wood reported for these species. With the exception of Pseudotsuga menziesii, growing conditions appeared to account for extraordinary values of φ. Increases in φ with stem height corresponded with reported decreases in E with height. The covariance between φ and E suggests some regulation of bending curvature by adjustments in cross-sectional area. These results suggest that stems taper to maintain a uniform bending curvature and that when E is relatively constant within and among stems, diameter along the stem or across stems can be predicted from bending moment using a simple power function.

Comparison of site index and biomass production of spruce-fir stands by soil drainage class in Maine

Abstract Site index and biomass production of natural, even-aged spruce-fir (Picea rubens Sarg., Picea mariana (Mill.) B.S.P., Abies balsamea L.) stands on four soil drainage classes were compared to determine the relationship between site index and biomass production. Biomass yield equations using stand age at breast height and site index as independent variables were developed by soil drainage class. Site index was determined from equations developed for each drainage class and revealed that site index for spruce and fir increased with improved drainage. Site index was found to be a poor predictor of stand biomass production on the soils studied as there was no significant change in the biomass production with increase in site index on the poorer drained soils. However biomass production decreased with increase in site index on the better drained soil suggesting that rates of self-thinning may increase on these soils with increase in site index. Site index, which measures only the height growth of the forest, may not be adequate in classifying sites into productivity classes, as it does not take into account the relation between soil and other stand properties. Stand biomass production was also compared among all soils, revealing that differences in biomass productivity among these soils is not evident at early ages. No significant differences in biomass productivity among stands were found until about stand age 60 years, where production of stands on the better drained soils became greater than those on the poorer drained soils.

Patterns of leaf area index during stand development in even-aged balsam fir – red spruce stands

Leaf area index (LAI) strongly controls forest stand production. Silviculturists can easily manage this biologically important variable by quantifying its relationship to more directly manageable s...

Disturbance history and stand dynamics in secondary and old-growth forests of the Southern Appalachian Mountains, USA

Abstract Understanding the patterns of past disturbance allows further insight into the complex composition, structure, and function of current and future forests, which is increasingly important in a world where disturbance characteristics are changing. Our objectives were to define disturbance causes, rates (percent disturbance per decade), magnitudes and frequency (time since last disturbance) for both secondary and old-growth mixed-oak stands, and to determine if all mixed oak stands experience similar disturbance history. The study was located in two southern Appalachian forests in western North Carolina, USA: Coweeta Hydrologic Laboratory, a 2,185 ha experimental forest with some history of harvesting, and the Joyce Kilmer Wilderness, a 6,805 ha old-growth forest with no known harvesting. We used dendroecological techniques to evaluate the disturbance histories and create chronologies of these mixed-oak forests. Average decadal disturbance rates ranged from 4.3% to 13.8%, similar to rates common in eastern temperate forests (5% to 20%). The decades of peak recruitment common to several stands were the 1840s, which coincides with the historical accounts of a hurricane; the 1900s through the 1940s, which coincide with logging and elimination of Castanea dentata (Marshall) Borkh. by chestnut blight; and the 1960s, which coincides with drought and an elm spanworm infestation. The large peaks of disturbance were often synchronous and widespread, affecting stands across both Coweeta and Joyce Kilmer. However, there were also scattered pulses of disturbance unique to single stands, suggesting that localized events also played a role in the disturbance dynamics. Periods of constant low rates of disturbance present in all stands also indicate the importance of small canopy gaps in these forests. We found that stands similar in disturbance regimes were also similar in species composition. Results from our study provide information on how past disturbances, both regional and local events, have shaped the current forest. This understanding could help inform models to better predict how forests might respond to future climate (e.g., rising temperatures and increasing precipitation variability) and disturbance patterns (e.g., more frequent and severe events).

Identification of low productivity sites for European larch (Larix decidua Miller) in Maine, USA

As silvicultural capital becomes limited, allocation of funds to high quality sites becomes critical and sites with low productivity potential should be avoided. Data collected from 31 plots located in 12 plantations established between 1930 and 1982 throughout central Maine were used to develop a soil-based rule to identify sites having a low productivity potential for European larch in Maine, thereby removing them from consideration for plantation establishment. Stepwise regression and discriminant analyses identified several variables that were associated with site index at an index age of 20 years breast height (SI20): solum thickness, B horizon clay content, and B horizon exchangeable K. Using these variables, a classification rule was developed to classify sites into 2 categories (poor, average SI20=14.7 m; and good, average SI20=17.7 m). Cross-validation demonstrated that the classification functions correctly identified 88% of the poor sites and 81% of the good sites.

Sixty Years of Silviculture in a Northern Conifer Forest in Maine, USA

In 1950, the US Forest Service initiated a cutting practice level (CPL) study on the Penobscot Experimental Forest in Maine on the basis of findings of a national appraisal of forestland management. Silvicultural treatments, including the selection system with 5and 15-year cutting cycles, fixed diameter-limit cutting, and variants of commercial clearcutting, were chosen to represent “High-order,” “Good,” “Fair,” and “Poor” cutting practices as then defined for eastern spruce–fir (Picea–Abies; northern conifer) forests. After 60 years, selection treatments have maintained a wide distribution of tree sizes, increased the proportion of mature spruce, and decreased the proportion of hardwoods; selection cutting on a 15-year cycle resulted in the highest current stand value. Fixed diameter-limit cutting resulted in the greatest harvest revenue but degraded residual stand composition, structure, and value. Outcomes of commercial clearcutting varied as a function of treatment (none, repeated harvest, or rehabilitation) after the first harvest. After 60 years, the proportion of acceptable growing stock was highest in selection and rehabilitation treatments and lowest in fixed diameter-limit and commercial clearcut treatments. Findings confirm that long-term outcomes of silvicultural treatments with attention to residual stand condition surpass those managed for short-term financial gains with regard to residual stand structure, value, and quality.

Leaf area and structural changes after thinning in even-aged Picea rubens and Abies balsamea stands in Maine, USA.

We tested the hypothesis that changes in leaf area index (LAI m2 m−2) and mean stand diameter following thinning are due to thinning type and residual density. The ratios of pre- to postthinning diameter and LAI were used to assess structural changes between replicated crown, dominant, and low thinning treatments to 33% and 50% residual density in even-aged Picea rubens and Abies balsamea stands with and without a precommercial thinning history in Maine, USA. Diameter ratios varied predictably by thinning type: low thinnings were <0.7, crown thinnings were >0.7 but 1.0 . LAI change was affected by type and intensity of thinning. On average, 33% density reduction removed 50% of LAI. Overall reduction of LAI was generally greatest in dominant thinnings (54%), intermediate in crown thinnings (46%), and lowest in low thinnings (35%). Upon closer examination by crown classes, the postthinning distribution of LAI between upper and lower crown classes varied by thinning history, thinning method, and amount of density reduction.