Uneven-aged Northern Hardwood Research Articles

Estimation of Northern Hardwood Forest Inventory Attributes Using UAV Laser Scanning (ULS): Transferability of Laser Scanning Methods and Comparison of Automated Approaches at the Tree- and Stand-Level

UAV laser scanning (ULS) has the potential to support forest operations since it provides high-density data with flexible operational conditions. This study examined the use of ULS systems to estimate several tree attributes from an uneven-aged northern hardwood stand. We investigated: (1) the transferability of raster-based and bottom-up point cloud-based individual tree detection (ITD) algorithms to ULS data; and (2) automated approaches to the retrieval of tree-level (i.e., height, crown diameter (CD), DBH) and stand-level (i.e., tree count, basal area (BA), DBH-distribution) forest inventory attributes. These objectives were studied under leaf-on and leaf-off canopy conditions. Results achieved from ULS data were cross-compared with ALS and TLS to better understand the potential and challenges faced by different laser scanning systems and methodological approaches in hardwood forest environments. The best results that characterized individual trees from ULS data were achieved under leaf-off conditions using a point cloud-based bottom-up ITD. The latter outperformed the raster-based ITD, improving the accuracy of tree detection (from 50% to 71%), crown delineation (from R2 = 0.29 to R2 = 0.61), and prediction of tree DBH (from R2 = 0.36 to R2 = 0.67), when compared with values that were estimated from reference TLS data. Major improvements were observed for the detection of trees in the lower canopy layer (from 9% with raster-based ITD to 51% with point cloud-based ITD) and in the intermediate canopy layer (from 24% with raster-based ITD to 59% with point cloud-based ITD). Under leaf-on conditions, LiDAR data from aerial systems include substantial signal occlusion incurred by the upper canopy. Under these conditions, the raster-based ITD was unable to detect low-level canopy trees (from 5% to 15% of trees detected from lower and intermediate canopy layers, respectively), resulting in a tree detection rate of about 40% for both ULS and ALS data. The cylinder-fitting method used to estimate tree DBH under leaf-off conditions did not meet inventory standards when compared to TLS DBH, resulting in RMSE = 7.4 cm, Bias = 3.1 cm, and R2 = 0.75. Yet, it yielded more accurate estimates of the BA (+3.5%) and DBH-distribution of the stand than did allometric models −12.9%), when compared with in situ field measurements. Results suggest that the use of bottom-up ITD on high-density ULS data from leaf-off hardwood forest leads to promising results when estimating trees and stand attributes, which opens up new possibilities for supporting forest inventories and operations.

Changes in growth dominance after partial cuts in even- and uneven-aged northern hardwood stands

The success of partial cut applications to northern hardwood stands is based upon the contribution of harvested and residual trees to overall stand growth, which can be captured by a stand-level criterion, i.e, the growth dominance coefficient (GDC). The main objective of this study was to test the relevance of GDC in evaluating the success of partial cuts in even-aged and uneven-aged stands that were composed of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britt.). We first determined whether GDC that was calculated with two indicators of light capture, stem mass and tree leaf area, produced similar results after partial cutting in both stand structures. Second, using linear mixed models, we determined the effect of selection cutting on GDC in uneven-aged stands over the short- and medium-term, and evaluated effects of tree vigour and size before cutting on diameter growth response of trees after cutting. GDC based upon stem mass was similar to GDC based upon tree leaf area in even-aged stands (p = 0.7200), but was lower in uneven-aged stands (p < 0.001). Selection cutting in uneven-aged stands did not change GDC in the short- (p = 0.7721) and medium- (p = 0.8363) term. Yet, several trees that grew rapidly before cutting responded negatively to partial cutting, while many trees that grew slowly before cutting responded positively. Overall, assessing the success of partial cuts appears to be difficult using GDC, a stand-level criterion. Such assessments, as well as a better understanding of the effects of partial cuts, could be obtained through more detailed growth analyses that are performed at the tree level.

Dynamics of the diameter distribution after selection cutting in uneven- and even-aged northern hardwood stands: a long-term evaluation

Selection cutting is defined as a tool for uneven-aged silviculture. Dependence on diameter distribution by forestry practitioners for identifying stand conditions has led to misuse of selection-like cuttings in even-aged northern hardwood stands. Our study used several long-term data sets to investigate the temporal stability in numbers of trees per diameter class in uneven-aged northern hardwood stands treated with single-tree selection and in 45-year-old second-growth stands treated with selection-like cuttings. We analyzed data from New York, Michigan, and Wisconsin to determine changes through time in number of trees across 2.5 cm diameter classes, shifts in the shape and scale of the three-parameter Weibull function used to describe the diameter distributions, and dynamics of associated stand attributes. Findings showed that single-tree selection cutting created and sustained stable diameter distributions and uniformity of conditions through consecutive entries in uneven-aged stands. By contrast, these characteristics varied through time in the second-growth stands that had been treated with selection-like cuttings. Analysis also showed that the Weibull shape and scale parameters for stands under selection system migrated towards those of the recommended target diameter distribution in the uneven-aged stands. These parameters diverged from the target with repeated use of selection-like cuttings in the second-growth even-aged stands.

The Composition and Height of Saplings Capturing Silvicultural Gaps at Two Long-Term Experiments in Managed Northern Hardwood Forests

Managing forests for mixtures of canopy species promotes future resilience and mitigates risks of catastrophic resource loss. This study describes the compositions, heights, and locations within openings of gap-capturing saplings in two long-term group-selection experiments in managed northern hardwoods. We expected opening size to affect the composition of gap-capturing saplings and that composition would match advance regeneration where relatively large stems remained following harvest. We also expected sapling height to respond positively to opening size, but plateau in gap areas above 200 m2, and legacy-tree retention to negatively affect sapling height. In two group-selection experiments, we found that the composition of gap-capturing saplings was not affected by opening size at 15 and 23 years post-harvest, respectively, and that composition matched advance regeneration only when larger stems (&gt;2.5 cm breast height, dbh) were removed during harvest. Gap-capturing sapling composition did not match the surrounding canopy in either study site. Sapling height was positively correlated with gap area, but, as we expected, plateaued in larger openings. In openings without legacy-retention, gap area did not significantly predict sapling height in openings larger than 100–200 m2, whereas this threshold was between 300–400 m2 in openings with single legacy-tree retention. Sapling height was negatively associated with distance into openings when legacy-trees were present. Group selection appears to recruit modestly higher proportions of shade-midtolerant and intolerant species to the canopy compared to adjacent unmanaged second-growth or managed, uneven-aged northern hardwoods.

Assessing small-stem density in northern hardwood selection system stands

Data from three uneven-aged northern hardwood stands in New York State were analyzed to assess the effects of understory American beech (Fagus grandifolia Ehrh.) density on changes of small stems within the 2.54 to 5.08 cm diameter class during the first cutting cycle under single-tree selection system. Findings show that the amount of understory American beech on a regeneration plot, quantified using a species index value (SIV), affects the abundance of other species. Results reveal the future plot-level stocking of these small trees for (i) all species (including American beech) as related to time since cutting and residual basal area or (ii) non-beech species as influenced by beech interference (SIV), time since cutting, and residual basal area. Findings indicate that for plots with limited understory beech, small-stem stem density will increase from postcut levels to a peak at 8–12 years after selection system cutting and then decrease. The higher the residual basal area is, the sooner the numbers of small trees reach a peak level and the fewer are present of that threshold size. Findings confirm that no or only minimal numbers of small non-beech trees develop on plots with high levels of understory American beech (SIV ≥ 0.5).

Beech shearing/high-stumping to control understory interference

The research evaluated opportunities for cutting off small understory beech root suckers at 0.6-0.9 m above ground. By six years (pilot test) after cutting with lopping shears in a closed-canopy uneven-aged northern hardwood stand, 91% of high stumps lacking a residual live branch had died. Of those cut above the lowest living branch, all survived. In a later operational trial by contract crews using chainsaws two growing seasons before a selection system cutting, 75% of high stumps without a residual live branch had died by the fifth year. Of stumps cut below the lowest living branch and that initially sprouted, only 7% had a live sprout by the sixth year. Among high stumps cut above the lowest branch, 84% survived.

Long-Term Susceptibility of Even- and Uneven-Aged Northern Hardwood Stands to Partial Windthrow

While uneven-aged silviculture may appear preferable to even-aged silviculture in terms of stand susceptibility to windthrow (major wind damage), the scientific evidence is equivocal on this issue, because the two systems do not operate over the same time frame. The goal of this study was to evaluate the windthrow susceptibility of even- and uneven-aged stands over a 100-year period. Susceptibility to windthrow of North American hardwood stands was evaluated by coupling a stand growth model (Forest Vegetation Simulator, or FVS) to stem windthrow probability equations from the literature. This coupling was straightforward given that FVS provides the diameter at breast height (DBH) of each tree within a stand over the simulation period. Windthrow susceptibility equations also use DBH to calculate stem windthrow probability. Our results show that average loss due to windthrow under uneven-aged management can be twice that observed under even-aged management at moderate wind severity for sugar maple-dominated stands. This result should be interpreted with caution because of the impossibility in our simulations of considering differences in tree form development between the two approaches. Nevertheless, this study clearly shows that even-/uneven-aged silviculture comparisons should be made on a long-term basis since uneven-aged stands are continuously susceptible to windthrow, while even-aged stands tend to be little affected by windthrow in their early developmental stages.

A new silvicultural approach to the management of uneven-aged Northern hardwoods: frequent low-intensity harvesting

We report a new silvicultural approach that is well suited for the management of uneven-aged forests in which timber production is an important objective. The approach recognizes two main components in the stand, i.e. a fiber production component, which provides veneer/sawlog quality products from the high-quality trees (HQT), and an ecological component, which contributes to the overall ecosystem functioning through the lower value stems. The objective of the study was to verify if it is possible to sustainably harvest only HQT in northern hardwood (NH) and therebyproduce aviable alternative to high-grading the stands. To do so, a simple stand growthsimulator, based on empirical growth rates of HQT in Sugar Maple/Yellow Birch stands in southwestern Quebec, was combined with an optimization tool. The optimization parameters aimed to identify possible tree marking regimes (TMRs) under 10-year rotation partial cutting, which would ensure that the basal area of HQT was maintained for 40 years. Results suggest that sustainability is achievable starting from very different initial stand structures and the application of a wide range of alternative TMRs. We argue that this new approach is one way to apply emerging concepts in forest management, such as ecological integrity, attempts to emulate natural disturbance regimes and provides new possibilities managing for resilience and for adaptation to climate change.

The impact of beech thickets on biodiversity

Beech bark disease has dramatically altered hardwood forest structure and composition across northeastern North America. Extensive overstory mortality has resulted in prolific root-sprouting in some stands leading to the development of understory thickets of clonal small-stemmed beech. Beech thickets may impact local forest biodiversity, but this has not been adequately evaluated. We hypothesized significant differences in diversity of groundcover flora, craneflies, amphibians, and small mammals between plots with and without beech thickets. Paired plots were established in uneven-aged northern hardwood forest stands with no recent management history at two sites in the Adirondack Mountains of New York State. Groundcover plants, terrestrial craneflies, amphibians and small mammals were sampled on twenty paired plots. Discriminant analysis showed a significant difference between thicket and non-thicket (control) areas; significant variables in plot type separation were beech sapling abundance, leaf litter depth, and coarse woody debris volume. Groundcover plant cover, richness, and diversity were significantly lower in thicket compared to non-thicket plots, while beech sapling density explained 17–38 % in groundcover plant species diversity. There were no significant differences between the diversity of cranefly, amphibian and small mammal communities of each plot type. Beech thickets are important determinants of local biodiversity.

Predicting Sugar Maple Mortality in Managed Uneven-Aged Northern Hardwood Stands

Predicting Sugar Maple Mortality in Managed Uneven-Aged Northern Hardwood Stands

Sampling design and precision of basal area growth and stand structure in uneven-aged northern hardwoods

Sampling design (distribution of plots, plot sizes, and number of sample plots) is an important consideration in planning a field study since it determines the bias and precision of information obtained. In this study, we evaluated the effects of two commonly used sampling designs on the precision of estimates of means related to stand basal area, considered as a whole or by diameter class, and 10-year growth components, in six 2-ha areas located in uneven-aged northern hardwood stands of Quebec managed under selection cutting. The study compares random and systematic sampling designs of one to 10 plots, with plots varying in size from 0.05 ha to 1.25 ha. Increasing the total area sampled (i.e., sampling area) in a stand from 0.05 ha to about 0.25 ha resulted in large precision gains; further increases in sampling area had more limited effects on precision. A sampling area of at least 0.5 ha would be required to obtain a minimally acceptable precision in estimating means of total basal area and 10-year growth components. A much larger sampling area would be required to obtain sufficient precision in estimating mean basal area by diameter class, often required to study stand structure. Precision can also be increased by dividing the sampling area into several smaller plots rather than using a single large plot. We found no clear difference in the results between random and systematic selection approaches.

Comparing selection system and diameter-limit cutting in uneven-aged northern hardwoods using computer simulation

Comparisons of selection system and diameter-limit cutting based on trials in specific settings have often yielded conflicting results. We used a simulation approach to evaluate sawtimber production over three cutting cycles on 10 sugar maple ( Acer saccharum Marsh.) dominated plots of varying initial forest structure. Treatments on each plot included light, moderate, and heavy intensities of selection system silviculture and diameter-limit cutting. Harvested sawtimber volumes were initially higher on all plots using diameter-limit cutting, but selection system outperformed diameter-limit cutting at later entries on 7 of the 10 plots. Volume differences between cutting types ranged among plots from 0.3 to 26 m3·ha–1, equating to a less than 1% to as much as a twofold difference. Average volumes from selection system at later entries were 20%–40% greater than diameter-limit cutting, due in part to consistent production in large sawtimber (≥46 cm). Yields from real stands could vary from these simulations where mortality losses (not modeled here) differ between treatments as a result of competition or logging damage. Findings suggest that cumulative sawtimber volumes from repeated selection system silviculture could eventually surpass that of diameter-limit cutting, but at a rate depending on initial stand conditions and harvesting intensity.

Minimum Tree Size and Interpretation of Stand Structure in Uneven-Aged Northern Hardwoods

Abstract Uneven-aged management of northern hardwoods frequently relies on diameter distributions in evaluating and controlling stand structure. However, the minimum tree diameter used to analyze diameter distributions varies among studies and may affect the interpretation of distribution shapes and, subsequently, stand structure. It has been suggested that the use of larger minimum diameters in the construction of these distributions would cause rotated sigmoid stand structures to exhibit a negative exponential distribution shape. To test this hypothesis, we constructed diameter distributions using minimum diameters of 3.0 and 13.0 cm for seven northern hardwood stands. Differences in distribution shape were observed only in unmanaged stands, in which three distribution shapes classified as rotated sigmoid using a 3.0-cm minimum diameter were classified as increasing-q when the 13.0-cm minimum diameter was used.

Modeling Tree Mortality in Low- to Medium-Density Uneven-Aged Hardwood Stands Under a Selection System Using Generalized Estimating Equations

Abstract Tree mortality was modeled through time in uneven-aged northern hardwood stands managed under a selection system using long-term remeasurement data. Two models were fitted and compared: a traditional logistic regression model that predicted the probability of individual tree mortality over discrete time periods and a logistic regression model estimated using generalized estimating equations (GEEs) to account for autocorrelation in the longitudinal data. Model evaluation was based on the mean prediction error, mean absolute prediction error, variance of prediction errors, and mean square error calculated using an independent validation data set. The GEE model produced smaller evaluation statistics, especially for the smaller diameter classes. The predicted probability of mortality from the two models was compared with the observed mortality across 5-cm diameter classes for two time periods. Our results indicate that the GEE model was better able to capture the change in the probability of mortality over time, especially for the smaller diameter classes, than the more traditional logistic model.

Relative effects of dendrometers on the estimation of diameter at breast height, stand basal area and stand volume in uneven-aged northern hardwoods

Four methods of estimating diameter at breast height (DBH) were used on 558 northern hardwood stems, located in 6, 0.25-ha plots, to compare their effects. The first method was a diameter tape reading and the second was a systematic reading, along a predefined axis, taken with a calliper. The last 2 methods were both obtained from the arithmetic mean of 2 different pairs of calliper readings taken at right angles. The first pair was directed along the major bole axis and the other pair along the minor axis. The 4 methods were compared in terms of mean tree diameter, plot basal area and plot volume. There were significant differences (p &lt; 0.05) in parameter estimates among methods, and the differences varied according to species (Acer saccharum Marsh. and Betula alleghaniensis Britt.) and tree size class. Differences among methods were more evident for larger DBH classes, which contribute most to the significant differences observed for both the plot and stand basal area and volume. Overall, tree DBH, plot basal area and plot volume provided by these 4 methods ranked in this ascending order: mean calliper reading directed along the minor axis, systematic calliper reading, mean calliper reading directed along the major axis, and diameter tape. The difference in stand basal area or volume between the estimates obtained with a systematic calliper reading and a diameter tape was 1.1 m2/ha with a mean basal area of about 21 m2/ha, or 10.5 m3/ha with a mean stand volume of 169 m3/ha. Because of the magnitude of these differences, it is recommended the method of DBH estimation be specified in studies, to be careful when crossing databases or models using different methods of DBH estimation, and to always maintain the same method of DBH estimation when remeasuring a plot. Key words: northern hardwoods, dendrometer, diameter tape, calliper, caliper, DBH measurement, basal area, volume

Evaluation of a tree classification system in relation to mortality risk in Québec northern hardwoods

A tree classification system was developed in the 1980s as part of a guide for tree-marking in the rehabilitation of unevenaged northern hardwood stands in Québec. It differentiates trees that are at high and low risk of mortality, trees with sawlog potential and cull trees. The risk class was assessed based on the presence of major crown and bole defects. The main objective of the present study was to evaluate this system with respect to its capacity to predict the probability of tree mortality. The variables used to classify the trees were observed in 88 experimental plots (0.5 ha) established between 1983 and 1999. Tree-level mortality probabilities were modelled for sugar maple (Acer saccharum Marsh.), American beech (Fagus grandifolia Ehrh.) and yellow birch (Betula alleghaniensis Britt.) to test the significance of the classification variables. The presence of decay, fungus or canker, wounds, uprooting, the death of at least 30% of the crown or of the roots, and the product class had significant (p &lt; 0.05) effects on mortality probabilities for at least one of the 3 species studied. In the main, the results supported the tree classification system. However, this system could be modified to differentiate not only trees with a high or low mortality risk, but also to identify some very high-risk trees. Key words: northern hardwood, mortality, defect, quality, classification, selection cutting, partial cut, sugar maple, American beech, yellow birch, uneven-aged, tree-marking

Spatial Scale and Stand Structure in Northern Hardwood Forests: Implications for Quantifying Diameter Distributions

Abstract Diameter distributions are frequently used to describe northern hardwood stand structure and serve as a structural guide for management. Several diameter distribution shapes have been found in managed and unmanaged uneven-aged northern hardwood stands, and it is unclear which patterns are due primarily to natural processes that could be self-perpetuating versus patterns that may be transient or aberrant. Variability in research methods has clouded consistent interpretation of resulting distributions. In this study, we evaluated northern hardwood stands in Michigan to determine how management and sample area affect interpretation of diameter distribution shape. Managed stands had a higher degree of variability in distribution shape and a tendency toward the increasing-q pattern. Unmanaged uneven-aged stands (i.e., old-growth remnants) were predominantly rotated sigmoid in shape. Patterns for each type of management became more consistent as sample area increased. Our data suggest a minimum sample area of 13% of total stand area may be needed to accurately capture diameter distribution shape; in this study that was 0.4 ha of a 3.2-ha sample plot. Additionally, greater heterogeneity in the structure of unmanaged stands resulted in the need for larger sample areas compared with that for managed stands.

Individual-tree diameter growth model for sugar maple trees in uneven-aged northern hardwood stands under selection system

An individual-tree diameter model was developed for sugar maple ( Acer saccharum Marsh.) in northern hardwood stands managed under selection system. We fitted long-term remeasurement data to a linear mixed model to account for the temporal autocorrelation of the remeasurements. The model was evaluated using independent data from two physiographic regions and representing a range of tree diameter classes, residual basal areas and years since cut. We compared our model to several individual-tree models based on data from stands with varied management histories. Several competition indices were also tested for an improvement in model fitting and prediction. Our model had lower bias and prediction error when compared to two previous models, as it better accounted for the increased diameter growth that occurred in trees from appropriately managed stands. The addition of a tree-specific competition index failed to improve model fit and predictive ability over stand-level basal area.

Rotated sigmoid structures in managed uneven-aged northern hardwood stands: a look at the Burr Type III distribution

Stand structures from a combined density manipulation and even- to uneven-aged conversion experiment on the Bartlett Experimental Forest (New Hampshire, USA) were examined 25 years after initial treatment for rotated sigmoidal diameter distributions. A comparison was made on these stands between two probability density functions for fitting these residual structures: the Burr Type III and a finite mixture of Weibulls. All stands exhibited some form of uneven-aged structure (either reverse J or rotated sigmoid) after 25 years. The Burr distribution fits the final distributions as well as the more complicated finite mixture model in all cases.

Ground-layer response to group selection with legacy-tree retention in a managed northern hardwood forest

We examined the effects of group selection with legacy-tree retention on ground-layer or understory diversity and composition in an uneven-aged northern hardwood forest in the Upper Peninsula of Michigan. We sampled 20 reference plots in the surrounding forest matrix and 49 openings with radii of 0.5 (n = 16), 0.75 (n = 17), and 1.0 (n = 16) times mean canopy tree height (22 m). Resultant opening areas were 321 ± 16 (mean ± SEs), 697 ± 21, and 1256 ± 39 m2, respectively. Each opening contained a centrally located legacy tree. Two years after harvesting, ground-layer diversity was significantly higher in openings than on reference plots (p &lt; 0.05) because of an influx of early seral, wetland, and weedy exotic species. The importance of aggressive ruderals (i.e., Carex ormostachya Wieg. and Rubus idaeus subsp. strigosus (Michx.) Focke) increased significantly (p &lt; 0.001) with increasing opening area. Although the importance and cover of several late-seral species were lower in openings compared with the forest matrix, few species found in the matrix were wholly absent from the openings. These results suggest that ground-layer plant communities in managed northern hardwood forests may display a high degree of resilience to intermediate-intensity disturbances.