North American Hardwood Research Articles

Increased tree water use with the development of a dense understory layer in a North American hardwood forest

AbstractThe formation of a single species, recalcitrant understory vegetation layer can limit tree regeneration and, in the long term, modify the composition of forests. Few studies have investigated how recalcitrant vegetation influences competition for water resources although the formation of a dense understory is likely to modify the forest water balance. In eastern North American hardwood forests, the development of a dense understory layer of American beech (Fagus grandifolia) has been observed in stands dominated by sugar maple (Acer saccharum), a phenomenon that shares many characteristics associated with recalcitrant vegetation. Given that water availability is generally negatively correlated with stand density, we hypothesized that the formation of a dense understory beech layer increased competition for water resources, thus leading to reduced water use by sugar maple trees in beech‐dominated stands. Using thermal dissipation sensors, we measured sap flux density (Fd) of two sugar maple trees at three beech‐dominated sites and three control sites. During the growing season, Fd of sugar maple trees was significantly larger at beech‐dominated sites compared to control sites, indicating a greater rate of water use by sugar maples in stands with a dense understory beech layer. We provide two hypotheses to explain our results at the tree scale: (i) reduced cover by forest floor vegetation could limit transpiration by this layer, thus allowing increased water availability to supply transpiration by overstory trees, or (ii) increased tree transpiration rate could be a mechanism to satisfy nutrient requirements in beech‐dominated stands often associated with lower soil fertility.

Impacts of spatial scale and resolution on species distribution models of American chestnut (Castanea dentata) in Pennsylvania, USA

American chestnut (Castanea dentata) was once a foundational tree species of eastern North American hardwood forests before its functional extinction due to an invasive chestnut blight first described at the beginning of the 20th century. Ongoing research to develop a blight-resistant American chestnut through traditional breeding or transgenic means is promising. However, restoration of American chestnut will require a thorough understanding of its spatial distribution and habitat suitability to both find surviving mature trees to enhance the genetic diversity of experimental blight-resistant trees and to identify optimal restoration sites. While many studies have modeled the spatial distribution of American chestnut trees, few have addressed how adjusting spatial scale and resolution impact the predictive ability of species distribution models (SDMs) for the tree species. In this study, we used Maxent to model American chestnut distribution at coarse resolution (238 m) across the state of Pennsylvania and fine resolution (3 m) within French Creek State Park in the southeastern portion of the state. Pennsylvania was chosen as our study area due to its central location in American chestnut’s pre-blight range. Results indicate that the spatial extent of the study area and ranges of environmental variables included in the models influence which variables are most predictive of chestnut distribution. This study suggests that environmental variables such as soil content, soil pH, and elevation are all strong predictors of American chestnut habitat, but their importance varies by resolution. This study also investigates how spatial scale and resolution influence the predictive ability of American chestnut SDMs and offers a workflow for locating restoration sites by first modeling species distribution at a coarse scale and broad extent to identify patches of suitable habitat, then focusing resources on fine scale modeling of species distribution local to those suitable patches.

Higher‐latitude spring‐flowering herbs advance their phenology more than trees with warming temperatures

Abstract The phenologies of co‐occurring trees and spring‐blooming understory herbs in northeastern North American hardwood forests appear to be regulated by different environmental drivers – air temperature and soil temperature/snowpack, respectively. Accordingly, it has been hypothesized that climate change–driven asymmetry in the advancement of canopy leaf‐out relative to the timing of understory growth could reduce photosynthetic rates and reproductive success of understory herbs through greater early‐season shading. To determine whether trees and spring‐flowering forest herbs are advancing their phenologies at different rates with respect to increasing global temperatures, we examined the phenological responses to warming of 10 species of trees and 11 species of spring‐flowering forest herbs (8045 observations from 965 sites) in northeastern North America using 13 years of data collected by citizen scientists under the auspices of the USA‐National Phenology Network. Contrary to expectation, the degree of advancement of leaf‐out as a function of temperature was greater in spring‐flowering forest herbs than in trees, with a mean response rate of −4.9 days/°C (95% BCI [−5.2, −4.6]) for spring‐flowering forest herbs vs. −3.3 days/°C (95% BCI [−3.5, −3.1]) for trees. However, the response to temperature was not consistent across the latitudinal range, with spring‐flowering forest herbs responding more strongly to warming than trees at middle (40–44°N) and higher (45–48°N) latitudes but not at lower latitudes (35–39°N). Synthesis. In contrast to previous suggestions, our study shows spring‐flowering forest herbs advancing their phenology at a higher rate than trees with respect to warming through most of the latitudinal range investigated, which could translate into a longer growing season and increased carbon uptake for spring‐flowering forest herbs as spring temperatures rise.

Genetic Structure of Natural Northern Range-Margin Mainland, Peninsular, and Island Populations of Northern Red Oak (Quercus rubra L.)

Plant populations at the leading edge of the species’ native range often exhibit genetic structure as a result of genetic drift and adaptation to harsh environmental conditions. Hence, they are likely to harbour rare genetic adaptations to local environmental conditions and therefore are of particular interest to understand climate adaptation. We examined genetic structure of nine northern marginal mainland, peninsular and isolated island natural populations of northern red oak (Quercus rubraL.), a valuable long-lived North American hardwood tree species, covering a wide climatic range, using 17 nuclear microsatellites. We found pronounced genetic differentiation of a disjunct isolated island population from all mainland and peninsular populations. Furthermore, we observed remarkably strong fine-scale spatial genetic structure (SGS) in all investigated populations. Such high SGS values are uncommon and were previously solely observed in extreme range-edge marginal oak populations in one other study. We found a significant correlation between major climate parameters and SGS formation in northern range-edge red oak populations, with more pronounced SGS in colder and drier regions. Most likely, the harsh environment in leading edge populations influences the density of reproducing trees within the populations and therefore leads to restricted overlapping of seed shadows when compared to more central populations. Accordingly, SGS was negatively correlated with effective population size and increased with latitude of the population locations. The significant positive association between genetic distances and precipitation differences between populations may be indicative of isolation by adaptation in the observed range-edge populations. However, this association was not confirmed by a multiple regression analysis including geographic distances and precipitation distances, simultaneously. Our study provides new insights in the genetic structure of long-lived tree species at their leading distribution edge.

Towards sustainable North American wood product value chains, part 2: computer vision identification of ring-porous hardwoods

Wood identification is vitally important for ensuring the legality of North American hardwood value chains. Computer vision wood identification (CVWID) systems can identify wood without necessitating costly and time-consuming off-site visual inspections by highly trained wood anatomists. Previous work by Ravindran and colleagues presented macroscopic CVWID models for identification of North American diffuse porous hardwoods from 22 wood anatomically informed classes using the open-source XyloTron platform. This manuscript expands on that work by training and evaluating complementary 17-class XyloTron CVWID models for the identification of North American ring porous hardwoods — woods that display spatial heterogeneity in earlywood and latewood pore size and distribution and other radial growth-rate-related features. Deep-learning models trained using 4045 images from 452 ring-porous wood specimens from four xylaria demonstrated 98% five-fold cross-validation accuracy. A field model trained on all the training data and subsequently tested on 198 specimens drawn from two additional xylaria achieved top-1 and top-2 predictions of 91.4% and 100%, respectively, and images devoid of earlywood, latewood, or broad rays did not greatly reduce the prediction accuracy. This study advocates for continued cooperation between wood anatomy and machine-learning experts for implementing and evaluating field-operational CVWID systems.

Stand and site characteristics affect the probability of stump sprouting in some eastern North American hardwoods

Stump sprouting is a widespread phenomenon in North American hardwood tree species following logging or severe crown damage from natural disturbances such as fire, wind, or insect attack. However, other than the effects of species and tree size, factors that influence the probability of sprouting are not well understood. Data from harvested stands in Pennsylvania and Connecticut, USA, were used to evaluate the effects of several variables on stump sprouting frequencies measured within the first year after cutting for several oak and other hardwood species. After adjusting for species and diameter of the “parent” tree (ortet), clearcut harvests were found to result in a higher frequency of sprouted stumps than shelterwood harvests or crop tree releases. In addition, the probability of sprouting was higher for trees in the Ridge and Valley province of Pennsylvania vs. the Appalachian Plateaus, and in general it was higher on dry vs. moist sites. After accounting for all measured variables, there remained statistically significant stand-to-stand differences in sprouting frequency that arose from unknown causes. Results should temper expectations of stump sprout contributions to regeneration stocking that are based on simple models derived from a small number of stands or a limited geographic area. On the stand-level basis at which most managers work, the actual contribution of sprouts to future stand development may vary widely from modeled expectations.

Towards Sustainable North American Wood Product Value Chains, Part I: Computer Vision Identification of Diffuse Porous Hardwoods.

Availability of and access to wood identification expertise or technology is a critical component for the design and implementation of practical, enforceable strategies for effective promotion, monitoring and incentivisation of sustainable practices and conservation efforts in the forest products value chain. To address this need in the context of the multi-billion-dollar North American wood products industry 22-class, image-based, deep learning models for the macroscopic identification of North American diffuse porous hardwoods were trained for deployment on the open-source, field-deployable XyloTron platform using transverse surface images of specimens from three different xylaria and evaluated on specimens from a fourth xylarium that did not contribute training data. Analysis of the model performance, in the context of the anatomy of the woods considered, demonstrates immediate readiness of the technology developed herein for field testing in a human-in-the-loop monitoring scenario. Also proposed are strategies for training, evaluating, and advancing the state-of-the-art for developing an expansive, continental scale model for all the North American hardwoods.

Influence of individual tree characteristics, spatial structure and logging history on tree-related microhabitat occurrence in North American hardwood forests

BackgroundTree-related microhabitats (hereafter, “TreMs”) are key components of forest biodiversity but they are still poorly known in North American hardwood forests. The spatial patterns of living trees bearing TreMs (hereafter, “TreM-trees”) also remain to be determined. As logging practices can lead to a loss of TreM-trees and of their associated biodiversity, it is essential to identify the factors explaining TreM occurrence to better integrate them into forest management. We therefore inventoried TreMs in 4 0.5-ha survey strips in northern hardwood forests in Quebec, Canada, while recording the spatial location of each tree. Two strips were located in unmanaged old-growth forests, and 2 were in forests managed under selection cutting. All 4 stands were dominated by sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia Ehrn.). Beech bark disease, an exotic pathology, was observed in all the strips.ResultsLarge diameter at breast height and low tree vigor were the main characteristics explaining the presence of TreMs at the tree scale. TreM-trees presented slight spatial aggregation patterns. These aggregates, however, were not well-defined and were generally constituted by a large number of trees bearing few different types of TreMs. Two TreM classes (broken branch or top and woodpecker lodge) also presented a spatial aggregation. Logging practices had no significant effect on TreM occurrence. Beech bark disease increased the frequency of senescent beeches. The impact of this pathology on TreMs was however mitigated by the small size of infected trees and probably by the short time elapsed since its appearance.ConclusionThe factors explaining the presence and abundance of TreMs on trees has so far been little studied in North American hardwood forests. Our results highlight that TreM-tree characteristics in the surveyed forests are consistent with those of previous studies conducted in other forest types and regions (e.g., Europe or Northwestern America). To our knowledge, this study is also the first to identify a spatial aggregation of TreM-trees and of specific TreM classes. It will be nevertheless necessary to determine whether the small impact of logging activities we observed results from current or past management practices.

Hybrid Breeding for Restoration of Threatened Forest Trees: Evidence for Incorporating Disease Tolerance in Juglans cinerea.

Hybridization is a potential tool for incorporating stress tolerance in plants, particularly to pests and diseases, in support of restoration and conservation efforts. Butternut (Juglans cinerea) is a species for which hybridization has only recently begun being explored. This North American hardwood tree is threatened due to Ophiognomonia clavigignenti-juglandacearum (Ocj), the causal fungus of butternut canker disease (BCD), first observed in 1967. Observational evidence in some wild J. cinerea populations indicates that naturalized hybrids of J. cinerea with Japanese walnut (Juglans ailantifolia) may be more tolerant to BCD than non-admixed J. cinerea, but this has not been formally tested in a controlled trial. We aimed to examine potential BCD tolerance within and between J. cinerea and J. cinerea × J. ailantifolia hybrids and to determine if there is a difference in canker growth between BCD fungal isolates. Five-year-old J. cinerea and hybrid trees were inoculated with two Ocj fungal isolates collected from natural infections found in two different sites in Indiana, United States, and a blank control (agar only). Measurements of both artificially induced and naturally occurring cankers were taken at 8-, 12-, 20-, 24-, and 32-month post-inoculation. Differences in canker presence/absence and size were observed by fungal isolate, which could help explain some of the differences in BCD severity seen between J. cinerea populations. Smaller and fewer cankers and greater genetic gains were seen in hybrid families, demonstrating that hybrids warrant further evaluation as a possible breeding tool for developing BCD-resistant J. cinerea trees.

Influence of invasive earthworms (Amynthas spp.) on Wisconsin forest soil microbial communities and soil chemistry

The invasion of Amynthas species complex of earthworms from Asia into North American hardwood forests poses a novel threat to these ecosystems. These species can competitively exclude previously-established earthworms, have novel effects on microbial communities, and influence the structural and chemical composition of soils differently than other earthworm species. However, because prior research has found that earthworm species is an important factor for microbial community composition, Amynthas species may also uniquely affect soil microbial community composition. To address this, we took earthworm and soil samples from 0.5-m quadrats along an active invasion front of Amynthas agrestis and A. tokioensis at the University of Wisconsin-Madison Arboretum. We tested all soil samples for basal metabolic rates, total carbon, and total nitrogen. We also extracted microbial DNA from cast samples, dissected worm guts, and soil samples, and we used high-throughput DNA sequencing to characterize Amynthas species-specific differences in bacterial and fungal community composition. We found that plots that had been occupied by Amynthas for longer than a year had higher total nitrogen, lower C/N, and different communities of soil bacteria and fungi when compared to newly-invaded plots. We also found that Amynthas had species-specific differences in bacterial communities adhered to worm guts and in worm casts. These results suggest that Amynthas species introduce novel assemblages of bacteria and fungi through their casts, and these differences are associated with the species-specific community composition of earthworm guts. These results also suggest that Amynthas species are likely affecting the soil microbial community during invasion.

Contribution to understanding the color development on wood surfaces treated with iron salts by a combination of analytical methods

Metal salts application on wood surface is a simple and nontoxic method that can be used for wood color modification, which is induced by the formation of colored complexes between wood extractives and metal ions. In this study, we have combined several analytical methods to understand the color development for four North American hardwood species: white oak, red oak, sugar maple and yellow birch. The phenolic composition of the ethanolic extracts of heartwood and sapwood of the four wood species was studied by UV-Vis spectrometry and liquid state phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR) and analyzed in relation to color development. The correlations between different OH types and the color development as measured with the CIEL*a*b* color coordinates, indicate that the descending order of correlation for phenolic hydroxyls was: di-substituted OH, mono-substituted OH and un-substituted OH while no correlation was found between COOH hydroxyls and total color change (ΔE).

North American Hardwoods Identification Using Machine-Learning

This technical note determines the feasibility of using an InceptionV4_ResNetV2 convolutional neural network (CNN) to correctly identify hardwood species from macroscopic images. The method is composed of a commodity smartphone fitted with a 14× macro lens for photography. The end-grains of ten different North American hardwood species were photographed to create a dataset of 1869 images. The stratified 5-fold cross-validation machine-learning method was used, in which the number of testing samples varied from 341 to 342. Data augmentation was performed on-the-fly for each training set by rotating, zooming, and flipping images. It was found that the CNN could correctly identify hardwood species based on macroscopic images of its end-grain with an adjusted accuracy of 92.60%. With the current growing of machine-learning field, this model can then be readily deployed in a mobile application for field wood identification.

Single-molecule sequencing yields the complete chloroplast genome sequence of Populus deltoides

Populus deltoides is a fast-growing, large tree and one of the largest North American hardwood trees. In this study, the complete chloroplast (cp) genome sequence of P. deltoides is characterized. The whole cp genome was assembled to 156,867 bp, including a large single copy (LSC) region of 85,534 bp, a small single copy (SSC) region of 16,513 bp and a pair of inverted repeats (IRs) region of 27,410 bp. The base content of the P. deltoides cp genome is A (32.0%), T (31.3%), C (18.0%), and G (18.7%), and AT bases occupy a large proportion of the cp genome. The neighbor-joining phylogenetic analysis with 20 cp genomes from the Salicaceae family showed that P. deltoides is sister to Populus davidiana. These will provide for the evolutionary and biological studies in Salicaceae family.

Changes in the quality of the northern U.S. hardwood timber resource from 2008 to 2017

High quality North American hardwood lumber and veneer is increasingly exported and used worldwide. A large portion of the timber resource utilized in the production of these products is contained within the northern region of the eastern United States. The cubic volume of this resource has increased by 15% since 2000, with most of this increase occurring in trees greater than 43.2 cm diameter at breast height. Although the volume of high-quality timber is likely at its highest level in over 100 years, 60% of the increase in the volume between 2008 and 2017 was in sawtimber-size (27.9 cm and larger) trees of low quality. Region wide, the species group “other white oaks” had the largest increase in high-quality volume. The largest increases in low-quality volume were for the soft maple and “other red oaks” species groups. While the volume of poletimber (12.7 to 27.7 cm) growing stock decreased between 2008 and 2017, the volume of cull poletimber-size trees increased more than 50%. These trends indicate a future decline in timber quality. Research is needed to determine the cause of these declines and how they may be reversed. One question that should be examined is the role of natural mortality and damage versus human disturbance on timber quality.

Effect of Prescribed Fire on Timber Volume and Grade in the Hoosier National Forest

AbstractLow-intensity surface fire is prescribed in eastern North American hardwood stands prior to overstory harvest in order to improve regeneration and recruitment of oak and other fire-tolerant and fire-adapted species. However, this use of prescribed fire potentially can reduce timber value. We inventoried overstory trees (>10 in. diameter at breast height) in 54 oak-dominated stands with varied prescribed fire histories and aspects in southern Indiana. We then documented the extent of prescribed fire damage (i.e., wounds) to overstory trees and quantified both the relative stand volume of timber loss and the proportion of trees that had tree grade reductions because of prescribed fire. Generally, as a stand received more prescribed fires, more trees were scarred, the relative volume lost increased, and a higher proportion of trees declined in grade. Overall, burned stands experienced less than 10 percent sawtimber volume loss, regardless of the number of prescribed fires and aspect. Less than 3 percent of trees, study-wide, had reduced grade because of prescribed fire. Grade and volume reductions varied by species, however. Our results suggest that prescribed fire has a minor economic impact on standing timber, particularly when timber is harvested within two decades of the first fire.

Challenges and Opportunities for North American Hardwood Manufacturers to Adopt Customization Strategies in an Era of Increased Competition

Much of the North American wood products industry was severely impacted by the recession of 2008–2009. In addition, many sectors within this industry face intense global competition. Against this backdrop, we examine economic opportunities for hardwood manufacturers to achieve greater competitive advantage via product customization, through a literature review and synthesis. We also discuss several related themes including agility, lean manufacturing, and clustering. We found that, in globally competitive environments, hardwood producers must be agile to adapt to economic conditions and dynamic customer demand. We discuss how some sectors of the hardwood industry have effectively exhibited customized production, and subsequently fared relatively well in the current economy. We conclude the synthesis by evaluating the importance of supply chains to achieving customization for hardwood producers. In the future, supply chains will need to be configured to rapidly respond to changing consumer demands, and pressure to provide more services will likely extend further back up the supply chain to hardwood sawmills. It is expected that sustainability practices, including green supply chain management, will impact operational and economic performance of hardwood firms as well.

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.

Generalized steady-state model for chlorine dioxide brightening of hardwood kraft pulps

A generalized, steady-state model for the first chlorine dioxide brightening (D1) stage is proposed for North American hardwood kraft pulps. The model predicts the post-D1 brightness obtained for a given chlorine dioxide (ClO2) charge based on three equation parameters. Data from laboratory D1 studies were examined with the model to determine the relationships of the parameters to extracted pulp properties. The asymptotic D1 brightness limit at extremely high bleach charges was ~93% ISO for most hardwood pulps. This parameter can be calculated from the limiting light absorption coefficient of D1 kinetic studies (≈ 0.10 m2/kg) and the unbeaten light scattering coefficient of the pulp. A second parameter was linearly correlated to the extracted kappa number of the hardwood pulp. This relationship seems to be affected by the chosen kraft pulping process (e.g., liquor or dissolved solids profiling). The final parameter is the extracted pulp brightness. The hardwood D1 model forecasted the brightness values of laboratory pulps that differ from measured values of ±0.5 to ±0.6 points. Daily averages of bleach plant data for a southeastern U.S. hardwood fiber line were examined with the model. Predictions were generally 0.5 points higher than the measured mill brightness values. This observation implied that the D0(EOP) pulp contained a small amount of extraction washer carryover (~0.4 kappa units on pulp) that was consuming ~1.2 kg ClO2/ton pulp, which was not accounted for in the hardwood D1 model.

Construction of a Framework Genetic Linkage Map in Gleditsia triacanthos L.

Genomic resources are sparse in most ecologically and economically important North American hardwood species. As part of the Hardwood Genomics project (http://www.hardwoodgenomics.org/), we evaluated the utility of restriction site associated DNA sequencing (RAD-Seq) for framework genetic linkage map construction in honeylocust (Gleditsia triacanthos L.), a leguminous tree common in eastern North America. Starting with a large open-pollinated family of progeny from a single tree, a mapping pedigree of 92 putative full-sibs was identified by kin group assignment and paternity analyses with microsatellite markers. RAD-Seq using Illumina next-generation DNA sequencing (NGS) generated over 117 M reads among the 92 plants. De novo reference genome clustering and alignment of samples to the reference genome revealed 5849 candidate single nucleotide polymorphisms (SNPs), of which 1570 were retained after quality filtering. Of the 1570 SNPs, 236 were in pseudo-testcross mapping configuration in the maternal parent and segregated approximately in the expected 1:1 ratio. The final map generated has a total length of 815.57 cM and consists of 178 markers on 14 linkage groups, corresponding to the haploid chromosome number in honey locust. Synteny and collinearity between honey locust and model legumes Glycine max, Medicago truncatula, and Phaseolus vulgaris were found for six of the honey locust linkage groups. RAD-Seq proved to be useful for framework linkage map construction in honey locust, a species for which no genomic resources had previously been available. However, greater sequence coverage and larger full-sib mapping pedigrees are necessary for the development of high-density linkage maps with future applications in quantitative trait locus (QTL) mapping.

Estimation of the S/G ratios of the lignins in three widely used North American hardwoods

Sugar maple (Acer saccharum), aspen (Populus tremuloides), and white birch (Betula papyrifera) are three hardwoods that are widely used by the North American pulp and paper industry. Because of their abundance, these species are also likely to be used by some of the biorefinery processes that are being developed. A significant amount of evidence indicates that the syringyl to guaiacyl (S/G) ratio of the lignin in a hardwood is a governing parameter regarding its ease of delignification. Credible data also show that among poplars the S/G ratio of the lignin significantly influences the ease of saccharification of the carbohydrate polymers to sugar monomers. Although the S/G ratio appears to be a key parameter for hardwoods, values accepted by most practitioners are not available for the three species. In this investigation, those ratios were estimated by an extensive literature review followed by S/G determination by nitrobenzene oxidation (NBO) and methoxyl analyses of organosolv lignin (OSL) from the ethanol/ water/sulfuric acid pulping process. The S/G values were approximately 1.4 for sugar maple, and 2.0 for aspen and white birch. Data are also included showing that sugar maple and white birch were equally reactive in kraft pulping. Thus, it is unclear whether or not the S/G ratio is indeed a governing parameter in this delignification process.