Bark Content of Two Shrub Willow Cultivars Grown at Two
Sites and Relationships with Centroid Bark Content and Stem
Spencer Eich1 & Timothy A. Volk1 & Mark H. Eisenbies1 # Springer Science+Business Media New York 2015
Abstract Understanding the characteristics of short rotation woody crops (SRWC), like shrub willow, that affect feedstock quality and its variability is a priority as this source of biomass is expanded. Because of its relationship to ash content, the allotment of bark and wood within the stem is often highlighted as a factor impacting feedstock quality. Bark-to-wood ratios are frequently determined in willow by measuring this ratio for the centroid and assuming it represents the entire stem. The objectives of this study were to determine how site and genetic factors influenced the allometry of whole stem bark (WSB%) content on a dry weight basis and if the centroid bark percentage (CB%) on a dry weight basis adequately represents WSB%. A multiple linear regression approach was used to model WSB% and centroid bark content (CB%) using diameter, cultivar, and site factors. Five candidate models were evaluated ranging from parsimonious to complex. The simplest models estimated WSB% from CB% (R2=0.76) and stem diameter (R2=0.44). The most complex model included all factors and had an R2 of 0.90. Two key relationships demonstrated by several models are that (1) CB% adequately predicts but underestimates WSB%, particularly for larger stems, and (2) WSB% increases as diameter decreases. However, empirical models can be enhanced with the inclusion of diameter, site, and cultivar information. The overall approaches can be useful for making relative comparisons between cultivars and sites. In a 3-year-old stand of willow stems less than 20 mm had high WSB%, the maximum being 23.8 %, but accounted for only 5 to 15 % of the total biomass. Large diameter stems accounted for the majority of the total and bark biomass, but the WSB% in large diameter stems is as low as 11.5 %. WSB% ranged from 12.9 to 14 % across the two cultivars at the stand level.
Keywords Short rotationwoody crops . ShrubWillow . Bark content . Feedstock quality . Bark allometry
Woody biomass is considered a renewable source of energy that can be used to generate electricity, heating, liquid transportation fuels, and bioproducts with a reduction of net greenhouse gas emissions in comparison to fossil fuels [1, 2].
Woody energy crops, like shrub willow, are one source of biomass that has been identified as having potential to produce large amounts of biomass in the future. Willow shrubs are a short rotation woody crop (SRWC) that can be grown on marginal agricultural land in the northeast and midwest regions of the USA  and across regions in Europe [4, 5].
Shrub willow has been studied and is being deployed as an energy crop because of its rapid growth rates, capability to regenerate after being harvested, broad and largely untapped genetic base, and relatively consistent feedstock quality when harvested . Willow biomass crops have also been observed to improve soil structure and prevent erosion [7, 8].
The bark-to-wood ratio of shrub willow is an important factor related to a number of feedstock quality characteristics such as ash content, lignin content, and sugar content [9–11].
The coppice growth form of shrub willow, where multiple stems are produced on each plant, has raised concerns about the amount of bark in this biomass source and how it might impact feedstock quality. Previous studies have indicated that the average number of stems per plant in 3-year-old shrub * Mark H. Eisenbies email@example.com 1 College of Environmental Science and Forestry, State University of
New York, 1 Forestry Dr., Syracuse, NY 13210, USA
DOI 10.1007/s12155-015-9617-y willow ranges from 4.6 to 12.5 stems . Bark and wood possess different chemical compositions. For example, in willow biomass feedstocks, the average ash content of stem bark is 4.8–6.0 % by weight and 0.52–0.89 % in stem wood. Typical extractive content in willow bark is 31–39 % compared to approximately 10 % in the wood with some variation in the values among different cultivars . Lignin content also differs within the bark and wood of willow shrubs with values of 27.4 % lignin in bark and 20 % lignin in wood [13, 14].
In general, the variability of ash and sugar content in woody materials is of larger practical significance to woody crop production over magnitude because conversion process efficiency is more closely tied to feedstock consistency when engineering conversion systems for large-scale facilities [15, 16]. Specific issues associated with ash include more frequent maintenance cycles, impair catalysts and contribute to slag formation, and decrease yields; ultimately, these issues can lead to lower revenues for growers or even rejection of their harvested material by end users.
Given that bark and wood possess different properties, understanding the factors that impact bark-to-wood ratios (e.g., diameter, cultivar, age, and site), as well as being able to predict the amount of bark in biomass, is an important objective for growers and end users of biomass. Expression of traits such as number of stems per plant or the distribution of stem diameters contributes to the amount of bark found in different shrub willow cultivars [10, 12]. However, the ash and bark content in specific cultivars is also seen to vary depending on the site conditions where the willow shrubs are grown [17, 18]. Environmental conditions can also influence how a plant allocates its resources in the formation of bark . Due to the mix of genetic and environmental factors that can influence the amount of bark in willow biomass crops and the impact that bark can have in conversion processes, it would be valuable to have a better understanding of how amounts of bark vary among shrub willow cultivars and across sites.
The process of measuring bark-to-wood ratios for willow stems involves physical removal of the bark from the wood, which is time consuming and labor intensive. Therefore, alternative sampling methods are often used to estimate the whole stem bark (WSB%) content on a dry weight basis. A common approach to estimate WSB% of individual stems is to measure the bark content of their centroid mass on a dry weight basis (CB%); the centroid being a 10–15-cm section of the stem across the balance point for the mass of the stem [9, 10, 20–22]. However, it is less clear from the literature how accurately the CB% represents the WSB% and whether this relationship varies across different cultivars grown at different sites. Given that CB% is often used to represent WSB%, its adequacy has not been specifically addressed in the literature, and others have found that the centroid is not a representative of moisture-dependent stem characteristics, ash content, and elemental contents [20, 23]. The primary objective of this study was to evaluate how bark-to-wood ratios are influenced by site conditions and genetic factors. A second objective was to determine if the bark-to-wood ratio of willow stem centroids and diameter were representatives of the bark-to-wood ratio of the entire stem.