Theo Verwijst

Growing fuel: a sustainability assessment of willow biomass crops

2to the atmosphere. The implementation of good management practices will maintain productivity over multiple rotations. Rural development and environmental benefits associated with deployment and use will accrue to the local community because of the willow system’s short supply chain. The economic valuation of these benefits are necessary for the deployment of woody crops, which in turn can help society become more sustainable.

Biomass estimation procedures in short rotation forestry

Biomass estimation procedures in short rotation forestry vary in their methodology, complexity and time demand depending on the specific aim of the estimation operation. In commercial short rotation forestry, estimates of harvestable biomass often are used to decide on harvest timing or continued management and to calculate economic gains under the actual market situation. For research purposes, weight estimates of single trees and their components are needed to describe and predict, e.g. leaf dynamics and the relative size differentiation that develops among individuals in a growing population. A biomass estimation procedure is defined as a planned chain of elements, integrated to comply with a specific aim. Elements such as defining a sampling unit, subsampling within primary units, model choice and conversions from single tree to stand area, fresh weight to dry weight and standing biomass to harvestable biomass, all may be included in a procedure. Those elements have to be modified according to the predefined aim and object of estimation. Consequently, a biomass estimation procedure is a compromise in which accuracy, speed and knowledge of the underlying biological reality together determine the final shape of the procedure and its results. This paper provides a short overview of current biomass estimation methods and gives details about the major elements in the procedures, especially on regression methods. Recent developments in biomass estimation methodology are exemplified with analyses on short rotation forests and attention is paid to the need for standardisation and automatization of the procedures.

Willow coppice systems in short rotation forestry: effects of plant spacing rotation length and clonal composition on biomass production.

Abstract Above ground biomass production was determined for ten Salix clones grown in pure and mixed stands at a square spacing of 1 m and seven rotation periods (1 to 6 and 8 years), and of one clone grown at four square spacings (0.5, 0.6, 0.7 and 1 m), with rotation cycles of 1 to 5 years. Most clones reached a maximum mean annual increment (8 to 14 tons dry matter ha −1 yr −1 ) under a rotation period of 4 to 5 years. Densely spaced stands exhibited a higher production than wider spacings during the first harvests under the shortest rotation periods. Neither in later harvests of short cycles (1 to 3 years) nor in any harvests of longer cycles (> 3 years) did spacing affect biomass production. Some clones suffered from leaf rust and grazing by roe deer. Clone mixtures showed a higher biomass production in the later stages due to the compensatory effect of the successful clones which, when growing in mixtures, could fill out the gaps left by individuals that suffered from impacts other than competition. We conclude that extremely short rotations (1 to 2 years) are unsuitable for Swedish conditions, and that 4- to 6-year rotations perform best. In such longer rotations, biomass production of stands with 2 × 10 4 plants per hectare equals the production of denser stands.

Water-use efficiency of willow : variation with season, humidity and biomass allocation

Information on the water-use efficiency (WUE) of a vegetation cover improves understanding of the interrelationship between the water and carbon cycles, and enables hydrological practices to be related to agricultural and silvicultural planning and management. This study determined seasonal and climatic variations of the WUE of a fertilized and irrigated short-rotation stand of Salix viminalis L. on a clay soil. The WUE was determined as the ratio of above-ground production to transpiration or, alternatively, to transpiration divided by the saturation vapour pressure deficit. Growth was estimated from a combination of destructive and non-destructive measurements for 10 day periods during the growing seasons of 1986 and 1988. Daily transpiration was estimated using a physically based evaporation model, tuned against energy-balance/Bowen-ratio measurements of total stand evaporation. Nutrients were adequate and climate conditions were similar in both years. In spite of irrigation soil-water deficits developed during midsummer and affected growth rates in different ways: in 1986, both stem and leaf growth decreased, while in 1988 only stem growth decreased. Exceptionally high stem growth rates, twice the total potential growth rates, were recorded after the drought of 1988. They were probably caused by root-allocated assimilates that were sent above-ground after the drought. In both years, stem growth ceased 2–3 weeks after the leaf area had reached its maximum. Since light and temperature were still sufficient to maintain assimilation, all growth presumably took place below ground towards the end of the season. Changes in root-shoot allocation caused large variations in the WUE in 1988. The WUE, weighted by the saturation vapour pressure deficit, was fairly constant in 1986. In both years, the WUE was correlated with the vapour pressure deficit. Towards the end of both growing seasons, when all assimilates were sent below ground, the WUE decreased rapidly to zero. The total WUE, estimated as the seasonally accumulated above-ground production divided by accumulated transpiration, was 4.1 g kg−1 in 1986 and 5.5 g kg−1 in 1988, which is relatively high in comparison with other species.

Above-ground Woody Biomass Production of Short-rotation Populus Plantations on Agricultural Land in Sweden

Although poplars are widely grown in short-rotation forestry in many countries, little is known about poplar growth performance in Sweden. In this study, above-ground biomass production was estimated for several hybrid aspen and poplar clones planted at different initial density at five locations across Sweden. Biomass assessments were based on allometric relationships between total above-ground woody dry weight and the diameter at breast height. According to a common harvest practice, tree biomass was partitioned into pulpwood and biomass for energy purposes. The percentage of pulpwood was strongly determined by clone for DBH >10 cm. The mean annual increment ranged from 3.3 Mg ha−1 yr−1 for balsam poplar in the north to 9.2 Mg ha−1 yr−1 for 9-yr-old ‘Boelare’ in southern Sweden. At the same age, hybrid aspen reached 7.9 Mg ha−1 yr−1. The results suggest that poplars and hybrid aspen are superior as biomass producers compared with tree species commonly grown on agricultural land at these latitudes. The results are discussed in the light of future wood supply for pulpwood and energy purposes in Sweden.

Influence of young poplar stands on floristic diversity in agricultural landscapes (Sweden)

Abstract Floristic diversity in 21 small short-rotation poplar plantations, 0.1 to 13 ha in size and aged 6 to 14 yr, was compared with adjacent agricultural fields of various crop types. The poplars were grown on former agricultural land and most of the study sites were located in southern and central Sweden. Surveys of ground vegetation were carried out by using nested quadrats (plots of 0.25 and 0.016 m 2 size) located along normally five transects running perpendicular to the habitat boundaries. Mean cumulative species number across all sites (all transects and plots included) was around 20 and similar in poplar stands and arable fields. However, the two habitat types shared only 3 to 12 species (mean: 8 species). Mean species number per survey plot was 4.5 in poplar stands compared to 5.3 in arable fields. Classification (TWINSPAN) and Detrended Correspondence Analysis (DCA) showed some floristic separation of the two habitat types. The results were interpreted in a landscape context. Mainly based on the low quantity of species shared by poplar stands and adjacent arable fields, the results support the hypothesis that small-scale poplar plantations increase floristic diversity in landscapes dominated by agriculture. Compared to old-growth mixed deciduous forests in different regions of Sweden, species richness of poplar plantations was similar or lower. Die floristische Artenvielfalt in 21 kleinflachigen Pappelbestanden (0,1–13 ha) im Kurzumtrieb wurde mit derjenigen benachbarter cker verglichen. Die Pappeln waren 6–14 Jahre alt und wuchsen auf ehemaligen Ackerflachen, vorwiegend in Sud- und Mittelschweden (ein Standort in Nordschweden). Die Bodenvegetation wurde mit Hilfe ineinander geschachtelter Aufnahmequadrate erfasst (Aufnahmeflachen 0,25 und 0,016 m 2 ). In jedem Bestand und Feld wurden funf parallel zueinander liegende Transekte angelegt, die senkrecht vom Bestandesrand ins Bestandesinnere verliefen. In jedem Transekt lagen in definierten Abstanden zum Bestandesrand funf Aufnahmeflachen. Die mittlere Gesamtartenzahl uber alle Bestande war 20 und etwa gleich in Pappelbestanden und ckern. Allerdings waren nur zwischen 3 und 12 (Mittelwert: 8) Arten gemeinsam in Pappelbestanden und benachbarten ckern. Die Artenanzahl pro Aufnahmeflache war kleiner in Pappelbestanden als in ckern. Klassifikation (TWINSPAN) und Ordination (DCA) zeigte eine gewisse Trennung der beiden Vegetationstypen. Die Ergebnisse wurden auf Landschaftsebene interpretiert und stutzen die Hypothese, dass kleinflachige Pappelplantagen die floristische Artenvielfalt v.a. in intensiv genutzter Agrarlandschaft erhohen kann. Im Vergleich zu naturlichen Laubmischwaldern verschiedener Regionen Schwedens war die Artenvielfalt in den Pappelplantagen entweder ahnlich (ostliches Mittelschweden) oder niedriger (Sudostschweden).

The influence of allometric variation, vertical biomass distribution and sampling procedure on biomass estimates in commercial short-rotation forests

Abstract Non-destructive biomass estimation methods for Salix which were developed for research purposes in Swedish energy-forestry are too time-consuming and expensive to be applied in commercial field-situations. Existing allometric equations (relating shoot weight to shoot diameter) may be used to exclude the destructive, time-consuming phase in biomass-estimation procedures. These relations were shown to be specific for species, age, clone and site, implying that generalized equations may lead to considerable errors for biomass estimations of particular stands. However, generalized equations were identified which, in general, caused deviations of less than 10% compared to the outcome of site-specific equations. Differences between estimated standing-biomass (> 5 cm above ground level) and biomass harvested at higher levels by commercial equipment can be corrected for and are species specific (− 0·6 and − 0·8% cm −1 for Salix viminalis and S. dasyclados , respectively). While performing a random sampling of stools for non-destructive estimation field workers tend to sample around an ‘ocular mean’, which leads to an over-estimation of standing biomass.

Shoot mortality and dynamics of live and dead biomass in a stand of Salix viminalis

Abstract Cumulative shoot mortality in a dense, productive stand of coppiced Salix viminalis amounted to nearly 90% of the initial shoot number after three years following a single cut. The decline in shoot number per area and the increase of stem dry weight per area indicated that mortality was due to self-thinning. Death occurred mainly among the smallest shoots. Although the proportion of living stems decreased drastically, the living biomass constantly accounted for almost 90% of the total standing biomass. Shoots of Salix viminalis lost about 25% of their biomass per year during the first two years after death. Weight loss was by loss of distal parts, which turned into litter, and through decrease of wood density. Cumulative weight loss as a percentage of cumulative biomass increased from 3.1% after the first season to 10.4% after the third season. For a rotation period of four years it is predicted that the proportion of standing dead and live biomass (0.1 and 0.9, respectively) would remain approximately constant, while the cumulative weight loss as a percentage of total biomass is expected to stay below 14%.

Cyclic and progressive changes in short-rotation willow coppice systems

Abstract In a willow coppice system, harvest-related cyclic changes occurred with regard to annual production, yield and number of shoots. The system however was also subjected to long-term directional changes. The relative weight variation between stools increased while the number of stools per hectare decreased over several cutting cycles. The long-term trends are able to affect the short-term cycles due to the interaction of the plant population with local and temporal site productivity factors. Stool mortality occurred in such a way that local density, calculated on the presence of the eight initially nearest neighbours, varied by one order of magnitude in the same stand after three cutting cycles. Such within-stand density variation may give rise to local yield-density effects, implying that differences in individual stool growth are determined by differences in available growing space. However, there were high positive correlations between above-ground stool weight at harvest on the one side and number and mean weight of shoots resprouting the year after harvest. Consequently, below-ground storage can be identified as a major determinant for the maintenance of a once-established competitive hierarchy over harvest, while the local density variation enforces the competitive hierarchy between consecutive harvests.

Stool mortality and development of a competitive hierachy in a Salix viminalis coppice system

Abstract Competition between individual plants in a dense willow coppice stand caused a high density-dependent mortality of stools. Negative growth and death occurred among the smallest stools, suggesting that self-thinning involves the death of individuals whose net assimilation rate falls below a specific threshold value. These observations are consistent with the asymmetric competition hypothesis. The average minimum stool weight in the beginning of the season, needed to gain weight during that season increased with stand development and was lowered by harvest, after which living stools that previously lost weight started to gain weight again. The relaxation of competitive pressure by harvest was only temporary and the competitive hierarchy which had become established during one cutting cycle became enhanced during the next cutting cycle. The observed high stool mortality is discussed in relation to differences between stool variation patterns which may occur in the stand.