Elsevier

Biomass and Bioenergy

Volume 23, Issue 5, November 2002, Pages 315-326
Biomass and Bioenergy

Effects of spacing, species and coppicing on leaf area, light interception and photosynthesis in short rotation forestry

https://doi.org/10.1016/S0961-9534(02)00060-0Get rights and content

Abstract

Red alder and Balsam Spire poplar were planted in May 1989 as single stems at 1.0m spacing on a converted agricultural site in central Scotland. Biomass production, canopy structure and light interception were quantified and the effects of coppicing and of planting at wider (1.5m) spacing were determined. Bowles hybrid willow was grown as coppice planted at 1.0m spacing and its canopy characteristics compared with those of coppiced alder and poplar.

Total above-ground biomass at 5 years old, ranged from 51 to 81Mgha−1 dry matter with an average annual production of 10Mgha−1 dry matter. Coppicing reduced poplar and alder biomass by 26% and 47%, respectively. Planting at the wider spacing reduced stocking density by 56% and led to a 35% reduction in biomass. Both coppicing and planting at wider spacing increased root:shoot ratios and leaf weight ratios in alder and poplar and reduced the seasonal interception of photosynthetically active radiation (PAR) for 2 years. Light interception was most influenced by early canopy development due to the annual distribution of PAR. Treatment effects on net photosynthesis were due, primarily, to changes in canopy structure affecting light levels.

After 3 years light interception was similar across all treatments suggesting that planting at wider spacing or early coppicing would require additional weed control for one more growing season.

Introduction

Short rotation forestry (SRF) is one option by which the European Union may achieve its stated objective of increasing the proportion of its energy obtained from renewable sources from 5% to 12% by 2010 [1]. SRF offers several advantages over the use of fossil fuels. It is carbon neutral and produces 80% less sulphur emissions during combustion than fossil fuels although NOx emissions are 10% greater [2]. In addition, SRF provides environmental benefits due to increased biodiversity when compared to arable systems. Insect and bird populations are enhanced due to the complex structure and physical diversity associated with SRF [3].

Rapid establishment of SRF is important both environmentally and economically. Early growth of at least some species has been shown to be highly sensitive to weed competition [4], [5]. To minimise the use of herbicides, with consequent environmental and economic benefits, canopy cover by the trees is needed to suppress the competing ground vegetation. Full site occupancy is achieved more rapidly at close spacing but the additional cost of planting material represents a major proportion of the investment during establishment [6], [7]. Maximum yields may not occur for 6–8 years, by which time intense inter-plant competition may increase the risk of damage from pests and disease due to plants being subjected to greater stress at closer spacing [8]. In addition to planting density, the decision to produce coppice or single stems may also affect the time taken to reach canopy closure. Few comparisons have been made regarding the early canopy development in coppice and single stems and those that do exist have often confounded site differences with cultural treatments [9].

The aim of the present study was to quantify the factors that influence early canopy development and growth in SRF on a converted agricultural site in Central Scotland. Particular focus was on species differences and how these are affected by planting density and by coppicing.

Section snippets

Site description

The experiment was established on agricultural land previously cropped for spring barley and located in central Scotland (55°40′N,3°52′W) at an elevation of 210m above sea level. Soils in the area are poorly drained eutric gleysols making them difficult to manage for agriculture due to poor trafficability during much of the year. The climate is cool-temperate with annual precipitation of 1100mm and mean monthly temperatures ranging from 0°C to 15°C.

Three species have been used in the present

Survival

Survival was high (>95%) in all treatments except for alder coppice which suffered 50% mortality following coppicing in winter 1989. This led to uneven stocking of around 5000ha−1 in alder coppice with an average spacing of 1.4m, similar to that in the 1.5m single stem treatments [10].

Biomass

Total above-ground biomass in July 1995 ranged from 51 to 81Mgha−1 dry matter with an average dry matter production of approximately 10Mgha−1yr−1 (Table 1). Values for alder and poplar single stems planted at 1.0m

Discussion

Annual yields in the present study lie in the range reported by Mitchell et al. [7] for similar species growing in the UK and agree with expected yields of 10–12Mgha−1yr−1 for SRF in temperate climates [12], [13]. Red alder is known to be prone to damage when coppiced [14], [15] with factors such as season, shoot diameter and age all affecting survival and resprouting following coppicing [14], [16]. In the present study, the mild winter prior to coppicing and the cut-back taking place in late

Acknowledgements

The authors acknowledge the co-operation of staff from the Central Scotland Countryside Trust and from Hartwood Home Farm in the establishment and maintenance of the experimental site. This work was funded by the Scottish Executive Environment and Rural Affairs Department.

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