Does whole-tree harvesting compromise soil nutrient supply in future rotations?
Current research in this area aims to identify those soils where nutrient resources may be compromised if tree residues are not left on site as is the case for whole-tree harvesting. This research shows:
- Significant decreases in second rotation growth associated with brash removal from a nutrient poor site
- Brash left on site has a positive effect on N, P, K availability and growth increment
- Brash retention alone appears unlikely to provide sufficient nutrition for optimum long-term tree growth on poorer sites.
Whole tree harvesting (WTH)
WTH is defined as the removal of most branches and needles from a harvesting site in addition to the stem wood that is removed in conventional harvesting. The stump is left in situ.
Advances in harvesting technology mean that all branches and foliage can be removed from a site for ease of extraction or for use as an energy source. Branches and foliage have much higher concentrations of nutrients than stem wood. Brash removal, particularly from nutrient poor sites, represents a substantial source of nutrients while its removal may also lead to soil acidification. However, brash left on site can lead to eutrophication on sites where nitrogen saturation is a potential problem.
A long-term experimental monitoring is ongoing on three sites specifically selected to form a well defined gradient of soil nutrient regime (from very poor to the upper boundary of poor) according to the ecological site classification.
An on-going experiment on brash management practices in Tugley Wood, Chiddingfold, West Sussex is investigating:
- The effects of brach management (see treatments below) on soil quality and chemical status
- Natural seedling regeneration under different brash management treatments
- Changes in ground flora, particularly the development of ‘weed’ species
- The effects of brash management on the decay of conifer brash.
- Mulch – created by a chain-flail chipper
- Heavy brash – harvesting residues removed from bare soil plots by an excavator onto adjoining plots and distributed evenly to a depth of up to 50 cm
- Light brash – harvesting residues moved to create a 15 – 20 cm layer of brash through which the soil is visible in places
- Bare soil – harvesting residues removed followed by soil scarification
- Grassed squares – grass seed sown onto bare soil in spring to encourage the development of a sward, creating competition for weeds.
The objective of the experiment is to identify the most suitable management of brash for encouraging broadleaf regeneration following clearfelling of conifers on lowland sites. In addition, advice will be given on the best brash management practices for long-term soil sustainability and optimal nutrition for seedling and tree growth.
Many lowland conifer plantations have recently been restored to broad-leaved woodland. The brash created by clear felling could be removed off site, or managed on site in several ways depending on public access and local nature conservation issues. If the brash is removed off site, a substantial amount of nutrients are removed with it. On the other hand, leaving conifer brash on sites being restored to broad-leaved woodland could cause problems such as soil eutrophication due to increased rates of mineralisation and nitrification.
However, as these sites are already on, largely, base-rich soils, nitrogen pollution of drainage water following brash decay is not as great a problem as in the uplands on more acidic soils. On the other hand, brash removal may decrease nitrogen leaching. In addition, the effect of brash management on ground vegetation, seedling regeneration and promotion of weed species is unclear.