Impacts of weed control on the survival, growth and carbon capture of Sitka spruce trees

Sitka Spruce, mixed agesWeeds compete for nutrients, water and light, and can severely threaten the survival and early growth of newly planted trees.  On sites where trees are being planted and during the trees’ early years when they are establishing themselves, it is very important to control the growth of weeds.  Weeds can be controlled using techniques such as hoeing, mowing and mulching or through herbicide applications.  This paper reports on an experiment to explore the effects of weed control on the establishment of Sitka spruce trees.  Six sites were used in the experiment and each was revisited 15–25 years after the trees were planted, and 6–22 years after weed control finished.

At half of the sites, weed control initially had some benefits to growth and, in some cases, to the survival of the trees.  On the other half of sites, weed control did not appear to have any effect.  When the trees reached 15–25 years old, although weed control did not appear to have any effect on tree growth, in three of the sites it did seem to increase the survival rate of the trees.  Using a mathematical model, estimates were made of the extent to which weed control would have an effect on tree survival until the year in which it was anticipated that the trees would be felled.  This indicated that weed control may increase the survival rate of trees right up until the year they are felled.  This would increase the amount of timber produced and the amount of carbon stored in the trees at some of the sites (although the presence and size of these benefits varied between sites).  The study demonstrates that there is potential for forest managers to use early weed control to increase timber yields and carbon storage in some British Sitka spruce forests; however to confirm this more strongly, there is a need to develop long-term weeding trials in the UK.

Stokes, V.J. and Willoughby, I.H. (2014). Early weed control can increase long-term growth, yield and carbon sequestration of Sitka spruce stands in Britain. Forestry 87, 425-435.