Field aspects of line intersect sampling for assessing coarse woody debris

Due to modern forestry practises, the amount of coarse woody debris has dec reased considerably in Swedish forests. Since many threatened species depen d on this substrate, assessment of it is central in the monitoring of fores t ecosystems. The plot-based methods normally used for timber cruising tend to be inefficient when the objects of interest are sparse and, consequentl y, line intersect sampling is an interesting alternative for assessing coar se woody debris on the ground. With this method there is a risk that the su rveyor, without being conscious about it, tends to move towards or away fro m a log instead of following a straight line. The result is biased estimate s. In this paper, a field test concerning the surveyors' influence on estim ates is presented. Line intersect sampling for assessing coarse woody debri s was tested by ii surveyors in four old conifer stands in northern Sweden. The study did not indicate any systematic differences between surveyors in their way of performing the inventory, although for the surveyors as a gro up a negative bias was found. The bias was, however, only due to a large un derestimation in one single stand, meanwhile the measurements in the other stands were close to reference values. Random measurements errors are, howe ver, likely to be introduced. The conclusion is that systematic errors shou ld not be a major problem in Line intersect sampling but that the dimension ing of surveys should consider the random error component introduced by sur veyors. (C) 1999 Elsevier Science B.V. All rights reserved.