Field aspects of line intersect sampling for assessing coarse woody debris

Due to modern forestry practises, the amount of coarse woody debris has decreased considerably in Swedish forests. Since many threatened species depend on this substrate, assessment of it is central in the monitoring of forest ecosystems. The plot-based methods normally used for timber cruising tend to be inefficient when the objects of interest are sparse and, consequently, line intersect sampling is an interesting alternative for assessing coarse woody debris on the ground. With this method there is a risk that the surveyor, without being conscious about it, tends to move towards or away from a log instead of following a straight line. The result is biased estimates. In this paper, a field test concerning the surveyors' influence on estimates is presented. Line intersect sampling for assessing coarse woody debris 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 group a negative bias was found. The bias was, however, only due to a large underestimation in one single stand, meanwhile the measurements in the other stands were close to reference values. Random measurements errors are, however, likely to be introduced. The conclusion is that systematic errors should not be a major problem in Line intersect sampling but that the dimensioning of surveys should consider the random error component introduced by surveyors. (C) 1999 Elsevier Science B.V. All rights reserved.