Variation in sapflow and stem growth in relation to tree size, competition and thinning in a mixed forest of pine and spruce in Sweden

The aim of this study was to analyse the relationship between different biometric parameters and sapflow and growth, respectively, in order to assess their potential for use in scaling. The effect of thinning on these relationships was also investigated. The studied stand consisted of Scots pine (ca. 60%), Norway spruce (ca. 35%) and downy birch (ca. 5%). Sapflow was measured by the tissue heat-balance method, and growth in circumference was measured by dendrometer bands, during a 3-year period. The thinning was carried out after 1 year of measurements. Regressions were calculated between sapflow and growth and tree diameter, needle mass estimated by allometry, and a competition index (CI), which depended on the distance and diameter of neighbouring trees. Both sapflow and growth were affected by strip-roads from a thinning ca. 8 years earlier. Before thinning, the variation in sapflow for spruce was best explained by CI when trees close to a strip-road were excluded; for pine, needle mass was equally as good as CI. In 1999 and 2000, needle mass generally explained the variation in sapflow best. Before thinning, the variation in pine growth was explained almost as well by diameter alone, as by CI. For spruce, needle mass explained most of the variation if all trees were included. If trees beside strip-roads were excluded, r(2) for CI increased and CI explained the variation best. Almost all spruces close to strip-roads had lower growth than was expected from their CI relationships; for pine, 13 of 23 trees had higher growth than expected. For the years after thinning, diameter was mostly at least as good as the other variables for predicting BA growth. It was concluded that a competition index is potentially useful for scaling, and that strip-roads affected both sapflow and growth of nearby trees. This should be considered in sampling design as well as in scaling up to stand level. (C) 2003 Elsevier B.V. All rights reserved.