- Guiterman, Christopher Henry
University of Maine Graduate School
Despite the commercial importance and widespread management of eastern white pine (Pinus strobus L.) in the Northeast, surprisingly little is known about the effects of thinning on even-aged stand development. To address this, patterns of leaf area, bole form, volume growth, and growth efficiency - defined as volume increment per unit leaf area - were examined over a 17-year period within a thinning study in central Maine designed to compare the conventional B-line and low density thinning regimes.
At the tree-level, many of the effects of thinning were as expected. Heavier, low density thinning resulted in significantly larger and deeper crowns with greater leaf area than equivalent trees in both the B-line and un thinned control treatments. These changes explained higher rates of diameter and volume growth. Thinning did not alter growth efficiency per se, but larger trees had slightly (but significantly) lower growth efficiency than smaller trees. Reconstruction of bole taper - quantified as Girard form class - showed that, surprisingly, B-line thinning produced more tapered butt-logs (first 5-meter) than low density thinning, resulting from a thinning-induced growth response at breast height but not at the top of the butt-log.
At the stand-level, an annual record of leaf area index (LAI) attained by litter fall collection showed that leaf area in the control treatment was relatively constant or slightly declined over the study period. Thinning significantly reduced stand leaf area and thus gross volume growth, but the thinned treatments had nearly equal LAIs for the ten years following the initial thinning. This explained the similar gross volume growth rates and growth efficiencies of the thinning treatments. Following the re-entry harvest, B-line leaf area increased until the stands reached crown closure, while the low density treatment continued rates of crown expansion and LAI increase without reaching a peak. Due to greater LAI, B-line gross stand volume growth and growth efficiency were significantly higher during the latter growth period; low density stand growth efficiency was still no different from the control. Growth efficiency of the un thinned stands was found to be positively related to stand density.
Results of this study have important implications to managers of eastern white pine. The contention that thinning below B-line stocking has deleterious effects on stand yield was in general not supported. On the contrary, only a minor loss of gross stand volume growth was found by thinning to a low density. In addition, low density trees were larger, faster growing, and had better stem form than comparable B-line trees. Therefore, low density thinning was found to be a viable alternative to conventional management.