OVERSTORY AND UNDERSTORY COMPETITION AFFECT UNDERPLANTED EASTERN WHITE-PINE

The importance of canopy and understory competition on white pine (Pin us strobus L.) seedlings was evaluated in stands with shelterwood trea tments. We sampled 20 stands in north-central Minnesota that were unde rplanted with white pine 3 to 10 years before sampling. We grouped the stands with various canopy and understory treatments into 6 stand typ es based on residual canopy composition. The effect of the shelterwood treatments, i.e., residual basal area and understory cover on white p ine seedling growth varied significantly among the stand types. Loweri ng basal area had a greater positive impact on seedling growth when sh ade tolerant softwood species, especially balsam fir (Abies balsemea ( L.) Mill.), were present in the canopy. Where shade tolerant softwood species were absent, the white pine seedling growth increased only sli ghtly or not at all in the presence of lowered residual basal area. We hypothesize that the exclusion of the understory by a shade tolerant midstory, indicated by the presence of balsam fir, prior to shelterwoo d treatment coated favorable conditions for white pine in the understo ry following the shelterwood treatment. The results suggest that shelt erwood treatments on mesic, more productive hardwood sites should be l inked to stand development stages where the understory is suppressed, e.g., following development of a shade tolerant midstory or during the stem exclusion phase. This method should complement present shelterwo od prescriptions for drier, low quality hardwood sites. This research indicates the importance of evaluating both vertical structure and sit e quality prior to designing white pine shelterwood treatments. (C) 19 98 Elsevier Science B.V.