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Aboveground growth interactions of paired conifer seedlings in close proximity

Devine, W.D. and T.B. Harrington, . 2011. New Forests 41: 163-178


Where belowground resources are relatively abundant, naturally established trees sometimes occur in very close proximity to one another. We conducted a two-year study to assess the aboveground interactions between Douglas-fir (Pseudotsuga menziesii) grand fir (Abies grandis) and noble fir (Abies procera) seedlings planted in closely spaced (stems 10 cm apart) conspecific and heterospecific pairs. Paired seedling growth also was compared to seedlings planted with no neighbor. Stem height growth was not affected by the presence of a neighbor seedling, although diameter growth was slightly reduced. Branch diameter growth and weight were reduced where seedling crowns overlapped; branch morphological data suggested that this was caused by shading rather than mechanical interactions. Light measurements showed the potential for significant shading, particularly by the relatively large, dense crowns of Douglas-fir. Heterospecific pairs including Douglas-fir demonstrated the competitive production principle in that their mean growth was greater than the average of conspecific pairs of both species. Neighbor seedling height significantly affected subject seedling growth; neighbor effects were similar whether the neighbor seedling was growing on the north or south side of the subject seedling. Light reflected from Douglas-fir crowns had a lower red: far-red ratio than that of noble fir, although there was no evidence of a phytochrome-mediated growth response to the neighbor seedling. While heterospecific seedling pairs showed reduced competition, we found no evidence of facilitation for seedlings growing in very close proximity.

Key Words

Seedling; competition; facilitation; light; Pseudotsuga menziesii; Abies grandis; Abies procera