Planting black walnut in southern Ontario: midrotation assessment of growth, yield, and silvicultural treatments.

We established 480 remeasurement plots in plantations originally laid out by F.W. von Althen in the 1970s and 1980s for his pioneering work on hardwood planting in southern Ontario. We used 110 of these plots to summarize the growth and yield of black walnut (Juglans nigra L.) plantations in southern Ontario. Overall, walnut growth averaged only 2.3 m3·ha–1·year–1, reflecting the less than optimal soil conditions at the majority of sites. However, good growth rates (i.e., >5 m3·ha–1·year–1) were recorded at sites with well-drained loamy soils, particularly when walnut was interplanted with other tree species. We also carried out a detailed analysis of four of the original experiments of F.W. von Althen to examine the long-term impacts of silvicultural treatments applied at an early stage in plantation development. Briefly, these analyses found (i) a significant long-term effect of controlling herbaceous competition at an early point in plantation development, although there is a suggestion that lower herbicide concentrations may be adequate in the long run, (ii) that fertilization effects, which were marginal at the time of application, were not apparent at age 32, (iii) generally better walnut growth when interplanted with other woody species, but autumn olive (Elaeagnus umbellata Thunb.), the species that stimulated the best walnut growth, grew invasively throughout the study area, thus ruling it out as a nurse species, and (iv) weak evidence that an initial spacing of 3 m × 3 m is optimal for walnut development in the absence of thinning up to age 30. This study provides rare insight into black walnut growth rates and best management practices in southern Ontario, although we recognize that the scope of these findings is limited by less than optimal soil conditions at many of the study sites. [ABSTRACT FROM AUTHOR]