Comparing productivity of pure and mixed Douglas-fir and western hemlock plantations in the Pacific Northwest

AMOROSO, MARIANO; TURNBLOM, ERIC

CANADIAN JOURNAL OF FOREST RESEARCH

NRC Canada

Año: 2006 p. 1484 - 1496

0045-5067

We studied pure and 50/50 mixtures of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) plantations to compare attained total yields between mixed species stands as opposed to monocultures of equal densities. Whether overall stand density influences this outcome has not been adequately investigated and for this we included three density levels (494, 1111 and 1729 Trees Per Hectare, or TPH) in the analysis.At age 12, as components of the mixed stands, Douglas-fir exhibited greater height, diameter, and individual tree volume than western hemlock at all densities. At 494 and 1111 TPH the monocultures resulted in higher volume per hectare than the mixed stand, but at 1729 TPH the mixed stand appeared to be just as productive as the pure stands. The increase in productivity by the mixture at high densities seems to have resulted from the partial stratification observed and most likely also from better use of the site resources. Because of this, less interspecific competition was probably experienced in the mixed stand than intraspecific competition in the pure stands. This study shows the important role density plays on the productivity of mixed stands, and thus in comparing mixtures and pure stands.