Are mixed species stands more productive than single species stands? Douglas-fir and western hemlock plantations in the Pacific Northwest

AMOROSO, MARIANO; TURNBLOM, ERIC

Aylmer, Canada

Workshop; 5th North American Forest Ecology Workshop; 2005

The Universit du Québec Montréal

Single species plantations have long been preferred when timber production is the primary management objective. Mixed species stands, on the other hand, are usually favored to achieve non-timber objectives. One reason mixed stands are avoided for meeting timber production objectives is an assumed sacrifice in their productivity compared to pure stands. However, ecological theory suggests that species in a mixture may utilize site resources more efficiently than a single species would be able to do, leading to a greater overall productivity. Whether overall stand density influences the outcome has not been adequately investigated. The objectives of this study were to assess differences in growth and productivity between Douglas-fir and western hemlock growing both in pure and mixed plantations across a range of planting densities. In order to study this, monocultures and 50/50 mixtures of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) were planted at a wide range of densities at two locations on the Olympic peninsula in the state of Washington. This research examined growth and yield performance 12 years after establishment at three density levels (200, 450 and 700 TPA). At age 12, in the mixture, Douglas-fir showed greater height, diameter, and individual tree volume than western hemlock at all densities. Compared to their performance in pure stands, Douglas-fir in the mixture exhibited better yield at the 700 TPA density level while western hemlock experienced a reduction in yield at that density. At 200 and 450 TPA the monocultures resulted in higher volume per acre than the mixed stands, but at 700 TPA the mixed stand appeared to be just as productive as the pure stands. At 700 TPA, Douglas-fir as a component averaged the same volume per acre as the pure hemlock stand. The increase in productivity seen in the mixture at high densities seems to be due, in part, to early stratification observed and probably also to fuller use of the site resources. This probably indicates less interspecific competition was experienced in the mixed stand than intraspecific competition was experienced in the pure stands. Even when interactions occurred at low and medium densities, they may not have been of large enough magnitude to cause the mixture to outyield the pure plantations. This shows the important role density plays on the productivity of mixed stands, and thus its importance in comparing mixtures to pure stands. Along with the timber production objectives, mixed planted stands can also achieve other management objectives (such as a broader range in wildlife habitats and increased aesthetic value) without the misconceived timber yield sacrifice.