Aboveground productivity of western hemlock and western redcedar mixed-species stands in southern coastal British Columbia

Mixed-species forest stands are generally thought to be more productive than single-species stands. However, empirical evidence is lacking. We tested the effects of species mixture on productivity based on variable compositions of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and western redcedar (Thuja plicata Donn ex D. Don) stands in southern coastal British Columbia. We selected 27 single-storied, even-aged stands that established naturally following clearcutting and slash burning, were at the advanced stem exclusion stage, 50-70-year-old at breast height (bh), and on equivalent sites with fresh soil moisture regime and medium soil nutrient regime. We used simple and multiple regression analyses to relate stand aboveground overstory biomass, understory biomass, and annual net primary productivity (ANPP) to tree species composition, stand age, stand density, and hemlock site index. Overstory biomass increased linearly with percentage of hemlock and stand age. No quadratic relationship was found between overstory biomass and percent hemlock, revealing that productivity was not higher in mixed-species stands. Understory biomass accounted for less than 0.1% of total stand biomass and decreased with stand density, increased with stand age and percent hemlock. Percent hemlock composition was positively related to stand age and negatively to stand density. Simple linear regression analysis showed no relationship between ANPP and percent hemlock. ANPP peaked at an age of 60 years at bh, increasing with stand density, and decreasing as hemlock site index increased. After separating the correlation among stand attributes, multiple regression analysis indicates higher ANPP with increasing hemlock percentage within a given stand age and density. We concluded that mixed-species composition in western hemlock and western redcedar stands in southern coastal British Columbia did not increase aboveground productivity. (C) 2003 Elsevier Science B.V. All rights reserved.