Effects of red alder on growth of Douglas-fir and western redcedar in southwestern British Columbia

Highlights

• Red alder density of up to 400 tph may be acceptable on nitrogen deficient sites.

• Douglas-fir growth may be enhanced when red alder density is relatively low at age 15.

• Competitive effects of red alder were lower than that of conifers in this study.

• The effectiveness of competition indices varies with tree species.

Abstract

Red alder (Alnus rubra [Bong.]), is the most common broadleaf species in the Pacific Northwest, and is widely distributed as a component of mixed stands with conifers. It rapidly overtops associated juvenile conifers and is a strong competitor. However, red alder is also a symbiotic nitrogen fixer and can contribute to improvement in soil nitrogen, as well as serving other roles such as improving biodiversity and forest health.

We examined data collected in mixed red alder-conifer plantations established between 1992 and 1999 to examine the effects of red alder on the growth of Douglas-fir (Pseudotsuga menziessi (Mirb.) Franco, Fd) and western redcedar (Thuja plicata Donn., Cw). Results suggest that densities of up to 400 alder trees per hectare may be acceptable and possibly desirable on nitrogen deficient sites. The growth of Fd may be enhanced when red alder density is relatively low at older ages. We found that the competitive effects of red alder on conifer growth were consistently lower than that of the conifers (Fd and Cw) while alder had stronger effects on light availability. The relationship between volume growth and competition indices varied between the two conifer species: (1) for Fd the highest correlations were obtained using a distance-dependent competition index based on the height differences, and (2) for Cw the highest correlations were with distance-independent Lorimer’s index. As also expected due to Cw’s greater shade tolerance, its volume growth showed less sensitivity to the presence of red alder than Fd.

Introduction

Red alder (Alnus rubra [Bong.]) is a deciduous tree species that grows both in pure and mixed stands in the Pacific Northwest region of North America. It is commonly found as a component of mixed stands with conifer species such as Douglas-fir (Pseudotsuga menziessi (Mirb.) Franco), Sitka spruce (Picea sitchensis [Bong.] Carr), western hemlock (Tsuga heterophylla [Raf.] Sarg.) and western redcedar (Thuja plicata Donn.) (Eyre, 1980, Deal and Harrington, 2006). In Canada, red alder occurs primarily in the Coastal Western Hemlock zone as a lowland species and generally grows within 200 km of the seacoast at elevations below 750 m. Red alder grows best on deep alluvial soils in river and stream flood plains and is limited by drought and low winter temperatures. Red alder has gained particular interest due to its various uses (furniture, cabinets and pallets), increasing commercial value, and special properties including nitrogen fixation capacity and tolerance of wet soil conditions (Harrington, 1984, Deal and Harrington, 2006).

Admixing red alder into conifer stands can contribute to long-term productivity (fixing nitrogen) (Binkley, 1983, Comeau and Sachs, 1992), enhanced complexity (Piccolo and Wipfli, 2002, Deal and Wipfli, 2004, Deal, 2007), mitigation of pest and pathogen infections of conifer species (McLean et al., 1989, Harrington et al., 1994, Hibbs and DeBell, 1994, McComb, 1994), as well as adaptation to climate change (Cortini et al., 2012). However, red alder is also a strong competitor in young conifer stands due to its rapid juvenile height growth, and detrimental effects on light availability to understory conifers. An understanding of both competitive and beneficial effects of red alder in mixture with conifers is fundamental to making decisions for sustainable management of complex forests in the Pacific Northwest.

A range of competition indices have been used for evaluating levels of competition and for examining the effects of competition on tree growth. For red alder at low densities, the distance to associated conifers would be expected to be important in accounting for competition, which suggests that we should use distance-dependent indices for better prediction of competitive effects. On the other hand, results vary depending on both the broadleaf and conifer species. Previous studies show that two distance-independent indices (Diameter’s Sum and Crown Surface Area) had the highest correlations with Douglas-fir stem volume growth, while for western redcedar the best indices were distance-dependent (Cortini and Comeau, 2008). In addition, Cortini and Comeau (2008) found simple indices such as height factor, total number of competitors, and competitor basal area performed well in their study. Selection of the ideal competition index is complicated due to the simultaneous beneficial and negative effects of red alder with both effects being dependent on site, soil and other factors. Therefore, it is necessary to evaluate a range of competition indices and select the most effective one for predicting radial growth of conifers growing in mixture with low to moderate densities of red alder.

In order to improve our understanding of the competitive effects of different amounts and spatial arrangements of red alder, and how these effects are influenced by factors such as site quality, conifer species, and stand-age, long-term studies of mixed alder-conifer plantings were established in 1992, 1994 and 1999 using both additive and replacement series designs as described by Thomas et al. (2005). The main objectives of the study presented herein are to use data from these experiments to: (1) examine the competitive effects of red alder on the growth of Douglas-fir and western redcedar; (2) evaluate the effectiveness of various distance-dependent and distance-independent competition indices; (3) explore the relationship between light level and alder density; (4) analyze the effects of alder density on soil and foliage nitrogen content, and (5) make recommendations for the management of alder in mixture with conifers.