Functioning of Mixed-Species Stands: Evidence from a Long-Term Forest Experiment

Abstract

The attitude of grant-giving bodies to long-term research programs over the past two decades has been ambivalent and, on occasions, such research has been equated to “stamp collecting”. Yet, when challenges to the environment have occurred such as acid rain, climate change, N deposition, land use change, it is to the “stamp collectors” that modelers have turned for reliable, temporal data on complex ecosystems. Nowhere is such complexity better illustrated than in old-growth native forests, with their mixture of different species and ages in the understory, intermediate, and canopy layers. In contrast, plantation forestry has traditionally consisted of even-aged monoculture stands, although evidence has accumulated that mixtures of tree species may have advantages over monocultures. The benefits include the potential for increased yield (e.g., Bartsch et al. 1996; von Lupke and Spellmann 1997; Luis and Monteiro 1998; Pretzsch, Chap. 3, this Vol.; Vilà et al., Chap. 4, this Vol.), resistance to pests and diseases (e.g., Su et al. 1996; Müller and Hallaksela 1998; Jactel et al., Chap 12, this Vol.; Pautasso et al., Chap. 13, this Vol.), reduced risk of wind-throw (e.g., Burkhart and Tham 1992; Dhôte, Chap. 14, this Vol.) and increased biological diversity (Lahde et al. 1999; Hartley 2002). However, while the area of mixed forest stands in western Europe has increased considerably in the last few decades (Gardiner 1999), few long-term experiments have attempted to provide clear answers about the potential advantages of mixtures over monocultures in well-defined and replicated conditions. Forest mixture experiments are often set up with the aim to understand mechanisms that maximize yield, and a number have studied the potential advantages of growing nitrogen-fixing and non-nitrogen-fixing tree species together (e.g., Tarrant 1961; Tarrant and Trappe 1971; Brozek 1990; Binkley 1992; Bi and Turvey 1994; DeBell et al. 1997). Kelty (1992) described this mechanism of interaction of species as facilitation (one species improving the growth conditions for the other), as opposed to complementarity, in which the two species exhibit differences in characteristics which cause interspecific competition to be significantly less than intraspecific. These would include, for example, differences in canopy structure, phenology, and root distribution. There have been some studies that have examined the potential benefits of mixing deciduous and coniferous species (e.g., Lockow 1998). Mixture effects have also been examined by comparing a variety of established native forest stands of different compositions of species (e.g.. Schuler and Smith 1988; Freist 1991; Nüsslein 1993; Longpré et al. 1994; Oyen and Tveite 1998). Vilà et al. (Chap. 4, this Vol.) have examined the impact of confounding factors when comparing such established stands. This chapter reviews the existing information in relation to one such experiment conceived in 1955, when the former Nature Conservancy (now Centre for Ecology and Hydrology) and the UK Forestry Commission (now Forest Research) set up a tree-planting program at Gisburn in the North Pennine area of NW England (Holmes and Lines 1956). The experiment was planned with two aims: Σ to compare tree growth in several two-species mixtures with their component monoculture stands, and Σ to measure and identify changes in a range of soil factors under the different species. As much of the information about Gisburn relates to that published by Brown (1992) on tree performance and soil changes in the first rotation, we have not repeated the detailed description of the methods and statistical analyses. However, Gisburn is perhaps unique in that the design, replication, and length of time over which the data have been collected have not been undertaken elsewhere.