Predicting understory light microclimate from stand parameters in young paper birch (Betula papyrifera Marsh.) stands

Abstract

On productive sites, young paper birch (Betula papyrifera Marsh.) outgrows most of the conifers with which it is associated. Due to its dominant position in the canopy, and the potentially high densities of birch regeneration found on some sites, its presence may lead to a reduction in the amount of light reaching subordinate conifers. To evaluate the potential use of stand attributes for estimating competition for light and understory light levels, we measured fractional transmittance of light (DIFN₁) using Licor LAI-2000 Plant Canopy Analyzers, at a height of 1 m under the canopies of nine 10–40-year-old paper birch stands in southern interior BC. Various measures of broadleaf and conifer abundance were made at each of the 55 sample points where light measurements were taken. Vertical light gradients were also measured to a height of 8 m within birch canopies at four of the nine sites.

Regression models were developed to predict understory light availability from broadleaf and conifer abundance. The models that best-fit the data were based on broadleaf basal area (BA_b), and included either conifer cover (C_c) (R²=0.83) or conifer basal area (BA_c) (R²=0.80). Broadleaf density (TPH_b) alone was a poor predictor of understory light, but in combination with broadleaf quadratic mean diameter (QMD_b) or height of the tallest broadleaf (HT_b) the models improved (R²>0.85). Light availability increased rapidly with relative height in birch canopies, especially where transmittance to the understory was low; at 35% of the height from live crown base to top of the canopy (relative height of 0.35), approximately 80% full light was available.

A diagrammatic representation of the relationship between DIFN₁, QMD_b and TPH_b is presented as a potential tool for the development of stand-tending prescriptions. DIFN₁, BA_b and TPH_b were all highly variable across individual sites, indicating that the models should be applied at the microsite level.

Introduction

Paper birch (Betula papyrifera Marsh.) is a common component of productive interior Cedar–Hemlock zone forests in the southern interior of BC (Lloyd et al., 1990), and following clearcut harvesting, it can regenerate abundantly from seed or by vegetative sprouting (Haeussler et al., 1990). In the past, birch was regarded primarily as a weed species that competed with juvenile conifers, and its presence was reduced using manual and chemical brushing treatments. Recent studies have shown that there are many benefits for retaining a birch component within conifer stands. Birch contributes to structural diversity within stands (Cameron, 1996), it contributes to soil nutrient cycling (Bradley and Fyles, 1995, Simard, 1995, Wang et al., 1996), and it helps to maintain a diverse ectomycorrhizal community (Jones et al., 1997). The presence of paper birch within young stands may also slow the spread of Armillaria root disease, which is an especially important consideration in southern interior BC forests, where the disease is common (Morrison and Mallett, 1996, Gerlach et al., 1997, Simard et al., 2001).

Paper birch grows rapidly in its juvenile phase (Haeussler et al., 1990, Simard and Vyse, 1992), and is a strong competitor for light (Comeau, 1996, Comeau et al., 1998). It can overtop concurrently established conifers for at least 25 years following regeneration. Numerous studies have shown that complete removal of paper birch improves conifer seedling growth in the short-term (Gregory, 1966, Andersson, 1985, Simard, 1990, Simard and Heineman, 1996, Simard and Hannam, 2000), but less information is available regarding threshold levels of birch that can be retained before conifer performance is negatively affected.

Efforts have been made to characterize levels of competition between broadleaves and conifers through the use of competition indices (e.g., Lorimer, 1983, Navratil and MacIsaac, 1993) and competition thresholds (e.g., Simard, 1990, Simard et al., 2001), although these tools are known to be somewhat limited by their static nature (Burton, 1993). Since competition for light is generally acknowledged to be of primary importance in young broadleaf-conifer stands (Coates and Burton, 1999), another approach to assessing the competitive environment has been to model light availability under the canopy on the basis of stand attributes. To be most useful, variables used to predict light availability should be easily measured. Broadleaf basal area, which requires only the collection of diameter data, has been strongly correlated with understory light in a number or studies (Jenkins and Chambers, 1989, Comeau, 1996, Comeau, 2001, Messier, 1996). Models have also been developed that predict light transmittance on the basis of crown or stand architecture (e.g., Grace et al., 1987, Brunner, 1998, Comeau et al., 1998, Stadt and Lieffers, 2000), but they involve the collection of a detailed and complex set of data for the stands to be modeled. Some competition indices have also been strongly correlated with light transmittance to the understory (MacDonald et al., 1990, DeLong, 1991, Comeau et al., 1993, Comeau et al., 1998).

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is the most commonly recommended conifer species for regeneration of harvested sites in moist, warm southern interior Cedar–Hemlock biogeoclimatic zone variants, followed by lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) and interior spruce (Piceaengelmannii×glauca) (Lloyd et al., 1990). Naturally regenerated mixtures in the ICH also typically include shade tolerant western redcedar (Thuja plicata Donn), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and subalpine fir (Abies lasiocarpa (Hook.) Nutt.). The literature suggests that, although Douglas-fir, lodgepole pine, western larch (Larix occidentalis Nutt.), and interior spruce can survive at light levels of 8–10% full sunlight, or lower (Carter and Klinka, 1992, Coates et al., 1994, Lieffers and Stadt, 1994, Chen, 1997, Chen and Klinka, 1998, Williams et al., 1999), extended periods of exposure to such low light is likely to result in substantial mortality. Kobe and Coates (1997) demonstrated that the probability of mortality increases in a non-linear fashion with decreasing diameter growth rates. Studies also indicate that height and diameter growth of these conifer species increases with light availability. For young interior Douglas-fir seedlings, Chen (1997) found approximately 87 and 50% of full sun were required to attain 70% of maximum radial and height growth, respectively. Height and diameter growth of coastal Douglas-fir also increased almost linearly with increasing light (Drever and Lertzman, 2001), and the authors suggest a minimum 40% full light is required before Douglas-fir can express differences in site productivity. Carter and Klinka (1992) also found that conifer growth was increasingly influenced by factors other than light where levels exceeded 30% full sun. For interior spruce in a moist temperate region of northwestern BC, radial increment increased almost linearly with increasing light, so that 60% of maximum growth was attained at approximately 82% light (Wright et al., 1998). In the same study, lodgepole pine growth increased even more steeply with increase in light levels; 60% of maximum radial growth was attained at approximately 65% full sunlight. For young Engelmann spruce (P. Engelmannii Parry ex Engelm.) seedlings on an interior site, 70% maximum diameter and height growth were attained at approximately 75% light (Chen, 1997). In contrast, Lieffers and Stadt (1994) report that white spruce (Picea glauca (Moench) Voss) growing under boreal aspen canopies achieved maximum height growth at 40% full light, while maximum diameter growth was achieved at full sunlight. Western redcedar is more shade tolerant, and can achieve 70% maximum radial growth at about 50% full light in northwestern BC (Wright et al., 1998). In coastal BC, maximum radial and height growth of western redcedar occurred at 30% full sun (Drever and Lertzman, 2001).

Juvenile birch stands increase in height and self-thin rapidly (Simard and Vyse, 1992, Kobe and Coates, 1997). In southern interior BC, where conifers tend to establish concurrently with paper birch and other broadleaves on clearcut areas, they may occupy intermediate or sub-dominant positions within the faster growing broadleaf canopy for a number of years. Consequently, variation in light availability within the canopy, as well as below it, is of interest for assessing the level of competition in mixed stands. Comeau (2001) showed that light availability increased rapidly with the relative height within aspen canopies, but similar information is not available for birch stands. Paper birch is utilized to only a limited extent in BC; it is harvested and milled for furniture and pallet manufacture, predominantly in small private sawmill operations, and it is widely used for firewood.

BC is large and climatically and ecologically diverse, making the development of silvicultural guidelines challenging. More information is available regarding broadleaf-conifer mixtures in boreal than temperate regions, and aspen has been more commonly studied than paper birch. Probable reasons are that boreal forests are more common across Canada and Scandinavia than moist temperate forests, and there has been increasing utilization of aspen from boreal forests in recent years (Massie, 1996). This report presents information about light availability in young birch stands in southern interior BC that can be compared with similar information that is available for broadleaf-conifer mixtures in other geographic regions. The overall objective of the study was to evaluate the potential use of easily measured stand attributes for estimating light levels in the understory of young (10–40-year-old) birch stands. This information is potentially useful in the development of stand tending prescriptions that balance conifer light requirements with the need to retain birch for its short- and long-term site benefits.