Three-Dimensional Canopy Structure of an Old-Growth Douglas-Fir Forest Bo Song, Jiquan Chen, and Janet Silbernagel ABSTRACT. In this study, the structurally heterogeneous canopy of a 12-ha plot in an old-growth Douglas-fir forest was analyzed using three-dimensional (3-D) canopy modeling, geographic information system (GIS), and spatial statistics. Using this approach, we were able to depict how species composition and spatial distribution affect the 3-D structure of canopies. We were also able to slice the canopy and calculate the canopy coverage at various heights, and therefore were able to calculate the cumulative canopy volume at individual crown bases. Information generated by GIS allowed us to calculate how much area canopies or canopy openings occupy. Total canopy volume (243,641 m3/ha) was dominated by western hemlocks (67.1%) and Douglas-firs (23.7%) in this plot. Western hemlocks and Douglas-firs also dominated the canopy coverage, with 65.7% and 25.5% average coverage, respectively. Average total canopy coverage was 84.3% with variations between 78.7% and 89.9% among the 12 1-ha subplots, implying that 15.7% of the stand consisted of canopy openings. Coverage of the canopy projection changed greatly from the lower to the upper canopy layers, with the maximum at about 20 m high. We also examined spatial relationships among groups of trees at different heights and at various scales using bivariate Ripley’s K analysis. We found that different species of the understory layer showed different spatial relationships relative to the overstory canopy layer. Likewise the same species in the understory layer showed different spatial relationships when the overstory species differed. This approach provided a useful tool for characterizing 3-D forest canopies, and the results will be very helpful for examining leaf distribution, understory light environ- ment, understory vegetation, and microclimate. FOR.SCI. 50(3):376–386. Key Words: Canopy opening, vertical structure, GIS, Ripley’s K analysis. EALISTIC REPRESENTATION OF CANOPY STRUCTURE ceive and release energy and materials (e.g., carbon dioxide in three-dimensional (3-D) space is a necessary and water) through photosynthesis, respiration, and evapo- R initial step in studying canopy-related ecological transpiration, and thereby create a microclimatic regime processes. Parker (1995) defined canopy as “the combina- within the canopy (Chen et al. 1993, 1999) that drives tion of all leaves, twigs, and small branches in a stand of understory processes (Song 1998, Van Pelt and Franklin vegetation; it is the aggregate of all the crowns.” As an 1999, 2000, Quinby 2000). Forest canopies also determine interface between forests and the atmosphere, canopies re- ecosystem processes and functions such as microhabitats for Bo Song, Assistant Professor, Belle W. Baruch Institute of Coastal Ecology and Forest Science, Clemson University, PO Box 596, Georgetown, SC 29442-0596—Phone: (843) 545-5673; Fax: (843) 546-6296; [email protected]. Jiquan Chen, Professor, Department of Earth, Ecological & Environmental Science, University of Toledo, Toledo, OH 43606—Phone: (419) 530-2664; [email protected]. Janet M. Silbernagel, Associate Professor, Department of Landscape Architecture, University of Wisconsin, Madison, WI 53706—Phone: (608) 265-8093; [email protected]. Acknowledgments: This research was partially supported by grants from the Pacific Northwest Research Station of the USDA Forest Service (Agreement No. PNW94-0541) and a Challenge Fellowship at Michigan Technological University. We thank William Conner, Carrie Lucas, Stephanie Beard, Tom Williams, and Chuck Gresham for their help in editing the drafts of the manuscript. We also thank Associate Editor and two anonymous reviewers for their constructive comments on the manuscript. Manuscript received March 18, 2002, accepted August 25, 2003. Copyright © 2004 by the Society of American Foresters 376 Forest Science 50(3) 2004 plants and wildlife (Dial 2004), properties of the forest floor Systems Research Institute, Redlands, CA) was used to (e.g., sun flecks, throughfall), and even belowground pro- analyze the canopy structure both horizontally and verti- cesses such as energy flux, regeneration, decomposition, cally. To quantify how stem distribution may affect canopy and respiration (Runkle 1991, Naumburg and DeWald structure, bivariate Ripley’s K analysis was used to examine 1999, Nadkarni and Sumera 2004). The spatial and temporal their spatial randomness. Details of how 3-D canopy mod- properties of these biophysical processes are directly related eling, GIS, and spatial statistics were used for analyzing 3-D to horizontal and vertical arrangement of forest canopies canopy structure are described below. and their changes at multiple temporal scales (Parker and Study Site Tibbs 2004). Our study site was located at the Wind River Canopy In some environments, foliage arrangement in 3-D space Crane Research Facility (WRCCRF) in the T.T. Munger may be much more important than the total amount of Research Natural Area of the Wind River Experimental foliage (Van Pelt 1995, Ishii et al. 2004). A tall, multi-lay- Forest, 45°48Ј N, 121°58Ј W, on the Gifford Pinchot Na- ered forest may have a much greater volume for foliage to tional Forest in southern Washington. This 450-year-old occupy, hence a more open canopy. Thus, understanding of forest stand is dominated by Douglas-fir (Pseudotsuga men- how crowns occupy the 3-D space has been the subject of ziesii) and western hemlock (Tsuga heterophylla), with the numerous studies (Van Pelt 1995, Song et al. 1997, Chen dominant trees averaging 55 to 65 m (maximum 67 m) tall. and Bradshaw 1999, North et al. 2004). One traditional Other tree species include western redcedar (Thuja plicata), approach to quantify the canopy structure is the leaf area western white pine (Pinus monticola), Pacific silver fir index (LAI; Gholz 1982, Franklin and Waring 1980). How- (Abies amabilis), grand fir (A. grandis), and noble fir (A. ever, these LAI values can be misleading in reflecting the procera). Understory trees include Pacific yews (Taxus potential light levels in the understory (Brown and Parker brevifolia). The site is in the Wind River valley at 355 m 1994). For example, an even-aged mono-layered forest may above sea level, within the southern Washington Cascade have the same LAI value as a multi-layered forest, but the Range (Shaw et al., in press). The forest represents the understory light conditions under these two forest canopies end-point of age, biomass, and structural complexity gradi- can be very different from each other. The traditional use of ents in coniferous biomes of the western Cascades (Franklin two-dimensional (2-D) canopy projection analysis is of little et al. 2002). use because forest stand biomass is unevenly distributed vertically. Hubbell and Foster (1986) measured the vertical Field Data Collection and Analysis distribution of foliage and the spatial distribution of stems, A 12-ha (400 ϫ 300 m) plot was established at the study but they didn’t integrate the canopy structure with 2-D site between 1994 and 1998 (Figure 1). Trees with diame- spatial pattern of trees. Effective methods are now available ters at breast height (dbh) greater than 5 cm were measured for analyzing the complex 3-D structures of canopies. These and identified to species. Their coordinates (x, y) and ele- methods include point pattern analysis (Chen and Bradshaw vation (z) were measured using a computerized total sur- 1999), 3-D canopy modeling (Silbernagel and Moeur 2001, veying station (WILD TC600 Total Station, Leica Geosys- Song et al. 1997, Song 1998, Van Pelt and Franklin 2000), tems AG, Heerbrugg, St. Gallen, Switzerland). Crown di- remote sensing (Harding et al. 2001, Lefsky et al. 2002), mension detail was collected from a canopy crane that had and application of geographic information systems (GIS; a 75-m vertical reach and an 85-m radius. The crown radii Song et al. 1997). of different species were measured at various vertical Description of crown architecture is the first step in heights from the crown’s top to its base, at up to four modeling forest canopies. Terborgh and Petren (1991) cardinal directions (sometimes the gondola of the crane treated crowns as cylinders, and Van Pelt and North (1996) could not access all four directions). Because only 26 trees treated crowns as several geometric shapes, such as cylin- were measured using the canopy crane, the maximum crown ders, cones, and paraboloids. Song et al. (1997) extended radii of the additional 194 trees were measured from the this approach by using a set of equations to simulate crown ground using a meter tape and a compass. The Laser Im- shapes. Our study objectives were to: (1) present a new pulse Rangefinder (Laser Technology Inc., Centennial, CO) approach of slicing forest canopies using a stem-mapped was used to measure the crown base heights. Regression database of an old-growth Douglas-fir forest in southern models developed from field measurements of dbh and the Washington, (2) understand how species composition and maximum crown radii were used to estimate the maximum spatial distribution of trees affect the characteristics of can- crown radii of all live trees (Ͼ5 cm in dbh) within this 12-ha opies in 3-D space, and (3) discuss potential applications of plot. To get the heights of all live trees (Ͼ5 cm in dbh), the methods and results in relevant forestry research. regression models of dbh and height relationships were developed by species from field measurements. One set of Methods tree height data was measured from the ground (225 trees) In this study, forest canopies were simulated by model- using the Criterion Laser (Laser Technology Inc.), and ing individual crowns, which were then placed onto the another set of height data was measured from the crane (307 spatial locations of corresponding trees. This process re- trees).