shorter trees ($1/3 as tall as the tallest height class) and the tree with the largest main ..... structural diagrams were completed and apparently error-free. 2.3.1. ... Adobe Photoshop, which was then exported to ImageJ for area and perimeter ...
Forest Ecology and Management 259 (2010) 976–994
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Increasing wood production through old age in tall trees Stephen C. Sillett a,*, Robert Van Pelt a, George W. Koch b, Anthony R. Ambrose c, Allyson L. Carroll a, Marie E. Antoine a, Brett M. Mifsud d a
Department of Forestry and Wildland Resources, Humboldt State University, Arcata, CA 95521, USA Department of Biological Sciences and the Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ 86011, USA c Department of Integrative Biology, University of California, Berkeley, CA 94720, USA d 7 Colston Avenue, Sherbrooke, Victoria 3789, Australia b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 October 2009 Received in revised form 1 December 2009 Accepted 6 December 2009
How long forest trees can sustain wood production with increasing age remains an open question, primarily because whole-crown structure and growth cannot be readily measured from the ground or on felled trees. We climbed and directly measured crown structures and growth rates of 43 un-suppressed individuals (site trees) of the two tallest species – Eucalyptus regnans and Sequoia sempervirens – representing a wide range of tree sizes and ages. In both species, ground-level measurements of annual growth, including height, ring width, and basal area increment, exhibited the oft-reported trend of decreasing growth (or no change in growth) with age, yet wood production of the entire main trunk and whole crown both increased with size and age up to and including the largest and oldest trees we measured. The balance between structural metrics of whole-crown respiratory demands (cambium area, inner bark volume, sapwood volume, and heartwood deposition area) and photosynthetic capacity (leaf area and green bark area) was statistically independent of size but not age. After accounting for the effect of size, trees with lower potential respiratory demands grew more than trees with higher potential respiratory demands per unit photosynthetic area. The strongest determinant of tree energy balance was the ratio of aboveground cambium area to leaf area. Among the site trees we examined, over 85% of the variation in annual wood production was explained by variation in size, and the proportion of total aboveground wood production in appendages (branches, limbs, and reiterated trunks) increased linearly with size. With increasing age in both species, the proportion of annual wood production converted to heartwood increased in main trunks and appendages. The oldest tree we measured produced more heartwood in its main trunk over 651 years (351 m3) than contained in any tree we measured 90 m height, >350 m3 wood volume) and age (>400 years) despite fundamental biological differences. The tallest species from each phylum, Eucalyptus regnans and Sequoia sempervirens, contrast in leaf morphology (large broad-leaved vs. small scale-like), physiology (light-demanding vs. shade-tolerant), rate of sexual reproduction (copious annual vs. sporadic decadal), wood anatomy (vessels vs. tracheids), and traits associated with fire and decay resistance (low vs. extremely high). Restricted to southeastern Australia, E. regnans is one of the fastest-growing species. Trees established after a standreplacing fire in 1939, for example, were up to 82 m tall in 2007 (pers. obs.). The greatest height documented for E. regnans (114.3 m, Mifsud, 2003) is similar to that of S. sempervirens (115.6 m, Sillett, unpublished), a species restricted to a narrow coastal strip in western North America. The typically even-age structure of E. regnans forests arises from synchronous seed germination following stand-replacing fire (Ashton, 2000), although in parts of its range lower intensity fires do occur, resulting in multi-aged E. regnans forests (Lindenmayer et al., 2000). In contrast, old-growth S. sempervirens forests typically have an all-age structure, because large trees survive fire and regeneration occurs continuously from vegetative sprouting as well as periodically from seeds (Lorimer et al., 2009). We compared the two tallest species to evaluate how biophysical and ecological factors interact to determine growth rates of trees as they approach maximum size. To overcome limitations of groundbased measurements, we climbed trees during consecutive years and conducted a complete inventory of crown elements by direct measurement. The resulting three-dimensional maps of aboveground structure allowed quantification of whole-tree biomass and annual growth. In addition, these measurements permitted verification of tree age and assessment of tree energy balance. Our study had the following specific objectives: (1) to derive accurate whole-tree estimates of aboveground structural attributes – leaves, bark, cambium, sapwood, and heartwood – for standing trees across a wide range of sizes and ages, (2) to determine how annual growth changes with increasing tree size and age, and (3) to assess when and if size- or age-related effects constrain wood production during a tree’s lifetime. 2. Methods The two sites chosen for this study had the tallest forest canopy known for each phylum (Fig. 1). The tallest angiosperm forest
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Fig. 1. Ground-level views of the two tallest forests. On left, monodominant Eucalyptus regnans forest in Kinglake National Park, Victoria – established after fire in 1707 – was tallest forest in Australia until stand-replacing fire in February 2009. On right, monodominant Sequoia sempervirens forest in Humboldt Redwoods State Park, California harbors >70% of known trees over 107 m tall.
(containing at least 25 trees >85 m) is located along Wallaby Creek on the Hume Plateau, Kinglake National Park, Victoria, Australia (378S, 1458E, 450–500 m elevation). The tallest conifer forest (containing >70% of living trees >107 m), grows on the alluvial terraces of Bull Creek, a tributary of the Eel River, in Humboldt Redwoods State Park, California, USA (408N, 1248W, 45–65 m elevation). Both forests are overwhelmingly dominated by a single species with climates characterized by cool wet winters and warm dry summers (Supplementary Appendix A). During summer, both forests receive moisture inputs from fog (Ashton, 2000; Dawson, 1998), but the E. regnans forest is occasionally desiccated by strong winds originating in the continent’s hot, arid interior—conditions conducive to tree-killing fires (McCarthy et al., 1999). In 2005, 15 primary study trees were selected from each forest—5 individuals from each of 3 height classes. The tallest height class included trees within 5% of the maximum height known for each species. The shorter height classes were chosen so that they were 4/5 and 2/3 as tall as the tallest height class. Portions of the E. regnans forest on the Hume Plateau had been burned at various times since European settlement, enabling us to find several cohorts of shorter height classes. In the all-aged S. sempervirens forest, horizontal variability caused by disturbances (i.e., treefalls, fire, and flooding) allowed us to locate shorter trees not overtopped by neighbors. For consistency, primary study trees were chosen so that their tops were un-shaded; all trees were canopy dominants or co-dominants. We avoided individuals growing near recently created gaps so that all trees likely had well-illuminated tops during much of the 20th century. Seven additional E. regnans trees, which were structurally mapped for a previous study (Van Pelt et al., 2004), were included in some analyses, including co-dominant, intermediate, and suppressed individuals. Six additional S. sempervirens trees, including five shorter trees (1/3 as tall as the tallest height class) and the tree with the largest main trunk in the forest, were selected in 2006 for inclusion in some analyses. These trees were all canopy dominants or co-dominants. The following subsections describe our field,
laboratory, and computer methods. A flowchart organizes and summarizes this information (Fig. 2). 2.1. Field 2.1.1. Whole-tree inventory Trees were rigged and climbed using rope techniques. A fiberglass measuring tape was secured near the treetop and lowered to average ground level to provide a height reference for all measurements. Tags were attached to the bark at 5 m intervals to aid in height determinations once the tape was removed. Main trunk diameters were measured at each tag and any other areas where rapid diameter changes or other anomalies occurred. Because E. regnans bark exfoliates each year above the dense understory layer to reveal smooth photosynthetic bark, the uppermost and lowermost heights of the smooth/rough bark transition were recorded for each tree to improve bark surface area calculations. The color of E. regnans smooth bark changes depending on its degree of hydration such that when wet it appears quite green, but this fades to a whitish color after drying. Just beneath the pale surface of dry smooth bark, however, is a rich green color indicating the presence of chlorophyll. To emphasize its photosynthetic capacity, we refer to the smooth bark of E. regnans as ‘green bark’ and the rough, non-photosynthetic bark of both species as ‘brown bark’ hereafter. Non-round regions of the lower trunk can extend up to 20 m above the ground. To avoid overestimations in bark, sapwood, and heartwood volumes as well as to facilitate accurate bark and cambium area calculations, detailed cross-sections of the nonround portions of each tree were prepared using a laser-based triangulation procedure. Three sturdy poles (or existing saplings) were used to create a triangular frame on which measuring tapes were horizontally stretched. The azimuth of each leg of the triangle was recorded to the nearest degree. The tape became the X value in a Cartesian mapping grid of the tree base. The Y value was the distance to the tree. Accuracy was maintained by setting a compass
S.C. Sillett et al. / Forest Ecology and Management 259 (2010) 976–994 Fig. 2. Flow chart summarizing field, laboratory, and computer methods. EURE: Eucalyptus regnans, SESE: Sequoia sempervirens. Black circles with abbreviations (e.g., SM) correspond to top categories (e.g., Structural Measurements) within computer methods. Hierarchical subsampling and whole-tree summations highlighted in black and gray boxes, respectively.
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to the tape azimuth +908 and using a survey laser (Impulse 200LR, Laser Technology Inc.) to measure horizontal distance to tree. The laser’s filter mode in combination with a reflector ensured that no spurious distances were measured. Within each third of trunk circumference, all major concavities and convexities were measured relative to the tape. This procedure was repeated for each of the three tape stretches. Between three and six heights were measured in this way, including at ground level and as high as possible (often with the filter mounted to a pole and held high). The corners of the triangle were the common element between measured portions and provided the basis for linking the thirds of trunk circumference to form a closed, complex polygon. In the few cases where additional cross-sections were needed at points even higher up the trunk, geometric interpolation was used to estimate these. Branches were defined as model-conforming units 4 cm diameter, where the basal portion could be reached by a climber. All portions of appendages proximal to these were called segments and were subdivided so that each segment consisted of a straight, non-bifurcating piece. The diameter, height, distance from main trunk center as well as azimuth from main trunk center were measured at each end of all segments, which were termed nodes. Each node was named, so that a segment name was based on the two nodes defining it. This permitted efficient construction of a three-dimensional framework with minimal accumulation of errors. Branches were uniquely numbered, and the basal diameter and node of origin were recorded. Total horizontal extension was measured using a survey laser or steel tape, and the overall branch azimuth was recorded to the nearest degree with a compass. Methods of foliage quantification varied between the two species. For E. regnans, additional branch measurements were vertical extension, total path length (i.e., linear distance along branch) of all portions 4 cm diameter, and number of foliar units (see below). Path length was divided into live and dead regions. For S. sempervirens, additional branch measurements were percentage foliated (5%) and overall slope recorded in 58 increments. In both species, branches
