Biomass and growth potential of Eucalyptus regnans up to 100 m tall
Introduction
A common approach to estimating tree biomass in forests involves measuring trunk diameter at breast height (DBH), applying species-specific allometric equations to predict each tree’s aboveground biomass as a function of DBH, and summing these values for all trees in a plot (e.g., Xu et al., 2012). Repeated measurements of DBH can then be used to calculate growth increments as biomass differences between time steps. This approach, which has been used in many forests worldwide, suggests that tree-level rates of biomass accumulation increase continuously with tree size (Stephenson et al., 2014). Two underlying assumptions of this approach, however, make its use problematic in old-growth forests. First, allometric equations are derived from sampling trees spanning a full range of sizes, including individuals near the maximum known for the species. Second, it is possible to measure accurately changes in DBH from year to year. Both assumptions are frequently violated, because large trees are difficult to measure.
Aboveground attributes of large trees require intensive effort to quantify with reasonable accuracy, leaving a dearth of robust allometric equations for many species. Lower trunks of large trees are often heavily buttressed and covered with unstable bark or epiphytes such that any measurable change in DBH from year to year may be a poor predictor of biomass growth increment. In at least two cases where sufficient effort has been expended to develop equations suitable for species in old-growth forests, measurements besides DBH (e.g., crown volume) provide stronger prediction of biomass and other aboveground attributes (Sillett et al., 2015). Overcoming these challenges is becoming increasingly important given the carbon sequestration capacity of old-growth forests (Luyssaert et al., 2008) and the global decline in large trees (Lindenmayer et al., 2012a).
Among giant species, Eucalyptus regnans is perhaps the fastest growing, with some trees <70 years old being >80 m tall (Sillett et al., 2010). The extreme growth rate of E. regnans stems from an evolutionary history shaped by infrequent but high-intensity fires, strong competition with regenerating vegetation after fire, and the need to raise seed capsules above the lethal zone before the next fire (Tng et al., 2012). Events on 7 February 2009 dramatically illustrate this aspect of E. regnans ecology. The so-called Black Saturday fire, which burned nearly 100,000 ha in 12 h (Cruz et al., 2012), killed every tree in our 270 × 27 m research plot in Kinglake National Park (Fig. 1). On visits to the plot 2, 34, and 58 months after the fire, we measured E. regnans densities of 39.9, 16.2, and 1.8 individuals per m2 and maximum heights of <0.1, 6.5, and 12.3 m, respectively (S.C. Sillett and R. Van Pelt, unpublished data). Such rapid growth, which is promoted by very high area- and mass-based rates of photosynthesis as well as annual shedding of outer bark that exposes photosynthetically active inner bark (Sillett et al., 2010), likely occurs at the expense of fire- and decay-resistance (Loehle, 1988). Thus, maximum longevity of E. regnans is considerably less than some other giant species (Wood et al., 2010, Sillett et al., in press).
Prior to Black Saturday, the 302-year-old stand of E. regnans in Kinglake National Park was Australia’s tallest forest with live-topped trees up to 92.4 m tall, but several substantially taller individual trees as well as many much larger (by volume) and older are known from other locations in Victoria and Tasmania (Hickey et al., 2000, Herrmann, 2006, Wood et al., 2010, Tng et al., 2012). Starting in the 19th century, the species was planted widely outside Australia, and E. regnans >60 m tall are known from New Zealand (Burstall and Sale, 1984). Whether the biomass growth increments we measured in large trees of Kinglake National Park are exceeded in even larger trees remains unknown.
In our previous study, we intensively measured the entire aboveground portion of 22 E. regnans >60 m tall to quantify leaves, bark, cambium, sapwood, heartwood, and biomass, and we re-measured 15 of these trees in 2006 to calculate growth increments (Sillett et al., 2010). The primary objective of our 2010 study was to compare tree-level performance of E. regnans and Sequoia sempervirens through old age, not to provide allometric equations for either species. Nevertheless, data from the 22 trees we studied were used to develop allometric equations for old-growth E. regnans forests (Keith et al., 2014) and included in an allometry database for woody plants (Falster et al., 2015). Here we extend E. regnans sampling by measuring five of the largest living trees in Tasmania, Victoria, and New Zealand. We have three main objectives: (1) to quantify tree-level productivity of the largest and oldest living E. regnans, (2) to develop allometric equations for E. regnans >60 m tall using the expanded dataset of 27 trees, and (3) to demonstrate the ability of these equations to predict biomass growth increments and to quantify aboveground attributes of large trees via ground-based measurements.
Section snippets
Tree selection
The trees we selected for detailed study added important geographic and size variation to our existing dataset, which included trees 61.1–92.4 m tall and 80–299 years old (i.e., 1926, 1851, and 1707 fire cohorts) at one location in Victoria. We avoided excessively leaning trees and those with highly irregular, fragmented crowns. The 86.9-m-tall tree (23) in Kurth Kiln Regional Park, near Melbourne, was one of the tallest known live-topped E. regnans in Victoria (at least one dead tree at Wallaby
Tree structure and age
Intensive direct measurements enabled us to quantify tree structure with minimal accumulation of errors (tree-level SE 1–10%, Table 1). These measurements revealed extreme structural variation among tall E. regnans (Fig. 2). The largest tree (27), which had a complex and highly reiterated crown, was nearly twice as massive as the tallest tree (26) and nearly three times as massive as the largest tree previously studied (19). When expressed as a percentage of total height, the three Tasmanian
Contrasting crown structures
Relatively deep crowns of the three Tasmanian trees (25, 26, 27) contrast with those of the other 24 trees. We suspect this difference is attributable, in part, to the fragmented nature of the old-growth forests in which they live. All three trees stand near enough to margins of previously clearcut forest that their crowns may have deepened in response to elevated light availability associated with the edge environment (Chen et al., 1993). Large-diameter appendages in lower crowns of trees 26
Acknowledgments
This research was supported by John Montague, the endowment creating the Kenneth L. Fisher Chair of Redwood Forest Ecology at Humboldt State University, a Grant from the National Science Foundation (IOB-0445277), and the Save the Redwoods League. We thank Marie Antoine, Jim Campbell-Spickler, Tom Greenwood, Brett Mifsud, Rikke Naesborg, and Cameron Williams for help with tree climbing. Brett Mifsud, Walter Herrmann, and Christopher Dean shared photographs of tree 28. Forestry Tasmania, Kinglake
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2021, Forest Ecology and ManagementCitation Excerpt :Even though heartwood proportions of aboveground biomass increase with tree age across species (Table 7), a relatively high heartwood increment (~6 Mg ha−1 yr−1) becomes increasingly unrealistic by the end of the Picea simulation, as heartwood decay is extensive in trunks of rainforest Picea > 200 yr old (Kimmey, 1956; Hennon, 1995; Kramer et al., 2018). Mass losses to decay may negate heartwood increments of tall Picea as suspected in tall E. regnans, whose heartwood is also poorly defended against fungi (Sillett et al., 2015a). In primary forest, initial tree ages for Sequoia are 2–3 centuries older than Picea and Pseudotsuga with aboveground biomass and biomass increment 0–25 Mg and 151–281 kg yr−1 higher, respectively, per tree for Sequoia (Fig. 14a-c).