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As trees grow larger, their heartwood proportion increases and eventually accounts for most of the biomass. Heartwood decay resistance depends on the type and amount of protective chemicals (extractives) deposited therein. Rates of heartwood accumulation and extractive content are major determinants of tree longevity and a forest's capacity for long-term carbon storage.

Heartwood development in the tallest tree species—Sequoia sempervirens (redwood)—is of particular interest because primary (old-growth) redwood forests hold global maximum biomass, and most of their aboveground carbon is stored in heartwood. Compared to primary forests, redwoods in secondary (young-growth) forests often make heartwood with lower extractive content and inferior decay resistance. Consequently, the long-term carbon storage potential of secondary redwood forests isn't equivalent to that of primary forests, but is this true throughout redwood's 460-mile-long range?

A new article in Forest Ecology and Management explores heartwood development in secondary redwood forests distributed across 18 locations representing three distinct regions. The "northern" region has extensive redwood-dominated forests and a cool, wet, foggy climate; the "central" region has forests fragmented by development and a cool but somewhat less foggy climate; and the "marginal" region covers southern and eastern range limits where isolated groves experience warmer, drier, and less foggy conditions. We sampled 77 dominant trees that were 96–202 years old. Trees were climbed and core-sampled at multiple heights to quantify growth increments and heartwood development.

Relative concentrations of heartwood extractives were estimated by comparing densities in well-replicated paired samples of sapwood and heartwood. We found clear northern versus marginal differences with central trees intermediate. Northern trees have the lowest wood densities, largest wood volume increments, fastest heartwood biomass accumulation, and lowest heartwood extractive content. Marginal trees are the opposite extreme, with highest wood densities, smallest wood volume increments, slowest heartwood biomass accumulation, and highest heartwood extractive content.

While overall growth efficiency (biomass made annually per unit leaf mass) is highest in northern trees and lowest in marginal trees, extractive production shows the opposite pattern. This is a trade-off, where investment in heartwood protection occurs at the expense of sapwood production. Marginal trees deposit twice as many extractives into their heartwood as central trees of the same age and five times more than northern trees. Why?

This is likely a climate effect. Warmer and drier conditions prevent trees from replenishing water supplies overnight, leaving insufficient turgor pressure for the cambium to make new sapwood cells. Heartwood extractives may be an alternative sink for photosynthetic sugars. Marginal redwoods are more climatically limited in size, but their heartwood is more enriched and durable from an earlier age. In marginal sites, heartwood extractive content is similar in trees of secondary and primary forests. This is in stark contrast to northern and central sites, where secondary forest trees have much lower extractive-enrichment than those in primary forests. Tree height enters the story here.

The taller trees get, the more leaves they have for photosynthesis, but greater height means ever-increasing biophysical challenges. Gravitational limits on water availability can constrain sapwood production in the tops of tall trees with heartwood extractives being an alternative sugar sink as with the aforementioned climate effect.

As a result, taller redwoods in northern and central primary forests have higher heartwood extractive content than shorter redwoods in nearby secondary forests. Extractive contents increase with height above ground in all regions, but marginal trunks have consistently more than northern and central trunks. In marginal forests, the height effect is overridden by climate such that even short trees have high heartwood enrichment.

What does this all mean from a practical standpoint? While redwoods' upper trunks generally have the highest heartwood density and extractive content, this is a small part of a tree. Most heartwood is in the trunk below the branches. The dense, well-protected "crown wood" has little practical timber value due to small quantities and many branch knots, but its ecological importance warrants consideration in forest management designed to promote non-timber values.

The higher density and extractive content of marginal site heartwood should make it far more durable than an equivalent mass produced in central or northern sites, and thus ideal for long-term carbon storage even in regenerating forests. In recent years, marginal redwoods actually produced more extractives annually than northern and central redwoods, despite having smaller heartwood increments overall. In fire-prone marginal sites, denser wood and enhanced extractive content contribute to the persistence of redwood-dominated vegetation. Despite a sparse distribution in steep terrain, marginal redwood forests retain a higher capacity for long-term carbon storage than any other vegetation in their vicinity.

Ironically, in more productive parts of the redwood range, secondary forests may be less effective at long-term carbon storage than those in marginal sites. In the cool and foggy northern range, heartwood decay resistance is much lower in short secondary forests than in tall primary forests. Silviculture to promote rapid height growth may be the best way to improve heartwood enrichment in young redwoods of regenerating forests in the core range.

Mature secondary redwood forests regenerating from pre-chainsaw era logging (1820s–1920s) are even more rare than primary forests because nearly all these forests have been logged again at least once. Treating redwood as a short-term rotation crop squanders the potential of a species that can reach heights over 380 feet and live beyond 2300 years. Shifting silvicultural practices to retain some of the tallest trees in perpetuity can maximize non-timber values like long-term carbon storage and biodiversity provisioning.

Redwood possesses a unique combination of attributes in addition to decay-resistant heartwood, including prolific clonal reproduction, high phenotypic plasticity of foliage that maximizes photosynthesis and foliar water uptake, thick resin-free bark that confers great fire resistance, and the ability to mobilize centuries-old dormant buds with decades-old carbon reserves for rapid refoliation after severe fire. These adaptive traits bestow extreme persistence on individual trees and grant redwood forests tremendous resilience, somewhat alleviating concerns about conversion to other vegetation types after disturbances or in response to a warming climate.

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Marie Antoine and Steve Sillett have dedicated their scientific careers to climbing, measuring, and understanding the world’s tallest and largest trees. Both are based in the Department of Forestry, Fire, and Rangeland Management at Cal Poly Humboldt in Arcata, California, U.S.A. Steve is the Kenneth L. Fisher Chair in Redwood Forest Ecology, and author or co-author of more than 60 scientific articles. Since 2001, Steve and Marie have climbed more than a thousand trees together. Their work has been featured in National Geographic Magazine (coast redwood, October 2009; giant sequoia, December 2012), and their early adventures in forest canopy exploration were documented in Richard Preston’s "The Wild Trees" (2007). Three co-authors were integral to this current publication: Alana Chin, Allyson Carroll, and Mark Graham.