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Even-aged forest management is geared towards timber production with ecosystem health as a lesser consideration. This creates a dichotomy where forests are treated either as plantations or reserves. Uneven-aged management can bring compromise to conflicting land uses by reducing ecosystem impacts while still allowing timber extraction. Whereas selection forestry focuses on which trees are taken, retention forestry focuses on protecting features that will remain after logging. These biological legacies provide ecosystem continuity.

Retained trees are often chosen based on their habitat value. Snags and living trees that are diseased, damaged, or dying provide cavities, decaying wood, and other microhabitats for a diversity of biota. Defects that make high-quality habitat trees tend to cause the collapse of large and old trees, so it's important to designate healthy recruitment trees for the future. Retention forestry that focuses only on habitat trees may be inconsistent with the goals of long-term carbon storage and ecosystem resilience.

An just published in Forest Ecology and Management explores the idea of "exceptional trees" and why we might consider choosing a subset of the most robust trees for permanent retention in managed forests. We present methods for precisely estimating aboveground biomass across the landscape and assess the contribution of exceptional trees to biomass and productivity. Our study focuses on Sequoia sempervirens (redwood) in California's Demonstration State Forests.

In each of the 20 locations, we found an exceptional redwood in a dominant canopy position with a broad crown and unusually large branches. Exceptional trees were paired with nearby trees that were co-dominant with smaller branches more typical of the stand. We climbed and core-sampled each tree at multiple heights such that growth histories could be reconstructed. All neighboring trees within a 20-meter radius were measured from the ground for trunk diameter and crown size.

The same areas were also scanned with airborne lidar to quantify tree-level characteristics. New algorithms were developed to segment the canopy into "tree approximate objects" (TAO), corresponding to individual trees plus subordinate vegetation. Segmenting lidar data into TAO makes it possible to identify exceptional crowns, defined as TAO larger than the 95th percentile. Linking lidar data to intensively measured trees allowed us to develop allometric equations for the estimation of forest biomass.

What did we learn? These forests have a higher density of smaller trees, less biomass, and lower productivity compared to other tall redwood forests we've surveyed using the same methods. Most exceptional trees were decades to centuries older than their paired typical trees. These trees survived the logging of taller neighbors, and their rings indicated growth releases following logging. Exceptional trees grow faster than typical trees.

Exceptional trees are concentrated in topographic concavities and close to creeks. This is partly a biological consequence of water being an important limit to tree height, but also a reflection of restrictions on timber operations within Watercourse and Lake Protection Zones (WLPZ) as required by California's Forest Practice Rules. These protective measures ensure that some large trees are left standing after logging, resulting in WLPZ serving as corridors of late successional riparian habitat. Clustering of exceptional trees in WLPZ has consequences for aboveground biomass distribution across the landscape.

Two-thirds of the surveyed hectares have very few, if any, exceptional trees, and biomass is less than 10% of primary redwood forests. Our recent rangewide analysis of tall redwood forests showed that only the range margins have such low biomass. We know from historical records that much larger and older trees once grew on state forest lands, so the scarcity of high-biomass hectares outside the WLPZ is sobering.

If long-term carbon storage is a management priority, then silviculture to restore large trees beyond the WLPZ should be considered. As redwoods enlarge with age, they make more biomass, and an ever-increasing proportion is decay-resistant heartwood that locks up carbon for centuries. Increasing the number of large, fast-growing trees across the landscape can increase carbon storage even if only a few such trees are retained per hectare.

Beyond carbon storage, a network of exceptional trees can promote habitat connectivity across managed forests. Biodiversity in secondary redwood forests is generally depauperate, but even in the most glorious primary forests, relatively few exceptional trees host the bulk of biodiversity because large appendages—branches, limbs, reiterated trunks—are otherwise in short supply. When trees have well illuminated crowns, the upper branches grow fast and are more likely to persist throughout the life of the tree and become ecologically significant.

Permanent retention of some exceptional trees isn't mutually exclusive with timber production. Even centuries-old redwoods experience competitive release through selective logging of neighbors. Of course, any silviculture approach that uses growing space for biological legacies presents a challenge; the opportunity cost of foregone timber must be balanced against benefits to ecosystems and society. Incentives rewarding landowners for indefinite retention of some trees are an obvious path forward.

This idea has particular appeal for redwoods because their extreme resistance to decay, fire, and herbivory allows individual trees to live over 2000 years and exceed 100 meters tall. Over 95% of redwood forests have been logged and converted to lower biomass systems across the species' range.

Designating exceptional trees in addition to protecting habitat trees in secondary forests will ensure that future forests have giant trees. These chosen trees will sequester carbon and increase ecosystem resilience for centuries, eventually becoming habitat trees themselves. Furthermore, exceptional redwoods are best equipped to resist losses of carbon and vegetative type conversion from fire. The larger the trees, the more resilient the forest.

Live tree retention isn't a new idea, but permanent protection for a subset of robust, undamaged trees is a twist on retention forestry well-suited to all forests where tree longevity exceeds rotation ages used in timber extraction. While managed forests can't recreate primary forests, they can nonetheless become healthy ecosystems. The methods we present provide a framework for how to identify exceptional trees. A combination of lidar scanning and boots-on-the-ground verification is essential for deciding which trees to retain indefinitely. Setting exceptional trees aside now is a profoundly hopeful action for the future.

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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 over 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: Russell Kramer, Bryan Fuentes, and Allyson Carroll.