Stephen C Sillett

Cal Poly Humboldt · Forestry, Fire, and Rangeland Management

Heavily exploited for its reddish, decay-resistant heartwood, the tallest conifer, Sequoia sempervirens, is a major component of coastal forests from extreme southwestern Oregon to California’s Santa Lucia Mountains. Primary Sequoia forests are now restricted to < 5 % of their former distribution, and mature secondary forests with trees over 60 m t...

Tracheid buckling may protect leaves in the dynamic environments of forest canopies, where rapid intensifications of evaporative demand, such as those brought on by changes in light availability, can result in sudden increases in transpiration rate. While treetop leaves function in reliably direct light, leaves below the upper crown must tolerate r...

Premise of the study: Trees in wet forests often have features that prevent water films from covering stomata and inhibiting gas exchange, while many trees in drier environments use foliar water uptake to reduce water stress. In forests with both wet and dry seasons, evergreen trees would benefit from producing leaves capable of balancing rainy-se...

The tallest conifers—Picea sitchensis, Pseudotsuga menziesii, Sequoia sempervirens, Sequoiadendron giganteum—are widely distributed in western North America, forming forests > 90 m tall with aboveground biomass ≥ 2000 Mg ha⁻¹. Here we combine intensive measurements of 169 trees with dendrochronology and allometry to examine tree and stand developme...

ContextWestern Olympic valley bottoms, disturbed by alluvial processes, are dominated by Picea sitchensis and isolated cohorts of Pseudotsuga menziesii, while upland contexts, disturbed by wind and fire, are dominated by P. menziesii. These forests have distinct structure and produce large trees with habitat for endangered birds.Objectives Describe...

Mature second-growth coast redwood (Sequoia sempervirens) forests—logged over 100 yr ago—are an important resource in the redwood region, but development of regenerating forests beyond rotation age (~50 yr) is not well understood. Continuous long-term data are especially lacking, considering that the maximum possible age of second-growth stands is...

The tallest species, Sequoia sempervirens, inhabits old-growth forests with global maximum biomass and leaf area. Here we determine if these forests also have maximum productivity. Intensive measurements of 114 trees 18–116 m tall and 115–2340 yr old were used to improve allometric equations for Sequoia. Applying the best available allometry to all...

The largest tree species, Sequoiadendron giganteum has a small native range restricted to California’s Sierra Nevada. Awe-inspiring stature contributed to its protection from logging, but anthropogenic climate change—particularly hotter drought—and over a century of fire suppression are possible threats. We measured 60 trees in seven forests to imp...

Large trees are critically important for structuring ecosystems and providing habitat, and trees with complex crowns provide more of these services than comparably sized trees with simple crowns. Forest managers are increasingly emulating old-growth structure by retaining various densities of aggregated and dispersed trees. This study explores how...

Mature second-growth forests dominated by Sequoia sempervirens occupy only two percent of the species’ current distribution yet represent an important benchmark for restoration management. Here we develop new allometric equations for these forests based on 44 trees 23–84 m tall, which can be used to estimate leaf, bark, cambium, sapwood, and heartw...

PREMISE OF THE STUDY: Leaves are the sites of greatest water stress in trees and a key means of acclimation to the environment. We considered phenotypic plasticity of Pseudotsuga menziesii leaves in their ecological context, exploring responsiveness to natural gradients in water stress (indicated by sample height) and light availability (measured f...

One of the five tallest tree species, Pseudotsuga menziesii has enormous economic and ecological importance, but rainforests dominated by this species are not as well understood as their drier montane counterparts. We climbed and measured 30 trees up to 97 m tall growing in coastal forests of the Olympic Peninsula and northern California to quantif...

Tree biomass is one of the most important variables for studying and managing forest ecosystems. With emphasis shifting from young forests grown for timber production to forests with old-growth characteristics, the need to quantify various components of individual trees in natural settings is increasing. Destructive methods are inherently limited b...

Data on leaf cross-sectional anatomy and morphology collected across the height gradient in 15 trees over 90 m tall along with light availability and d13C data for each sample. Samples represent 5 trees each of Sequoia sempervirens, Picea sitchensis, and Pesudostuga menziesii.

Dendrochronological studies of large and old Sequoia sempervirens are limited by access and complex crossdating, but core sampling at regular height intervals along the main trunks of five standing trees allowed for reconstruction of growth, height, and age while providing within-tree replication for crossdating. We developed a crossdated ring-widt...

In old-growth Sequoia sempervirens forests, reiterated trunks and limbs provide required habitat elements for specialized arboreal species, including an endangered seabird, Brachyramphus marmoratus. The oldest second-growth redwood forests—established after 19th century logging—lack species dependent on complex structure, presumably because redwood...

Fires that burn through forests cause changes in wood anatomy and growth that can be used to reconstruct fire histories. Fire is important in Sequoia sempervirens (D. Don) Endl. (coast redwood) forests, but fire histories are limited due to difficulties crossdating annual rings of this species. Here we investigated three fires (1985, 1999, 2008) in...

Leaf-level anatomical variation is readily apparent within tall tree crowns, yet the relative importance of water and light availability in controlling this variation remains unclear. Sitka spruce (Picea sitchensis, (Bong.) Carr.) thrives in temperate rainforests of the Pacific Northwest, where it has historically reached heights >100 m, despite ra...

Large trees are the most prominent structural features of old-growth forests, which are considered to be globally important carbon sinks. Because of their large size, estimates of biomass and growth of large trees are often based on ground-level measurements (e.g., diameter at breast height, DBH) and little is known about growth dynamics within the...

Crossdated tree-ring chronologies for the Arcata Community Forest (ACF) and Muir Woods National Monument (Muir Woods) expand the spatial coverage of dated coast redwood (Sequoia sempervirens (D. Don) Endl.) series. Crossdating relies on the common pattern of ring-width variation among tree populations, and dated chronologies have many applications,...

Forests >80 m tall have the highest biomass, and individual trees in these forests are Earth’s largest with deep crowns emerging above neighboring vegetation, but it is unclear to what degree these maxima depend on the emergent trees themselves or a broader-scale forest structure. Here we advance the concept of _emergent facilitation_, whereby emer...

PREMISE OF THE STUDY: Leaves respond to environmental signals and acclimate to local conditions until their ecological limits are reached. Understanding the relationships between anatomical variation in leaves and the availability of water and light improves our ability to predict ecosystem-level impacts of foliar response to climate change, as it...

Tree-level productivity is largely determined by crown size and aboveground vigor, but light availability ultimately controls growth. Competition indices are typically used in modeling instead of actual measurements of light. Our goals were to determine which measure of light best predicts trunk growth increments of Sequoia sempervirens, to quantif...

Predicting tree biomass and growth increments via allometric equations is routine in forestry, but this approach is problematic in old-growth forests unless equations are derived from trees spanning the full size range. Using intensive measurements of 27 standing Eucalyptus regnans trees 61.1–99.8 m tall and 80–430 years old in Tasmania, Victoria,...

Understanding how plants are constructed—i.e., how key size dimensions and the amount of mass invested in different tissues varies among individuals—is essential for modeling plant growth, carbon stocks, and energy fluxes in the terrestrial biosphere. Allocation patterns can differ through ontogeny, but also among coexisting species and among speci...

Understanding how plants are constructed—i.e., how key size dimensions and the amount of mass invested in different tissues varies among individuals—is essential for modeling plant growth, carbon stocks, and energy fluxes in the terrestrial biosphere. Allocation patterns can differ through ontogeny, but also among coexisting species and among speci...

As the only species exceeding 90 m in height and 2000 years of age, Sequoia sempervirens and Sequoiadendron giganteum provide the optimal platform upon which to examine interactions among tree structure, age, and growth. We climbed 140 trees in oldgrowth redwood forests across California, USA, spanning a broad range of sizes and including the talle...

Structural and physiological changes that occur as trees grow taller are associated with increased hydraulic constraints on leaf gas exchange, yet it is unclear if leaf-level constraints influence whole-tree growth as trees approach their maximum size. We examined variation in leaf physiology, leaf area to sapwood area ratio (L/S), and annual above...

Extremely decay-resistant wood and fire-resistant bark allow California's redwoods to accumulate millennia of annual growth rings that can be useful in biological research. Whereas tree rings of Sequoiadendron giganteum (SEGI) helped formalize the study of dendrochronology and the principle of crossdating, those of Sequoia sempervirens (SESE) have...

We investigated how bats use habitat structure along the vertical gradient of an old-growth Sequoia sempervirens (redwood) forest. Ground-based detection methods would underrepresent bats that use the canopy and above-canopy airspace in this forest as they reach far beyond practical netting and acoustic detection range. We equipped two tall trees w...

The constraint on vertical water transport is considered an important factor limiting height growth and maximum attainable height of trees. Here we show evidence of foliar water storage as a mechanism that could partially compensate for this constraint in Sequoia sempervirens, the tallest species.We measured hydraulic and morpho-anatomical characte...

Redwood branches provide all the carbohydrates for the most carbon-heavy forests on Earth, and recent whole-tree measurements have quantified trunk growth rates associated with complete branch inventories. Providing all of a tree's photosynthetic capacity, branches represent an increasing proportion of total aboveground wood production as trees enl...

Background/Question/Methods As part of a larger study addressing the response of Sequoia sempervirens to changing environmental conditions throughout its range, three 1-hectare plots (plot dimensions were 10:1, 316.23 m x 31.623 m) were installed in upland portions of old-growth rain forests at the northern end of the species range. In these rain...

Background/Question/Methods Giant sequoia (Sequoiadendron giganteum) are the largest and among the oldest living organisms on Earth, with many individuals possessing massive and complex crowns. Spatial and temporal heterogeneity in climatic factors interact with changes in xylem water potential and tree structure throughout individual crowns to d...

Background/Question/Methods Among unsuppressed trees in the world’s tallest forest (i.e., along Bull Creek in Humboldt Redwoods State Park, California), the annual rate of aboveground growth increases with tree size through old age such that the tallest and largest trees produce the most wood annually. Here we extend whole-crown measurements of S...

Alternatives to clear-cutting are being implemented to increase biodiversity of managed forests in the Pacific Northwest. Lichens are an integral component of old growth, but lichen biomass develops slowly in forests. We evaluated the long-term potential of live tree retention for lichen conservation in Douglas-fir (Pseudotsuga menziesii (Mirb.) Fr...

Treetops become increasingly constrained by gravity-induced water stress as they approach maximum height. Here we examine the effects of height on seasonal and diurnal sap flow dynamics at the tops of 12 unsuppressed Sequoia sempervirens (D. Don) Endl. (coast redwood) trees 68-113 m tall during one growing season. Average treetop sap velocity (V(S)...

Unlabelled: Premise of the study: Leaves at the tops of most trees are smaller, thicker, and in many other ways different from leaves on the lowermost branches. This height-related variation in leaf structure has been explained as acclimation to differing light environments and, alternatively, as a consequence of hydrostatic, gravitational const...

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 –...

Organic soils up to I in thick cover tree surfaces within the canopies of old-growth redwood [Sequoia sempervirens (D. Don) Endl] forests 50 in or more above the ground. Very little is known about litter quality and litter decomposition processes within these canopies. A combination of solid-state cross-polarization magic-angle spinning (13)C nucle...

Background/Question/Methods While the importance of old trees and ancient forests as reservoirs of carbon is well established, their capacity to respond to changing climate and rising atmospheric carbon dioxide is poorly understood. We used a combined dendrochronological and tree mapping approach to reconstruct whole-tree wood production for the p...

This study examined relationships between foliar morphology and gas exchange characteristics as they vary with height within and among crowns of Sequoia sempervirens D. Don trees ranging from 29 to 113 m in height. Shoot mass:area (SMA) ratio increased with height and was less responsive to changes in light availability as height increased, suggest...

We examined changes in branch hydraulic, leaf structure and gas exchange properties in coast redwood (Sequoia sempervirens) and giant sequoia (Sequoiadendron giganteum) trees of different sizes. Leaf-specific hydraulic conductivity (k(L)) increased with height in S. sempervirens but not in S. giganteum, while xylem cavitation resistance increased w...

Here we respond to the communication in American Journal of Botany (96: 542-544 in this issue) by Netting, who proposes several ways in which our paper "The Limits to Tree Height" (Nature 428: 851-854) may have erred in estimating the biophysical limits to height growth in Sequoia sempervirens. We first explain that because embolism repair requires...

In 1995, we installed surface-sterilized, rough-barked and smooth-barked tree branches in clearcuts, young forests, and old growth. Half of the experimental branches were inoculated with propagules of the epiphytic cyanolichen, Lobaria oregana. In 1997, we concluded that L. oregana was associated with old-growth Douglas-fir forests because of dispe...

We studied changes in morphological and physiological characteristics of leaves and shoots along a height gradient in Sequoia sempervirens, the tallest tree species on Earth, to investigate whether morphological and physiological acclimation to the vertical light gradient was constrained by hydrostatic limitation in the upper crown. Bulk leaf water...

Seventy trees from seven stands 50-650 years old were selected for this investigation of crown structural development in Pseudotsuga menziesuii All branches, limbs, and trunks were nondestructively measured for size, structure, and location while climbing the trees with ropes. These data were used to generate a computer model of each tree's crown t...

The tropical monodominant tree Dicymbe corymbosa reiterates via epicormic shoots and roots, resulting in multistemmed trees with complex pseudotrunks and root mounds. In 2 ha of primary forest on the Guiana Shield, we quantified the reiterative structure and aboveground soil development of 307 D. corymbosa individuals ≥ 10 cm dbh and investigated t...

We used rope techniques to access epiphyte communities on nine large and structurally complex redwoods (Sequoia sempervirens) occupying old-growth forest reserves of northwestern California. All species of epiphytic lichens, bryophytes and vascular plants were recorded, biomass of dominant vascular epiphytes (Polypodium scouleri and Vaccinium ovatu...

Sequoia sempervirens (redwood) is a long-lived, shade-tolerant tree capable of regeneration without disturbances and thus often present in all sizes within a single forest. In order to evaluate functional linkages among structures, plant distribution, and biodiversity in the canopy, we quantified all vascular plants from ground level to the treetop...

Abstract.—We investigated habitat use and movements of the wandering salamander, Aneides vagrans, in an old-growth forestcanopy. We conducted a mark-recapture study of salamanders in the crowns of five large redwoods (Sequoia sempervirens) inPrairie Creek Redwoods State Park, California. This represented a first attempt to document the residency an...

Within the canopy of old-growth redwood (Sequoia sempervirens (D. Don) Endl.) forests, accumulations of plant debris in crotches and on massive limbs serve as parent materials for ''arboreal soils'' up to 1 m thick and over 50m above the forest floor. These soils are important habitats and water sources for desiccation-sensitive organisms in the fo...

Many aspects of tree physiology, epiphyte ecology, and stand-level forest dynamics can greatly benefit from whole-tree estimates of surface area, wood volume, and biomass. Surface area estimates are needed to determine carbon production for trees with photosynthetic bark and to estimate epiphyte habitat with tree crowns. Although most conifers carr...

Exploration of tree crowns worldwide often reveals rich epiphyte communities composed of lichens, bryophytes, and vascular plants. In very old, wet forests, nearly every tree surface, from outermost twigs to thick branches and the main trunk, is covered by epiphytes. An arboreal botanist faced with this diversity quickly realizes that these plants...

Trees grow tall where resources are abundant, stresses are minor, and competition for light places a premium on height growth. The height to which trees can grow and the biophysical determinants of maximum height are poorly understood. Some models predict heights of up to 120 m in the absence of mechanical damage, but there are historical accounts...

Canopy access includes primarily vertical techniques for reaching the canopy from the ground, and canopy movement includes primarily horizontal techniques for moving through the canopy. Both directions of motion are necessary to fully explore and sample the forest canopy. Walkways, cranes, and rafts offer extensive canopy movement but involve subst...

Redwood forests contain some of the largest and most structurally complex trees on Earth. The most abundant vascular epiphyte in these forests is the fern Polypodium scouleri (Polypodiaceae). We measured dimensions of all 765 P. scouleri mats on 32 trees (27 Sequoia sempervirens, 5 Picea sitchensis). Eighteen P. scouleri mats from 11 trees were ran...

Ancient redwood trees (Sequoia sempervirens) often have complex crowns consisting of multiple, resprouted trunks. This study focuses on a single redwood tree, which is known as the Redwood Creek Giant, growing in Tall Trees Grove of Redwood National Park, CA. It is the sixteenth tallest (109.8 m) known living tree and the twentieth largest (744.7 m...

The canopies of old-growth Douglas-fir forests support a diversity of epiphytic lichens and bryophytes. We quantified the vertical distributions of six dominant epiphytes throughout the crowns of large Tsuga heterophylla (46-57 m tall) and Pseudotsuga menziesii (60-78 m tall) trees in five forest stands of the central Oregon Cascades. There were no...

Epiphytic lichen biomass accumulates slowly in forest canopies. We eval- uated three alternative hypotheses for the slow accumulation of epiphytic lichens, using two experiments in tree crowns from 15 Douglas-fir forest stands representing three age classes: old growth, young, and recent clearcuts. The first experiment evaluated whether forest age,...

In the coastal forests of northern California, redwood (Sequoia sempervirens) reigns supreme. Individual trees can exceed 112 m in height, and individual forest stands can have a biomass over 3,000 metric tons per hectare. Ancient redwoods often have complex crowns consisting of many reiterated trunks, some of which are larger than full-size trees...

In the Oregon Cascades, epiphytic cyanolichens are abundant in old-growth forest canopies, but they accumulate very slowly in young forests. We evaluated whether epiphytic cy- anolichens require old growth and/or thick, underlying moss mats to achieve normal rates of growth and mortality. We transplanted over one thousand mature thalli of two old-g...

Lichens were cultured by attaching a thallus fragment to a nylon monofilament loop with silicone sealer. Two effective methods for adjusting lichen mass to a standard moisture content were developed (the ‘reference-sample’ and ‘sacrificial’ methods). These corrections for moisture content allow detection of very small changes in dry mass without ha...

Canopy epiphyte research in the Pacific Northwest has entered its third decade, and three basic patterns of epiphyte distribution are now known. First, epiphyte biomass and diversity slowly increase during forest succession. This trend is most obvious with cyanolichens, which dominate epiphyte communities in old-growth forests but are scarce in you...

A total of 65 epiphyte species (macrolichens, bryophytes, and a fern) were found on the branches of four 700-year-old Douglas fir trees, two growing in the forest interior and two growing on the edge of a 20-year-old clearcut. The moss Antitrichia curtipendula and the cyanolichen Lobaria oregana dominated the epiphyte assemblages. Branch epiphyte a...

A total of 127 bryophyte species (50 mosses, 76 liverworts, and 1 hornwort) was encountered in the inner crowns of six Ficus tuerckheimmi trees in a lower montane wet forest landscape: 109 on three intact forest trees and 76 on three isolated trees. Fifty-two species were found only on the intact forest trees, while only 18 species were exclusive t...

Thesis (Ph. D.)--Oregon State University, 1995. Includes bibliographical references (leaves 109-115). Photocopy.

Thalli of two epiphytic lichens, Lobaria oregana (Tuck.) Müll. Arg. and Pseudocyphellaria rainierensis Imsh., were collected from the canopy of an old-growth forest. Thalli were individually attached to nylon monofilament and transplanted into the crowns of four trees, two growing in the forest interior and two growing on the edge of a clear cut, a...