Stephen C. Sillett

The limits to tree height

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 of taller trees. Current hypotheses of height limitation focus on increasing water transport constraints in taller trees and the resulting reductions in leaf photosynthesis. We studied redwoods (Sequoia sempervirens), including the tallest known tree on Earth (112.7 m), in wet temperate forests of northern California. Our regression analyses of height gradients in leaf functional characteristics estimate a maximum tree height of 122–130 m barring mechanical damage, similar to the tallest recorded trees of the past. As trees grow taller, increasing leaf water stress due to gravity and path length resistance may ultimately limit leaf expansion and photosynthesis for further height growth, even with ample soil moisture.

DISPERSAL LIMITATIONS OF EPIPHYTIC LICHENS RESULT IN SPECIES DEPENDENT ON OLD‐GROWTH FORESTS

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, bark roughness, or dispersal rate limits the establishment of the dominant old- growth-associated lichen, Lobaria oregana. Surface-sterilized branches with either rough or smooth bark were repeatedly inoculated with propagules and compared 1 yr after the last inoculation. Dispersal affected rates of establishment: inoculated branches had 27X more newly established thalli than controls. Establishment on smooth bark was highest in clearcuts, intermediate in young forests, and lowest in old growth. There was as much or more establishment of sown propagules on smooth-barked branches as on rough-barked branches in all age classes. In the second, transplant-performance experiment, Lobaria oregana grew as rapidly in young forests as in old growth but lost biomass and suffered more injuries in clearcuts. In contrast, L. pulmonaria performed at least as well in clearcuts as in young forests and old growth. Poor dispersal and establishment limit the development of L. oregana populations in Douglas-fir forests. Particular substrates and microenviron- ments found only in old growth are not essential for Lobaria establishment and growth. Maximizing the number and dispersion of remnant trees in cutting units should maximize the rate of accumulation of L. oregana biomass in the regenerating forest. The single most important action promoting the accumulation of old-growth-associated epiphytes will be the retention of propagule sources in and near all cutting units.

Effects of tree height on branch hydraulics, leaf structure and gas exchange in California redwoods

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 with height in both species. Despite hydraulic adjustments, leaf mass per unit area (LMA) and leaf carbon isotope ratios (delta(13)C) increased, and maximum mass-based stomatal conductance (g(mass)) and photosynthesis (A(mass)) decreased with height in both species. As a result, both A(mass) and g(mass) were negatively correlated with branch hydraulic properties in S. sempervirens and uncorrelated in S. giganteum. In addition, A(mass) and g(mass) were negatively correlated with LMA in both species, which we attributed to the effects of decreasing leaf internal CO(2) conductance (g(i)). Species-level differences in wood density, LMA and area-based gas exchange capacity constrained other structural and physiological properties, with S. sempervirens exhibiting increased branch water transport efficiency and S. giganteum exhibiting increased leaf-level water-use efficiency with increasing height. Our results reveal different adaptive strategies for the two redwoods that help them compensate for constraints associated with growing taller, and reflect contrasting environmental conditions each species faces in its native habitat.

SURVIVAL AND GROWTH OF CYANOLICHEN TRANSPLANTS IN DOUGLAS-FIR FOREST CANOPIES

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 cyanolichens 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-growth associated species (Lobaria oregana and Pseudocyphellaria rainierensis) into the crowns of Douglasfir trees in thirteen forest stands representing four age classes: old-growth (400-700 yr), mature (140-150 yr), young (35-40 yr), and recent clearcut. Wooden racks were used instead of trees in the clearcuts. Half of the cyanolichen thalli were transplanted onto thick moss mats and half were transplanted onto bare bark. After one year, both species grew at least as well in younger forests as they did in old growth (20 to 30% increase in mass), but growth rates were significantly lower in clearcuts. Mortality rates were very low (<10%) in young, mature, and old-growth forests, but high (50 to 90%) in clearcuts. Pseudocyphellaria rainierensis grew significantly better on moss than on bare bark (30 vs 23% increase in mass). Since epiphytic cyanolichens can survive and grow in a broad range of forest age classes, silvicultural treatments that facilitate their colonization of young forests have great conservation potential. Epiphyte biomass increases slowly during succession in temperate forests (Esseen et al. 1996; Lesica et al. 1991; McCune 1993; Neitlich 1993; Rose 1992; Selva 1994). Old-growth forests support much higher epiphyte loads than younger, managed forests. In the Douglas-fir forests of Oregons western Cascades, the most striking epiphytic difference between old-growth and managed forests involves cyanolichens. These nitrogen-fixing lichens dominate the old-growth forest canopy (Pike et al. 1975; Sillett 1995a), but are scarce or absent in younger forests (McCune 1993; Neitlich 1993; Spies 1991). Many epiphyte species, including several endemic cyanolichens, are seldom if ever, found in forests less than a century old (Rosentreter 1995; Sillett & Neitlich 1996). Conservation of old growth-associated epiphytes is becoming an important consideration in the Pacific Northwest, where most old-growth forest habitat has been destroyed by logging (Norse 1990). A recent emphasis on ecosystem management (Swanson & Franklin 1992) has stimulated efforts to promote epiphytes in managed forests. Recovery of epiphytic cyanolichens is particularly important because of their potential contributions to forest productivity. Nitrogen fixed by epiphytic cyanolichens can represent an important nutrient input in humid old-growth forests (Denison 1979; Pike 1978; Sollins et al. 1980). Epiphytes also constitute a significant part of biodiversity. For example, the number of lichens and bryophyte species often equals or exceeds the number of vascular plant species in old-growth forests (e.g., Lesica et al. 1991). The importance of epiphytic lichens has been recognized under a comprehensive new ecosystem management plan (FEMAT); populations of many oldgrowth associated species must now be monitored and protected (Rosentreter 1995). Reasons for the slow accumulation of epiphytic lichens in forests are poorly understood. Unsuitable microclimates in younger forests is one of several possible explanations (Sillett & Neitlich 1996). Microclimatic differences between old-growth and younger forests presumably arise from structural differences such as the frequency and size of gaps, abundance of standing dead material, and complexity of canopy architecture. Unlike more open, structurally complex old-growth forests, much of the canopy in dense, young, even-aged Douglas-fir forests, is shaded and sheltered from direct sunlight and precipitation (Kuiper 1988; Spies & Franklin 1991; Van Pelt & North 1996). Furthermore, bryophytes develop in thick mats on branches in large Douglas-fir trees, and many epiphytic cyanolichens are closely associated with these mats (Sillett 1995a). Perhaps these water storing mats ameliorate temperature and moisture fluctuations of epiphytes in their immediate vicinity, making these microhabitats more suitable to desiccation sensitive species (Lawrey 1991). 0007-2745/98/20-31

Growth Rates of Two Epiphytic Cyanolichen Species at the Edge and in the Interior of a 700-Year-Old Douglas Fir Forest in the Western Cascades of Oregon

1.35/0 This content downloaded from 207.46.13.119 on Wed, 30 Mar 2016 05:39:46 UTC All use subject to http://about.jstor.org/terms 1998] SILLETT & McCUNE: CYANOLICHEN TRANSPLANTS 21 TABLE 1. Characteristics of study sites in the Willamette National Forest in western Oregon. Trunk diameter (dbh) and height are listed for the two Douglas-fir trees climbed in each stand. Stand number Age class Elevation (m) Estimated age Tree dbh (m) Tree height (m) 1 old growth 610 700 2.77, 2.08 68.0, 66.0 2 old growth 830 560 1.84, 1.65 82.5, 78.0 3 old growth 490 450 1.56, 1.41 74.0, 66.5 4 old growth 590 400 1.62, 1.31 71.0, 67.5 5 mature 890 150 0.87, 0.76 48.5, 49.5 6 mature 560 140 1.30, 1.26 59.5, 62.5 7 mature 650 140 1.02, 0.93 54.5, 48.0 8 young 780 40 0.40, 0.38 25.0, 23.5 9 young 760 37 0.46, 0.38 25.5, 20.0 10 young 500 36 0.33, 0.30 25.0, 26.0 11 clearcut 670 12 clearcut 460 13 clearcut 790 This study evaluates one explanation for the slow development of this critical component of Douglasfir forest--that epiphytic cyanolichens require the environment created by the structure of old-growth forest canopies. We also consider the possible effects of moss mats on cyanolichen growth and mortality in the forest canopy. Two cyanolichen species

Trunk reiteration promotes epiphytes and water storage in an old-growth redwood forest canopy

Thalli of two epiphytic lichens, Lobaria oregana (Tuck.) Miill. Arg. and Pseudocy- phellaria 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 clearcut, at heights between 35 and 45 m above the ground. Half of the transplants in each tree were derived from thalli collected in the interior and half were derived from thalli collected on the edge. After one year, L. oregana transplants grew significantly less on the edge than they grew in the interior. The source of P. rainierensis thalli significantly affected transplant growth rate. Thalli that came from the interior and were transplanted to the edge did not grow, on average, while thalli that came from the edge and were transplanted back to the edge grew well.

Epiphyte Communities on Sitka Spruce in an Old-Growth Redwood Forest

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 treetops in an old-growth redwood forest (Prairie Creek Redwoods State Park, California, USA). This involved mapping terrestrial and epiphytic trees, shrubs, and ferns as well as climbing 27 trees up to 101 m tall within a 1-ha plot. We monitored canopy microclimates using sensor arrays that collected hourly data for up to 30 months. The plot held 4283 Mg/ha of aboveground dry mass in living plants, 95.4% of which was contributed by redwood. A high degree of structural complexity and individuality was evident in the crowns of the 14 largest trees in the form of reiterated trunks arising from main trunks, other trunks, and limbs. Thirteen species of vascular plants occu...

CROWN DEVELOPMENT OF COASTAL PSEUDOTSUGA MENZIESII, INCLUDING A CONCEPTUAL MODEL FOR TALL CONIFERS

Abstract Using rope techniques for access, we surveyed epiphytes on five Sitka spruce trees up to 92 m tall in an old-growth redwood forest. We quantified epiphyte diversity by sampling 5% of each trees surface area of axes (branches >5 cm diameter) and branchlets (branches <5 cm diameter, including foliage). Epiphyte communities included 57 macrolichen, 15 crustose lichen, 17 bryophyte, and two fern species. The five most abundant species—Isothecium myosuroides, Polypodium scouleri, Polypodium glycyrrhiza, Lobaria pulmonaria, and Frullania nisquallensis—contributed 42.1, 13.3, 8.4, 6.7, and 4.7% of the total epiphyte biomass, respectively. There was an average of 36.2 kg of bryophytes, 9.9 kg of lichens, 12.7 kg of ferns, and 131 kg of associated dead organic matter per tree. Axes supported 83% of the biomass and 98% of the dead organic matter. At the whole-tree level, bryophyte biomass was 11.3 times higher and lichen biomass was 2.5 times lower on axes than branchlets. Ferns were restricted to axes. Ordination analysis revealed one dominant gradient in epiphyte composition that was positively correlated with height and lichen diversity, and negatively correlated with bryophyte diversity. Chlorolichens dominated the exposed portion of the gradient with equivalent amounts of cyanolichens and bryophytes. Mosses dominated the intermediate portion of the gradient with equivalent amounts of liverworts, cyanolichens, and chlorolichens. There was very little lichen cover in the sheltered portion of the gradient, which was dominated by bryophytes. Extensive bryophyte mats with large quantities of dead organic matter promote biological diversity on Sitka spruce in redwood forest canopies by storing water and serving as habitats for desiccation-sensitive organisms.

Bryophyte Diversity of Ficus Tree Crowns from Cloud Forest and Pasture in Costa Rica

Seventy trees from seven stands 50–650 years old were selected for this investigation of crown structural development in Pseudotsuga menziesii. 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 trees crown that was error-checked trigonometrically. Leaves, bark, cambium, and wood were quantified by using limited destructive sampling to develop predictive equations that were applied to the complete inventory of structures in each trees crown. Summations of these values yielded whole-tree estimates of several structural variables. A second set of equations was then developed to predict these whole-tree parameters from simple, ground-based measurements. Principal components analysis of 24 tree-level variables revealed two orthogonal dimensions of structure that accounted for 71.3% and 12.4% of total variation in the 70 trees. The first dimension represented a gradient...

Physiological consequences of height-related morphological variation in Sequoia sempervirens foliage

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 to the isolated trees. Bryophyte species richness, bryophyte cover, and the frequency of pendents, tall turfs, tails, and fans were significantly higher in intact forest trees. Inner crowns of isolated trees had higher rates of evaporation, had higher macrolichen cover, and were more exposed to sunlight than inner crowns of intact forest trees. Ordination analysis revealed one dominant pattern in bryophyte composition in the inner canopy: a desiccation gradient ranging from sheltered sites in the intact forest trees to exposed sites in the isolated trees.