Manipulating tree crown structure to promote old-growth characteristics in second-growth redwood forest canopies

doi:10.1016/j.foreco.2018.02.036

Forest Ecology and Management

Volume 417, 15 May 2018, Pages 77-89

https://doi.org/10.1016/j.foreco.2018.02.036 Get rights and content

Highlights

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We imposed a factorial experiment on 24 redwood trees in second-growth forests.
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Treetop removal (topping) stimulated trunk reiteration, and new tops grew rapidly.
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Branch tip removal combined with topping resulted in the formation of limbs.
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Compartmentalization of wounds explained negative effects of pruning on growth.

Abstract

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 redwoods maintain simple, model-conforming crowns for centuries unless damaged by wind or fire. We imposed a factorial experiment on 24 redwoods 59–75 m tall in six second-growth forests to determine if trunk reiteration and limb formation can be induced by removing treetops (topping) and branch tips (tipping) to disrupt apical control. We also increased light availability in the upper crown by pruning branches. After intensively mapping trunks and branches and imposing treatments, we re-mapped trees three years later to quantify growth increments. Topping stimulated trunk reiteration from the cut, and reiterated tops gained height more rapidly than controls. Tipping also stimulated trunk reiteration from branches, especially when combined with topping, resulting in formation of limbs (i.e., branches giving rise to reiterated trunks). Pruning had consistently negative effects on trunk and branch growth increments even after accounting for post-treatment variation in leaf area and light availability, suggesting that photosynthate was diverted to compartmentalization of wounds. Strategic injury of trees may have long-term conservation value in second-growth redwood forests if limbs can be initiated high enough in the crown to persist as trees approach maximum height. Topping and tipping treatments should be combined with silvicultural thinning of neighboring trees to increase light availability far more than can be achieved by pruning.

Introduction

Simplification of forest structure that follows conversion of old-growth to younger forests or plantations is a major contributor to extinction, because many species require habitats created by the structure of old trees (Lindenmayer and Franklin, 2002). Northern redwood (Sequoia sempervirens) forests are the most structurally complex, carbon-dense, and productive in the world (Jones and O’Hara, 2012, Van Pelt et al., 2016) with individual trees exceeding 2500 years of age, 400 Mg aboveground biomass, and one hectare of projected leaf area (Sillett et al., 2015a). Tree-level abundance of arboreal biota in these forests is positively correlated with abundance of trunk reiterations and amounts of decaying wood in the crown (Sillett and Van Pelt, 2007). Soil accumulations in crotches between multiple trunks and on large limbs support massive epiphyte loads (Sillett and Bailey, 2003). Limbs—branches giving rise to trunk reiterations—are important structures because they become much thicker than branches, which rarely exceed 25 cm diameter. The largest redwood limbs are >200 cm diameter (Sillett and Van Pelt, 2007) and >2000 years old (Sillett et al., 2015a). The Marbled Murrelet (Brachyramphus marmoratus), an endangered seabird, is an old-growth specialist that requires large-diameter appendages for nesting platforms (Hamer and Nelson, 1995, Baker et al., 2006). Trunk reiterations and limbs develop as redwoods age, but undamaged trees can retain a simple crown form for centuries. In young forests without remnant trees, redwoods almost always lack reiterations and possess dense, conical crowns with small branches, explaining why old-growth associated species are now virtually restricted to parks and reserves (Sawyer et al., 2000, Cooperrider et al., 2000).

Like many conifers, the genetically determined architectural model of redwood consists of an orthotropic (i.e., vertical) trunk bearing numerous plagiotropic (i.e., horizontal) branches (Tomlinson, 1983). Wind-induced crown breakage appears to promote structural complexity by creating gaps in otherwise dense, model-conforming crowns. Trunk reiterations eventually arise from the damaged main trunk and limbs, giving each tree an individualized and more complex structure. The vast majority of crown-mapped redwoods in old-growth forests possess broken tops with internal decay, and nearly every limb measured in these trees has a broken branch axis beyond the reiteration (Sillett et al., 2015a). These observations and the linkage between spatial aggregation of trunks and canopy water storage (Sillett and Van Pelt, 2007) suggest that injuries to model-conforming trees may stimulate trunk reiteration and limb formation, thereby providing critical habitat elements to old-growth associated species.

Injuring trees—blasting tops, hollowing main trunks, inoculating scars with decay fungi—is employed to enhance wildlife habitats in some forests (Lewis, 1998, Brandeis et al., 2002). Approaches that stimulate pathogenesis, while effective in the short term for cavity-nesting species, operate at the expense of long-term tree health. Silvicultural thinning has been proposed as a way to accelerate limb development on retained redwoods for marbled murrelet conservation (Franklin et al., 2007). While thinning does stimulate trunk diameter growth in young forests, the largest branches (up to 9 cm) on trees <100 cm DBH consistently occur directly below points of previous damage on the trunk (Keyes, 2011). We are unaware of any controlled attempts to promote crown-level complexity in healthy forest trees via arboricultural techniques.

If redwood responds like other trees, a variety of methods may be effective in stimulating trunk reiteration and accelerating limb formation. Many conifers exhibit strong apical control in which the intact treetop suppresses reiteration and regulates the export of photosynthate from branches (Blake et al., 1980, Cline, 1997). When the treetop is removed, apical control is disrupted, and branches retain photosynthate, which results in more rapid diameter growth (Wilson, 1981, Wilson, 1998). Rapid branch growth can lead to upward bending and formation of a new leader trunk (Wilson, 2000). What is unclear, however, is whether treetop removal will stimulate multiple reiterations within the crown or merely replacement of the leader trunk. If the treetop exerts apical control over branches, branch tips may also exert apical control over higher-order stems arising from the branch. Support for this idea comes from a study of Fraxinus in which death of branch tips resulted in the formation of forks (Remphrey and Davidson, 1992). If this occurs in redwood, branch tip removal might stimulate limb formation, especially when apical control of the main trunk is also disrupted.

In addition to apical control, branch growth is driven by light availability. Branches compete with each other for light, and well-illuminated branches grow faster than shaded branches (Stoll and Schmid, 1998, Kramer et al., 2014). Thus, pruning may lead to higher rates of growth in remaining branches by reducing shade within the crown. Branches also compete for photosynthate they export to the trunk, which is stored and then re-allocated at the beginning of the growing season, and well-illuminated branches are more competitive sinks than shaded branches (Sprugel, 2002). Experiments involving topping and pruning have been restricted to very small trees, so the extent to which these results apply to redwood is unknown. If trunk reiteration and limb formation can be induced in redwood, however, their ecological value may be small unless trees used in experiments are sufficiently tall to retain these structures into old age and not lose them to self-pruning. In old-growth redwood forests, trunk reiterations and limbs supporting arboreal biota are only prevalent above 50 m as a consequence of deep shade in the lower canopy (Sillett and Van Pelt, 2007).

Here we perform a factorial experiment that manipulates apical control and within-crown light availability of redwoods in second-growth forests. Recognizing that any limbs formed below 50 m would likely be lost in future centuries as trees approach maximum height, we performed the experiment in forests over 60 m tall. Our primary objective is to determine if trunk reiteration and limb formation can be induced by removal of treetops (hereafter ‘topping’) and branch tips (hereafter ‘tipping’) as well as branch pruning. We predict that topping will stimulate trunk reiteration by disrupting apical control and allowing replacement of the decapitated leader. Release of apical control should reduce export of photosynthate from branches to the trunk, allowing branches of topped trees to grow faster. Moreover, if branch-level apical control suppresses limb formation, only tipping will stimulate orthotropic reiteration from branches. Alternatively, if apical control is unimportant, topping and tipping will have little effect, pruning will diminish trunk growth by reducing the amount of leaves, and pruning will promote branch growth by increasing crown-level light availability. Our secondary objective is to quantify the magnitude of treatment effects on growth increments of the study trees by intensively measuring entire trunks and every branch. It may be possible to guide the developmental trajectory of redwood crown structure without causing severe injury or greatly diminishing tree-level productivity.

Section snippets

Methods

Study area and tree selection—Second-growth forests with trees at least 60 m tall are rare in northern California, as nearly all of the forests first logged in the late 19th and early 20th centuries were logged again at least once. In Del Norte and Humboldt Counties, however, we found suitable second-growth forests in six locations (blocks): one in Jedediah Smith Redwoods State Park (A), two in Del Norte Coast Redwoods State Park (B, C), two in Arcata Community Forest (D, E), and one in

Results

Tree structure, age, and efficiency—Despite having similar ages, heights, trunk diameters, and undamaged crowns, the 24 trees selected for this experiment spanned a wide size gradient, ranging in total aboveground mass from 5.5 to 23.0 Mg and leaf mass from 68 to 813 kg (Table 3). Crowns of some trees (e.g., 20) were relatively narrow and sparse while others (e.g., 5) were much wider and denser (Fig. 1). Such high tree-to-tree structural variation resulted in growth increments varying by

Discussion

The rarity of mature second-growth forests in the northern redwood range made finding sufficiently tall trees challenging. Locations we selected were logged in the late 19th and early 20th centuries, but we could not determine exact dates, as the old-growth forests were apparently not clearcut. At least three trees (8, 9, 23) are examples of advance regeneration decades older than others in their locations. An increment core sample from the trunk of tree 5 at 10 m has an inner section of very

Acknowledgments

Funding for this research was provided by the US Fish and Wildlife Service, California Department of Fish and Wildlife, the Save the Redwoods League, and Kenneth L. Fisher. California Department of Parks and Recreation—Jedediah Smith Redwoods State Park, Del Norte Coast Redwoods State Park, Humboldt Redwoods State Park—and the Arcata Community Forest granted permission to conduct research in these reserves. Ben Iberle and Bob Van Pelt provided field assistance and useful discussion of ideas.

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Manipulating tree crown structure to promote old-growth characteristics in second-growth redwood forest canopies

Highlights

Abstract

Introduction

Section snippets

Methods

Results

Discussion

Acknowledgments

For. Ecol. Manage.

For. Ecol. Manage.

For. Ecol. Manage.

For. Ecol. Manage.

For. Ecol. Manage.

Model Based Inference in the Life Sciences: A Primer on Evidence

Nesting habitat characteristics of the marbled murrelet in central California redwood forests

J. Wildl. Manage.

Estimating branch production in trembling aspen, Douglas-fir, jack pine, black spruce, and balsam fir

Can. J. For. Res.

Ethylene, gibberellins, auxin, and apical control of branch angle in the conifer, Cupressus arizonica

Planta

Cavity nester habitat development in artificially made Douglas-fir snags

J. Wildl. Manage.

Model Selection and Multimodal Inference: A Practical Information-Theoretic Approach

Millennium-scale crossdating and inter-annual climate sensitivities of standing California redwoods

PLoS ONE

Concepts and terminology of apical dominance

Am. J. Bot.

Terrestrial fauna of redwood forests

The hydraulic architecture of balsam fir (Abies balsamea)

Physiologia Plantarum

The role of epicormic shoot production in maintaining foliage in old Pseudotsuga menziesii trees

Can. J. Bot.

The role of epicormic shoot production in maintaining foliage in old Pseudotsuga menziesii trees II. Basal reiteration from older branch axes

Can. J. Bot.

Carbon density in managed coast redwood stands: implications for forest carbon estimation

Forestry

Thinning promotes restoration of branch structure in second-growth redwoods at Redwood National Park

Ecol. Restoration

Structural development of redwood branches and its effects on wood growth

Tree Physiol.