Elsevier

Forest Ecology and Management

Volume 270, 15 April 2012, Pages 183-188
Forest Ecology and Management

In-situ measurement of twig dieback and regrowth in mature Acer saccharum trees

https://doi.org/10.1016/j.foreco.2012.01.020Get rights and content

Abstract

Crown shyness is thought to influence both the productivity and dynamics of forests, but few studies have examined the underlying causes of this common phenomenon. The few studies that exist suggest that crown shyness is caused by the reciprocal abrasion of neighboring tree crowns, resulting in the death of peripheral buds and/or the breakage of peripheral twigs (referred to here as twig dieback). However, twig dieback has not been directly observed due to the difficulty of accessing the crowns of mature canopy trees. In this study, we used a mobile canopy lift to obtain in-situ measurements of twig dieback in mature sugar maple (Acer saccharum Marsh.) trees. We measured: (1) percent dieback (% year−1) – the proportion of twigs that broke off or died; (2) dieback (cm year−1) – the length of the broken or dead portion; and (3) regrowth (cm year−1) – the length of the new twigs that sprouted from axillary buds. One third of the twigs suffered dieback over the course of 1 year, resulting in 1.41 cm of dieback, on average. Percent dieback was significantly higher in tree crowns located within 3 m of another crown, indicating that dieback is the result of the reciprocal abrasion of neighboring crowns. Percent dieback also increased with tree size, suggesting that tree sway increases as trees grow larger, resulting in more frequent and more intense abrasion. These trends were exacerbated by similar trends in regrowth, which was slower both in large trees and in trees located with 3 m of another crown. Our results suggest dieback may represent a substantial drain on both tree growth and stand productivity – a drain that increases as trees grow larger and stands mature.

Highlights

► One-third of twigs suffered dieback over the course of 1 year. ► Percent dieback increased significantly with stem diameter and slenderness. ► Regrowth decreased significantly with stem diameter. ► Percent dieback was significantly higher in crowns located within 3 m of one another. ► Regrowth was significantly slower in crowns located within 3 m of one another.

Introduction

It is widely recognized that local variation in canopy leaf area influences both the productivity and dynamics of forests. However, research on this topic has largely focused on the dominant source of variation: the large gaps formed by the death of canopy trees (Brokaw, 1982, Valverde and Silvertown, 1997, Vepakomma et al., 2008). As a result, forest canopies are often characterized as a high-contrast mosaic of light-filled gaps, where most of the regeneration occurs, surrounded by a continuous overstory canopy, where most of the production takes place (Lieberman et al., 1989).

This characterization neglects two important sources of variation within the overstory canopy: (1) age-related variation in the foliage density of individual tree crowns (Ishii and Wilson, 2001, Nock et al., 2008), the result of a process commonly called crown thinning; (2) the web of narrow spaces between adjacent tree crowns, a pattern commonly called crown shyness (Putz et al., 1984, Rudnicki et al., 2003, Rudnicki et al., 2004, Meng et al., 2006, Goudie et al., 2009).

Both crown thinning and crown shyness are thought to influence the productivity and dynamics of forests, because less light is intercepted by the canopy as a whole, while more light is intercepted by seedlings and saplings regenerating in the understory (Ryan et al., 1997, Smith and Long, 2001, Goudie et al., 2009, Nock et al., 2008, Digregorio et al., 1999). In maturing lodgepole pine stands, for example, the onset of crown shyness coincides with a simultaneous decline of leaf area and productivity (Long and Smith, 1992, Smith and Long, 2001). Yet, surprisingly little research has examined the underlying causes of crown thinning and crown shyness (Nock et al., 2008, Meng et al., 2006). The dearth of research on the causes of crown shyness is particularly surprising, because it is both prevalent and prominent, occurring to some extent in most forest types and very evident in certain forest types.

Crown shyness is most evident in even-aged conifer stands (Putz et al., 1984, Rebertus, 1988, Ng, 1977). For example, in mature pine stands, inter-crown spaces (excluding tree-fall gaps) are observed to cover 3–30% of the ground area (Rudnicki et al., 2003, Rudnicki et al., 2004). However, crown shyness is prevalent in other forest types as well. For example, inter-crown spaces cover 15–17% of the total ground area in moist tropical forests (Rebertus, 1988).

It is often assumed that crown shyness is caused by the reciprocal shading of neighboring trees (Koike, 1989, Sorrensen-Cothern et al., 1993, Umeki, 1995, Makela, 1997). However, there is growing evidence that crown shyness is caused by reciprocal abrasion (Long and Smith, 1992, Rudnicki et al., 2003, Rudnicki et al., 2004, Meng et al., 2006), resulting in the death of peripheral buds and/or the breakage of peripheral twigs. In mangrove forests, for example, Putz et al. (1984) found that the branches bordering inter-crown spaces had a higher proportion of broken twigs compared to those inside the crown, suggesting that crown shyness is caused by the breakage of peripheral twigs. However, we are not aware of any study that has observed this process directly, and there is no single term that is consistently used to describe it. Here, we use the term twig dieback (or simply dieback) to describe both the breakage of peripheral twigs and the death of terminal buds that do not break off.

Despite the lack of direct observations, it has also been inferred that tree size and slenderness may determine the amount of twig dieback (Rudnicki et al., 2003). This is because tree sway increases with tree size and slenderness (Rudnicki et al., 2004, Meng et al., 2006), resulting in more frequent and intense abrasion (Rudnicki et al., 2003, Long and Smith, 1992). Thus, the proportion of twigs that suffer dieback may be greater in large and/or slender trees, and the amount of breakage may be greater as well. Indeed, crown shyness has been shown to increase both with tree height and stem slenderness (Rudnicki et al., 2004).

A tree’s size may also influence its ability to replace peripheral twigs by growing new twigs from dormant axillary buds. Trees allocate a greater proportion of resources to reproduction (Gross, 1972, Tappeiner, 1969, Leal and Thomas, 2003) and defense (Boege and Marquis, 2005, Loehle, 1988, Thomas, 2010) as they grow larger and older. Thus, the compensatory growth of twigs that sprout from dormant axillary buds may decline with tree size and/or age. The rate of regrowth may also decline with age due to senescence, the progressive decline in the physiological functioning of an organism through time (Day et al., 2001, Day et al., 2002, Thomas, 2010).

In this study, we used a mobile canopy lift to obtain direct, in-canopy measurements of twig dieback and regrowth in mature sugar maple trees. The goal of the study was to address the following four questions: (1) What percent of peripheral twigs suffer dieback over the course of 1 year? (2) How much of the twig breaks off or dies, on average? (3) Does the amount of dieback vary with tree size, tree age, or the proximity of neighboring tree crowns? (4) Does the rate of regrowth vary with tree size, tree age, or the proximity of neighboring tree crowns?

Section snippets

Methods

Below, we first describe the field site and sampling methods, including the criteria used to select the branches and twigs to be sampled. Then, we describe the methods used to quantify the annual rate of dieback, as well as various tree- and branch-level predictor variables. Finally, we describe statistical analyses used to examine whether dieback increases with tree size, proximity to neighboring tree crowns, tree age, or height.

Results

On average, 34% of the twigs on a tree suffered dieback over the course of 1 year, but there was large variation from tree to tree, with percent dieback varying from 7% to 61% (Table 1). Thirty-five percent of the twigs actually broke off, whereas 65% of the twigs simply died without breaking. Among twigs that suffered dieback, the average rate of dieback was 1.41 cm year−1, while the average rate of regrowth was 0.92 cm year−1.

As expected, percent dieback increased significantly with stem diameter

Discussion

In this paper, we employed direct, in-situ measurements to demonstrate that more than one third of the twigs suffered dieback over the course of a year, resulting in 1.41 cm of dieback, on average. Furthermore, our results indicate that reciprocal inter-crown abrasion does contribute to dieback. In particular, we found that percent dieback in opposed crowns was more than double that observed in unopposed crowns (Fig. 2).

Percent dieback also increased with diameter and slenderness (Table 2),

Acknowledgements

This research was supported by an NSERC Discovery grant to John Caspersen. The authors wish to thank Haliburton Forest and Wildlife Reserve for their support to conduct this research, Philip Rudz and Lazar Pavlovic for their field assistance, as well as Tomasz Gradowski for providing his lift training. The authors also thank Sean Thomas and Tat Smith of the University of Toronto and Bill Cole of the MNR, Ontario for comments on an earlier version of this manuscript.

References (34)

  • M.E. Day et al.

    Age- and size-related trends in woody plant shoot development: regulatory pathways and evidence for genetic control

    Tree Physiology

    (2002)
  • M.E. Day et al.

    Age-related changes in foliar morphology and physiology in red spruce and their influence on declining photosynthetic rates and productivity with tree age

    Tree Physiology

    (2001)
  • L.M. Digregorio et al.

    Radial growth trends of sugar maple (Acer saccharum) in an Allegheny northern hardwood forest affected by beech bark disease

    Journal of the Torrey Botanical Society

    (1999)
  • H.L. Gross

    Crown deterioration and reduced growth associated with excessive seed production by birch

    Canadian Journal of Botany

    (1972)
  • H. Ishii et al.

    Crown structure of old growth Douglas-fir in the western Cascade Range, Washington

    Canadian Journal of Forest Research

    (2001)
  • M.J. Jaffe

    Thigmomorphogenesis: the response of plant growth and development to mechanical stimulation

    Planta

    (1973)
  • M.J. Jaffe et al.

    Thigmomorphogenesis: the effect of mechanical perturbation on plants

    Plant Growth Regulation

    (1993)
  • Cited by (0)

    View full text