Tree Wound Responses: Healthy Wound Closures by Callus and Woundwood

Summary: Christopher J. Luley of Urban Forestry LLC in Naples, New York, describes callus and woundwood as tree wound responses whose wound closures denote health.

Rotten Eastern white pine (Pinus strobus L.) heartwood section exhibits inwardly curving curls known as ram's horns that lower woundwood's heal-and-seal ability: Randy Cyr/Greentree/Bugwood.org, CC BY 3.0, via Forestry Images

The article Biology and Assessment of Callus and Woundwood in Arborist News for April 2015 acknowledges the causes that form and grow tree wound responses and their contributions to tree risk assessments.

Wound responses break down into closure, as new tissue formation after wounding, and Dr. Alex Shigo's (May 8, 1930-Oct. 6, 2006) compartmentalized "responses of preexisting tissues." Closure considers callus tissue that converges around wounds shortly after wounding and that anatomically and functionally different, subsequently formed woundwood ultimately covers within weeks or months. Woundwood develops from "callus or from uninjured vascular cambium at the margin of [non-shallow] injuries that have damaged or exposed the phloem, vascular cambium, or sapwood."

Callus emerges from dividing (hyperplasic) or enlarged (hypertrophic) cells adjacent to wounds while bark parenchyma cellular tissue and phloem vascular tissue engender both callus and woundwood.

Most woody plant organs, such as soft, succulent parenchyma-rich fruits, leaves, roots and stems, form callus, which functions totipotently to fashion the entire plant from parts. The "varying ratios" of auxin and cytokinin, as respectively cell-dividing and cell-elongating plant hormones, guide what type of organs callus generates: buds, leaves, roots, shoots, stems.

Woundwood with inner bark and wood, called phloem and xylem, heals larger wounds than callus even though callus and woundwood sometimes have incomplete, naturally cracked seals. Its initially scarce medullary rays and vessels and short cells indicate anatomical dissimilarities with xylem sapwood even though woundwood, unlike callus, ultimately imitates normal sapwood's appearance. Sapwood and woundwood juggle annual growth rings by which master arborists, master gardeners, master naturalists and tree stewards judge age for tree wound responses and trees.

Sapwood without tree wound responses knows decay and discoloration, particularly on 1-plus-square-foot- (929.03-plus-square-centimeter-) large wounds, while woundwood keeps them in check toward the wound's "open face."

Ram's horns, as inward-curving curls into decay- or wound-formed cavities or openings or as thick bark on opposing ribs, lower woundwood's success at healing and sealing. Timing meddles with tree wound responses since woundwood manages fastest, greatest production before or beginning active tree-growing seasons and since wounds in autumn manifest greatest dieback. Wound dressings and wound wraps likewise narrow the heal and seal success rates of tree wound responses by nurturing steady decay, not callus and woundwood formation.

The absence or the presence, the age and the amount and rate of callus and woundwood formation, as tree wound responses, offer tree risk assessment-related information.

Cold weather, improper pruning cuts, included bark, restricted flow of carbohydrates and slow radial growth prevent callus and woundwood from presenting themselves initially or progressing properly.

Woundwood, as the "tissue that seals larger wounds in trees and returns the function to the stem," qualifies as a "type of response growth" when present. Its absence reflects tree health by revealing compromised defense mechanisms against infections, pathogens and pests, damaged tissues for tree wound responses" and inadequate vitality for growth. The "elevated strength and toughness of woundwood, and its use in hand tools where these characteristics are advantageous," support decay resistance when present, not when absent.

Callus and woundwood, as tree wound responses, tell of environmental and physical impacts upon tree growing seasons, health and species, according to author Christopher J. Luley.

Properly cleaned and shaped wounds (2) on a cherry tree (Prunus spp.) facilitate callus formation (3), which is helpful for wound healing and for preventing decay: US Forest Service Northeastern Area-St. Paul Field Office, Public Domain, via USDA Forest Service-Northeastern Area

Acknowledgment

My special thanks to:
talented artists and photographers/concerned organizations who make their fine images available on the internet;
University of Illinois at Urbana-Champaign for superior on-campus and on-line resources.

Image credits:

Rotten Eastern white pine (Pinus strobus L.) heartwood section exhibits inwardly curving curls known as ram's horns that lower woundwood's heal-and-seal ability: Randy Cyr/Greentree/Bugwood.org, CC BY 3.0, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=1238007

Properly cleaned and shaped wounds (2) on a cherry tree (Prunus spp.) facilitate callus formation (3), which is helpful for wound healing and for preventing decay: US Forest Service Northeastern Area-St. Paul Field Office, Public Domain, via USDA Forest Service-Northeastern Area @ https://www.na.fs.fed.us/spfo/pubs/misc/tree_care/aib372-h.htm

For further information:

Gilman, Ed. 2011. An Illustrated Guide to Pruning. Third Edition. Boston MA: Cengage.

Hayes, Ed. 2001. Evaluating Tree Defects. Revised, Special Edition. Rochester MN: Safe Trees.

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