Fostering regeneration: gap creation strategies in mature Monterey pine stands in coastal California

Document Type

Article

Publication Date

6-30-2026

Abstract

Monterey pine, a regionally endemic species with the greatest abundance on the Monterey Peninsula in central coastal California, is the most widely planted conifer species worldwide. Many native Monterey pine forests in the wildland–urban interface (WUI) are overmature and exhibit poor regeneration, leading to closed canopies and increasingly dense understories, a situation that has been exacerbated by the introduced disease pitch canker, which kills Monterey pines of all age classes. This fire-adapted species is dependent on canopy gaps for regeneration, but restrictions on controlled burns in the WUI prevent the use of this management tool. Alternative methods are needed to sustain Monterey pine in its native range and to safeguard this valuable genetic resource in a warming and drying climate. We hypothesized that Monterey pine regeneration could be enhanced by opening stand structure through the creation of canopy gaps in mature forest stands. At four sites representing a range of local understory species compositions, we assigned overstory Monterey pines to one of three treatments: single or triple girdled trees and one ungirdled control tree. Trees were girdled in 2004 to induce gradual tree decline and reduce canopy closure. Before girdling, we counted and removed all Monterey pine seedlings within a 5m radius of each monitored tree, then periodically assessed canopy closure, understory plant species composition, and percent cover, and counted Monterey pine seedlings by height class through 2010. Girdled plots had greater proportions of seedlings in the taller classes than control plots in 2007 and 2010. Total seedling counts in girdled treatments were positively correlated with increased woody but not herbaceous species cover. In 2021, some Monterey pine saplings were more than 6 m tall in girdled plots. Reducing canopy density increased seedling survival for the cohort that escaped initial mortality factors, such as pitch canker disease and herbivory. Consistent site-level differences in regeneration success were evident. Girdling primarily affected regeneration by facilitating seedling growth under favorable conditions, rather than by directly increasing seedling numbers. Gap creation thus emerges as a valuable strategy for preserving native Monterey pine forests and safeguarding the genetic variability of a globally important tree species.

Publication Title

Frontiers in Forests and Global Change

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