Retention patch size and conservation of saproxylic beetles in boreal white spruce stands

I am very pleased to introduce my relatively recent paper, Retention patch size and conservation of saproxylic beetles in boreal white spruce stands, published in Forest Ecology and Management in 2015. So, let me briefly talk about the beginning of this study. Back in the day I started my PhD study at the Invertebrate Ecology lab at the University of Alberta, Canada, there were several interesting projects in which I could be involved. Among many possibilities, I was strongly attracted by the project that studies relationships between forest retention patch size and beetle diversity, because it was a truly interesting opportunity to test the island biogeography theory (IBT) in the real world. Although a direct testing of IBT was impossible with previously designed retention patches in industrial harvest blocks (i.e., each patch was so close to either each other or nearby intact forest), this study brought me great insights about IBT and a real world forest management for conservation purposes.

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A trogossitid beetle, Peltis fraterna (Randall), found under the bark of white spruce downed deadwood.

I investigated my all-time favourite group of insects, beetles! Among them, I decided to study deadwood-associated beetles (so-called saproxylic beetles) since these beetles are well known to be vulnerable by large-scale industrial forestry. Also, there were no such studies that considered retention patch size and saproxylic insects in the western boreal forest of Canada.

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Three harvest blocks studied. Green color indicates harvested matrix (HM). Grey color indicates intact forest (IF). Orange color indicates retention patches categorized as small (S): 0.63-1.06 ha, medium (M): 1.43-2.93 ha, and large (L): 3.34-5.93 ha.

To determine if there is an optimal retention patch size that minimizes loss of saproxylic beetles on harvested landscapes in boreal white spruce forests in NW Alberta, I selected three different scales of retention patches (Small, 0.63-1.06 ha; Medium, 1.43-2.93 ha; and Large, 3.34-5.93 ha) 10 years after harvest. Harvested matrix and intact forests were also chosen as comparisons.

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Ground photos of different sizes of retention patches, harvested matrix, and intact forest (Forest detectives in the last photo are me and Bill Sperling in one rainy day).

These harvest blocks were designed under the natural disturbance management paradigm inspired by the Ecosystem Management Emulating Natural Disturbance (EMEND) project, and provided valuable opportunities to understand biodiversity responses in the ‘real-world’ situation.

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Window traps on a girdled tree and a snag.

I installed window traps on both girdled trees and snags with my friends (Jeremy Katulka and Lindsay Dent) to sample saproxylic beetles in both interior and edge of patches to examine possible edge effects on saproxylic beetle assemblages.

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Redundancy analysis (RDA) of saproxylic beetle assemblages of (a) all species combined. Ellipses show 95% confidence intervals around group centroids. Significant and non-significant environmental variables are indicated with black and grey arrows, respectively. Colors of ellipses: grey (harvested matrix), red (small patch), orange (medium patch), beige (large patch), and green (intact forest). Abbreviations: CL (center large), CM (center medium), CS (center small), DCs (decay classes), EL (edge large), EM (edge medium), ES (edge small), HM (harvested matrix), and IF (intact forest). *Lee et al. (2015)

The results from the study strongly supported the positive role of retention patches regardless of patch size. However, my co-authors and I found negative edge effects in both small and medium patches. Interestingly, beetle fauna in the center of small patches was entirely incorporated into those in the edge of large patches. This suggests that retention of large patches (> 3.33 ha) will better maintain saproxylic beetle assemblages similar to those in intact forests.

I recognize that there is no such threshold patch size that can be universally applied to all forest organisms. However, when considering current rates of large-scale and continuous harvest by human being, it will be necessary to determine an optimal patch size that minimizes negative effects on biodiversity, especially on sensitive species associated with intact forests. In the case of saproxylic beetle assemblages, large patches in harvested landscapes seem to be optimistic for conserving vulnerable saproxylic beetles against forestry practices.

*If anyone wants to read more detailed research findings with discussions, please find (or ask) the original paper linked below:

Lee, S.-I., Spence, J.R., Langor, D.W., and Pinzon, J., 2015. Retention patch size and conservation of saproxylic beetles in boreal white spruce stands. Forest Ecology and Management, 358: 98–107.

*Also there is a short version of the original research written for publics, industry partners and public officers published in EMEND Insights:

Lee, S.-I., Spence, J.R., Langor, D.W, Pinzon, J., 2016. EMEND Insights #9. Retention patch size influences saproxylic beetle conservation in white spruce stands. Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada, pp. 1–4.