Ten‐year responses of ground‐dwelling spiders to retention harvest in the boreal forest

Abstract

The Ecosystem Management Emulating Natural Disturbances (EMEND) project tests the hypothesis that varying levels of green tree retention maintain and retain forest biodiversity better than conventional clear-cutting. We studied epigaeic spiders to assess biodiversity changes 2, 5, and 10 yr following a range of partial retention harvests (clear-cut, 10-75% retention) and unharvested controls in four boreal mixedwood cover types. A total of 56 371 adult spiders representing 220 species was collected using pitfall traps. Lasting effects on forest structure were proportional to harvest intensity. These changes strongly influenced spider richness, abundance, and species composition, as well as assemblage recovery. Distinctive assemblages were associated with disturbance level, especially with partial harvests (≤50% retention), and these were dominated by open-habitat species even 10 yr after harvest. Assemblages were more similar to those of controls in the highest (75%) retention treatment, but significant recovery toward the structure of pre-disturbance assemblages was not detected for any prescription in any cover type. Although early responses to retention harvest suggested positive effects on spider assemblages, these are better explained as lag effects after harvest because assemblages were less similar to those of unharvested controls 5 yr post-harvest, and only minor recovery was observed 10 yr following harvest. Retention of forest biodiversity decreased over time, especially in conifer stands and the lower (10-50%) retention treatments. Overall, retention harvests retained biodiversity and promoted landscape heterogeneity somewhat better than clear-cutting; however, there was a clear gradient of response and no retention "threshold" for conservation can be recommended on the basis of our data. Furthermore, results suggest that retention harvest prescriptions should be adjusted for cover type. We show that low retention ameliorated impacts in broadleaved forests characteristic of earlier stages in mixedwood succession, but only higher retention was associated with less impact in successionally older conifer forests. Although these short-term responses (10 yr) of spider assemblages support use of retention harvests, understanding the true conservation merit of these practices, relative to conventional approaches, requires evaluation over longer time scales, with work more focused on recovery of biodiversity than on its preservation after harvest.

Bryophyte abundance, diversity and composition after retention harvest in boreal mixedwood forest

“…There were notable differences among forest types in terms of bryophyte responses to variable‐retention harvesting, with mixed and conifer‐dominated forests appearing to be more sensitive. This is consistent with results of studies suggesting that both conservation and recovery of taxa affected by forest harvest will be more difficult in forests on mixedwood landscapes that have higher coniferous components (Pinzon et al., ). Thus, we recommend that harvest prescriptions be adjusted specifically for each forest type and that conifer‐dominated forests, in particular, be considered for higher levels of retention or prioritized in bryophyte conservation planning in boreal mixedwood landscapes.…”

“…The efficacy of retention harvest scaled with harvest intensity and we saw many examples of notable differences in response between the lower (clearcut, 10%, 20%) retention levels as compared to those >20%. These results concur with previous work on effects of retention harvesting on understorey vascular plants (Craig & Macdonald, 2009) and forest floor invertebrates (Pinzon, Spence, Langor, & Shorthouse, 2016;Work et al, 2010) at EMEND. While our results support the contention that even low levels of retention have value for ameliorating negative impacts of forest harvesting on forest floor biodiversity, the interactions we observed with harvest intensity suggest that higher levels of retention (>10%) would even better meet biodiversity conservation objectives.…”

Ecosystem memory of wildfires affects resilience of boreal mixedwood biodiversity after retention harvest

“…strength of retention). However, other studies at EMEND show that our study groups are all increasingly affected by increasing harvest intensity and even 10% retention is better than none (Harrison et al , Macdonald and Fenniak , Gradowski et al , Solarik et al , Work et al , Caners et al , Pinzon et al ). This suggests that higher retention levels promote greater EM in post‐harvest landscapes but further work is required for rigorous scientific demonstration.…”

“…). Pre‐harvest forest conditions can contribute to reorganization of forest ecosystems even after they are further disturbed by harvest (Perry et al , Pinzon et al ). Our results suggest that EM effects of historical wildfire regime may result in information and material legacies that confer resilience to ecosystems by setting biodiversity recovery toward the pre‐disturbance state even in the face of a disturbance with which the system has no previous experience; i.e.…”

“…). Individuals of both taxa are highly mobile through aerial ballooning or flight respectively, and some species quickly invade post‐harvest environments in high numbers (Gram et al , Pinzon et al , , Haché et al ). Ten years post‐harvest, however, this large opportunistic response of r‐strategist species seemed to decrease as EM effects on bird and spider recovery increased (Fig.…”