Combining aggregated and dispersed tree retention harvesting for conservation of vascular plant communities

Abstract

Retention harvesting (also called tree retention or structural retention), in which live mature trees are selectively retained within harvested stands at different retention levels and in different patterns (aggregated to dispersed), is increasingly being used to mitigate the negative impacts of forest harvesting on biodiversity. However, the effectiveness of combining different patterns of retention harvesting for conservation and recovery of understory vascular plants in the long term is largely unknown. To address this gap, we compared understory vascular plant diversity, abundance, and composition between aggregated retention and five levels of surrounding dispersed retention (0% [clearcut], 10%, 20%, 50%, 75%) 15 yr postharvest. We also investigated the influence of dispersed retention on the ability of embedded retention patches to support plant communities characteristic of unharvested forests, and whether it varies by patch size of aggregated retention (0.20 ha or 0.46 ha) and position within patches (edge or interior). Species richness, diversity, and cover were higher in the dispersed retention than in the patch retention as the harvested areas favored early-seral plant species. Graminoid cover was greater at the edges than in the interior of large patches. Retention patches as small as 0.2 ha more effectively supported shade-tolerant (forest interior) plant communities when they were surrounded by higher levels of dispersed retention (as compared to patches retained within clearcuts). Overall, the combined use of both aggregated and dispersed retention within a given cutblock benefits both late- and early-seral plant species and thus could effectively conserve understory plant assemblages in harvested landscapes. Sustainable forest management should therefore consider using a range of retention patch sizes combined with varying levels of surrounding dispersed retention in harvest designs to achieve objectives for plant conservation.

Regeneration patterns of key pine species in a mixed-pine forest indicate a positive effect of variable retention harvesting and an increase in recruitment with time

“…Since the aggregated retention consists of both open patches and patches with tree clusters, plots in these stands were established by assigning three plots in the open patches and three within the tree clusters. Although edge effects can be an important factor in the aggregated retention (Powers et al 2008;Franklin et al 2018), our objective was to capture responses that represent the average conditions of both the open and retained tree patches represented in the aggregated treatment. To establish plots, we established a grid system over each stand in ArcGIS 10.1 (2012) and generated several grid intersections.…”

“…It is also possible that there are just no marked differences in the responses of these two pine species between the aggregated and dispersed retention patterns when VRH is implemented at the 30% retention level. If the latter is the case, and depending on the scale and management objectives, such similarities in species responses between retention patterns may provide managers with the flexibility to accommodate multiple management objectives (Franklin et al 2018), especially where such objectives are complimentary.…”

Level and spatial pattern of overstory retention impose trade‐offs for regenerating and retained trees

“…Managers must weigh multiple, competing objectives in the design of VR prescriptions. Our results suggest that a diversity of approaches are needed to achieve balance among these objectives, e.g., by distributing aggregates, clearings, and areas of dispersed retention within or among harvest units (e.g., Lee et al 2017, Franklin et al 2018, mimicking to some extent, the heterogeneity of habitats, structures, and resource environments created by natural disturbance. At the same time, prescriptions can be tailored to local conditions, capitalizing on stand features (e.g., seed sources, advance regeneration, or other biodiversity values), edaphic conditions, or topography (e.g., Rudolphi et al 2014) that enhance ecological or other objectives.…”

“…Although DEMO was not designed to test aggregate stability as a function of size, patches smaller than 1 ha, with greater edge-to-area ratios, should be more susceptible to wind damage (Esseen 1994, J€ onsson et al 2007, Steventon 2011) and less likely to provide the life-boating function of larger aggregates (Perhans et al 2009, Gustafsson et al 2020. Risk of windthrow could also be reduced by embedding smaller aggregates among dispersed trees (Franklin et al 2018), or by a staged approach to harvest consisting of a partial "preparatory" cut, a period of adjustment, then a final reduction to the target density or basal area.…”

Designing Timber Harvesting to Enhance New Zealand Falcon Populations

“…Heterogeneous landscapes provide greater prey diversity or prey abundance for raptors and are preferentially used by a wide range of raptors including falcons and hawks (Jullien and Thiollay 1996, Petty 1998, Steenhof et al 1999, Ganusevich et al 2004, Atuo and O'Connell 2017). Therefore, conservation measures facilitating avian diversity in managed forests will often involve the creation of heterogeneous landscapes by establishing retention patches, leaving live mature trees on the harvesting sites (Rosenvald and Lõhmus 2008, Franklin et al 2018, Franklin et al 2019), or by rotational (Drapeau et al 2000) small‐scaled patch harvesting throughout the landscape (Petty 1998, Barrientos and Arroyo 2014).…”