Disturbance-based silviculture for habitat diversification: Effects on forest structure, dynamics, and carbon storage

Highlights

• We analysed disturbance-based silvicultural treatments at 217 inventory plots.

• Treatments enhanced the variation in individual structural elements.

• However, treatments did not fully approximate partial wind disturbances.

• Carbon storage was positively associated with stand structural complexity.

• Managing for bird habitats improves forest heterogeneity while maintaining high carbon stocks.

Abstract

Disturbance-based silviculture is of increasing interest as an approach to provide multiple ecosystem services and beta diversity in habitat conditions. One such approach increasingly employed in the eastern U.S. is a set of forestry practices developed to diversify forested bird habitats, called Silviculture with Birds in Mind (SBM). While strongly appealing to many private landowners, empirical data have not yet been reported regarding the effects of SBM treatments on forest structure and dynamics and how they compare to natural disturbances. Moreover, the potential of bird-oriented silviculture like SBM to enhance co-benefits, for instance, by retaining high carbon stocks in managed forests, has not been investigated. The objectives of our study were thus (i) to analyze the effects of SBM treatments on forests and compare them with natural disturbances, and (ii) to assess the co-benefits of multiple habitat indicators and carbon storage within three years of silvicultural treatment in mature northern hardwood-conifer forests.

We derived 14 stand structural variables as well as carbon storage from 217 SBM inventory plots, and compared them with the effects of intermediate-severity wind disturbance using non-metric multidimensional scaling (NMDS). Subsequently, we applied multi-hierarchical Bayesian models to investigate SBM treatment effects on aboveground carbon storage, as well as on four key habitat indicators. We also used Bayesian models to derive the relationships between habitat indicators and carbon storage.

SBM treatments created a diversity of post-harvest stand conditions and, while having lower values for some structural characteristics in comparison to controls, significantly enhanced the variation in individual structural elements. Moreover, the treatments were closer in ordinal space to the irregular structure associated with intermediate-severity wind disturbance than untreated control plots. However, the NMDS indicated that SBM treatments did not fully approximate partial wind disturbances. Carbon storage was positively associated with stand structural complexity.

Disturbance-based approaches like SBM help diversify habitat conditions and we expect these effects to become more pronounced as stands respond to the treatments over time. If applied more broadly, treatments targeted at diversifying habitats would also help maintain high carbon stocking at landscape scales. However, as the treatments do not fully emulate the region’s natural disturbance regime, we propose widening the portfolio of multi-cohort, retention, and gap-based silvicultural approaches in landscape-scale management.

Introduction

Recent decades have witnessed growing interest in the development of silvicultural approaches designed to mimic natural disturbances in many parts of the world (Brang et al., 2014, Franklin et al., 2007, Seymour et al., 2002). These approaches, often referred to as ‘disturbance-based’, ‘nature-oriented’, ‘ecological’, ‘close-to-nature’, ‘multi-functional’ or ‘retention’ forestry (Puettmann et al., 2015), share the objective of perpetuating the full range of stand scale structures and landscape patterns which organisms require, assuming adaptation to the natural disturbance regimes driving stand and landscape dynamics (Franklin et al., 2007). A further goal is to provide a broader array of habitat conditions and ecosystem services than may be associated with practices, such as short rotation industrial forestry or over reliance on even-aged management, that tend to simplify and homogenize stand and landscape structures (Kuuluvainen and Grenfell, 2012, Puettmann and Tappeiner, 2014). Some disturbance-based silvicultural systems were specifically designed to encourage both stand structural complexity and landscape patch diversity (Keeton, 2006, Bauhus et al., 2009, Stanturf et al., 2014). These approaches have great potential to balance the interests of society in biodiversity and the supply of multiple ecosystem services. For instance, management for structural complexity in mixed northern hardwood forests of eastern North America has been found to increase elements of late-successional biodiversity (Dove and Keeton, 2015, Gottesman and Keeton, 2017, McKenny et al., 2006) and carbon storage (Ford and Keeton, 2017) while, at the same time, providing wood products from timber harvests (Nunery and Keeton, 2010).

Disturbance-based approaches have struggled to gain traction within the forestry community in the eastern United States (Fahey et al., 2018). However, recently interest has developed around silvicultural approaches specifically tailored to promote a diversity of bird habitats (Sallabanks and Arnett, 2005), reflecting the nation-wide interest among non-industrial private landowners in birds as a primary motivation for owning forestland (Butler et al., 2007). A leading example is a set of approaches collectively called Silviculture with Birds in Mind (SBM), currently being demonstrated on a variety of ownerships in northeastern North America (Hagenbuch et al., 2012). Variants of SBM are under development for other regions of North America as well, such as the U.S. Southeast and Pacific Northwest (see, for example, Wood et al., 2013). In the Northeast, six innovative silvicultural treatments adapted to three stand development conditions have been promoted, of which five had been implemented at the time of this study. The treatments are largely based on disturbance-oriented concepts (see Frelich and Lorimer, 1991, Seymour et al., 2002, Keeton, 2006, Franklin et al., 2007), such as gap creation, legacy tree retention, variable density thinning, and dead wood enhancement, and are applied to deciduous hardwood, coniferous, and mixed forest types. Some of the treatments are explicitly intended to emulate the multi-aged (or “multi-cohort”) stand structures associated with partial disturbance events, particularly intermediate (or moderate) intensity windthrow (Hanson and Lorimer, 2007, Meigs and Keeton, 2018). Multi-cohort systems, such as the irregular shelterwood method and expanding gaps with retention, are of increasing interest and usage in the region (D’Amato et al., 2018, Kern et al., 2017, Raymond et al., 2009).

The disturbance-based concepts guiding SBM postulate that by adjusting silvicultural systems to benefit a broader array of taxa, including avian diversity, other co-benefits may accrue. These include ecosystem services associated with a diversity of successional and stand development conditions (Swanson et al., 2011), including a greater portion of forested area in late-successional or old-growth conditions (Thom et al., 2019). These conditions are often associated with a high structural complexity and high carbon density (Gunn et al., 2014, Keeton et al., 2011, McGarvey et al., 2015, Urbano and Keeton, 2017).

To date the SBM approaches have had broad appeal (Keeton, unpublished landowner survey data), both because they integrate wildlife goals (game and non-game) with economic objectives and because landowners overwhelmingly support bird conservation on non-industrial private forests (Butler et al., 2007). In northern New England, the prescriptions target 12 indicator species as proxies for the diversity of habitat conditions required by 40 songbird species identified as high priorities for conservation (Hagenbuch et al., 2012). A strong suit is the incorporation of explicit operational considerations rendering the guidelines readily implementable across a variety of ownerships, including private non-industrial forestlands (Hagenbuch et al., 2012).

The 12 indicator species in SBM were selected because they (i) require a wide range of forests types and conditions; (ii) are declining in population size or are at risk; (iii) have a large portion of their global breeding population in the region; and (iv) are relatively easy to identify by non-professional observers. Habitat conditions supporting feeding and breeding of different species include, for instance, irregularly sized and structured canopy gaps with early successional vegetation (e.g., American woodcock (Scolopax minor [Gmel.])), tall trees (e.g., scarlet tanager (Piranga olivacea [Gmel.])), and downed (e.g., Canada warbler (Cardellina canadensis [L.])) and standing deadwood (e.g., yellow-bellied sapsucker (Sphyrapicus varius [L.])) (Hagenbuch et al., 2012). SBM treatments have been designed to collectively create these habitat conditions by mimicking disturbances of different severities and sizes.

Some of the silvicultural approaches investigated here, such as variable density thinning (in SBM termed “variable retention thinning” or VRT), have been employed in many different forest systems in North America (Carey, 2003, Stanturf et al., 2014), but have been poorly studied in the eastern U.S. Others are better understood in terms of implications for long-term stand development trajectories (see, for example, Halpin et al., 2017) but have not been assessed relative to other disturbance based approaches. Though the subject of on-going monitoring, effects of the SBM treatments on bird populations are still largely uncertain. Stand improvement thinning, a variant of one of the SBM treatments, was successful at fostering songbird occupancy and abundance (Rankin and Perlut, 2015). In addition, Nareff et al. (2019) found that territory densities and population abundances for Cerulean Warblers responded positively to a range of retention-based, bird-oriented silvicultural practices in Central Appalachian hardwoods. However, the success of bird-oriented treatments at emulating natural disturbances, and their ability to achieve management objectives other than bird habitat, have not been previously assessed. This also holds true for associations between silviculturally enhanced bird habitat diversity and ecosystem services such as carbon storage.

In this study, we quantify the co-benefits of disturbance-based treatments oriented towards birds in terms of their effects on habitat conditions, stand structure associated with natural disturbance impacts, and carbon storage, the latter being a service fundamentally linked to climate regulation (Bonan, 2008, Schwaab et al., 2015, Thom et al., 2017). The objectives of our study are to: (i) describe the outcome of SBM treatments and compare them with natural disturbances typical for the region; and (ii) analyze the co-benefits of multiple habitat indicators and carbon storage within three years of silvicultural treatment. We hypothesize that SBM treatments have varying effects on individual structural indicators, and as a suite of techniques emulate a range of low to intermediate severity natural disturbance influences on forest structure (Morrissey et al., 2014). Hence, we expect that these treatments enhance the availability of key structural attributes supporting habitat conditions for numerous forest-dwelling species. Moreover, we expect that carbon density remains high at most SBM stands after treatment which has been observed after low severity natural disturbance (Reinikainen et al., 2013). We also anticipate a positive relationship between key structural variables and carbon storage as structurally complex northern hardwood-conifer forests have been found to exhibit a high carbon density (Thom and Keeton, 2019, Urbano and Keeton, 2017).