Forest succession along a productivity gradient following fire exclusion

Highlights

• Historically, fire equalized forest density across a range of site productivities.

• Mixed conifer forests have experienced greater change than ponderosa pine forests.

• Moister sites with western larch should be a priority for restoration.

Abstract

Numerous studies have documented significant change in conifer forests of the American West following the cessation of recurrent fire at the end of the 19th century. But the successional dynamics that characterize different forested settings in the absence of fire remain poorly understood. This study reconstructs structural and compositional change over 150 years across a productivity gradient that includes both mixed conifer and ponderosa pine forests within a 688,000 ha study area in the southern Blue Mountains of eastern Oregon. Moister and more productive forest stands dominated by grand fir did not support more tree basal area and were not appreciably denser in 1860 than drier and less productive stands dominated by ponderosa pine. The greatest magnitude of change over the last 150 years has occurred in the most productive mixed conifer stands. Drier ponderosa pine dominated stands with significant live old-growth structure have experienced relatively little change even after more than a century without fire. Reconstruction of historical forest density in productive mixed conifer forests suggests these sites were historically coupled to the broader dry forest landscape pattern via frequent fire disturbance and should be a priority for restoration if a return to historical conditions is a goal of management.

Introduction

Recurrent fire truncates succession, resulting in marked discrepancies between edaphic and climatic potential and typical biotic expression (Bond et al., 2005, Pyne, 1982). Accumulation of biomass and shifts in species composition associated with disruption of historical fire patterns following Euro-American settlement have been documented across boreal, eastern deciduous, Rocky Mountain evergreen, Pacific coastal, southeastern mixed evergreen, and Mexican montane forest biomes of North America (e.g., Brown et al., 2015, Stephens et al., 2015, Boucher et al., 2009, Nowacki and Abrams, 2008, Friedman and Reich, 2005, Moore et al., 2004, Heyerdahl and Alvarado, 2003, Gilliam and Platt, 1999, Fulé and Covington, 1994). The effects of fire exclusion on successional dynamics are often compounded by land use practices, climate change, pollution, and introduction of invasive species (Wimberly and Liu, 2014, King et al., 2013, Littell et al., 2010, Brooks et al., 2004, Fenn et al., 2003, Hemstrom, 2001).

In the inland Pacific Northwest (IPN), extensive logging, livestock grazing, and fire suppression has resulted in a decline in large and old trees, an increase in the relative abundance of fire intolerant tree species, and an increase in overall forest density (Spies et al., 2006, Hessburg and Agee, 2003). Dendroecological reconstructions (Merschel et al., 2014, Everett et al., 2008, Perry et al., 2004, Wright and Agee, 2004, Camp, 1999), analyses of historical timber inventories (Hagmann et al., 2014, Hagmann et al., 2013), and interpretation of historical photography (Hessburg et al., 2007, Hessburg et al., 1999) have documented significant changes to IPN forest structure and composition relative to historical conditions. Successional dynamics in the absence of fire have been reported for a variety of different regions throughout the American West (O’Connor et al., 2016, Margolis and Malevich, 2016, Abella et al., 2015, Taylor, 2010, Taylor, 2000, Fulé et al., 2009), but to date only a few studies have carefully examined how the magnitude and direction of forest change varies among different forest types in the IPN (Merschel et al., 2014, Heyerdahl et al., 2001, Camp, 1999).

Historical forest dynamics in the IPN and appropriate management strategies to restore historical conditions are almost always inferred from existing forest structure and composition (Stine et al., 2014, Franklin and Johnson, 2012, Hessburg et al., 2007, Franklin et al., 2002). But there is little empirical data that links contemporary conditions to past successional and disturbance dynamics, especially in moister and more productive forest types (Stine et al., 2014). Examining the characteristics of succession in the absence of fire in different IPN forest types will help inform management decisions and predict the potential for forests to return to stable structural and compositional configurations (Halpern, 1988), or experience uncharacteristically severe disturbance or state changes (Miller et al., 2009, Savage and Mast, 2005, Beisner et al., 2003).

This study uses detailed dendroecological reconstructions of forest structure and composition across a broad productivity gradient in the southern Blue Mountains of eastern Oregon to better understand the variability in forest response to disturbance regime shifts. Objectives of this study include: (1) Quantifying historical structure and composition in diverse forest types; and, (2) describing the magnitude and direction of succession in different forest types in the absence of fire.