Academic journal article The Geographical Bulletin

Disturbance and Successional Dynamics in an Old-Growth White Pine-Mixed Hardwood Forest in the Great Lakes Region of Minnesota

Academic journal article The Geographical Bulletin

Disturbance and Successional Dynamics in an Old-Growth White Pine-Mixed Hardwood Forest in the Great Lakes Region of Minnesota

Article excerpt

INTRODUCTION

Eastern white pine (Pinus strobus L.) is a tree symbolic of the Great Lakes region. Prior to Euro-American settlement, it was widely distributed over approximately 4 million hectares (Frelich 1995). However, contemporary coverage of white pine has been reduced to 0.6% of its presettlement extent (Frelich 2002), mostly relegated to scattered remnant old-growth trees often found occupying the emergent canopy strata of pine-mixed hardwood forests (Abrams et al. 1995, Carleton et al. 1996, Abrams 2001, Burgess et al. 2005). This dramatic decline in areal extent and changes in stand-structural conditions raise important concerns regarding the future status of white pine in Great Lakes forests (Cornett et al. 1998).

Determining the disturbance history of a forest stand is crucial to understanding local forest dynamics (e.g., Lorimer and Frelich 1989, Veblen et al. 1991). This is particularly important for a species such as white pine diat exists across a broad geographic range and in diverse environmental settings (Wendel and Smith 1990; Abrams 2001). Research conducted in the Great Lakes region suggests that fire was the dominant disturbance agent in pre-settlement white pine forests (Heinselman 1981, Frelich 2002). However, the disturbance regimes of many white pine forests were radically altered with the onset of Euro-American settlement in the late 180Os. This period included land-use changes and extensive white pine logging followed by severe slash fires that nearly eliminated white pine seed source over much of its range (Heinselman 1973, Cornett et al. 1998, Weyenberg et al. 2004). The changes wrought in die ecology of white pine forests by these practices have been followed by additional modifications to the function of these forests diat have arisen both directly and indirectly from human activity.

Effective fire suppression in the Great Lakes has altered disturbance interactions in white pine forests by shifting species composition and increasing the relative importance of other disturbances, such as wind (e.g., Webb 1989) and deer browsing (e.g., Ziegler 1995). Historically, surface fires and occasional stand-replacing fires were the primary mechanisms that reset succession and created suitable sites for white pine regeneration (Abrams et al. 1995, Frelich 2002, Weyenberg et al. 2004), but twentieth century fire suppression effectively removed fire as a disturbance agent in this area (Frissell 1973). In the absence of fire, increasing levels of late-successional species such as balsam fir (Abies balsamea (L.) P. Mill) and sugar maple (Acer saccharum Marsh.) have developed in many pine forest understories (e.g., Carleton et al. 1996). This shift in species composition often results in more deeply shaded, moister understory conditions that reduce the likelihood of future fire occurrence. Wind disturbance events are relatively common in this region (Frelich and Lorimer 1991, Frelich and Graumlich 1994), yet unlike fire, they generally accelerate succession, particularly when the understory is dominated by more wind-firm hardwood species (Webb 2001; Rich et al. 2007). Coinciding with an increase in hardwood species abundance throughout pine forest understories, widespread agriculture and forest fragmentation expanded the preferred habitat of white- tailed deer (Odocoileus vtrginianus [Boddaert]) and facilitated a remarkable increase in their population since near extirpation in the early 190Os (Alverson et al. 1988, Augustine and Jordan 1998). In the presence of large deer populations, browsing of white pine seedlings has become a primary form of disturbance in many forests (Ziegler 1995, Frelich 2002).

The alteration of historical disturbance regimes over the past 150 years in existing old-growth white pine communities has placed them in a tenuous position and increased the demand for research focused on this historically important ecosystem (cf., Abrams et al. 1995). Many suggest that declines in white pine forests throughout the Great Lakes region are the result of anthropogenic alterations to disturbance regimes following Euro-American settlement, most notably the lack of fire (e. …

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