Wood net primary production resilience in an unmanaged forest transitioning from early to middle succession

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Abstract

The mixed deciduous forests of the upper Midwest, USA are approaching an ecological threshold in which early successional canopy trees are reaching maturity and beginning to senesce, giving way to a more diverse canopy of middle and late successional species. The net primary production (NPP) of these forests is generally considered past peak and in decline, but recent studies show a striking resilience in the NPP trajectories of some middle and late successional forests; yet, the mechanisms controlling such temporal changes in NPP are largely unknown. At the University of Michigan Biological Station in northern Michigan, we used a ≥9-year continuous record of wood net primary production (NPP), leaf area index (LAI), canopy composition, and stem mortality in 30 forested plots to identify the constraints on wood NPP as a mixed forest transitions from early to middle succession. Although wood NPP decreased over time in most stands, the rate of decline was attenuated when the canopy comprised a more diverse assemblage of early and middle/late successional species. The mechanism for sustained NPP in stands with more species diverse canopies was the proliferation of LAI by intact later successional tree species, even as stem mortality rates of early successional trees increased. We conclude that projections of carbon sequestration for the aging mixed forests of the upper Midwest should account for species composition shifts that affect the resilience wood NPP.

Introduction

The importance of early successional forests to the North American terrestrial carbon (C) sink is firmly established, with post-disturbance regrowth transferring C from the atmosphere to forest biomass. Prolific colonization and high growth rates by early successional species generally cause a rapid post-initiation rise in forest net primary production (NPP), which is followed by a gradual decline as early successional species senesce (Berger et al., 2004, Gough et al., 2008a). Widespread deforestation in the U.S. Midwest and East during the late 19th and early 20th centuries broadly and near-synchronously reset succession by abruptly transforming old-growth forests into early successional ecosystems (Gough et al., 2007). Following a post-disturbance release of C to the atmosphere, extensive forest regrowth prompted a North American C sink into woody plant production that increased throughout the 20th century and now is thought to be in gradual decline (Birdsey et al., 2006).

While the consequences of historical disturbances on NPP are well understood, the future trajectory of regional C sequestration remains highly uncertain as disturbance patterns shift and forests rapidly transition from early to middle succession (Pregitzer and Euskirchen, 2004, Birdsey et al., 2006). Many forests of the upper Midwest and Eastern U.S. are likely to undergo a subtle transition to middle succession that includes the emergence of a more biologically and structurally complex forest, but does not involve complete canopy replacement that occurs in fire prone and intensively managed ecosystems (Frelich and Reich, 1995, Stearns and Likens, 2002). This less severe transition to a more heterogeneous secondary forest is likely to be increasingly common because fire suppression and less aggressive forest harvesting have greatly reduced stand replacement in the region (Caspersen et al., 2000). Further complicating predictions of future forest C sequestration are recent studies showing modest increases in NPP by older forests (Urbanski et al., 2007, Luyssaert et al., 2008). Despite an ongoing transition in forest composition across much of the region, considerable uncertainty remains in whether middle successional forests will sequester C in biomass at rates comparable to that of their early successional predecessors.

Maintenance of NPP in forests transitioning from early to middle succession will depend on the resilience of forest growth as canopy dominant early successional species senesce and give way to later successional species. Forest thinning studies show that partial canopy removals have a negligible effect on NPP because they are offset by improved growth of residual canopy or understory trees (Mund et al., 2002, Sendak et al., 2003, Chiang et al., 2008, Sabo et al., 2008, Campbell et al., 2009, Harmon et al., 2009). In unmanaged forests undergoing abrupt declines in early successional canopy species, NPP thus may be better maintained in stands that contain an assemblage of longer-lived, middle and late successional species that are released upon the senescence of early successional competitors (Caspersen and Pacala, 2001, Luyssaert et al., 2008). Few studies have examined NPP in unmanaged forests during the transition from early to middle succession, and those that have relied on forest chronosequences rather than continuous long-term data from a single location (Caspersen and Pacala, 2001, Pregitzer and Euskirchen, 2004). Only recently has the accumulation of long-term, continuous C cycling data from forested ecosystems allowed for the examination of forest NPP over timescales that are relevant to successional processes (Urbanski et al., 2007, Gough et al., 2008a).

Our primary objective was to elucidate canopy controls on wood NPP trajectories during transition from early to middle succession for a broadly distributed, unmanaged aspen-dominated forest in northern lower Michigan, USA. Stand-replacing forest harvesting and wildfire at our study site and throughout the upper Midwest during the early 20th century established a widespread cohort of aspen (Populus spp.) and birch (Frelich, 1995, Friedman and Reich, 2005). Aspen-dominated forests at our site and across the region are now approaching or are past maturity and beginning to decline (Stearns and Likens, 2002, Wolter and White, 2002, Hill et al., 2005, Bergen and Dronova, 2007). We took advantage of a ≥9-year continuous record of wood NPP, leaf area, canopy composition, and stem mortality in 30 forested plots nested within this regionally important ecosystem to examine how near-decadal changes in canopy characteristics relate to the resilience of an important ecosystem function, NPP, as mortality of early successional species increases.

Section snippets

Site description

Our study was conducted at the University of Michigan Biological Station (UMBS) in northern lower Michigan, USA (45°35.5′N 84°43′W). The forest is a secondary successional mixed northern hardwood ecosystem that naturally regenerated following the late 19th century harvesting of old-growth white pine (Pinus strobus L.), red pine (Pinus resinosa Ait.), and eastern hemlock (Tsuga canadensis (L.) Carr.). Currently, over half of the standing live tree mass consists of bigtooth aspen (Populus

Landscape trends over time in mortality, wood NPP, LAI, and tree canopy diversity

The forest, at the landscape scale, exhibited substantial changes over 9 years in tree species composition because of declining early successional aspen and birch trees and relatively low mortality of later successional species. Live tree (d  8 cm) stem density in 1998 was highest for bigtooth aspen and red maple, which comprised >200 trees per hectare. Middle/late successional white pine, red oak, and American beech had stem densities of <100 trees per hectare (Fig. 1A). Ten-year stem mortality

Discussion

A primary objective of this work was to identify constraints on trajectories of wood NPP as mixed forests transition from early to middle succession, a phenomenon that is occurring broadly across the Midwestern and Eastern U.S. (Birdsey et al., 2006). Findings of a general decline at the plot scale in NPP in our ∼90-year-old forest are consistent with numerous studies that show production tends to decrease as early successional species senesce (Luyssaert et al., 2008). Our study is one of the

Acknowledgements

We thank two referees for their reviews of this manuscript. This research was supported by the U.S. Department of Energy's Office of Science (BER) through the Midwestern Regional Center of the National Institute for Global Environmental Change under Cooperative Agreements No. DE-FC03-90ER610100, and the Midwestern Regional Center of the National Institute for Climatic Change Research at Michigan Technological University, under Award No. DE-FC02-06ER64158. We acknowledge the University of

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