Recent shifts in shade tolerance and disturbance traits in forests of the eastern United States

Brice B Hanberry

doi:10.1186/s13717-019-0187-3

Abstract

BackgroundCurrent forests of the eastern USA have the potential to succeed in composition to more shade-tolerant species. However, long-term processes of transition from fire-tolerant tree species to fire-sensitive species and effects of current land use on forests may interfere with successional progression.MethodsI examined if forests in three regions have increased in shade tolerance and if life history strategy groups that represent response to disturbance (i.e., fire-tolerance, early-successional species based on low shade tolerance, mid-successional species, late-successional species, and trees valued for traits related to short harvest rotations) have changed, using Forest Inventory and Analysis surveys, adjusted for comparison, and generalized linear mixed models to assess approximately 30 year trends, with adjustments to equalize different survey methods.ResultsAlthough statistically significant, a slight increase of 2 to 4% in regional mean shade tolerance may not be ecologically significant. In the central East, mid-successional species replaced early-successional species and early-successional species replaced fire-tolerant oaks, resulting in an overall shift from fire-tolerant oaks to mid-successional species. Decreased fire-tolerant pine species and increased planted pine species were the major changes in the northern Southeast and Coastal Plain.ConclusionsThe successional process of increased composition by shade-tolerant species over time was overshadowed by land use changes that resulted in decreased fire-tolerant species and increased planted pine. Furthermore, frequent land use disturbance may continue to prevent the slow progress of compositional succession to very shade-tolerant species.

Highlights

Succession in composition to more shade-tolerant species was rare in forests of the eastern USA before EuroAmerican settlement (1600s to 1800s; e.g., Hanberry et al 2018a)
Excluding boreal forests, infrequent severe disturbance, generally by wind and fire, occurred on rotations of hundreds of years that were longer than tree lifespans; when stand-replacing disturbance occurred, canopy loss was diffuse (Seymour et al 2002)
Large-scale, long-term comparisons in US forests would not be possible without use of Forest Inventory and Analysis (FIA) surveys, which can be improved by use of adjustments to make more accurate estimates

Summary

Introduction

Succession in composition to more shade-tolerant species was rare in forests of the eastern USA before EuroAmerican settlement (1600s to 1800s; e.g., Hanberry et al 2018a). Excluding boreal forests, infrequent severe disturbance, generally by wind and fire, occurred on rotations of hundreds of years that were longer than tree lifespans; when stand-replacing disturbance occurred, canopy loss was diffuse (Seymour et al 2002). Other disturbances, such as frequent surface fires of less than 35 years that removed small trees, tended to produce mortality in a few overstory trees, resulting in gaps that allowed recruitment of tree regeneration, albeit with some amount of intermediate severity disturbance (Hart and Kleinman 2018). Long-term processes of transition from fire-tolerant tree species to fire-sensitive species and effects of current land use on forests may interfere with successional progression

Methods

Results

Conclusion