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

Abstract

Background: Current 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. Methods: I 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.Results: Although 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. Conclusions: The 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.

Rapid digitization to reclaim thematic maps of white-tailed deer density from 1982 and 2003 in the conterminous US

“…In addition, despite variable browse preferences, almost all tree species, including species such as northern white cedar (Thuja occidentalis) identified as preferred browse reduced by deer browsing, have increased in relative abundance of trees (diameter ≥12.7 cm) in the eastern US between the 1800s and the current decade of 2010 (Hanberry, Palik & He, 2013;Hanberry & Abrams, 2019;Hanberry & Dey, 2019). Increases in most tree species probably preceded increases in deer densities, but recent trends during the past decades are similar to historical trends (Hanberry, 2019; northern white cedar increased slightly in northern mixed forests, B Hanberry, pers. obs., 2019).…”

Tree recruitment is determined by stand structure and shade tolerance with uncertain role of climate and water relations

“…Klopcic et al., 2014 ). There is little empirical evidence on how the tolerance to shade or drought modulates the response of tree recruitment to climate and stand structure at larger scales (Hanberry, 2019 ; Klopcic et al., 2014 ), although the importance of shade or drought tolerance for tree recruitment is beyond doubt from a qualitative and process‐based perspective (Leuschner & Ellenberg, 2017 ; Price et al., 2001 ; Shen & Nelson, 2018 ).…”

Timing of Tree Density Increases, Influence of Climate Change, and a Land Use Proxy for Tree Density Increases in the Eastern United States

“…Indeed, the tree species that have increased during the past century, along with tree density increases, due to changing land use including agriculture, probably have made eastern U.S. forests less resilient to climate change and extreme events. Historically dominant, widespread oak and pine species are drought-and fire-tolerant, but these species have been replaced by more mesic, drought-and fire-sensitive species, such as maples, ashes, and elms [46][47][48]. Historically dominant forests occurred as low-density savannas and woodlands, which provide benefits of reduced water demand and also reduced chance of insect outbreaks and spreading crown fires compared with dense forests [44].…”