Legacy forest structure increases bird diversity and abundance in aging young forests.

Marlyse C. Duguid,Mark S. Ashton,Juliana Hanle

doi:10.1002/ece3.5967

Abstract

Many studies have demonstrated the importance of early‐successional forest habitat for breeding bird abundance, composition, and diversity. However, very few studies directly link measures of bird diversity, composition and abundance to measures of forest composition, and structure and their dynamic change over early succession. This study examines the relationships between breeding bird community composition and forest structure in regenerating broadleaf forests of southern New England, USA, separating the influences of ecological succession from retained stand structure. We conducted bird point counts and vegetation surveys across a chronosequence of forest stands that originated between 2 and 24 years previously in shelterwood timber harvests, a silvicultural method of regenerating oak‐mixed broadleaf forests. We distinguish between vegetation variables that relate to condition of forest regeneration and those that reflect legacy stand structure. Using principal components analyses, we confirmed the distinction between regeneration and legacy vegetation variables. We ran regression analysis to test for relationships between bird community variables, including nesting and foraging functional guild abundances, and vegetation variables. We confirmed these relationships with hierarchical partitioning. Our results demonstrate that regenerating and legacy vegetation correlate with bird community variables across stand phases and that the strength with which they drive bird community composition changes with forest succession. While measures of regeneration condition explain bird abundance and diversity variables during late initiation, legacy stand structure explains them during stem exclusion. Canopy cover, ground‐story diversity, and canopy structure diversity are the most powerful and consistent explanatory variables. Our results suggest that leaving varied legacy stand structure to promote habitat heterogeneity in shelterwood harvests contributes to greater bird community diversity. Interestingly, this is particularly important during the structurally depauperate phase of stem exclusion of young regenerating forests.

Highlights

Breeding bird communities reflect their habitat (Cody, 1985)
Our results show that, during early stem exclusion—the most depauperate phase of early stand development in regard to stand structure—leaving legacy stand structure correlates with bird community variables (Table 2) (Hilmers et al, 2018)
Our results support temperate mixed broadleaf management to promote ground-story diversity and retain varied legacy stand structure. They agree with prior research in demonstrating that the ephemerality of early seral habitat requires landscape-level management—harvests timed in intervals of 7–10 years across a forest—but we demonstrate the importance of legacy stand structure during early stem exclusion and its relationships with a variety of bird functional guilds

Summary

| INTRODUCTION

Breeding bird communities reflect their habitat (Cody, 1985). Over the last century, dramatic changes of the age and structure of forests due to the suppression of disturbance regimes, agricultural abandonment, and declines in timber harvesting have drastically altered bird populations in eastern North America (Askins, 1998; Askins, Zuckerberg, & Novak, 2007; Brawn, Robinson, & Thompson, 2001; DeGraaf & Yamasaki, 2003; Litvaitis, 1993) and many temperate broadleaf forests worldwide (Block & Brennan, 1993; Gustafsson et al, 2012; MacArthur & MacArthur, 1961). Shrublands provide critical habitat for shrub-nesting bird species, many of whom are at risk or have seen sharp declines in their populations (DeGraaf & Yamasaki, 2003; King & Schlossberg, 2014; North American Bird Conservation Initiative US Committee, 2014; Schlossberg & King, 2007). Studies have demonstrated that regenerating shelterwoods host the greatest diversity and abundance of breeding birds in northeastern North American forests, attributing this to shelterwoods’ varied structure and composition providing habitat for multiple bird functional guilds (Ashton & Kelty, 2018; Duguid, Morrell, Goodale, & Ashton, 2016; Goodale, Lalbhai, Goodale, & Ashton, 2009; Keller, Richmond, & Smith, 2003; King & DeGraaf, 2000; Labbe & King, 2014; Perry & Thill, 2013). Shelterwoods’ varying arrangements of retained legacy trees facilitate the regeneration of heavy-seeded and poorly dispersed tree species that require partial shade for germination and seedling establishment, such as oaks, hickories, and maples (Ashton & Kelty, 2018). Using a combination of bird point counts and detailed vegetation surveys, we examine the following questions: (a) How does regenerating vegetation influence bird community composition and abundance in irregular shelterwood harvests over time? and (b) how does this differ from legacy vegetation influence on bird community composition and abundance?

| METHODS

Findings

| DISCUSSION

| CONCLUSION