Converging forest community composition along an edaphic gradient threatens landscape‐level diversity

Abstract

Aim Plant communities across the temperate zone are changing in response to successional processes and human‐induced disturbances. Here, we assess how upland forest under‐ and overstorey community composition has changed along an edaphic gradient. Location Northern Wisconsin, USA. Methods Forest sites initially sampled in the 1950s were resampled for overstorey composition and diversity, basal area, and understorey composition and diversity. We used clustering methods to identify groups of stands based on overstorey composition, and we used similarity indices, ordination and diversity indices to evaluate changes in species abundance and overall community structure. Results Sites clustered into four overstorey groups along the edaphic gradient: ‘hemlock’ sites dominated by hemlock in 1950, ‘mesic’ sites dominated by northern hardwoods, ‘dry’ sites with a significant pine inclusion in the canopy and diverse ‘dry‐mesic’ sites in the middle. Collectively, forests gained maple, ash and cherry while losing pines, birches and red oaks. The hemlock forest sites gained hardwoods, while the dry‐mesic sites shifted towards a more mesic hardwood composition. Only the driest sites have remained relatively stable in species composition. Main conclusions These trends reflect both ‘mesification’ and homogenization among northern forests. Highly diverse mid‐gradient and mesic hemlock‐dominated stands are transitioning to maple dominance. Fire suppression may be favouring invasions of more mesic plants into historically drier sites, while high deer abundance likely limits hemlock regeneration. If current trends continue, maples will dominate the majority of northern forests, with significant losses of local native species richness and substantial shifts in understorey composition.

Trait–environment relationships remain strong despite 50 years of trait compositional change in temperate forests

“…As a result, changes in understory composition can have important implications for both site richness and ecosystem function. In the north, graminoids and some ferns are increasing while evergreen and biotically pollinated species are decreasing (Rooney et al 2004, Wiegmann and Waller 2006, Amatangelo et al 2011. In the north, graminoids and some ferns are increasing while evergreen and biotically pollinated species are decreasing (Rooney et al 2004, Wiegmann and Waller 2006, Amatangelo et al 2011.…”

“…Prior to European settlement, forest composition across Wisconsin reflected a distinct climatic gradient, differential fire regimes, and soil characteristics that varied with glacial history and topology (Curtis 1959, Rhemtulla et al 2007). Since then, forests across the state have transitioned toward more uniform composition reflecting fire suppression, deer overabundance, and habitat fragmentation (Rooney et al 2004, Rogers et al 2008, Amatangelo et al 2011. Since then, forests across the state have transitioned toward more uniform composition reflecting fire suppression, deer overabundance, and habitat fragmentation (Rooney et al 2004, Rogers et al 2008, Amatangelo et al 2011.…”

“…Since then, forests across the state have transitioned toward more uniform composition reflecting fire suppression, deer overabundance, and habitat fragmentation (Rooney et al 2004, Rogers et al 2008, Amatangelo et al 2011. These changes in Wisconsin forest overstories often strongly affect understory community composition, yet the linkages between over-and understories have weakened (Rogers et al 2008, Amatangelo et al 2011, Sonnier et al 2014. These changes in Wisconsin forest overstories often strongly affect understory community composition, yet the linkages between over-and understories have weakened (Rogers et al 2008, Amatangelo et al 2011, Sonnier et al 2014.…”

Scarification and gap size have interacting effects on northern temperate seedling establishment

“…Some of the most noticeable compositional changes include a decrease in overstory evenness (Zhang et al, 2000), an expansion of a few shade tolerant species (Schulte et al, 2007;Nowacki and Abrams, 2008;Amatangelo et al, 2011;Hanberry, 2013), a large decline in conifers (including hemlock and white pine) and less shade tolerant species (including paper birch and yellow birch), and seedling and sapling size classes dominated by a few shade tolerant species (Neuendorff et al, 2007;Matonis et al, 2011). Some of these demographic shifts (e.g.…”

“…Unfortunately, due to changes in species composition including declining tree species diversity and evenness (Schuler and Gillespie, 2000;Lawrence, 2004;Amatangelo et al, 2011), the variety of goods and services some forest ecosystems can provide may soon decline (Chapin et al, 2000;Folke et al, 2004;Drever et al, 2006;Fischer et al, 2006). Complicating matters further, lower diversity may lead to declining resilience to novel disturbances, such as climate change and invasive pests and pathogens (Sturrock et al, 2011;AndersonTeixeira et al, 2013;Duveneck et al, 2014).…”