Changes in beta diversity and species functional traits differ between saplings and mature trees in an old‐growth forest

• Published in: Ecology and evolution 2021-01, Vol.11 (1), p.58-88
• Main Authors: Kirk, David Anthony, Brice, Marie‐Hélène, Bradstreet, Michael S., Elliott, Ken A.
• Format: Article
• Language: English
• Subjects: Abundance; Acer rubescens; Acer rubrum; Acer saccharinum; Carolinian forest; Climate change; community traits; Composition; Coniferous forests; Coniferous trees; Conifers; Cornus florida; Drought resistance; Forest ecosystems; Forests; Fraxinus americana; Groundwater; Groundwater levels; Hardwoods; Homogenization; Invasive species; Lakes; old‐growth forests; Ontario; Original Research; Pathogens; Plant diversity; Plant species; Population decline; Populus grandidentata; protected areas; Regeneration; Shade; species at risk; Species composition; Species diversity; temporal change; Trees; Water levels; β‐diversity
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.6913

Invasion by generalist tree species can cause biotic homogenization, and such community impoverishment is likely more important in rare forest types. We quantified changes in tree diversity within Carolinian (range in Central Hardwood Forest), central (range in Central Hardwood Forest and Northern Hardwood‐Conifer Forest), and northern species [range reached Northern‐Conifer‐Hardwood/closed Boreal (spruce‐Fir) Forest] in an old forest tract in southern Canada at points surveyed 24 years apart. We asked: How did mature tree and sapling composition and abundance change for the three species’ groups? Did those changes lead to biotic homogenization? Can species’ changes be explained by community traits? We tested for differences in temporal and spatial tree β‐diversity, as well as forest composition and structure, using univariate/multivariate analyses and a community trait‐based approach to identify drivers of change. Major increases occurred in abundance for mature Acer rubrum (northern), while other species decreased (Fraxinus americana, Populus grandidentata); declines were found in A. saccharinum (central) and Cornus florida (Carolinian). Species composition of saplings, but not mature trees, changed due to replacement; no evidence for biotic homogenization existed in either cohort. As a group, northern mature tree species increased significantly, while central species decreased; saplings of pooled Carolinian species also declined. Shade tolerance in mature trees increased, reflecting successional changes, while drought tolerance decreased, perhaps due to changing temperatures, altered precipitation or ground water levels. Saplings showed declines in all traits, probably because of compositional change. Our results demonstrated that saplings can more closely reflect change in forest dynamics than mature trees, especially over short time periods. Based on sapling trends, this remnant could ultimately transition to a mesophytic hardwood stand dominated by A. rubrum and other shade‐tolerant species, creating a more homogeneous forest. While encouraging regeneration for Carolinian and central tree species could ensure high levels of diversity are conserved in the future, it is important to balance this with the primary management goal of maintaining the forest's old‐growth characteristics. Our results highlight the importance of monitoring tree β‐diversity in long‐term forest plots to examine changes in homogenization, species composition, and functional trai