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

1. 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 Hardwoods), northern (range reached Northern
hardwood-conifer/Boreal-spruce-fir) and central species (range in Central
Hardwood region and Northern hardwood-conifer) in an old forest in
southern Canada at points surveyed 24 years apart. 2. We asked: How did
mature tree and sapling composition and abundance change for the 3
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. 3. Major
increases occurred in abundance for mature Acer rubrum (northern), while
others 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 declined; saplings
of Carolinian species declined.  4. 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.   5. 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.  6. While
encouraging regeneration for Carolinian and central tree species could
ensure high levels of diversity are conserved in the future, it is
important that this is balanced with the primary management goal of
maintaining the older-growth characteristics of the forest.