DOI: 10.5061/dryad.5tb2rbp3v
title: Savannas are not old fields: functional trajectories of forest expansion in a fire-suppressed Brazilian savanna are driven by habitat generalists

description: Under fire suppression, many tropical savannas transform into forests.
 Forest expansion entails changes in environmental variables and plant
 community structure. We hypothesized that forest expansion into savanna
 results in a shift in community-weighted mean functional traits from
 stress tolerance to competitiveness, with generalist species having trait
 values intermediate between those of specialists of savanna and forest
 habitats. We studied 30 plots distributed over three savanna-forest
 boundaries undergoing forest expansion in the Brazilian Cerrado, capturing
 a gradient from open savanna to recently formed forest. We measured
 functional traits of 116 woody species of savanna specialist, generalist,
 and forest specialist functional groups and quantified changes in species
 composition and mean traits across the basal area gradient. We identified
 two main axes of species traits. The first separated forest and generalist
 species from savanna specialists, with the latter possessing traits
 associated with resistance to disturbance and stress – such as thick
 leaves, thick bark, slower height growth and lower shade tolerance. Our
 second trait axis separated shrubs and understory trees from pioneer
 species. Generalist species’ traits did not differ substantially from
 forest species, nor did they tend to have a typical pioneer strategy.
 Community-weighted trait means changed linearly with forest development.
 There was a steady increase in traits associated with competitive
 dominance rather than stress tolerance and fire resistance, indicating a
 wholesale shift in the selective environment. Several of these patterns –
 e.g., increasing height and decreasing light requirements – are common in
 old-field succession. In contrast to old-field succession, we found that
 specific leaf area increased, leaf thickness decreased, and wood density
 stayed constant. The assembly of forests appears to be shaped by
 environmental filters that contribute to a functional trajectory distinct
 from most other studied ecosystems. Our results highlight the importance
 of the functional composition of the early community and of the early
 colonizers of the open environment. Differences between savanna and forest
 specialists reflect the selective effects of the contrasting environments,
 while the traits of generalists – and their interaction with environmental
 filters – drive dynamics of forest expansion.
description: Tree inventories are from 30 permanent plots established at Santa
 Barbara Ecological Station in 2015. Each plot is 20 x 50 m, in which all
 trees with a diameter of 30 cm height (D30) greater than 5 cm were
 measured. 10 subplots of 5 x 5 m were sampled for smaller trees, with D30
 &gt; 2 cm. These need an expansion factor of 4 for plot-level
 analysis. Heights were measured with telescoping height poles. See
 metadata sheet for inventory data.  Trait data was
 measured on individual trees. For specifics of measurements, see metadata
 tab for trait data.