Résumé
1. Considering intraspecific trait variability (ITV) in ecological studies
has improved our understanding of species persistence and coexistence.
These advances are based on the growing number of leaf ITV studies over
local gradients, but logistical constraints have prevented a solid
examination of ITV in root traits or at scales reflecting species’
geographic ranges. 2. We compared the magnitude of ITV in above- and
below-ground plant organs across three spatial scales (biophysical region,
locality, plot). We focused on six understory species (four herbs, two
shrubs) that occur both in disturbed and undisturbed habitats across
boreal and temperate Canadian forests. We aimed to document ITV structure
over wide ecological and geographical scales by asking: 1) What is the
breadth of ITV across species range-scale?; 2) What proportion of ITV is
captured at different spatial scales, particularly when local scale
disturbances are considered? and 3) Is the variance structure consistent
between leaf and analogous root traits, and between morphological and
chemical traits? 3. Following standardized methods, we sampled 818
populations across 79 forest plots simultaneously, including disturbed and
undisturbed stands, spanning four biophysical regions (~5200 km). Traits
measured included specific leaf area (SLA), specific root length (SRL),
and leaf and root nutrient concentrations (N, P, K, Mg, Ca). We used
variance decomposition techniques to characterize ITV structure across
scales. 4. Our results show that an important proportion of ITV occurred
at the local scale when sampling includes contrasting environmental
conditions resulting from local disturbance. A certain proportion of the
variability in both leaf and root traits remained unaccounted for by the
three sampling scales included in the design (36% on average) and a
largest amount for SRL (54%). Substantial differences in magnitude of ITV
were found among the six species, and between analogous traits, suggesting
that trait distribution was influenced by species strategy and reflects
the extent of understory environment heterogeneity. 5. Even for species
with broad geographical distributions, a large proportion of
within-species trait variability can be captured by sampling locally
across ecological gradients. This has practical implications for sampling
design and trait selection for both local studies and continental scale
modeling.