Root traits reveal safety and efficiency differences in grasses and shrubs exposed to different fire regimes

Roots are key components of terrestrial ecosystems, yet little is known
about how root structure and function vary across a broad range of
species, functional groups, and ecological gradients in situ. We assessed
how woody and grass root anatomical traits vary among soil depths and
different fire frequencies to better understand the water-use strategies
exhibited by these two functional groups in tallgrass prairie experiencing
woody encroachment. Specifically, we asked: (1) Do root anatomical traits
differ with fire frequency or soil depth? (2) Do relationships between
anatomical traits that confer hydraulic safety versus efficiency vary by
fire frequency or soil depth? (3) Is root anatomy associated with
integrative root traits (e.g., root diameter, specific root length (SRL),
and root biomass)? (4) When scaled by root biomass, do root water-use
traits impact the capacity for water uptake? We collected grass and woody
roots from 10, 30, and 50 cm deep soil in areas burned every 1, 4, and
20-years. We then measured xylem conduit diameter, conduit cell wall
thickness, conduit number, conduit mechanical safety (t/b), stele area,
endoderm thickness, hydraulic diameter, theoretical hydraulic
conductivity, and root-system theoretical hydraulic conductance. We
observed: (1) Woody roots had high hydraulic conductance in shallow soils
and greater mechanical strength in deeper soils, which may provide a
competitive advantage in less frequently burned, more diverse plant
communities; (2) Shallow grass roots had unique trait combinations at the
anatomical and root-system levels (thinner, more numerous conduits and
higher root-system hydraulic conductance compared to deeper roots) that
likely allow these plants to rapidly use water but tolerate dry soils
under multiple fire regimes; and (3) hydraulic safety versus efficiency
tradeoffs translate between different hierarchical scales (i.e., from
anatomical to integrative root traits). These results provide anatomical
evidence to explain water-use dynamics in tallgrass prairie and also
provide novel insight regarding functional strategies that may facilitate
the conversion from grassland to shrubland in less frequently burned
tallgrass prairie. Future work should investigate these dynamics in situ,
as they may explain current and future patterns of woody-grass coexistence
in tallgrass prairies.