Bridging the flux gap: sap flow measurements reveal species-specific patterns of water-use in a tallgrass prairie

Predicting the hydrological consequences following changes in grassland
vegetation type (i.e., woody encroachment) requires an understanding of
water flux dynamics at high spatiotemporal resolution for predominant
species within grassland communities. However, grassland fluxes are
typically measured at the leaf or landscape scale, which inhibits our
ability to predict how individual species contribute to changing ecosystem
fluxes. We used external heat balance sap flow sensors and a hierarchical
Bayesian state-space modeling approach to bridge this “flux-gap” and
estimate continuous species-level water flux in common tallgrass prairie
species. Specifically, we asked: 1) How do diurnal and nocturnal water
fluxes differ among woody and herbaceous plants? (2) How sensitive are
woody and herbaceous species to environmental drivers of diurnal and
nocturnal water flux? We highlight three results: (1) Cornus drummondii,
the primary woody encroacher in this grassland, exhibited the greatest
canopy-level water loss, (2) nocturnal transpiration was a large component
of the water lost in this ecosystem and was driven primarily by C4 grasses
and C. drummondii, and (3) the sensitivity of canopy transpiration to
environmental drivers varies among plant functional types and throughout a
24-hour period. Our data reveal important insights regarding the water-use
strategies of woody versus herbaceous species in tallgrass prairies, and
about the potential hydrological consequences of ongoing woody
encroachment. We suggest that the high, static flux rates observed in
woody species will likely deplete deep water stores over time, potentially
creating hydrological deficits in grasslands experiencing woody
encroachment and concomitantly increasing the vulnerability of these
ecosystems to drought.