Bridging the Flux Gap: Sap Flow Measurements Reveal Species‐Specific Patterns of Water Use in a Tallgrass Prairie

Abstract

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 the following: (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‐hr 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.

Spatio-temporal differences in leaf physiology are associated with fire, not drought, in a clonally integrated shrub

“…Although leaf water potential was not measured in 2018, stomata were similarly insensitive to intra-annual variation in microclimate during the drought. Cornus drummondii is deep-rooted and has access to a consistent water source ( Ratajczak et al 2011 ; Nippert et al 2013 ), which likely allows leaves to maintain similar g s throughout the growing season despite intra-annual fluctuations in precipitation, atmospheric temperature or vapour pressure deficit ( O’Keefe et al 2020 ). However, if deep water stores are depleted during extreme or prolonged future drought, C. drummondii may experience a leaf water potential threshold that induces stomatal closure ( Klein 2014 ; Meinzer et al 2016 ) and overall gas exchange rates may decline.…”

“…In mesic grasslands, shrub encroachment often increases above-ground net primary productivity ( Lett et al 2004 ; Knapp et al 2008 ) and soil organic carbon ( Li et al 2016 ). Deep-rooted shrubs may also shift grassland ecohydrology by using deep soil water relative to herbaceous species ( Nippert and Knapp 2007 ) and by transpiring more water than other co-occurring species ( O’Keefe et al 2020 ). Considering grasslands will likely become warmer and drier as climate change progresses, a shift in vegetation cover from herbaceous species to deep-rooted woody shrubs may deplete deep water stores and increase evapotranspiration, altering ecosystem water flux ( Huxman 2005 ; Scott et al 2014 ; Logan and Brunsell 2015 ; Honda and Durigan 2016 ; Acharya et al 2017 ; O’Keefe et al 2020 ).…”

“…Leaves were allowed to stabilize to chamber conditions before recording the measurement. Gas exchange measurements were collected between 10:00 h and 15:00 h. This is a similar time window used in other studies on this species ( Muench et al 2016 ; O’Keefe and Nippert 2018 ; O’Connor et al 2020 ) and there is no evidence of midday decline in sap flux throughout the growing season ( O’Keefe et al 2020 ). Sampling order was random for each sampling date to minimize variability contributed by diurnal effects.…”

“…Increased cover of deep-rooted clonal shrubs in grasslands is predicted to shift ecosystem carbon cycling and ecohydrology through increased above-ground biomass and evapotranspiration ( Knapp et al 2008 ; Logan and Brunsell 2015 ; O’Keefe et al 2020 ). Given that clonal shrubs are more likely to expand across grasslands and savannas than non-clonal shrubs ( Ratajczak et al 2011 ; Formica et al 2014 ), understanding how this growth form uses resources and responds to environmental variation will be key for predicting how shrub encroachment may alter carbon and water cycling in the future.…”

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

“…grasslands, hydraulic conductivity, hydraulic efficiency, hydraulic safety, roots contribute to its spread across these landscapes. Cornus drummondii utilizes soil water from deeper soil depths than grasses to support consistently high leaf gas exchange rates, which may provide this species a competitive advantage over herbaceous species in a variable environment (O'Keefe et al, 2020). However, how woody and grass root anatomical traits vary through the soil profile, particularly in response to fire, is unknown.…”