TY - JOUR
T1 - Time variable hydraulic parameters improve the performance of a mechanistic stand transpiration model. A case study of Mediterranean Scots pine sap flow data assimilation
AU - Sus, Oliver
AU - Poyatos, Rafael
AU - Barba, Josep
AU - Carvalhais, Nuno
AU - Llorens, Pilar
AU - Williams, Mathew
AU - Vilalta, Jordi Martínez
PY - 2014/11/1
Y1 - 2014/11/1
N2 - Tree transpiration is regulated by short-term physiological adjustments and long-term shifts in hydraulic architecture in response to fluctuating evaporative demand and water supply. Despite the tight interdependence of plant water loss and carbon uptake and its crucial implications for plant growth and survival under drought conditions, the underlying mechanisms remain incompletely represented in most state-of-the-art mechanistic models. Important process information is resolved in tree transpiration (sap flow) data, which are the measurable outcome of water transport through the soil-plant-atmosphere continuum under variable environmental conditions. Here, we assimilated sap flow data measured in two Scots pine stands from climatically contrasting sites - one of which experiencing a strong drought during the study period - in NE Spain into a process-based ecophysiological model (SPA) using the Ensemble Kalman Filter (EnKF) in order to: (1) distinguish differences in hydraulic characteristics between sites and between healthy and defoliated individuals within a site; (2) identify possible structural model deficiencies, particularly regarding temporal changes in plant hydraulic conductance which the model assumes constant; and (3) derive implications for gross photosynthesis and carbon cycling. In terms of stomatal control, the assimilation of sap flow data into SPA showed a more conservative water use under dry conditions. Time-varying plant conductivity substantially improved model performance under severe drought, while seasonally varying capacitance and stomatal efficiency only resulted in marginal improvements. Not accounting for this seasonal variability would translate into a 30-60% overestimation of modelled GPP during drought. Our results suggest that an explicit representation of mechanisms leading to temporal changes in hydraulic conductivity (i.e., xylem embolism) is required for models to reproduce tree functioning under extreme drought.
AB - Tree transpiration is regulated by short-term physiological adjustments and long-term shifts in hydraulic architecture in response to fluctuating evaporative demand and water supply. Despite the tight interdependence of plant water loss and carbon uptake and its crucial implications for plant growth and survival under drought conditions, the underlying mechanisms remain incompletely represented in most state-of-the-art mechanistic models. Important process information is resolved in tree transpiration (sap flow) data, which are the measurable outcome of water transport through the soil-plant-atmosphere continuum under variable environmental conditions. Here, we assimilated sap flow data measured in two Scots pine stands from climatically contrasting sites - one of which experiencing a strong drought during the study period - in NE Spain into a process-based ecophysiological model (SPA) using the Ensemble Kalman Filter (EnKF) in order to: (1) distinguish differences in hydraulic characteristics between sites and between healthy and defoliated individuals within a site; (2) identify possible structural model deficiencies, particularly regarding temporal changes in plant hydraulic conductance which the model assumes constant; and (3) derive implications for gross photosynthesis and carbon cycling. In terms of stomatal control, the assimilation of sap flow data into SPA showed a more conservative water use under dry conditions. Time-varying plant conductivity substantially improved model performance under severe drought, while seasonally varying capacitance and stomatal efficiency only resulted in marginal improvements. Not accounting for this seasonal variability would translate into a 30-60% overestimation of modelled GPP during drought. Our results suggest that an explicit representation of mechanisms leading to temporal changes in hydraulic conductivity (i.e., xylem embolism) is required for models to reproduce tree functioning under extreme drought.
KW - Data assimilation
KW - Drought
KW - Ecosystem modelling
KW - Mediterranean
KW - Plant hydraulics
KW - Sap flow
UR - http://www.scopus.com/inward/record.url?scp=84908192383&partnerID=8YFLogxK
U2 - 10.1016/j.agrformet.2014.08.009
DO - 10.1016/j.agrformet.2014.08.009
M3 - Article
AN - SCOPUS:84908192383
SN - 0168-1923
VL - 198
SP - 168
EP - 180
JO - Agricultural and Forest Meteorology
JF - Agricultural and Forest Meteorology
ER -