Improving ecosystem‐scale modeling of evapotranspiration using ecological mechanisms that account for compensatory responses following disturbance

Abstract

Mountain pine beetle outbreaks in western North America have led to extensive forest mortality, justifiably generating interest in improving our understanding of how this type of ecological disturbance affects hydrological cycles. While observational studies and simulations have been used to elucidate the effects of mountain beetle mortality on hydrological fluxes, an ecologically mechanistic model of forest evapotranspiration (ET) evaluated against field data has yet to be developed. In this work, we use the Terrestrial Regional Ecosystem Exchange Simulator (TREES) to incorporate the ecohydrological impacts of mountain pine beetle disturbance on ET for a lodgepole pine‐dominated forest equipped with an eddy covariance tower. An existing degree‐day model was incorporated that predicted the life cycle of mountain pine beetles, along with an empirically derived submodel that allowed sap flux to decline as a function of temperature‐dependent blue stain fungal growth. The eddy covariance footprint was divided into multiple cohorts for multiple growing seasons, including representations of recently attacked trees and the compensatory effects of regenerating understory, using two different spatial scaling methods. Our results showed that using a multiple cohort approach matched eddy covariance‐measured ecosystem‐scale ET fluxes well, and showed improved performance compared to model simulations assuming a binary framework of only areas of live and dead overstory. Cumulative growing season ecosystem‐scale ET fluxes were 8 – 29% greater using the multicohort approach during years in which beetle attacks occurred, highlighting the importance of including compensatory ecological mechanism in ET models.

Decadal-Scale Reduction in Forest Net Ecosystem Production Following Insect Defoliation Contrasts with Short-Term Impacts of Prescribed Fires

“…Stomatal conductance and whole-tree transpiration measured using sap flux gauges increased for many of the remaining canopy oaks, suggesting that light use efficiency and photosynthetic rates also increased [23,61]. As canopy gaps persisted, increased productivity of sub-canopy oaks and pine seedlings and saplings became increasingly important in maintaining GEP and ANPP at the oak stand, similar to results reported for sub-canopy species following non-stand-replacing disturbance in other forests (e.g., [18,[22][23][24]62]). Although canopy oak species in the PNR have twice the average foliar N concentrations, higher maximum stomatal conductance, and greater water use efficiencies (WUEs) compared to pitch and shortleaf pines, pines and oaks have similar light compensation points, quantum yields, and maximum assimilation rates on a leaf area basis [61].…”

“…Recently, it has been hypothesized that relatively high rates of GEP and NEP in intermediate-age forests can be maintained by frequent low-intensity disturbances which delay the onset of steady-state conditions [18,20,21]. Two primary mechanisms are thought to be involved: increased canopy heterogeneity following disturbance allows greater light penetration into the sub-canopy and results in greater light use efficiency and compensatory photosynthesis by the remaining trees, saplings, and understory vegetation [22][23][24]; and nutrients released during and following disturbance are rapidly reabsorbed by recovering vegetation, stimulating foliage production and enhancing photosynthetic rates [18,25].…”

Distributed Plant Hydraulic and Hydrological Modeling to Understand the Susceptibility of Riparian Woodland Trees to Drought‐Induced Mortality

“…The source/sink term q r is expressed as where I is the flux of water infiltrating at the land surface (m/hr), E is soil evaporation (m/hr), and T is transpiration (m/hr). E is calculated using the Penmann‐Monteith equation, with the conductance term assumed to be proportional to soil water content using an analog for Darcy's law (Millar et al, ). T is solved by the parsimonious plant hydraulics scheme in TREES, which predicts a steady‐state relation between T and xylem pressure ( P c ) at a given soil water potential ( P s ; equation ; Sperry et al, ).…”

Model–data fusion approach to quantify evapotranspiration and net ecosystem exchange across the sagebrush ecosystem at different temporal resolutions

“…W varies from 0 to 1, with a value of 1 indicative of no stress. A n was used to further constrain stomatal conductance (Katul et al, ; Loranty et al, ), which was then used as an input (as canopy conductance) in the Penman–Monteith equation to quantify transpiration ( E t ; Millar et al, ). Other components of the water balance, including canopy ( E c ) and soil evaporation ( E s ), were also calculated using the Penman–Monteith equation (Monteith, ).…”

“…Other components of the water balance, including canopy ( E c ) and soil evaporation ( E s ), were also calculated using the Penman–Monteith equation (Monteith, ). In case of E c , aerodynamic conductance quantified diffusive conductance, while in case of E s , conductance was proportional to soil moisture (based on Darcy's law; Millar et al, ). Total ET was expressed as follows: …”