Can leaf-to-ecosystem-scale process modeling resolve differences in multi-scale intrinsic water use efficiency estimates?

Intrinsic water use efficiency (iWUE) quantifies the trade-off between carbon gain and water loss providing an indicator of stomatal regulation in response to environmental change. iWUE can be estimated for different temporal and spatial scales, from sub-daily to annual and from leaf to ecosystem scale. Tree-level iWUE estimates are commonly derived from stable carbon isotope compositions (δ¹³C) in tree rings. Laser ablation technology facilitates the analysis of δ¹³C at fine intra-ring resolution, providing insights to intraseasonal iWUE variations. Despite the promise of intraseasonal iWUE derived from tree-ring archives, links and discrepancies between iWUE estimates representing different scales are poorly understood.

This study investigates intraseasonal iWUE over 2002–2019 in a Scots pine dominated stand (Hyytiälä, southern Finland) at three spatial scales: shoot, tree and ecosystem scale. Empirical iWUE estimates are derived from shoot gas exchange, tree-ring δ¹³C and eddy covariance (EC). iWUE estimates were integrated to temporal resolution corresponding to tree-ring subsections using growth modeling and assimilation-based weighting. To understand differences in these iWUE estimates, we apply a multi-layer, multi-species, soil-plant-atmosphere-transfer model (pyAPES).

The level differences between shoot-, tree- and ecosystem-scale iWUE estimates were in line between measurement- and model-based estimates. Both showed that ecosystem-scale iWUE was 40% lower than shoot- or tree-scale iWUE. Model results suggested half of this difference was explained by the presence of other species in the stand and understory. Most of the remaining difference was attributed to neglecting the difference between leaf and air temperature in the calculation of ecosystem-scale iWUE.

δ¹³C-based iWUE correlated moderately with EC-based iWUE at inter- and intra-annual resolutions (r=0.58). δ¹³C-based iWUE correlated more strongly with modelled iWUE (ecosystem, tree, shoot) at both inter- and intra-annual resolutions (r=0.74–0.87), suggesting modelled iWUE may be more robust over multi-decadal timeframes than EC-based iWUE. Clearest miss-matches between intraseasonal δ¹³C-based iWUE and EC-based iWUE (or modelled iWUE) were during the two dryest years of the study period. This may be caused by remobilization of reserves, or other drought related processes affecting isotopic fractionation or, alternatively, uncertainties in dating wood formation processes during drought. Plausibly this indicates that tree-ring δ¹³C is not a robust indicator of iWUE during severe drought but rather provides means to pinpoint periods of such conditions.