Estimating canopy gross primary production by combining phloem stable isotopes with canopy and mesophyll conductances

Abstract

Gross primary production (GPP) is a key component of the forest carbon cycle. However, our knowledge of GPP at the stand scale remains uncertain, because estimates derived from eddy covariance (EC) rely on semi-empirical modelling and the assumptions of the EC technique are sometimes not fully met. We propose using the sap flux/isotope method as an alternative way to estimate canopy GPP, termed GPP iso/SF , at the stand scale and at daily resolution. It is based on canopy conductance inferred from sap flux and intrinsic water-use efficiency estimated from the stable carbon isotope composition of phloem contents. The GPP iso/SF estimate was further corrected for seasonal variations in photosynthetic capacity and mesophyll conductance. We compared our estimate of GPP iso/SF to the GPP derived from PRELES, a model parameterized with EC data. The comparisons were performed in a highly instrumented, boreal Scots pine forest in northern Sweden, including a nitrogen fertilized and a reference plot. The resulting annual and daily GPP iso/SF estimates agreed well with PRELES, in the fertilized plot and the reference plot. We discuss the GPP iso/ SF method as an alternative which can be widely applied without terrain restrictions, where the assumptions of EC are not met. K E Y W O R D S boreal forest, intrinsic water-use efficiency, mesophyll conductance, nitrogen fertilization, phloem δ 13 C, PRELES, sap flux, stand transpiration 1 | INTRODUCTION Gross primary production (GPP) represents a key flux in the carbon (C) budget of a forest ecosystem. GPP has been commonly estimated using many approaches, such as eddy covariance (EC), empirical models and upscaling ecophysiological measurements at stand scale

Whole‐tree mesophyll conductance reconciles isotopic and gas‐exchange estimates of water‐use efficiency

“…(2011) and Vernay et al . (2020). Fortunately, owing to methodological advances (Pons et al ., 2009), but more importantly due to the recognition of the crucial role of g m in regulating vegetation C–H 2 O tradeoffs (Flexas et al ., 2012; Sun et al ., 2014; Raczka et al ., 2016), measurements of g m are becoming increasingly available for many species under varying climatic conditions.…”

“…iWUE calculated from measurements of gas exchange reflects the [CO 2 ] gradient between the atmosphere and the substomatal cavity, which depends on boundary and stomatal (g s ) conductances; by contrast, iWUE from δ 13 C of plant material is proportional to the [CO 2 ] gradient between the atmosphere and the site of carboxylation, which additionally depends on g m (Warren & Dreyer, 2006;Michelot et al, 2011). Many studies comparing iWUE estimates neglect g m limitations to photosynthesis (Medlyn et al, 2017;Guerrieri et al, 2019;Tarin et al, 2020) although see Michelot et al (2011) and Vernay et al (2020). Fortunately, owing to methodological advances (Pons et al, 2009), but more importantly due to the recognition of the crucial role of g m in regulating vegetation C-H 2 O tradeoffs (Flexas et al, 2012;Sun et al, 2014;Raczka et al, 2016), measurements of g m are becoming increasingly available for many species under varying climatic conditions.…”

A new generation of sensors and monitoring tools to support climate-smart forestry practices

“…mensuration of stomatal conductance as regulator of transpiration over a short time period) and stable isotope data (i.e. carbon isotope ratio, δ 13 C, to derive water use efficiency) may also provide an individual-tree based estimate of gross primary productivity (GPP) (Klein et al 2016;Vernay et al 2020). Transpiration could also be further scaled-up, e.g., from stand to watershed, using remote sensing images (Cermak et al 2015).…”

Continuous Monitoring of Tree Responses to Climate Change for Smart Forestry: A Cybernetic Web of Trees

“…At heavily instrumented sites, field-portable instruments for analyzing stable isotope compositions may become useful for determining spatial patterns of root water extraction at varying soil depths with succeeding phenological stages (Liu et al 2019), thus complementing plant transpiration measurements (Nadezhdina et al 2010;Rothfuss and Javaux 2017). Canopy transpiration flux can be combined with water-use efficiency, as inferred from carbon isotope analysis, to infer gross primary productivity (GPP) of forest canopies (Klein et al 2016;Vernay et al 2020).…”