Separating transpiration and stem water storage in a large boreal tree using Heat Ratio Method sap flow sensors

Morgane Merlin1 ([email protected]), Kevin Solarik2 & Simon Landhäusser1

1Department of Renewable Resources, University of Alberta, Edmonton AB, CANADA2National Council for Air and Stream Improvement NCASI, Montreal QC, CANADA

Landhäusser Research Group

landhausser.ualberta.ca

1. Estimating forest water use 3. Measured sap flow poorly matches with gravimetric flow without appropriate data corrections

2. Sap flow sensors & Cut-tree method

Thank you for field support to Iain Jamieson, Fran Leishman, Andy Fitzsimmons, Julie Zettl, and all membersof the Landhäusser’s lab and Newton Tran (ICT International Ltd.)

Trees play a major role incontrolling the water fluxes ofterrestrial ecosystems, thusobtaining accurate estimates ofwater use is crucial under currentand future climatic conditions.

Lags between canopy and stemmeasurements of water fluxeshighlight the importance of stem waterstorage as a significant water pool inforests.

Objective 1. Are estimates of sap flow accurately matching gravimetric flow?

Objective 2. Can we identify the dynamics of stem water storage versustranspiration in hourly sap flow measurements?

Heat Ratio Method sap flow sensors (ICT International Pty Ltd.) were installed ona 60 years old and 20 m tall aspen in July 2017 at 2.5, 5 and 10 m along thetrunk across four cardinal directions.

Water uptake / Evapotranspiration

Input

Infiltration / Runoff

Objective 2. The influence of height location (2.5, 5 and 10 m trunk height) onmeasured sap flow across cardinal orientations was assessed and therelationship with VPD estimated.

Objective 1. A zero flowcorrection and an appropriatesapwood area calculation wereapplied to estimate sap flow,which was then compared togravimetric flow measured bythe scale. HRM sap flow sensor

10 m

5 m

2.5 m

Objective 1. Data corrections (appropriate zeroflow correction and correct extrapolationto the whole sapwood area) are crucial forthe accuracy of the sensors at varioustime scales.

Objective 2. It appears that large volumes ofwater are exchanged laterally within thesapwood but little is withdrawn, whichmay indicate significant exchanges ofwater with the phloem and theheartwood which are currently notaccounted for.

4. Stem water storage represents up to 45% of daily water use in this mature aspen trunk

→ Proper sapwood areaestimation and zeroflow correction arenecessary forimproving the matchwith gravimetricflow.

→ Even after correction,sensors overestimatehigh flows during theday.

→ A relatively smallover/under-estimationof hourly flowsignificantly impactthe accuracy of themeasured cumulativewater uptake over only10 days.

→ Measured sap flowat 5 and 10 malong the trunkshows limitedmaximum flowcompared to thesap flow measuredat 2.5 m.

→ The placement of the sapflow sensor at differenttrunk heights will impactthe parameter estimationfor correlating sap flowwith climatic variablessuch as vapor pressuredeficit.

→ There is a large volume ofwater daily exchanged withthe wood, with littlewithdrawal before the cut.

→ In temperate broadleavedtrees, the trunk representsthe largest reservoir of storedwater, however, over thedays show here, aspendoesn’t seem to rely onstored water for earlymorning transpiration.

The tree was cut August 12th

2017 and the base of the trunkwas suspended in a large waterbucket resting on a scale torecord water flowgravimetrically.

Objective 1. Data corrections (appropriate zeroflow correction and correct extrapolationto the whole sapwood area) are crucial forthe accuracy of the sensors at varioustime scales.

Objective 2. It appears that ~16% of daily waterfluxes come from internal water storage,which may come significant exchanges ofwater with the phloem and theheartwood which are currently notaccounted for.

4. Stem water storage represents up to 20 % of daily water use in this mature aspen trunk

→Measured sap flowat 5 and 10 malong the trunkshows limitedmaximum flowcompared to thesap flow measuredat 2.5 m.

→ The placement of the sapflow sensor at differenttrunk heights will impactthe parameter estimationfor correlating sap flowwith climatic variablessuch as vapor pressuredeficit.

→ There is a significant volumeof water daily exchangedwith the wood.

→ In temperate broadleavedtrees, the trunk representsthe largest reservoir of storedwater. From the low trunk tobelow the canopy, ~16% ofdaily water fluxes come fromdaily water storage in thisaspen tree.

Corrigendum