Download Understanding tree water transport measurement techniques and

Understanding tree water transport measurement techniques and their detailed information
related to night time variations in stem diameter
1
M. I. Hannukainen, 2T. Hölttä, 3S. Sevanto, 2E. Nikinmaa, 1T. Vesala
1
Division of Atmospheric Sciences, Department of Physics, University of Helsinki
2
Division of Atmospheric Sciences, Department of Forest Ecology, University of Helsinki
3
Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New
Mexico, USA
Keywords: stem diameter, sap flow, evaporation
INTRODUCTION
Tree stem diameter depends on the total water content in the stem and tension inside the xylem and the
phloem conduits. Sap flow in the xylem is driven by evaporation, which is controlled by temperature,
atmospheric humidity and radiation through stomatal conductance. For this reason xylem diameter has a
clear diurnal cycle with largest diameter occurring at night when stomata are closed and evaporation is at
minimum and smallest diameter being observed at daytime when stomata are open and evaporation is at
maximum (Figure 1). Very high evaporation can cause embolism in xylem cells. Embolization is most
likely to happen during daytime when maximum tension exceeds that of previous day (Hölttä et al.
2005). Embolized cells can refill with water after high tension eases (Hollbrook et al.2001). Phloem
water transport depends on osmotic efficiency and xylem water supply. All the water pulled to the
phloem by this osmotic force is taken from the xylem, and phloem flow effectively competes for water
with the transpiration stream. Consequently phloem diurnal cycle outlines follow xylems diurnal cycle.
Changes in radiation or rain conditions cause instant changes in stem diameter. At daytime these changes
can be quite large and fast (Sevanto 2003). Night time behavior is more stable in the absence of radiance,
which is the main driving force of all fluxes and flows. I general stem diameter increases steadily at
night. This increase is a result of stem refilling with water from the soil.
Measurements however show frequently a short-term decrease in night time diameters (Figure 1). These
rapid changes took place mostly in the xylem. Phloem often follows, but it can also behave completely
independently. Night time diameter shrinkage is not a result of rain or temperature changes, and cannot
be cannot be easily explained with existing theories (Hannukainen 2010). Understanding the mechanism
behind this diameter night time behavior in stem and xylem could expose details about the tree water
transport system.
Water transport in trees is difficult to measure non-destructively or without compromising the quality of
the measurement data. Several techniques exist, with different advantages and weaknesses. This study
aims to compare data from different measurement techniques to find some clarification to unexplained
night time tree diameter behavior.
METHODS
All measurement used in this study were carried out at SMEAR II station (Station for Measuring
Ecosystem-Atmosphere Relations, University of Helsinki), in central Finland. The measurement station
is located in a rural area, in a Scots pine (Pinus Sylvestris L.) -dominated forest. The forest was planted
from seed in 1962, and is now about 15 tall.
Diurnal stem diameter variations were measured with linear variable displacement transducers (LVDT;
Solartron AX/5.0/S, Solartron Inc., West Sussex, UK) simultaneously both on the xylem and on the bark.
The system consisted of sensors attached to a metal frame (Sevanto 2003). One sensor was placed on the
surface of the xylem by inserting a small screw through the phloem and the bark. The other measured on
the whole stem after removal of dead bark. The screw required for the xylem measurement was very
small and should cause only small local embolization. Therefore, water transport or stem diameter
behavior should not be significantly influenced by this measurement equipment. Stem diameter change
data can be obtained as frequently as needed, and noise is almost nonexistent. In this study data was
collected once per minute.
Transpiration was with chambers that were installed on the same trees were stem diameter was measured
(Altimir 2002). There were several chambers in one tree, each holding a whole shoot inside. For
transpiration measurement a sample of air was collected and transported to gas analyzer (URAS 4, infra
red absorption, Hartmann Brown, Germany). Water vapor flux was then obtained by a change in water
vapor concentration inside the chamber when the chamber was closed for a minute. Chambers were open
between the collections. This measurement becomes unreliable if ambient relative humidity (RH) is too
high. In those conditions water vapor can condense on the chamber walls creating too high water vapor
concentration to the sample. 70% is a rough RH limit, and values obtained above RH of 70% were
therefore omitted in this study. This makes chamber flux measurement very difficult to use at night time,
because RH is usually exceeds this limit.
Sap flow in the xylem was measured with Granier method, which is based on convective heat transport
(Granier 1985). Two thin needles were inserted to the xylem. The upper one was heated, and temperature
difference between the needles was the measured quantity. Sap flow cools the heated needle via
convection and thus smaller temperature difference between needles means greater sap flow rates.
Weakness of this system is that heat is also conducted to surrounding tissue. With slow sap flow rate a
larger fraction of the heat is spread to the surroundings and measurement exaggerates the flow rate.
When the flow increases and needle cools rapidly below the temperature of the surrounding tissue and
heat is conducted back to the needle from the surroundings. This results in underestimates of the flow
rate until temperature between the needle and its environment has stabilized. The time lag and error in
flow rate cased by this is not well known. This makes Granier type measurement unreliable in low flow
rate conditions. Sap flow is at its lowest at night time, and therefore measurement data should be
considered with caution.
Currently there is no accurate non-destructive method to quantify embolization or its recovery process in
field conditions. The only method available for measuring embolism formation is based on the detection
of pressure waves cased by cavitation events, which can be measured in the ultrasound frequency using
microphones (Hölttä et al. 2005). It is believed that acoustic emissions that cavitation events produce
these acoustic emissions that can be counted, but the exact relation of them to embolism is not well
known. Microphones can detect sound only from a very small area, about one squire centimeter, and
therefore this measurement is very localized.
CONCLUSIONS
These measurements show different pieces of the whole tree water transport system. Evaporation
measurement show only current water flux from the leaves. After stomata are closed and evaporation
considerably lessened, water keeps rising in the stem for some time. Upward water flow will start to
decrease after the xylem water supplies are sufficiently filled. This point or its quantification is not well
known. Sap flow time lag depends on the heat conductive properties of the tissue surrounding the
needle, which varies with tree species, age, and individual. Low sap flow rates are very difficult to
reliably determine with this method. Embolization measurements were somewhat experimental in nature,
and were probably done too sparsely to produce a proper overall picture of the whole stem behavior.
Evaluation of diameter night time changes with these water transport measurements must be done with
care. Best result is probably obtained by modeling the needed sap flow and evaporation that could cause
measured night tie diameter decreasing, and compare calculated values to measured ones. This should
help at least rule out the possibility of night time stomata opening being the cause of stem shrinking.
Also embolization data and previous day evaporation data should be compared to night time shrinking to
determine the possibility of refilling of xylem cells being the reason of temporary decrease in diameter.
Figure 1. Diurnal behavior of stem diameter (black line), water vapor (thicker grey line), PAR
(Photosynthetically Active Radiation, thinner grey line) and sap flow (black dots). Stem diameter show a
sudden decrease after sunset at about 10 pm and a slight increase in sap flow occurs simultaneously.
ACNOWLEDGEMENTS
The financial support by the Academy of Finland Centre of Excellence program (project no 1118615) is
gratefully acknowledged.
REFERENCES
Altimir N., Vesala T, Keronen P., Kulmala M., Hari P. 2002. Methodology for direct field measurements
of ozone flux to foliage with shoot chambers. Atmospheric environment 36 : 19-29.
Granier, A. 1985. Une nouvelle méthode pour la mesure du flux de sève brute dans le tronc des arbres. Annales des
Sciences Forestieres 42:193-200.
Holbrook NM, Ahrens ET, Burns MJ, Zwieniecki MA. 2001. In vivo observation of cavitation and
embolism repair using magnetic resonance imaging. Plant Physiology 126:27-36.
Hölttä T, Vesala T, Nikinmaa E, Perämäki M, Siivola E, Mencuccini M. 2005. Field measurements of
ultrasonic acoustic emissions and stem diameter variations. New insight into the relationship between
xylem tensions and embolism. Tree Physiology 25: 237–243.
Sevanto, S. (2003). Tree Stem Diameter Change Measurements and Sap Flow in Scots Pine. Report
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