Impact of rooting depth on transpiration over a hill evergreen forest in northern
Thailand*Katsunori Tanaka 1, Hideki Takizawa 2, Tomonori Kume 3, Jianqing Xu 1, Chatchai Tantasirin4, andMasakazu Suzuki3(1: FRCGC/JAMSTEC, 2: Nihon University, 3: The university of Tokyo, 4: Kasetsart University)* Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Japan,
3173-25 Showamachi, Kanazawa-ku,
Yokohama, Kanagawa, Japan. e-mail: ktanaka@jamstec.go.jp
Abstract
Possible impacts of rooting depth on vegetative processes have also been discussed because rooting depth limits soilwateruse. Previous research showed that transpiration in an evergreen broad-leaved forest in northern Thailand (18o48’ N, 98o54’E) peaked at the end of the dry season. However, rooting-depth limitations on soilwater use were not investigated. This
study examined the impact of rooting depth and soil hydraulic properties on transpiration using a soil plant air continuum(SPAC) model. The soil texture at the site was classified as silty sand from the measured relationship between thevolumetric soilwater content and soilwater potential at 0.1-m depths. To effectively simulate heat-pulse velocity variation
corresponding to dry-season transpiration and annual discharge, a rooting depth of 4-5 m was needed, assuming a silty sandsoil texture under unsaturated conditions. This value is less than the reported maximum rooting depth of evergreen trees andis considered reasonable. A penetration test showed that soil became harder at depths of 4-5 m.
Keyword: rooting depth, transpiration, hill evergreen forest.

1. Introduction
Tanaka et al. [2003] concluded that transpiration in an
evergreen forest in northern Thailand peaked in the late dry
season. They suggested that reduced canopy wetness
lowered evaporation; however, stomatal conductancedeclined only slightly, even under the driest conditions andhighest net radiation. These results counter previous
reports of an evapotranspiration decline in Thailand s dry
season for evergreen forests [Pinker et al., 1980] and othervegetation [Toda et al., 2002]. The transpiration peaksuggests that sufficient water transpires between thesurface and base rock; this trend was found every year.
Observations of the heat pulse velocity near a ridge, whereno water table seemed to form, also showed that the wateruse of individual trees was largest at the end of the dry
season. This indicates that soilwater is likely to maintainthe transpiration peak even in an unsaturated area.
However, research on this subject is lacking.
This study used a soil plant air continuum (SPAC)
numerical model to investigate how rooting depth affects
evapotranspiration in an evergreen forest under unsaturatedconditions. Seasonal variations in the heat pulse velocitycorresponding to transpiration and annual discharge werecompared with simulated results to determine the optimum
rooting depth. A penetration test was also carried out in theforest to estimate the strength of the soil.

2. Method
2.1. Site
Since February 1997, the hydrological andmeteorological parameters of a sub-watershed of theKog-Ma Experimental Watershed have been measured.
The sub-watershed is situated 1265-1420 m above meansea level on Mount Pui (18°48N, 98°54E) near
Chiang Mai in northern Thailand. A 50-m meteorologicaltower has been built, and instruments for measuringmeteorological parameters, such as radiation, wind velocity,
and air temperature, have been installed on the tower.
Evergreen forest covers the hills of the experimental
watershed. Fagaceae dominate, and the species include
Lithocarpus, Quercus, and Castanopsis [Bhumibhamon andWasuwanich, 1970]. The LAI is approximately 4.5, with aseasonal range from 3.5 to 5.5 [Tanaka et al., 2003]. Forest
floor soils derive from granitic materials and are classified
as Reddish Brown Lateritic [Tangtham, 1974].
To estimate the strength of the soil, a penetration test wascarried out with a simplified dynamic cone penetrometer(S06-M, Tsukuba Maruto Co.) at a point near the tower inthe sub-watershed. The equipment used consisted of a pairof rods (external diameter: 16 mm) coupled verticallyusing a knocking head. Using this method, the strength (orcompaction) was expressed as Nc, which was defined as thenumber of times a 5-kg slide hammer had to be dropped
from a height of 0.5 m above the knocking head for the tipof the cone to penetrate a depth of 0.1 m vertically.

2.2. Model
A one-dimensional SPAC model was used to investigate
the impact of rooting depth on transpiration [Tanaka et al.,2004]. The model combines a soil multilayer model forthermal heat and soilwater transfer [Kondo and Xu, 1997]
and a canopy multilayer model for the heat and water
budget [Tanaka et al., 2003]. The soil multilayer model
considers the change of albedo and evaporation efficiency
with the soil moisture at the top of the soil column [Kondoand Xu, 1997]. The canopy multilayer model consists of asecond-order closure model for atmospheric diffusion
coupled with a radiation transfer model, a rainfall
interception model, a Farquhar-type photosynthesis model[Farquhar et al., 1980], and Ball s stomatal conductancemodel [Ball, 1988]. Combined, the two models consider
the loss of soil moisture by water uptake (or transpiration)and the impact of soilwater content on stomatal closure.

3. Result
3.1 Soil conditions

The modeled volumetric soilwater content ( θ )soilwater potential (ψ) relationship for silty sand proposedby Kondo and Xu [1997] is very similar to the measuredvalues at a depth of 0.1 m. Of the four soil textures
proposed by Kondo and Xu [1997], the soil texture at thesite is best classified as silty sand. Therefore, numericalsimulations were carried out assuming a silty sand soil
texture under unsaturated conditions.
Fig.1 shows the values of Nc in the watershed at a point
near the tower. The values gradually increased to a depth of~4.5-m, although there was a peak at a depth of ~2.8 m; Nc
exceeded 10 at depths of 4-5 m, and was ~20 at depths
exceeding 5 m. The soil layers clearly became harder there.

March (late dry season), May (early rainy season), andSeptember (late rainy season). These values are the averagevalues for each month in the years 1998-2000. The
simulated transpiration increased with rooting depth in thelate dry season. On the other hand, transpiration values
changed only slightly with rooting depth in the rainyseason (Fig. 2) as compared to the late dry season. At arooting depth of 4 m, transpiration increased more in thelate dry season than in the rainy season (the monthly valuesof the rainy season transpiration for June to August were
41-49 mm mh-1 at rooting depths of 1-12 m), and thetranspiration clearly peaked at the end of the dry season atrooting depths of 4-5 m.

80

Et (mm month- 1 )Tanaka et al. [2004] detail these assumptions.
Transpiration (Et), canopy interception (Ei), and soil
weresimulatedusingevaporation(Es)hydro-meteorological variables, such as rainfall, downward
short-wave and long-wave radiation, air temperature,humidity, and wind velocity observed above the canopy
from 1998 to 2000. The simulated discharge (D) wascompared with measured value.

601020May ( R ai n y)

200012345678Rooting depth (m)3001Fig. 2: Relationship between rooting depth andtranspiration Et in the late dry season and rainy season,assuming silty sand as the soil texture. The values are
the averages of monthly values for each month in2
345610 0 0(mm year - 1 )S oi l d e p th (m )Se p . ( R a i ny )40N c- v al ue0Mar . ( L a te d r y)

Mea su r e d D

80 0D60 0Et

40 020 0EiEs07012345678Rooting depth (m)Fig. 1: Nc near the tower. Nc corresponds to thestrength of the soil.

3.2 Simulation
The study area has three seasons: a rainy season, andearly (cool) and late (hot) dry seasons. Fig. 2 shows therelationship between rooting depth and transpiration inFig. 3: Relationship between rooting depth and annual
water budget component amounts (i.e., discharge D,
transpiration Et, canopy interception Ei, and soil
evaporation Es), assuming silty sand as the soil texture.
The values are the annual averages for a 3-year period(1998-2000).

As expected, rooting depth had little influence on theannual amounts of soil evaporation and canopy
interception, particularly canopy interception, but it did
affect the annual amounts of transpiration and discharge(Fig. 3). Therefore, a rooting depth of 4-5 m should be
used to agree with the annual discharge (Fig. 3).

4. Consideration
The Nc profile showed that the strength of the soil clearlychanged at depths of 4-5 m. In the study, the soil texture at
the site was classified as silty sand [Kondo and Xu, 1997].
To effectively simulate transpiration peak in the dry season
and annual discharge, a rooting depth of 4-5 m was needed
for the silty sand soil texture. This value is lower than thereported maximum rooting depth of evergreen trees[Canadell et al., 1996; they reviewed the maximum rooting
depth of trees from many references and calculated theaverage and standard deviation as 7.0 ± 1.2 m.] and isreasonable. The value also approximates the depth at which
Nc clearly changed. These results emphasize that thelate-dry-season transpiration peak is theoretically possible
based on rooting-depth limitations to soilwater use.
As , The results show that the rooting depth should be set
to exceed the tree rooting depths that are often used in land
surface models within global circulation models (GCMs)(e.g., 1-2 m) to simulate transpiration peaks over forests inthe late dry season using a one-dimensional model.
The finding that a deeper rooting depth is necessary tosimulate the transpiration peak over forests in the late dry
season is similar to the results for an Amazonian forest
under dry conditions [Arain et al., 1997, Sen et al., 2000].
Generally, evergreen trees can develop roots that penetrate
to deeper soil layers so that they can maintain leaves andtranspiration under dry conditions. As Canadell et al.
[1996] showed, tropical evergreen forests likely have
deeper rooting depths than do tropical deciduous forests,
which have no leaves under dry conditions. Jackson et al.
[1999] showed that only evergreen species grew roots
deeper than 10 m in an ecosystem containing deciduousand evergreen species.

5. Summary
Numerical simulations were carried out using a SPAC
model to investigate the impact of rooting depth and soiltexture, which limit soilwater use, on transpiration. The
model assumed that unsaturated soil conditions weredictated by the boundary conditions. The soil texture at thesite was classified as silty sand by Kondo and Xu [1997],
based on comparisons of modeled and measured θ -ψrelationships. The results showed that a rooting depth of
4-5 m effectively simulated transpiration variation during
the dry season and the annual discharge, assuming silty
sand. The value of 4-5 m is within the reported maximum
rooting depths of trees, and approximates the depth of 4-5m at which the Nc-value corresponding to the strength ofsoil clearly changed and the soil became harder. Therefore,
the late-dry-season transpiration peak is theoreticallypossible based on rooting-depth limitations to soilwater

use.

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