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Modelling water balance in fertilised and unfertilised

Cupressus lusitanica and Pinus radiata grown across

an environmental gradient

Michael S. Watt a,*, Alicia L. Kiyvyra b, Peter W. Clinton a, Graham Coker a,Roger L. Parfitt c, Robyn Simcock d, John Dando c, Murray R. Davis a,

Stephen H. Schoenholtz e

a Ensis, PO Box 29237, Fendalton, Christchurch, New Zealandb Oregon State University, Corvallis, OR 97331, USA

c Landcare Research, Private Bag 11052, Palmerston North, New Zealandd Landcare Research, Private Bag 92170, Auckland, New Zealand

e Virginia Tech, Blacksburg, VA 24061-0001, USA

Received 29 June 2007; received in revised form 6 October 2007; accepted 8 October 2007

Abstract

As erosion control and water conservation are almost always important issues, information on water use between different plantation species

may be of significant management value for guiding decisions on species selection. The main and interactive effects of species and fertilisation on

annual water loss and drainage in three-year old Cupressus lusitanica Mill. and Pinus radiata D. Don were examined across an environmental

gradient at eight sites in New Zealand using small highly stocked plots designed to compress the rotation length. Annual average fractional

available volumetric water content (ua), total annual water loss and total annual drainage for each treatment at each of the eight sites was determined

using a daily water balance model, validated against two-monthly measurements of ua.

Modelled annual average ua, total annual water loss and total annual drainage were all significantly influenced by site and species (P < 0.001 for

all variables), but not fertilisation or the interaction between species and fertilisation (P > 0.05 for all variables). Averaged across the eight sites,

annual average ua for C. lusitanica was higher than that of P. radiata by 0.15 (0.58 vs. 0.43). Total annual water loss for P. radiata was on average

104 mm higher, than that of C. lusitanica (893 mm vs. 789 mm), while total annual drainage for C. lusitanica was on average 129 mm higher than

that of P. radiata (285 mm vs. 156 mm). Combining average values for drainage and water loss indicated that there was on average 233 mm more

available water per annum in soils under C. lusitanica than P. radiata, which equated to an average of 22% of total annual rainfall.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Cupressus lusitanica; Fertilisation; Pinus radiata; Radiata pine; Water balance

1. Introduction

Pinus radiata D. Don is the most widely planted plantation

species throughout the southern hemisphere (Lewis and

Ferguson, 1993). The predominance of P. radiata is due to

good growth rates, adaptability of the species to a wide range of

site types, its responsiveness to tree breeding and silviculture

and versatility of the species to a wide range of end-uses (Cown,

1997).

During recent years Cupressus lusitanica Mill. has become a

popular alternative to P. radiata. This species has good growth

rates and is suitable for a wide variety of end-uses including

panelling, and exterior joinery (Haslett, 1986). C. lusitanica

requires moderately fertile, moist, well drained soils and

exhibits a preference for mild climates and reasonably sheltered

sites (Clifton, 1994; Miller and Knowles, 1996).

Species selection is an important management decision. As

erosion control and water conservation are almost always

important issues, information on water use between different

plantation species may be of significant management value for

guiding decisions on species selection. Previous research,

undertaken across sites with a wide range in annual rainfall, has

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Forest Ecology and Management 255 (2008) 1104–1112

* Corresponding author. Tel.: +64 3 364 2949; fax: +64 3 364 2812.

E-mail address: michael.watt@ensisjv.com (M.S. Watt).

0378-1127/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2007.10.047

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shown that P. radiata uses between 62 and 72% of annual

rainfall (Whitehead and Kelliher, 1991a,b; Fahey et al., 2001).

Comparisons on a dryland site suggest that this water use is less

than that of the common plantation species, Pseudotsuga

mensiezii (Mirb.) Franco (Fahey et al., 2001). Although water

use by P. radiata has been well documented, little research has

investigated water use in C. lusitanica and how this compares

with P. radiata.

Water use by plantation species is dependant on a number of

factors of which the most important include annual rainfall

(Pearce and Rowe, 1979), leaf area index, (Watt et al., 2003) and

available root-zone water storage (Whitehead and Kelliher,

1991b). Consequently, to be robust, species comparisons of water

use should be undertaken across a broad climatic and edaphic

gradient, which includes a wide range in stand leaf area index.

This type of diversity is covered by a set of eight site quality

plots established across a wide edaphic and climatic gradient

within New Zealand. These plots form a representative subset

of a national network of plots grown over a period of four years

at high stand density to identify the key edaphic determinants of

productivity (Watt et al., 2005). Through establishing these

plots at high density this approach aims to compress the

experimental period over which the stand develops, by

increasing intra-tree competition and the rate at which leaf

area develops and the base of the green crown recedes. In these

plots canopy closure is typically attained at most sites by two

years after planting. This technique has significant advantages

over conventional experimental methods as time frames over

which measurements can be taken are shortened and relatively

large numbers of plots can be installed permitting sampling of a

wide range of environments.

Using this dataset, the objective of this study was to examine

the main and interactive effects of species and fertilisation on

annual water use in plots of three-year-old C. lusitanica and P.

radiata. This was achieved by modelling daily water balance,

over the fourth year after planting, for both species from

detailed measurements of input parameters at each site, and

validating these estimates against two-monthly gravimetric

measurements.

2. Materials and methods

2.1. Determination of water balance

A daily water balance equation was used to calculate root-

zone water storage (W) on the ith day as,

Wi ¼ Wi�1 þ Pi � Eti � Etwi � Egi � Fi (1)

where Pi is rainfall, Eti is transpiration from the dry tree canopy,

Etwi evaporation of intercepted rainfall from the tree canopy, Egi

evaporation from the soil, and Fi drainage from the root zone

(Whitehead et al., 2001). Surface runoff was assumed to be

insignificant.

Transpiration from the tree canopy was calculated using the

simple diffusion equation,

Et ¼ ’DgstLt (2)

where D is vapour pressure deficit, gst is average stomatal

conductance for the canopy and Lt is projected leaf area index,

and w will be defined below. The inverse relationship between

stomatal conductance and vapour pressure deficit was modelled

using the function described by Lohammer et al. (1980) as,

gst ¼ gstmax

�1þ ðD� DsminÞ

D0t

��1

(3)

where gstmax describes maximum stomatal conductance at low

vapour pressure deficit, and D0t is the sensitivity of gst to D,

when D > Dsmin (value of D below which gs is constant).

Evaporation from the soil, Eg, was calculated from the

available energy beneath the tree canopy, Gg, as,

Eg ¼’tsGg

lðsþ gÞ (4)

where the coefficient t (=1.4) describes the degree of coupling

of the soil surface with the air above the canopy (Kelliher et al.,

1990), s is the slope of the relationship between saturated

vapour pressure and temperature at a given air temperature,

l is the latent heat of vaporisation, g is the psychometric

constant, and w will be defined below. Values for s, g, and l

are temperature-dependant and calculated from standard

meteorological tables (Goudriaan and van Laar, 1994). Using

Beers Law, Gg was calculated from (e�kLtGa) where Ga is the

available flux density above the canopy (assumed to be 70% of

shortwave radiation) and k is the light extinction coefficient,

which is assumed to be 0.5 for a spherical leaf angle distribution

(Goudriaan and van Laar, 1994).

The coefficient w was used to reduce Eg and Et as soil water

storage declined. Root-zone volumetric water content on the ith

day, ui (=Wi/[r(1 � c)]), was calculated from root-zone water

storage (Wi), root depth (r), and fractional stone content of the

soil (c). The value of the coefficient w was set to 1 at maximum

values of u (umax), and was not reduced until u declined to a

threshold value (ut). As u progressively declined below this

threshold w was linearly reduced from 1 to 0 at minimum values

of u (umin). The threshold values (ut) for reducing Eg and Et,

taken from Watt et al. (2003), were assumed to be respectively

60 and 55% of the fractional available volumetric water

content, ua, determined as,

ua ¼ui � umin

umax � umin

(5)

Both Eg and Et were reduced to 75% of their potential rates

on days when rain occurred, and Et was also reduced by 50% on

days when an air temperature <0 8C was recorded (Watt et al.,

2003). Drainage from the root zone was assumed to be zero

when ui � umax and equal to rainfall reaching the soil when

ui > umax.

Daily meteorological data required for the water balance

model include total rainfall and solar radiation, mean air

temperature, minimum air temperature, and average vapour

pressure deficit. The model also requires values for the

parameters gstmax, D0t, ut, the site specific variables, r, c,

available volumetric water content (umax � umin), a function for

M.S. Watt et al. / Forest Ecology and Management 255 (2008) 1104–1112 1105

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determining Etw and seasonal measurements of Lt. Site level

average values for r, c, Lt and available volumetric water

content are given in Table 1 and treatment level values for D0t,

ut, and gstmax, which were used in the model across all sites, are

given in Table 2.

2.2. Site description and treatments

The water balance model was used to estimate daily water

balance during the fourth year after planting at eight sites

distributed throughout New Zealand. Sites were selected to

represent the wide range in climatic conditions and soil

properties on which New Zealand plantations are established

(Fig. 1; Table 1). Across sites, total annual rainfall ranged three-

fold (665–1729 mm) and annual average temperature varied

two-fold (9.4–16.3 8C). The eight sites encompassed four soil

orders (as classified by the New Zealand Soil Classification

(Hewitt, 1998)), and occurred on sites ranging in texture from

sand to clay loam (Table 1).

At each of the selected eight locations a series of eight plots

was installed using a factorial design with the following three

factors: species (P. radiata and C. lusitanica), fertilisation (no

fertiliser and nutrients supplied in excess of crop requirements)

and surface disturbance (no compaction and compaction from

previous harvesting operations). As the main and interactive

effect of species and fertilisation on water use was of most

interest in this paper, reported values are restricted to the four

subplots from the undisturbed treatment only, in which the soil

was not compacted from previous harvesting operations.

Each sub-plot was small in size (3 m � 3 m) and contained

nine measurement trees spaced at 0.5 m � 0.5 m

(40,000 trees ha�1) surrounded by a two-row buffer. Regular

applications of herbicide were made to ensure weed-free

conditions. All sites were planted with seed stock of the same

origin (growth and form factor of 19 (Vincent and Dunstan,

1989) for P. radiata and 99/275 for C. lusitanica) sourced from

the Scion nursery in Rotorua. The eight site quality plots were

installed during winter 2002.

In fertilised plots broadcast application of fertiliser was

undertaken three times a year during the first year (winter,

spring, summer) and thereafter during spring at annual

intervals. Over the four years of the study the elemental

quantity of nitrogen, phosphorus, potassium, sulphur, magne-

sium and calcium applied to each fertilised plot was 690, 200,

558, 160, 40 and 160 kg ha�1, respectively. At sites identified

by soil analysis (at time of planting) as being deficient in boron,

magnesium and copper, additional applications of these

nutrients to fertilised plots were made during summer in

sufficient quantities to ensure these nutrients were non-limiting.

2.3. Model parameterisation

Daily estimates of rainfall, minimum and maximum

temperature for each site were obtained from temporally and

spatially splined values. Total daily solar radiation, determined

from hourly measurements, was obtained from nearest

Table 1

Site level variation in climate, stand structure, water balance input variables and soil variables

Variable Site average Site number (see Fig. 1 for location of sites)

1 2 3 4 5 6 7 8

Climate

Total annual rainfall (mm year�1) 1078 1226 1729 1362 856 1046 665 729 1012

Annual average temperature (8C) 12.8 16.3 15.1 14.3 11.9 11.9 11.6 11.7 9.4

Annual average PAR (MJ PAR m�2 day�1) 7.0 7.4 7.0 7.6 7.7 7.7 6.9 6.9 4.7

Annual average VPD (kPa) 0.41 0.43 0.39 0.32 0.40 0.42 0.49 0.46 0.38

Stand structure

Tree height, age 4 (m) 3.2 3.6 3.7 4.4 3.8 4.0 1.9 1.8 2.3

Tree ground-line diameter, age 4 (mm) 42 37 40 62 52 41 36 29 41

Annual average Lt, age 3–4 (m2 m�2) 4.2 3.7 3.5 6.6 4.9 6.0 2.9 1.9 4.1

Root depth, age 4 (m) 0.42 0.43 0.40 0.64 0.42 0.43 0.33 0.36 0.33

Water balance input variables

Stone content (%) 13 0 0 0 20 10 10 40 21

umax � umin (m3 m�3) 0.24 0.16 0.21 0.30 0.23 0.27 0.29 0.29 0.20

Wmax �Wmin (mm) 91 68 84 193 76 106 86 63 50

Soil variables

Soil order – Podzol Ultic Brown Brown Pallic Brown Brown Brown

Soil texture – Sand Silt Silt loam Silty clay loam Silt Silt loam Sandy loam Clay loam

For each variable the value for all eight sites is shown, and where applicable the site average is also given.

Table 2

Values used for parameters in the water balance model, for unfertilised (Unfert.)

and fertilised (Fert.) Cupressus lusitanica and Pinus radiata

Parameter Units Pinus radiata Cupressus

lusitanica

Fert. Unfert. Fert. Unfert.

gstmax mol m�2 s�1 0.26 0.33 0.15 0.18

D0t kPa 0.94 0.94 0.94 0.94

ut(Eg) m3 m�3 0.60ua 0.60ua 0.60ua 0.60ua

ut(Et) m3 m�3 0.55ua 0.55ua 0.55ua 0.55ua

See text for explanation of the symbols.

M.S. Watt et al. / Forest Ecology and Management 255 (2008) 1104–11121106

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meteorological stations (NIWA, n.d.). Average daily tempera-

ture was determined as the mean of maximum and minimum

temperature. Average vapour pressure deficit was determined

from minimum and average daily temperature using standard

formulae (Goudriaan and van Laar, 1994).

Prior to plot establishment, a soil pit was excavated at each

site in undisturbed soil to the estimated depth of the roots,

which averaged 57 cm. Root depth was estimated from residual

roots left from previously harvested trees, or the presence of an

impermeable pan, below which roots were unable to penetrate.

From this pit measurements of stone content (c) were taken by

soil horizon to the estimated root depth. Following Gradwell

(1972), umax and umin were determined at tensions of 10 kPa and

1500 kPa, respectively. Where these values differed from

gravimetric measurements (see below for description), umax and

umin were adjusted to the gravimetric value.

Trees were harvested four years after planting (winter 2006)

and plot level estimates of root depth, r, were determined by

measuring the depth of the deepest root on five trees per plot.

These measurements of r, and estimates of c, umax and umin, over

the measured root depth, were used to determine maximum

(Wmax) and minimum (Wmin) root-zone water storage from

W = ur(1 � c). Available root-zone water storage was then

determined as Wmax �Wmin.

During the fourth year after planting, leaf area index (Lt) of

each plot was measured directly using a canopy analyser (LAI-

2000, Li-Cor Inc., Lincoln, NE, USA) every four months.

Measurements were taken at four locations in each plot and

averaged to obtain a plot level estimate. Daily estimates of Lt were

obtained by fitting a spline function to these plot level estimates.

Evaporation of intercepted rainfall from the tree canopy was

determined as above canopy rainfall, less stemflow and

throughfall. Measurements of monthly net throughfall were

taken during four-monthly rainfall periods at site five, within a

single fertilised plot for both species. Throughfall beneath the

canopy was determined at five locations within each plot using

125 mm diameter plastic funnels attached to 3 L plastic

reservoirs. Stemflow was measured on five trees per plot using

channels formed by PVC pipe. The pipe was split long-

itudinally to form an upward facing concave surface and run

down the stem over a distance of approximately 300 mm into

20 L containers. To broaden the range in leaf area index

sampled, these data were supplemented with measurements

taken during a total of five-monthly rainfall periods from P.

radiata plots on two additional sites.

At site five, evaporation of intercepted rainfall from the tree

canopy (Etw) was correlated to monthly rainfall, with a slope of

0.36 for both species (Fig. 2). As analysis of covariance

indicated that species had no significant effect on either

intercept or slope for this relationship a combined interception

equation was developed for both species.

Evaporation of intercepted rainfall from the tree canopy

expressed as a percentage of rainfall, (Etw/P), was modelled as a

function of Lt, using the equation form developed by Landsberg

and Waring (1997), using data obtained from the three sites at

which measurements were taken (data not shown). For both

species, Etw/P increased linearly with a slope of 0.039 from 0 at

an Lt of 0 m2 m�2 to 0.35 at the maximum Lt of 8.89 m2 m�2.

During the fourth year after planting gravimetric measure-

ments were taken to a depth of 150 mm from all four sub-plots

within the eight sites, every two months, and converted to an

Fig. 2. Relationship between total monthly rainfall interception by the canopy

and total monthly rainfall at site five for Pinus radiata (filled circles) and

Cupressus lusitanica (open circles). Linear lines with no intercept have been

fitted to data for Pinus radiata (thick line) and Cupressus lusitanica (thin line)

Linear equations for both species were highly significant (P < 0.0001,

R2 = 0.98 for both species), and both lines have a slope of 0.36.

Fig. 1. Location of the eight study sites throughout New Zealand.

M.S. Watt et al. / Forest Ecology and Management 255 (2008) 1104–1112 1107

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fractional available volumetric water content (ua), using site

specific measurements of bulk density, umax and umin (see

Eq. (5)).

Maximum stomatal conductance (gstmax) for both species

was determined from the literature (Watt et al., 2003) and

measurements made in the field. All values of gstmax are

expressed on the basis of half the total surface area. For P.

radiata values of gstmax were obtained from Watt et al. (2003),

who took detailed measurements for unfertilised two-year-old

trees, which had a mean gstmax of 0.28 mol m�2 s�1 under well

watered conditions. For C. lusitanica measurements of gstmax

were taken from nine plants in the unfertilised treatment at site

7, under well watered conditions. Measurements were taken

from shoots distributed throughout the canopy under ambient

conditions by placing foliage in a clear top curvette attached

to a gas exchange system (model LI 6400, Li-Cor Inc.,

Lincoln, NE, USA). Leaf area of the C. lusitanica shoots

was determined using an area meter (model LI 3100,

Li-Cor Inc., Lincoln, NE, USA). All measurements were

taken under well lit conditions at a vapour pressure deficit of

less than 0.5 kPa, and ambient external CO2 concentration

(�360 mmol mol�1), prior to mid-day. Values of gstmax for C.

lusitanica, expressed on a half surface area basis, averaged

0.16 mol m�2 s�1.

Using these initial values as starting parameters gstmax was

empirically determined separately for the four combinations of

fertiliser and species by minimising the sums of squares

between measured and modelled ua across the eight sites.

Following Watt et al. (2003), D0t was set to 0.94 kPa for all

treatment combinations. Fitted values of gstmax showed good

agreement with measurements and were estimated to be 0.15

and 0.18 mol�2 m�2 s�1, respectively, for fertilised and

unfertilised C. lusitanica and 0.26 and 0.33 mol m�2 s�1

respectively for fertilised and unfertilised P. radiata (Table 2).

2.4. Model fit to measurements

Modelled values of ua fitted measurements well. When

averaged across the course of the year for all 32 site by

treatment combinations (total of 8 sites � 4 treatments)

modelled values of ua accounted for 66% of the variance in

measured ua (Fig. 3). Measured values were slightly under-

predicted by the model at low ua and slightly overpredicted by

modelled values at high ua. Analysis of covariance showed

there were no significant differences between either intercepts

(P = 0.07) or slopes (P = 0.93) in this relationship for the four

treatments, indicating that the modelled ua were unbiased with

respect to treatment (Fig. 3).

2.5. Data analysis

All analyses were undertaken in SAS (SAS Institute, 1996),

using plot-level data. The main and interactive effects of site,

species and fertilisation on average measured ua, model inputs

(Lt, r, Wmax � Wmin) and outputs (annual average ua, total

annual drainage and total annual water loss) were assessed

using analysis of variance. All variables were tested for

normality and homogeneity of variance and found to meet the

underlying assumptions of ANOVA.

3. Results

3.1. Site level variation in tree dimensions and water

balance characteristics

Site level variation in tree dimensions and site characteristics

affecting water balance were considerable (Table 1). Both

ground-line diameter, and height, at age four, ranged two-fold

across sites (Table 1). Average site Lt ranged three-fold, from

1.9 to 6.6 m2 m�2. Soil stone content, over the root-zone,

ranged from 0 to 40%. Available volumetric water content

(umax � umin), and root depth ranged two-fold, while there was

four-fold range in available root-zone water storage

(Wmax �Wmin) from 50 to 193 mm.

3.2. Influence of fertilisation and species on measured ua

Measurements of average fractional available volumetric

water content (ua) were significantly affected by site

(P = 0.0004) and species (P = 0.004), but not fertilisation

(P = 0.87) nor the interaction of fertilisation and species

(P = 0.96). Across sites, average measured ua ranged two-fold

from 0.32 to 0.64. Average measured ua in plots of C. lusitanica

exceeded values of ua in P. radiata plots by 24% (0.52 vs. 0.42).

3.3. Influence of fertilisation and species on model inputs

and outputs

Model inputs exhibited little sensitivity to either the main or

interactive effects of species and fertilisation (Table 3). Neither

root depth nor available root-zone water storage were

significantly affected by species or fertilisation. While leaf

area index did not significantly vary between species,

Fig. 3. Relationship between average measured and modelled fractional avail-

able volumetric water content, ua, for plots of unfertilised Pinus radiata (open

circles), fertilised Pinus radiata (closed circles), unfertilised Cupressus lusi-

tanica (open squares), and fertilised Cupressus lusitanica (closed squares). The

solid line shows the 1:1 relationship.

M.S. Watt et al. / Forest Ecology and Management 255 (2008) 1104–11121108

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application of fertiliser significantly increased values, by on

average 19% (4.60 m2 m�2 vs. 3.85 m2 m�2).

Both site and species had a highly significant influence on

annual average ua, total annual water loss and total annual

drainage (Table 3). However, none of these variables were

significantly affected by fertilisation or the interaction between

fertilisation and species. Given that fertilisation did not

significantly affect these variables, further analyses use site

level data which is averaged across fertilisation treatments for

both species.

The significant site range in annual average ua, total annual

water loss and total annual drainage was wide and scaled

significantly and positively with annual rainfall for all three

variables (P < 0.001 for all variables). Averaged across

species, annual average ua increased two-fold from 0.39 to

0.73 across the two sites representing the extremes in annual

rainfall (Fig. 4a). Total annual water loss increased two-fold

across the annual rainfall range (Fig. 4b), and as a percentage of

annual rainfall, remained relatively constant at approximately

0.83 up to a rainfall threshold of 1000 mm year�1, before

declining to 0.58 at 1729 mm year�1 (Fig. 5a). Total annual

drainage increased 14-fold across the annual rainfall range

(Fig. 4c), and as a percentage of rainfall, remained relatively

constant at approximately 0.15 up to a rainfall threshold of

1000 mm year�1, before increasing to 0.42 at 1729 mm year�1

(Fig. 5b).

Annual average ua for C. lusitanica was higher than that of P.

radiata at all eight sites (Fig. 4a) by on average 0.15 (0.58 vs.

0.43; Table 3). Total annual water loss for P. radiata was on

average 104 mm higher, than that of C. lusitanica (893 mm vs.

789 mm; Table 3). When expressed as a percentage of annual

rainfall this species difference in water loss averaged 10% (0.85

vs. 0.75; Fig. 5a). Conversely, total annual drainage for C.

lusitanica was on average 129 mm higher than that of P. radiata

(285 mm vs. 156 mm; Table 3). Expressed as a proportion of the

annual rainfall, treatment differences in drainage averaged 0.12

(0.12 vs. 0.24), across the eight sites (Fig. 5b).

Treatment differences in water loss were mainly attributable

to variation in transpiration. Estimated annual water loss

through transpiration in P. radiata significantly exceeded that of

C. lusitanica by 165 mm (Table 4). Annual losses through

interception were very similar and not significantly different

between species (180 mm vs. 186 mm), while estimated annual

losses through evaporation from the soil in C. lusitanica

significantly exceeded those of P. radiata by on average 55 mm

(Table 4).

4. Discussion

Conclusions drawn from this study are based on data from

plots grown at high initial stand density over a short time

period. Through establishing these plots at high density this

approach aims to compress the experimental period over which

the stand develops by increasing intra-tree competition and the

Table 3

Variation in water balance model inputs and outputs by species and fertilisation

Model inputs Model outputs

Lt (m2 m�2) Root depth (m) Wmax �Wmin (mm) ua Water loss (mm year�1) Drainage (mm year�1)

Cupressus lusitanica

Fertilised 4.8 (0.6) 0.43 (0.04) 94 (17) 0.58 (0.04) 818 (73) 256 (69)

Unfertilised 3.6 (0.8) 0.38 (0.03) 83 (13) 0.58 (0.04) 759 (73) 313 (87)

Pinus radiata

Fertilised 4.4 (0.4) 0.42 (0.04) 92 (18) 0.46 (0.05) 889 (76) 157 (67)

Unfertilised 4.1 (0.6) 0.44 (0.05) 96 (18) 0.41 (0.05) 897 (77) 155 (85)

Significance of treatment and site

Site 17*** 12*** 40*** 39*** 55*** 69***

Species (Sp.) 0.03ns 1.4ns 1.2ns 108*** 28*** 50***

Fertiliser (Fert.) 8.1** 0.6ns 0.5ns 3.3ns 1.7ns 2.3ns

Sp. � Fert. 2.6ns 2.3ns 2.2ns 2.2ns 3.0ns 2.6ns

Each value is the mean � standard error from eight sites. Also shown are ANOVA results describing the influence of treatment and site on each variable. Values

presented for the ANOVA include the F-value followed by the P category, in which asterisks ***, ** represent significance at P = 0.001 and 0.01, respectively,

ns = non-significant at P = 0.05.

Table 4

Variation in transpiration from the dry tree canopy (Et), evaporation of inter-

cepted rainfall from the tree canopy (Etw), and evaporation from the soil (Eg) by

species and fertilization

Water balance component

Et (mm year�1) Etw (mm year�1) Eg (mm year�1)

Cupressus lusitanica

Fertilised 509 (62) 213 (39) 96 (36)

Unfertilised 421 (69) 159 (42) 179 (49)

Pinus radiata

Fertilised 630 (52) 188 (25) 71 (13)

Unfertilised 630 (60) 172 (31) 95 (30)

Significance of treatment and site

Site 26*** 23*** 6.9***

Species (Sp.) 52*** 0.2ns 6.0*

Fertiliser (Fert.) 3.7ns 6.0* 6.1*

Sp. � Fert. 3.7ns 2.0ns 1.8ns

Each value is the mean � standard error from eight sites. Also shown are

ANOVA results describing the influence of treatment and site on each variable.

Values presented for the ANOVA include the F-value followed by the P

category, in which asterisks ***, *, represent significance at P = 0.001 and

0.05 respectively, ns = non-significant at P = 0.05.

M.S. Watt et al. / Forest Ecology and Management 255 (2008) 1104–1112 1109

Author's personal copy

rate at which leaf area develops and the base of the green crown

recedes. As the validity of this approach in replicating processes

such as water use at conventional scales is uncertain, further

research should be undertaken to ensure results are applicable

to mature stands grown at operational stand densities.

With the aforementioned caveat in mind, our data clearly

show that species differences in annual average ua, total annual

drainage and total annual water loss were considerable, highly

significant and consistent across a wide environmental gradient.

Combining annual totals for drainage and water loss demon-

strated there was on average 233 mm more available water per

annum in soils under C. lusitanica than P. radiata, which

constitutes 22% of average annual rainfall. Differences of this

magnitude could have considerable influence on species

selection, when issues such as erosion control or water

conservation in dryland areas are of importance.

Model simulations suggest that greater water loss in P.

radiata was primarily due to higher annual transpiration which

exceeded values for C. lusitanica by 165 mm or 35% (630 mm

vs. 465 mm). As leaf area was very similar between species,

transpirational differences were primarily attributable to the

higher estimated values of maximum stomatal conductance

(gstmax) for P. radiata, compared to C. lusitanica

(0.165 mol m�2 s�1 vs. 0.30 mol m�2 s�1). Model estimates

of gstmax broadly agree with values obtained from the field for

C. lusitanica and previous measurements for P. radiata of the

same age, measured by Watt et al. (2003). Further research is

needed to determine if the stomatal response for C. lusitanica to

increases in D and declining ua follows the functions assumed in

this study. Annual estimates of transpiration for P. radiata are

similar to those obtained by Whitehead and Kelliher (1991a) for

a closed canopy stand (630 mm vs. 704 mm). The low values of

transpiration for C. lusitanica are consistent with a study in

Ethiopia (Fetene and Beck, 2004) which showed that rates of

transpiration per unit leaf area were considerably lower in C.

lusitanica than two indigenous (Croton macrostachys Hochst.

ex. Del. and Podocarpus falcatus (Thunb.) Endl.) and one

exotic species (Eucalyptus globulus Labille.).

Results suggest that evaporation of intercepted rainfall from

the tree canopy may not significantly differ between species.

Fig. 4. Relationship between total annual rainfall and (a) annual average

fractional available volumetric water content (ua), (b) total annual water loss

and (c) total annual drainage, for plots with Pinus radiata (filled circles) and

Cupressus lusitanica (open circles).

Fig. 5. Relationship between total annual rainfall and (a) the quotient of total

annual water loss and total annual rainfall and (b) the quotient of total annual

drainage and total annual rainfall, for plots with Pinus radiata (filled circles)

and Cupressus lusitanica (open circles).

M.S. Watt et al. / Forest Ecology and Management 255 (2008) 1104–11121110

Author's personal copy

However, this result should be interpreted with caution as it is

based on data collected at a single site with high Lt. Further

research should be undertaken to confirm these results in stands

with lower Lt, covering a broad gradient in rainfall.

Measured evaporation of intercepted rainfall from the tree

canopy for P. radiata averaged 21% of rainfall across three

sites, with average Lt of 6 m2 m�2. These results are consistent

with previously measured values for P. radiata, grown at

conventional stand densities and ranging in age from 6 to 35

years, which show evaporation of intercepted rainfall from the

tree canopy (Etw) ranging from 19 to 30% of rainfall (Smith,

1974; Feller, 1981; Langford and O’Shaughnessy, 1978;

Crockford and Richardson, 1990; Myers and Talsma, 1992;

Fahey et al., 2001). For P. radiata there was a positive linear

relationship between Lt and the fraction of rainfall intercepted

and evaporated from the tree canopy (Etw/P). Using this

function, annual modelled values of Etw were on average a

relatively low proportion of annual rainfall at 16% and 17% for

C. lusitanica and P. radiata, respectively, reflecting the low

values of Lt for these plots compared to mature stands. Use of Lt

to determine Etw/P is consistent with the method used within

the process-based growth model 3-PG (Landsberg and Waring,

1997; Sands and Landsberg, 2002).

In the P. radiata plots modelled annual evaporation from the

soil surface (Eg) averaged 8% of annual rainfall. These low

values for Eg are typical of closed canopy stands such as those

reported here, and are consistent with previously reported

values in 11- to 13-year-old closed canopy P. radiata which

range from 6 and 7% of annual rainfall (Whitehead and

Kelliher, 1991a,b). Although the model suggests that C.

lusitanica has higher rates of Eg than P. radiata, absolute

differences were relatively small at an average of 55 mm, and

offset by considerably lower rates of transpiration in C.

lusitanica. Species differences in Eg were primarily attributable

to the low Lt of unfertilised C. lusitanica, which had a

disproportionate effect on Eg, as transmitted radiation and Eg

declines exponentially with increasing Lt.

Measurements of fractional available volumetric water

content (ua) clearly show that P. radiata uses more water than C.

lusitanica. As modelled values of ua fitted these two-monthly

measurements well, the daily modelled values are likely to

accurately represent species differences in annual average ua,

total annual water loss and total annual drainage. However, the

components of water loss estimated in this study may be subject

to some error. Potential sources of error in the estimation of dry

canopy transpiration include inaccurate estimation of stomatal

conductance, and its response to vapour pressure deficit. Of

most concern is the estimation of these parameters for C.

lusitanica as there is little information in the literature to

substantiate the values used in this study. Estimates of leaf area

index, obtained from the leaf area meter were not calibrated

against actual values. Although this is unlikely to affect

estimates of total annual water loss, it may influence estimates

of stomatal conductance and the partitioning of water loss

between water balance components. An assumption of the

modelling, which requires verification, is that parameters used

in all model components were generally applicable across all

sites. Further detailed research, conducted across a range of

sites for both species, is needed to check parameter values,

validate model components, and determine if assumptions

regarding the general applicability of parameters are valid.

In conclusion, these results show that despite overall species

similarity in leaf area index and root depth, C. lusitanica

consistently used less water and had higher drainage than P.

radiata, at sites covering a wide environmental gradient.

Further research should be undertaken to ascertain whether

these results are applicable to mature stands grown at

conventional stand densities. If findings are confirmed, these

differences in water use may become an important considera-

tion in species selection.

Acknowledgements

We thank the numerous forest companies and private owners

for providing sites for the trial series, and Hugh Wilde, Trevor

Webb, Amy Taylor, Wim Rijkse, Craig Ross and Malcolm

Mcleod who assisted in selection, identification and classifica-

tion of soils. The help of technicians at the Landcare Research

environmental chemistry lab in analysing samples is gratefully

acknowledged. We are also grateful for the useful comments

made by two anonymous reviewers. This project was funded by

the New Zealand Foundation for Research Science and

Technology under contract No. C04X0304. ‘Protecting and

Enhancing the Environment through Forestry’.

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