Defining sustainability of plantation forests through

identification of site quality indicators influencing

productivity—A national view for New Zealand

Michael S. Watt a,*, Graham Coker a, Peter W. Clinton a, Murray R. Davis a,Roger Parfitt c, Robyn Simcock d, Loretta Garrett b, Tim Payn b,

Brian Richardson b, Andrew Dunningham b

a Centre for Sustainable Forest Management, Forest Research, P.O. Box 29237, Fendalton, Christchurch 4800, New Zealandb Centre for Sustainable Forest Management, Forest Research, Private Bag 3020, Rotorua, New Zealand

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

Received 1 April 2005; received in revised form 19 May 2005; accepted 20 May 2005

Abstract

New Zealand is committed to developing sustainable forest management practices as evidenced through Government

involvement in international forestry agreements such as the Montreal Process, and the forestry sector’s adoption of forest

certification mechanisms. In support of this commitment, it has been identified that there is little quantitative evidence of the

interactions of plantation forestry on site quality and long-term site productivity. To address this issue, a nationwide study of site

quality was initiated at 35 key sites covering the range of edaphic and environmental conditions representing the productivity

envelope for New Zealand plantation forests. At each location, within the productivity envelope, eight short-term site quality

plots were planted at a very high stand density (40,000 stems ha�1) to rapidly identify key soil indicators of growth which may

be useful for determining site sustainability. In addition, a permanent sample plot was established by planting seedlings at

conventional stem densities (500–1100 stems ha�1). At each site, a factorial design was applied with the following three factors:

species (Pinus radiata and Cupressus lusitanica), fertiliser (no fertiliser and nutrients supplied in excess of crop demands) and

disturbance (low and high disturbances). After two years of increment, initial analyses are presented which partition treatment

and site effects on increment and identify key soil properties that influence increment of the two species.

Volume increment over the two-year period was most strongly influenced by site, ranging 50-fold and 15-fold across sites for

C. lusitanica and P. radiata, respectively. For the treatments, species accounted for most of the variance in increment, with mean

volume increment across all sites of P. radiata significantly exceeding that of C. lusitanica by 56%. Fertilisation also

significantly influenced volume increment inducing mean gains of 30%. Disturbance had a significant, but comparatively weak

effect, reducing mean volume increment by 9%. After correction had been made for environment (temperature and rainfall), soil

properties that were most strongly related to volume increment for both species included CN ratio, total soil nitrogen, total soil

www.elsevier.com/locate/foreco

Forest Ecology and Management 216 (2005) 51–63

* 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 # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2005.05.064

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–6352

1. Introduction

New Zealand is a member of the 12 country

Working Group on Criteria and Indicators for the

Conservation and Sustainable Management of Tem-

perate and Boreal Forests. ‘‘The Montreal Process’’

(Anon., 1995) and the New Zealand Government have

agreed to report on progress towards sustainable forest

management as measured by indicators grouped

within seven criteria. Criterion 2, ‘Maintenance of

Productive Capacity of Forest Ecosystems’, and

criterion 4, ‘Conservation and Maintenance of Soil

and Water Resources’, address the important issue of

whether plantations can be grown for an indefinite

number of rotations without adversely affecting soil

quality or the site’s capacity for net primary

production. Although the Montreal Process and

certification schemes identify potential indicators that

can be used to describe site quality, foresters need

tools, guidelines or management systems to enable

them to turn these high-level, subjective sustainability

goals into indicators that can be measured quantita-

tively (Richardson et al., 1999).

Despite the importance of sustainability for the

long-term viability of the forestry sector, there is

little direct evidence to indicate how successive

rotations, characteristic of plantation forestry, influ-

ence site productivity. As well-designed long-term

field trials for investigating sustainability are rare,

evidence demonstrating changes in productivity are

uncommon, even at a regional level (Dyck and Cole,

1990). Within New Zealand, there are considerable

data that may be used to examine temporal changes

in productivity of the most commonly planted

plantation species Pinus radiata, as annual growth

has been measured in permanent sample plots in

some plantations for over 60 years (Woollons, 2000).

However, these data are of questionable value in

assessing temporal changes in soil quality because

they are invariably confounded by changes over time

in management practices, tree genotype and climate

(Morris and Miller, 1994; Richardson et al., 1999;

Woollons, 2000). Mechanistic models are also of

limited use in assessing changes in site productivity

as our knowledge of many of the key ecosystem

processes is incomplete (Yarie, 1990).

The importance of well-designed long-term field

experiments for detecting changes in site productivity

over time has been emphasised by many researchers

(Adlard et al., 1984; Dyck and Cole, 1990; Richardson

et al., 1999). Ideally, these trials should be installed

across a wide environmental range and run over a

number of successive rotations. Within the trials, it is

vital to control management and genotype factors and

monitor climatic parameters, so that any changes in

productivity can be correctly attributed to alterations

in site quality (Morris and Miller, 1994).

Although long-term field trials are important for

determining if site productivity changes over time,

these trials do little to elucidate the mechanisms

causing alterations to site quality (Adlard et al., 1984;

Richardson et al., 1999). Understanding these

mechanisms is essential as it provides the information

necessary for managers to predict and prevent or

ameliorate declining site quality. Measurements of

soil properties are useful in this regard as indicators of

changes in site quality and provide considerable

information on factors which result in changes in site

quality and productivity over time. As forest

productivity is an important indicator of sustainability,

these soil properties must be related to measures of

performance such as net primary production (Richard-

son et al., 1999). Use of this information and the

development of relationships between various man-

agement practices and soil properties would enable

site-specific recommendations to be made on manage-

ment practices necessary to maintain the productive

capacity of the site (Richardson et al., 1999).

One approach that could be used to rapidly

determine the key soil properties influencing site

phosphorus, organic phosphorus and depth of the A horizon. When soil properties were included in combination, the best

predictive models of volume increment formulated for both species included rainfall, temperature, the product of total soil

nitrogen and total soil phosphorus and depth of the A horizon.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Sustainability; Forest productivity; Soil quality; Indicators; Pinus radiata; Cupressus lusitanica; New Zealand

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–63 53

productivity is experimentation in miniature. This

method, which uses small scale, highly stocked plots

to rapidly induce site productivity limitations and

compress the length of the rotation, has considerable

merit (Amateis et al., 2003a). Previous research shows

that trees grown at these high stand densities over short

periods mimic growth patterns of trees growing

over longer periods at conventional stand densities

(Amateis et al., 2003a,b; Sharma et al., 2003). As the

experimental cycle is shortened, this approach is more

efficient and cost effective than the same experiment

conducted at an operational scale. These greater

efficiencies allow relatively large numbers of plots to

be installed, which permits increased coverage across

a wide range of environments. Another key advantage

of this approach is that the small plot area used is

subject to less edaphic variation, which makes it easier

to characterise soil properties within the plot (Amateis

et al., 2003a).

Although these small plots are relatively short-

lived, the use of high stand densities should ensure that

there is significant demand for resources from the site

relatively quickly, thus enabling the key soil properties

influencing productivity to be identified. Experimen-

tation in miniature has been successfully used to link

productivity with changes in key soil properties

induced by different management practices at a single

site (Kelting et al., 1999). In this study, small highly

stocked plots (spacing 0.3 m � 0.3 m; stand density of

111,111 stems ha�1) of Pinus taeda grown over a one-

year period were used to validate a soil quality index

(SQI) model developed from literature based suffi-

ciency curves for five key growth determining

attributes of forest soils. Using predetermined

sufficiency curves developed from juvenile and

mature stands (see Kelting et al., 1999 for references),

the SQI accounted for 60% of the variation in tree

volume, with three of the five attributes (water table

depth, soil aeration depth and net N mineralisation)

exhibiting significant relationships with volume.

These results suggest that experimentation in minia-

ture may provide a useful means of rapidly identifying

key soil properties influencing tree growth and site

sustainability.

Following this rationale, a nationwide study of site

quality using mini plantations was initiated at 35 key

sites covering the range of edaphic and environmental

conditions for New Zealand plantation forests. An

important component of this study is the establishment

of new permanent sample plots at each study site to

determine the long-term effects of forest and forest

operations on soil quality and site productivity and

validate the short-term experimentation.

This paper describes the plot location, layout and

measurements taken and documents the range of

climatic and edaphic properties occurring across the

trial series. Using data collected from two years of

increment in the productivity envelope plots, addi-

tional objectives were to: (i) investigate how treat-

ments (species, fertilisation and disturbance) and site

influence productivity; (ii) identify key soil indicators

of productivity; (iii) examine whether these soil

indicators vary between species.

2. Materials and methods

2.1. Location of site quality plots

Sites were selected to represent the range in soil

properties on which plantation forests are currently

grown in New Zealand (Fig. 1). Soils were grouped

according to the New Zealand soil classification

(Hewitt, 1998), which recognises 15 soil orders.

Plantation forests within New Zealand occur on 9 of

these 15 soil orders, namely the Allophanic, Brown,

Oxidic, Pallic, Podzol, Pumice, Raw, Recent and Ultic

soil orders. The trial series was established on all of

these soil orders apart from Oxidic, which represents a

small proportion of the area (0.7%) on which the forest

estate is planted (R. Simcock, personal communica-

tion). The number of sites established on each soil

order was weighted to be representative of the

corresponding plantation area on which the soil order

is found (Fig. 1).

Sites were further screened using climatic surfaces

to ensure that selected areas represented the con-

siderable range in meteorological conditions found

throughout New Zealand forest plantations. Long-

term climate data obtained from thin-plate spline

surfaces (Hutchinson and Gessler, 1994) fitted to

meteorological station data (Leathwick and Stephens,

1998) were used to determine climatic conditions at

each selected site. When compared to long-term

average values for all plantation forests (Anon., 1983),

the 35 selected sites almost completely encompass the

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–6354

Fig. 1. Distribution map of soil orders in New Zealand, showing location of site quality plots.

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–63 55

Table 1

Comparison of the range in mean annual temperature, solar radiation, relative humidity (recorded at 09:00 NZST) and total annual rainfall, at the

selected 35 sites, with that found throughout New Zealand plantation forests

Selected 35 sites Plantation forests

Minimum Mean Maximum Minimum Maximum

Temperature (8C) 8.6 11.5 (0.3) 15.5 8.0 15.6

Solar radiation (MJ m�2) 12.0 14.3 (0.16) 15.4 12.0 15.4

Relative humiditya (%) 69 79 (0.8) 87 66 87

Rainfall (mm) 609 1560 (121) 3718 609 3718

The mean � standard error for the selected 35 sites is shown.a Recorded at 09:00 NZST.

range in total annual rainfall and mean annual

temperature, relative humidity and solar radiation

(Table 1).

2.2. Experimental design

Site quality plots were located at each of the 35

selected locations. At each location, a series of eight

plots was installed using a factorial design with the

following three factors: species (P. radiata and

Cupressus lusitanica), fertiliser (no fertiliser and

nutrients supplied in excess of crop demands) and

disturbance (low and high disturbances). P. radiata

was chosen as it is the most common plantation

species within New Zealand, while C. lusitanica was

selected as it is a commonly planted species on more

fertile farm sites. Quantities of fertiliser were applied

to treated plots to ensure that there were no nutrient

limitations. Disturbed plots were located on areas

compacted by previous harvesting operations. Each

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

nine measurement trees spaced at 0.5 m � 0.5 m

(40,000 stems 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 Forest Research nursery in Rotorua. At each of

the 35 selected locations, permanent 40 m � 40 m

sample plots were installed adjacent to the site

quality plots at conventional stand densities (500–

1100 stems ha�1) using P. radiata seedlings with a

growth and form factor of 19. The site quality plots

were installed over a three-year period from 2000 to

2002.

2.3. Measurements

Elevation, longitude, latitude, aspect and slope

were recorded at each location. Within the site quality

plots tree ground-line diameter, height and crown

width are measured annually. Measurements of

photosynthetically active photon flux density, air

temperature and relative humidity are taken from

sensors installed on a 3 m tower located adjacent to the

experimental plots. A tipping bucket rain gauge

positioned on top of the tower records above-canopy

rainfall.

A comprehensive set of soil chemical and physical

measurements were taken from the site quality plots

prior to planting and fertilisation. Soil chemistry

samples were taken from 0 to 100 mm depth at five

points in each of the four undisturbed plots to give 20

cores per bag. This process was repeated for the four

disturbed plots at each location. Sampling points were

at least 1 m apart near the centre of the plots. These

samples were bulked by disturbance class, then

analysed for soil moisture, pH in water, total carbon

(C), total soil nitrogen (N), total soil phosphorus (P),

organic P, Bray P, Olsen P, P retention, exchangeable

bases and CEC, following the methods described by

Blakemore et al. (1987). Results are expressed on an

oven dry basis (105 8C). Soil physical properties weremeasured on samples of mineral soil taken from the

edges of disturbed and undisturbed plots. Air capacity

(at 10 kPa), macroporosity (at 5 kPa), bulk density,

particle density, cone penetration resistance (at

10 kPa) and total available water were determined

from 5 to 200 mm depth, following the procedures

described by Gradwell (1972).

A soil pit was excavated in undisturbed soil

adjacent to each site quality plot to supplement soil

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–6356

property measurements taken from the upper mineral

horizons. From this pit, all previously described

physical and chemical soil properties were determined

by horizon to depths of up to 1 m. Particle size was

measured by dispersing the field-moist soil in water

with an ultrasonic probe and separating the <2, 2–63

and >63 mm fractions by sedimentation. The depth of

all horizons, profile drainage class (Hewitt, 1993),

rooting depth and depth to mottling were also

recorded. Each soil was described pedologically and

classified to subgroup level following the New

Zealand Soil Classification (Hewitt, 1993). As organic

horizons were not always present and were usually

less than 30 mm thick, data from these layers have not

been included.

2.4. Data analyses

Analyses presented in this paper use data collected

from the complete series of 35 site quality plots over the

first two years of the experiment. Data for four plots

were excluded from all analyses due to the excessive

mortality within these plots. All analyses were under-

taken in SAS (SAS Institute, 1996). Variables were

tested for normality and homogeneity of variance and

transformations made as necessary to meet the under-

lying statistical assumptions of the models used.

A two-way analysis of variance (ANOVA) was

used to test for the main effects of disturbance and site

on soil physical and chemical properties within the top

100 mm of the mineral horizon. The main and

interactive influence of site and treatment (fertiliser,

species and disturbance) on height, diameter and

volume increment over the first two years (defined as

final values less initial values), was tested using a

generalised linear model (GLM). Initial size was

included as a covariate in the model when found to be

significantly related to increment.

Key soil properties influencing volume increment

were determined for each species using data from

unfertilised plots for which soil property measure-

ments were available. Stem volumewas selected as the

dependant variable as it is considered to be a better

measure of productivity than either height or diameter

(Jackson and Gifford, 1974). Stem volume (V) was

determined from tree height (h) and diameter

measured at ground level (D) for both P. radiata

and C. lusitanica using the following equation

previously found to be applicable to stands covering

a range of ages and stockings (Beets, unpublished

data):

V ¼�p

�D

2

�2

h

�0:25 (1)

To remove the effect of climate, base models of

volume increment were separately constructed for each

species using temperature and rainfall as independent

variables. Univariate relationships between corrected

volume increment and soil properties were then

examined by singly adding soil properties to the two

base models. Significance, functional form and strength

of these univariate relationships were compared

between species.

A multiple regression model for each species was

also constructed using a combination of edaphic

properties. Using appropriate functional forms and

any necessary transformations, variables were sequen-

tially introduced into each base model. Variables were

only retained if inclusion significantly improved the

model and parameter values for the included variables

were significant (P < 0.05). In constructing the two

final models, emphasis was placed on developing

simple equations with very little apparent bias.

3. Results

3.1. Impact of site and disturbance on soil

properties

The selected sites included extremes in soil texture

ranging from single-grained scoria and sandy soils to

clay loam soils. Variation in soil texture was most

pronounced for the sand (5–96%) and silt fractions (3–

79%). All physical and chemical properties sampled

significantly (P < 0.001) varied across sites (Table 2).

Disturbance had a significant impact on all soil

physical properties apart from total available water

(Table 2). The closely related variables, air capacity

and macroporosity, were most strongly affected by

disturbance (partial r2 = 0.16 and 0.12, respectively).

Of the soil physical properties examined, macro-

porosity exhibited the greatest range with values in the

undisturbed treatment exceeding those in the disturbed

treatment by 47% (21 versus 14 v/v). For chemical

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–63 57

Table 2

Characteristics of soil physical and chemical properties in the top 100 mm of the mineral soil for disturbed and undisturbed plots sampled at 31

sites before planting

Undisturbed plots Disturbed plots Analysis of variance

Mean Range Mean Range Site Dist.

Physical properties

Bulk density (g cm�3) 1.0 0.5–1.4 1.1 0.5–1.6 0.90*** 0.04***

Particle density (g cm�3) 2.5 2.2–3.0 2.5 2.2–3.0 0.99*** 0.005*

Penetration resistance (MPa) 0.8 0.3–2.1 1.1 0.2–2.1 0.81*** 0.07***

Total porosity (%, v/v) 62 45–78 58 46–79 0.85*** 0.07***

Macroporosity (%) 21 4–49 14 4–32 0.69*** 0.16***

Air capacity (%) 24 4–45 18 5–38 0.78*** 0.12***

Total available water (%, v/v) 19 9–32 19 4–35 0.91*** ns

Chemical properties

Carbon (%) 6.0 0.7–21.6 5.2 1.0–28.9 0.86*** 0.02*

Total N (%) 0.29 0.02–0.67 0.25 0.03–0.79 0.89*** 0.02*

CN ratio 22 11–46 22 11–38 0.97*** ns

pH 5.2 4.1–6.0 5.2 4.2–6.2 0.94*** ns

CEC (cmol g�1) 22 2–72 21 3–76 0.97*** ns

Exchangeable Na (cmol g�1) 0.19 0.02–0.77 0.16 0.01–0.56 0.85*** ns

Exchangeable K (cmol g�1) 0.47 0.01–1.15 0.43 0.03–0.85 0.88*** ns

Exchangeable Mg (cmol g�1) 1.9 0.1–12.0 1.7 0.3–9.8 0.97*** ns

Exchangeable Ca (cmol g�1) 4.4 0.2–18.7 4.1 0.4–18.7 0.94*** ns

Sum bases (cmol g�1) 7.0 0.3–23.6 6.4 0.8–22.2 0.95*** ns

Base saturation (%) 33 7–95 34 5–89 0.96*** ns

P retention (%) 42 3–96 40 0–89 0.97*** ns

Olsen P (mg g�1) 8.3 2.3–29.0 7.6 0.9–24.2 0.88*** ns

Bray P (mg g�1) 20 3–82 20 3–106 0.94*** ns

Inorganic P (mg g�1) 150 15–326 140 18–457 0.93*** ns

Organic P (mg g�1) 314 21–675 281 28–595 0.93*** ns

Total P (mg g�1) 464 37–937 420 62–838 0.93*** 0.01*

The influence of disturbance (Dist.) and site on each soil property is shown by partial r2-values followed by significance levels. ns: non-

significant at P = 0.05.* Significant at P = 0.05.

*** Significant at P = 0.001.

properties, disturbance significantly reduced concen-

trations of total soil N, P and C (Table 2).

3.2. Site and treatment influences on tree

increment

Increment over the two-year period was most

strongly influenced by site (Table 3). Site significantly

(P < 0.001) influenced increment in all characteristics

accounting for 61, 76 and 49% of the total variance for

tree diameter, height and volume increment, respec-

tively. Stem volume increment was a more sensitive

indicator of site quality than either tree height or

diameter increment, exhibiting 50-fold and 15-fold

variations in volume increment, respectively, for

C. lusitanica and P. radiata (Fig. 2).

Treatment influences on tree characteristics

became more pronounced as the experiment pro-

gressed, especially for stem volume (Fig. 3). The

maximum treatment differences for stem volume

(averaged across sites) increased from 33% (4.8–

6.4 m3 ha�1) at the end of year 1 to two-fold (7.5–

17.4 m3 ha�1) at the end of year 2. Treatment effects,

particularly those associated with species, were least

marked for tree height increment (Table 3).

For all characteristics examined increment of

P. radiata significantly (P < 0.001) exceeded that of

C. lusitanica (Tables 3 and 4). Fertilisation also

significantly influenced all tree characteristics (P <0.001), inducing gains of 30, 8 and 13% for stem

volume, height and diameter increment, respectively

(Tables 3 and 4). Disturbance significantly influenced

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–6358

Table 3

Main and interactive effects of initial size, site and treatments

(species, disturbance and fertilisation) on tree diameter, height

and volume increment over two years

Dependant variable

Diameter Height Volume

Initial size ns ns 0.25*

Site 0.61*** 0.76*** 0.49***

Species (Sp.) 0.10*** 0.02*** 0.05***

Fertilisation (Fert.) 0.04*** 0.01*** 0.03***

Disturbance (Dist.) ns 0.01** 0.01*

Site � Sp. 0.07*** 0.06*** 0.05***

Site � Fert. ns 0.03*** 0.03*

Site � Dist. 0.06*** 0.04*** 0.04***

Sp. � Fert. ns ns ns

Sp. � Dist. ns ns ns

Dist. � Fert. ns ns ns

Sp. � Dist. � Fert. ns ns ns

The partial r2 of each significant variable is shown with the

significance level.* Significant at P = 0.05.** Significant at P = 0.01.*** Significant at P = 0.001.

Fig. 2. Boxplots showing site variation in growth increment over

two years for: (a) tree diameter, (b) tree height and (c) tree volume

for Cupressus lusitanica and Pinus radiata. The boundaries of the

boxplot closest to zero indicates the 25th percentile, the centre lines

within the boxes indicate the median and the boundaries of the boxes

farthest from zero indicate the 75th percentile. Error bars are the 5th

and 95th percentiles, and filled symbols represent data outlying the

5th and 95th percentiles.

tree height (P < 0.01) and volume increment

(P < 0.05) but not diameter increment. Reductions

in increment induced by disturbance were consider-

ably lower than gains obtained through fertilisation for

all tree characteristics apart from height. In contrast to

fertilisation, tree height was more strongly influenced

by disturbance than tree diameter (Table 3).

The interactions were significant for almost all

combinations of site by treatment, with the single

exception being the fertiliser by site interaction for tree

diameter (Table 3). Site interactions were strongest for

species, intermediate for disturbance and weakest for

fertiliser. In contrast, none of the interactions examined

between the species, fertiliser and disturbance treat-

ments were significant (P > 0.05) suggesting that

disturbance and fertilisation have independent influ-

ences on increment and both species respond similarly

to fertiliser and disturbance. This was confirmed by

examination of treatment means which show a

consistent ranking in disturbance and fertiliser treat-

ments (UF > DF > UN > DN) for both species

(Table 4).

3.3. Identification of key soil properties

For both species, temperature and rainfall were

significantly related to volume increment. Inclusion of

both variables in the model as second-order poly-

nomials accounted for 46 and 33% of total variation in

volume increment for P. radiata and C. lusitanica,

respectively.

After correction had been made for climate, the soil

properties most strongly related to increment for both

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–63 59

Fig. 3. Changes in: (a) tree diameter and (b) tree volume over time

for all treatments for both Pinus radiata (dotted line) and Cupressus

lusitanica (solid line). Closed circles represent undisturbed and

fertilised (UF), open circles are disturbed and fertilised (DF), closed

squares are undisturbed and unfertilised (UN) and open squares are

disturbed and unfertilised (DN).

Table 4

Variation in tree height, diameter and volume increment between

species (Sp.), fertilisation (Fert.) and disturbance (Dist.) treatments

Tree characteristic

Diameter

(mm)

Height

(m)

Volume

(m3 ha�1)

RUF 26.2 1.43 17.3

RDF 25.1 1.28 16.2

RUN 22.8 1.28 14.5

RDN 22.7 1.21 12.8

CUF 21.3 1.26 11.8

CDF 20.6 1.18 11.0

CUN 17.6 1.14 8.6

CDN 16.6 1.00 7.5

Treatment-induced changes (%)

Sp. (R/C) +27 +8 +56

Fert. (F/N) +13 +8 +30

Dist. (D/U) �3 �7 �9

Also shown are treatment-induced changes. Abbreviations: R, Pinus

radiata; C, Cupressus lusitanica; U, undisturbed; D, disturbed; F,

fertilised; N, unfertilised.

Table 5

Species comparison of edaphic variables significantly related to

volume increment, after correction was made for climatic effects

P. radiata C. lusitanica

Similarities between species

CN ratio 0.17*** 0.30***

Total soil N 0.16*** 0.20***

Total soil P 0.19*** 0.17***

Organic P 0.19*** 0.17***

‘‘A’’ horizon depth 0.12*** 0.08**

Particle density 0.09*** 0.05*

Inorganic P 0.08** 0.05*

Fine sand percent 0.06* 0.08*

Exchangeable K 0.06* 0.05*

Bulk density 0.06** 0.05*

Differences between species

Drainage class 0.11*** ns

Depth to mottles 0.05** ns

Exchangeable Na ns 0.05*

The partial r2 of each significant variable is shown with the

significance level. ns: non-significant.* Significant at P = 0.05.** Significant at P = 0.01.*** Significant at P = 0.001.

species were soil CN ratio, total soil N, total soil P and

soil organic phosphorus (Table 5). Although sig-

nificant, depth of the A horizon, particle density,

inorganic P, exchangeable potassium, fine sand

percentage and bulk density displayed weaker

relationships with volume increment. For all of these

properties, the functional forms of these univariate

relations exhibited considerable similarity between

the two species (see Fig. 4 for relations between

increment and main variables). Drainage class and

depth to mottles were significantly related to volume

increment for P. radiata but not C. lusitanica. In

contrast, exchangeable sodium was significantly

related to volume increment for C. lusitanica but

not P. radiata.

When included in combination the best models

formulated for volume increment included rainfall,

temperature, the product of total soil N and P and

depth of the A horizon for both species (Table 6). The

total amount of variation in volume increment

explained by these models for P. radiata and C.

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–6360

Fig. 4. Response curves of volume increment for Pinus radiata (thick line) andCupressus lusitanica (thin line) plotted against: (a) total soil P, (b)

total soil N, (c) CN ratio and (d) A horizon depth. All other variables in the model were held at mean values when each response curve was

generated.

lusitanica was 72 and 57%, respectively. Both models

exhibited little apparent bias when residuals were

plotted against either predicted values or independent

variables.

Table 6

Summary of statistics for the final predictive models of volume

increment for Pinus radiata and Cupressus lusitanica

P. radiata C. lusitanica

Rainfall 0.31*** 0.23***

Temperature 0.15*** 0.08*

N � P 0.20*** 0.18***

‘‘A’’ horizon depth 0.06*** 0.06**

Total r2 0.72 0.57

The partial r2 of each significant variable is shown with the

significance level of the variable tested using the F-test. Also shown

is the total r2 for the complete model.* Significant at P = 0.05.** Significant at P = 0.01.*** Significant at P = 0.001.

4. Discussion

Indicators of productivity found in this study have

been regularly cited as determinants of site quality in

both forestry and agricultural settings (see Schoen-

holtz et al., 2000 for review). Total soil N and P

concentrations are the most commonly cited elemental

indicators of productivity. Although not as widely

employed, soil CN ratio has been used to model

growth (Page, 1976) and provides an index of the rate

of nitrogen mineralisation. Soil bulk density is

commonly used as an important determinant of

productivity as it can affect root growth and a host

of soil properties and processes which influence water

and oxygen supply (Schoenholtz et al., 2000). Depth

to mottling and total porosity have been previously

used as variables for determining sufficiency of

aeration within site quality models (Gale et al., 1991).

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–63 61

For P. radiata, the variables selected for the final

model in this study (Table 6) are very similar to those

found to be the dominant influences on increment in

mature stands. In a nationwide study covering a

similar range of sites to this study, Jackson and Gifford

(1974) found 66% of periodic volume increment was

attributable to mean annual rainfall, seasonal dis-

tribution of rainfall, temperature departures from

optimum, soil depth, total soil N and available P.

Although A horizon depth was not measured in the

Jackson and Gifford study, more recent analysis

(Hunter and Gibson, 1984; Woollons et al., 2002) has

found average depth of the A horizon to be a

significant determinant of site index (mean height of

the 100 largest trees per hectare in terms of diameter,

at age 20 years) for P. radiata. The close correspon-

dence in soil properties related to productivity found

between our study and mature P. radiata stands

supports findings by Kelting et al. (1999) and suggests

that experimentation in miniature may provide a

useful and rapid means of assessing sustainability.

Research strongly suggests that trees grown at very

high stand densities follow a similar developmental

pattern as operational stands. Studies using P. taeda

show that the pattern of diameter and height growth for

trees grown at operational stand densities over 16

years closely corresponds to the pattern of develop-

ment over 4 years when the distance between trees is

reduced to 1/16th of the operational stand distance

(Amateis et al., 2003a,b; Sharma et al., 2003).

Recently obtained data from our oldest site quality

plots (Watt, unpublished data) confirm this similarity

in growth pattern and also show that the average rate of

volume increment (mean annual increment) in site

quality plots over the first four years closely matches

that of stands growing at final operational stockings

(300 stems ha�1) over a 30-year period. These results

suggest that young trees grown at very high stockings

may impose the same annual demand for resources on

a site as older trees grown at operational stockings.

An important consideration in modelling site

quality is to determine how selected soil properties

differ between species. In this study, both species

exhibited very similar growth responses to the most

important soil chemical properties, and the variables

included in the final model of volume increment were

identical between species. Despite these similarities,

P. radiata exhibited greater sensitivity to the drainage

class of the soil and depth to mottles, which suggests

that this species has a greater requirement for a well

aerated soil than C. lusitanica. Subtle differences in

responses to soil properties between species were also

apparent for total soil N, P and the CN ratio. Total soil

N and CN ratio were more important indicators of

productivity for C. lusitanica than total and organic P,

whereas the opposite held for P. radiata. This may

reflect their contrasting mycorrhizal status, with P.

radiata associating with ectomycorrhizal species,

whereas C. lusitanica associates with endomycor-

rhizal species. As well as facilitating the acquisition of

nutrient ions, especially of N and P, recent studies have

shown ectomycorrhizae may mobilise N from organic

polymers as well, enabling the host species to obtain N

directly from plant and microbial detritus sources

(Read and Perez-Moreno, 2003). Thus, N is likely to

be less limiting for P. radiata than C. lusitanica.

Soil disturbance from harvesting induced significant

and substantial differences in soil physical properties.

Results indicate that most of the variation between

disturbed and undisturbed treatmentswas attributable to

macroporosity, air capacity, cone penetration resistance,

total porosity and to a lesser extent bulk density, a

finding which is consistent with previous research

(McMahon et al., 1999; Merino and Edeso, 1999;

Merino et al., 1998). In the present study, concentrations

of total soil C, N and P were reduced by disturbance.

Other researchers have also found that disturbance

significantly reduces soil concentrations of N (Merino

and Edeso, 1999) and P (Mroz et al., 1985; Tuttle et al.,

1985). Reductions in concentrations of these elements

may have been caused through removal of the humus

layer and topsoil in disturbed plots during harvesting,

reducing the quantity of available nutrients.

Despite the clear disturbance-induced alterations to

soil physical properties, increment reductions attri-

butable to disturbance were relatively small, and did

not appear to be strongly related to the most influenced

properties, macroporosity or cone penetration resis-

tance. This may be due to the low number of sites in

which macroporosity was below the 10% threshold

thought to be limiting to root growth (Greacen and

Sands, 1980). Similarly, cone penetration resistance

values did not exceed values (>2.5 MPa) cited by

Greacen and Sands (1980) as being limiting to root

growth. In contrast, bulk density was more strongly

related to increment for both species between sites

M.S. Watt et al. / Forest Ecology and Management 216 (2005) 51–6362

than air-filled porosity, macroporosity or cone

penetration resistance. High bulk densities have been

shown to cause low porosity and low water infiltration

(Rab, 1996) and the values found at some of these

sites were sufficiently high (>1.3 g cm�3) to cause

impedance to root elongation and therefore a

reduction in plant growth (Froehlich, 1979; Gale

et al., 1991). Disturbance-induced differences for

increment were also attributable to total soil N, a result

which is consistent with the strong influence of this

variable on increment across sites.

The strong and significant influence of fertilisation

on volume increment found in this study is consistent

with previous research across a broad range of species

(see Fox, 2000 for review). Gains obtained through

fertilisation were three-fold higher than reductions in

volume increment induced by disturbance. In contrast,

the site interaction was weaker for fertilisation than

disturbance, indicating that fertiliser effects were

relatively consistent across sites. The relative impor-

tance of fertilisation compared to disturbance is

reinforced by the modelling which shows significant

soil indicators to be predominantly chemical proper-

ties. These results highlight the importance of fertiliser

as a silvicultural treatment which can be used to

improve site quality.

Although preliminary, results from this study

highlight soil properties likely to be most important

for monitoring sustainability of site quality within

New Zealand plantations. The soil properties included

in the final model are consistent with key drivers of

productivity within mature stands. If these relation-

ships are confirmed by final measurements in the site

quality plots, the key soil properties will be monitored

within the long-term permanent sample plots. This

monitoring should provide a basis for site-specific

recommendations on management practices necessary

to maintain the productive capacity of New Zealand’s

plantation forests.

Acknowledgements

We are indebted to 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 classification

of soils. The help of technicians at the Landcare

Research Environmental Chemistry Laboratory in

analysing samples is gratefully acknowledged. Dr. E.

Mason and Mr. M. Kimberley provided advice on the

statistical analyses. We are also grateful to helpful

comments provided by the two anonymous referees.

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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