Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol16.1996:

Tree Physiology 16, 527--536
© 1996 Heron Publishing----Victoria, Canada

Responses of carbon gain and growth of Pinus radiata stands to
thinning and fertilizing
D. W. SHERIFF
Plantation Forest Research Center, Division of Forestry and Forest Products, CSIRO, P.O. ox
B 946, Mount Gambier, SA 5290, Australia

Received March 2, 1995

Summary Thinning of forest stands is widely carried out to
minimize the slowing of growth of individual stems that follows from increasing competition among trees as they become
bigger. After thinning, there is an increase in the growth rate of
remaining trees because of an increase in the availability of
resources per tree. Often, there is also an increase in foliar
efficiency (biomass increase/foliage amount). On sites where
mineral nutrient supply is limiting, fertilizers may be applied,
often in association with thinning, to boost productivity.
Growth responses to fertilizer application depend on an adequate supply of other resources, but also involve nonlinear
interactions among mineral nutrients and between nutrients

and other growth-limiting environmental factors. The effects
of thinning and fertilizing on the carbon gain and growth
responses of Pinus radiata D. Don to availability of resources
(light, mineral nutrients and water) and to changes in the
canopy are discussed.
Keywords: foliar efficiency, light, mineral nutrients, productivity, water.

Introduction
Thinning of forest stands is carried out to minimize the slowing
of growth of individual stems that follows from increasing
competition among trees as they increase in size. Fertilizers are
often applied in association with thinning, where mineral nutrient availability limits productivity. Responses to thinning
and fertilizing vary in magnitude among stands even of the
same species. Mensurational data obtained from field trials is
valuable for predicting responses of stands of a particular age
and species under particular edaphic and climatic conditions,
but cannot be applied generally. Thus, empirical fertilizer and
thinning trials, which contribute little to understanding of the
processes involved in growth responses, have frequently been
undertaken to allow prediction of the responses of specific

stand types to particular silvicultural treatments. Such trials
have led to a large accumulation of information but little
advance in understanding about the mechanisms that underlie
silvicultural treatment responses. Without such understanding,
accurate predictions about stand responses to management
actions cannot be made in many circumstances.
Thinning and fertilizing both have potential for increasing

stand productivity. Thinning potentially reduces limitations by
all resources, whereas fertilizing alters only nutrient supply.
Fertilizing has the same effect on the productivity of both trees
and stands, whereas thinning has different effects on productivity at the tree and stand scales. A mechanistic explanation of
these responses requires an understanding of the process of
carbon gain and assimilate allocation. The scarcity of information about belowground responses often limits the scope for
mechanistic interpretations of silvicultural treatment effects.
In Pinus radiata D. Don, stem biomass increase, stem growth
and net primary production (NPP) are closely correlated over
a wide range of conditions (Sheriff and Rook 1990, McMurtrie
and Landsberg 1992). Moreover, there are more published data
on stem growth than productivity. Consequently, in this analysis of how environmental factors and silvicultural treatments

affect P. radiata I have used stem growth as a surrogate for
productivity. Many of the data used are from monoculture
stands at Eyrewell, Kaingaroa, Puruki and Woodhill forests,
New Zealand, and at the Biology of Forest Growth Experiment
(BFG), ACT, Australia, which have large differences in aboveground productivity (Figure 1).
The focus of this review is on stand responses to thinning
and fertilizing as determined by resource availability, interactions between resources, and foliage. Specifically, I consider:
(1) the effects of resource availability on productivity and stem
growth; (2) the relative importance of foliage mass and foliar
efficiency; and (3) the allocation of assimilate to different
processes and end uses.

Effects of resource availability on productivity
Resource availability and biomass productivity of individual
trees are closely but nonlinearly related. The best combination
of site and individual tree productivity occurs at an intermediate stocking, the magnitude of which depends on tree size and
resource availability. At high stocking, productivity of individuals is small because of competition for resources, but site
productivity is high. At low stocking, inter-tree competition is
small so individual productivity is high, but inherent limitations to physiological activity and the maximum size individuals can attain limit site utilization, so that site biomass
productivity is low (e.g., Woollons and Whyte 1989, Whyte

and Woollons 1990).

528

SHERIFF

Figure 1. Relationships between aboveground biomass production and
foliar biomass at several sites. The lines (fitted with functions of the
form: Productivity = a(1 − e −kfb ); where a and k are parameters and fb
is foliar biomass) denote: a = Puruki site (ages 2--12, minimum water
or nutrient limitations, Beets and Pollock 1987); b = Woodhill site
(ages 7--10.5, with fertilizer, Beets and Madgwick 1988); c = same as
b, but without fertilizer; d = Kaingaroa site (ages 2--22, with thinning
at 8 years, Madgwick et al. 1977); e = Eyrewell site (ages 7--11 years,
with and without thinning and fertilizer, Mead et al. 1984); f = Rotorua
site (ages 5--13, close-spaced stands, Madgwick and Oliver 1985); g =
BFG site (with irrigation + nutrients (IL and IF), Snowdon and Benson
1992); h = same as g, but with irrigation only; and i = same as h, but
unirrigated (C and F).

Light
The relationship between aboveground biomass production
and intercepted light can be represented as a linear regression:
P = a + bI,

(1)

where P is aboveground biomass productivity, I is intercepted
PPFD, b is biomass production per unit of intercepted PPFD,
and a is a negative quantity related to losses of carbon from the
aboveground portion. Parameter a is composed of two components: (1) photosynthate respired aboveground, and (2) the
proportion of assimilate allocated belowground. Differences in
I, due to differences in LAI, will have a greater effect on P than
on a, so for a size and age class, light use efficiency (LUE,
equivalent to the parameter b) of aboveground biomass production increases with light interception.
Although aboveground biomass production of conifer
stands is often linearly related to light interception (e.g., Stenberg et al. 1995), carbon gain per unit intercepted light is
reduced by various constraints, which change the values of
parameters a and b in Equation 1. For example, following
thinning, LUE (biomass gain/light absorbed) of Eucalyptus

regnans J.F. Muell increased and stand biomass production
was unchanged in the absence of weeds, whereas in the presence of weed competition LUE was unchanged and stand
biomass production was reduced (West and Osler 1995).
Water deficits
There is a negative correlation between water deficits in the
plant or in the air surrounding conifers and foliar carbon
assimilation (ACO 2) (e.g., Bennett and Rook 1978, Whitehead

et al. 1983, Sheriff 1995, Sheriff and Mattay 1995, Teskey et
al. 1995). Sheriff and Whitehead (1984) found ACO 2 of container-grown P. radiata began to decline at a foliar water
potential (Ψ) of −1.6 to −1.8 J g −1, and reached zero at a Ψ of
−2.0 to −2.4 J g −1, over a narrower range than other measured
conifers (e.g., Teskey et al. 1995). On the other hand, Sheriff
(1995) found ACO 2 of field-grown P. radiata changes as a result
of stomatal and nonstomatal effects (e.g., Bunce 1977), so that
it is constant at a soil water potential (Ψs) above about −0.6 J
g −1, and is greater than zero at a Ψs of −3.0 J g −1.
The negative relationship between leaf to air vapor pressure
difference (D) and ACO 2 results from a negative effect of D on
gs, and sometimes also from a direct effect of D on ACO 2

(Sheriff 1995).
Lowering of ACO 2 by a reduction in gs will increase assimilatory transpiration efficiency (ATE = assimilation/transpiration), but when there is a direct effect of water deficits on
ACO 2, ATE will decline (Sheriff 1995).
Water use
Biomass productivity and transpirational water loss are generally positively associated (e.g., Leith 1976, Ritchie 1983) because both processes are driven by diffusion and climatic
factors, especially radiation and water. Greater availability of
water increases aboveground productivity of water-limited
stands (Figure 1). Figure 2 provides examples of interrelationships between water supply and yield based on data analyzed
by Nambiar (1994) from 10--15-year-old Pinus radiata stands
across a range of sites and on data from Puruki and Woodhill
(comparisons between data sets are only valid for sets of either
MAI or CAI data). At one site (denoted by solid circles), where
only water availability was varied, water supply and yield
changed together (Snowdon and Waring 1991). A linear regression for this relationship produced an r2 of 0.97. A linear

Figure 2. Effect of water supply on stem volume increment of 11--15year-old P. radiata for a range of sites. Based on a graph of Nambiar
(1994). Data for CAI, except the data denoted by open and solid
squares which are for MAI. References are given only for data not
added by the author. Symbols: d = Australian Capital Territory; s =
BFG control; m = BFG fertilized (Snowdon and Benson 1992); n =

BFG irrigation only; , = BFG irrigation + nutrients; .= Gippsland,
effluent irrigation; h = Yield plots: New South Wales, Gippsland and
Western Australia; j = Mount Gambier region, South Australia; e =
Puruki (Beets and Brownlie 1987); r = Woodhill (Jackson et al.
1983).

CARBON GAIN AND GROWTH OF PINE AFTER THINNING

regression calculated for the CAI data in Figure 2 produced:
V = 8.5 + 0.019 W (r2 = 0.35)

(2)

where V is the annual volume stem increment, and W is the
annual water input. Thus, 65% of the variance in the data can
be ascribed to factors other than differences in water supply. At
the water-limited BFG site, additional water alone increased
stem growth less than irrigation + nutrients, whereas nutrients
alone had little effect. Variations in productivity and stem
growth at the well-watered, fertile Puruki site are attributable

to different thinning and pruning regimes. A reliable indication
of how much climatic factors other than precipitation affect
increment at any sites in Figure 2 is not possible because, apart
from the Woodhill data, they were collected in a single year
(e.g., Snowdon and Waring 1991) or had matching data from
different treatments for each of several years (e.g., BFG data)
when there were large, varying responses to treatment. However, r2 values calculated from data collected over several years
on individual P. radiata trees (MacDougal 1938) indicate that
80--90% of the variation in stem growth can be attributed to
factors other than variation in rainfall (Figure 3).
There are strong interactions between fertilizer and water in
water-limited environments. At the water-limited BFG site,
fertilization increased diameter growth and estimated NPP of
irrigated trees by 42 and 18%, respectively, whereas corresponding values for unirrigated trees were only 14 and 5%,
respectively (McMurtrie et al. 1990a). Mineralization and absorption are higher in wetter soils and when periods of higher
soil water are longer (e.g., Snowdon and Waring 1990). Therefore, fertilization may not alter productivity unless accompanied by thinning if the response to fertilization is constrained
by a low soil water content (e.g., Butcher 1977, Donald 1987,
Turner and Lambert 1987, Snowdon and Waring 1990). However, fertilizing a thinned, nutrient-limited stand may not appreciably increase water use in the short to medium term if
WUE is increased (Sheriff et al. 1986, Squire et al. 1987).
Fertilization will improve tree water status and so increase

growth when improved nutrition causes greater water uptake
(e.g., Myers 1988) by enhancing root hydraulic conductance
(Minshall, 1975).
Nutrients
Increased mineral nutrient availability can increase productiv-

529

ity if the nutrient in question is limiting carbon gain, provided
that the greater supply causes an increase in nutrient uptake.
Carbon gain has been widely linked to the nitrogen status of
individual leaves and canopies (Field and Mooney 1983, Evans
1989, Field 1991, Teskey et al. 1995). In conifers there may be
a strong positive association between foliar ACO 2 and [N] (both
per foliage area or mass) (e.g., Nambiar et al. 1984, Sheriff and
Mattay 1995), but at times there is little (e.g., Teskey et al.
1994) or no (e.g., Sheriff 1995) association. Sometimes the
relationship may be present only when other limitations are
small, for example at BFG ACO 2 and foliar [N] were interrelated
only when foliar conductance was greater than 75 mmol m −2

s −1 (Thompson and Wheeler 1992).
The observation that site productivity is often influenced
more by plant nitrogen content than by light interception or
foliage mass led Ågren (1985) to formulate the nitrogen productivity concept, namely that ‘‘the amount of biomass produced is directly related to the amount of nitrogen in that
biomass.’’ This relationship is often strong because nitrogen is
often a growth-limiting nutrient (Ågren 1985). Nitrogen contributes positively to carbon gain by increasing both LUE and
LAI, i.e., light interception (e.g., Linder and Rook 1984).
Although relationships between tissue nitrogen content and
biomass productivity are generally good at a particular site, the
relationships vary among sites because of differences in other
factors that affect productivity (Figure 4).
Positive associations between ACO 2 and many of the other
elements required for growth have been reported (Terry and
Rao 1991). Of these, effects of phosphorus on ACO 2 (e.g.,
Brown 1981, Conroy et al. 1986, Sheriff et al. 1986, Black
1988, Reich and Schoettle 1988, Conroy et al. 1990, Rousseau
and Reid 1990, Sheriff 1995, Teskey et al. 1995) and on growth
(Raupach et al. 1975) are the most studied in conifers as well
as in other tree species (e.g., Fei et al. 1990, Kirschbaum and
Tompkins 1990, Cromer et al. 1993).
Responses of NPP and yield to variation in supply of a
nutrient depend on potential limitations by other factors, including other nutrients, immobilization in the soil and interactions among nutrients (e.g., MacLeod 1969, Dey and Rao
1989). Interactions among nutrients may involve interactions
in nutrient uptake where, for example, addition of a limiting
nutrient can stimulate root growth, and therefore increase
uptake of other mineral nutrients (Hopmans and Clerehan
1991). In trees, the best known interaction in nutrient utiliza-

Figure 3. Stem volume increase of
two P. radiata trees in California.
Letters within the symbols are in
sequence of the years of measurement, with initial tree ages of 31-32 years in (a), and 8--9 years in
(b) Adapted from data of MacDougal (1938).

530

SHERIFF

is reduced when foliage is removed at this time (Rook and
Whyte 1976). Replotting Rook and Whyte’s (1976) data as the
proportions of maximum foliage and stem volume increment
produces a 1/1 agreement over a wide range of values (Figure 6); but a close relationship is not found for low growth rates
(Rook and Whyte 1976). These interrelationships complicate
analysis of mechanistic connections between growth and assimilation.
Respiration

Figure 4. Aboveground biomass production in relation to aboveground
nitrogen content of fertilized (d) and unfertilized (s) P. radiata
seedlings. Numbers next to data points indicate the width of the
weed-free strip along the rows of plants. The relationships are: Fertilized: Biomass increase = 2.37 + 9.79N, r 2 = 1.00; and Unfertilized:
Biomass increase = 17.52 + 3.83N, r 2 = 1.00. Adapted from data of
Woods et al. (1992).

tion is the N × P interaction (Figure 5). Thus, fertilization may
only increase productivity if both N and P are applied (Donald
1987), or it may reduce productivity if nitrogen is added to
P-deficient soils (Hunter et al. 1986). Surfaces in Figure 5
indicate stem growth is limited by the nutrient in shortest
supply relative to the amount needed. Thus, to use foliar
nutrient content to predict growth we need information on a
range of foliar elements (Ingestad 1962). Failure to recognize
limitations by non-nutrient factors can also lead to unpredictable responses of productivity to fertilization and tissue
nutrient concentrations (e.g., Hunter and Hoy 1983).
Sink--source interactions
Sink activity, for example growth rate, and source size, the
amount of assimilatory tissue, can both regulate source activity
(Sweet and Wareing 1966, Luxmoore et al. 1995, Teskey
1995). Because current assimilate is important for growth
during the main growing season (when growth is high), growth

Figure 5. Interacting effects of foliar phosphorus and foliar nitrogen
on biomass production of Araucaria cunninghamii. Adapted from data
of Richards and Bevedge (1969).

Daily integrals of plant respiration and carbon assimilation are
often linearly related, but several factors, for example light
intercepted over the previous few days (Hansen and Jensen
1977), affect the magnitude of this relationship. More specifically, growth respiration is usually proportional to ACO 2 and
maintenance respiration is usually proportional to protein turnover (Amthor 1984). The latter probably explains the positive
relationship between maintenance respiration and protein or N
content (Kawahara et al. 1976, Sprugel et al. 1995), although
in conifers a better relationship has been found between respiration and intercepted light (Sprugel et al. 1995). However,
these simple relationships break down when conditions vary;
for example different genotypes, species, or N/protein ratios
(Greenwood and Barnes 1978, Greenwood et al. 1978, Cropper and Gholz 1991).
Foliage quantity and efficiency
Productivity depends on the quantity of foliage, and on foliar
efficiency (FE = (increase in biomass)/(foliage quantity)). Foliar efficiency can be calculated per leaf area (FE la ) or per
foliage mass (FElb). Thus, FE is partly derived from light
interception per foliage quantity and partly from the efficiency
of converting intercepted light into biomass. Foliar efficiency
is almost always related to aboveground productivity, so a
component of FE can result from the proportional allocation of
assimilate aboveground versus belowground. Thus, although it
is often used in discussion, FE is defined by several parts of the
system and does not explain the system or how it operates.
Light interception is nonlinearly related to LAI (e.g., Gower et

Figure 6. Relationship between the amount of foliage on a P. radiata
tree and its stem volume increment during the period of maximum
growth, both as a proportion of a maximum. Adapted from data of
Rook and Whyte (1976).

CARBON GAIN AND GROWTH OF PINE AFTER THINNING

al. 1995), which explains at least in part the observed increases
in FEla following thinning, because at high LAI, thinning
changes LAI more than light interception.
At the well-watered, fertile Puruki site, where biomass production was little constrained by water or nutrients, thinning
increased both FE (FEla = 1.9 Mg ha −1 year −1, FElb = 1.6 Mg
ha −1 year −1) and LUE (1.6 Mg ha −1 per GJ m −2) (Figures 7a
and 7b). The increase in LUE may have resulted from pruning
of shaded foliage, from greater partitioning of biomass aboveground or from less tissue desiccation, which can be appreciable even on well-watered sites. For example, during a wet
midwinter period in southeastern South Australia, Ψ of P. radiata on sunny days was −0.4 (predawn) to −1.25 J g −1 (minimum) (Sheriff, unpublished observations), which could reduce
growth considerably (e.g., Rook et al. 1977).
Where productivity is not seriously limited by resource
availability before thinning, site productivity is generally reduced by thinning. However, even under these conditions,
greater resource availability will enable productivity to recover
to near prethinning values in the second year after thinning
(Table 1). Productivity of individual trees usually increases
rapidly after thinning because foliar biomass increases and has
a higher FE (Table 2). The contribution of FE to productivity

531

Figure 7. Effects of thinning, indicated by vertical dotted lines, at
Puruki on (a) FEla and (b) LUEi. Adapted from data of Beets and
Pollock (1987), Grace et al. (1987) and Whitehead (1986).

Table 1. Effects of thinning P. radiata on aboveground biomass production and foliar biomass. Except where indicated in the footnotes,
comparisons are between pre-and post-thinning values. Where possible, values are given for the first and second years after thinning, which are
indicated by the superscript numerals 1 and 2, respectively.
Before thinning
Site

Thinned at
age (years)

Puruki
(Tahi)
6
(Beets and Pollock 1987)
10

After thinning

Stocking
(stems ha −1)

Productivity
(Mg ha −1 year −1)

Foliar biomass
(Mg ha −1)

Stocking
(stems ha −1)

1960

28.4

15.7

495

495

28.2

11.5

159

(Rua)

7

1843

24.2

12.6

550

(Toru)

8

1969

28.8

9.7

540

540
1866

31.3
18.1

10.7
5.7

292
544

15403

2224

13.91
17.22
13.91
17.22
9.6

2224

10.5

11.4

7

2224

16.1

10.1

7

2224

19.6

12.9

11
Kaingaroa
8
(Madgwick et al. 1977)
Eyrewell
7
(Mead et al. 1984)
7
Woodhill
7
(Beets and Madgwick 1988)
7

3

15404

6.71
8.22
6.71
8.22
7.4

810
810
14835
7416
14837
7418
14839
74110
148311
74112

Productivity
(Mg ha −1 year −1)

Foliar biomass
(Mg ha −1)

15.51
26.72
21.01
20.92
20.51
19.92
17.91
26.42
21.71
6.11
29.72
9.71
11.42
9.21
18.92
8.5
5.6
9.7
7.2
18.8
13.9
11.5
15.6

4.21
6.72
4.11
5.72
4.41
6.52
4.81
7.62
8.11
3.01
4.62
3.61
5.32
4.01
6.82
6.9
6.2
11.3
8.1
10.8
9.0
11.2
11.7

The same year’s data, between unthinned and thinned stands; 4 the same year’s data, between unthinned and thinned, nitrogen fertilized stands;
the same year’s data, between thinned and unthinned stands with no added nutrients;7,8 the same year’s data, between thinned and unthinned
stands with lupins; 9,10 the same year’s data, between thinned and unthinned stands with fertilizer; 11,12 the same year’s data, between thinned
and unthinned stands with fertilizer and lupins.
5,6

532

SHERIFF

Table 2. Effects of thinning P. radiata on aboveground biomass production of trees and foliar efficiency at four sites. Details of treatments are in
Table 1.
Before thinning
Site

Productivity
(kg tree −1 year −1)

Foliar efficiency
FElb (year −1)

Productivity

Foliar biomass

Foliar efficiency

6

14.5

1.8

10

57.0

2.4

(Rua)

7

13.1

1.9

(Toru)

8

15.9

2.9

Kaingaroa

11
8

51.0
9.7

2.6
3.2

Eyrewell

7

7

9.01
11.22
9.01
11.22
4.4

2.11
2.12
2.11
2.12
1.3

7

4.7

0.9

7

7.2

1.6

7

8.8

1.5

2.21
3.72
2.31
2.32
2.81
2.82
2.11
3.12
1.51
1.11
5.62
1.31
1.32
1.31
2.12
1.3
1.7
1.4
2.1
1.8
2.6
0.9
2.4

1.11
1.72
1.11
1.52
1.21
1.72
1.61
2.62
1.41
1.81
2.82
1.01
1.22
1.11
1.62
1.4
2.5
1.5
2.1
1.6
2.7
1.3
2.7

2.01
2.22
2.11
1.52
2.41
1.62
1.31
1.22
1.01
0.61
2.02
1.31
1.02
1.11
1.32
0.9
0.7
0.9
1.0
1.1
1.0
0.7
0.9

Puruki
(Tahi)

Age at thinning
(years)

After thinning/Before thinning

7
Woodhill

is greatest in the first year after thinning, before the canopy has
expanded greatly. Before thinning, potential for carbon gain
explains 93% of the variance in aboveground productivity at
the sites in Table 2 (foliage biomass contributes 62% and FElb
contributes 31%). One to two years after thinning, these values
were 88, 26 and 62%, respectively, indicating that FElb is an
important factor driving aboveground productivity after thinning, but the data do not indicate causes or mechanisms. Data
from other pine species indicate an important part of the
thinning response is explained by greater carbon gain per unit
foliage (e.g., Donner and Running 1986, Ginn et al. 1991).
This contributes both to the general growth response and to
crown expansion as remaining trees reoccupy the site. Crown
expansion enhances the increase in productivity that results
from more foliage.
At BFG, adding nutrients to a nutrient-limited stand well
supplied with water increased foliage mass and FE, such that
they contributed an estimated 70 and 30% to the greater productivity of the fertilized stand (McMurtrie et al. 1990a).
Generally, FElb increased as soil resources increased: control,
fertilized, irrigated, irrigated and fertilized, and irrigated with
nutrient solution treatments had FElb values of 2.01, 2.34, 2.33,
2.75, and 2.82 Mg ha −1 year −1 per Mg ha −1, respectively
(Snowdon and Benson 1992).
Stand characteristics defined by FElb and foliar biomass
before thinning explain 90% of the variance in data for the
same characteristics after thinning. Thus the factors driving

prethinning productivity are strongly related to the factors that
drive postthinning productivity.

Effects on biomass partitioning below- and aboveground
Most studies have indicated that greater availability of soil
nutrients or water causes the proportion of assimilate allocated
to belowground biomass to fall. Thus, greater aboveground
productivity as a result of fertilization, irrigation and thinning
can be caused by changes in assimilate allocation. Many studies of this effect have been on annual crops, but the principles
are generally applicable, even though controls on allocation
may be species specific (e.g., Gower et al. 1995). Thus, although greater assimilate allocation to aboveground biomass
can contribute to greater aboveground productivity following
fertilization, irrigation or thinning, the magnitude of this contribution is species specific. For example, fine root turnover is
a major sink for assimilate after canopy closure in conifers, but
effects of changed soil nutrient availability on assimilate allocation to fine roots vary in size and direction in different
studies (e.g., Gower et al. 1995).
In P. radiata there seems to be a close negative link between
biomass partitioning to fine roots and to stems (Santantonio
1989), but variation in nutrient supply has produced contradictory reports of changes in assimilate allocation to roots.
Thomas and Mead (1992) observed a 9 and 37% greater
biomass partitioning belowground and to fine roots, respec-

CARBON GAIN AND GROWTH OF PINE AFTER THINNING

533

Figure 8. Effects of thinning and
fertilizing on stem growth as a
fraction of the increase in aboveground dry matter. Vertical dotted
lines indicate times of thinning at
Puruki and Kaingaroa. Treatments
at Eyrewell were unthinned control (s), unthinned fertilized (d),
thinned unfertilized (,) and
thinned fertilized (.). At Woodhill all treatments were thinned
with an unfertilized control (s),
lupins (d), fertilizer (,) and fertilizer + lupins (.). Replotted
from data of Beets and Pollock
(1987), Madgwick et al. (1977),
Mead et al. (1984) and Beets and
Madgwick (1988), respectively.

tively, in N-fertilized trees, which is qualitatively the same as
found by Snowdon and Waring (1985) and Smith et al. (1994),
whereas Nambiar (1980) found a smaller carbon allocation to
roots of fertilized than unfertilized trees and Squire et al.
(1978) observed a small, but similar effect on mycorrhizal
roots. Barker (1973) found no clear relationships between
biomass production with variying nutrient status and partitioning belowground. Differences between modeled net carbon
gain and aboveground biomass production in P. radiata indicate a 50% reduction in allocation of assimilate to roots in
response to an increase in nutrient supply (McMurtrie and
Landsberg 1992). Similar results have been obtained for other
species (Santantonio 1989). Reasons for these contradictions
are unclear, but could result from genetic variation in
root/shoot ratios or efficiencies of nutrient uptake and utilization (Theodorou and Bowen 1993), or responses to nutrition
(Snowdon and Waring 1985), or to ontogenic changes in partitioning (e.g., Madgwick 1981).
Thinning probably increases carbon partitioning belowground in the short term (Santantonio 1989). At Puruki, thinning increased carbon allocation to fine roots by 33%.
However, fine roots were a minor component (4.6%) of
biomass production immediately after thinning (Santantonio
and Santantonio 1987a, 1987b), whereas maintenance of
grafted roots after thinning requires carbon from an initially
smaller canopy (Will 1966).

resources allowed individual trees to grow more rapidly, but
did not affect stand characteristics. However, a large increase
in supply of soil resources can alter stand characteristics. At
BFG, which is a nutrient- and water-limited site, Ps was greater
with more water and nutrients (Figure 9). Simulations indicate
the proportion of assimilate allocated to stem biomass in high
nutrient stands at BFG was almost twice that in low nutrient
stands (McMurtrie et al. 1990b). Fertilizing, especially with
nitrogen, generally increases the proportion of biomass in the
crown and reduces that in stems (e.g., Smith et al. 1994).

Conclusions
To improve silvicultural management we need to develop
techniques for making reliable, quantitative predictions of
stand responses. Of particular importance is an understanding
of the mechanisms and interactions involved, including the
way in which productivity responds to interactions between
resource and non-resource factors, and of how FE is implicated

Partitioning of aboveground biomass to stem
Thinning increased partitioning of aboveground biomass to
stem (Ps) at Puruki and (beginning one year after thinning) at
Kaingaroa, it lowered Ps at Eyrewell, and had little effect on Ps
at Woodhill (Figures 8a--d). Aboveground biomass production
and Ps were negatively related. This was significant both before thinning and in the second year after thinning, with r 2
values of 0.36 and 0.59. There was a positive correlation
between aboveground biomass production before and after
thinning. There was also a positive correlation between Ps
before thinning and one year or two years after thinning (r2 =
0.62 P < 0.001, r2 = 0.63 P < 0.003). Reduced competition for

Figure 9. Effects of irrigation and fertilization on stem growth as a
fraction of the increase in aboveground dry matter at BFG with the
following treatments: control (.), fertilized (h), irrigated (,), irrigated + fertilized (d), and irrigated with liquid fertilizer (s). Replotted from data of Snowdon and Benson (1992).

534

SHERIFF

in this. Increased FE in response to thinning appears to result
from greater carbon gain per unit foliage, whereas responses
of FE to fertilizing may be caused by changes in carbon
partitioning and carbon gain. We need to understand factors
that control both carbon gain and carbon partitioning if we are
to predict stand responses reliably.

Acknowledgments
I thank Joe Landsberg and Peter Snowdon for their helpful and
thoughtful comments on this paper.

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