Tree Physiology 28, 187–196
© 2008 Heron Publishing—Victoria, Canada
Retranslocation of foliar nutrients in evergreen tree species planted in
a Mediterranean environment
D. N. FIFE,1 E. K. S. NAMBIAR2,3 and E. SAUR4
1
Ensis, CSIRO Plantation Forest Research Centre, P.O. Box 946, Mount Gambier, SA 5290, Australia
2
Ensis, CSIRO Forestry and Forest Products, P.O. Box E4008, Kingston, ACT 2604, Australia
3
Corresponding author ([email protected])
4
École Nationale d’Ingénieurs des Travaux Agricoles de Bordeaux, 1, cours du Général de Gaulle, CS 4021, F. 33175 Grandigan Cedex, France
Received November 23, 2006; accepted September 3, 2007; published online December 3, 2007
Summary Internal nutrient recycling through retranslocation (resorption) is important for meeting the nutrient demands
of new tissue production in trees. We conducted a comparative
study of nutrient retranslocation from leaves of five tree species
from three genera grown in plantation forests for commercial
or environmental purposes in southern Australia—Acacia mearnsii De Wild., Eucalyptus globulus Labill., E. fraxinoides H.
Deane & Maiden, E. grandis W. Hill ex Maiden and Pinus
radiata D. Don. Significant amounts of nitrogen, phosphorus
and potassium were retranslocated during three phases of leaf
life. In the first phase, retranslocation occurred from young
leaves beginning 6 months after leaf initiation, even when
leaves were physiologically most active. In the second phase,
retranslocation occurred from mature green leaves during their
second year, and in the third phase, retranslocation occurred
during senescence before leaf fall. Nutrient retranslocation occurred mainly in response to new shoot production. The pattern
of retranslocation was remarkably similar in the leaves of all
study species (and in the phyllodes of Casuarina glauca Sieber
ex Spreng.), despite their diverse genetics, leaf forms and
growth rates. There was no net retranslocation of calcium in
any of the species. The amounts of nutrients at the start of each
pre-retranslocation phase had a strong positive relationship
with the amounts subsequently retranslocated, and all species
fitted a common relationship. The percentage reduction in concentration or content (retranslocation efficiency) at a particular
growth phase is subject to many variables, even within a species, and is therefore not a meaningful measure of interspecific
variation. It is proposed that the pattern of retranslocation and
its governing factors are similar among species in the absence
of interspecies competition for growth and crown structure
which occurs in mixed species stands.
Keywords: Acacia, Eucalyptus, interspecies variation, pine.
Introduction
Internal nutrient recycling by retranslocation (resorption)
from leaves is an important factor in the supply of nitrogen,
phosphorus and potassium for new growth in tree species at
different phases of foliage development (Nambiar and Fife
1991, Millard 1994, Fife and Nambiar 1995, Aerts 1996, Saur
et al. 2000, van Heerwaarden et al. 2003). Calcium is largely
immobile within the plant. Nutrient retranslocation studies
help to quantify the net amounts of nutrients available for reuse by new growth and provide information for nutrient management in production forests (Fife and Nambiar 1997). Understanding retranslocation efficiency is important because
retranslocation provides a mechanism for nutrient conservation at the species level and does not differ strongly among
growth forms of perennials (Aerts 1996).
Most soil nutrients taken up by trees are used in annual production of foliage, which serves as a reservoir of reusable nutrients. Nutrients in one generation of foliage can be retranslocated to support the production of the next generation of foliage, irrespective of the rate of soil nutrient supply (Nambiar
and Fife 1987). In this process, nutrient retranslocation occurs
not only from senescing leaves after the stand has fully occupied the site, as has been commonly reported, but also from
young leaves in open stands, including those of transplanted
seedlings (Nambiar and Fife 1991). At a nitrogen-deficient
site in a 13-year-old Pinus radiata D. Don stand, application of
nitrogen increased wood production by up to 45%, and the
amount of nitrogen retranslocated from needles was increased
4.5-fold, showing that the higher the growth rate, the greater
the amount of nutrients retranslocated (Fife and Nambiar
1997).
Several studies have reported interspecies variation in nutrient retranslocation from foliage. Hawkins and Polglase (2000)
compared retranslocation during senescence among 15 species of eucalypts, each species grown in separate plots supplied with non-limiting water and nutrients, and found that
about 50% of the nitrogen was retranslocated during senescence, in contrast to 11–24% of the phosphorus. Kumar and
Singh (2005) examined variation among eight tropical species
grown over mine spoils and reported retranslocation efficiency
during leaf senescence from 38 to 69% for nitrogen and 50 to
71% for phosphorus. Studies have shown that leaf lifespan is a
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FIFE, NAMBIAR AND SAUR
key determinant of retranslocation efficiency in several species (Escudero et al. 1992, Niinemets and Tamm 2005). In
mixed species stands of deciduous trees, variation in nutrient
retranslocation efficiency and resorption kinetics among species was related to differences in leaf longevity (lifespan based
on timing of leaf fall) and the nature of the biochemical pool of
nutrients (Niinemets and Tamm 2005). However, these studies
have two potential limitations. First, substantial amounts of
retranslocation occur from young foliage well before senescence (Fife and Nambiar 1991, 1995, Millard 1994). Second,
leaf mass is lost during senescence that affects mass-based
concentration values and affects the estimation and interpretation of retranslocation (van Heewaarden 2003).
Based on studies of individual species, Pinus radiata (Nambiar and Fife 1991, Fife and Nambiar 1997) and Eucalyptus
globulus Labill. (Saur et al. 1997), we have suggested that, despite the contrasting genetic and morphological characteristics
of these two species, retranslocation from both young and
senescing leaves is governed by the same set of tree variables
when the trees are grown in the same environment.
The range of species being planted for commercial or environmental benefits in southern Australia has increased in recent years, and there is value in understanding their nutritional
physiology. In the research reported here, we extended our nutrient retranslocation studies by focusing on interspecies variation. Our three main objectives were as follows. First, to compare the nature and extent of retranslocation from leaves of genetically diverse species planted at the same time at the same
site, but in separate stands. This experimental design provided
a more robust basis for interspecies comparison and avoided
the confounding effects of interspecies competition on growth
and crown structure that occur in mixed species stands. Second, to follow nutrient accumulation and retranslocation continuously during the full lifespan of leaves rather than during
senescence (leaf fall) alone, as has been done in other studies.
Third, to examine nutrient retranslocation efficiency calculated on the basis of both nutrient concentrations and contents
in leaves. With this information we tested the hypothesis that
the pattern of retranslocation and the factors governing it are
similar among species in the absence of interspecies competition on growth and crown structure.
and southwest Victoria; 37°45′ S, 140°47′24″ E, 63.0 m a.s.l.),
Australia. The growing environment is Mediterranean (characterized by winter rainfall and dry summers) with a 5-m deep
sandy soil. The principal profile form is Uc 2.2 (Northcote
1979) and described as a Spodic Quartzipsamment (Soil Survey Staff 1975) or a bleached vertic subplastic yellow chromosol (Isbell 1996). Before establishing the experimental stand,
the site was under pasture that had improved soil fertility.
Characteristics of the top 30 cm of soil are presented in Table 1.
Long-term mean annual rainfall and evaporation are 710
and 1350 mm, respectively. On average, 39% of rain falls in
winter (June to August), 26% in spring, 13% in summer (December–February) and 23% in autumn. During our study,
rainfall was 700 mm year – 1.
Stand
The experimental stand was planted with six tree species: Acacia mearnsii De Wild. (a nitrogen fixing species); Eucalyptus
globulus; E. fraxinoides H. Deane Maiden; E. grandis W. Hill
ex Maiden; Pinus radiata; and Casuarina glauca Sieber ex
Spreng. Nursery-grown seedlings of all species, about 10 cm
high, were planted in May after autumn rains in plots containing 120 trees 2.4 m apart (1700 trees ha –1 ), equally spaced
within and between rows. Three replications of each species
were arranged in a randomized block design. Weeds were controlled with herbicide before planting, and subsequently by
mowing until weeds were suppressed by the tree canopy.
Tree sampling
The study began when the trees were 27 months old. Within
each plot (species), two groups of 16 trees (representing the
size classes but avoiding those that were heavily suppressed or
had damaged crowns) were marked. Within each group, five
trees were marked for regular foliar sampling. Thus measurements within species were treated as six replications. The results for C. glauca have been reported separately (Saur et al.
1997) and are discussed in this paper in relation to the results
for the other species.
Growth measurements
Materials and methods
Height and diameter at breast height over bark (DBH) of all
trees were measured every month over one year.
Site
Foliar sampling
The experimental site was located 10 km north of Mount Gambier (the Green Triangle region of southeast South Australia
Morphology and growth form of leaves differed considerably
among genera and species (Figure 1). Acacia mearnsii leaves
Table 1. Selected soil properties. Mineralizable nitrogen (N) measured after 60 days of incubating undisturbed soil cores in the laboratory.
Soil depth (cm)
pH in water
Total C (%)
Total N (%) Mineralizable N (mg kg – 1 ) Total P (mg kg – 1 )
Exchangeable Ca (cmol kg – 1 )
0 –5
5–10
10–15
15–30
6.2
5.9
5.7
5.7
1.8
1.0
0.7
0.6
0.13
0.06
0.04
0.03
1.68
0.84
0.61
0.51
54.4
18.7
9.4
6.8
112.7
97.7
89.8
77.5
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RETRANSLOCATION OF FOLIAR NUTRIENTS IN EVERGREENS
189
branches and so could not be collected.
On the five trees marked for foliar sampling, branches at
1.3 m height were selected. From each branch, a set of young
leaves (recently initiated) and a set of fully expanded mature
leaves were marked by attaching a label to their petioles. Sufficient numbers of leaves were marked to provide samples at
monthly intervals for 12 months. At each sampling, two leaves
per tree were removed from each age class and bulked for a
composite sample. The leaves in these composite samples
were recounted, dried at 70 °C for 48 hours and weighed.
Plant analysis
Figure 1. Mature green leaves of Acacia mearnsii, Eucalyptus globulus, Eucalyptus fraxinoides, Eucalyptus grandis and Pinus radiata.
in adult form are described as “bipinnate with 8–21 pairs of
pinnae and 15–70 pairs of leaflets” (Boland et al. 1985).
Eucalypt leaves are described as “ovate to broad-lanceolate” in
E. fraxinoides, “elliptic-ovate” in E. globulus and “ovate” in
E. grandis (Boland et al. 1985). All eucalypt leaves sampled
were of adult form. The foliage of P. radiata consisted of fascicles with three or four needles. The results for P. radiata are
expressed on a needle basis because the number of needles per
fascicle varies. Hereafter, the unit of foliage for all species is
referred to as a leaf.
From all species, leaves at three phases of development
were sampled regularly. Phase 1: young, still expanding,
leaves were sampled from initiation to about 12 months of age.
Phase 2: mature leaves that had attained full size at the time of
the first sampling (at an age of about 12 months) were followed as long as they remained green (to the age of about 22
months). Phase 3: senesced leaves still attached to branches
were collected just before leaf fall together with samples of adjacent live green foliage. This allowed reliable comparison of
leaf mass and nutrient contents between pairs of green and
senesced leaves. However, in A. mearnsii, pinnae were too
fragile at the onset of senescence and rarely remained on
A subsample of each dried leaf sample was digested in sulfuric
acid. Nitrogen and phosphorus were measured colorimetrically, potassium by flame photometry and calcium by atomic
absorption spectroscopy. Foliar nutrient concentrations and
contents (mass leaf –1 ) were calculated.
Relationships investigated were: (1) the maximum nutrient
contents in young and mature leaves and the amounts subsequently retranslocated; (2) the nutrient contents in mature live
leaves and the amounts retranslocated from senescing leaves;
and (3) height, diameter increments and branch elongation and
the amount of nutrients retranslocated over the same interval.
Data analysis
To test the potential bias from the location of two sample
groups within plots, we analyzed the data (see Tables 2 and 3)
in a split plot design, treating each sampling group as a sub
plot. Results were not significantly affected by sampling
method. Therefore, we used analysis of variance (ANOVA) or
regression analysis for interpreting data based on six replications (sample groups) for each species.
Results
Tree growth
When the study began, there were large differences among
species in height and stem diameter of the 27-month-old trees
(Table 2). Height ranged from 2.2 m for P. radiata to 5.1 m for
A. mearnsii, and DBH ranged from 2.0 cm for E. grandis to
5.3 cm for A. mearnsii. Between age 27 and 39 months, height
and diameter increments differed significantly between species, but the seasonal patterns of growth were similar. For all
species, height and diameter growth peaked during summer
Table 2. Mean height and diameter at breast height over bark (DBH) of trees at 27 and 39 months after planting. Increment equals growth between
27 and 39 months. The main effect of species was significant (P < 0.001) in all cases. Within a column, values followed by different letters differed
significantly at P < 0.05.
Species
Acacia mearnsii
Eucalyptus globulus
Eucalyptus fraxinoides
Eucalyptus grandis
Pinus radiata
Height (m)
DBH (cm)
27 months
39 months
Increment
27 months
39 months
Increment
5.1
3.5 a
3.3 a
2.6 b
2.2 b
7.8
6.9 a
6.7 a
5.4
4.4
2.7 ab
3.4 c
3.4 c
2.8 b
2.2 a
5.3
3.2 a
2.8 a
2.0 a
2.8 a
8.6 a
7.2 ab
7.8 a
6.1 b
7.3 ab
3.3
4.1 b
4.9 a
4.1 b
4.4 ab
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FIFE, NAMBIAR AND SAUR
from January to March (data not shown) and subsequently decreased. Height growth over the 12 months was greatest for
E. fraxinoides and E. globulus and least for P. radiata, whereas
diameter growth was greatest for E. fraxinoides and least for
A. mearnsii (Table 2).
Leaf growth
Leaf morphology varied considerably among species (Figure 1), and there were major differences in crown configuration among the genera and among eucalypt species. The
change in mass of young leaves of all species followed a similar pattern from the first sampling in early spring (September,
Figure 2). Mass per leaf increased rapidly over the first four
months, with little change subsequently. During the peak
growing season (October to December), leaf mass increased
35-fold for A. mearnsii and 3-fold for P. radiata. Individual
young leaves of A. mearnsii reached 1.03 g leaf – 1. Young
leaves of E. globulus, E. fraxinoides, E. grandis and P. radiata
attained mean biomasses of 0.80, 0.44, 0.42 and 0.03 g leaf – 1,
respectively.
The mass of mature leaves changed little during the
12 months after the first sampling; A. mearnsii, E. globulus,
E. fraxinoides, E. grandis and P. radiata weighed 1.20, 0.76,
0.37, 0.31 and 0.04 g leaf – 1 respectively. Beginning in June
(early winter when trees were 36 months old), some of the old
leaves began to senesce. There were no clear interspecific differences in leaf longevity (lifespan) or timing of senescence.
Furthermore, senescence occurred unevenly and not all leaves
of the same age in the same species senesced at the same time.
The estimated lifespan of leaves ranged from 20 to 23 months
for all species, but the longevity of E. globulus leaves may be
less than that of other species. There was a net reduction in dry
mass of leaves at senescence, 9–10% for each of the three
eucalypt species and 32% for P. radiata.
Foliar nutrient concentrations
Young leaves The pattern of changes in foliar concentrations
of nitrogen, phosphorus, potassium and calcium in young
leaves (Figure 3A) was similar among species. Nitrogen concentrations were consistently higher in A. mearnsii than in the
other species, and decreased in all species from October to January as leaves increased in size. Similar patterns were found for
Figure 2. (A) Changes in mean biomass of young leaves of Acacia
mearnsii (䊊); Eucalyptus globulus (䊏); E. fraxinoides (䊉); E. grandis
(䉱); and Pinus radiata (䉭). (B) Changes in mean biomass of young
leaves of P. radiata on an enlarged scale.
foliar concentrations of phosphorus and potassium (Figure 3),
and there were no significant differences among species. In
contrast to other nutrients, calcium concentrations increased
continuously, and there were significant differences among
species. The ranking in increasing order of foliar calcium concentration was P. radiata, E. fraxinoides, A. mearnsii, E. globulus and E. grandis. By the final sampling, there was a more
than 4-fold difference in foliar calcium concentration between
P. radiata (0.5%) and E. grandis (2.2%) (Figure 3B). There
were also significant differences in calcium concentrations
among eucalypt species.
Mature leaves The concentrations of nitrogen, phosphorus
and potassium in mature leaves decreased gradually from August to April (Figure 3B). The nitrogen concentration in A. mearnsii was significantly higher than in the other species (Figure 3B) but differed little among species despite the differences
in leaf size (Figure 1). Foliar phosphorus concentrations decreased continuously over the 12 months with no significant
differences among species.
Senescing leaves At senescence, there was a further decrease
in the concentrations of nitrogen, phosphorus and potassium in
all species (Figure 3B). Nitrogen concentrations in senesced
leaves were highest in A. mearnsii (1.39%) and lowest in
E. fraxinoides (0.28%).
Interspecific differences in nutrient concentrations in senesced leaves did not parallel those observed in live mature
leaves, because the relative decline in concentration during senescence varied among species. Furthermore, the coefficients
of variation of foliar concentrations in green leaves were between 1.7 and 22.7% compared with between 5.1 and 98.2% in
senesced leaves. Consequently, the correlations between foliar
nutrient concentrations of green and senesced leaves was low
(r 2 = 0.13 for phosphorus; r 2 = 0.05 for potassium). The correlation was higher for nitrogen, because the value for A. mearnsii dominated the regression.
Leaf nutrient contents and retranslocation
Young leaves Contents of nitrogen (Figure 4A), phosphorus
(Figure 4B) and potassium (Figure 4C) in young leaves increased rapidly during the study in all species. Rates of accumulation were highest between October (spring) and January
(midsummer), after which, the pattern of change varied depending on species and nutrients.
The amounts of nitrogen in young leaves of A. mearnsii and
E. globulus (Figure 4A) decreased between February (late
summer) and May–June (early winter). In contrast, there was
little change in foliar nitrogen contents in E. fraxinoides,
E. grandis and P. radiata. Phosphorus content increased in
young leaves of all species, reaching maximum values in summer (January–February, Figure 4B), followed by significant
net reductions in A. mearnsii, E. globulus and P. radiata but
not in E. fraxinoides or E. grandis. The seasonal changes in
potassium content for each species (Figure 4C) were similar to
those observed for nitrogen. Calcium content of young leaves
of all species increased progressively over the study period
(data not shown), but the rate of accumulation differed among
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191
Figure 3. Changes in nutrient concentrations in
(A) young and (B) mature leaves of Acacia
mearnsii (䊊); Eucalyptus globulus (䊏);
E. fraxinoides (䊉); E. grandis (䉱); and
Pinus radiata (䉭). Separate symbols on the
right are for senesced foliage collected at the
time of the last collection of live foliage. Vertical bars are least significant differences at P =
0.05 for the interaction between time and species.
species. At the final sampling in August, the calcium content
in young foliage ranged from 0.1 mg leaf –1 in P. radiata to
14.1 mg leaf –1 in A. mearnsii. There was great variation in calcium content among the three eucalypt species (3.03, 9.02 and
13.4 mg leaf –1 in E. fraxinoides, E. grandis and E. globulus,
respectively).
Mature leaves The contents of nitrogen (Figure 5A), phosphorus (Figure 5B) and potassium (Figure 5C) in mature
leaves declined significantly in all species during the study,
typically from September to May of the following year, when
there were no changes in unit leaf mass and all sampled leaves
(Figure 5) were green. The net reduction in nutrient content reflects nutrient retranslocation from green leaves. These
amounts represented a large proportion of those present initially when mature leaves were first sampled (Table 3). The
proportions retranslocated ranged as follows: nitrogen from
31% in E. globulus to 60% in P. radiata; phosphorus from 54%
in E. globulus to 63% in P. radiata; and potassium from 18% in
E. globulus to 38% in A. mearnsii and P. radiata.
There were further decreases in nutrient contents during leaf
senescence (Figure 5). The net amounts retranslocated during
leaf senescence as a proportion of the pre-senescent values
ranged as follows: nitrogen 76% in P. radiata and 56% in
E. globulus; phosphorus 78% in P. radiata and 37% in
E. globulus; and potassium 88% in P. radiata and 54% in
E. globulus.
From first sampling of mature needles until after senescence
(the sum of the above two processes; see Figure 5) there were
significant differences among species in both amount and proportion of the initial pool of both nitrogen and phosphorus that
were withdrawn. The total amount of nitrogen withdrawn
ranged from 0.67 mg leaf – 1 (or 87% of the initial pool) for
P. radiata to 7.90 mg leaf – 1 (or 70% of the initial pool) for
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FIFE, NAMBIAR AND SAUR
Figure 4. Changes in (A) nitrogen, (B)
phosphorus and (C) potassium contents of
young foliage during their first growing
season for Acacia mearnsii (䊊); Eucalyptus globulus (䊏); E. fraxinoides (䊉);
E. grandis (䉱); and Pinus radiata (䉭).
Arrows mark maximum and minimum values. Vertical bars are least significant differences for changes with time (P = 0.05).
E. globulus. The total amount of phosphorus withdrawn
ranged from 0.06 mg leaf – 1 (or 89% of the initial pool) for
P. radiata to 7.90 mg leaf – 1 (or 63% of the initial pool) for
E. globulus.
Relationships between initial nutrient contents and net
nutrient retranslocation
In both young and mature leaves, there were significant posi-
tive linear relationships between maximum nutrient contents
of both young and mature leaves while they remained green
during Phases 1 and 2 and the amounts subsequently retranslocated (Figure 6). All species fitted within the common relationships for nitrogen (Figure 6A), phosphorus (Figure 6B)
and potassium (Figure 6C).
Similarly, there were strong positive linear relationships between the amounts of nitrogen (Figure 7A), phosphorus (Fig-
Figure 5. Changes in (A) nitrogen, (B)
phosphorus and (C) potassium contents of
mature foliage during their first growing
season for Acacia mearnsii (䊊); Eucalyptus globulus (䊏); E. fraxinoides (䊉);
E. grandis (䉱); and Pinus radiata (䉭).
Arrows mark beginning and end of
retranslocation calculations. Vertical bars
are least significant differences for
changes at P = 0.05. Symbols on the right
are for dead leaves sampled during the last
collection of live needles.
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Table 3. Net amounts of nitrogen (N), phosphorus (P) and potassium (K) retranslocated from green leaves between about 12 and 22 months of age
(marked by arrows in Figure 5). Leaves remained green throughout the sampling period. The main effect of species was significant (P < 0.001) in
all cases except for P (% initial) where the effect was not significant. Within a column, values followed by different letters differed significantly at
P < 0.05.
Species
N (mg leaf –1 )
N (% initial)
P (mg leaf – 1 )
P (% initial)
K (mg leaf – 1 )
K (% initial)
Acacia mearnsii
Eucalyptus globulus
Eucalyptus fraxinoides
Eucalyptus grandis
Pinus radiata
11.72
3.53 a
3.53 a
3.43 a
0.45
34.2 a
31.0 a
51.9 b
54.0 bc
59.9 c
0.73
0.46
0.22 a
0.26 a
0.04
59.8
53.8
58.1
60.7
62.9
3.03
0.74 a
0.43 ab
0.73 a
0.12 b
38.2 a
18.3
30.8 a
36.8 a
37.8 a
ure 7B) and potassium (Figure 7C) in pre-senescent mature
leaves and the amounts of those nutrients retranslocated at senescence. This significant relationship existed despite a significant loss in leaf mass during senescence. Thus, at all stages of
the leaf lifespan, the net amounts of nutrients retranslocated
increased as the amounts in the corresponding pre-withdrawal
stage increased, indicating that the sizes of the initial nutrient
pools determined the amounts retranslocated in all species.
sium contents, in both young and mature leaves of all species,
coincided with the period of maximum growth rates. However,
there were no consistent relationships between amounts retranslocated and growth increments (data not shown).
Relation between tree growth and net nutrient
retranslocation
Our study species represent wide genetic diversity. Acacia
mearnsii (a nitrogen-fixing species) and the three eucalypts
are indigenous to Australia, but they do not occur in natural
stands in the region where this experiment was conducted.
Pinus radiata is an exotic species but has been grown in the region for more than 120 years and has been subjected to several
Relationships between growth increments (measured as height
or diameter growth or branch extension) and nutrient retranslocation for the corresponding growth period were examined.
We observed that decreases in nitrogen, phosphorus and potas-
Discussion
Leaf forms
Figure 6. Relationship between
maximum nutrient contents of (A)
nitrogen, (B) phosphorus and (C)
potassium in young and mature
leaves and the amounts retranslocated in Acacia mearnsii (䊊);
Eucalyptus globulus (䊏); E. fraxinoides (䊉); E. grandis (䉱); and
Pinus radiata (䉭). For each species, one value represents retranslocation during Phase 1 (Figure 4)
and the other during Phase 2 (Figure 5). Values are means of six
replicates. Broken lines are 95%
confidence limits of each regression.
Figure 7. Relationship between
contents of (A) nitrogen, (B) phosphorus and (C) potassium in mature leaves before senescence and
amounts retranslocated during senescence in Eucalyptus globulus
(䊏); E. fraxinoides (䊉); E. grandis
(䉱); and Pinus radiata (䉭). Broken lines are 95% confidence limits of each regression.
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FIFE, NAMBIAR AND SAUR
cycles of genetic selection. This set of species has a wide range
of leaf forms (Figure 1), crown characteristics and rates of leaf
(Figure 2) and tree growth (Table 2), all of which influence
nutrient retranslocation. Despite the large variation in attributes among species, the seasonal pattern and duration of leaf
growth were remarkably similar and leaves of all species did
not increase in mass after 4–5 months of age (Figure 2). A
similar pattern of growth was found in studies with P. radiata,
even when nitrogen application increased basal area growth by
40% (Fife and Nambiar 1997). Furthermore, based on sequential sampling (Figure 2) and other observations, the estimated
leaf lifespan of all species in our study was typically 20–23
months after initiation, a narrow range given the species diversity. These results suggest that the growing environment
strongly affected the timing and duration of foliage growth
across species.
Three phases of foliar nutrient retranslocation
Most studies have examined nutrient retranslocation and its efficiency based on comparison of green and abscised leaves.
This approach yields an incomplete picture, because retranslocation occurs copiously from young green leaves as well as
senescing leaves (Fife and Nambiar 1984, Sheriff et al. 1986,
Nambiar and Fife 1987, Saur et al. 1997). We examined
interspecific variation in the dynamics of changes in nutrient
concentration and content in three distinct phases during the
lifespan of leaves. Phase 1 occurred during the first year of leaf
development when leaves were still growing (Figures 2, 3A
and 4). Initially, foliar nutrient concentrations declined (Figure 3) as leaves expanded and gained mass in the first 5 months
(Figure 2A). However, nutrient contents per leaf increased
(Figure 4) until net nutrient retranslocation began from these
young (6–7 months old) leaves (Figure 4).
Phase 2 was measured during a 10-month period in fully expanded (still green) leaves aged from about 12 months to
22 months. Net amounts of nutrients retranslocated differed
among species and were largely determined by leaf size (Figure 2), but the patterns and processes were similar across species. We have previously found similar results in P. radiata
(Fife and Nambiar 1997) and E. globulus (Saur et al. 2000).
Phase 3 occurred during leaf senescence, which we measured using matched adjacent pairs of green and senesced
leaves. This provides a more accurate measure of retranslocation than a comparison of nutrient concentrations in fallen
leaves and in leaves of the green crown. The latter approach is
especially difficult in evergreen species that simultaneously
bear leaves of different age classes. Senescence also leads to
mass loss. In pine needles, this was three times greater (32%)
than in eucalypt leaves and would confound the comparison of
nutrient retranslocation between species. Changes in leaf mass
at senescence due to retranslocation of carbohydrates have
been found in many species, including P. radiata (Fife and
Nambiar 1997), Alaskan birch (Betula papyrifera var. humilis
(Reg.); Chapin and Moilanen 1991) and mangroves (Kandelia
candel (L.); Lin and Wang 2001).
One factor contributing to the decline in foliar nutrients, especially potassium, is leaching by throughfall. However, we
can exclude a role of leaching in our study, because significant
declines in nutrient contents occurred from December to March
when rainfall (as light showers) ranged from 18 to 38 mm
month –1 and potential evaporation ranged from 170 to 200 mm
month –1 and there was no throughfall or stem flow. Furthermore, we have previously demonstrated significant reductions
in foliar nutrient contents (including potassium) during dry
rainless summers in studies with P. radiata (Fife and Nambiar
1982, 1984), C. glauca (Saur et al. 1997) and E. globulus (Saur
et al. 2000).
The key finding is the close similarity between contrasting
species in foliar nutrient dynamics, especially the cycles of
nutrient accumulation, depletion and retranslocation. This leads
to the question: what factors determine the amounts of nutrients retranslocated during the leaf lifespan and are they common across species? The nurient content of a leaf is strongly
influenced by its mass which varies among species. Results
show that the amount of retranslocated nutrients correlates
strongly with the amount present just before retranslocation.
This is consistent with an earlier observation in P. radiata
(Nambiar and Fife 1991) and is supported by evidence from
other species, including Picea sitchensis (Bong.) Carr.
(Millard and Proe 1992, 1993) and E. globulus (Saur et al.
2000).
In contrast, Niinemets and Tamm (2005) studied nutrient
retranslocation in mixed species stands by comparing nutrient
concentrations in green leaves from the mid-canopy with
freshly fallen leaves and concluded that mid-seasonal foliar
nutrient concentrations were unrelated to nutrient retranslocation. Several methodological factors contribute to this difference in conclusions. Niinemets and Tam (2005) used a
single mid-season sample representing the mid-crown from
mixed species stands, and therefore, the match between green
leaves and fallen leaves may not be as close as in our study.
Our method is based on monthly sequential sampling of groups
of leaves from the same set of branches to define the phases of
nutrient accumulation and retranslocation as a continuous process, throughout the leaf lifespan. In mixed species stands,
interspecific variation in dominance influences nutrient uptake, growth rates and the relative shading in the crown. Leaf
shading stimulates leaf abscission and significant retranslocation even when shading is only partial and leaves are young
(Saur et al. 2000). These considerations are important in interpreting results of different studies and of interspecific difference in retranslocation.
We found a common relationship between the pre-retranslocation amount of leaf nutrients and the amount of nutrients
retranslocated across a range of contrasting species. This common relationship holds for all phases of retranslocation (representing different leaf ages) and for nitrogen, phosphorus and
potassium (Figures 6 and 7). A similar pattern of nutrient
movement has been found in the phyllodes of C. glauca grown
at this site (Saur et al. 1997). We conclude that the amount of
nutrients retranslocated during any phase in the life of leaves is
strongly dependent on the amount (content) present at the start
of the retranslocation phase, and that this is a generic relationship applicable across species when grown in plantations. In
TREE PHYSIOLOGY VOLUME 28, 2008
RETRANSLOCATION OF FOLIAR NUTRIENTS IN EVERGREENS
addition to leaf size, several studies have drawn attention to the
role of various nutrient pools in retranslocation (Chapin and
Moilanen 1991, Hawkins and Polglase 2000, Wang et al. 2003,
Niinemets and Tamm 2005).
Retranslocation efficiency
Nutrient retranslocation efficiency has been expressed as a
proportional change in foliar nutrient concentration or content
from the pre-retranslocation value. Niinemets and Tamm
(2005) estimated retranslocation efficiency during leaf senescence in deciduous trees and attributed variation among species to differences in leaf longevity (lifespan based on timing
of leaf fall) and the nature of nutrient pools, both leading to differences in the kinetics of retranslocation. In an ecological
context, it has been shown that leaf lifespan is related to leaf
mass per area; i.e., plants with low leaf mass per unit area have
leaves with a short lifespan and high nitrogen concentration
(Wright and Westoby 2004). This relationship would be important in mixed species stands containing species differing in
leaf longevity. However, we found no evidence of a ranking for
species by nutrient retranslocation efficiency, and there was no
reliable variation in leaf longevity among our study species. In
the Australian environment, leaf longevity is strongly dependent on site factors including soil water and nutrient availability.
These considerations raise the question of whether the notion of retranslocation efficiency, expressed as a proportion of
a somewhat arbitrarily defined initial value, is a useful explanatory measure for ranking species? To be so, the ratio must
have a degree of stability within species. In our study, percentage retranslocation (content based) for the mature green leaves
of P. radiata were 60% nitrogen and 63% phosphorus. In an
earlier study applying the same method of sampling and calculations for comparable needle age, the percentage for nitrogen
varied between 26 and 41% and percentage phosphorus between 19% and 40% (Nambiar and Fife 1991). Importantly,
the rate of nitrogen applied to the stand, and the resulting response to growth, had a strong influence on percent retranslocation of nitrogen, phosphorus (Fife and Nambiar 1997) and
potassium (Fife and Nambiar, unpublished data). Clearly, the
percentage retranslocation varies considerably depending on
the phase of retranslocation. For example, Table 3 shows no
significant difference in percentage retranslocation for phosphorus in mature green leaves among species, but the corresponding species’ values were further apart when based on estimates during leaf senescence.
Saur et al. (2000) studied retranslocation in E. globulus
growing at another site using identical methods to those used
here and found the following percentage retranslocation from
mature green leaves: nitrogen 51%; phosphorus 54%; and potassium 31% (compared to 31% nitrogen and 18% potassium
in our study). Thus the percentage retranslocation, even within
a species, is highly dependent on site and stand characteristics,
age of leaves during their life cycle and factors such as leaf
shading. Consequently, it is unlikely to be a reliable measure
for interspecies comparisons of retranslocation efficiency.
Caution is required in relating resorption efficiency and the
195
resorption proficiency derived from limited datasets to tree
growth of different species in mixed stands (cf. May et al.
2005). Such analyses do not provide a reliable basis for exploring interspecific divergence in nutrient retranslocation.
In conclusion, when genetically diverse species are grown
in the same environment without interspecies competition for
resources, there is a remarkable similarity in the pattern of, and
the mechanism governing, nutrient retranslocation. Retranslocation occurred in three sequential phases during the life of
leaves in all species. The interspecific variation in net amounts
retranslocated during these phases was primarily due to the
size of the foliage unit, and secondarily to the difference in foliar nutrient concentrations. There was no consistent difference between, or ranking in, retranslocation efficiency among
species. Retranslocation was largely governed by seasonal effects on the nutrient requirement for shoot growth and factors
that determine nutrient concentrations and contents of leaves.
Acknowledgments
We thank David Gritton for assistance with experimental work, Julian
Mattay for advice on statistical analysis and presentation of the paper
and Dr. Chris Beadle and Dr. Barrie May for constructive comments
on the manuscript.
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