Some factors influencing the
increment of forests
A. R. LUDLOW
Imperial College at Silwood Park, Silwood Park, Ascot, Berkshire, SL5 7DE, England
Summary
~^
It is humbling to read the paper by R.N. Aldrich-Blake (1927) because he deduced many truths
that we have since confirmed, and he deduced them with the most subtle of arguments. His article could still be used if one were to start building a computer model to capture the physiological processes of tree growth. His ideas alone would provide a suitable framework for modelling
• and most of the internal and external factors affecting assimilation and respiration would be
included. The model would lack a modern understanding of biochemistry, particularly of photosynthesis and the role of nutrients, and Aldrich-Blake did not consider the important problem of
allocating assimilated carbon to the growth of different parts of the tree. Our ideas on allocation
are now changing and there is emerging evidence for the role of plant signals in allocation
which, in turn, affects the increment in timber volume.
The importance of the rate of assimilation
of raw materials
Aldrich-Blake began by pointing out that variations in wood density made it difficult to compare species unless dry weight was used. He
went onto view the tree as 'capital' while the
increment could be thought of as 'interest' and
he proposed that we should use the 'momentary
rate of dry-weight increment' to denote the rate
'in a period so short that the compound interest
effect is not appreciably felt'. In this he anticipated the short time steps needed in modern
computer models of the processes of forest
growth.
He considered the phases of photosynthesis
and growth throughout the year and identified
the winter as a time when 'the capital earns little or no interest' because of a direct reduction
in photosynthesis but, also, because 'preparations have to be made to face the winter and the
O Institute of Qurtered Fi
ensuing spring'. We now understand some of
the details of frost hardiness and the preparation of overwintering buds but Aldrich-Blake
realized, 70 years ago, that there must be a cost
to these investments.
He was interested, too, in the fact that spring
growth must rely on assimilates from the previous year and cited evidence that 'the length of
annual shoots of evergreen conifers is controlled
by the weather of the previous summer: while
the weather of the current year influences only
the time at which this growth takes place, and
the width of the annual ring' (Hesselman, 1904;
Cieslar, 1907; Romell, 1925; Burger, 1926).
Flowering and fruiting, too, reduced the
interest rate temporarily yet,
judging from Hartig's observations on starch
storage in beech, it appears that this tree is
unnecessarily wasteful in carrying out seed
production (Hartig, 1897). Hartig found that
Fomny, Vol. 70, No. 4, 1W7
382
FORESTRY
the wood parenchyma of the outer twenty
annual rings of these trees was densely packed
with starch during winter and that this starch
was dissolved from only the two outer rings
in ordinary summers, the remainder being
stored untouched against the coming of a
mast year. All this capital earned no interest
for several years.
Aldrich-Blake went on to compare the almost
inactive capital invested in heartwood with the
efficiency of 'more humble plants' and advised
that foresters should study the factors governing
the momentary dry-weight increment of their
plants. The rest of his paper reviews the results
of physiological investigations which have a
bearing on the dry-weight increment of plants.
tissues accumulate in the life of the tree so that
the relative losses through respiration become
smaller as the tree grows.
He was concerned that, 'in the present state
of knowledge' he saw respiration as a loss without understanding the benefit the plant gained
from the energy released.
Aldrich-Blake went on to cite studies by
Lundegardh (1924a); Stalfelt (1923) showing
that respiration rose more rapidly with temperature than photosynthesis. He concluded that
the optimum temperature for growth must
therefore be less than the optimum for photosynthesis. High night temperatures, he concluded should reduce growth and he cited
evidence in favour of this from Lundegardh
(1924a). Aldrich-Blake was aware, too, that respiration was higher in the spring than at other
times of year (Willstatter and Stoll, 1918).
Water and salt requirement of trees
Aldrich-Blake followed Brigs and Shantz (1913)
in defining a measure of water use efficiency: the
weight of water transpired in producing a particular increment divided by the dry-weight of
the increment. He drew attention to the fact that
drought reduced diameter growth in spruce in
the year of the drought but brought poor height
growth in the following year.
The largest gap in his knowledge was in the
effect of nutrients. Moller (1904) had described
the symptoms of deficiencies of nitrogen, sulphur, potassium, phosphorus and magnesium
but almost no quantitative data were available.
Nor was there any hint of the effect of nitrogen
on the root:shoot ratio nor of any of the later
work of Ingestad and his colleagues on the optimal balance of nutrient supply (Ingestad, 1962;
Ingestad and Lund, 1979; Agren and Ingestad,
1987).
Respiration
In this section, Aldrich-Blake explained that
dry-weight increment must be 'largely dependent on the difference between the mean rates at
which assimilation and respiration proceed'. He
appreciated that respiration went on in all living
tissues but that cell walls and dead tissues are
'excused the tax of respiration' and that these
Photosynthesis, and the factors which
influence the rate of this process
In 1927 there was very little understanding of
the chemistry of carbon assimilation but
Aldrich-Blake discussed the factors affecting its
rate:
1 Internal
• 'chlorophyll content, or, more accurately
the active chloroplast surface'
• an internal 'protoplasmic factor'
2 External
• previous conditions affecting the accumulated carbohydrates in the leaf
• all factors influencing stomatal aperture
• salt (or nutrient) supply
• water supply
• atmospheric carbon dioxide
• temperature
• light intensity and wavelength
There have been huge strides, since 1927, in
working out the chemistry of photosynthesis
and our modern understanding of the factors
limiting photosynthesis is summarized in a valuable review by Long (1991). It is now possible to
predict assimilation rates accurately in a wide
range of conditions but Aldrich-Blake had anticipated the framework of these models and the
factors they have to take into account.
SOME FACTORS INFLUENCING THE INCREMENT OF FORESTS
Chlorophyll content and the 'protoplasmic
factor'
There may be little surprise that the effect of
chlorophyll content was well understood in
1927 but Willstattcr and Stoll (1918) had
observed that the assimilation of any species did
not remain constant throughout the season.
They therefore deduced that some internal 'protoplasmic factor', besides chlorophyll content,
influences the leaPs capacity for assimilation.
We now know that assimilation rate depends
directly on many biochemical components
including enzymes such as rubisco. But in 1927
such a deduction was astute.
Accumulation of carbohydrate in the leaf
It is slightly strange that Aldrich-Blake treated
the previous conditions affecting accumulated
carbohydrate as external factors instead of
treating the accumulated carbohydrate as internal. But he was years ahead of his time in arguing that accumulation of carbohydrates in the
leaf might slow down photosynthesis. The biochemistry is now better understood and the rate
of photosynthesis may be reduced if end-products bound with phosphorous are not converted
rapidly enough to release the phosphorous
which is needed for ATP synthesis in the chloroplast (Stitt, 1991). Other forms of end-product
limitation may occur, illustrated by an increase
in the level of ambient CO2 which: (1) is followed by an accumulation of carbohydrates in
leaves; (2) has a greater effect on plants with
many active sinks; (3) may promote activity in
sinks, especially increasing bushiness and
branching, and so have a direct effect on allocation. End product limitation is greater after
short-term exposure to low temperature (Labate
and Leegood, 1988; Stitt, 1991).
Stomatal aperture
In contrast to the chemistry of photosynthesis,
there has been rather little progress in understanding the facts controlling stomatal aperture
and no sound physiologically based model is
available, even today. It is becoming clear that
stomatal aperture is sensitive to signals from the
roots as well as, or instead of the water balance
383
of the leaf and the vapour pressure deficit (e.g.
Khalil and Grace, 1992; Aiken and Sraucker,
1996).
Nutrients
There was very limited understanding in 1927 of
the effect of nutrients on photosynthesis but
Aldrich-Blake realized that magnesium and iron
were essential in the production of chlorophyll.
He suggested that, since phosphorus was an element of proteins, its absence might slow down
protein production so that assimilates were
diverted into carbohydrates which in turn
slowed down photosynthesis. In modern terms,
a shortage of phosphorus reduced sink strength
leading to accumulated carbohydrates. Neither
the existence of ATP nor the activity of enzymes
nor their protein nature was then understood.
Aldrich-Blake would no doubt have been
delighted to learn that phosphorus was necessary for the activity of his 'protoplasmic factor'.
Water supply
Aldrich-Blake understood clearly that the effect
of water balance on stomata would, in turn,
alter the rate of assimilation. He also discussed
the role of water in maintaining rurgor and,
indeed, as a raw material for many biochemical
processes. Although we have hugely more data,
it is still difficult to predict the quantitative
effect of water uptake on photosynthesis, with a
physiological process model. A recent comparison of European forest models showed substantial differences in their response to fluctuations
in precipitation (Ludlow and Randle, 1997).
Carbon dioxide, temperature and light
Aldrich-Blake discussed the interaction between
these three factors and whether they should be
regarded as limiting factors (Blackman, 1905).
He cited Harder (1921) and Lundegardh (1921)
showing that 'the factor most in minimum
always exercises the strongest controlling influence, but an increase in the other factors has
some small effect'.
Carbon dioxide. In introducing the detailed
discussion of carbon dioxide Aldrich-Blake
384
FORESTRY
wrote: 'Some may think that an investigation of
the relations between the atmospheric CO2 content and the rate of assimilation is a fruitless
study, owing to the small variations in the quantity of this gas in the atmosphere, and the
impossibility of controlling it.' One wonders
how he would have viewed the man-made rise
in CO2 and the huge investment in studies of
plant responses to elevated CO2.
Aldrich-Blake suggested that 'shade bearers'
might be more sensitive to increasing CO2 than
'light demanders'. His immediate interest, however, was more practical: vertical gradients of
CO2 had been observed in forests, especially close
to dawn. In one study of tropical rain forest near
Rio de Janeiro, McClean (1919) had observed
concentrations of 340 Pascals at breast height at
6.30 a.m. That is ten times the normal atmospheric content. Aldrich-Blake concluded that this
was of little benefit to trees in an even-aged stand
because they had so few branches at low levels
and the difficulties of managing an uneven-aged
stand might make the task impractical.
Temperature. Much of the work on temperature and photosynthesis had been done at
extremely high CO2 levels and Aldrich-Blake,
like Lundegardh (1924b), was sceptical of the
relevance of such studies. Lundegardh himself
worked at normal levels of CO2 and concluded
that potato leaves had an optimal assimilation
rate at 20°C.
Light intensity and wavelength. Aldrich-Blake
began by pointing out that experimenters measured the light falling on the leaf and not the
light used in photosynthesis. Only half this light
was used in the leaf and most of that was 'converted into heat for the vaporisation of water in
transpiration'. Only 0.6-2.7 per cent of the solar
energy was used in photosynthesis (Puriewitsch,
1914). Aldrich-Blake was struck with the wastefulness of light use: 'it therefore seems certain
that a leaf can spare only a small proportion of
its complement of light for the most important
function which it subserves.' As above, he was
concerned with the poor return on the physiological capital of the tree.
Aldrich-Blake was fully aware of the differences in response between sun and shade leaves.
Figure 1 after Boysen-Jensen (1923) is taken
from Aldrich-Blake's paper. The curves represent the apparent (gross) hourly assimilation of
sun and shade leaves of elder {Sambucus nigra)
at 20°C and, presumably, in an atmosphere containing 30 Pascals CO2. Similar curves have
1
1
1
1
1
— —'
^
-
-
"
-
t
/
/
^
SHADE LtAr.
-
I
I-
l
o /
/
1
5
10
1
If
1
20
LlCHT iMTENIITT AI
1
IS
" 'A
30
|
•40
or SUNLIGHT
Fic. 2.
Figure 1. A comparison of gross hourly photosynthesis of elder {Sambucus nigra) sun and shade leaves.
(Facsimile of Figure 2 in Aldrich-Blake (1927), plotted from experiments by Boysen-Jensen (1923).)
SOME FACTORS INFLUENCING THE INCREMENT OF FORESTS
385
been found for many species since and AldrichBlake's discussion has a modern sound. He
pointed out that the curves began below the
abscissa because respiration exceeded assimilation at very low light levels. And he went on to
discuss the light intensity at which assimilation
and respiration were equal (now called the compensation point).
Aldrich-Blake pointed out that the differences
between the two curves were intelligible because
the sun leaves were thicker and, for a given area
of tissue, there was a higher absolute respiration
cost in sun leaves, so the compensation point
was at a higher light intensity. The thinner
shade leaf, on the other hand, became light saturated at about 12 per cent of sunlight:
tered light and this was true of broadleaved
stands (Knuchel, 1914). However,
for in such a light intensity all the assimilating cells are presumably sufficiently illuminated to yield their maximum output. The
lower tissues of the thicker sun leaf are
shaded by the superincumbent tissues, and are
therefore unable to yield their maximum output until the upper surface of the leaf receives
about 40 per cent of sunlight.
In his final section, Aldrich-Blake discussed the
amount of timber produced by a given mass of
foliage. The range was considerable and five
times as much timber might be produced from a
given leaf mass on good soils as on poor. We
now know that trees allocate far more to roots
on a poor soil, and less to stems (Keyes and
Grier, 1981). Similarly, 33-year-old trees were
estimated to be 2.7 times more efficient than 90year-old trees.
Aldrich-Blake cited Stalfetl (1921) who found
that light assimilation curves of sun and shade
needles in pine and spruce 'ascended up to a
light intensity of full sunlight'.
He pointed out that the compensation point
determined the habitat of a plant and he published values of the compensation point collected for a number of species by Lundegardh
(1924a). In an astonishingly modern passage
Aldrich-Blake argued:
The most productive species must use the
sunlight to the greatest possible extent. It
must produce a series of leaves, graded in
thickness from the periphery of the crown
inwards; the thick outermost leaves making
good use of full sunlight, the innermost being
so thin that they can show a dry weight gain
in the densest shade.
He went on to discuss the effect of wavelength and that light passing through outer
leaves was depleted in the photosynthetically
active wavelengths. Light reflected from leaves,
on the other hand, was more uniform in wavelength. The light reaching the forest floor should
therefore be a mixture of transmitted and scat-
the light under coniferous stands possessed
nearly the same relative spectral composition
as the light outside. The difference between
broad-leaved and coniferous stands indicates
that little light is transmitted through, or
reflected from conifer needles; and that such
light as reaches the ground beneath a coniferous stand does so through gaps in the needle
mosaic.
The incremental efficiency of leaves and
needles
Sections that Aldrich-Blake did not include
Up to now I have summarized Aldrich-Blake's
account and shown that, in places, he is astonishingly modern in outlook. There has been much
work in each of the areas he covered. For example, we can now measure gas exchange on small
time-scales at different points in the tree simultaneously. In addition, we understand the biochemistry of photosynthesis and can calculate the
effect of temperature, CO2, light and biochemical
components with reasonable confidence. Such calculations are built into modern computer models
but the models still differ widely in their predictions (Ludlow and Randle, 1997). There seem to
be two main types of problem.
Where the theory is well advanced, we still
lack relevant data. Most quantities have been
measured but not on the same stand. Modellers
are forced to take values measured in different
geographical areas and sometimes different
species. We have vastly more data than AldrichBlake could cite, but the gaps are still too large.
386
FORESTRY
The second type of problem is that important
processes are still not understood. For example,
our knowledge of stomatal control is incomplete
and yet it has a large effect on the growth of
trees in a changing climate.
Another major gap is our understanding of
processes below ground. Aldrich-Blake focussed
on assimilation and respiration but forest increments also depend on nutrient and water uptake
which in turn depend on the pattern of root
growth.
Perhaps the most important gap in our understanding is the allocation of assimilate to different parts of the tree. In Aldrich-Blake's terms
this has a large effect on the efficiency of the tree
and therefore on the return from capital. A tree
which puts too much into roots will have less
foliage and will assimilate less carbon. One that
invests too little in roots may suffer water or
nutrient deficiency which also reduces its assimilation rate. Because of the compound interest
effect, small departures from the optimum can
lead to large losses. At the moment we know
too little about allocation processes to calculate
them accurately.
There has been a great deal of work on the
balance between root and shoot growth and the
way this is affected by nutrients. Ingestad and
his colleagues have already been cited (Ingestad,
1962; Ingestad and Lund, 1979; Agren and
Ingestad, 1987); and there has been theoretical
work by Thornley and more recently by Dewar
(Thornley, 1972, 1976, 1991; Reynolds and
Thornley, 1982; Dewar, 1993). However, these
models would fail to predict some of the observed
seasonal changes in allocation and growth, which
suggests that there are fundamental gaps in our
understanding (Dewar etal., 1994). The problems
have been more fully illustrated and discussed by
Ludlow and Atkinson (1996).
A second well-established feature of allocation is that xylem growth and foliage growth are
correlated. This has been expressed in the pipemodel theory (Shinozaki et al., 1964a, b) and
built into growth models by Valentine (1985)
and Makela (1986) who assumed that the ratio
of total foliage mass to total cross-sectional area
of sapwood remained constant. Ludlow et al.
(1990) assumed that the ratio of new foliage to
the area of new sapwood remained constant.
McWilliam (1972), in his Ph.D. thesis on 22-
year-old Sitka spruce (Picea sitchensis), showed
that the start and end of shoot extension were
almost perfectly correlated with the start and
end of xylem cell division at different heights
down the tree. Ludlow and Atkinson (1996) presented and reanalysed his data and argued that
auxins produced in the expanding foliage during
shoot extension stimulated xylem cell division.
Such a mechanism would explain the observed
correlation between growth of foliage and
growth of sapwood area.
New evidence confirms earlier postulates that
root signals to shoots modify whole plant
growth processes and the signals include abscisic
acid, nitrate flux, and cytokinins (Aiken and
Smucker, 1996). The next generation of tree
growth models will have to take into account the
way sink strength in particular parts of the plant
may be influenced by signals from other parts.
We may be about to move from models where
growth is driven by substrate (e.g. Reynolds and
Thornley, 1982) to ones which give more appropriate weight to the environmental and internal
factors affecting sink strength.
Appraisal
For the first issue of Forestry, Aldrich-Blake
wrote an astonishingly far-sighted review.
Much of the work he cited was German or
Scandinavian and probably not accessible to the
British forestry community. In later years forest
research in Britain turned away from tree physiology in favour of mensuration and questions
of immediate importance to forest practice. The
pendulum has swung sharply back as we now
try to calculate the effect of climate change, rising CO2 and acid rain. It is humbling to realize
how much Aldrich-Blake and his contemporaries knew and how unerringly they guessed. It
was an outstanding paper when he wrote it and
is worth reading still.
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