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OCR for page 127
RESOURCE ALLOCATION IN TREES AND ECOSYSTEMS
R.H. Waring
Forest Science Department
Oregon State University
Corvallis, Oregon 97331
ABSTRACT
As ecosystems are subjected to various kinds of stresses, the
availability of resources required to support life is altered. As a
consequence, plants, animals, and microbes alter the way in which
resources are expended. When a particular ecosystem is
subjected to an unusual combination of stresses, or an unusual
intensity of one type of stress, biologically catalyzed reactions are
initiated that permeate throughout the system. A record of how
the biological components of an ecosystem reacted to various
stresses is often encoded in the tissue composition of trees. For
example, a nutritional imbalance may alter the normal
essential minerals to one another in foliage or in roots.
types of stresses affect the amount of carbon stored in
organs, the possibility of seed production, and the form of
bole. Structural indices of leaf/bole, leaf/phloem, and leaf/root
allocation may aid in interpreting other signals of environmental
change associated with mineral and stable isotope composition.
INTRODUCTION
ratio of
Other
various
a tree's
The allocation of carbon into various components of trees and ecosystems changes
depending on the environment (Waring 1983~. Trees on an exposed headland exhibit
extreme stem taper and an extensive supporting root system. Such trees also contribute a
larger proportion of fresh foliage and limbs to annual litterfall than normal. If pollutants
affect tree growth and the decomposition process, then deviations from normal patterns
observed. Recognition of deviate responses depends
on analysis along environmental gradients, historical reconstructions of changing
environment, and experiments. I draw on these sources in suggesting production ratios
most likely to change in response to pollutants.
in carbon allocation should also be
Foresters, as an aid to estimating the value of trees, construct tables and equations
that predict the wood volume in stems of specified basal diameters. Depending on the
type of tree and the environment in which it grows, the taper of the stem varies. This
fact has led foresters to develop local volume tables for many commercially valuable
trees. The approach has been extended to estimating the weight of foliage, branches, and
large diameter roots from the measurement of stem diameter (Whittaker and Woodwell
1968~. Production rates are predicted by assessing the calculated change in weights of
various organs in relation to measured changes in stem diameter. Because many of these
allometric relationships were determined from trees sampled before pollution was
extensive, they serve as a benchmark for comparing any change attributed to pollutants.
In the last decade the basic approach has been refined by recognizing that the
cross-sectional area of conducting sapwood is often proportional to the foliage it
127
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Representative terms from entire chapter:
circles represent
~ -
~ ~ -
~ -
~ -
2
a
a
-
1
O I
128
supports at the time of peak leaf development (Waring 1983~. This relationship is more
accurate than that determined from diameter alone. Sometimes the historical development
of tree crowns can be reconstructed by knowledge of what age sapwood converts to
heartwood and from branch scars when limbs die (Margolis et al. 1988~. By coring a tree
it is possible to compare the amount of wood produced annually from a certain
complement of foliage. This "growth efficiency" ratio decreases abruptly when the
environment becomes less favorable and more slowly as trees age and the cost of
maintenance respiration increases (Fig. 1~.
129
1000
-
-
o
~ _
Y
~ 500 .
a
._
~ _
.
o
_-~~~ r2 0 91
tD ~ -- a _--6-
._ _
_—
__—
_— .
__—
0 1000 2000 3000
net photosynthesis ~ kg C ha1 whorI~1)
Figure 2. Annual net photosynthesis by whorls of branches in a Scots pine stand
contributes a fixed fraction to branch growth. On fertilized and irrigated plots (upper
line) the fraction is higher than on control plots (bottom line). After Linder and
Axelsson (1982~.
PHLOEM/SAPWOOD AREA
If the transport of photosynthate through phloem is inhibited, then the
cross-sectional area of sieve cells should be reduced in relation to leaf area or the
surrogate, sapwood cross-sectional area. Scots pine trees provided with optimum water
and nutrients exhibit a ratio of phloem/sapwood area in the stem, half that of
unfertilized and unirrigated trees (Dr. Erik Mattson-Djos, University of Uppsala, Sweden).
This corresponds with a similar reduction in the fraction of photosynthate allocated to
fine-root production (Alexsson and Alexsson 1986). If pollutants reduce the functional
area of phloem down the bole, annual wood increment should mirror this, resulting in
reduced taper as noted by Schutt and Cowling 1985~.
LEAF AREA/BOLE MAINTENANCE
As trees grow, the maintenance cost of parenchyma cells in the conducting tissue
becomes proportionately larger. These cells make up nearly 30% of the sapwood in oak
and 5% or more in the sapwood of other species (Waring and Schlesinger 1985). For
conifers with a fairly similar percent of living cells in sapwood, pioneer species usually
support fewer leaves with a given amount of sapwood. Species that follow in succession
130
tend to support more leaves with less sapwood and to have lower light-compensation
points for photosynthesis. Pollutants that reduce photosynthesis make shaded branches no
longer self-sufficient and they die. The original sapwood serving those branches, however,
remains alive until a large fraction of foliage is lost (Margolis et al. 1988~. Advanced
successionaly species have a slower turnover time of foliage than pioneer species, and
thus take much longer to adjust to changing conditions. For these reasons, pioneer trees
are likely to be favored in heavily polluted areas.
LEAF/LITTER PRODUCTION
Increased damage to foliage will increase the normal turnover, resulting in an
increased fraction of new/total foliage on evergreens, and a temporary increase in
litterfall. As the canopy becomes more open, it intercepts less precipitation and radiant
energy. This favors a microclimate conducive to improved decomposition. If, however,
heavy metals or nutrient imbalances are associated with conditions favoring canopy
opening, the rate of carbon breakdown and mineral release may be reduced below that
expected. Deviations from expected rates may be indicative of pollutants affecting heavy
metal and nutrient balances (O'Neill et al. 1977~. Imbalance in N:P:S ratios in foliage and
litter also are indicative of nutritional problems affecting tree growth and litter
decomposition (Waring 1985, Staaf and Berg 1982~.
BARK BEETLE ATTACKS/ GROWTH EFFICIENCY
In many forests, bark beetle attacks follows a reduction in tree vigor. Christiansen
et al. (1987) illustrated that any stress that critically reduced the amount of
photosynthate being translocated down the bole during the period of insect attack lowers
production of defensive compounds. In general, growth efficiency provides a good index
to the threshold at which trees are killed by a particular density of attacking beetles
(Fig. 3~.
CONCLUSION
Changes in carbon allocation must be based on some reference to normal. Local
volume tables, stem analyses, and methods that quantify changes in climate and
atmospheric deposition can assist in interpreting the significance of observed alterations
in allometric relationships. Sometimes it is possible to reconstruct the development of
tree canopies by correlation with sapwood cross-sectional area. Shifts in how
photosynthate is allocated to branches and bole may be indicative of changes associated
with pollution load. Alterations in phloem conducting area may also result. Analysis
across pollution gradients may be useful in assessing the value of proposed techniques.
RECOMMENDATIONS
Healthy forests contain trees able to allocate a considerable fraction of
photosynthate to wood production. Any environmental stress decreases the fraction of
wood produced/ unit of foliage. This index of vigor correlates with a tree's ability to
withstand a fixed amount of defoliation, bark beetle attacks, pathogenic infection, and
dose of air pollutants. Sustained exposure to new stresses will subsequently lower vigor,
reduce carbohydrate reserves, and tree resistance to a variety of pests and pathogens.
131
200
0
on I so
m
:~E I Do
in
i>
-
~ so
-
m
~
.
· .
S
To
a-:- ~
Of
° ° to 1
~ o 1
a. of 0
·/
·/ 0
do o
oaf m^~ O ~ 1,,,,t,
0 so 100 1 so
o 1
n I
1
Growth Efficiency
wood/m2 leaf/yr
Figure 3. Growth efficiency provides an index to the density of bark beetle attack
required to kill lodgepole pine trees. Filled or partly filled circles represent the
proportion of conducting tissue killed on attacked trees. Open circles represent trees able
to halt all beetle attacks before any conducting tissue was killed. The dotted vertical line
indicates the boundary above which beetle attacks are unlikely to cause tree mortality.
After Waring and Pitman (1985~.
Tree vigor, defined as grams of wood produced annually per square meter of foliage,
can be assessed by extracting wood cores and determining growth, sapwood thickness,
and tree diameter. I recommend that all forest studies develop the constants for
applying these relationships and use them as a general frame of reference.
In specific cases where low vigor is recorded and air pollution is expected to be a
contributing cause, further analyses are warranted. From what we know about ozone,
excess nitrate, and sulfur dioxide effects, allocation of wood to branches should increase/
unit of foliage while that to the lower stem and roots should decrease. These expected
alterations in branch and bole wood allocation should be sampled because the pattern of
response differs from that initiated from most other kinds of environmental stresses.
More detailed analyses of phloem, starch reserves, and nutrient balances could also
be made in foliage, twigs, and sapwood but are seasonally dependent variables that are
best studies in experimental programs involving stable isotope and remote sensing
techniques (see papers by B. Fry and B. Rock).
132
REFERENCES
Axelsson, E., and B. Axelsson.1986. Changes in carbon allocation patterns in spruce and
pine trees following irrigation and fertilization. Tree Physiol. 2:189-204.
Caldwell, M.M., H.-P. Meister, J.D. Tenhunen, and O.L. Lange. 1986. Canopy structure,
light microclimate and leaf gas exchange of Quercus coccifera L. in a Portuguese
macchia: measurements in different canopy layers and simulation with a canopy
model. Trees 1 :25-41.
Christiansen, E., R.H. Waring, and A.A. Berryman. 1987. Resistance of conifers to bark
beetle attack: searching for general relationships. Forest Ecol. & Management
22:89- 106.
O'Neill, R.V., B.S. Ausmus, D.R. Jackson, R.I. Van Hook, P. Van Voris, C. Washburne, and
A.P. Watson. 1977. Monitoring terrestrial ecosystems by analysis of nutrient export.
Water, Air, Soil Pollut. S:271 -277.
Linder, S., and B. Axelsson. 1982. Changes in carbon uptake and allocation patterns as a
result of irrigation and fertilization in a young Pinus sylvestris stand. Pp. 38-44 in
Carbon uptake and allocation in subalpine ecosystems as a key to management, R.H.
Waring (ed.~. Proceedings of an I.U.F.R.O. Workshop. Forest Research Lab., Oregon
state Univ., Corvallis, OR.
Margolis, H.A., R.R. Gagnon, D. Pothier, and M. Pineau. 1988. The adjustment of growth,
sapwood area, heartwood area, and sapwood saturated permeability of balsam fir
after different intensities of pruning. Can. J. For. Res. (in press).
Schutt, P., and E.B. Cowling. 1985. Waldsterben, a general decline of forests in central
Europe: Symptoms, development, and possible causes. Plant Disease 69:548-558.
Staaf, H., and B. Berg. l 9X2. Accumulation and release of plant nutrients in decomposing
Scots pine needle litter. Long-term decomposition in a Scots pine forest. II. Can. J.
Bot. 60:1561 - 1568.
Waring, R.H. 1983. Estimating forest growth and efficiency in relation to canopy leaf
area. Adv. Ecol. Res. 13:327-354.
Waring, R.H. 1985. Imbalanced forest ecosystems: assessment and consequences. Forest
Ecol. & Management 12:93- 112.
Waring, R.H., and G.B. Pitman. 1985. Modifying lodgepole pine stands to change
susceptibility to mountain pine beetle attack. Ecology 66:~89-897.
Waring, R.H., and W.H. Schlesinger. 1985. Forest ecosystems: concepts and management.
Academic Press, Orlando, F1.
Whittaker, R.H., and G.M. Woodwell. 1968. Dimension and production relations of trees
and shrubs in the Brookhaven forest, New York. J. Ecol. 56:1-25.
