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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
~ - ~ ~ - ~ - ~ - 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~. <at \ LAI Figure 1. When shrub cover was removed (O) from beneath an arid zone ponderosa pine forest, the growth efficiency of trees significantly increased over that observed with shrub cover present Ail. As the canopies developed, the leaf-area index (LAI) eventually shaded out shrub cover and trees competed with one another to a similar extent. After Waring (1983~. By comparing changes in other selected ratios of structural biomass it may be possible to distinguish more specific causes of stress and to correlate these with parallel changes in the pollution load on forests. PHOTOSYNTHESIS/BRANCH PRODUCTION The capture of radiant energy (400-700nm) in the process of photosynthesis differs depending on the exposure of leaves and their biochemical capacities. Photosynthesis can be predicted for different portions of tree crowns (Caldwell et al. 1986~. When branch growth is compared with photosynthesis, a linear relationship results (Fig. 2~. The slope of the relationship appears to differ slightly depending on the nutritional balance. Where pollutants are likely to damage phloem and restrict translocation, a higher than normal allocation of photosynthate to branch production would be expected.
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.
