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EVALUATION OF ROOT GROWTH AND FUNCTIONING OF TREES EXPOSED TO AIR POLLUTANTS J.H. Richards Department of Range Science and Ecology Center Utah State University Logan, Utah 84322-5230 ABSTRACT Reduced carbon allocation to roots caused by air pollution exposure results in reduced root growth, turnover and capacity for water and nutrient uptake. Reduced root activity may be a sensitive indicator of the effects of air pollution exposure and should be detectable by the techniques reviewed. Minirhizotrons and rhizotrons allow nondestructive assessment of seasonal patterns of root growth, experimental treatment effects on growth and observation of root development and morphology. The 14C dilution technique may provide a useful alternative to repeated core sampling for long-term study of root turnover. Soil psychrometry is potentially useful for immediate detection of localized water uptake by roots and standard neutron-attenuation techniques can detect cumulative water extraction by roots. Application of a 32p/33p dual-isotope labeling technique would allow determination of root and mycorrhizal nutrient uptake capacity in the field as affected by experimental air pollution exposure. Root ingrowth cores enriched with limiting nutrients may be useful in evaluation of the degree of nutrient stress or root growth capacity of trees in the field. The many techniques suitable for field measurement of root functional and morphological parameters would be useful in research situations, but require some development before application to broad-scale monitoring would be feasible. Application of these techniques in the field is dependent on the availability of suitable controls to distinguish pollutant effects from variation due to other stresses or genetic and environmental variation. Gaseous pollutants have direct effects on photosynthetic capacity and translocation (see reviews by McLaughlin 1985, Mansfield and Jones 1985, Carlson and Bazzaz 1985, Cooley and Manning 1987~. Reduced allocation of carbohydrates to root systems as a result of either of these mechanisms will have similar effects. The effects of reduced carbon allocation to roots should be measurable not only as reduced root growth and turnover but also as reduced capacity for nutrient and water uptake. A recent review of assimilate partitioning in plants exposed to Of found that for 20 species root growth was reduced an average of 35% while shoot growth was reduced only 21% (Cooley and Manning 1987). While fumigation with low concentrations of SO2 caused no visible shoot symptoms in Norway spruce, root growth was reduced by 50% in the year following the fumigation (Keller 1985~. Similar results for four broad leaf tree species 169
170 and red pine were also cited by Keller (1985~. The conclusion that root biomass is reduced more than shoot biomass upon exposure to Of or SO2, is also supported by Lechowicz ( 1987~. He noted, however, that NO2 did not have a differential effect on roots and shoots. While carbohydrate concentrations in roots of plants exposed to pollutants have been determined (see reviews by Cooley and Manning 1987, Lechowicz 1987), pollutant effects on root physiological activity, as distinct from growth, have not been directly investigated. The reviews cited above and others (e.g., Go~zik and Krupa 1982) indicate clearly that root production, which reflects the partitioning of assimilates to root growth, is usually reduced before effects are detected in the shoot system or to a greater extent than shoot growth. Numerous studies on herbaceous plants and tree seedlings suggest that root growth and physiological activity are dependent upon a continuing supply of current assimilate. For example, Thorgeirsson (1988) showed that within 1-2 days after shading or defoliation root respiration of soil-grown perennial grass plants was reduced in proportion to reductions in shoot carbon gain. Hydroponic studies with maize plants indicated that a single day of shading resulted in more than 60°h reduction in root respiration and ammonium, nitrate and potassium uptake (Massimino et al. 1981~. Following the day of shading, recovery of root respiration and nutrient uptake required 3 days while shoot function recovered within a single day. Studies of both warm- and cool-season grasses demonstrated that root growth ceased within 24 hours of defoliation and did not resume until substantial shoot regrowth had occurred (Crider 1955, Troughton 1957~. In these studies, root growth often ceased for several weeks following a single defoliation event. Finally, current photosynthate may be the primary carbon source for new root growth in seedlings of several coniferous forest species (van den Driessche 1987, Ritchie 1982~. In larger plants, such as forest trees, root activity may persist longer on stored reserves. Nevertheless, several studies suggest that root growth is often coupled to current photosynthesis and allocation of carbohydrates. This coupling may be particularly strong between flushing of leaves, when depletion of stored carbohydrates may be substantial (Dougherty et al. 1979, Wargo 1979, McLaughlin et al. 1980), and replenishment of stored reserves. For example, rhizotron studies of mature white oak by Reich et al. (1980) showed that root growth was reducer! by more than 50% when competing shoot sinks (shoot, leaf and acorn growth) were most active. Chronic exposure of trees to pollutants may result in reduced allocation to roots and lower carbohydrate reserves, thereby increasing the dependence of root activity on current photosynthate supply. Reduced root growth and impaired functioning may be factors predisposing pollutant-exposed trees to damage from other stresses. Quantification of the impacts of pollutants on root function in forest trees under field conditions would provide important information for evaluating tree susceptibility to secondary damage. Moreover, because the effects of some gaseous pollutants have been detected in roots prior to shoots, roots may be early and sensitive indicators of damage before damage is detectable in the shoot system (see reviews by Mansfield and Jones 1985, Carlson and Bazzaz 1985~. Although root systems may be sensitive indicators, the effects are unlikely to be specific to pollution damage. Other stresses reducing photosynthetic activity or translocation to the root system would have similar effects. In this paper, I discuss several techniques that have proved useful in measuring root growth and functioning in field situations and may prove applicable to assessing pollutant damage through effects on root systems of forest trees.
ROOT PRODUCTION AND TURNOVER Fine root production, which has a central role in carbon and nutrient cycles in forest ecosystems, is usually estimated by periodic sampling of the forest floor and mineral soil and determining the mass of live and dead root and soil litter fractions. Fine root production is calculated by a variety of methods with different assumptions concerning transfers of biomass among the root and litter fractions in the belowground system (Persson 197S, McClaugherty et al. 1982, Hansson and Steen 1984~. Depending on the method, calculated values of total fine root production may vary three- to four-fold even when the same sequential harvest data are used (McClaugherty et al. 1982, Singh et al. 1984). Errors in the overall estimates can be large due to variation among samples (Singh et al. 1984), simultaneous root production and mortality (Kurz and Kimmins 1987), or failure to incorporate estimates of losses due to decomposition, herbivory or other processes (Hansson and Steen 1984, Milchunas et al. 1985, Vogt et al. 1986a). Careful tuning of the sampling schedule to the phonology of root growth at the site of interest and appropriate application of statistical constraints are necessary to allow reliable estimates of fine root production (McClaugherty et al. 1982, Persson 1983, Aber et al. 1985, Vogt et al. 1986a, 1986b). For example, root production calculated from sequential sampling may seriously underestimate the true value when fine root production and mortality occur simultaneously and at high rates (Aber et al. 1985, Kurz and Kimmins 1987~. Two other techniques that depend upon modified sequential sampling may be useful for calculating root production. These techniques are based upon ingrowth into root-free soil cores (e.g., Fabiao et al. 1985) or root starch concentration measurements combined with soil temperature monitoring (Marshall and Waring 1985~. Analysis of errors in these techniques has been limited (see Caldwell and Virginia 1988~. Because a large investment in sampling and sample processing would be required to obtain the data necessary to calculate fine root production in forests, it is unlikely that this parameter could serve as a practical indicator of air pollution damage. Standardized methods could not be used on many sites because of the need to adapt the sampling scheme to the particular growth conditions of the site. Careful quality control would be required to assure consistent processing of samples through the years that would be necessary to integrate the variation in fine root production due to environmental fluctuations. Even if sampling could be conducted to insure reliable estimates of fine root production through time, large spatial variation, including variation due to stand age, species composition, understory composition and site nitrogen availability (e.g., Persson 1983, Aber et al. 1985, Vogt et al. 1986b), would require simultaneous sampling of interspersed (Hurlbert 1984) exposed or damaged stands and control stands of unexposed or undamaged trees (see Cape, this volume). An approach to determining root production that requires less intensive sampling than the usual series of sequential harvests involves labeling the structural carbon of the root system with 14C and assessing the subsequent dilution by 12c incorporated into new root structure during the period in which production is to be determined (Caldwell and Camp 1974~. A turnover coefficient is derived from the ratios Of 14c/12C in root structural tissue at the beginning and end of the period. Root production during the period is the product of the turnover coefficient and the total root biomass at the beginning. This approach has been used successfully in field studies of cold-desert shrubs (Caldwell et al. 1977), but it has not been utilized for forest trees. Application to trees
172 is impractical because of the difficulty of labeling large plants and because of the potential errors resulting from incorporation of 14C from labile carbon pools into new root structural material during the time between the first and final sampling. This incorporation would compromise one of the critical assumptions underlying this technique and, if uncorrected, would result in severely underestimating root Production (Milchunas et al. 1985~. A second assumption underlying this technique is that 4C and 12C are lost from the root system, by decomposition, herbivory, etc., in the same proportion as they occur in the structural carbon in the root system. Milchunas et al. ( 1985) and Caldwell and Eissenstat (1987) discussed the errors associated with this technique and concluded that the isotope dilution technique can accurately estimate root production in the field if precautions are taken to insure that both the incorporation and decomposition assumptions are not violated. Because the isotope dilution technique is sensitive to errors resulting from carbon fluxes from labile to structural pools and the ratio of isotopes in material lost from the root system, it would be more useful as a research tool to study whole-plant carbon allocation patterns than as a monitoring method. Separate turnover coefficients would undoubtedly need to be developed for fine and large roots. The greatest barrier to using this technique in forests is how to label root tissues of large trees practically and adequately. The hazard associated with labeling trees might be alleviated if CO: highly enriched or depleted in the stable carbon isotope, 13C, was substituted for 14CO2. Highly enriched NaHCO3 (ninety-nine atom percent 1 3C) is available commercially and could be used to produce 13Co2; and some industrial sources of CO2 are substantially depleted in l3c (B. Fry, pers. comm.). If all the practical problems of labeling trees could be overcome, the isotope dilution technique might be useful in assessing air pollution effects on fine root production in forests. While this technique does not completely alleviate spatial and temporal variation problems associated with sequential harvest sampling, it could be applied to interspersed damaged and healthy trees. ROOT GROWTH AND DEVELOPMENT Nondestructive observation of the growth of roots against glass or plastic walls or tubes has been extensively utilized in root system studies for over a century, especially in crop plant root systems. Construction, installation and utilization of large root observation chambers (rhizotrons) and glass or plastic tubes (minirhizotrons) have been reviewed by Bohm (1979), Taylor (1987), and Caldwell and Virginia (1988~. Rhizotrons have been effectively used to study the seasonal dynamics of root growth in relation to shoot growth of large trees and to determine the relationships of root growth to soil temperature and soil and plant water status (Reich et al. 1980, Teskey and Hinckley 1981, Kuhns et al. 1985~. Rhizotrons could potentially be used to determine how these relationships might change in trees exposed to air pollution. Rhizotrons could be placed so that roots of both damaged and undamaged trees, or experimentally exposed and control trees, could be monitored on the same site in the same year. In addition root morphology and development (e.g., root elongation rates and the longevity of individual root elements, etc.) could be directly monitored in rhizotrons. As Marshall (1986) showed that fine roots of Douglas fir died as their carbohydrate reserves were depleted, in trees where carbohydrate concentrations of fine root elements are affected by air pollution exposure, longevity of fine root elements should be reduced compared with longevity in undamaged trees. The relationship between root elongation and suberization and development of root hairs as well as mycorrhizal or pathogenic relationships could also be studied with
173 rhizotrons. Fluorescence techniques might be helpful in some of these observations (see review by McMichael and Taylor 1987~. Root observation tubes (minirhizotrons) have advantages over larger rhizotrons in inexpensive replication and increased placement flexibility. Minirhizotrons can generally be used for a longer period of time compared to large glass plates before accumulation of roots dictate replacement. Large numbers of tubes are required, however, to compensate for the reduced area of the observing surface. Observations in minirhizotrons can be made with instruments ranging from simple, inexpensive periscopes (Richards 1984) to expensive, color-video systems (Upchurch and Ritchie 1984~. Minirhizotrons have been used in agronomic situations to determine root depth distributions, root length density and density dynamics (see reviews in Taylor 1987~. Applications of minirhizotrons in forests would likely be less important for determining these parameters, however, than for determining root morphological and developmental patterns. Recording high-resolution, color images in minirhizotrons means that virtually all of the morphological and developmental parameters studied in rhizotrons can also be obtained with minirhizotrons at much less expense. Replicate placements of minirhizotrons would allow researchers to deal with spatial variation problems. Image analysis of video recordings of root systems from minirhizotrons could facilitate data analysis and determination of morphological parameters (Smucker et al. 1987~. Minirhizotrons also provide an efficient way to examine treatment effects on root growth and other root parameters because non-destructive observations can be made on the same root elements before and after treatment. Minirhizotron tubes could be installed in experimental soils of known characteristics to investigate the interaction between roots and soils in the field. Application of minirhizotrons to studying roots of forest trees appears extremely promising. Lack of previous use of minirhizotrons in forests, however, would require much baseline work. Some morphological and developmental parameters related to pollutant acclimation or damage might better be determined on root samples from cores or on roots of uniform seedlings than through observations of roots growing against transparent barriers. Branching patterns of roots afflicted with aluminum toxicity, including stubby, brittle roots with thickened and brown root tips and an overall reduction in fine branching (Foy et al. 1978), have been found in damaged trees (e.g., Glatzel et al. 1986~. Subtle changes in branching patterns of roots could be quantified by utilizing the recently described topological approach of Fitter (1985, 1987~. Fitter (1986) showed that root systems exhibiting distinct topologies could be related to their space-filling and water transport efficiencies. Root samples from cores could also be utilized to determine mycorrhizal development in fine roots and element concentrations in fine root tissues. These topics are addressed by other papers in these proceedings (Antibus and Linkins; Shortle; Marx and Shafer). Finally, as root growth potential determined under standard conditions is an accepted measure of seedling physiological status (e.g., Ritchie 1982, 1985, DeWald and Feret 1987), root growth potential of uniform seedlings differentially exposed to air pollutants should provide an excellent, integrative measure of pollutant effects on root system developmental potential.
174 ROOT FUNCTIONING: WATER UPTAKE The presence of active roots in soil is commonly inferred by loss of water from the soil as determined by standard techniques utilizing neutron attenuation measures of water content (e.g., van Bavel et al. 196S, Nnyamah and Black 1977, Rambal 1984~. This technique can detect subtle differences in the seasonal and depth patterns and extent of water uptake. Unfortunately, it is ineffective in very shallow soil layers and is complicated to some extent by drainage. Nevertheless, if the depth distribution or seasonal pattern of water uptake of forest trees was affected by air pollution, this could be determined through neutron attenuation or other techniques for detecting water uptake by root systems (see Caldwell and Virginia 1988~. Here again, as with determinations of root production, temporal and spatial variation and availability of interspersed controls sets a limit on the inference that can be made from these measurements. Soil psychrometers, which measure water potential in a small volume of soil adjacent to the sensor, can also be used to infer the presence of active roots. Utilizing soil psychromters, Richards and Caldwell (1987) detected substantial diet fluctuations in the soil water potential of shallow, dry soil layers beneath transpiring sagebrush plants. These fluctuations were related to daytime depletion and nocturnal resupply of water conducted through roots from deep moist soil layers and leaked from active roots into the dry soil layers. The diet fluctuations were not present when active roots were not present. These diet patterns depend on the root morphology of sagebrush, which has a dense shallow root system and a relatively sparse deep root system. For trees with similar types of root systems, growing in situations where the shallow soil dries while the deep root system remains in moist soil, similar transport phenomena would be expected. If found, this phenomena, termed hydraulic lift (Richards and Caldwell 1987), could be used as an indicator of the activity of roots in localized zones in the soil. Since root water uptake can be detected on a daily basis with psychrometers, they would be most useful in short-term pollutant exposure experiments. ROOT FUNCTIONING: NUTRIENT UPTAKE Radioactive or rare chemical tracers have been used to determine the locations of active roots in soil (see Caldwell and Virginia 1988 for a review). Reduced allocation of photosynthate to roots because of air pollution exposure could result in reduced overall rooting depth, as has been noted following defoliation of forage species (Crider 1955, Troughton 1957), or temporary cessation of root activity at some depths. Such an effect on grass roots was noted following defoliation when uptake of labeled phosphate from the 1 5-25-cm soil layer was inhibited for 19-30 days (Oswalt et al. 1959~. While techniques such as these do not provide quantitative information about the intensity of root activity, they are easy to use and can be applied repeatedly on the same site, thus allowing evaluation of chronic reductions in root depth or spread. Dual labeling with radioactive or rare chemical tracers allows efficient determination of the relative importance of roots in different locations or at different times for nutrient uptake (Calc~well and Virginia 1988~. These techniques can be applied not only to investigating root depth but more importantly to providing a quantitative measure of the relative intensity of nutrient uptake by roots at different depths. An advantage of multiple tracer techniques is that the results they provide are an integration of processes in the soil, rhizosphere and plant. This is much more efficient and reliable at providing an ecological perspective than utilizing analyses of individual
175 processes. However, it does not provide information on the mechanisms affecting plant nutrient acquisition at these smaller scales. A dual-phosphorus-isotope labeling technique has been developed to investigate the relative competitive effectiveness of plants. This technique examines the phosphate uptake of an indicator plant simultaneously competing with two experimental plants (Caldwell et al. 1985, 1987~. The effectiveness of phosphate uptake by roots of the indicator Plant competing with different plants on opposite sides was determined by the ratio Of 2p and 3P in tissues of the indicator plant following labeling with carrier- free phosphate on opposite sides with one or the other isotope. Advantages of this technique were discussed in detail by Caldwell and Eissenstat (1987~. This technique could be adapted to investigate the effects of air pollution exposure on the nutrient uptake capacity of roots in trees of moderate size. Using an experimental design similar to that used by Caldwell et al. (1987), the phosphate uptake capacity of fumigated and control trees or trees exposed to ambient versus clean air could be compared. An isotope pair of iron (55Fe, 59Fe), although presenting different radiological hazards, might also be useful in this type of study. An important process for nutrient uptake by plants is proliferation of roots into nutrient-enriched patches in the soil (St. John et al. 1983, Robinson and Rorison 1985~. Recent experiments by Friend et al. (1987) have shown that nitrogen-deficient Douglas fir selectively proliferated roots in nitrogen-enriched ingrowth cores when the enrichment in the core was high enough to stimulate compensatory root growth. Because numerous ingrowth cores can be utilized for the same tree or for damaged and undamaged trees on the same site, this technique has potential for monitoring both the development of nutrient stress in trees that might result from chronic pollutant exposure as well as revealing changes in the capacity to respond to nutrient-enriched ingrowth cores. If ingrowth cores were enriched in a nutrient that became more limiting as air pollution damage increased (e.g. Mg, Beyschlag et al. 1987, Lange et al. 1987), the relative ingrowth into those cores should increase with damage until the level of damage finally reduced ingrowth capacity. For a nutrient that is already limiting, the capacity to grow roots into ingrowth cores would likely be reduced as the degree of damage increased. Potential advantages of this technique include the internal control provided by analysis of unenriched ingrowth cores, the ability to utilize damaged and undamaged trees within the same stand and the possibility of monitoring the same tree or site through time without substantial disturbance of the soil or plant system. CONCLUSIONS The processes of carbon fixation and allocation are directly affected in trees exposed to O3 and SO2. Substantial reductions in root production commonly result. In addition, nitrogen deposition on canopies or soil acidification reduces root growth and production. Because of this common response of trees to several of the effects of air pollution, measures of root growth or activity would be useful indicators of forest exposure to potentially damaging levels of air pollution. Root indicators may be particularly important in detection of effects of air pollution damage to trees because the effects on roots have been reported to occur before or with larger magnitudes than effects on shoot systems. While long-term or high intensity exposure of forests causes reductions in root growth, root depth and eventually total root mass, it is more important to detect changes in tree or stand function before severe damage has occurred. Root activity can be measured in the short term by techniques reviewed in this paper, among others, and some of these techniques could be adapted for use as indicators of
176 stress in forests exposed to pollution but not yet apparently damaged. Changes in patterns of root growth or development, water uptake and nutrient uptake would provide clues that damage was occurring. Interpretation of such changes would require concomitant data on tree exposure and studies of whole plant carbon and nutrient balance. Finally, technique attention in the application forest trees. development and sampling considerations will require careful of root measures as indicators of air pollution damage to SUMMARY 1. Techniques that have been discussed have promise for determining effects of air pollution exposure on roots of forest trees. All of these, however, need validation in research situations before they would be useful in monitoring. 2. Most potential effects of air pollution damage on root function are unlikely to be specifically symptomatic of pollutant damage because they could also result from other stresses. Some root morphological or developmental processes may, however, be symptomatic of toxicity to specific ions in the soil system or indicative of particular physiological perturbations. 3. Of all the techniques and research questions reviewed, four appear most promising for assessing root system damage in large trees due to air pollution exposure. These are: a. Minirhizotron observations of root growth rates and morphological and developmental parameters. b. Determination of root growth potential of uniform seedlings. c. Evaluation of root ingrowth into nutrient-enriched ingrowth cores. d. Dual-isotope-labeling techniques to evaluate nutrient uptake capacity. Assessments made using these techniques would only be reliable when applied to damaged and undamaged trees or experimentally exposed and control trees on the same sites. Utilizing multiple root measurements will provide much more reliable information than attempting to base root damage assessments on a single measure. Integration of root system studies with study of whole-plant functioning would provide additional benefits. ACKNOWLEDGMENTS I appreciate the many helpful comments on this paper provided by R.A. Black and R.T. Richards. Many of the techniques discussed were developed with support from NSF (BSR-8207171 and BSR-8705492) and the Utah Agricultural Experiment Station.
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