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OCR for page 169
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
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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.
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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
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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
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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.
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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
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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
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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.
OCR for page 177
321.
177
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Persson, H. 1978. Root dynamics in a young Scots pine stand in Central Sweden.
Oikos 30:508-519.
Persson, H.A. 1983. The distribution and productivity of fine roots in boreal
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Rambal, S. 1984. Water balance and root water uptake by a Quercus coccifera L.
evergreen shrub. Oecologia (Berlin) 62:18-25.
Reich, P.B., R.O. Teskey, P.S. Johnson, and T.M. Hinckley. 1980. Periodic root and
shoot growth in oak. Forest Science 26:590-598.
Richards, J.H. 1984. Root growth response to defoliation in two Agropyron
bunchgrasses: Field observations with an improved root periscope. Oecologia
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Richards, J.H., and M.M. Caldwell. 1987. Hydraulic lift: substantial nocturnal water
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73:486-489.
Ritchie, G.A. 1982. Carbohydrate reserves and root growth potential in Dou~las-fir
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Ritchie, G.A. 1985. Root growth potential: principles, procedures and predictive
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principles, procedures, and predictive abilities of major tests. Proceedings of a
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Robinson, D., and I.H. Rorison. 1985. A quantitative analysis of the relationships
between root distribution and nitrogen uptake from soil by two grass species.
Journal of Soil Science 36:71-85.
Singh, I.S., W.K. Lauenroth, H.W. Hunt, and D.M. Swift. 1984. Bias and random
errors in estimators of net root production: a simulation approach. Ecology
65:1760-1764.
Smucker, A.J.M., J.C. Ferguson, W.P. DeBruyn, R.K. Belford, and J.T. Ritchie. 1987.
Image analysis of video-recorded plant root systems. Pp. 67-80 in H.M. Taylor
(ed.), Minirhizotron observation tubes: Methods and applications for measuring
rhizosphere dynamics. ASA Special Publ. No. 50. ASA-CSSA-SSSA, Madison,
WI.
St John, T.V., D.C. Coleman, and C.P.P. Reid. 1983. Growth and spatial distribution
of nutrient-absorbing organs: selective exploitation of soil heterogeneity. Plant
and Soil 71:487-493.
Representative terms from entire chapter:
root production
