mon and apparently well adapted large life form in semi-arid ecosystems in the western United States where shortage of water and nutrients and extreme.
P0res~~;olog-y Management ELSEVIER
Forest Ecology and Management 86 (1996) 229-240
Root system morphology and development of seedling and juvenile Juniperus occidentalis S. Krimer ‘, P.M. Miller * , L.E. Eddleman Department
of Rungeland
Resources,
Oregon
State University,
Corvallis,
OR 97331, USA
Accepted 22 February 1996
Abstract Root systems of 55 Juniperus occidentalis Hook. ssp. occidentalis (4-65 cm tall, 3-28 years old) were excavated at two sites in central Oregon. Above- and below-ground tree components were measured, and root system morphology was delineated into three phases of development. The prominence of tap root length and biomass declined as trees grew. In the tallest trees, excavated tap roots penetrated to 130 cm; lateral roots l-5 mm diameter accounted for most of lateral root biomass, but lateral root length was dominated by roots of less than 1 mm diameter. Total root length for the tallest trees was 152 m; lateral roots extended 5.7 m from the trunk, occupied 102 m2 of soil, and were concentrated in the upper 25 cm of the profile. Tree height was a good predictor of root parameters of biomass, length, tap root penetration, and lateral root extension with r2 values of 0X2-0.96. The period of major changes in foliage morphology and root system structure coincide with a significant decrease in fine root/foliage biomass ratio. Keywords: Root/shoot biomass; Root distribution; Root length; Regressions
1. Introduction
Evergreen, woody perennials are the most common and apparently well adapted large life form in semi-arid
ecosystems
in the western
United
States
where shortage of water and nutrients and extreme fluctuations in temperature and rainfall prevail (Solbrig, 1982). Seedling establishment and juvenile growth are critical periods in the life cycle of perennial species. Morphological and physiological attributes during these periods are important factors
* Corresponding author. ’ Present address: Department of Botany, Arizona State.University, Tempe, AZ, USA. 0378-1127/%/$15.00 PII SO378-
determining ecological relationships such as succession, competition, invasion, and dominance. Juniper-us occidentalis Hook. ssp. occidentalis has encroached on and replaced many shrub-grass dominated plant communities of the intermountain northwestern region of North America (Burkhardt and Tisdale, 1969; Burkhardt and Tisdale, 1976; Miller et al., 1994). Successful establishment and growth of J. occidentalis in semi-arid environments suggest the presence of adaptive morphological and physiological features such as extensive root systems (Caldwell et al., 1977) and large root/shoot ratios (Monk, 1966). Juniperus occidentalis must have evolved adaptive features to tolerate or avoid adverse abiotic and biotic conditions during the seedling and juvenile period of its life cycle. One such feature
Copyright 0 1996 Elsevier Science B.V. All rights reserved.
1127(96)03769-3
230
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Ecology
may be related to biomass allocation and root system development. The objectives of this research were to describe the structural and spatial development of root systems of seedling and juvenile J. occidentalis and to examine possible growth strategies based on the relative contribution of plant organs to total biomass.
2. Methods We collected data at two sites in central Oregon. The Barr Road site, located 12.8 km west of Redmond (44”16’N, 12 l”20’ W), is a west-facing slope at 1050 m elevation. The Powerline Road site is 10 km south of Prineville (44”12’N, 120”55’W) and is relatively flat at 1055 m elevation. Soils were similar at both sites, a well drained loamy-skeletal, Aridic Haploxeroll derived from Mazama volcanic ash over tuff, with a uniform silty loam texture down to a discontinuous hard pan at about 70 cm. The plant communities at the sites were similar to the Juniperus/Artemisia/Agropyron-Chaenactis association described by Driscoll (1964). Long-term annual precipitation at Redmond, OR, the nearest recording station which is about 10 km from the sites, averages 217 mm, mostly occurring as snow from November to January and rain in May and June (NOAA, 1982). The root systems of 55 J. occidentalis were excavated during August and September. All individuals were growing under Artemisia tridentata ssp. uaseyana shrubs. We randomly selected a minimum of five trees from each of seven height classes (l-4.9, 5-9.9, 10-19.9, 20-29.9, 30-39.9, 40-49.9, and 50-75 cm) for excavation. Each tree was marked at ground level, and its root system exposed using hand tools. In most cases, lateral roots and fine roots were completely extracted. Tap roots of several individuals in the larger height classes could not be completely excavated because they grew in cracks of the underlying volcanic tuff. During excavation, we characterized overall morphology of each root system. Maximum rooting depth was measured as the vertical distance from ground level to the tip of the deepest penetrating root. Lateral extension was recorded as the average horizontal distance between the tips of the five to six longest lateral roots and the center of the tap root.
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Root systems were washed in water to remove any remaining soil. Photographs were taken of each tree laying on a 10 cm* grid. The root system was carefully arranged on the horizontal to indicate the natural vertical distribution of the roots within the soil profile. The two dimensional arrangement KUfleeted the occurrence of roots with in each 10 cm grid as accurately as possible. Above-ground components of each tree were divided into juvenile awl-like foliage, adult scale-like foliage. branches, trunk, and dead tissue. Dead tissue was determined by color and included all grey, brown, and yellow foliage. Branches were considered dead if they did not bear any green foliage. Below-ground components were separated into tap and lateral roots. Roots were further divided based on diameter size classes into fine roots (1 mm or less). medium roots (1.1-5 mm), coarse roots (5.1- 10 mm), and large roots (over 10.1 mm?. The diameter of each lateral root and its location on the tap root relative to the soil surface were recorded during separation. We did not distinguish between dead and live roots. Dry mass of each component was measured after drying at 80°C to a constant mass. Root length was calculated from length/mass ratios of fine and medium root classes. Subsamples of fine and medium roots consisted of randomly selected, 5-cm-long root sections which were counted and weighed. When possible, ten sections were used; however. sample size varied with the amount of root material available in each diameter class. Root lengths of coarse and large roots were determined by direct measurement. Trees were aged by counting rings on a polished cross section from the base of the stem. Growth rings were counted independently under a microscope by two persons. Since ring counts were similar between persons, age was taken as the maximum number of growth rings counted. Summary statistics were calculated for each of the seven height classes. Results are presented as the mean -t 1 standard error. Predictive equations for below-ground parameters were developed by relating data on each belowground variable to tree height. To comply with the basic assumptions of parametric regression analysis, log-log transformed data (base-e values) were used
S. Kriimer
et d/Forest
Ecology
for computing regression constants when residual analysis indicated the necessity for transformation (Sokal and Rohlf, 198 1). The linearized form of the allometric model, In y = a + blnx, provided the best mathematical relationship between variables. Coefficients of determination can be relatively ineffective for expressing the closeness of fit and reliability of estimates for data that differ greatly in their magnitude (Whittaker and Woodwell, 1968). To express the spread of points from the regression lines more effectively, estimates of relative errors (E) were calculated. To eliminate the systematic bias introduced in the predicted values by logarithmic transformation of the variables (Baskerville, 1972, Sprugel, 19831, correction factors (CF) were calculated for each variable. The correction factor s2/2 accounts for the bias in the predicted value y in arithmetic units as y = e(‘ny+s ‘2), where s* is the estimated residual variance of the regression.
3. Results The number of trees in each height class ranged from 5 to 13 (Table I>. Average height and age ranged from 4.3 cm and 3.2 years in class 1 to 65.1 cm and 28.2 years in class 7. As trees grew taller, accumulated above- and below-ground biomass increased at an exponential rate. The smallest trees had a total above-ground biomass of 0.05 g, belowground biomass of 0.04 g, and a total root length of 34 cm. Trees in height class 7 had an average of 73.3 g foliage, 61.8 g branch/trunk, 26.0 g tap roots, 58.3 g lateral roots, and a mean total root length of 152.2 m. Trees in height classes 1 and 2 had 100% juvenile foliage. Juvenile foliage decreased steadily as the plants aged; the tallest trees in height class 7 had only 24% juvenile foliage (Table 2). The contribution of branches and trunk to above-ground biomass increased as the trees grew taller, while dead tissue declined. Total above-ground biomass was more than 50% of the total tree biomass in all size classes. As the trees grew taller, the relative mass of tree components and root length changed (Tables 1 and 2). The tap root contributed 49-55% of the belowground biomass in trees up to 20 cm tall. For trees over 30 cm tall, the tap root was 12% of the below-
and Manclgement
86 (1996) 229-240
231
ground biomass. Penetration of tap roots increased from 18 cm in 4-cm-tall trees to over 100 cm in trees with an average height of 65.1 cm. Average tap root length was greater than tap root penetration in all height classes because many tap roots branched. The length of the tap roots as a percentage of total root length declined from 59 to 3% as trees grew taller. Total root/total foliage biomass ratio was 2.1 in height class 1 trees, and ranged from 1.6 to 1.1 for trees in height classes 2-7 (Table 2). The more functional ratio of fine roots (under 1 mm in diameter, available for water and nutrient uptake) to green foliage (capable of carbon dioxide assimilation) declined from 2 in height class 1 trees to 0.18 in class 7 trees (Table 2). Root system development of seedling and young J. occidentalis progressed through three phases. Root systems of trees in height classes 1 and 2 were dominated by a long slender tap root that grew more or less straight downward, with a few laterals which were much shorter than the tap root (Fig. l(a)). Tap roots penetrated to 18 + 0.4 and 26 + 2 cm for trees in height classes 1 and 2, respectively (Table 1). The shortest tap root measured was 17 cm long. Class 1 and 2 trees had a similar number of lateral roots (Fig. 21, but lateral root mass (Fig. 31, length (Fig. 4), and root penetration of class 2 trees were twice those of class 1 trees. In the second phase (height classes 3, 4, and 5) the general character of the root system changed (Fig. l(b)). Some plants still had a dominant tap root, but the tap root of others had branched, and the deepest roots were branches originating from the tap root. Tap roots penetrated from 38 f 4 to 82 + 9 cm, but biomass and length of lateral roots surpassed that of tap roots (Table 1). During this phase, biomass of tap roots increased 740%, but biomass of laterals increased 1720%. The number of lateral roots increased and larger diameter laterals became more abundant as trees aged (Fig. 2). Trees with an average height of 35.7 + 1 cm (height class 5) had a root system that extended through the soil profile to a depth of 82 cm and had a maximum extension of lateral roots of 500 cm (Fig. 4). During phase 2 of J. occident&s root system development, the majority of lateral roots emerged from the tap root at soil depths of between 10 and 30
232
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et (II./ Forest Ecology
und Manqement
86 ~19961229-240
I Number of trees (n), height, and dry biomass of above- and below-ground tree components of the 55 J. occi&nrcrlis specimens excavated at the Barr Road and Powerline Road sites in central Oregon (mean + I SE). Mean total length of tap roots and lateral roots. penetration of tap roots (Tap pen.), and lateral root extension (Lat. ext.) are also given l__ll____Height class Table
il-4.9
cm)
2 (5-9.9
n Height km) Age (years,) Biomass (g) Juvenile foliage Adult foliage Branch Trunk Dead Total above Tap root Lat. root Total below Total tree Root length (cm) Tap root Lat. root Root ext. km) Tap pen. km) Lat. ext. (cm)
7
cm)
4.3 + 0.1
I3 6.9 f 0.4
3.2 f 0.2
5.2 + 0.81
0.02 0 0
f 0.01
0.07 0 0.02
+ 0.01
0.01 0.01
* ,004 + ,003
0.04 0.02
* 0.01 + 0.01
0.04 0.02
i. 0.01 * .003
0.15 0.06
0.02 0.04 0.08
+ .005 + .Ol k 0.02
3
4
(IO-19.9cm)
(20-29.9
9
8
12.7 * I.5 2.1 i 1.8
25.6 IX.5
0.73 0.11
cm)
6
7
(30-39.9cm)
(40-49.9cm)
(SO-75
5
8
5
i- 1.5
i 0.8 * 0.5
5.0 + I.1 1.69 + 1.5
6.5 rf; 0.X4 4.42 f 2.3
lo.i
_t 2.2
1X.2
Ir 5.7
0.2 k 0.09 0.37 * 0.1
1.7 + 0.8 2.46 f 0.63
2.69 + 0.72 5.0 + I.3
9.4 I
F 0.04 IO.02
0.12 -t 0.06 1.53 + 0.59 0.58 i 0.18
0.87 + 0.32 II.72 jl 3.6
1.24 rt 0.44 19.85 f 5.1
3.7 5 0.7 52.9 rt 10.4
0.05 f 0.01 0.1 I kO.03 0.25 + 0.06
0.61 -I: 0.29 1.19 f 0.45 2.72 i I
2.9X 4.60 7.59
+ 0.91 & 1.37 + 2.27
4.30 10.5
* 0.81 F 3.79
10.3 20.5
i 2.9 * 4.5
19.3
t 5.X
14.8 t 4.14 34.65 + 9.2
30.x 83.7
* 6.8 i 17.1
20 -i 2.4 14k 4.4
29 + 5.3 35+ II
84i22 294t
18 * 0.4 4.4 + 0.6
26f2 IO,2
8’ + 9
73 + 7
254+51
355--1-
* 0.01
cm. Laterals over 5 mm diameter at their origin accounted for an increasing percentage of lateral root number and mass. Most of these larger roots tapered rapidly near their origin and then continued out more or less horizontally with little or no taper. Smaller, secondary and tertiary laterals grew at many angles
Juvenile foliage (% total foliage) Branch + trunk (% total aboveground) Dead tissue (o/o total aboveground) Aboveground biomass (% total tree) Tap root (% total belowground) Tap root length (or0 total root length) Total root biomass/total foliage biomass Roots less than 1 mm dia./total foliage biomass
100 20 20 56 49 59 2.1 2.0
t 3.5 + 7.4
I.? I.8
il.45
h
24
44: 112.1 :-
32
-
to the vertical and some grew upwards towards the root crowns of Festuca idahoenses plants, where the 1. occidentalis roots proliferated and branched into many fine roots. Very few woody laterals grew vertically and formed sinker roots. During the third phase of root development (height
Table 2 Relative percentages of individual tree components and root/shoot Barr Road and Powerline Road sites in central Oregon, USA
I
65.i 28.2
cm)
257 i X5 1761 + 463
114
Height
35.7 21.4
i 1.0
44.8 27.0
+ 0.42 + 0.09
i I.0 + 1.6
5
ratios by height
class of 55 J. occidentalis
specimens
growing
class a 2
3
4
5
6
7
100 I4 I4
87 42 8
75 35 7
60 39 6
36 39 7
24 43 4
55 55 44
58 49 25 1.4 0.7
61 I5 I7 I.1 0.4
57 I2 10 I.4 0.4
63 12 7 1.1 0.2
63 I2 4 I.2 0.2
1.6 1.1
at the
S. Kriimer
et al./ Forest Ecology
Fig. 1. The phase 1 root system typical of J. occidentah 3, 4 and5 (b) pictured against a 10 cm* grid.
and Management
86 (1996) 229-240
233
from height classes 1 and 2 (a) and phase 2 root systemsof trees in height classes
234
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and Manugement
classes6 and 71, root systems resembledthe shape and gross morphology of those in phase 2 (Fig. I(b)), but root system dimensions increased as a result of both tap and lateral root growth (Fig. 5). Tap roots weighed 10 + 3 and 26 + 6 g in treesfrom the largest two height classes;lateral roots weighed 21 + 5 and 58 f 13 g (Table 1). The number of larger lateral roots continued to increase and larger laterals appearedat all soil depths(Fig. 2). Tap roots penetratedto a maximum depth of 130 cm. Roots 1-5 mm in diameter dominated the vertical distribution of lateral root biomassof trees in height classes6 and 7 (Fig. 31, but the vertical distribution of root lengths indicated the predominancein the soil profile of roots less than 1 mm in diameter (Fig. 4).
0
2
4
Number 0246
6
of Lateral Roots 02468
i
Class 2
Class 1
-m b
0246
Class 3
5 65
I
Class 4
I
0
2
4
6
8
IO
45 55 65
Class 5
75 ; 85
229-240
Root biomass continued to be concentrated in the upper 30 cm of the soil profile (Fig. 3). A 34-cm-tall J. occidenralis specimen had lateral roots that extended 5.7 m from the trunk of the tree and occupied a surrounding area of 102 m2. Fine lateral roots branched repeatedly over a short distanceand formed a dense mat of roots extending beyond the tree crown. This root mat occupied a volume of soil approximately one third the diameter of the total root system by about 20 cm thick (Fig. 5). Root systems of J. occidentalis overlapped and intermingled with root systems of various sized neighboring junipers, sagebrush,and other plants in the community. Root systemswere active during the August/September excavation period: we encoun-
i i 45
86 (1994)
’
0
02468
i
24681012
Class 7
Class 6 EJ P
m c Imm dia. ?Q 1 - 5 mm dia.
Fig. 2. The number of J. occidenfu~~~ lateral roots less than 1 mm and I-5 mm in diameter from trees in height classes 1-7 and their distribution at different depths in the soil.
S. Kriimer
et al. / Forest
Ecology
Weight 0 5r
0.008 --.--
and Munugement
of Lateral
Roots,
0.016 0.024 ‘--L- ‘-1 I
15
86 (1996) 229-240
0 fl
235
g 0.008
L-L0.016 .-
0.024 1
25 35 I !
Class 2
Class 1
0.8
0
0.4
0.8
1.2
]
Class 6
Fig. 3. The weight of lateral roots less than I mm and I-5 mm in diameter depths in the soil.
4
8
12
Class 7
from trees in height classes I-7
and their distribution
at different
S. Kriimrr
236
et cd./
Forest
Length 0 5-F
5 --
IO
15
ati
Ecology
of Lateral 20
Mxqement
86 (19961229-240
Roots,
cm
25
15 25 I Class 1
35
2
Class 2
45 -:
fs 551 z
1
’
~
65 i Class 3
m
Class 4
i ':
Class 5
I
75
0
1 800
1600
2400
5 15 25 35
55 65F Class 6
Fig. 4. The length of lateral roots less than 1 mm and I-5 depths in the soil.
/
mm in diameter
Class 7
from trees in height classes l-7
and their distribution
at~diffenmt
S. Kriimer
et d/Forest
Fig. 5. The phase 3 root system typical
Ecology
of J. occidenralis
tered numerous succulent white and pink roots. Coarse lateral roots frequently exhibited increased diameter growth with increasing distance from the root trunk. Coefficients of determination (r2) using tree Table 3 Relationships Tree component
between
tree component
( y)
Tap root biomass (g) Lateral root biomass (8) Fine root biomass (g) Coarse root biomass (g) Total belowground biomass (g) Tap root length km) Lateral root length (cm) Fine root length (cm) Coarse root length (cm) Total root system length (cm) Tap root penetration (cm) Lateral root extension (cm)
and Managemenr
86 (1996)
229-240
from height classes 6 and 7 pictured
237
against a 10 cm2 grid.
height as the independent variable indicated that height was closely correlated with all root parameters considered, with r* values of 0.82-0.96 (Table 3). Predictive capability was highest for total belowground biomass (r* = 0.96); the lowest relative error
( y) and tree height (x) N
Intercept
40 40 40 27 40 40 40 40 27 40 41 41
- 7.89 -9.16 - 6.92 -9.31 - 7.80 0.55 - 1.82 - 0.20 - 3.45 -0.40 1.87 -2.17
a
Slope b
r2
E
CF
2.62 3.13 2.29 3.21 2.89 1.50 2.79 2.09 2.90 2.43 0.68 1.90
0.95 0.95 0.95 0.90 0.96 0.88 0.95 0.95 0.82 0.95 0.92 0.94
0.57 1.20 1.68 1.93 1.77 1.72 I .86 1.68 2.34 1.67 1.22 1.59
0.16 0.24 0.14 0.22 0.16 0.15 0.19 0.14 0.36 0.13 0.02 0.11
238
S. Kriimer
et al./ Forest
Ecolo.gy and Munugement
was associated with the prediction of tap root biomass (E = 0.57).
4. Discussion Resistance of plants to desiccation is often a function of root penetration depth (Oppenheimer, 1960). Rooting depth of .I. occidentalis averaged 20 cm during the initial years of development and increased to over 100 cm in trees older than 30 years. However, J. occidentalis appears to have a shallower root system than similar-aged woody species from moisture limited environments. One-year-old Chrysothamnus naseousus and Chtysothamnus viscidifzorus seedlings, excavated at a site with soils and climatic conditions similar to those found in this study, had roots penetrating more than 76 cm deep (McKell, 1956). A rooting depth of over 1 m was reported for l-year-old Purshia rridenrata (Hubbard. 1957; Stanton, 19591, and Artemisia tridentata seedlings grown in a greenhouse had roots at 85 cm depth within 12 weeks (McKell, 1956). As J. occidentalis grows, the demands for water, mineral nutrients, and carbon dioxide to support growth increase. To maximize the uptake of water and minerals, the investment of a high proportion of biomass into roots appears advantageous for plants in semi-arid systems. However, in young J. occidentalis declining root/shoot ratios with increasing age and the consistently high contribution of foliage to total biomass suggest that other adaptive features are important. In contrast to some other evergreen species of the same age range, J. occident&s is capable of maintaining foliage biomass as the largest fraction of total standing mass throughout its seedling and juvenile growth periods (Ovington, 1957; Chew and Chew, 1965). The higher rates of CO, assimilation and lower investment of biomass and nitrogen per unit of foliage area of juvenile foliage compared with adult foliage (Miller et al., 1995) provides the carbon to support above-ground growth and allocation to fine roots during phase 1 of root system development. Biomass allocation in the root system shifted from the tap root to fine lateral roots with increasing size. An extensive fine lateral root system located in the upper soil layers represents a minimal cost both in
86 (1996)
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terms of construction and maintenance while pruviding maximum nutrient and water absorb&ng capacity (Miller et al., 1990; Fitter, 1994). The dense lateral root system of J. occidentalis allows moisture absorption from light rains and thunder showers when moisture only penetrates into the surface horizons of the soil. The tap root may serve primarily for water absorption from deeper soil layers, especially after moisture in the upper parts of the profile is depleted (Miller et al., 1992) and during the winter months when the topsoil is frozen (Caldwell and Richards, 1986). Juvenile awl-like foliage has certain advantages which enhanceestablishmentand early growth of J occidentalis, but which appear to become disadvantageousfor a larger tree (Miller et al., 1993; Miller et al., 1995). High rates of water loss, associatedwith high rates of CO, assimilation of juvenile foliage. appearto be a liability for a large tree in the semi-arid environment of eastern Oregon. The transition to :t more conservative use of resources associatedwith adult scale-like foliage is consistent with the overall resource use efficiency and performance of a stress tolerant long-lived evergreen tree. The period of major changes in foliage morphology and root sys tern structure coincide with a significant decreasein fine root/foliage biomass ratio. Access IO soil resourcesthroughout the year in combination with the evergreen nature of juniper foliage could allow 3. occident&is to grow and transpire year-around. once it is establishedon a site (Miller et al.. 1992). The positive spatial association of seedling and sapling J. occidentalis with sagebrush (Burkhardt and Tisdale, 1976; Eddleman. 1987) indicates that sagebrush may act as a nurse plant for juniper. Researchhas indicated nocturnal hydraulic lift in big sagebrush(Richards and Caldwell, 1987: Caldwell and Richards, 1989). Additional soil moisture available from hydraulic lift, in combination with decreased soil evaporation and decreased transpirational lossesdue to shadingfrom sagebrushcanopies, may be sufficient to sustaintranspiration and growth in J. occident&s seedlingsduring drying cycles. Juniper root systems are located in the zone of highest sagebrush root concentration (Sturges and Trlica, 1978; Richards and Caldwell, 1987) and therefore in the zone where moisture from hydraulic lift is likely to be most abundant. The additional
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moisture may allow seedling and sapling J. occidenr&s to maintain an extensive fine root system during short-term droughts and to resume below-ground activities with the onset of favorable conditions without the cost of replacing roots sloughed due to dry soils. Hydraulic lift promotes soil nutrient mineralization and absorption by roots (Richards and Caldwell, 1987) and is therefore likely to improve the nutrient status of J. occidentalis seedlings and saplings. Sagebrush concentrates nutrients from a large soil volume into the upper soil horizons under the crown (Doescher et al., 1984). Seedling J. occident&is growing in the proximity of sagebrush can absorb and incorporate these nutrients into long-lived tissues. Sequestering nutrients that were mined by sagebrush reduces nutrient acquisition costs for seedling and sapling J. occidentalis and perhaps augments the competitiveness of the juniper. Juniper-us occidentalis takes 20-30 years to develop root system extensions comparable to mature sagebrush plants (Robertson, 1943; Tabler, 1964; Sturges and Trlica, 1978). During this period, seedling and sapling juniper appear to be little influenced by competition from sagebrush and are able to eventually gain dominance in established sagebrush stands. Over the life span of the J. occidentalis, continuous immobilization of otherwise available nutrients by an evergreen tree may deprive sagebrush of a critical resource base and probably results in reduced nitrogen, phosphorus, and calcium turnover in sagebrush/juniper communities. Increases in dead sagebrush on sites with abundant juniper saplings indicate the definite competitive advantage of J. occidentalis. With time, the sagebrush dies out in many stands leaving a community composed of J. occidentalis and a few species of perennial grasses (Miller et al., 1994). Above- and below-ground structural changes observed in this research combined with documented functional changes may help to explain the competitive ability of J. occidentalis seedings and saplings. However, the adaptive significance of structural development and biomass allocation patterns in young J. occident&s cannot be fully explained by the analysis of a limited data set from two study sites. Growth patterns of J. occidentalis must be examined relative to its whole autecology, the functioning of
urul Manqement
86 (1996) 229-240
239
associated species, abiotic factors of the environment, and the interactions and changes of all these factors through time. Extrapolation will require verification with similar data sets from other sites. However, these growth patterns and relationships can be used as a preliminary model of the growth and development of J. occident&is in its natural environment. The regression equations developed can be used to estimate root system parameters from simple height measurements. Use of regression equations reduces the effort and expense in sampling and predicting below-ground biomass and root system components in seedling and juvenile J. occidentalis.
Acknowledgements
This research was supported in part by Crook County, Oregon and by the Agricultural Experiment Station, Oregon State University and is Technical Paper 10-932. We thank the Bureau of Land Management for the use of the research site and P. Doescher, G.W. Cox, J. Kummerow and one anonymous reviewer for reviewing drafts of the manuscript. References Baskerville, G.L., 1972. Use of logarithmic regression in the estimation of plant biomass. Can. J. For. Res., 2: 49-53. Burkhardt, J.W. and Tisdale, E.W., 1969. Nature and successional status of western juniper vegetation in Idaho. J. Range Manage., 22: 264-270. Burkhardt, J.W. and Tisdale, E.W., 1976. Causes of juniper invasion in southwestern Idaho. Ecology, 57: 472-484. Caldwell, M.M. and Richards J.H., 1986. Competing root systems: morphology and models of absorption. In TJ. Givnish (Editor), On the Economy of Plant Form and Function. Cambridge University Press, Cambridge, pp. 25 l-273. Caldwell, M.M. and Richards, J.H., 1989. Hydraulic lift: Water efflux from upper roots improves effectiveness of water up take by deep roots. Cecologia, 79: l-5. Caldwell, M.M., White, RX, Moore, R.T. and Camp, L.B., 1977. Carbon balance, productivity, and water use of cold-winter desert shrub communities dominated by C, and C, species. Oecologia, 29: 275-300. Chew, R.M. and Chew, A.E., 1965. The primary productivity of a desert-shrub (Larren tridentata) community. Ecol. Monogr., 35: 355-375. Doescher, P.S., Miller, R.F. and Winward, A.H., 1984. Soil chemical patterns under eastern Oregon plant communities
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