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0 USDA Forest Service, Rocky Mountain Research Station, 240 West Prospect Road, Fort Collins, Colorado 80526 USA
Received for publication August 31, 1999. Accepted for publication February 15, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1600 m at Pawnee Buttes to >3300 m at Rollins Pass. In this study we investigated two possible explanations for limber pine's success across a broad range of elevations: (1) the sites on which it is found, although separated by >1000 m elevation, may not be very different with respect to environmental factors that affect tree growth, and (2) limber pine growth is insensitive to environmental factors that change with elevation. We compared site characteristics of 12 limber pine stands at elevations ranging from 1630 to 3328 m as well as the growth and morphology of trees in each of these stands. Mean daily air temperature in July decreased linearly with the elevation of the site from 22.8° to 12.6°C. The growth and morphology of limber pine leaves, shoots, and trees were, in general, not related to the elevation or July mean air temperature of the sites. There was, however, a significant decrease in stomatal density with increasing elevation, which may be an acclimational response to restrict water loss at high elevations. Our data suggest that the fundamental and realized niche of limber pine is broad with respect to air temperature. In light of the high gene flow and only slight genetic differentiation among populations of species with bird-dispersed seeds, such as limber pine, it is especially unusual to see similar growth throughout an environmental gradient. Physiological and anatomical plasticity or wide physiological tolerance ranges may enable limber pine to uncouple its growth from its environment.
Key Words: acclimation • adaptation • altitude • fundamental niche • limber pine • phenotypic plasticity • Pinaceae • Pinus albicaulis • realized niche • stomatal density
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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3400 m in Colorado (Burns and Honkala, 1990
). In the northern Rocky Mountains and west, limber pine is generally found at lower elevations with whitebark pine (Pinus albicaulis Engelm.) occupying the higher elevations. In the southern Rocky Mountains limber pine grows at high-elevation sites and is displaced at the lower elevations by southwestern white pine (Pinus strobiformis Engelm.). In the central Rocky Mountains limber pine grows from below the lower tree line to the upper tree line, from 1600 m at Pawnee Buttes to >3300 m at Rollins Pass. Limber pine's elevational range is wider than any co-occurring tree species (Table 1). In the central Rocky Mountains, it is associated at low elevations with ponderosa pine (Pinus ponderosa Dougl. ex Laws.), Rocky Mountain juniper (Juniperus scopulorum Sarg.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), whereas at high elevations its associates include lodgepole pine (Pinus contorta Dougl. ssp. latifolia Engelm.), subalpine fir (Abies lasiocarpa (Hook.) Nutt.), bristlecone pine (Pinus aristata Bailey), and Engelmann spruce (Picea engelmannii Parry ex Engelm.). Why is limber pine able to exist in such a variety of locations, alongside species that are much more restricted in their elevational distribution?
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). It prefers to cache seeds on wind-exposed sites that accumulate little snow (Vander Wall and Balda, 1977
) and uses geometric relationships between landmarks to facilitate cache retrieval during the winter and spring (Kamil and Jones, 1997
). A single nutcracker may cache up to 30 000 seeds per hectare in one season (Lanner and Vander Wall, 1980
) but retrieves and consumes only 80% of them leaving the rest to be eaten by rodents or to germinate.
The caching patterns of nutcrackers not only affect where seeds are deposited but also influence the growth form of limber pine. Bird-dispersed trees grow as single-stemmed trees or in clumps or clusters (following the terminology of Tomback, Hoffman, and Sund, 1990
). If one seed from a cache germinates, an individual tree becomes established at the tree site. A clump of stems can be the product of a single germinant that has lost apical dominance, resulting in a multiple-stemmed growth form or a cluster of individual germinants from one cache. Genetic analysis is required to determine whether a clump of stems is one genet with multiple stems or a cluster of genets. Approximately half of all clumps are individuals with multiple stems (Carsey and Tomback, 1994
) revealing that loss of apical dominance at a young age is common for limber pine.
Another consequence of being bird-dispersed is the genetic structure of populations, which is often less differentiated among populations than that of wind-dispersed species (Bruederle et al., 1998
). Estimates of gene flow from the lowest to the highest elevation stands of limber pine in the central Rocky Mountains using molecular genetic analyses are inconsistent and depend on the technique applied (Schuster, Alles, and Mitton, 1989
; Mitton, 1995
; Latta and Mitton, 1997
). The necessary reciprocal transplant studies to examine the genetic-by-environment interaction on the phenotype of limber pine along an elevation gradient have not been done. In general, however, pines that are dispersed by nutcrackers tend to lack the family structure found in pines with wind-dispersed seeds (Furnier et al., 1987
; Bruederle et al., 1998
).
If limber pine lacks elevational races, we hypothesize that the effects of elevation on growth and resultant morphology of limber pine would be more obvious than for species that have undergone adaptations to local environments. A complex array of physical factors that affect plant growth vary with elevation: air temperature and atmospheric pressure decrease and precipitation and wind increase (see Friend and Woodward, 1990
). While there are numerous provenance studies with conifer species over a range of elevations (see Mitton, 1995
), there are surprisingly few studies that have quantified growth of mature trees at different elevations (see Tranquillini, 1979
). Native populations of erect trees of Picea engelmanni (Hansen-Bristow, 1986
), Pinus sylvestris L. (Grace and Norton, 1990
; James, Grace, and Hoad, 1994
), Pinus contorta (Schoettle, 1990
), Pinus pumila Regel. (Kajimoto, 1993
), Abies lasiocarpa (Hansen-Bristow, 1986
), and Abies koreana (Kang et al., 1990
) have shown reduced leaf length, shoot growth, and leaf production per year with increasing elevation. Similar effects of elevation on growth have been observed in native populations of a tree species in the tropics as well (Cordell et al., 1998
). High-elevation populations of Betula have reduced sexual reproduction compared to populations growing at lower elevations (Holm, 1994
), yet differences in shoot and leaf growth were not evident across an elevation range of 560 m (Kudo, 1995
). Herb, shrub, and broad-leaf tree species at high elevations have decreased specific leaf area, increased stomatal density, and different ratios of 13C to 12C compared to those at low elevations (Körner, Farquhar, and Roksandic, 1988
; Körner et al., 1989
; see Friend and Woodward, 1990
, and references therein; Vitousek, Field, and Matson, 1990
). The decrease in growth with increasing elevation is interpreted to be a symptom of increasing environmental stress.
In Colorado, limber pine tends to dominate stands only in dry locations (Peet, 1981
; Rebertus, Burns, and Veblen, 1991
), so it is unlikely that the usual increase in soil moisture with elevation is obvious at these sites. It is possible that through variation of site exposure with elevation, the expected variation in air temperature with elevation could be minimized. In this study we tested two possible hypotheses for limber pine's success across a broad range of elevations: (1) the sites on which it is found, although separated by >1000 m elevation, do not differ much with respect to environmental characteristics that affect tree growth, and (2) the growth and morphology of limber pine are not affected by the differences in environment with elevation.
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MATERIALS AND METHODS |
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Stand densities, by tree species, were measured using the point-centered quarter method (Mueller-Dombois and Ellenberg, 1974
). Tree height (HT, in metres), measured with a height pole and diameter at 1.37 m (DBH, in centimetres) were recorded for the nearest single tree or clump of trees in each quadrant with respect to each of the 10 sample points. A small core was also extracted from the north side of each tree stem at 1.37 m above the ground for quantification of bole growth for each of the previous four years. In addition, for each stem, the presence or absence of male and female cones was noted to determine whether the tree was reproductive.
Soil samples were collected from every other point-centered quarter-sample point for five samples from each site, (except the three sites within Rocky Mountain National Park) and analyzed for texture and nutrients. In the laboratory, the samples for a site were composited and sieved to separate soil (the particles < 2 mm in diameter) from gravel (particles > 2 mm). Visible organic matter was removed from the composited samples. Textures were determined using the hydrometer method of Gee and Bauder (1986)
as modified by Stohlgren (T. Stohlgren, personal communication USGS, Fort Collins, Colorado). Percentage nitrogen of the soils was determined using a LECO CHN-1000 element analyzer (LECO Corporation, St. Joseph, Michigan, USA). In addition pH (from paste), percentage organic matter (combustion), and P, K, Zn, Fe, Mn, and Cu (AB-DTPA extract) were quantified.
Tree and shoot morphology measurements
Because we know that tree size (Schoettle, 1994
) and growth form (Feldman, Tomback, and Koehler, 1999
) can affect growth, we standardized the size of the trees used for among-site comparisons; we selected ten single-stemmed limber pine trees per site that were between 5 and 7 m tall. Tree height, diameter at breast height (DBH), radial annual bole growth (COREGR), and the presence/absence of male and female cones were recorded for each tree. To avoid within-crown variation in shoot characteristics (Schoettle and Smith, 1991
), three shoots were collected from the south side of the upper third of the crown of each tree. The shoots were clipped at the base of the oldest annual growth increment with live needles. We determined the length of the foliated portion of the twig (FOL L, in centimetres), age of the oldest leaves (LL, in years), annual shoot extension growth (INCR, in centimetres), needle length (NL, in centimetres) and number of fascicles (FASC#) produced each year, and the dry mass of each annual leaf cohort (LEAFWT, in grams) and twig increment (TWIGWT, in grams) for each shoot (see Schoettle and Smith, 1991
, for details). From these data the mass per fascicle (WTFASC, in grams) and fascicle density (FD, number of fascicles per centimetre of twig) were calculated for the leaf cohorts formed in each of the years from 1993 through 1997. Statistical analyses (see below) were conducted on the average of these values from the three shoots per tree for years 1993–1997. In addition, the specific leaf area (SLA, in square centimetres of total leaf surface area per gram dry mass of leaf tissue), a measure of the robustness of the leaf, was quantified for current year (1997) needles. The total leaf surface area was calculated geometrically from measurements of needle length and width (measured at 8x magnification with Optimus v. 6.1 image analysis software). On those same leaves, the number of stomata per leaf surface area (SDEN) and number of stomata per needle volume (STVOL) were quantified (according to Illingworth [1975
] using the Optimus v. 6.1 image analysis software). All shoot and leaf samples were collected between 20 August and 24 September 1997.
Data analysis
All of the statistical analyses were performed with SPSS. We applied regression analysis to test for significant relationships between site variables (average daily air temperature in July and soil characteristics) and elevation. The botanical differences among sites were assessed by comparing the species presence data at all sites using Detrended Correspondence Analysis (DCA) (PC-ORD; McCune and Mefford, 1995
).
We applied ANOVA to detect whether the tree variables (FOL L, INCR, NL, FASC#, LEAFWT, TWIGWT, LL, FD, WTFASC, SDEN, STVOL, SLA) were affected by the elevation of the sites. An average value for each variable was computed for each tree for a sample number of ten for each site except for the Pond View site where only five trees were sampled. For those variables that were significantly different among sites, we used ANOVA with polynomial contrasts (based on the elevation or Tair of the sites) to test whether the relationship was linear, quadratic, or cubic. The assumption of normally distributed residuals and homogeneous variances among sites was tested and confirmed for each variable. All analyses were conducted on the full data set (all 12 sites) as well as on a data set where the lowest elevation site (Dave's Draw) was omitted. Unless otherwise noted, omitting Dave's Draw did not affect the interpretation of results. For those factors where the linear contrast with elevation was significant, a linear regression analysis was performed on the site means (N = 12) to generate a regression line (r2 = 0.33 is significant at a P = 0.05 level).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The growth characteristics of ten single-stemmed limber pine trees of similar height (5–7 m) at each site varied significantly among sites, although a relationship with elevation or Tair was not always clear (Table 5). Bole growth, as measured by mean annual ring width (COREGR), was variable and not simply related to elevation or Tair (Table 5). The length of the new twig produced each year (annual shoot growth, INCR) was only weakly related to elevation (r2 = 0.137) or Tair (Table 5). Leaf life span increased linearly with increasing elevation (Fig. 4). However, if the lowest elevation site (Dave's Draw) is omitted and the analysis is restricted to the forested elevation range of limber pine (2450–3328 m), the linear relationship between leaf life span and elevation is lost (P = 0.238). The length of the shoot that retained foliage (FOL L) varied among sites, yet was only weakly, nonlinearly, related to the elevation or Tair of the site (Table 5).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The 12 limber pine-dominated sites were typical for their elevations with respect to air temperature. The rate of change in mean July air temperature, Tair, with increasing elevation was -5.6°C/km, which is similar to the average lapse rate of -6.91°C;shkm for a 10-yr period calculated from 39 Colorado weather stations across a similar elevation range east of the Continental Divide (r2 = 0.93; data obtained from the Colorado Climate Center). The mean daily temperatures in July for the sites were also not unusual. The Jenny Lake site, at upper tree line, had a July mean temperature of 12.6°C, whereas the mean air temperatures at Red Feather and Woods Landing, near the lower tree line, was 18.2°C and 16.6°C, respectively (Table 2). Around the world, the mean July temperature for the upper tree line is 13°C and for the lower tree line is 17°C (Cogsbill and White, 1991
). Therefore the tree line sites that support limber pine are relatively typical with respect to air temperature. In addition, the other plant species (trees, shrubs, and herbs) varied predictably with elevation among the 12 sites, further suggesting that the growing conditions varied among sites.
Every site studied had limber pine growing singly and in clumps. The percentage of locations occupied by limber pine of the multistemmed growth form averaged 45% among our stands. This is similar to the percentage of clumps in limber pine stands in Utah (Lanner, 1980
) and Colorado (Carsey and Tomback, 1994
), but it is slightly higher than other stands in Colorado observed by Tomback and Linhart (1990)
and Schuster and Mitton (1991)
. Growth form did not vary predictably with elevation, therefore it is unlikely that the distribution of limber pine growing singly vs. in clumps contributes to the species' broad elevational range.
The environmental stress of increasing elevation that is apparent in the growth patterns of other tree species was less obvious for limber pine. Most of the growth characteristics of ten single-stemmed limber pine trees per site (tree heights of 5–7 m) were not strongly related to the elevation or air temperature of the site. The length of the new twig produced each year (annual shoot growth) was only weakly related to site elevation or Tair for limber pine in this study, in contrast to annual shoot growth, which decreased dramatically with increasing elevation for Engelmann spruce and subalpine fir (Hansen-Bristow, 1986
) and lodgepole pine (Schoettle, 1990
) in the central Rocky Mountains and to a lesser degree for whitebark pine in the Northern Rocky Mountains (Schoettle, unpublished data) (Table 7), and other species found elsewhere (see Tranquillini, 1979
). Shoot growth probably indexes the carbon gain of the shoot (Schoettle and Smith, 1991
) or the branch (Sprugel, Hinckley, and Schaap, 1991
; Stoll and Schmid, 1998
) during the current and prior year to its growth. The weak relationship between shoot growth and elevation suggests that carbon gain for limber pine may not be affected by elevation.
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; Schoettle and Fahey, 1994
; Reich et al., 1996
). Thus increased leaf life span with elevation has been suggested to be an acclimational response to stress, compensating for reduced shoot growth and foliage production per year and enabling trees with different growth capabilities to retain similar crown architectures and photosynthetic surface areas (Schoettle, 1990
; Schoettle and Fahey, 1994
). For most Pinus species, including limber pine (this study), leaf life span increases with increasing elevation (Ewers and Schmid, 1981
; Schoettle, 1990
). However, if the exceptionally low elevation site (Dave's Draw) is omitted and the analysis is restricted to the forested elevation range (2459–3328 m), the linear relationship between leaf life span and elevation is lost for limber pine. Limber pine also did not exhibit the same relationships with elevation and shoot morphological characteristics (decreased annual shoot growth and similar foliage retention per shoot) as other pine species (Weidman, 1939
; Schoettle, 1990
). This also suggests that limber pine is less stressed than other species by the elevation and Tair gradient.
Two fascicle and needle characteristics of limber pine varied with elevation in a different manner than has been observed in other species, implying either less stress or broader tolerances than other species. First, we saw an increase in specific leaf area of current-year leaves at the highest elevations for limber pine. In contrast, in natural populations of herb, shrub, and broad-leaved tree species, the specific leaf area of leaves decreases (Körner et al., 1989
; Vitousek, Field, and Matson, 1990
; Körner, Farquhar, and Wong, 1991
) or remains constant (Kudo, 1995
; Sveinbjörnsson, Nordell, and Kauhanen, 1992
) with increasing elevation. Similarly, in common gardens, specific leaf area of conifers decrease or remain unchanged from populations originating from high to low elevations (Zhang, Marshall, and Jaquish, 1993
; Zhang and Marshall, 1995
; Oleksyn et al., 1998
). Second, needle length decreased with elevation for limber pine to a lesser degree than observed for other conifers (Hansen-Bristow, 1986
; Steele, Coutts, and Yeoman, 1989
; Kang et al., 1990
; James, Grace, and Hoad, 1994
) (Table 7). Needle length has been shown to vary along other environmental gradients including growing-season length (Armstrong et al., 1988
), availability of water during the growing season (Fritts, Smith, and Stokes, 1965
; Isik 1990
; Raison, Myers, and Benson, 1992
), and growing-season temperature (Mikola, 1962
; Junttila and Heide, 1981
; Armstrong et al., 1988
). It appears that needle length is sensitive to the most limiting factor in any given environment but does not provide insight to help to identify that factor.
How can limber pine uncouple its growth from temperature changes from the upper to below the lower tree line, i.e., from mean daily air temperatures in July under 13°C to over 22°C? The rates of most physiological and biochemical processes are a function of temperature. Therefore, for growth to be insensitive to variation in air temperature, there must be (1) an adjustment of the morphology or physiology of the plant such that the temperature of the plant is not that of its surroundings, (2) an adjustment of the temperature optima for biochemical processes or the biochemical capacity directly, and/or (3) unusually broad temperature optima.
Leaf clustering reduces the wind speed around needles, increasing the boundary layer resistance, which enables the temperature of the leaves to be well above air temperatures during the day (Hadley and Smith, 1987
). Foliar density, a measure of the clustering of needles or fascicles on a shoot, increases near the upper tree line for Engelmann spruce and subalpine fir (Hadley and Smith, 1987
), Japanese stone pine (Pinus plumila; Kajimoto, 1993
), and others species (see Tranquillini, 1979
). But fascicle density did not increase along the elevation gradient for limber pine. This suggests that the lapse rate for the temperature of limber pine leaves is likely to be similar to that of ambient air temperature, as suggested by McNaughton (1984)
.
Whereas a close coupling of limber pine leaf temperatures to air temperatures throughout the elevation gradient may not overcome the temperature limitations of biochemical processes, it may avoid the increase in the leaf-to-air vapor pressure deficit with elevation predicted by Gale (1973)
and Smith and Geller (1979)
. Even so, the potential for greater water stress at high elevations, especially on dry sites, remains since the diffusivity of water vapor in air increases with elevation due to reduced atmospheric pressure. Stomatal density is correlated with stomatal conductance to water vapor (Nobel, 1983
) and can vary between individuals growing on dry vs. moist sites (Pinus ponderosa; Monson and Grant, 1989
). The decrease with elevation in stomatal density and number of stomata per leaf volume in limber pine is consistent with an acclimational response to reduce water loss at high elevations.
Stomatal density is inversely correlated with the concentration of carbon dioxide in the atmosphere for many species, including limber pine (Van de Water, Leavitt, and Betancourt, 1994
; Beerling and Kelly, 1997
, and references therein). Carbon dioxide concentration (mass per volume) decreases with increasing elevation, however carbon dioxide availability may decline only slightly with elevation due to the counteracting effects of decreased atmospheric pressure and temperature on gaseous diffusivity (Gale, 1972
; Smith and Donahue, 1991
; Terashima et al., 1995
). Studies of non-water-stressed species along elevation gradients have reported that stomatal densities among species increase (Woodward, 1986
) or remain unchanged (Körner et al., 1989
) with increasing elevation. Stomatal densities among broad-leaved woody species decrease with increasing elevation (Körner, Allison, and Hilscher, 1983
). The few studies that have examined variation in stomatal density of a tree species growing at different elevations have shown both unchanged and decreased stomatal densities with increasing elevation (Zelawski and Niwinski, 1966, as cited by Tranquillini, 1979
; Illingworth, 1975
; Hultine and Marshall, 1998
). Stomatal densities also decrease with elevation in limber pine. The decrease in stomatal density with elevation for limber pine suggests that conserving water may be more advantageous than facilitating carbon dioxide uptake for this species on dry, high-elevation sites.
Limber pine differs from most of its associated species in that its seeds are more widely dispersed due to birds. Species with bird-dispersed seed are less genetically differentiated across environmental gradients than those species whose seeds are dispersed only by wind (Bruederle et al., 1998
). Consequently, we might have expected limber pine growth to be affected to a greater extent by the factors that change with elevation than associated wind-dispersed species that are adapted (genetically differentiated) to local conditions along the elevation gradient (Grant and Mitton, 1977
; Rehfeldt, 1983, 1994a
). However, this was not the case, and the variation in growth of limber pine with elevation resembled that of whitebark pine, another bird-dispersed pine, to a greater degree than the other species. Limber pine may be an example of a genetic generalist, as described by Rehfeldt (1994b)
, characterized by wide tolerance ranges. A high capacity for physiological plasticity or broad physiological tolerances could be adaptive for a species with long-distance seed dispersal across elevations and low genetic differentiation. Alternatively, high physiological plasticity may mask or prevent the development of genetic differences among populations (Sultan, 1992
) and be responsible for a stabilized growth pattern and morphological phenotype along environmental gradients. Previous studies have suggested that limber pine has an especially wide range of physiological plasticity within and among individuals and sites (Mooney, Wright, and Strain, 1964
; Mooney, Brayton, and West, 1968
; Lepper, 1974, 1980
; Barrick and Schoettle, 1996
; Schoettle and Rochelle, in press
). Without long-term transplant studies to specifically address the genetic-by-environment interaction, we cannot determine conclusively the relative role of physiological plasticity vs. genetic differentiation. The variation in the physiology of natural populations of limber pine trees across the elevation gradient will be addressed in a companion paper (Schoettle, unpublished data), and the establishment of a reciprocal transplant study along the elevation gradient is in progress.
In summary, the fundamental and realized niche for limber pine is broad with respect to air temperature. Not only can the species persist on sites experiencing a wide range of air temperatures, limber pine growth and resultant morphology were not related to the elevation or seasonal air temperature at the sites. These data suggest that limber pine has a high degree of physiological plasticity.
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FOOTNOTES |
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2 Author for reprint requests (phone: 970 498-1333; FAX: 970 498-1010; e-mail: Schoettl{at}lamar.colostate.edu
).
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