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(American Journal of Botany. 2001;88:1643-1649.)
© 2001 Botanical Society of America, Inc.

Ecology

Comparative vessel anatomy of arctic deciduous and evergreen dicots1

Dennis M. Gorsuch2, Steven F. Oberbauer2,4 and Jack B. Fisher3

2Department of Biological Sciences, Florida International University, Miami, Florida 33199 USA 3Fairchild Tropical Garden, 11135 Old Cutler Road, Miami, Florida 33156 USA

Received for publication October 12, 2000. Accepted for publication February 13, 2001.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Arctic tundra plant species exhibit striking variation in leaf character and growth form. Both are likely related to differences in vessel anatomy, and all may affect responses to climate changes in the Arctic. To investigate the relationships among leaf character, growth form, vessel anatomy, and susceptibility to freeze-thaw-induced xylem cavitation, xylem vessel characteristics were compared among six deciduous and six evergreen arctic dicot species of erect and prostrate growth forms. We hypothesized that deciduous and erect species would have larger and longer vessels than evergreen and cushion/mat-forming species. Vessel lengths, diameters, and densities were measured for each species. Theoretical vessel flow rates were calculated using Poiseuille's law for ideal capillaries. Flow rates were used to determine the susceptibility of vessels to cavitation induced by freeze-thaw events that may become more frequent with global warming. Vessel diameters were larger in deciduous species compared to evergreens, and in shrubs/trees vs. cushion/mat-forming plants. Vessel length distributions, however, did not differ for growth form or leaf character. Vessel density was greater in cushion/mat-forming species than in shrub/tree species. Deciduous plants showed a greater contribution to total conductivity by relatively larger vessels than evergreens. One of the deciduous species, Vaccinium uliginosum, is predicted to be susceptible to freeze-thaw-induced cavitation. These results have important implications for future arctic species composition and plant community structure.

Key Words: cavitation • cushion plant • tundra • vessel density • vessel length


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Global temperatures are expected to rise as a result of increasing atmospheric concentrations of greenhouse gases (Keeling et al., 1995 ). General circulation models (GCMs) predict climate warming will be most profound at polar latitudes, and evidence suggests that temperatures in the Arctic are increasing (Mitchell et al., 1990 ; Maxwell, 1992, 1997 ). The GCMs predict a 6–15°C increase of spring surface air temperatures in the Arctic within the next 50 yr (Gates et al., 1992 ; Manabe and Stouffer, 1993 ). Vegetation changes associated with elevated temperatures in the Arctic are expected to be substantial (Chapin, Hobbie, and Shaver, 1997 ).

Climate changes coupled with increased temperatures include the possibility of growing season freezes, drought, or increased precipitation during the growing season. Mean annual temperature increases are associated with greater seasonal temperature fluctuations, leading to potentially severe frosts (Cannell and Smith, 1986 ; Maxwell, 1997 ). The negative effects of freeze-thaw events are likely to be greater as the growing season progresses and plant activity is at its maximum. Although rainfall in the Arctic has recently been increasing, there is also a potential for increased variability in precipitation, which may lead to drought conditions.

Elevated and increasingly variable temperatures combined with longer growing seasons will have direct effects on the phenology and physiology of arctic species (Shaver and Kummerow, 1992 ; Arft et al., 1999 ; Starr, Oberbauer, and Pop, 2000 ). Arctic tundra plants show remarkable diversity in growth form and leaf traits. To some extent, growth form can be predicted from microhabitat. Deciduous and evergreen dwarf shrubs are often most abundant in wet tussock tundra, while deciduous and evergreen cushion and creeping plants are found mostly on drier, exposed ridges.

The degree to which arctic plants can tolerate or take advantage of these changing climatic conditions will depend on characteristics such as leaf character, growth form, phenology, allocation, and storage (Sørenson, 1941 ; Shaver and Kummerow, 1992 ). Under simulated climate warming experiments, deciduous plants in the Alaskan Arctic have been shown to take advantage of elevated temperatures more successfully than other growth forms (Chapin and Shaver, 1996 ). Among possible mechanisms to explain this ability are characteristics of their hydraulic architecture (Tyree and Ewers, 1991 ). The xylem of deciduous species is dominated by larger vessels that allow for rapid and efficient uptake of water and nutrients. These characteristics are associated with higher rates of photosynthesis and growth (Oberbauer and Oechel, 1989 ; Semikhatova, Gerasimenko, and Ivanova, 1992 ; Chapin and Shaver, 1996 ).

Blockage caused by embolism formation can severely reduce plant hydraulic conductivity and ultimately photosynthesis and growth (Sperry, 1986 ; Sperry and Pockman, 1993 ). Several studies indicate different embolism mechanisms in water stress vs. freeze-thaw events. There is strong evidence that water-stress-induced cavitation is caused by air seeding between interconduit pit membranes under high tensions and is a function of the diameter of the largest pores on the pit membranes rather than vessel dimensions (Sperry and Tyree, 1990 ; Sperry and Saliendra, 1994 ). Freeze-thaw-induced embolism, however, has been shown to be highly correlated with vessel dimensions (Sperry and Sullivan, 1992 ; Davis, Sperry, and Hacke, 1999 ). Thus, the same xylem anatomy characteristics associated with transport efficiency are also associated with increased cavitation susceptibility after freeze-thaw events (Sperry and Tyree, 1990 ; Sperry and Sullivan, 1992 ).

All xylem sap has dissolved gases that are forced out of solution during freezing to form bubbles in the ice. Upon thawing, these bubbles can either dissolve back into the sap or expand to obstruct the entire conduit (either vessel or tracheid). Embolism follows when the vapor-filled conduit continues to fill with gases diffusing from the surrounding tissue (Davis, Sperry, and Hacke, 1999 ). In order for freeze-thaw-induced cavitation to be avoided, the bubble pressure (pb) must exceed water vapor pressure (pwv). The pb is a function of the bubble's radius of curvature (R) and the surface tension (T) of the xylem sap. Therefore, cavitation will unequivocally occur for px ≤ pwv – (2T/R) where px is the xylem pressure following the thaw (Davis, Sperry, and Hacke, 1999 ). Susceptibility to freezing-induced cavitation should increase for more negative px and larger R. The px depends largely on transpiration rate and soil water potential, while R depends on the size of the xylem conduit.

Because plants with an evergreen leaf character are unable to avoid unfavorable freeze-thaw events, they are likely to possess hydraulic architecture dominated by tracheids and smaller vessels with narrow diameters and short lengths. These characteristics are advantageous under freezing conditions because smaller conduit volumes should result in smaller dissolved air contents and bubbles with a smaller R, and thus require a lower px to cause cavitation (Sperry and Tyree, 1990 ; Sperry and Saliendra, 1994 ; Sperry et al., 1994 ). Although several comparative anatomical studies of temperate deciduous and evergreen plants have been done, no observations of arctic plants exist that attempt to link hydraulic architecture to leaf character, growth form, and freeze-thaw-induced cavitation.

In this paper we evaluate the relationship among leaf character, growth form, and xylem vessel anatomical characteristics in arctic tundra dicots. We compare vessel diameter, length, and density in six deciduous and six evergreen species. Vessel diameter is also used to predict the contribution of small, medium, and large vessels to overall flow in each species. Because of a limited growing season length, we predict that deciduous species will have larger vessel diameters and lengths than evergreen species. Since vessels with larger diameters are likely to take up more space in the xylem, we expect vessel density to be greater in evergreen species. Lastly, we expect the proportion of flow contributed by larger vessels to be greater in deciduous species.

Cushion and mat-forming plants are expected to have smaller diameter vessels with lower density than shrub species. Since these plants are found on dry ridge sites, they are more exposed to high winds and hard freezes and experience these conditions over a longer period during winter than plants in less exposed sites. It is likely that they compensate for harsh growing conditions by developing fewer and smaller vessels with a greater proportion of fibers to withstand strong winds compared to shrubs growing in wet tussock tundra. Anatomical differences between plants of the two growth forms may have important implications for the overall structure of future arctic plant communities, particularly in the face of climate warming.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Samples for the study were taken from tundra in the northern foothills of the Brooks Range, Alaska near Toolik Lake (68°28' N, 149°34' W, 760 m above sea level). Vegetation communities near the site have been described in detail by Walker, Walker, and Auerbach (1994) . For vessel diameter determination, single branches were clipped from each of 30 plants of each species in June 1999. Sample plants were separated by at least 10 m to insure that branches were of different genotypes. Clippings varied from 6 to 10 cm in length and were preserved in airtight plastic bags containing a 70% ethanol solution. Ten branches of each species were collected for vessel length determination in August 1999. Samples were placed in airtight plastic bags with moistened Sphagnum moss and transported to the laboratory in Miami in a cooler within 48 h of collection. Upon arrival, plants were immediately placed in plastic cups in a growth chamber with the basal ends under water to maintain transpiration. The species used are listed in Table 1.


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  		Table 1. Classification of study species for leaf character, growth form, and habitat where collected. Species with upright branches (above the surface of the soil) were considered dwarf shrubs or small trees. Cushion/mat-forming species are horizontally spreading with meristems close to the soil surface

 
Xylem vessel diameter and density
Vessel diameters were measured on thin cross sections (10–50 µm) cut at the base of the current season's growth. Species with small-diameter stems (0.7–1.0 mm) were sectioned using a rotary microtome (Reichert-Jung, Model 2030, Nubloch, Germany). Stems >1 mm were sectioned by hand. An image analysis system (AgVision, Decagon Devices, Pullman, Washington, USA), attached to a Nikon microscope was used to measure vessel diameters at 100x magnification. Measurements were made by tracing the maximum and minimum distances across vessel lumens and taking the mean of these two values. At six equally spaced intervals (60°), all vessels within a pair of rays were measured. Means were calculated for each branch (N > 15 vessels/branch cross section), and a grand mean was determined for each species (N = 7 branches/species).

Mean vessel diameter and the Hagen-Poiseuille law were used to calculate the relative contribution of each conduit to hydraulic conductivity by dividing the diameter of each vessel raised to the fourth power by the sum of all vessel diameters raised to the fourth power. Vessels were then categorized into small, medium, and large size classes to determine the importance of each size class to total conductivity. Each vessel in the small size class contributed <1% to total flow. Each medium-sized vessel contributed between 1 and 2% to total flow, while each large vessel contributed >2% to total flow. These data were used to determine the relative vulnerability of plants from each growth form to embolism caused by freeze-thaw events. Vessel density at the base of the current season's growth per 1 mm2 of xylem area (N > 20) for all species was also measured (Ewers and Fisher, 1989 ).

Xylem vessel length
The latex paint infusion method (Ewers and Fisher, 1989 ) was used to determine distribution of vessel lengths of plants of each species (N = 7 stems/species). For all plants, the longest unbranched stems were selected for study. Based on preliminary measurements, these segments were longer than the longest vessels. Stems were defoliated with shears before they were cut off from the plant, and the cut proximal end was immediately recut under water and kept submerged to avoid introduction of embolisms into the xylem conduits. A dilute latex paint solution was then fed into the stem (Ewers and Fisher, 1989 ).

A 100:1 water to latex paint dilution was filtered through Whatman number 1 filter paper. The filter paper prevented particles with a diameter >5 µm from obstructing vessels and allowed particles with diameters of at least 0.2 µm (too large to pass through pit membranes) to enter the stems. The latex emulsion was gravity fed into the proximal end of the 2-cm stem segment from a 2-m column with a pressure of 0.02 MPa. The solution was allowed to pass through the stem until flow completely stopped, as indicated by the position of the meniscus at the top of the column. The stem segments were cut into four uniform lengths of 2.5 mm, giving a total of four size classes (2.5, 5, 7.5, and 10 mm). The basal (proximal) end of each stem surface was then shaved smooth with a razor blade, and the number of paint-containing vessels was counted (Ewers and Fisher, 1989 ). This procedure gave the raw vessel count in each of the four size classes. Vessels were counted as paint filled even if only partially filled with the latex paint. Vessel length distribution was calculated as shown in Table 2 and described below.


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  		Table 2. Example of a calculation of vessel length in a Salix alaxensis stem with a diameter of 1 mm

 
The raw vessel count at each increment represents the number of vessels continuous from the basal end of the stem. The difference between the raw vessel counts of a segment and the next distal segment represents the number of vessel ends in that segment. Assuming a random distribution of vessels in the stem, the difference between the number of vessel ends in a segment and the number of vessel ends in the next distal segment represents the rate of linear increase for vessels of this length class. This difference multiplied by the number of increments from the base (steps to zero) gives the number of vessels of that length class. This number can then be expressed as a percentage of the paint-filled vessels at the zero point. The sum of the calculated numbers of vessels in each size class should equal the raw vessel count at the zero point (Ewers and Fisher, 1989 ).

Statistical analysis
Data sets were tested for normality using the Shapiro-Wilk test. Percentage values for vessel length data were arcsine transformed prior to data analysis. Homogeneity of variance was determined by Levene's test statistic. A one-way ANOVA was used to compare variables between growth forms (at P < 0.05). The Bonferroni post hoc test was used to compare differences among species within each growth form. The SPSS software package (SPSS, 1998 ) was used for all tests.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Vessel anatomy of deciduous and evergreen species
Vessel diameters of most species were relatively small, with mean values between 10 and 15 µm (Fig. 1). Mean vessel diameter ranged from 8 µm for Cassiope tetragona to 26 µm for Vaccinium uliginosum. The mean diameter of deciduous species was 1.3 times greater than the mean diameter of evergreen species (Table 3, P < 0.05). A Bonferroni post hoc test attributed within-group differences among deciduous species to Vaccinium uliginosum and Salix alaxensis, both of which had significantly greater mean vessel diameters (Fig. 1, P < 0.05). The post hoc test also showed significant within-group differences in the evergreens caused mainly by Ledum palustre, Vaccinium vitis-idaea, and Cassiope tetragona (Fig. 1, P < 0.05).



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  		Fig. 1. Vessel diameter distributions of study species (N > 125 vessels/species). Vertical bars delimit range of the values, lower and upper edges of the boxes indicate the first and third quartiles, and inner horizontal bar indicates mean. Post hoc comparisons for species within leaf character groups are shown as letters below the boxes (lowercase for deciduous, uppercase for evergreens) by the Bonferroni multiple-comparisons test (P < 0.05)

 

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  		Table 3. Mean vessel diameter and vessel density (± 1 SE) for deciduous and evergreen shrubs and results of ANOVA comparing the two leaf character groups for both traits

 
The contribution of vessels of different size classes to total conductivity was calculated for each species based on vessel diameter (Fig. 2). For most species, the small size class was responsible for the majority of the conductivity. Vaccinium uliginosum was the only species in which the greatest contribution to flow was from the large size class (>40 %). For two of the evergreen species, Ledum palustre and Cassiope tetragona, the large size class represented <10% of the total flow, and >50% of the flow was due to the small size class. Overall, the deciduous species had a greater proportion of large vessels contributing to total flow than evergreens (Fig. 2, P < 0.05).



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  		Fig. 2. Contribution of three vessel size classes (small, medium, and large) to total conductivity in each study species and leaf character groups. Percentages indicate the amount of flow contributed by each vessel within each size class (N > 125 vessels/species)

 
Vessel density differed markedly among species (Fig. 3). The two species with the largest diameter vessels, Vaccinium uliginosum and Salix alaxensis, had the lowest vessel density. The species with the highest vessel density was the evergreen Empetrum hermaphroditicum. Because of large variation in vessel distribution of species within the evergreen and deciduous groups, vessel density was not significantly different between the two groups (Table 3). A correlation analysis showed a strong negative relationship between maximum vessel density and maximum vessel diameter (Fig. 4, r = –0.70), but the significance of the relationship is driven by values for V. uliginosum.



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  		Fig. 3. Vessel density distributions of study species (N > 20 cross sections/species). Vertical bars delimit range of the values, lower and upper edges of the boxes indicate the first and third quartiles, and inner horizontal bar indicates mean

 


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  		Fig. 4. Scatterplot of mean vessel density and mean vessel diameter for study species (r = –0.70)

 
The distribution of vessel lengths also varied considerably among species (Table 4). Maximum vessel lengths did not exceed 10 mm; in one species, Cassiope tetragona, vessel lengths were <2.5 mm. Because of the large variation among species, vessel length distributions of evergreens and deciduous growth forms were not significantly different (Fig. 5, P > 0.05). Maximum vessel diameter was not significantly correlated (r = 0.18, P > 0.05) with maximum vessel length for each species, in part because of the high ratio of diameter to vessel length in V. uliginosum.


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  		Table 4. Vessel length distributions for deciduous and evergreen species. Numbers under each size class are % (N = stems/species)

 


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  		Fig. 5. Mean vessel length distributions in four size classes for deciduous and evergreen species (N = 7 stems/species). Error bars are + 1 SE of the mean

 
Cushion vs. shrub vessel anatomy
Shrubs had significantly larger mean vessel diameters than cushion plants (Table 5, P < 0.05). Mean vessel diameters among cushion and mat-forming species showed less variability than shrub species (Table 5). Mean vessel diameters of this growth form varied from 11 to 13 µm, compared to a range of 8–26 µm in shrubs. Vessel density of cushion/mat-forming plants was significantly greater than in shrub plants (Table 5, P < 0.05). Variability among species of the same growth form followed the same trend as diameter (Table 5). Vessel length distributions were not significantly different between the two growth forms (Fig. 6, P > 0.05).


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  		Table 5. Results of ANOVA comparing cushion/mat-forming and dwarf shrub/tree growth forms for (A) mean vessel diameter and vessel density and (B) mean values (± 1 SE) for both variables. Post hoc comparisons for species within leaf character groups are shown for both variables. Letters indicate differences among species within each group by the Bonferroni multiple-comparisons test (P < 0.05)

 


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  		Fig. 6. Vessel length distributions in four size classes for cushion and shrub species (N = 7 stems/species). Error bars are + 1 SE of the mean

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Our results indicate that leaf character and growth form can predict some aspects of vessel anatomy. Mean vessel diameters and maximum vessel lengths were larger in deciduous species than in evergreens. As expected, mean vessel diameters were large and density was low in the riparian deciduous tree growth form (S. alaxensis), but it was difficult to explain these same trends in one of the deciduous shrubs (V. uliginosum).

Based on the Hagen-Poiseuille law, deciduous species were also determined to have a greater contribution from large vessels to overall theoretical flow than evergreen species (Fig. 2). This finding supports our hypothesis that deciduous species rely on larger vessels for water and nutrient uptake and is consistent with studies reporting greater rates of photosynthesis and growth found in these same species (Oberbauer and Oechel, 1989 ; Semikhatova, Gerasimenko, and Ivanova, 1992 ; Chapin and Shaver, 1996 ).

We also compared xylem anatomy of cushion and mat-forming plants vs. the shrub growth form. As predicted, vessel diameters in plants of the cushion and mat-forming growth form were significantly smaller than diameters of shrub plants. Unexpectedly, vessel density was greater in cushion plants than in shrubs. Although the smaller diameter vessels of cushion plants occupy less area within the xylem, the higher density results in similar proportional areas occupied by vessels in the cushion plants and in the erect shrubs. This result contradicts our hypothesis that cushion plants would have lower vessel density because of a greater investment in fibers per xylem area to withstand the strong winds along ridges. Because the prostrate growth forms remain within the surface boundary layer, these plants may not require the high proportion of fibers that we had initially expected.

Xylem conduit anatomy is often used to explain differences in conductivity and cavitation susceptibility (Ewers, 1985 ; Sperry and Sullivan, 1992 ; Langan, Ewers, and Davis, 1997 ). Davis, Sperry, and Hacke (1999) used conduit diameter to calculate an empirical threshold mean value (30 µm) above which plants are extremely sensitive to cavitation induced by freezing. The vessel diameter distributions (Fig. 1) for each species show that Vaccinium uliginosum has some vessels above this range, making it the most vulnerable species to freeze-thaw-induced cavitation. Indeed, following a mid-August 2000 freeze-thaw event at Toolik Lake, V. uliginosum and S. alaxensis exhibited wilting and leaf burning symptoms consistent with cavitation damage.

Sperry and Sullivan (1992) argued that bubble size in thawing conduits is determined by conduit volume rather than diameter, because the volume of water determines the volume of air frozen out of solution and the ultimate size of bubbles. Vessel length distributions did not differ among leaf character or growth forms, but they did differ considerably among species. Susceptibility to cavitation based on vessel volume would suggest that the two willow species with the longest vessels, Salix alaxensis and S. pulchra, might be more susceptible than predicted based on vessel diameter.

Because vulnerability to water-stress-induced cavitation depends on pit membrane properties rather than vessel dimensions, it is more difficult to predict the effects of drought on cavitation. A comparative anatomical study by Wagner, Ewers, and Davis (1998) of evergreen chaparral shrubs from temperate environments with much lower risks of frost but higher risk of drought showed much larger mean vessel diameters in comparison with the evergreen shrubs from this study. Xylem tension may be a better predictor of conductivity and cavitation during water stress events than dimensions of xylem conduits, which can be highly plastic.

The species in this study may have plastic growth and physiological responses to alterations in growing conditions so that anatomical parameters measured under present conditions may not represent those under future warmed conditions. Chapin and Shaver (1985) have shown that total biomass of the woody, graminoid, moss, and lichen functional groups in tussock tundra changes under different environmental manipulations. Although the greatest shifts in biomass allocation among functional groups occurred under nutrient addition, temperature increases also had large effects. A comparative anatomical study of Salix pulchra in controlled-environment chambers set at +5°C above current growing-season temperatures showed vessel diameters twice as large as those of the field grown plants reported in this study (Gorsuch, 2000) . These results indicate the potential for alterations in community structure and composition under future climate conditions based on differences in the plasticity of physiology and anatomy of evergreen and deciduous species. It is important to note that roughly 30% of low arctic vegetation is made up of graminoids, mosses, and lichens that are functional groups not represented in this study, but may play a significant role in community dynamics under predicted climate change conditions.

Changes at the plant community level may have important implications at the ecosystem level that may affect individual species responses. Any change in canopy height among the dominant deciduous species due to increased temperatures could affect snow retention and the albedo of the surface and therefore affect canopy and soil temperatures (Bonan, Pollard, and Thompson, 1992 ). These effects on the ecosystem energy budget are large enough that, if extensive, could permanently alter regional temperatures (Bonan, Pollard, and Thompson, 1992 ). Changes in species composition will have effects on nutrient cycling as well through differences in characteristics such as tissue turnover times and litter quality (Chapin, Hobbie, and Shaver, 1997 ). An overall increase in carbon storage is also expected to occur with increased temperatures caused by higher productivity of the deciduous species than the evergreens they replace (Shaver et al., 1992 ). However, if these increased temperatures are associated with an increase in the incidence of freeze-thaw events or drought during the growing season, evergreen shrub and cushion species will likely continue to be important components of arctic plant communities.


   FOOTNOTES
 
1 The authors thank the National Science Foundation Office of Polar Programs for partial support of this project through grant OPP-9615845. We also thank Jennifer H. Richards and Keith Condon for technical advice. Greg Starr, Lorraine Ahlquist, Flavio Moreno, Maureen Donnelly, and three anonymous reviewers made helpful comments on the manuscript. Back

4 Author for reprint requests (oberbaue{at}fiu.edu ). Back


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 DISCUSSION
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