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Ecology |
University of New Mexico, Biology Department, Albuquerque, New Mexico 87131 USA
Received for publication May 23, 2002. Accepted for publication August 8, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: competition • flooding disturbance • invasion • Populus deltoides subsp. wislizinii • riparian management • Salicaceae • Tamaricaceae • Tamarix ramosissima
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Tilman, 1982
; Latham, 1992
). This can be especially true at the seedling stage, when the interaction of the environment with competitive pressures may result in subtle interspecific differences in growth that may increase in magnitude over time, determining the structure of the mature population. Understanding the ways in which competition acts in plant communities is especially important in the context of plant invasions. While it is frequently assumed that nonnative species have invaded a habitat through competitive exclusion of native species, often these invaders are competitively weak and therefore depend on disturbance to remove competitively superior native species (Crawley, 1987
; Hobbs, 1989
; Cronk and Fuller, 1995
; Burke and Grime, 1996
; Sher and Hyatt, 1999
). Identifying conditions under which this will occur is important ecologically as well as for management application.
Previous research has shown that substrate texture (defined by particle size [USDA, 1951
]) can be one of the most important environmental variables for determining seedling survival and growth and therefore potentially competitive success (Knox, Harcombe, and Elsik, 1995
; Reich et al., 1997
). Substrate type, including both particle sizes and amount of organic material present, will affect water-holding capacity, aeration, resistance to penetration by roots, and nutrient availability (Pantastico-Caldas and Venable, 1993
; Zak et al., 1994
; Knox, Harcombe, and Elsik, 1995
; Freitas and Mooney, 1996; Reich et al., 1997
). Although the relationship between substrate and plant productivity has been intensely investigated, how this relationship affects plant competition is still under debate, particularly in terms of how competition intensity is affected by resource availability (for reviews see Goldberg and Barton [1992]
and Bengtsson, Fagerstrom, and Rydin [1994]
). We investigated the impact of substrate type on competitive outcomes using species that naturally experience a range of substrate textures and nutrient levels and that have distributions likely to be affected by this interaction between competition and the environment.
The southwestern U.S. floodplain is an ecosystem in which the relationship between substrate type and competition may be especially important. Native plains cottonwood, Populus deltoides subsp. wislizenii (Eckenwalder, 1977
; these populations have been previously identified as P. fremontii), and introduced saltcedar, Tamarix ramosissima (identified previously as T. chinensis), co-occur as seedlings in high numbers where over-bank flooding has created a moist, open substrate (Taylor, Wester, and Smith, 1999
). Varying intensities and frequencies of floods deposit sediments of different textures, creating a gradient from sand to heavy clay substrates (Taylor, Wester, and Smith, 1999
). Previous research indicates that both species may prefer coarse to fine-textured substrates (McBride and Strahan, 1984
; Mahoney and Rood, 1992
) even when the coarse textures are lower in available nutrients (Sher, Marshall, and Taylor, 2002
). However, in a field study, Tamarix disappeared over time in the sandy sites where Populus was most dense (Sher, Marshall, and Taylor, 2002
). This suggested that Populus may be capable of competitively excluding Tamarix from these optimal sites, leaving Tamarix in areas where the substrate is heavier and perhaps where Populus does not grow as well. This research tests this hypothesis by investigating the effect of substrate environment on competitive dynamics between Populus and Tamarix.
Populus occurs primarily along rivers and is phreatophytic (i.e., uses groundwater). It produces wind- and water-dispersed seed in the spring and requires overbank flooding to create substrate for establishment (Horton, Mounts, and Kraft, 1960
; Fenner, Brady, and Patton, 1984
; Rood, Kalischuk, and Mahoney, 1998
). This flooding dependence may account for the decline of Populus along southwestern rivers since the extensive damming and channelization of the rivers (Howe and Knopf, 1991
; Crawford et al., 1993
; Busch and Smith, 1995
; Stromberg, 1998
). This species of Populus appears to depend more on sexual reproduction than vegetative spread (Howe and Knopf, 1991
), and seedlings are very vulnerable to desiccation during establishment (Fenner, Brady, and Patton, 1984
; Mahoney and Rood, 1998).
Tamarix was introduced to the American Southwest in the late 1800s and was described as invasive by the mid 1900s (Campbell and Dick-Peddie, 1964
; Robinson, 1965
). Although similar to Populus in several respects, Tamarix has several characteristics that may have allowed it to become so successful. It has been argued that Tamarix may be better able to take advantage of unsaturated soil sources than many native species because it is facultatively phreatophytic (Busch, Ingraham, and Smith, 1992
; Smith et al., 1998
); however, recent evidence suggests that Populus can also be facultatively phreatophytic (Snyder and Williams, 2000). Like Populus, Tamarix produces seed in the spring and it also readily exploits the same habitats for germination (Horton, Mounts, and Kraft, 1960
; Taylor, Wester, and Smith, 1999
). However, Tamarix produces seeds for many more months of the year, extending well past the historical spring flooding season, and seedlings may not be as flood-dependent as Populus (Horton, Mounts, and Kraft, 1960
). In addition, Tamarix has physiological and morphological adaptations that allow it to survive the high salinity, drought, and fire associated with decreased over-bank flooding (Horton, Mounts, and Kraft, 1960
; Busch and Smith, 1993
, 1995
; Segelquist, Scott, and Auble, 1993
; Shafroth, Friedman, and Ischinger, 1995
; Cleverly et al., 1997
; Smith et al., 1998
). All of these differences may contribute to establishment success of Tamarix over much of the Rio Grande floodplain since the cessation of over-bank flooding.
Promoting reinstatement of over-bank flooding to stimulate Populus establishment requires that we understand how these species interact as seedlings. Experimental work has confirmed that Populus can exert intense interspecific competitive stress on Tamarix in sand with a shallow water table (Sher, Marshall, and Gilbert, 2000
). However, it is not known whether Populus can grow well in clay and maintain its competitive superiority over Tamarix. Decreased flows of southwestern rivers can mean less deposit of large-particle sediments during flooding events; small-particle sediments are very common in the Middle Rio Grande and may be symptomatic of regulated rivers. It should also be noted that loss of fine sediments also can be symptomatic of regulated rivers, as they are transported downstream in the reach below the dam and are not replenished due to sediment entrapment in reservoirs.
The productivity or stressfulness of a given sediments texture depends in large part on the availability of water. In the floodplains of the southwestern U.S., floodwaters recede quickly and high rates of evaporative water loss from the surface create potential drought conditions soon after establishment. The root growth of seedlings of both Tamarix and Populus follows the declining water table that occurs after spring flooding (Shafroth et al., 1998). Populus experiences water stress when these draw-down rates are too rapid (Mahoney and Rood, 1991
; Segelquist, Scott, and Auble, 1993
; Stromberg, Tiller, and Richter, 1996
). Studies on the interaction of substrate type and draw-down rate on Populus show that growth is better in sand than in gravel when draw-down rates are moderate, but that growth in sand with no draw-down is poor (Mahoney and Rood, 1992
). There is less information about Tamarix response to draw-down and substrate texture; however, Tamarix does appear to also grow best with moderate water table decline (Horton and Clark, 2000
). Empirical observations also have suggested that they are negatively affected by shallow water tables (Horton, Mounts, and Kraft, 1960
; Campbell and Dick-Peddie, 1964
).
It is not known how Populus responds to draw-down in the range of finer substrate textures that are found in southwest floodplains or how Tamarix growth is affected by either substrate texture or draw-down rate across different textures. Due to the sensitivity of these species to water availability, draw-down rates are likely to affect competitive ability, but may do so differently depending on substrate type.
Our research addresses the following specific questions: (1) How do substrate texture and draw-down rate affect plant germination and growth of Tamarix and Populus, independent of density? (2) How is competition between Tamarix and Populus affected by different substrate texture and draw-down (i.e., resource availability) treatments? (3) What are the implications of these patterns of growth across substrate textures and competitive regimes for riparian management?
If the competitive superiority of Populus depends on a low-stress environment, we may expect the native's advantage to differ across substrates. Our predictions were that neither Populus nor Tamarix would grow well in sand with a shallow water table but that Tamarix would grow better than Populus in clay. Therefore, we expected competition of Populus over Tamarix to be greatest in sand with a declining water table and for this competitive hierarchy to be reversed in clay.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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).
Each pot consisted of a 1 m tall x 20 cm diameter PVC pipe closed at the bottom with a perforated plastic disc. Two millimeter diameter holes were drilled at 10-cm intervals on opposite sides of each pot so that water could enter. Pots were placed in 46 cm tall aluminum stock tanks and bottom-watered. This setup mimics the shallow water table of the floodplain environment and allowed us to control draw-down rate (Mahoney and Rood, 1991
, 1992
; Sher, Marshall, and Gilbert, 2000
). There were 11 tanks of 15 pots each: 5 sand (2 no draw-down, 3 draw-down) and 6 clay (3 draw-down and 3 no draw-down), which we randomly positioned within a matrix of 3 x 6 tanks (extra tanks were due to initial loss of rapid draw-down treatment; see above). Although tanks contained pots with only one substrate texture, there were three tanks for each substrate texture/draw-down treatment. The tanks were placed outdoors at the Los Lunas Agricultural Science Center, Los Lunas, New Mexico, USA. As tanks were outdoors and not insulated, they were therefore subject to seasonal conditions and solar radiation.
Within each substrate/water treatment, seeds were planted in pots to create one of 39 different density treatments in an additive series (Firbank and Watkinson, 1985
; Connolly, 1986
; Law and Watkinson, 1987
). We chose to maximize the number of different density-mixture treatments in the experiment rather than have fewer treatments that were replicated. The latter approach would have increased our predictive power for a few specific combinations, whereas our goal was to have power to describe the shape of the response. The exception to this was our lowest density, "no-competition" treatments for which we did need more specific predictive power; therefore, we replicated these (1 : 0, 0 : 1, and 1 : 1) three times for each species. Also, we included a greater number of different low-density treatments, for which there is likely to be the greatest amount of variation between densities.
Density treatments included seven monocultures for each species and 25 mixtures. Therefore, within each of the four substrate/water treatments, there were 45 pots representing 39 different density treatments ([7 mono. Tamarix + 7 mono. Populus + 25 mixtures of Populus and Tamarix = 39] + 6 additional low-density replicates = 45 pots). Planting density treatments were divided into three groups that had a range of densities and mixtures; each group shared a tank. Pots with similar densities were placed adjacent to each other within a tank to minimize edge effects. Due to a five-fold mass difference between newly germinated Populus and Tamarix, Tamarix planting densities were five times that of Populus. Also, this provided treatments that reflected the high ratios of Tamarix to Populus commonly observed in the field at germination (Sher, Marshall, and Taylor, 2002
). Pot densities ranged between 1 and 40 individual(s) per pot (Populus) and between 5 and 200 individuals per pot (Tamarix); the greatest total density treatment was a mixture of 40 Populus with 200 Tamarix.
Unfortunately, the number of density treatments in sand had to be reduced to accommodate low germination rates. Density treatments in sand therefore included 5 monocultures per species and 17 mixtures (vs. 7 and 25 in clay). For the treatment of sand with draw-down, there were enough seedlings to have replicates for eight of the mixtures and two monocultures per species in addition to the six low density replicates. Therefore within the sand/draw-down treatment, there were 45 pots representing 27 density treatments ([5 mono. Tamarix + 5 mono. Populus + 17 mixtures = 27] + 18 replicates = 45 pots), and the in sand/no draw-down treatment there were 30 pots representing 27 different density treatments ([5 mono. Tamarix + 5 mono. Populus + 17 mixtures = 27] + 3 replicates = 30 pots).
Seeds were collected in the field a month before planting. For Tamarix, the chaff was separated from the seed by hand, and for Populus the seed hairs were separated from the seed by blowing air through a sieve. We planted seeds on 4 July 1997, which coincided with the timing of flood-water recession and natural seedling establishment in the field. Seeds and seedlings were protected from extreme heat and desiccation for the first 2 wk with shade cloth and with top watering for the first 3 wk.
We planted seeds at ten times final needed density and counted total germinated individuals daily for 2.5 wk. Individual seeds could not be tracked due to their small size and their movement within a pot when watered. Germination scoring was conducted on a subsample of 66 pots (2 substrate textures x 3 replicates x [3 densities 50/50 mixture + 4 densities Populus + 4 densities Tamarix]). We then thinned (or transplanted) to treatment densities after 1 mo.
We initiated draw-down treatments on 25 August, after the seedlings' root systems were established. Starting water level for all tanks was 23 cm below the substrate surface (i.e., a full tank), therefore we were not able to simulate draw-down from a fully flooded state earlier in the season. Beginning 25 August, we reduced water level in the draw-down tanks 10 cm each day until there was only 10 cm remaining. This water level (10 cm) was maintained for the remainder of the experiment. The water level of no draw-down treatments was maintained at 23 cm below the substrate surface for the duration of the experiment.
We measured heights of a random sample of five individuals of each species from the center of each pot on 27 August, 6 October, and 5 November. Because there is evidence that these species are shade-intolerant (Howe and Knopf, 1991
; Taylor, Wester, and Smith, 1999
), change in height over time is a potentially important measure of competitive effect and response (sensu Goldberg, 1990
). At the end of one growing season (5 November 1997), we harvested aboveground biomass. Although a belowground harvest was also initially planned, it was not feasible due to the large size of the pots and the inability to distinguish between individuals' roots. These samples were dried at 35°C for 6 d. Within each pot, the five randomly selected individuals measured for height were weighed individually for an estimate of mean individual biomass per pot, and the remainder was weighed for total pot biomass by species.
Statistical analyses and model fitting
All data were checked for normality and heterogeneous variances and transformed as necessary. The relationship between the independent variables (species, substrate type, water treatment, and their interactions) and response variables (germination densities over time, heights over time, and final aboveground biomass) were analyzed using ANOVA (general linear model analysis of variance) and repeated measures ANOVA (PROC GLM SAS, 1990).
We assessed response to competition using three-dimensional response surface models created from mean height and biomass across density treatments (Table 1). In these models, the two axes representing the independent variables were densities of Populus and Tamarix. The third axis was the response variable of mass or height. Each species and treatment was modeled separately. If a good fit was not found using nonlinear models, we developed linear models (Sher, Marshall, and Gilbert, 2000
). To create these, we performed a constructed variable test (Weisberg, 1985
) based on the knowledge that the response (Y) decreases monotonically and therefore the shape is best described with a curved line, defined by X to some negative power (e.g., X–1). The specific negative power (X–1 or X–0.75) creates the best possible fit to the raw data. Thus the purpose of fitting the data to these models is to create a response surface that can be easily visually interpreted and with which predictions can be made regarding yield in different competitive environments.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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 | (1) |
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 | (2) |
), and A is a competition coefficient equivalency term. The high value of A for both height and biomass response reflects the greater competitive impact of one Populus individual relative to one Tamarix (Table 4). Tamarix response to competition was generally greater than that of Populus, but the patterns among substrates were similar. Competition was more intense for Tamarix in clay than in sand (Fig. 5 vs. Fig. 6), and like Populus, greater in draw-down (as reflected by the larger value for C in Table 4). While densities of Populus had a strong negative effect on Tamarix growth, intraspecific competition was often nearly as intense (as reflected in A). Within the clay treatments, Tamarix mass was strongly negatively impacted by competition with both species, while this competitive effect was more gradual for height response (Fig. 5A–C vs. B–D). Although both response variables of height and mass were best fit with the same type of model, the coefficient values differed such that response surfaces were very different in shape for height vs. biomass. In clay, predicted values for the space at which competition becomes appreciable (C) were greater for mass than for height, whereas the coefficient equivalency term (A) was greater for height than for mass (Table 4). This reflects the fact that increasing the density of all neighbors had a more immediate negative effect on Tamarix mass than on height. However, it took more Tamarix individuals to decrease height than it did to decrease mass with the same effectiveness of a single Populus.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). However, the floodplain environment where these species establish is highly variable; in order for a competitive advantage to be ecologically meaningful, it must be maintained over a range of conditions. Important sources of environmental variation for Populus and Tamarix in the field are substrate type and draw-down rate of the water table. We found significant effects and interactions between substrate type and draw-down on growth as well as on the competitive relationship of these species.
How do substrate texture and draw-down rate affect plant germination and growth of Tamarix and Populus, independent of density?
Both Populus and Tamarix had greater germination and growth on clay than in sand. While our first-year seedlings grew best in clay, field observations have found second-year Populus and Tamarix growing better in sandier substrates (Sher, Marshall, and Taylor, 2002
).
This difference between preferences of seedlings versus saplings may be due to abiotic demands differing at these stages. At establishment, Populus is highly sensitive to water availability, which is optimal in fine-textured sediments. It is possible that our low rates of Populus germination on sand were exacerbated by our removal of the seed hairs prior to planting. This was necessary to keep seeds from blowing out of the pots, but in the field these hairs may reduce desiccation of seeds. In any case, germinating seeds are clearly sensitive to moisture conditions, which can be affected by substrate texture. Once Populus seedlings' taproots reach the water table, aeration and penetration of the substrate (which is best in coarse sediments) may become more important for growth than water-holding capacity of the substrate. Evidence of this switch in response to substrate also can be seen in the growth trajectories of the different substrate types; while initial Populus growth was best in clay, growth decreased during the final sampling period, while continuing to increase in the sandy substrate.
Draw-down of the water table increased growth for both species, although Populus was more strongly affected. Our results are consistent with previous observations that both Tamarix (Horton, Mounts, and Kraft, 1960
; Campbell and Dick-Peddie, 1964
; Horton and Clark, 2000
) and Populus (Mahoney and Rood, 1991
, 1992
; Segelquist, Scott, and Auble, 1993
; Shafroth, Friedman, and Ischinger, 1995
) growth is stimulated by a moderate draw-down and poor in shallow water tables. Our direct comparisons of the two species suggests that Tamarix is less sensitive than Populus to shallow water tables, although Populus was still larger than Tamarix in this treatment. The source of the stress to the plants in experimental shallow water tables cannot be determined from this experiment but is likely to have been anaerobic conditions.
How is competition between Tamarix and Populus affected by different substrate texture and draw-down (i.e., resource availability) treatments?
Populus was a better competitor than Tamarix across experimental treatments; however, competition intensity did differ among these treatments. Our prediction was that competition would be most intense in the most productive environment; this was supported, even though our prediction of which environment this would be was not. Substrate texture and hydrology interacted such that competition was greatest in the clay and with draw-down of the water table—the highest productivity environment for both species.
Another important aspect of the effect of substrates on Tamarix was in the competitive response of biomass vs. height. Tamarix tended to lose its lateral growth first in response to increasing neighbor density, suggesting that light, and therefore height, was important. This pattern of mass being more rapidly diminished than height was seen across substrate types. However, in sand with draw-down, there appeared to be an effect of facilitation by intraspecific neighbors for Tamarix. It is interesting to note that the other case in which evidence of facilitation was seen was for Populus growing in clay with no draw-down. How inverse density dependence would occur under either condition cannot be determined in this experiment and does not appear to be a strong effect in any case.
Although Tamarix competitive ability (as reflected by B0 in the linear models and A in the nonlinear models) was improved in clay with no draw-down, Tamarix always had less of an effect on Populus than intraspecific neighbors, and in most cases Populus had a greater competitive effect on Tamarix than intraspecific neighbors.
What are the implications of these patterns of growth for riparian management?
These results suggest that during establishment, the native Populus may have the greatest competitive advantage in a wet, high nutrient environment where there is a healthy draw-down of the water table. Clay was a more productive substrate in this experiment; yet even in sand with no draw-down and the lowest productivity conditions, Populus competitively suppressed Tamarix, as measured by aboveground growth. Before using these results to make any inferences to management of populations, however, it is important to note that as this is a test of individual performance, thus there can be only limited inference to population-level responses. Furthermore, we did not measure root growth or uptake of water or nutrients in this experiment. We began our draw-down treatment late relative to the field conditions, insuring that most individuals' roots had reached the water table. Phreatophytes such as these are unlikely to compete for water if the taproots reach the groundwater; however, if the groundwater is not reached, the outcome of competition may be very different from what was observed in this experiment. Also, a measure of competition that included belowground biomass may have yielded a different interpretation.
Therefore, the applicability of this work must be in the context of actual field observations. Previous work had reported low growth and high mortality of Tamarix seedlings in areas where Populus seedlings were dense (Sher, Marshall, and Taylor, 2002
). Another study of mature populations also found slower growth and fewer stems of Tamarix when growing under Populus (Lesica and Miles, 2001
). These results as well as those of another pot experiment (Sher, Marshall, and Gilbert, 2000
) support the hypothesis that these patterns are due to Populus outcompeting Tamarix. It is important that in this experiment, even under abiotic conditions that were stressful for Populus growth, Populus had a measurable negative effect on Tamarix.
Water-table dynamics are likely to be of key importance for Populus' competitive success. Moderate water table declines are symptomatic of a healthy floodplain that has high water tables in the spring and then lowers at a speed appropriate to Populus seedling root growth (Mahoney and Rood, 1991
). Alteration of the rivers in the southwest with dams and levees has been cited as the predominant cause of declining Populus seedling establishment (Campbell and Dick-Peddie, 1964
; Howe and Knopf, 1991
; Crawford et al., 1993
; Busch and Smith, 1995
). It appears that river alteration can be dangerous to Populus not only when there are no overbank floods and/or water tables recede too quickly but also when it results in a stagnant water table. In our experiment, Populus' apparent competitive superiority is the most pronounced when the water table recedes. That Populus was the strongest competitor under high productivity conditions lends support to river management that stimulates Populus seedling establishment and growth, even without control for Tamarix establishment (e.g., appropriately timed floods, draw-down rates; Auble et al., 1993; Shafroth et al., 1998; Stromberg, 1998
). Preliminary efforts to do this have, in fact, proved successful (Taylor et al., 1999
).
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FOOTNOTES |
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2 Author for correspondence (anna{at}sher.com
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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