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1 Division of Biology, Kansas State University, Manhattan, Kansas 66506 USA; and 2 Department of Biology, Principia College, Elsah, Illinois 62028 USA
Received for publication January 26, 1999. Accepted for publication June 8, 1999.
ABSTRACT
Yucca glauca is a C3 evergreen rosette species locally common in the C4-dominated grasslands of the central Great Plains. Most congeners of Y. glauca are found in deserts, and Y. glauca’s morphological similarities to desert species (steeply angled leaves, evergreen habit) may be critical to its success in grasslands. We hypothesized that the evergreen habit of Y. glauca, coupled with its ability to remain physiologically active at cool temperatures, would allow this species to gain a substantial portion of its annual carbon budget when the C4 grasses are dormant. Leaf-level gas exchange was measured over an 18-mo period at Konza Prairie in northeast Kansas to assess the annual pattern of potential C gain. Two short-term experiments also were conducted in which nighttime temperatures were manipulated to assess the cold tolerance of this species. The annual pattern of C gain in Y. glauca was bimodal, with a spring productive period (maximum monthly photosynthetic rate = 21.1 ± 1.97 µmol·m·s) in March through June, a period of midseason photosynthetic depression, and a fall productive period in October (15.6 ± 1.25 µmol·m·s). The steeply angled leaves resulted in interception of photon flux density at levels above photosynthetic saturation throughout the year. Reduced photosynthetic rates in the summer may have been caused by low soil moisture, but temperature was strongly related (r = 0.37) to annual variations in photosynthesis, with nocturnal air temperatures below -5°C in the late fall and early spring, and high air temperatures (>32°C) in the summer, limiting gas exchange. Overall, 31% of the potential annual carbon gain in Y. glauca occurred outside the "frost-free" period (April–October) at Konza Prairie and 43% occurred when the dominant C4 grasses were dormant. Future climates that include warmer minimum temperatures in the spring and fall may enhance the success of Y. glauca relative to the C4 dominants in these grasslands.
Key Words: climate change • cold tolerance • evergreen • grasslands • Liliaceae • photosynthesis • tallgrass prairie • water relations • Yucca.
The North American genus Yucca, family Liliaceae, has 30–40 species, which range widely throughout the United States (McCleary, 1973
). Although a majority of these species grow in the arid southwest, the Great Plains Yucca, Yucca glauca Nutt., has a distribution that extends east into the Great Plains and north into southern parts of North Dakota (Great Plains Flora Association, 1986
). This species is unique because it is the only evergreen subshrub in these grasslands, yet its success may be related to several traits more commonly found in plants adapted to desert environments.
Two prominent characteristics of Yucca glauca, its evergreen habit and steeply angled leaves, co-occur commonly in desert plants but are seldom found together in grasslands. The evergreen habit is considered adaptive, in part, because the retention of photosynthetically active tissue throughout the year permits carbon gain during favorable climatic periods when other species are dormant (Monk, 1966
; Ormsbee, Bazzaz, and Boggess, 1976
; Waring and Franklin, 1979
; Lassoie et al., 1983
; Aerts, 1995
). Steeply angled leaves decrease light interception and leaf temperatures in the summer (Ehleringer and Werk, 1986
), but substantially more solar radiation may be intercepted during the winter and early spring. This is the period when most of the annual carbon gain occurs in a congener of Y. glauca, Y. brevifolia, as well as several other desert species (Sisson, 1983
; Smith, Hartsock, and Nobel, 1983
; Roessler and Monson, 1985
; Rasmuson, Anderson, and Huntley, 1994
).
A number of factors may limit photosynthesis in evergreen species in temperate climates, but annual as well as diurnal temperature variations may be particularly important. Periods of photosynthetic depression have been noted in many desert evergreen perennials, including Yucca species. These may occur in the summer in response to temperatures that exceed the optimum for C3 photosynthesis concurrent with periods of water stress (Sisson, 1983
; Smith, Hartsock, and Nobel, 1983
; Nobel and Smith, 1983
; Huxman et al., 1998
). However, Roessler and Monson (1985)
documented that high temperature, not water stress, was responsible for low midday photosynthetic rates for Y. glauca in northeastern Colorado.
In addition to the negative effects of high temperature, low temperatures may influence plant responses severely, ultimately determining species distribution patterns (Grace, 1987
; Woodward, 1987
). For example, low rather than high temperature tolerance was suggested to control local distribution of 14 evergreen Agave species in the southwestern United States and northern Mexico (Nobel and Smith, 1983
). In the central and northern Great Plains, where maximum temperatures may not be as severe as in desert environments, minimum (nocturnal) temperatures may be more important in determining annual carbon budgets and affecting the distribution and success of Y. glauca.
In this study, we document seasonal patterns of carbon gain for Y. glauca and present data from short-term manipulations of nocturnal air temperatures to identify some of the potential mechanisms that enable this species to persist in a highly productive C4-dominated grassland in northeastern Kansas. Our general hypothesis was that Y. glauca’s morphological and physiological similarities to "desert" species are key to the success of this species in this grassland. Specifically, we hypothesized that the evergreen habit of Y. glauca, coupled with its ability to remain physiologically active at cool temperatures, would allow this species to gain a substantial portion of its annual carbon budget during that time of the year when the dominant C4 grassland species were dormant. The annual pattern of carbon gain for Y. glauca has not been assessed, nor have the environmental factors that determine this pattern, yet both are important to understand in Y. glauca because it is the only species within a functional group (evergreen subshrubs) that is underrepresented in most temperate grasslands (Tilman et al., 1997
; Freeman, 1998
). Moreover, cold temperature limitations to plant success are of particular interest in light of recent reports of global increases in temperature minima and plant responses to these alterations (Easterling et al., 1997
; Alward, Detling, and Milchunas, 1999
).
MATERIALS AND METHODS
Research was conducted at the Konza Prairie Research Natural Area, a 3487-ha tallgrass prairie preserve in northeast Kansas (39°05' N, 96°35' W; 320–440 m elevation). Average monthly temperatures for Konza Prairie (30-yr record; Kansas State University Weather Data Library) range from a January low of -2.7°C to a July high of 26.6°C with an average annual total precipitation of 835 mm, 75% falling from April through October (Bark, 1987
; Hayden,1998
). The climate is continental and noted for its extremes, as well as significant interannual variability in temperature and precipitation (Knapp et al., 1998b). Study sites were located along two upland limestone outcrops, which consisted of stony, steep slopes of 20–30%. These were dominated by a matrix of perennial, warm-season C4 grasses [Andropogon gerardii Vitman., Sorgastrum nutans (L.) Nash., and Andropogon scoparius Michx.; Freeman and Hulbert, 1985
] and were typical of the types of habitats in which Y. glauca is found in this grassland. Soils (Benfield and Florence series) varied from shallow to moderately deep with numerous rock fragments both on the surface and throughout the profile.
Long-term studies
To determine the annual pattern of potential carbon gain for Y. glauca, field measurements of leaf-level gas exchange were made for an 18-mo period (January 1996–May 1997). Five mature plants were randomly chosen from the populations, tagged, and measured at least twice a month under saturating light conditions at midday. Leaves of plants angled upward at varied angles and were arranged in a symmetrical rosette. Typically, leaves varied from 20 to 40 cm in length; inflorescences were 1–1.5 m tall. Net photosynthesis (A) was measured at ambient conditions on a naturally oriented, mid-rosette leaf in each of the cardinal directions with a portable gas exchange system (LI-COR, Model 6200, LI-COR, Inc., Lincoln, Nebraska, USA) at midday (1100–1300). Clear, sunny days were selected such that photon flux density (PFD) was >1000 µmol·m-2·s-1 during all measurements. Environmental and plant parameters that were measured throughout the study period included: PFD, air temperature (Ta), leaf temperature (Tl), A, stomatal conductance (gs), and vapor pressure deficit (VPD). Air and leaf temperatures were measured with fine-wire copper-constantan thermocouples (shielded from direct sunlight for Ta and contacting the abaxial leaf surface for Tl; N = 4 per plant). PFD was measured with a quantum sensor oriented parallel to the sunlit leaf surface for leaves pointing in the four cardinal directions. Usually, PFD was measured on adaxial surfaces (N = 4 per plant) except in the winter when the abaxial surfaces of north-facing leaves were sunlit. Midday leaf water potential (
) was determined throughout the study period with a pressure chamber (PMS Model 1000, Plant Moisture Stress, Inc., Corvallis, Oregon, USA) on five excised leaves, each from a different individual.
After annual patterns of physiological activity were documented, the relationship between A and PFD was established for south-facing leaves (those exposed to maximal PFD) on five plants in May (a period of moderately high A prior to flowering). Neutral-density screens were used to alter PFD in the field. Plants were allowed to fully equilibrate (based on periodic gas exchange measurements) to alterations in PFD prior to recording data for the A-PFD relationship.
Short-term studies
To assess the importance of low-temperature limitations for A in Y. glauca and to establish the temporal dynamics of recovery from cold nocturnal temperatures, five mature plants were transplanted in December 1996 from the field into large containers (40 cm in diameter and 34 cm deep). These were grown in native soil under laboratory conditions beneath high-intensity (PFD > 1000 µmol·m-2·s-1) sodium lamps (Sunbrella, Environmental Growth Chamber, Chagrin Falls, Ohio). The growth photoperiod was adjusted to mirror the seasonal photoperiods that occurred over the course of the study. Plants were well watered and fertilized with a half-strength nutrient solution (Hyponex 15–30–15). After transplanting, plants were allowed to acclimate for 1 mo before gas exchange measurements began. All measurements were made at ambient growth conditions in the laboratory (Ta = 26.2°–31.6°C, mean = 29.6° ± 0.63°C, RH = 19.7–31.6%, mean = 25.1 ± 0.8%). To evaluate the effects of nocturnal low air temperatures, pretreatment A was measured prior to placement of the plants in a growth chamber (Percival Model I-35D, Pervical, Inc., Boone, Iowa, USA) at 0°C for the dark period (~14 h), and then 2 wk later at -5°C for a similar period of time. After each of these cold treatments, A was measured on ten random leaves at >1000 PFD at 20-min and daily intervals until A recovered to pretreatment levels.
To assess the effect that cold nocturnal air temperatures may have on A in Y. glauca in the field, nocturnal leaf temperatures were increased by covering plants at night. Ten mature plants were chosen at the field site; five were the same plants that were followed throughout the study period (control plants) and the remaining five plants (randomly selected from among individuals that were similar in size to the control plants) were used as the treatment plants. Treatment plants were covered at night with rigid polypropylene "domes" that were 30 cm tall by 30 cm in diameter (Northern Tier Co., Sheridan, Wyoming, USA). Nocturnal covering of plants resulted in a moderation of minimum Ta and reduced heat loss by infrared reradiation. Every day for 60 d, beginning 4 March 1997, the domes were placed on the treatment plants within 1 h of sunset and removed within 1 h of sunrise. Air temperature (Ta) was monitored with fine-wire copper-constantan thermocouples and a data-logger at the site (LI-COR, Model LI-1000). Nocturnal air and leaf temperatures (measured by attaching thermocouples to plants such that the junctions were pressed against the leaf surface) were equivalent when measured simultaneously in dome plants. Soil temperature (Ts) was measured at 5 and 10 cm depths with a digital thermometer (Omega HH-25TC, Omega Engineering, Inc., Stamford, Connecticut, USA) and a thermocouple immediately after domes were removed in the morning and immediately before domes were replaced at night. Midday A was measured intermittently on sunny days throughout this period on both treatment and control plants.
Statistical analysis
For the long-term studies, mean values of A presented are the mean of all sample dates (N = 3–5) by month. Means are an average of measurements for leaves from all four directions (N, S, E, W) unless otherwise indicated. Multiple regression analysis and stepwise procedures were performed to determine whether individual or combinations of the abiotic/biotic factors measured (Ta, Tl, , and VPD) were related to variations in net photosynthesis. For the short-term studies, laboratory and field measurements of A (N = 5 plants) were averaged for each time interval. Repeated-measures and split-plot, mixed-model procedures were used to determine whether treatment effects were significant (SAS, 1997
). Whole plot represented treatment and control effects in a completely random design, and subplot equaled time with each plant a block. Subplot analysis addressed the effects due to treatment, time, and treatment x time interaction.
RESULTS
Long-term field study
During this study, average monthly temperatures ranged from a January low of -9.0°C to a July high of 30.6°C, with an annual total precipitation of 738.1 mm (72% falling during the frost-free period, April–October; Fig. 1). These values compared favorably with the 30-yr monthly mean for Konza Prairie Research Natural Area. The mean monthly daily maximum PFD (from a horizontal sensor at an on-site meteorological station) varied from a low of 1081 and 1015 µmol·m-2·s-1 in January and December, respectively, to a high of 1984 µmol·m-2·s-1 in June. This pattern contrasts with that of PFD measured incident on south-facing Y. glauca leaves (Fig. 1). On these leaves, a bimodal pattern was evident with peaks occurring in March and November, outside the typical growing period of the dominant C4 grassland species (May–September). Photosynthetic light saturation in Y. glauca occurred at ~1000 PFD (Fig. 2). Both the adaxial surfaces of south-facing leaves and the abaxial surfaces of north-facing leaves were exposed to PFD above this level in both summer and winter (Fig. 2). Thus, PFD was seldom limiting to A on clear days any time during the year.
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). The midseason photosynthetic depression during mid-June through mid-September occurred when daytime air temperatures were above 32.0°C. Mean monthly A of >5 µmol·m-2·s-1 for clear days was measured in ten out of 12 mo. Annual patterns of stomatal conductance mirrored patterns of A throughout the study. In contrast, ranged from -1.4 MPa in April to a low of -2.4 MPa in June (Fig. 3). This low value occurred during the month with the highest mean A, suggesting that water stress did not strongly limit A at this time. Overall, 31% of the potential annual carbon gain for Y. glauca was estimated to occur outside of the "frost-free" period (November–April) in this grassland and 43% occurred in the months when the dominant C4 grass, A. gerardii, is not physiologically active (October–April; Knapp, 1985
).
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), and vapor pressure deficit (VPD) did not reveal any significant correlations between A and any additional individual or combinations of these factors. Analyses were performed for both the entire sampling period and for the frost-free period.
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DISCUSSION
The distribution of Yucca glauca in tallgrass prairie is restricted to uplands where grassland productivity is lowest (Briggs and Knapp, 1995
). The success of Y. glauca here is likely due to a reduction in competitive interactions with the dominant species. Competitive interactions may be minimized by temporal separation of the physiological activity of this evergreen species and the dominant grasses. The annual pattern of carbon gain in Y. glauca was bimodal, with spring and fall periods of higher photosynthetic activity (Fig. 3). Seasonal maximum A in Y. glauca was only slightly higher than maxima measured in Yucca congeners found to the southwest (Huxman et al., 1998
). The period of midsummer decline in A for Y. glauca coincides with the period of maximum biomass accumulation in the dominant grassland species (Knapp et al., 1998a). This atypical (for grasslands) C3 evergreen uses an extended period for C uptake, much of which falls outside the frost-free period or the typical growing season of the dominant C4 grassland species (May–Sept.; Knapp, 1985
). This pattern reflects a temporal separation of growth, a result of differences in photosynthetic temperature responses between the dominant species and Y. glauca. Indeed, the dominant C4 grass, A. gerardii, has a temperature optimum for photosynthesis of ~34°C (Knapp, 1985
) vs. ~23°C in Y. glauca. Niche separation based on physiological responses to seasonal temperature gradients has been identified previously as an important mechanism of resource partitioning in grasslands (Kemp and Williams, 1980
).
At upland sites in tallgrass prairie, water is often the resource that most strongly limits productivity during the growing season (Knapp et al., 1993
; Briggs and Knapp, 1995
). However, low midday 
in Y. glauca did not appear to impose a significant limitation on A during the summer months of this study. In fact, the lowest measured during the study occurred in the month of highest mean A. However, seasonal minima in soil moisture typically occur during the months of July and August in this grassland (Knapp, 1985
). Stomatal closure may have ameliorated midday during this period, thus, we cannot discount the possibility that A may have been partially limited by soil water availability at this time as well. In the rocky, shallow soils at this site, attempted measurements of soil moisture in the rooting zone were unsuccessful. When temperature was regressed with A in Y. glauca, a positive relationship was evident, and this relationship was improved by omitting temperatures >32°C (Fig. 4), suggesting that high temperatures limit A in midsummer. Similarly, in the shortgrass steppe in eastern Colorado, Roessler and Monson (1985)
concluded that high temperatures were responsible for the midseason photosynthetic depression in Y. glauca. Although only 36% of the seasonal variation in A in Y. glauca could be explained by this single factor (Tl), combinations of other environmental factors were not statistically significant.
Positive carbon gain in Y. glauca (A > 5 µmol·m-2·s-1) was measured in ten out of 12 mo and A > 1 µmol·m-2·s-1 was measured at daytime temperatures as low as 6°C (Fig. 3). From monthly mean values, we estimated that 31% of the potential annual carbon gain occurs outside the "frost-free" period and 43% occurs when the dominant C4 grasses are dormant. Although these estimates are certainly high due to the selection of clear, sunny days for measurements, clearly the potential exists for substantial carbon gain to occur in Y. glauca in the early spring and fall in the central Great Plains. The two months (January and February) where positive carbon gain did not consistently occur are months that historically have mean minimum daily air temperatures well below 0°C (based on 30-yr averages for Manhattan, Kansas).
Nocturnal temperature experiments in the laboratory and field reinforced the importance of cold nighttime temperatures in limiting photosynthetic activity in Y. glauca (Adams and Barker, 1998
). In the laboratory, net photosynthesis was reduced only briefly after exposure to a nocturnal Ta of 0°C, and full recovery was achieved within 5–20 min. Nocturnal air temperatures of -5°C resulted in an extended recovery period (3 d; Fig. 5) and visible leaf damage. Tissue damage from exposure to low temperatures was indicative that low temperatures may limit agave distributions at high elevations (Nobel and Smith, 1983
). Overall, these results suggest that nighttime temperatures at or near -5°C may delimit the nonphotosynthetic period in Y. glauca. These laboratory plants were not allowed to become cold-hardened, however, and it is possible that even lower nighttime temperature may be tolerated by Y. glauca in the field without substantial negative effects on photosynthesis the next day.
Results from temperature manipulations in the field supported the conclusion that exposure of Y. glauca to nighttime temperatures of 0°C did not substantially reduce photosynthetic activity the next day. The buffering of minimum air temperature by nocturnal dome treatments (treatment plants were always kept at or above 0°C) led to increased A in treatment plants, relative to the control plants (maximum increase of 136 and 26% overall, respectively) in the spring. Therefore, laboratory and field studies support the hypothesis that a significant environmental determinant of carbon gain in Y. glauca in these grasslands is temperature: low nocturnal temperatures in the fall and spring and high daytime temperature in the summer.
In summary, several plant and environmental factors may help explain the successful extension of Y. glauca's distribution from the arid Southwest into the upland grasslands of the Great Plains. The ability of Y. glauca to tolerate the most xeric sites within grasslands (where grass productivity is low) may be an important mechanism by which competitive interactions with the C4 grasses are reduced. The evergreen habit of Y. glauca contributes to its persistence in the tallgrass prairie through the retention of photosynthetically active leaves that can extend that portion of the year in which positive carbon gain can occur. In addition, the nearly vertical leaf orientation of Y. glauca allows increased solar interception in the fall, winter, and spring. Finally, the ability of Y. glauca to tolerate potentially freezing air temperatures at night and maintain photosynthetic activity on days when temperatures are moderate extends the period of potential C gain in the early spring and fall. Together, these attributes increase the potential for "nongrowing" season carbon gain. Thus, it appears that Y. glauca uses its desert traits as important mechanisms for persistence and success in central Great Plains grasslands. Of course, the long-term persistence of this species also depends on successful vegetative reproduction and seedling establishment. In a demographic study of the population used in this study, as well as another Kansas population, both asexual reproduction and seedling establishment were noted, with matrix models indicating overall population stability under the current climate (Maragni, 1997
).
Implications for climate change
Climatic limitations of species distribution are well supported, especially when considering the effect of minimum temperatures on plant survival (Grace, 1987
; Woodward, 1987
). Larcher (1995)
; suggested that the poleward spread of physiognomic vegetation types will be most affected by minimum temperatures and how well particular species can tolerate changes. Since minimum air temperatures may strongly limit potential carbon gain in Y. glauca in the "dormant season," changes in the climate of the Great Plains, especially increases in nighttime minima, could benefit this species more than the dominant grasses.
Current climate change predictions include a rise in the mean annual global temperature of 2°–4°C (Bazzaz, 1990
; deGroot, Ketner, and Ovaa, 1995
; Hall et al., 1995
; Teughels et al., 1995
). The most recent trends in temperatures show fairly constant maximum temperatures in the United States, with only small increases. In contrast, minimum temperatures are increasing at a faster rate almost everywhere (Easterling et al., 1997
; Alward, Detling, and Milchunas, 1999
). Such temperature alterations may remove limitations to A in Y. glauca earlier in the spring and later in the fall, periods when soil moisture is high (Knapp et al., 1998a). Because the dominant C4 grasses would still be limited by cool daytime temperatures, they would be less likely to respond. Thus, an increase in growth and productivity of Y. glauca may result if the dominant grasses are unaffected. In some Yucca species, an increase in plant size has been correlated with increased seed production (Huxman and Loik,1997
). How alterations in temperature may affect reproduction via seedling establishment (Huxman et al.,1998
) or clonal expansion is unclear, however. Nonetheless, an expansion of evergreen agave species has been predicted due to increased temperatures associated with global climate change (Nobel, 1996
), and perhaps species such as Y. glauca, which represent a relatively uncommon growth form in grasslands, and use a unique temporal window for C gain, can serve as bellwethers for climate change.
FOOTNOTES
1 Research supported by the NSF Konza Prairie LTER Program, The Nature Conservancy and the Kansas Agricultural Experiment Station (00-241-J). Shawn Conard provided valuable field assistance. 
LITERATURE CITED
Adams, W. W., and D. H. Barker. 1998 Seasonal changes in xanthophyll cycle-dependent energy dissipation in Yucca glauca Nuttal. Plant, Cell and Environment 21: 501–511. [CrossRef]
Aerts, R. 1995 The advantages of being evergreen. Trends in Ecology and Evolution 10: 402–406. [CrossRef]
Alward, R. D., J. K. Detling, and D. G. Milchunas. 1999 Grassland vegetation changes and nocturnal global warming. Science 283: 229–231.
Bark, D. 1987 Konza Prairie Research Natural Area. In D. Greenland [ed.], The climates of the long-term ecological research sites, 45–50. Occasional Paper number 44, Institute for Arctic and Alpine Research, University of Colorado, Boulder, Colorado, USA.
Bazzaz, F. A. 1990 The response of natural ecosystems to rising global CO2 levels. Annual Review of Ecology and Systematics 21: 167–169. [CrossRef][ISI]
Briggs, J. M., and A. K. Knapp. 1995 Interannual variability in primary production in tallgrass prairie: climate, soil moisture, topographic position and fire as determinants of aboveground biomass. American Journal of Botany 82: 1024–1030. [CrossRef][ISI]
deGroot, R. S., P. Ketner, and A. H. Ovaa. 1995 Selection and use of bio-indicators to assess the possible effects of climate change in Europe. Journal of Biogeography 22: 935–943. [CrossRef][ISI]
Easterling, D. R., B. Horton, P. D. Jones, T. C. Peterson, T. R. Karl, D. E. Parker, M. J. Salinger, V. Razuvayev, N. Plummer, P. Jamason, and C. K. Folland. 1997 Maximum and minimum temperature trends for the globe. Science 277: 364–366.
Ehleringer, J. R., and K. S. Werk. 1986 Modifications of solar-radiation absorption patterns and implications for carbon gain at the leaf level. In T. J. Givnish [ed.], On the economy of plant form and function, 57–82. Cambridge University Press, New York, New York, USA.
Freeman, C. C. 1998 The flora of Konza Prairie: a historical review and contemporary patterns. In A. K. Knapp, J. M. Briggs, D. C. Hartnett, and S. L. Collins [eds.], Grassland dynamics: long-term ecological research in tallgrass prairie, 69–80. Oxford University Press, New York, New York, USA.
———, and L. C. Hulbert. 1985 An annotated list of the vascular flora of Konza Prairie Research Natural Area, Kansas. Transactions of the Kansas Academy of Sciences 88: 84–115.
Grace, J. 1987 Climatic tolerance and the distribution of plants. New Phytologist 106: 113–130. [ISI]
Great Plains Flora Association. 1986 Flora of the Great Plains. University Press of Kansas, Lawrence, Kansas, USA.
Hall, D. O., D. S. Ojima, W. J. Parton, and J. M. O. Scurlock. 1995 Response of temperate and tropical grasslands to CO2 and climate change. Journal of Biogeography 22: 537–547. [CrossRef][ISI]
Hayden, B. P. 1998 Regional climate and the distribution of tallgrass prairie. In A. K. Knapp, J. M. Briggs, D. C. Hartnett, and S. L. Collins [eds.], Grassland dynamics: long-term ecological research in tallgrass prairie, 19–34. Oxford University Press, New York, New York, USA.
Huxman, T. E., E. P. Hamerlynck, M. E. Loik, and S. D. Smith. 1998 Gas exchange and chlorophyll fluorescence responses of three south-western Yucca species to elevated CO2 and high temperature. Plant, Cell and Environment 21: 1275–1283. [CrossRef]
———, and M. E. Loik. 1997 Reproductive patterns of two varieties of Yucca whipplei (Liliaceae) with different life histories. International Journal of Plant Science 158: 778–784. [CrossRef][ISI]
Kemp, P. R., and G. J. Williams III. 1980 A physiological basis for niche separation between Agropyron smithii (C3) and Bouteoua gracilis (C4). Ecology 61: 846–858. [CrossRef][ISI]
Knapp, A. K. 1985 Effect of fire and drought on the ecophysiology of Andropogon gerardii and Panicum virgatum in a tallgrass prairie. Ecology 66: 1309–1320. [CrossRef][ISI]
———, J. M. Briggs, J. M. Blair, and C. Turner. 1998a Patterns and controls of aboveground net primary production in tallgrass prairie. In A. K. Knapp, J. M. Briggs, D. C. Hartnett, and S. L. Collins [eds.], Grassland dynamics: long-term ecological research in tallgrass prairie, 193–221. Oxford University Press, New York, New York, USA.
———, ———, D. C. Hartnett, and S. L. Collins. 1998b Grassland dynamics: long-term ecological research in tallgrass prairie. Oxford University Press, New York, New York, USA.
———, J. T. Fahnestock, S. J. Hamburg, L. B. Statland, T. R. Seastedt, and D. S. Schimel. 1993 Landscape patterns in soil-plant water relations and primary production in tallgrass prairie. Ecology 74: 549–560. [CrossRef][ISI]
Lassoie, J. P., P. M. Dougherty, P. B. Reich, T. M. Hinckley, C. M. Metcalf, and S. J. Dina. 1983 Ecophysiological investigations of understory eastern redcedar in central Missouri. Ecology 64: 1355–1366. [CrossRef][ISI]
Larcher, W. 1995 Physiological plant ecology. Springer-Verlag, New York, New York, USA.
Maragni, L. A. 1997 Ecophysiology and demography of Yucca glauca in the central Great Plains. Master's thesis, Kansas State University, Manhattan, Kansas, USA.
McCleary, J. A. 1973 Comparative germination and early growth studies of six species of the genus Yucca. American Midland Naturalist 90: 503–508.
Monk, C. D. 1966 An ecological significance of evergreenness. Ecology 47: 504–505. [CrossRef][ISI]
Nobel, P. S. 1996 Responses of some North American CAM plants to freezing temperatures and doubled CO2 concentrations: implications of global climate change for extending cultivation. Journal of Arid Environments 34: 187–196. [CrossRef][ISI]
———, and S. D. Smith. 1983 High and low temperature tolerances and their relationships to distribution of agaves. Plant, Cell and Environment 6: 711–719.
Ormsbee, P., F. A. Bazzaz, and W. R. Boggess. 1976 Physiological ecology of Juniperus virginiana in oldfields. Oecologia 23: 75–82. [ISI]
Rasmuson, K. E., J. E. Anderson, and N. Huntly. 1994 Coordination of branch orientation and photosynthetic physiology in the Joshua tree (Yucca brevifolia). Great Basin Naturalist 54: 204–211.
Roessler, P. G., and R. K. Monson. 1985 Midday depression in net photosynthesis and stomatal conductance in Yucca glauca. Oecologia 67: 380–387.
SAS. 1997 SAS / STAT software: changes and enhancement through release 6.12. SAS Institute, Cary, North Carolina, USA.
Sisson, W. B. 1983 Carbon balance of Yucca elata Engelm. during a hot and cool period in situ. Oecologia 57: 352–360.
Smith, S. D., T. L. Hartsock, and P. S. Nobel. 1983 Ecophysiology of Yucca brevifolia, an arborescent monocot of the Mojave Desert. Oecologia 60: 10–17.
Tilman, D., J. Knops, D. Wedin, P. Reich, M. Ritchie, and E. Siemann. 1997 The influence of functional diversity and composition on ecosystem processes. Science 277: 1300–1302.
Teughels, H., I. Nijs, P. Van Hecke, and I. Impens. 1995 Competition in a global change environment: the importance of different plant traits for competitive success. Journal of Biogeography 22: 297–305. [CrossRef][ISI]
Waring, R. H., and J. F. Franklin. 1979 Evergreen coniferous forests of the Pacific Northwest. Science 204: 1380–1386.
Woodward, F. I. 1987 Climate and plant distribution. Cambridge University Press, New York, New York, USA.
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S. Niu, Z. Yuan, Y. Zhang, W. Liu, L. Zhang, J. Huang, and S. Wan Photosynthetic responses of C3 and C4 species to seasonal water variability and competition J. Exp. Bot., November 1, 2005; 56(421): 2867 - 2876. [Abstract] [Full Text] [PDF]
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