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Distribution of verbesina virginica (asteraceae, frost weed) in central texas and possible causes.

Abstract.-Verbesina virginica (Asteraceae, Frost Weed) is a tall, perennial, herbaceous plant mostly found in the eastern United States. In central Texas, it occurs mainly under the canopy of Qitercits virginiana (live oak) or Uimns crassifolia (cedar elm). Its ecological niche and factors affecting its distribution are not well recognized. To better understand conditions limiting its distribution, a combination of transects and leaf level gas exchange measurements were completed. Transects were from below a woodland canopy into an adjacent grassland. Mean density below the canopy was 700 [+ or -] 120 (mean [+ or -] one SD) plants/100 [m.sup.2] decreasing to zero in the associated grassland. Ninety-nine percent of the plants were below the canopy. Soil depth along the transect was highly variable but not significantly different. A light gradient from 207 [+ or -] 53 [mu]mol/[m.sup.2]/s below the canopy to 2126 [+ or -] 71 [mu]mol/[m.sup.2]/s in the grassland was found. Maximum photosynthetic rates ([A.sub.max]) were 12.68 [+ or -] 1.40 [mu]molC[O.sub.2]/[m.sup.2]/s at the highest light level and changed very little as light levels increased from 400 to 2000 [mu] mol/[m.sup.2]/s. Dark respiration ([R.sub.d]) was 0.76 [+ or -] 0.07 [mu]molC[O.sup.2]/ [m.sup.2]/s. Stomatal conductance was 0.311 [+ or -] 0.110 mol[H.sub.2]O/[m.sup.2]/s at the [A.sub.max] and did not change significantly over the light levels tested. Transpiration was 6.10 [+ or -] 1.40 [mu]mo1[H.sup.2]O/[m.sup.2]/s at the [A.sub.max] and decreased with light levels, but not significantly until 100 [mu] mol/[m.sup.2]/s. Light saturation ([L.sub.sat]) was 288 [mu]mol/[m.sup.2]/s and light compensation ([L.sub.cp]) was 16 [mu]mol/[m.sup.2]/s. Results suggest that V. virginica is a shade adapted plant that can carry out gas exchange below the woodland canopy. Its [A.sub.max] is higher than typical understory plants and results do not explain why this species is not found in associated grasslands.

When a species is found in a given location, it is because that species can tolerate or requires the environmental conditions present in that area. Measuring population density of terrestrial plants is relatively easy to do (Van Auken et al. 2005), but sorting out the characteristics or factors that determine why a species is present or dominant where it is found is much more challenging (Begon et al. 2006). It is just as taxing to ascertain why similar species fit together in communities. Certain species are limited to open habitats, some to woodlands or forests, others seem to occur at the edge of communities, while others do not seem constrained. Limiting factors could be biotic or abiotic, but are not always easy to define. Certainly light levels, soil depth, soil moisture, nutrient levels, competition, biotic characteristics, or combinations of these factors are possibilities (Valladares & Niinemets 2008).

In the central Texas Edwards Plateau region, savannas are associated with upland or riparian woodlands or forests in many places (Van Auken et al. 1981; Van Auken & McKinlcy 2008; Van Auken & Smeins 2008). A species found in some of these communities is Verbesina virgimca L. (Asteraceae, Frost Weed) (Correll & Johnston 1979). It seems to be an understory species, forming almost mono-specific communities in some understory habitats especially on deeper soils, but isolated plants are found below the canopy in some communities (Enquist 1987). It can establish below some trees, but no studies were found concerning its light requirements, needs for establishment or successional status.

Comparison of other species growing in shady habitats have been done, and physiological differences between plants found in shade compared to those found in full sun are fairly well known (Begon et al. 2006; Valladares & Niinemets 2008). Plants growing in low light usually have reduced photosynthctic rates when exposed to high light levels, light saturation occurs at lower light levels, light compensation points are lower (photosynthetic rate equals respiration rate) and dark respiration is lower (Boardman 1977; Larcher 2003; Valladares & Niinemets 2008). Sun plants on the other hand have higher photosynthetic rates at high light levels, and also have higher transpiration and stomatal conductance rates (Young & Smith 1980). Adaptive crossover is displayed by some species allowing them to acclimate to high or low light environments, consequently they could have a broader ecological niche (Givnish 1988; Givnish et al. 2004).

In the present study, linear transects, perpendicular to the canopy edge, were carried out from below the canopy into the adjacent grassland to determine where the highest density of V. virginica plants were located. In addition, light levels and soil depth were measured along each transect. Based on most information about this species, it was hypothesized that it was a shade plant and would have characteristics of a shade plant. Consequently, photosynthetic rates, light saturation point, light compensation point, respiration, conductance, and transpiration were expected to be low when compared to sun adapted plants.

METHODS

Study species.--Verbesina virginica (Asteraceae, Frost Weed) is a 0.9 to 1.8 m tall, erect, unbranched, perennial, herbaceous plant mostly found in the eastern United States with Kansas, Oklahoma and Texas as its western limit of distribution (Correll & Johnston 1979; USDA 2009). In central Texas, it occurs mainly under the canopy of Quercus virginiana (live oak) or Ulmus crassifolia (cedar elm) and on deeper soils in some of these communities. It sometimes forms almost mono-specific communities in understory habitats especially on deeper soils including some riparian soils. In addition, isolated plants are found below the canopy in some upland communities (Enquist 1987). Its main stem has four to five prominent wings, which are usually absent from the highly branched head region. It has large ovate to oblong-lanceolate, pubescent leaves and it flowers from late summer through fall. The flower heads usually have three to four white to greenish white ray flowers and up to 15 disk flowers. It seems to tolerate high temperatures but not dry or compacted soil. The type of rooting system of the plants is unreported.

Study tfrax-This field study was carried out on the southern edge of the Edwards Plateau region of central Texas just south of the Balcones Escarpment in northern Bexar County (Correll & Johnston 1979; Van Auken et al. 1981; Van Auken & McKinley 2008). The Balcones Escarpment consists of a rough, well-drained area, with elevations increasing from approximately 250 m above mean sea level (AMSL) at the southern edge to between approximately 500 and 700 m AMSL near the center, but in most places the increase in elevation is abrupt. This study area was about 350 m AMSL near the low end of the escarpment and at the upper edge of the Cibolo Creek floodplain. Most of the subsurface of the area is Cretaceous limestone, and soils are usually shallow, rocky or gravelly on slopes, and deeper in broad valleys and flats (Taylor et al. 1962; NRCS 2006). Soils are dark colored and calcareous with usually neutral or slightly basic pH.

Mean annual temperature of the area is 20.0 [degrees] C with monthly means ranging from 9.6 [degrees] C in January to 29.4 [degrees] C in July (NOAA 2004). Mean annual precipitation is 78.7 cm and bimodal, with peaks occurring in May and September (10.7 cm and 8.7 cm, respectively), with little summer rainfall and high evaporation (Thornthwaite 1931; NOAA 2004). However, rainfall is highly variable and rarely average.

Area vegetation-Juniperus-Quercus savanna or woodland is the community type in the study area and is representative of savannas and woodlands found throughout this region, but higher in woody plant density than savanna communities farther to the west (Van Auken et al. 1979; 1980; Van Auken et al. 1981; Smeins & Merrill 1988). The high density woody species are Jimipenis ashei (Ashe juniper) and Quercus virgin/ana (=Q. fusiformis, Live oak) followed by Diospyros texana (Texas persimmon) and Sophora secundijlora (Texas mountain laurel). Ulmus crassifolia (cedar elm) is found in these communities, but usually at lower density and on the deeper soils. Associated with these woodlands are relatively small grasslands and sparsely vegetated intercanopy patches or gaps (openings in the woodlands) (Van Auken 2000). The major herbaceous species below the canopy is Carex planostachys (cedar sedge) (Wayne & Van Auken 2008) or in the current study sites it was V. virginica. In the grasslands and gaps Aristida longiseta (red three-awn), Bouteloua curtipendula (side-oats grama), Bothriochloa (=Andropogon) laguroides (silver bluestem), B. ischaemum (KR bluestem), various other [C.sub.4] grasses, and a variety of herbaceous annuals are common (Van Auken 2000).

Transect measurements-Each study site consisted of a stand of V. virginica plants located under a canopy of Q. virginiana or U.crassifolia and an adjacent grassland. There were five V. virginica stands sampled. A transect was established through the approximate center of each stand. Stands ranged in size from approximately 400 [m.sup.2] to over 10,000 [m.sup.2]. Transects were 10 m in length, perpendicular to the canopy edge and centered on the canopy edge or drip line. Contiguous 0.5 [m.sup.2] quadrats were sampled along each transect. Each quadrat was 0.25 m wide and 2.00 m long with the long axes parallel to the canopy edge. All V. virginica plants within each quadrat were counted and standardized to plants/100 [m.sup.2].

Light levels and soil depth were also measured along each transect. Light levels were measured at 0.50 m intervals along each transect using a LI-COR [R] LI-190 SA integrating quantum sensor. A total of 105 measurements were made, 21/transect, and values were averaged for each location along the transects. Soil depth was measured at the same points along the transects. A 1.5 cm diameter iron bar was driven into the ground until it wouldn't penetrate any deeper, removed, depth was measured, and values were averaged (Van Auken 2000).

Gas exchange measurements.-Gas exchange rates were measured as a function of light level or photosynthetic flux density (PFD) and plotted for leaves of V. virginica plants growing in shade (Hamerlynck & Knapp 1994; Furuya & Van Auken 2009; Wayne & Van Auken 2009). There were five separate plants or replications and one leaf was measured per plant. Plants sampled were approximately 1.5 m tall. All plants were in the field and below the canopy of either Q. verginiana or U. crassifolia trees. The fifth leaf from the newest fully expanded leaf from the plant apex was measured. Ambient PFD was measured with the Li-Cor [R] portable photosynthetic meter with an integrating quantum sensor at the approximate surface of each leaf at the time the light response curves were initiated (LI-COR, Inc, Lincoln, NE).

Measurements were made within [+ or -] three hr of solar noon with a Li-Cor [R] 6400 portable photosynthetie meter. Irradiances were generated by the Li-Cor LED red-blue light source using a modified light curve program with the Li-Cor [R] 6400. A gas flow rate of 400 [mu] mol/s and a C[O.sub.2] concentration of 390 [mu] mol/mol was used. One mature, undamaged, fully expanded leaf per replication was used with the two x three cm chamber. The Li-Cor [R] 6400 was operated at approximate ambient summer, midday, daytime temperature (34 [degrees] C) and relative humidity (50%), and was calibrated daily. Response data were recorded after at least two minutes when a stable total coefficient of variation was reached (---0.3%), usually less than five minutes. Light response curves were started at a PFD of 2000 [mu]mol/[m.sup.2]/s and then reduced stepwise to 1800, 1600, 1400, 1200, 1000, 800, 600, 500, 400, 300, 200, 150, 100, 75, 50, 25, 10, and 0 [mu] mol/[m.sup.2]/s (19 total measurements per leaf).

Measurements included net photosynthesis, stomatal conductance, and transpiration. Separate one way ANOVAs were used to determine if net photosynthesis, stomatal conductance, and transpiration were significantly different over the PFD's tested (Sail et al. 2001). A repeated measures ANOVA was not used because only one leaf type was examined. If experiment wide differences were found, Tukey's HSD was used to delect differences between PFD's examined. Shapiro-Wilks tests were used to test for normal distributions and the Bartlett's Test was used to test for homogeneity of variances. If data were not normal or the variances were not homogenous, and could not be corrected with transformations, non-parametric Kruskal-Wallis ANOVA and Dunn's multiple range test were used.

Maximum photosynthesis ([A.sub.max]), PFD at [A.sub.max], transpiration at [A.sub.max]. conductance at [A.sub.max], light saturation point, dark respiration, light compensation point, and the quantum yield efficiency (initial slope) were determined for each replicate, and means were calculated (Wayne & Van Auken 2009). The [A.sub.max] was the highest net photosynthesis rate. Light saturating photosynthesis was the PFD when the slope of the initial rate line reached the [A.sub.max]. Dark respiration was the gas exchange rate at a PFD of 0 [mu]mol/[m.sup.2]/s (y-intercept of the line for the initial slope or rate). The light compensation point was calculated as the PFD when the photosynthetic rate = 0 [mu] mol C[O.sub.2]/[m.sup.2]/s (x-intercept of the line for the initial slope or rate). The quantum yield efficiency or initial slope was calculated using the dark value and increasing PFDs until the regression coefficient of the slope decreased (150 [mu]mol/[m.sup.2]/s PFD). Significance level for all tests was 0.05.

RESULTS

The mean density of V. virginica varied significantly by position along the transects (Fig. 1, Kruskal-Wallis ANOVA, P=0.0001). Plants were mostly distributed under the canopy as opposed to being in the associated open grassland. The density of V. virginica plants decreased from about 700 plants/100 [m.sup.2] below the canopy to zero at the edge of the grassland (Fig. 1). No V. virginica plants were found in the open grassland beyond 0.75 m from the dripline outward into the grassland in any of the 5 transects sampled. The highest density of V. virginica was 850 plants/100 [m.sup.2] which occurred 3.5 m from the drip line, under the canopy. A total of 283 plants were counted, with 99% below the canopy and only three in the grassland or at the drip line.

[FIGURE 1 OMITTED]

The mean light level (PAR, [mu]mol/[m.sup.2]/s) varied significantly by position along the transects (Fig. 2, Kruskal-Wallis ANOVA, P=0.0001). Measurements from the open grassland positions were higher than those below the canopy. The lowest mean light level was 207 [+ or -] 53 [mu]mol/[m.sup.2]/s and was found five meters from the dripline, below the canopy. The highest mean light level was 2126 [+ or -] 71 [mu]mol/[m.sup.2]/s and was found four meters from the dripline into the associated grassland. The mean light level at the canopy edge was 721 [+ or -] 325 [mu]mol/[m.sup.2]/s and was significantly different from the light levels in the grassland and most of the positions below the canopy (Fig. 2, Dunn's multiple range test). There were few significant differences in light levels in the open grassland.

[FIGURE 2 OMITTED]

Mean soil depth along the five transects from under the canopy into the open grassland was patchy but did not vary significantly by position (Fig. 3, ANOVA, F=1.4936, P=0.1054). The deepest mean soil depth was 42.4 [+ or -] 18.2 cm and was 2.5 m from the drip line below the canopy. The shallowest mean soil depth was 20.7 [+ or -] 14.4 cm and was found 0.5 m from the drip line below the canopy.

[FIGURE 3 OMITTED]

The mean ambient light level under the canopy at the level of the leaves of V. virginica was 110 [+ or -] 17 [mu]mol/[m.sup.2]/s (Table 1). This light level was lower than the light saturation point but above the light compensation point and the carbon assimilation rate was 35% of the [A.sub.max]. The photosynthetic response of the V. virginica leaves was significantly different over the light levels tested (one-way AN OVA, F=178.9, P<0.0001, Fig. 4). At PFD's above approximately 300 [mu]mol/[m.sup.2]/s, rates were fairly constant and there were few differences (Tukey's HSD; P [less than or equal to] 0.05), while at PFD's lower than 300 [mu]mol/[m.sup.2]/s, leaves generally had lower and significantly different rates at most of the light levels tested (Tukey's HSD; P < 0.05). Verbesina virginica had a fairly high maximum photosynthetic rate ([A.sub.max]) at 12.7 [+ or -] 1.4 [mu]mol C[O.sub.2]/[m.sup.2]/s at the highest light level tested, 2000 [mu]mol/[m.sup.2]/s (Table 1). Light saturation ([L.sub.sat]) was 288 [mu]mol/[m.sup.2]/s and the light compensation point [L.sub.cp] was 16 [mu]mol/[m.sup.2]/s (Table 1). The dark respiration (Rd) was low at 0.76 [mu]mol C[O.sup.2]/[m.sup.2]/s. The quantum yield efficiency or the initial slope (slope of the line from 0-150 [mu]mol/[m.sup.2]/s) was 0.0375 [+ or -] 0.0019 [mu]mol C[O.sub.2]mol quanta. Mean stomatal conductance ([g.sub.leaf]) of V. virginica plants below the canopy was not significantly different over the light levels examined (ANOVA, F=1.16, P=0.3126, Fig. 5; Table 1). The lowest stomatal conductance was 0.190 [+ or -] 0.080 mol [H.sub.2]O/[m.sup.2]/s at the lowest light level tested (zero [mu]mol/[m.sup.2]/s), while the highest mean stomatal conductance was 0.311 [+ or -] 0.110 mol [H.sub.2]O/[m.sup.2]/s at the highest light level tested (2000 [mu]mol/[m.sup.2]/s). Transpiration rates (E) varied significantly over the light levels tested (ANOVA: F=5.09, PO.0001, Fig. 6). The lowest mean transpiration rate was 3.46 [+ or -] 0.96 mmol [H.sub.2]O/[m.sup.2]/s and was found at the lowest light level tested (zero [mu]mol/[m.sup.2]/s. Table 1). The highest mean transpiration rate was 6.10 [+ or -] 0.99 [mu]mol [H.sub.2]O/[m.sup.2]/s at 2000 [mu]mol/[m.sup.2]/s or full sunlight.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]
Table 1. Means and standard deviations for the ambient canopy light
levels (PFD) and ecophysiological characteristics for shade leaves
of Verbesina virginica plants growing in shade.

Parameter                                              Sun
Ambient canopy Light - PFD                         110 [+ or -] 17
([mu]mol/[m.sup.2]/s)
[A.sub.can] - Canopy C[O.sub.2] Uptake            4.5 [+ or -] 0.5
([mu]molC[O.sub.2]/[m.sup.2]/s)
[A.sub.max] - Maximum C[O.sub.2] Uptake          12.7 [+ or -] 1.4
([mu]molC[O.sub.2]/[m.sup.2]/s)
PFD - Light Level at [A.sub.max]                   2000 [+ or -] 0
[A.sub.can] - Light saturation                      288 [+ or -]32
([mu]mol/[m.sup.2]/s)
[A.sub.max] - Compensation point                     16 [+ or -] 2
([mu]mol/[m.sup.2]/s)
Rd - Dark Respiration                           0.76 [+ or -] 0.07
([mu]molC[O.sub.2]/[m.sup.2]/s)
IS or QY - Initial slope                     0.0375 [+ or -]0.0019
([mu]molC[O.sub.2]/[mu]mol quanta)
[g.sub.leaf] - Conductance                    0.311 [+ or -] 0.110
(mol[H.sub.2]O/[m.sup.2]/s)) at [A.sub.max]
[g.sub.leaf] - Conductance                    0.190 [+ or -] 0.080
(mol[H.sub.2]O/[m.sup.2]/s)) at [L.sub.cp]
E - Transpiration                               6.10 [+ or -] 0.99
(mmol[H.sub.2]O/[m.sup.2]/s) at [A.sub.max]
E - Transpiration                               3.46 [+ or -] 0.96
(mmol[H.sub.2]O/[m.sup.2]/s) at [L.sub.cp]


DISCUSSION

Verbesina virginlea was found below or at the edge of the canopy of Q. virginiana or U. crassifolia (Fig 1) but not in associated grasslands. Transects were short and focused on the canopy edge of these woodland communities. No V. virginica plants were seen in the grassland beyond the end of the transects. In addition, density below the canopy, where the transects ended, did not appear to decrease. Light levels measured appeared to be important, with almost no V. virginica plants found m the high light open grassland habitat (Fig. 2). Soil depth did not seem to be a factor determining the presence or density of V. virginica (Fig. 3), but they were not expected in the shallow soils of the more arid woodland communities (Van Auken et al. 1981).

Shade leaves of V. virginica plants growing in the low light environments of various canopy trees (Enquist 1987), had a high maximum photosynthetic rate ([A.sub.max]), which is atypical of many species found growing below a woodland or forest canopy habitat (Begon et al. 2006). Shade adapted leaves of one spring green eastern deciduous forest understory species was 1.16 times higher than rates for V. virginica, but a second spring green species was 52.4% lower (Hull 2002). Summer green eastern deciduous forest understory species had lower [A.sub.max] rates at 44.4% and 30.9% of the V. virginica [A.sub.max] rates (Hull 2002).

Other photosynthetic parameters, including light saturation, light compensation, and dark respiration were in the range expected for shade adapted plants (Boardman 1977; Hull 2002; Larcher 2003; Givnish et al. 2004; Begon et al. 2006; Valladares & Niinemets 2008). Conductance and transpiration were relatively high for shade adapted leaves. These responses are not completely consistent with findings for shade plants, but are suggestive that V. virginica is a shade species (Boardman 1977; Hull 2002; Larcher 2003; Givnish et at. 2004; Begon et al. 2006; Valladares & Niinemets 2008). In addition, conductance and transpiration rates suggest that V. virginica was not water limited and that photosynthetic rates reported were light dependent and typical for this species (Matzner et al. 2003; Vilagrosa et al. 2003).

Verbesina virginica is a native species with a fairly broad distribution, especially in the eastern United States. However, very little is known about its photosynthetic capability. No studies have been identified which evaluate the physiological responses or growth responses of this species to light levels or other environmental factors. Most of the parameters measured for shade leaves suggest that this species is a shade adapted species, and its growth is best in full or at least partial shade. Verbesina encelioides, a related species, had an [A.sub.max] of 12.3 [mu]mol C[O.sub.2]/[m.sup.2] /s, which is within the range reported here for V. virginica, but V. encelioides is a disturbance species and not expected to do well at low light levels below a canopy (Gleason et al. 2007).

Usually, true understory species like summer green species (Hull 2002) have much lower photosynthetic rates than the rates reported for V. virginica in the current study. Photosynthetic Amm rates of three European montane understory forests species were 3.4 - 5.5 [mu]mol C[O.sub.2]/[m.sup.2]/s) (Hattenschwiler & Korner 1996). These forest understory species reached light saturation at 210 [+ or -] 20 [mu]mol/ [m.sup.2]/s compared to 288 [+ or -] 32 [mu]mol/[m.sup.2]/s for V. virginica. Arnica cordifolia, an herbaceous perennial which grows in the understory of lodgepole pine forests in southeastern Wyoming, also had maximum photosynthetic rates lower than V. virginica. However, light saturation was higher at 350 [mu]mol C[O.sub.2]/ [m.sup.2]/s (Young & Smith 1980). Polygonum virginianum, an herbaceous perennial found in the forest understory and at the forest's edge in the eastern United States, had an [A.sub.max] of- 3 [mu]molC[O.sub.2]/[m.sup.2]/s at a light saturation of-500 [mu]mol/[m.sup.2]/s (Zangerl & Bazzaz 1983). Carex pianostachys from the central Texas Edwards Plateau Juniperas woodland understory had an [A.sub.max] value of 4.9 [+ or -] 0.3 [mu]molC[O.sub.2]/[m.sup.2]/s which was lower than the shade leaves of V. virginica and reached light saturation at 151 [+ or -] 43 [mu]mol/[m.sup.2]/s (Wayne & Van Auken 2009). While V. virgimca in central Texas is typically found growing in shaded habitats or the edge of woodlands, its high [A.sub.max] for shade adapted leaves compared to other herbaceous shade plants would suggest it could grow in a variety of light environments including open habitats, but it was not found there.

True sun plants are adapted to high light conditions and have high rates of gas exchange. For example, Abut Hon theophrasti an early successional herbaceous perennial, had [A.sub.max] rates between 15-25 [mu]molC[O.sub.2]/[m.sup.2]/s (Wieland & Bazzaz 1975; Bazzaz 1979; Munger et al. 1987a; Munger et al. 1987b; Hirose et al. 1997; Lindquist & Mortensen 1999). Two oaks of gallery forest in tall grass prairies of northeastern Kansas, Quercus muehlenbergii and Q. macrocarpa had [A.sub.max] rates of 11-13 [mu]molC[O.sub.2]/[m.sup.2]/s for shade leaves (Hamerlynck & Knapp 1994) which is comparable to 12.7 [+ or -] 1.4 [mu]molC[O.sub.2]/[m.sup.2]/s reported for V. virginica in the present study.

Plants can acclimate to the variability of the light environment where they are found, including some early successional species or plants from disturbed (open) communities (Bazzaz & Carlson 1982). For example, Polygonum pensylvanicum, a colonizing annual of open fields, had an [A.sub.max] of ~ 12 [mu]molC[O.sub.2]/[m.sup.2]/s at ~ 1500 [mu]mol/[m.sup.2]/s when plants from a shaded-habitat (200 [mu]mol/[m.sup.2]/s) were measured (Bazzaz & Carlson 1982; Zangerl & Bazzaz 1983); however the rate was ~ 24 [mu]mol/[m.sup.2]/s at ~ 1800 [mu]mol/[m.sup.2]/s when plants from a full sun habitat were examined (Bazzaz & Carlson 1982). Species like V. virginica that have a relatively high [A.sub.max] and one that does not change significantly over a wide range of light levels would do well in canopy shade especially in the presence of various sunflecks (Hull 2002). Further studies would be needed to determine if V. virginica does acclimate to variability in the li environment as reported for other understory species.

The dark respiration of shade leaves of V. virginica (0.76 [+ or -] 0.07 [mu]molC[O.sub.2]/[m.sup.2]/s) is also similar to other shade-adapted plants (Hamerlynck & Knapp 1994). This rate is 33% of the [R.sub.d] of shade adapted leaves of S. secundiflora found in the same area (Furuya & Van Auken 2009). The [R.sub.d] for shade adapted leaves of one spring green eastern deciduous forest understory species was about 1.58 times higher than rates for V. virginica. The rate for another spring green species studied was the same as the rate for V. virginica (Hull 2002). The [R.sub.d] for shade adapted leaves of summer green easteni deciduous forest understory species was about 57% and 38% of the rates for V. virginica (Hull 2002). Dark respiration for shade-adapted species is typically lower than sun-adapted species, due to the lower metabolism of shade-adapted species (Bjorkman 1968; Bazzaz & Carlson 1982). Polygonum pensylvanicum grown at 200 [mu]mol/[m.sup.2]/s had a respiration rate of ~ 0.5 [mu]molC[O.sub.2]/[m.sup.2]/s, although the rate was twice as high when plants from full sun were measured (Bazzaz & Carlson 1982). Some suggest that low dark respiration is the best predictor of a species ability to exist in shaded environments (Valladares & Niinemets 2008).

Other photosynthetic parameters reported in this study for V. virginica are similar to those values reported in the literature. Quantum yield efficiency reported here (0.038 [mu]molC[O.sub.2]/[mu]mol quanta) for shade leaves is within the range of values (0.035 -0.052 [mu]molC[O.sub.2]/[mu]mol quanta) reported for other shade adapted species (Hirose ct al. 1997; Hull 2002). Stomatal conductance and transpiration reported in the current study were similar to other studies, however many factors affect these parameters (Wieland & Bazzaz 1975; Zangerl & Bazzaz 1984; Yun & Taylor 1986; Mungeretal. 1987a; Munger et al. 1987b; Stafford 1989).

The shade leaves of V. virginica showed interesting photosynthetic responses. These physiological responses to various light levels more than likely are contributors to the apparent niche observed for this species in the field. In general, resource utilization is spatially partitioned among species along complex environmental gradients, such as changes in light from open areas to woodland or forest edges (Wayne & Van Auken 2009). The ability of V. virginica to reach high photosynthetic rates at lower light level, its light saturation, and light compensation point allow it to exist in shaded environments. At light levels below 300 [mu]molC[O.sub.2]/[m.sup.2]/s, data suggests that other more shade tolerant species such as C. planostachys would probably be able to out-compete V. virginica (Wayne & Van Auken 2009), but not after V. virginica was established. At light levels above 300 [mu]mo1C[O.sub.2]/[m.sup.2]/s, V. virginica could dominate, in part because it has photosynthetic rates as high as or higher than most co-occurring species and its large leaves would reduce light levels to very low values below its canopy (Grunstra 2008; Furuya & Van Auken 2009; Wayne & Van Auken 2009). However, its absence in associated grasslands is not explained. The established C4 grasses would have equal or higher photosynthetic rates and perhaps be more tolerant of higher light levels and lower soil water levels than V. virginica.

ACKNOWLEDGEM ENTS

We would like to thank M. Grunstra and J. K. Bush for help during various stages of the study.

LITERATURE CITED

Bazzaz, F. A. 1979. The physiological ecology of plant succession. Ann. Rev. Ecol. Syst.? 10(1);351-371.

Bazzaz, F. A. & R. W. Carlson. 1982. Photosynthetic acclimation to variability in the light environment of early and late successional plants. Oecologia, 54 (3):313-316.

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OWVA at: oscar.vanauken@utsa.edu

Jason W. Gagliardi and O. W. Van Auken Department of Biology, University of Texas at San Antonio San Antonio, Texas 78249
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Author:Gagliardi, Jason W.; Van Auken, O.W.
Publication:The Texas Journal of Science
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Date:Aug 1, 2010
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