where: SE (X) is the standard error for osmotic potential at 0 MPa (control) and SE (Yi) is the standard error of osmotic potential at -0.2 to -0.8 MPa, respectively. Symbols for comparison: sp. type-species

Figure 2

Figure 3
types (xeric and mesic spp.), wp-water potential, ns- not significant, * p<0.05, ** p<0.01, *** p<0.005 -------------------------------------- 25 Figure 4 Relative root elongation rate (mean +SE) of congeneric species of A) Fraxinus, B) Nyssa, C) Vaccinium, and D) Quercus grown in various water potentials --------------------------------------- 26 Correlation between (A & C) absolute root length (mm) and (B & D) relative root elongation rate taken 6 days after initial germination and root turgor potential (MPa) across all species examined. Rate of root elongation (6DA) = (final root length (mm)-initial root length)/maximum root length of the control.

Figure 5

Turgor potential = substrate osmotic potential (MPa) minus the root osmotic potential (MPa). There was no significant difference between xeric and mesic species in all cases presented ------------------------------------------------------------------- 27
CHAPTER III. Drought Tolerance of Xeric and Mesic Southeastern US Mixed Forest Species Figure 1 Mean leaf water potential (-MPa) at different wilt stages of the species tested ------------------------------------------------------Wilt stagesof A) F. americana, B) F. pennsylvanica, C) N. sylvatica, D) N. aquatica, E) Q. alba, and F) Q. nigra, respectively: a) normal, b) slightly wilted, c) wilted, d) severely wilted, e) nearly dead, and f) presumed dead ----------Complete stem survival of A) F. americana, B & C) F. pennsylvanica, D) N. sylvatica, E) N. aquatica, F & G) Q. alba, and H) Q. nigra after re-watering subsequent to drought exposure tested --------------------------------------------------Partial stem survival of A) F. pennsylvanica, B) N. aquatica, C) Q. nigra after re-watering subsequent to drought exposure ------

Figure 4

Survival rates of congeneric species pairs of A) Fraxinus, B) Nyssa, and C) Quercus, wp- water potential, ns- not significant, *p<0.05, **p<0.01, ***<0.005 ----------------------------------------------------- 49 Percent complete stem survival (CSS) and percent partial stem survival (PSS) of congeneric species pairs of A & B) Fraxinus, C & D) Nyssa, and E & F) Quercus, wp- water potential, ns - not significant, *p<0.05, **p<0.01, ***<0.005 --------------------

Figure 6

CHAPTER IV.Effect of Changes in Water and Nutrient Availability on Seedling Growth Performance of Xeric and Mesic Southeastern US mixed Forest Species Figure 1 Growth performance experiment set-up. Circle indicates the flooded treatment ------------------------------------------------------------------- 65 Biomass allocation patterns: total plant biomass, stem mass ratio (SMR), root mass ratio (RMR), leaf mass ratio (LMR), fine root xi

RESULTS Relative final germination rates declined with substrate water potential for each congeneric species pair of Fraxinus, Nyssa, Vaccinium, and Quercus examined (Fig. 1, p<0.0001). The low germination was more apparent when the levels in water potential were below -0.4 MPa though the reduction in germination percentage was also evident even at -0.2 MPa. There was a significant interactive effect between species type and substrate water potential in all species (Fig. 1A, C-D, p<0.0001 for all congeners), except Nyssa (Fig. 1B, p=0.452) although the trends are contrasting. The interaction effect reflects the relative germination rate response to drought between the xeric and mesic species. There was no consistent tendency for mesic species to be more sensitive to water stress. While the mesic species of Fraxinus showed a significantly higher relative germination percentage than their xeric species pair (Fig. 1A, p<0.0001), Vaccinium exhibited the opposite trend (Fig. 1C, p<0.0001). Although the xeric and mesic Quercus spp. responded differently to water potential (Fig. 1D, p<0.0001), the direction of the difference was not consistent. However, we found a sharp decline in the germination of mesic Q. nigra at -0.6 MPa relative to xeric Q. alba, but at -0.8 MPa, Q. alba had the lower germination. The relative germination rates of xeric and mesic Nyssa spp. did not vary significantly (Fig. 1 B, p=0.252) in response to decreasing water potential. It was also evident that germination was delayed by drought stress (Fig. 2A-C, p<0.0001), except for Quercus (Fig. 2D, p=0.204). There were significant differences in germination time between the xeric and mesic species within genera (Fig. 2A-D, p<0.018),
but the direction of this difference was not consistent. The only significant interactions between species type and different levels of water stress were in Fraxinus spp. and Nyssa spp. (Fig. 2A, p=0.027 and B, p<0.001, respectively).

100 80

F. americana (Xeric) F. pennsylvanica (Mesic) sp. type*** 100 wp *** sp. type x wp***

-0.2 -0.4 -0.6

N.sylvatica (Xeric) N. aquatica (Mesic) ns sp. type wp *** sp. type x wpns
Relative final Germination (%)

0 -0.2 -0.4 -0.6 -0.8

Q. alba (Xeric) Q. nigra (Mesic) sp. type*** wp *** sp. type x wp***

C 0 -0.2 -0.4

V. stamineum (Xeric) 100 V. corymbosum (Mesic) sp. type*** wp *** sp. type x wp***80
Substrate water potential (MPa) Figure 1. Relative final germination percentage (mean + SE) of congeneric species of A) Fraxinus, B) Nyssa, C) Vaccinium and D) Quercus grown in various water potentials. Relative final germination percentage is the mean final germination (%) observed in each species grown in various substrate water potential relative to mean the final germination (%) of the control. Symbols for comparison: sp. type-species types (xeric and mesic spp.), wp-water potential, ns- not significant, * p<0.05, ** p<0.01, *** p<0.005

The xeric and mesic species varied widely in their survival response after drought exposure. When species were slightly wilted to wilted and had complete stem survival (Fig. 3 A-D, F, & H), they re-sprouted new leaves from the shoot apex. Conversely, species which were extremely wilted shed leaves prior to new leaf production at the shoot apex (Fig. 3 G). Partial stem survival after drought exposure was manifested by new growth that occurred at some point below the shoot apex (Fig. 4). Some stems that were presumed to be dead also showed re-sprouting at the base of the stem. Basal re-sprouting indicates that while stems were more vulnerable to death during dry-down, the root systems are still functional. Thus, functional roots are likely able to transport water to newly developed leaves at the base of the stem. A B C D
Figure 3. Complete stem survival of A) F. americana, B & C) F. pennsylvanica, D) N. sylvatica, E) N. aquatica, F & G) Q. alba, and H) Q. nigra after re-watering subsequent to drought exposure. Circles indicate new growth that occurred subsequent to re-watering after drought.
Figure 4. Partial stem survival of A) F. pennsylvanica, B) N. aquatica, C) Q. nigra after rewatering subsequent to drought exposure. Circles indicate new growth that occurred subsequent to re-watering after drought.
Overall, different levels of water stress significantly affected rates of complete stem survival and partial stem survival of all species (Fig. 5 and Fig. 6, p<0.001, for each congener). There was only a significantly higher survival rate in xeric Fraxinus sp. relative to mesic Fraxinus sp. In all cases, xeric species had a significantly higher complete stem survival than their respective mesic species pair (p<0.001, data not shown) using the simple model of likelihood ratio test. There was a significantly higher partial stem survival of all xeric species than mesic species (p<0.001), except for Nyssa spp. (p=0.925), using the simple model of likelihood ratio test. The relatively higher drought survival of species commonly associated with dry sites than those from moist sites was also observed by Engelbrecht et al. (2005) in the tropical moist forest. Either species type or the interaction between species type and leaf water potential showed a significant effect on survival across leaf water potentials. A significant interactive effect of species and water potential on the overall percent survival
was only observed in Nyssa (Fig. 5B, p=0.006). There was a significant interaction effect of species by water potential on complete stem survival of Fraxinus (Fig. 6A, p<0.001) and Quercus (Fig. 6E, p=0.042) congeners but not in Nyssa (Fig. 6C, p=0.136). On the other hand, partial stem survival was significantly affected by the interaction of species and water potential, in Nyssa and Quercus (Fig. 6D, p=0.006; Fig. 6F, p=.050, respectively) congeners and not in Fraxinus (Fig. 6B, p=0.066).

F. americana F. pennsylvanica sp. * wp *** sp. x wp ns

Survival (%)

N. sylvatica N. aquatica sp. ns wp *** sp. x wp **
Q. alba Q. nigra sp. ns wp *** sp. x wp ns
Leaf water potential (-MPa) Figure 5. Survival rates of congeneric species pairs of A) Fraxinus, B) Nyssa, and C) Quercus. sp.- species, wp- water potential, ns- not significant, *- p<0.05, **- p<0.01,

***-p<0.005.

.Xeric

.Mesic sp

F. americana F. pennsylvanica sp.*** wp*** sp. x wp***
F. americana F. pennsylvanica sp.** wp.*** sp. x wpns
Complete Stem Survival (%)
N. sylvatica N. aquatica sp.*** wp.*** sp. x wpns
Partial Stem Survival (%)
Q. alba Q. nigra sp.ns wp.** sp. x wp*
Q. alba Q. nigra sp.ns wp*** sp. x wp.*
Leaf water potential (-MPa) Figure 6. Percent complete stem survival (CSS) and percent partial stem survival (PSS) of congeneric species pairs of A & B) Fraxinus, C & D) Nyssa, and E & F) Quercus. Sp.-species, wp- water potential, ns- not significant, *-p< 0.05, **-p<0.01, ***p<0.005.
DISCUSSION The ability of a species to tolerate drought often determines the distribution of a species in a landscape (Crawford and Braendle, 1996; Lopez and Kursar, 2003; Van Nieuwstadt and Sheil, 2005). Even if seeds germinate in a particular site, but drought continues to act as a filter to plant survival after germination and initial seedling establishment. I performed a dry-down experiment to evaluate the drought tolerance of three congeneric pairs of xeric and mesic species commonly found in the SE US mixed forest and observed that different wilt stages closely corresponded to the different leaf water potentials across species (Fig.1). Future seedling establishment might also be negatively impacted by the occurrence of periodic drought because of a decrease in the availability of soil moisture necessary for seedling establishment. The levels of water stress imposed in the experiments significantly reduced the survival of the seedlings of SE US mixed forest species examined. Most of the xeric species had higher drought-tolerance than mesic species as manifested by their higher survival rates especially when both contrasting species were exposed to extreme water stress. The results of the simple model of likelihood ratio test revealed that the entire stems of the xeric species were less vulnerable to wilting than the mesic species especially at extreme drought stress. This may suggest that xeric seedlings are better able to withstand the low water potentials in well-drained sites (xeric sites). The drought-tolerance strategy that these xeric seedlings exhibited allowed them to have a higher complete stem survival (CSS) under drought.

Leuscher, C., K. Backes, D. Hertel, F. Schipka, V. Schmitt, D. Terborg and M. Runge. 2001. Drought response at leaf, stem and fine root levels of competitive Fagus sylvatica L. and Quercus petraea (Matt.) Leibl. trees in dry and wet years. Forest Ecol. and Management 149:33-46. Poorter, L. and Y. Hayashida-Oliver. 2000. Effects of seasonal drought on gap and understorey seedlings in a Bolivian moist forest. J. of Trop. Ecol. 16:481-498. Prider, J. N. and J. M. Facelli. 2004. Interactive effects of drought and shade on three arid zone chenopod shrubs with contrasting distributions in relation to tree canopies. Functional Ecol. 18:67-76. Radford, A. E., H. E. Ahles and C. R. Bell. 1964. Manual of the Vascular Flora of the Carolinas. UNC Press, Chapel Hill. 1183 p. Schafale, M. P. and A. S. Weakley. 1990. Classification of the Natural Communities of North Carolina. 3rd Approximation. NC Natural Heritage Prog., Div. of Parks and Recreation, NC Dept. of Environment, Health and Natural Resources. p. 35-80. Slot, M. and L. Poorter. 2007. Diversity of tropical tree seedling responses to drought. Biotropica 39:683-690. Smirnoff, N. 1993. Tansley review no. 52. The role of active oxygen in the response of plants to water deficit and desiccation. New Phytol. 125:27-58. Sperry, J. S. and U. G. Hacke. 2002. Desert shrub water relations with respect to soil characteristics and plant functional type. Functional Ecol. 16:367-378. Steege, H. T. 1994. Flooding, drought tolerance in seeds and seedlings of two Mora species segregated along a soil hydrological gradient in the tropical rain forest of Guyana. Oecologia 100:356-367. Tyree, M. T., G. Vargas, B. M. J. Engelbrecht and T. A. Kursar. 2002. Drought until death do us part: a case study of the desiccation tolerance of a tropical moist forest seedlingtree, Licania platypus (Hemsl.) Fritsch. J. of Expt. Bot. 53:2239-2247. Tyree, M. T., B. M. J. Engelbrecht, G. Vargas, and T. A. Kursar. 2003. Desiccation tolerance of five tropical seedlings in Panama. Relationship to a field assessment of drought performance. Plant Physiol. 132:1439-1447. Van Nieuwstadt, M. G. L. and D. Sheil. 2005. Drought, fire, and tree survival in Borneo rainforest, East Kalimantan, Indonesia. J. of Ecol. 93:191-201.
Veenendaal, E. M., M. D. Swaine, Agyeman V. K., D. Blay, I. K. Abebrese, and C. E. Mullins. 1995. Differences in plant and soil water relations in and around a forest gap in West Africa during the dry season may influence seedling establishment and survival. J. Ecol. 83:83-90.
CHAPTER IV Effect of Changes in Water and Nutrient Availability on Seedling Growth Performance of Xeric and Mesic Southeastern US Mixed Forest Species
INTRODUCTION Plant species composition in a landscape can be constrained by environmental factors that influence the germination, survival, and seedling performance of individual species. The habitat in which a plant grows influences its growth independent of the influence of its genetic make-up, which changes over longer time scales. Growth performance continues to act as a filter after germination due to differences in environmental requirements between different life-stages, since conditions and traits that are advantageous for one life history stage may not be beneficial for another developmental stage (Broncano et al., 1998). More specifically, the environmental conditions that favor seed germination may not always be favorable for seedling survival and growth (Schupp, 1995). Growth responses to resource availability may play an important role in determining the success of seedling recruitment in an area (Gordon and Rice, 2000; Padilla et al., 2007), species distributions (Dalling et al., 2004), and the confinement of the xeric species and mesic species in their respective edaphic condition. Plant growth performance is an interplay of all physiological processes including water relations, photosynthesis, respiration, and mineral nutrition (Lambers et al., 1998). Therefore, I can expect that the success of plants in a particular area will be determined by the whole suite of traits that govern the uptake of water and nutrients (Park, 1990). Plants are continuously adjusting their physiological responses and allocation patterns to match changes in the environment. Plants adjust allocation of biomass to shoots and roots in ways that improve acquisition of a limiting resource as well as survival under

environmental stress (Long and Jones, 1996). This phenotypic plastic response is an important strategy that affects carbon allocation in the plant. Some carbon allocation responses in woody plants are reflected in the distribution of plants along a resource gradient (Long and Jones, 1996). For example, plants from xeric sites tend to allocate relatively more biomass to roots than those from mesic sites (Matsuda et al., 1989). Considerable work has been done to investigate seedling performance of woody plant as a function of light availability (Tyree et al., 1998; Lin et al., 2002; Dalling et al., 2004; Delagrange et al., 2004; Baltzer and Thomas, 2007; Quero et al., 2007) and the interaction between irradiance and water availability on growth performance (Black et al., 2005; Baraloto, 2006; Kobe, 2006; Niva et al., 2006; Pardos et al., 2006; Sanchez-Gomez et al., 2006; de Gouvenain et al., 2007; Feng and Li, 2007; Lavinsky et al., 2007), but in the southeastern United States mixed forest, water availability is one of the primary factors exerting control on vegetation distribution (Bahari et al., 1985; Woodward, 1987), although light is an important influence on seedling growth performance. The SE US mixed forest is dominated by broad-leaved deciduous trees and is classified by Delcourt and Delcourt (1981) into 3 vegetation types: oak-hickory-southern pine (Quercus-Carya-Pinus), southern pine (Pinus), and cypress-gum (TaxodiumLiquidambar) forests. Deciduous forest in the SE US is composed of tree genera that are diverse and widespread, both geographically and ecologically (Barnes, 1991). For example, the oaks (Quercus), many of which are xerophytic (Q. incana, Q. prinus, Q. marilandica), are also adapted to mesic conditions (Q. nigra, Q. palustris, and Q. phellos) whereas Q. alba, Q. rubra, and Q. velutina are species that are widely distributed (Barnes, 1991) along
gradients of water availability. On the other hand, the hickories (Carya), which are typically associated with oaks, are also abundant and widespread on xeric and mesic sites (Barnes, 1991). Carya glabra dominates the xeric sites, while C. aquatica and C. cordiformis dominates the mesic sites. Other genera that dominate the mesic sites are Acer, Fagus, and Tilia (Barnes, 1991). In the SE US, xeric forest is typically dominated by drought-tolerant species. Conversely, mesic forest is dominated by drought-sensitive hardwood forest species (Braun, 1950; Schafale and Weakley, 1990). Therefore, water availability may play a role in the segregation of xeric and mesic species. Water plays an essential role in all physiological plant processes. At a whole plant level, water is a medium for the transport of nutrients needed for plant growth and development. While the nutrient level of the soil may not be dependent on soil moisture, the availability of nutrients for plant uptake may be dependent on soil moisture. Nutrient content of the soil is also determined by the leaf litter and organic matter it possesses in addition to other factors such as soil erosion, soil pH, and soil microorganisms (Fitter and Hay, 2002). Plant responses to different nutrient levels can be influenced by water regime (Gusewell, 2003) and may be dependent on the ability of the plant to acquire these resources (Tyree et al., 1998). In a similar habitat nearby, nutrient concentrations decreased from mesic to xeric sites in an Appalachian oak forest in southwest Virginia (Martin et al., 1982). Mesic sites were dominated by mesic hardwood species with relatively higher nutrient concentrations than the xeric pine stands that dominated the xeric sites (Martin et al., 1982). Therefore, I may expect that upland and well-drained sites are more nutrient-limited than those in the bottomland and moist sites. Hence, this study focused on growth performance of xeric and

mesic species as affected by changing water and nutrient availability. I studied early seedling growth, since this stage is particularly sensitive to resource shortages (Kramer and Kozlowski, 1979) and is also critical in the establishment of any plant species that are significantly influenced by water (Kozlowski and Pallardy, 1997; Turk et al., 2004) and nutrient availability. Forest composition is dictated by establishment success (Ackerly, 2004), which may depend in turn on the tolerance of the species to the limiting resource in a particular area (Valladares, 2003). The differential seedling performance under various water and nutrient levels may potentially cause a separation of xeric and mesic species along gradients of resource availability. I hypothesized that relative to mesic species, growth performance of xeric species is less sensitive to chronic water stress but is more sensitive to flooding. A two factorial experiment (3 water levels x 2 nutrient levels) was done to: a) determine differences in growth performance, photosynthetic electron transport rate, and stomatal conductance between xeric and mesic species, b) determine biomass allocation patterns in response to changes in resource availability, and c) determine the effects of water and nutrient availability on biomass allocation, electron transport rate, and stomatal conductance.
MATERIALS AND METHODS Study species Congeneric species pairs of Fraxinus (F. americana-xeric sp.; F. pennsylvanicamesic sp.), Nyssa (N. sylvatica-xeric sp.; N. aquatica-mesic sp), Quercus (Q. alba-xeric sp.; Q. nigra- mesic sp.), and Vaccinium (V. stamineum- xeric sp.; V. corymbosum- mesic sp.) were studied. The Fraxinus, Nyssa, and Quercus seeds were obtained from Sheffields seed company, who collected seeds from the Southeastern US. Vaccinium seeds were obtained from the Department of Horticultural Science, NCSU. Congeners were used to ensure phylogenetic independence, which is an important consideration for making inferences in comparative studies and in improving the statistical power of comparison between two groups (Ackerly, 1999). In each pair, one species is adapted to xeric conditions, while another is adapted to mesic conditions (Radford et al., 1964 and Schafale and Weakley, 1990). We used xeric and mesic in relative terms, wherein xeric species are those common in well-drained sites, while mesic species are those which are more frequent in moist sites. Table 1 shows the wetland indicator status of the study species. Fraxinus americana is a facultative upland species and is typical of rich upland soil (Lance, 2004), which is dry and well-drained (Radford et al., 1964; Burns et al., 1990; Schafale and Weakley, 1990). Natural stands of Fraxinus pennsylvanica, a facultative wetland species, are almost completely confined to moist bottomlands (Burns et al., 1990), swamps, and along streams (Lance, 2004). Another facultative upland species is Quercus alba, which is found on either sandy plains, gravelly ridges, rich uplands and well-drained soils, but is common in driest shallow soil (Burns et al., 1990; Schafale and Weakley, 1990). Moreover, Q. alba grows in

Effect of water and nutrient availability on ETR and stomatal conductance Genera significantly differed in ETR (F3, 387 = 3.31, p=0.02) and gs (F

3, 386=

p<0.001). There was no significant difference in the electron transport rate (ETR) between xeric and mesic species (Fig. 3A, F1, 387 = 1.00, p = 0.318). The interactive effect between species type and water level also did not significantly affect ETR (Fig. 3B, F2, 387 = 2.66, p = 0.072). ETR was affected by the interaction between species type and nutrient availability (Fig. 3C, F1, 387= 4.51, p=0.034). ETR increased under high nutrient levels for both species types. The change in the ETR response to nutrient availability was greater in mesic species than in xeric species. Mesic species also had a relatively higher stomatal conductance than xeric species (Fig. 3D, F1, 386 = 11.60, p=0.0007). Stomatal conductance was affected by the species type and water level interaction (Fig. 3E, F2, 386 = 4.30, p=0.014) and not by species type and nutrient level interaction (Fig. 3F, F1,
= 3.25, p= 0.072). Both species types
reduced stomatal conductance under drought, but a greater reduction was observed in mesic species relative to xeric species. The opposite trend was found under the flooded treatment; stomatal conductance of mesic species was much less reduced than xeric species.

(umol electrons m-2 s-1)

sp. type x nutrient*

(mmol m-2 s-1)

sp. type x water *
Dry Well- Flooded watered

Nutrient level

Figure 3. Electron transport rate (ETR) and stomatal conductance (gs) of xeric and mesic species; and the effect of different water and nutrient levels on ETR and gs.
-not significant, *p<0.05, **p<0.01, ***<0.005

0.45 0.40 0.35

r2=0.61 p=0.023

LMR (g g )

0.30 0.25 0.20 0.15

Ns Qa Na

Qn Fa Fp
0.10 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24

FRMR (g g )

Figure 4. Correlation between leaf mass ratio (LMR) and fine root mass ratio (FRMR) of xeric () and mesic (o) woody plant species. Each point is a mean for 60 individual plants. Species codes: Fa, Fraxinus americana; Fp, Fraxinus pennsylvanica; Ns, Nyssa sylvatica; Na, Nyssa aquatica; Qa, Quercus alba; Qn, Quercus nigra; Vs, Vaccinium stamineum; Vc, Vaccinium corymbosum 73

transport of water and nutrients, the mesic species may need to increase their FRMR more than xeric species due to their lower CRMR. On the other hand, the higher CRMR of xeric species (Fig. 2P) was balanced by a lower FRMR of xeric species than mesic species (Fig. 2M), which could help xeric plant species maximize transport of water and nutrients. The compensatory mechanism between CRMR and FRMR may improve the balance in the acquisition, transport and storage of carbon and nutrients among xeric and mesic species.
Effect of water and nutrient availability on ETR and stomatal conductance I measured electron transport rate (ETR) as a surrogate for photosynthetic capacity of the plants, since ETR is usually correlated with photosynthesis rate (Flexas at al., 1999 and Delagrange et al., 2004). The high nutrient level may have contributed to the higher ETR for both species types (Fig. 3C), since high nutrient availability is expected to yield greater amounts of the photosynthetic apparatus, such as chlorophyll, carboxylation enzymes, and chloroplasts, where electron transport occurs (Rieger and Duemmel, 1992. If I compare the species types under different nutrient levels, the mesic species had a greater increase in ETR under high nutrients. The low stomatal conductance of the xeric species (Fig. 3F) may have contributed to the lower increase in ETR under high nutrients relative to low nutrients (Fig. 3C). This would indicate that photosynthesis of xeric species may have a low capacity for their response to high nutrients due to inherently low stomatal conductance. When there is low stomatal conductance, photosynthesis may be limited by insufficient CO2. When photosynthesis is reduced, ETR is lower since there would be lower demand for the products of the light reactions. Moreover, it is important to note that species in resource-poor environments tend not to respond strongly to resource availability. 78
Stomatal closure, a mechanism that reduces transpirational water loss, was apparent in both species types in droughted conditions as manifested by their drop in stomatal conductance (Fig. 3E). A greater drop in stomatal conductance among the mesic species relative to the xeric species may indicate that mesic species are more stressed under such conditions. However, mesic species maintained their stomates open even under flooded conditions, unlike xeric species. The higher stomatal conductance under flooding among the mesic species may be advantageous in their adaptation to moist habitats. On the other hand, low stomatal conductance of xeric species under flooding may result in poor growth performance (Fig. 2B). Thus, flooded habitats may not favor the adaptations of xeric species. It can be deduced that growth performance of plants is not only controlled by water deficit, but by excessive water as well. In the previous chapters of this thesis, I observed that drought negatively impacted germination and plant survival. Plant survival under drought stress is a determinant in the current distribution of xeric and mesic species. In this study, flooding more markedly influenced the growth performance of both species types than drought. The drought treatment that I used might not be sufficient to impose substantial drought stress. Growth performances of xeric and mesic species in response to their current habitats are likely due to phenotypic plasticity that will allow them to adjust carbon allocation to acquire resources in their particular habitat. Traits that allow xeric species to perform well in xeric habitats and traits that allow mesic species to perform well in mesic habitats may cause the segregation of xeric and mesic species along a resource gradient. Having a lower plant biomass among xeric species, could be a consequence of their growth response to resource

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cell walls. Once air enters the vessel, it disrupts the cohesion of water molecules and the water column breaks and retracts, filling the vessel first with water vapor. Ultimately, as air comes out of solution from the surrounding water, the vessel completely fills with air (Kozlowski and Pallardy, 1997). Xylem cavitation is a major potential problem in plants (Dickison, 2000), causing xylem dysfunction (Tyree and Sperry, 1989) and disrupting the hydraulic pathway from the soil to the leaf (Zimmerman, 1983). The resulting increased resistance to water flow in the sapwood can eventually limit plant growth and lead to death of the plant (Tyree and Sperry, 1988) since cavitation reduces the ability of plants to transport water to the leaves and induces greater water stress, and reduces transpiration and photosynthesis (Pockman et al., 1995 and Sperry, 1995). Drought, as a form of water stress, is one of the environmental factors that influence growth and distribution of plants because it affects the physiological processes such as water transport. Plants surviving in habitats with low water availability have numerous traits for coping with water stress. A trait of particular importance for drought tolerance is cavitation resistant xylem, which allows water transport under condition of water deficit. Plant survival in water-stressed conditions depends on the ability of the plants to conduct water, which requires the ability to either resist or repair cavitation. In addition, the ability of xylem conduits to maintain the high xylem tensions required for water transport in plants is limited by their tendency to become embolized (Sperry and Tyree, 1990). Hence, drought-induced cavitation can limit the distribution of woody plants. This study focused on woody plant species found in the SE US mixed forest dominated by broad-leaved deciduous trees and is classified by Delcourt and Delcourt (1981)
into 3 vegetation types: oak-hickory-southern pine (Quercus-Carya-Pinus), southern pine (Pinus), and cypress-gum (Taxodium-Liquidambar) forests. Deciduous forest in the SE US is composed of tree genera that are diverse and widespread, both geographically and ecologically (Barnes, 1991). For example, the oaks (Quercus), many of which are xerophytic (Q. incana, Q. prinus, Q. marilandica), are also adapted to mesic conditions (Q. nigra, Q. palustris, and Q. phellos), whereas Q. alba, Q. rubra, and Q. velutina are species that are widely distributed (Barnes, 1991) along gradients of water availability. On the other hand, the hickories (Carya), which are typically associated with oaks, are also abundant and widespread on xeric and mesic sites (Barnes, 1991). Carya glabra dominates the xeric sites, while C. aquatica and C. cordiformis dominates the mesic sites. Other genera that dominate the mesic sites are Acer, Fagus, and Tilia (Barnes, 1991). Although the SE US mixed forest is not considered to be a water-limited ecosystem, such distribution of species suggests that water availability does exert a strong control on the success of the woody plants in their current distribution. This study could be used to inform modeling of the future landscape of the SE US mixed forest to predict how the global distribution of vegetation is likely to respond to climate change. Understanding what factors restrict species distributions, the consequences for plant species survival and the associated physiological adjustments could help explain natural patterns in productivity of natural ecosystems. I hypothesized that xeric species are better able to withstand drought conditions than mesic species by having greater cavitation resistance. I aimed to: 1) compare the water potential at 50% loss of conductivity (PLC50) of xeric and mesic species, 2) test for

1.6e-4 1.4e-4

Huber value (HV)

1.2e-4 1.0e-4 8.0e-5 6.0e-5 4.0e-5 2.0e-5 0.0 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70

r =0.0004 p=0.9452

SLA (cm2/g)

r2=0.256 p=0.064

Figure 1. Regression plot showing the relationship between wood density and A) specific conductivity (Ks), B) leaf specific conductivity (Kl), C) Huber value (HV) and D) specific leaf area (SLA) for xeric () and mesic (o) woody plant species.

Loss of Conductivity (%)

C. glabra (M) C. tomentosa (X) 5 Pressure (-MPa) 8
F. pennsylvanica (M) F. americana (X) Pressure (-MPa) 8
Loss of Conductivity (%) Pressure (-MPa) 8 I. glabra (M) I. opaca (X)
Loss of Conductivity (%) 2
M. tripetala (M) L. tulipifera (X) 5 Pressure (-MPa) 8
Loss of Conductivity (%) Pressure (-MPa) 8 Q. nigra (M) Q. alba (X)
Pressure (-MPa) 8 N. aquatica (M) N. sylvatica (X)
Loss of Conductivity (%) Pressure (-MPa) 8 V. fuscatum (M) V. pallidum (X)
Figure 2. Vulnerability curves in closely related pairs of woody plants. Species name with (X) is from xeric sites while species name with (M) are from the mesic sites.
Water Potential at PLC 50 (-MPa)

p=0.011

Quercus

Magnolia

Fraxinus

Vaccinium

Figure 3. Water potential at 50% loss of conductivity ( PLC50) of xeric and mesic species pair.

PLC 50 (-MPa)

Ks, p=0.201 Sp. type, p=0.016

0 0.3 0.4 0.5

WD, p=0.113 Sp. type, p=0.013 0.6 0.7

Wood density (g/cm3)

Figure 4. Relationship between A) Specific conductivity (Ks) and B) Wood density and the xylem water potential inducing the 50% loss of conductivity ( PLC50) in the xeric and woody plants. The regression lines correspond to the significant difference of PLC50 between species types.

DISCUSSION

As hypothesized, there was a strong tendency for xeric species to be more resistant to cavitation than closely-related mesic species. In most genera, resistance to drought-induced cavitation was greater in species from xeric sites than in species from mesic sites (Fig. 2A-F). The only exception to this finding was Vaccinium (Fig. 2G). Xeric species attained 50% loss in hydraulic conductivity at a lower (more negative) water potential than the mesic species (Figure 3). All xeric species examined, except Vaccinium, are better able to withstand drought conditions by having vessels that are less vulnerable to cavitation. Previous studies have shown greater cavitation resistance of xeric than mesic species and populations. Differences were found between populations of Pseudotsuga menziesii (Kavanagh, 1999), Pinus halepensis (Tognetti et al., 1997), and Cordia alliodora (Choat et al., 2007). Differences were also found between species of Eucalyptus in Australia (Franks et al., 1995), Acer in French Alps (Tissier et al., 1997), Juniperus and Pinus in Utah (Linton et al., 1998), Pinus, Cedrus, and Cupressus in France (Froux et al., 2002), Cordia in Costa Rica and Panama (Choat et al., 2007), Quercus in United Kingdom (Higgs and Wood, 1995) and in Florida (Cavander-bares and Holbrook, 2001), and subspecies of Artemesia tridentata in Utah (Kolb and Sperry, 1999). Xeric species are able to withstand a more negative water potential prior to catastrophic xylem cavitation than the mesic species. Having a higher water potential threshold may also allow the xeric species to continue water transport even in drier sites. Physiological processes necessary for growth such as transpiration and photosynthesis are dependent on water transport capacity. Since xeric species are still able to conduct water

distribution in the near future. A greater resistance to catastrophic xylem cavitation among xeric species may result to their higher survival rates relative to mesic species. A greater survival rate of xeric species than mesic species in drier habitats may favor the recruitment of xeric species in habitats with low water availability. In some sites, there may be a decline or displacement in mesic species, especially if their migration rate is so slow that it cannot cope with rapid changes in forest climate zones. Therefore, these results will help us understand the future shifts in the species distribution of SE US mixed forest in the face of future climate change.

LITERATURE CITED

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