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PubMed1992

Complete congruence between morphological and rbcL-based molecular phylogenies in birches and related species (Betulaceae).

Estimations of phylogenies from morphological and molecular data often show contrasting results. We compared morphological and molecular phylogenies in an ancient family of woody dicots, the Betulaceae (birch family). The phylogeny of the family was estimated from parsimony analysis of morphological characters in the genera Alnus, Betula, Carpinus, Corylus, Ostrya, and Ostryopsis and from parsimony and distance-matrix analyses of DNA sequences of the chloroplast gene encoding the large subunit of ribulose-1,5-biphosphate carboxylase (rbcL) in the genera Alnus, Betula, Carpinus, Corylus, and Ostrya and in two outgroups, Quercus and Liquidambar. The topologies obtained by the different methods were completely congruent, and bootstrapping strongly supported the division of the family Betulaceae into two major clades, Betuleae (Alnus and Betula) and Coryleae (other members). Only slightly more homoplasy was present in the rbcL sequence data set than in the morphological set. Relative-rate tests indicated that the Coryleae clade had a faster rate of rbcL evolution than did the Betuleae clade. Heterogeneity of rates of morphological evolution also paralleled those for rbcL.

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PubMed1992

Carbon exchange rates, chlorophyll content, and carbohydrate status of two forest tree species exposed to carbon dioxide enrichment.

Seedlings of yellow-poplar (Liriodendron tulipifera L.) and white oak (Quercus alba L.) were exposed continuously to one of three CO(2) concentrations in open-top chambers under field conditions and evaluated after 24 weeks with respect to carbon exchange rates (CER), chlorophyll (Chl) content, and diurnal carbohydrate status. Increasing the CO(2) concentration from ambient to +150 or +300 microl l(-1) stimulated CER of yellow-poplar and white oak seedlings by 60 and over 35%, respectively, compared to ambient-grown seedlings. The increases in CER were not associated with a significant change in stomatal conductance and occurred despite a reduction in the amounts of Chl and accessory pigments in the leaves of plants grown in CO(2)-enriched air. Total Chl contents of yellow-poplar and white oak seedlings grown at +300 microl l(-1) were reduced by 27 and over 55%, respectively, compared with ambient-grown seedlings. Yellow-poplar and white oak seedlings grown at +300 microl l(-1) contained 72 and 67% more morning starch, respectively, than did ambient-grown plants. In contrast, yellow-poplar and white oak seedlings grown at +300 microl l(-1) contained 17 and 27% less evening sucrose, respectively, than did plants grown at ambient CO(2) concentration. Diurnal starch accumulation and the subsequent depletion of sucrose contributed to a pronounced increase in the starch/sucrose ratio of plants grown in CO(2)-enriched air. All seedlings exhibited a substantial reduction in dark respiration as CO(2) concentration increased, but the significance of this increase to the carbohydrate status and carbon economy of plants grown in CO(2)-enriched air remains unclear.

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PubMed1992

Radial velocity profiles of water flow in trunks of Norway spruce and oak and the response of spruce to severing.

Trunk-tissue heat balance, volumetric and staining methods were used to study xylem water flow rates and pathways in mature Norway spruce (Picea abies (L.) Karst.) and pedunculate oak (Quercus robur L.) trees. The radial profile of flow velocity was confirmed to be symmetrical in spruce, i.e., maximum flow velocity was in the center of the conducting xylem and tailed with low amplitude (about 30 cm h(-1)) in the direction of the cambium and heartwood. Variability around the trunk was high. In contrast, in oak, the radial profile of flow velocity was highly asymmetrical, reaching a peak of about 45 m h(-1) in the youngest growth ring and tailing centripetally for about 10 rings, but variability around the trunk was less, under non-limiting soil water conditions, than in spruce. In spruce, the flow rate increased abruptly within seconds when the tree was severed while immersed in water, and then decreased gradually, showing significant root resistance. We conclude that water flow through an absorbing cut surface differs from the flow higher in a tree trunk because of the presence of hydraulic capacitances in the conductive pathways. The staining technique always yielded higher estimates of flow velocity than the non-destructive tree-trunk heat balance method.

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PubMed1993

Variation in seedling hydraulic architecture as a function of species and environment.

Northern red oak (Quercus rubra L.) and yellow-poplar (Liriodendron tulipifera L.) were grown for two years in full sunlight (unshaded) or 20% of full sunlight (shaded) under either well-watered or drought conditions. There was a close association between evaporative flux (in situ) and leaf-specific conductivity (LSC) in stem segments of both species. Shaded, drought-stressed seedlings of both species had significantly reduced leaf area, evaporative flux, volume flow rate in xylem, flow velocity, potentially functional xylem area, and LSC than unshaded, well-watered seedlings. Reductions in LSC associated with drought or shade were similar for both species; and within a treatment, both species had similar LSC. Species differed in the manner of LSC adjustment to drought and shade. Reductions in leaf area associated with drought or shade were accompanied primarily by reductions in potentially functional xylem area for L. tulipifera, and by reductions in flow velocity for Q. rubra. These results suggest (1) the existence of a homeostatic balance between evaporative flux and LSC, (2) that species with widely different growth patterns and xylem anatomies may develop similar LSC within the same environment, and (3) a possible hydraulic basis for differences in habitat between ring- and diffuse-porous species.

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PubMed1993

Photosynthesis of oak leaves under water stress: maintenance of high photochemical efficiency of photosystem II and occurrence of non-uniform CO(2) assimilation.

Net CO(2) assimilation rates (A), stomatal conductance to water vapor (g(w)), photosynthetic O(2) evolution in 5% CO(2) (A(max)) and photochemical efficiency of photosystem II were monitored in leaves of young oak saplings (Quercus petraea Matt. Liebl.) in response to increasing drought. Both A and g(w) declined rapidly as soon as predawn leaf water potential dropped below -1.0 MPa. The calculated intercellular concentration of CO(2) first declined and then increased again as drought intensity increased, suggesting that both stomatal closure and a decreased ability of mesophyll chloroplasts to fix available CO(2) were involved in the drought-induced reductions in A and g(w). However, this assumption was not supported by the observations that, with increasing drought, the decline in A(max) was limited, and the photochemistry of photosystem II and the quantum yield of light-driven electron transport remained stable. Autoradiograms of (14)CO(2)-fed leaves revealed non-uniform assimilation rates during water stress. The consequences of a potential artifact induced by this patchiness on the calculation of intercellular CO(2) concentration are discussed.

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PubMed1994

Frost tolerance and bud dormancy of container-grown yellow birch, red oak and sugar maple seedlings.

Container-grown seedlings of red oak (Quercus rubra L.), sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton) in their first year of growth were overwintered outdoors. Tolerance of roots and stems to freezing was compared from late summer to the following spring. Mitotic activity in the apical bud was related more closely to air temperature than to bud dormancy as defined by days to bud break. In all species, stem hardening was observed before days to bud break reached a maximum. Dormancy release (days to bud break equal to zero) of yellow birch coincided with loss of stem hardening in the spring. Roots hardened more slowly, had a lower frost tolerance than stems in fall and winter, and dehardened earlier than stems in the spring. There were differences in stem and root hardiness among the species, with yellow birch being the most tolerant, followed by sugar maple and red oak. Primarily because of root sensitivity to frost, winter was a critical period for all three species, but particularly for red oak.

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PubMed1994

Axial and radial water flow in the trunks of oak trees: a quantitative and qualitative analysis.

Axial water flow in the trunks of mature oak trees (Quercus petraea (Matt.) Liebl. and Q. robur L.) was studied by four independent techniques: water absorption from a cut trunk, sap flowmeters, heat pulse velocity (HPV) and thermoimaging. Estimation of the total water flow with sap flowmeters, HPV and water absorption yielded comparable results. We concluded from dye colorations, thermograms and axial profiles of sap flow and heat pulse velocity that, in intact trunks, most of the flow occurred in the current-year ring, where early-wood vessels in the outermost ring were still functional. Nevertheless, there was significant flow in the older rings of the xylem. Total water flow through the trunk was only slightly reduced when air embolisms were artificially induced in early-wood vessels, probably because there was little change in hydraulic conductance in the root-leaf sap pathway. Embolization of the current-year vessels reactivated transport in the older rings.

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PubMedDate not supplied

Interactive effects of elevated CO(2) and soil drought on growth and transpiration efficiency and its determinants in two European forest tree species.

The responses of growth and transpiration efficiency (W = biomass accumulation/water consumption) to ambient and elevated atmospheric CO(2) concentrations (350 and 700 micro mol mol(-1), respectively) were investigated under optimal nutrient supply in well-watered and in drought conditions in two temperate-forest tree species: Quercus petraea Liebl. and Pinus pinaster Ait. Under well-watered conditions, doubling the CO(2) concentration for one growing season increased biomass growth by 138% in Q. petraea and by 63% in P. pinaster. In contrast, under drought conditions, elevated CO(2) increased biomass growth by only 47% in Q. petraea and had no significant effect on biomass growth in P. pinaster. Transpiration efficiency was higher in Q. petraea than in P. pinaster in all treatments. This difference was linked (i) to lower carbon isotope discrimination (Delta), and thus lower values of the intercellular/ambient CO(2) concentration (c(i)/c(a)) ratio, in Q. petraea, (ii) to lower values of leaf mass ratio (LMR, leaf mass/whole plant mass), which we suggest was positively related to the proportion of daytime carbon fixation lost by respiration (Phi), in Q. petraea, and (iii) to slightly lower C concentrations in Q. petraea than in P. pinaster. The CO(2)-promoted increase in W was higher in Q. petraea (+80%) than in P. pinaster (+50%), and the difference was associated with a more pronounced decrease in Phi in response to elevated CO(2) in Q. petraea than in P. pinaster, which could be linked with the N dilution effect observed in Q. petraea. Because Phi also directly affects growth, the CO(2)-induced enhancement of Phi in Q. petraea is a crucial determinant of the growth stimulation observed in this species. Leaf gas exchange regulation was not the only factor involved in the responses of growth and W to elevated CO(2) and drought, other physiological processes that have crucial roles include carbon and N allocation and respiration.

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PubMedDate not supplied

Genotypic and phenotypic variation as stress adaptations in temperate tree species: a review of several case studies.

Species that occupy large geographic ranges or a variety of habitats within a limited area deal with contrasting environmental conditions by genotypic and phenotypic variation. My students and I have studied these forms of ecophysiological variation in temperate tree species in eastern North America by means of a series of field and greenhouse experiments, including controlled studies with Cercis canadensis L., Fraxinus pennsylvanica Marsh., Acer rubrum L., Prunus serotina Ehrh. and Quercus rubra L., in relation to drought stress. These studies have included measurements of gas exchange, tissue water relations and leaf morphology, and have identified genotypic variation at the biome and individual community levels. Xeric genotypes generally had higher net photosynthesis and leaf conductance and lower osmotic and water potentials at incipient wilting than mesic genotypes during drought. Xeric genotypes also produced leaves with greater thickness, leaf mass per area and stomatal density and smaller area than the mesic genotypes, suggesting general coordination among leaf morphology, gas exchange and tissue water relations. Leaf phenotypic plasticity to different light environments occurred in virtually every study species, which represented a wide array of ecological tolerances. In a study of interactions of genotypes with environment, shade plants, but not sun plants, exhibited osmotic adjustment during drought and shade plants had smaller reductions in photosynthesis with decreasing leaf water potential. In that study, sun, but not shade, plants had significant genotypic differences in leaf structure, but with certain variables phenotypic variation exceeded genotype variation. Thus, genotypic variation was not expressed in all phenotypes, and phenotypes responded differentially to stress. Overall, these studies indicate the importance of genotypic and phenotypic variation as stress adaptations in temperate tree species among both distant and nearby sites of contrasting environmental conditions.

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PubMed1995

Ecophysiological attributes of the native Acer saccharum and the exotic Acer platanoides in urban oak forests in Pennsylvania, USA.

When the exotic Acer platanoides L. (Norway maple) and the native A. saccharum Marsh. (sugar maple) grow together in the understories of urban Quercus forests in the eastern USA, average annual height growth increments are nearly twice as large in A. platanoides as in A. saccharum, 19.26 +/- 3.22 versus 10.01 +/- 1.69 cm. We examined several ecophysiological mechanisms that might be associated with the superior invasive ability and growth of A. platanoides in two urban oak forests in Pennsylvania. Leaf longevity was 12 days greater in A. platanoides than in A. saccharum. In addition, leaf mass/leaf area ratio was greater in A. platanoides than in A. saccharum (2.67 +/- 0.03 versus 2.32 +/- 0.02 mg cm(-2)); however, leaf thickness was significantly lower in A. platanoides than in A. saccharum suggesting that A. platanoides contains more dense palisade and mesophyll cell layers than A. saccharum. Field net photosynthesis (mass basis) and photosynthetic light response curves (area basis) indicated significantly greater carbon assimilation, and nitrogen and phosphorus use efficiencies in A. platanoides than in A. saccharum. Acer platanoides also exhibited higher water use efficiency than A. saccharum (0.88 +/- 0.12 versus 0.32 +/- 0.09 mmol CO(2) mol(-1) H(2)O). Acer platanoides exhibited significantly lower osmotic potentials than A. saccharum, but a similar relative water content at zero turgor. We conclude that A. platanoides utilizes light, water and nutrients more efficiently than A. saccharum.

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PubMedDate not supplied

Leaf, branch, stand and landscape scale measurements of volatile organic compound fluxes from U.S. woodlands.

Natural volatile organic compound (VOC) fluxes were measured in three U.S. woodlands in summer 1993. Fluxes from individual leaves and branches were estimated with enclosure techniques and used to initialize and evaluate VOC emission model estimates. Ambient measurements were used to estimate above canopy fluxes for entire stands and landscapes. The branch enclosure experiments revealed 78 VOCs. Hexenol derivatives were the most commonly observed oxygenated compounds. The branch measurements also revealed high rates of isoprene emission from three genera of plants (Albizia, Chusqua and Mahonia) and high rates of monoterpene emission from three genera (Atriplex, Chrysthamnus and Sorbus) for which VOC emission rates have not been reported. Measurements on an additional 34 species confirmed previous results. Leaf enclosure measurements of isoprene emission rates from Quercus were substantially higher than the rates used in existing emission models. Model predictions of diurnal variations in isoprene fluxes were generally within +/- 35% of observed flux variations. Measurements with a fast response analyzer demonstrated that 60 min is a reasonable time resolution for biogenic emission models. Average daytime stand scale (hundreds of m) flux measurements ranged from about 1.3 mg C m(-2) h(-1) for a shrub oak stand to 1.5-2.5 mg C m(-2) h(-1) for a mixed forest stand. Morning, evening and nighttime fluxes were less than 0.1 mg C m(-2) h(-1). Average daytime landscape scale (tens of km) flux measurements ranged from about 3 mg C m(-2) h(-1) for a shrub oak-aspen and rangeland landscape to about 7 mg C m(-2) h(-1) for a deciduous forest landscape. Fluxes predicted by recent versions (BEIS2, BEIS2.1) of a biogenic emission model were within 10 to 50% of observed fluxes and about 300% higher than those predicted by a previous version of the model (BEIS).

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PubMedDate not supplied

Survival strategy of hiba (Thujopsis dolabrata var. hondae) and two broad-leaved trees in northern Japan in response to creep and glide pressures of snow.

We compared the tolerance of hiba (Thujopsis dolabrata var. hondae Makino), beech (Fagus crenata Blume) and oak (Quercus serrata Thunb.) to creep and glide pressures of snow. The top/root (T/R) ratio of hiba trees ranged between 7.1 and 41.4 compared with 3.6-10.9 and 1.0-12.4 for beech and oak, respectively. However, if prostrate stems bearing adventitious roots were included in the root component, the T/R ratio of hiba trees was comparable to the range observed for the co-occurring broad-leaved species (1.2 to 3.9). The adventitious roots originating from the trunks of hiba trees extended horizontally during the early growth stage. However, when stem diameter at ground level reached 6-7 cm, some large-diameter adventitious roots developed vertically to form, so-called, sustentacular roots. Because all of the dead hiba trees that we examined lacked sustentacular roots, we conclude that sustentacular roots are necessary to enable young hiba trees to withstand the pressures of snow movement. All of the large beech trees formed sustentacular roots and no dead beech trees were observed. Not all the oak trees formed sustentacular roots, which may explain why this species was found only on gentle slopes.

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PubMed1996

Measuring stem water content in four deciduous hardwoods with a time-domain reflectometer.

New technologies in time-domain reflectometry offer a reliable means of measuring soil water content. Whether these same technologies can be used or adapted to estimate the water content of other porous media, such as the woody tissue of forest trees, has not been thoroughly addressed. Therefore, curves relating the apparent dielectric constant (K(a)) to volumetric water content (g cm(-3)) were constructed for large-diameter stems of red maple (Acer rubrum L.), white oak (Quercus alba L.), chestnut oak (Q. prinus L.), and black gum (Nyssa sylvatica Marsh.). This information was combined with previously published data and a proposed "universal" calibration equation for wood was derived. Stainless-steel rods (15-cm wave guides) were inserted into 160 trees (30 to 49 per species) growing in an upland oak-hickory forest and stem water contents estimated monthly during 1994 and 1995 with a time-domain reflectometer (TDR). Volumetric water contents in April ranged from 0.28 g cm(-3) for red maple to 0.43 g cm(-3) for black gum, with no evidence that water content changed as a function of stem diameter. Stem water contents estimated during 1994 (a wet year) increased from May to July, reached a maximum in midsummer (0.41 to 0.50 g cm(-3)), and then decreased in November. During 1995 (a dry year), stem water contents for red maple and black gum (two diffuse-porous species) decreased from May to August, reached a minimum in September (0.29 to 0.37 g cm(-3)), slightly increased in October and November, and then decreased in December. A different trend was observed during 1995 for white oak and chestnut oak (two ring-porous species), with water contents remaining fairly stable from May to August, but decreasing abruptly in September and again in December. Stem water contents estimated with a TDR broadly agreed with gravimetric analyses of excised stem segments and increment cores, although there was evidence that overestimation of water content was possible with TDR as a result of wounding following wave guide installation. Nonetheless our results hold promise for the application of TDR to the study of stem water content and to the study of whole-plant water storage.

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PubMed1996

Carbon partitioning and allocation in northern red oak seedlings and mature trees in response to ozone.

Northern red oak (Quercus rubra L.) seedlings and trees differ in their response to ozone. Previous work reported reductions in net photosynthesis, carboxylation efficiency and quantum yield of mature tree leaves, whereas seedling processes were unaffected by the same ozone exposure. To further characterize differences in ozone response between seedlings and mature trees, we examined carbon partitioning and allocation in 32-year-old trees and 4-year-old seedlings of northern red oak after exposure to subambient (seasonal SUM00 dose (sum of all hourly ozone exposures) = 31 ppm-h), ambient (SUM00 dose = 85 ppm-h) and twice ambient (SUM00 dose = 151 ppm-h) ozone concentrations for three growing seasons. For mature trees, ozone exposure decreased foliar starch partitioning, increased starch partitioning in branches and increased (14)C retention in leaves. In contrast, starch partitioning in leaves and branches, and foliar (14)C retention in seedlings were unaffected by ozone exposure, but soluble carbohydrate concentrations in coarse and fine roots of seedlings were reduced. Differences in carbohydrate demand between seedlings and mature trees may underlie the higher leaf ozone uptake rates and greater physiological response to ozone in mature northern red oak trees compared with seedlings.

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PubMed1996

Effects of elevated CO(2) and light availability on the photosynthetic light response of trees of contrasting shade tolerance.

Photosynthetic light response curves (A/PPFD), leaf N concentration and content, and relative leaf absorbance (alpha(r)) were measured in 1-year-old seedlings of shade-intolerant Betula papyrifera Marsh., moderately shade-tolerant Quercus rubra L. and shade-tolerant Acer rubrum L. Seedlings were grown in full sun or 26% of full sun (shade) and in ambient (350 ppm) or elevated (714 ppm) CO(2) for 80 days. In the shade treatments, 80% of the daily PPFD on cloud-free days was provided by two 30-min sun patches at midday. In Q. rubra and A. rubrum, leaf N concentration and alpha(r) were significantly higher in seedlings in the shade treatments than in the sun treatments, and leaf N concentration was lower in seedlings in the ambient CO(2) treatments than in the elevated CO(2) treatments. Changes in alpha(r) and leaf N content suggest that reapportionment of leaf N into light harvesting machinery in response to shade and elevated CO(2) tended to increase with increasing shade tolerance of the plant. Shifts induced by elevated CO(2) in the A/PPFD relationship in sun plants were largest in B. papyrifera and least in A. rubrum: the reverse was true for shade plants. Elevated CO(2) resulted in increased light-saturated A in every species x light treatment combination, except in shaded B. papyrifera. The light compensation point (Gamma) decreased in response to shade in all species, and in response to elevated CO(2) in A. rubrum and Q. rubra. Acer rubrum had the greatest increases in apparent quantum yield (phi) in response to shade and elevated CO(2). To illustrate the effects of shifts in A, Gamma and phi on daily C gain, daily integrated C balance was calculated for individual sun and shade leaves. Ignoring possible stomatal effects, estimated daily (24 h) leaf C balance was 218 to 442% higher in the elevated CO(2) treatments than in the ambient CO(2) treatments in both sun and shade seedlings of Q. rubra and A. rubrum. These results suggest that the ability of species to acclimate photosynthetically to elevated CO(2) may, in part, be related to their ability to adapt to low irradiance. Such a relationship has implications for altered C balance and nitrogen use efficiency of understory seedlings.

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PubMed1996

Estimates of net photosynthetic parameters for twelve tree species in mature forests of the southern Appalachians.

Leaf gas exchange, temperature, and incident radiation were measured in situ for 20 mature trees of 12 deciduous species spanning a range of heights from 7.9 to 30.1 m and growing in the southern Appalachian Mountains. Air temperature, water vapor pressure, total radiation, photosynthetically active radiation, and carbon dioxide concentration were also measured. Estimated mean, light-saturated net assimilation rates ( micro mol m(-2) s(-1)) were: Quercus coccinea Muenchh. (10.3), Q. prinus L. (9.9), Q. rubra L. (8.9), Betula lenta L. (8.1), Liriodendron tulipifera L. (7.9), Q. alba L. (7.6), Carya glabra Mill. (7.2), Acer rubrum L. (5.6), Nyssa sylvatica Marsh. (3.9), Cornus florida L. (3.5), and Acer pensylvanicum L. (1.7). There were significant differences in both net assimilation rates and quantum yield efficiencies between species, with the understory species C. florida and A. pensylvanicum exhibiting lower net assimilation rates at saturation and higher estimated quantum yield efficiencies than the other species. Average temperature and light decreased from the canopy top to bottom, whereas ambient CO(2) concentration increased, and vapor pressure and vapor pressure deficits were inconsistent. We observed curvilinear effects of temperature and vapor pressure deficit on net assimilation response to light, and these effects varied by species. Errors in predicted net assimilation ranged from 1 to 3 micro mol m(-2) s(-1) under the environmental conditions prevailing during the study.

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PubMed1996

Leaf gas exchange and growth of flood-tolerant and flood-sensitive tree species under low soil redox conditions.

Seedlings of Taxodium distichum L., Quercus lyrata Walt. and Q. falcata var. pagodaefolia Ell. were grown for 22 days in a rhizotron system providing two soil redox potential regimes, +170 mV (low Eh) and +560 mV (high Eh). Leaf chlorophyll concentration and gas exchange, root alcohol dehydrogenase (ADH) activity, root and leaf ethylene production, and growth and biomass partitioning were measured. In response to the low Eh soil treatment, stomatal conductance was reduced in Q. falcata and Q. lyrata but not in T. distichum, whereas net photosynthesis was reduced significantly in all species; however, net photosynthesis in T. distichum began to recover within 2 weeks of treatment initiation. Within each treatment, mean stomatal conductance and net photosynthesis were significantly greater in T. distichum than in the oak species. Leaf chlorophyll concentration was not affected by the soil treatments. All species showed significant reductions in root and leaf dry weights in response to the low Eh soil condition. The low Eh soil treatment resulted in increased root ADH activity and ethylene production in T. distichum, but had no effect on root ADH activity and ethylene production in the oak species.

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PubMed1996

Long- and short-term flooding effects on survival and sink-source relationships of swamp-adapted tree species.

About 95% of swamp tupelo (Nyssa sylvatica var. biflora (Walt.) Sarg.) and sweetgum (Liquidambar styraciflua L.) seedlings survived continuous root flooding for more than two years, whereas none of the swamp chestnut oak (Quercus michauxii Nutt.) and cherrybark oak (Q. falcata var. pagodifolia Ell.) seedlings survived one year of flooding. Death of oak seedlings occurred in phases associated with periods of major vegetative growth, e.g., after bud burst in spring, after summer stem elongation, and during the winter deciduous stage, suggesting that stored reserves and sources were inadequate to maintain the seedlings when vegetative sinks were forming. Additional evidence that flooding induced a source deficiency in oak was that leaves of flooded oak were 65 to 75% smaller than leaves of nonflooded oak. Flooded swamp tupelo seedlings had a normal leaf size and patchy stomatal opening compared with nonflooded seedlings. Flooding caused increases in alcohol dehydrogenase (ADH) specific activity in taproot cambial tissues and increases in starch concentrations of swamp tupelo seedlings that were reversed when seedlings were removed from flooding. Flooding had little effect on soluble sugar concentrations in swamp tupelo or sweetgum. In the long-term flood-dry-flood treatment, in which all species had survivors, upper canopy leaf photosynthetic rates were higher in all species during the dry period than in nonflooded controls, whereas their starch and soluble sugars concentrations were similar to those of nonflooded controls. Based on seedling survival and the sink-source relationships, the order of flood tolerance was: swamp tupelo > sweetgum > swamp chestnut oak > cherrybark oak.

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