Variation in water relations of black spruce stock types planted in Ontario

Upland, intermediate and lowland sites in northeastern Ontario were planted between May 28 and June 8 with three types of black spruce (Picea mariana (Mill.) BSP) nursery stock: (1) spring-lifted, 1.5 + 1.5 bareroot plants (BR); (2) 24-week-old, winter-sown, container stock (CWS); and (3) spring-sown, overwintered, container stock (CO). At the beginning of the growing season, the BR stock had the lowest xylem pressure potentials (Psi(x)), stomatal conductances (g(wv)), and net photosynthetic (P(n)) rates. By the end of the growing season, the BR stock still had lower g(wv)s than the container stock types, but had higher shoot Psi(x) values. In August, the turgor loss points for the BR, CO and CWS stock types were -2.8, -1.93 and -1.6 MPa, respectively, while the minimum observed shoot Psi(x) values were -1.4, -1.7 and -1.9 MPa, respectively. The BR stock produced the greatest dry weight of new shoots and unsuberized roots. No new shoots were produced by the CWS stock, but they produced a greater dry weight of unsuberized roots than the CO stock. As a percent of the dry weight of suberized roots, the greatest production of unsuberized roots was by the CWS stock, the least by the BR trees.     

Planting stress in newly planted jack pine and white spruce. 1. Factors influencing water uptake

Bareroot jack pine (Pinus banksiana Lamb.) seedlings (2 + 0) and bareroot white spruce (Picea glauca (Moench) Voss) transplants (1 1/2 + 1 1/2) were taken from cold storage and planted on a clearcut forest site in northeastern Ontario on several dates between May 6 and June 5 during which period soil temperature at 15 cm depth increased from 0 to 18 degrees C. Additional cold-stored trees were transferred to a greenhouse where they were grown in pots for 0, 7 or 28 days and then placed with their roots in aerated water maintained at one of a range of constant temperatures between 0 and 22 degrees C. In both species, daytime xylem pressure potentials (Psi(x)) and needle conductances (g(wv)) decreased with decreasing soil or water temperature. At all root temperatures, g(wv) was lower, and Psi(x) higher, in jack pine than in white spruce. After 28 days in the greenhouse, g(wv) of jack pine seedlings, and Psi(x) of white spruce, was higher than in plants just removed from cold storage. In both species, water-flow resistance through the soil-plant-atmosphere continuum (RSPAC) increased as root temperature decreased. At all root temperatures, RSPAC was higher in plants just removed from cold storage than in plants grown in the greenhouse for 28 days, during which time many new unsuberized roots were formed. At root temperatures above 10 degrees C, RSPAC of both species was higher in trees newly planted in mineral soil than in trees with roots in aerated water; presumably because the roots of planted trees had limited hydraulic contact with the soil. On the day following removal from cold storage, relative plant water flow resistance increased, in both species, more rapidly with declining root temperature than could be accounted for by the change with temperature in the viscosity of water, thus indicating an effect of temperature on root permeability. The same effect was evident in jack pine seedlings, but not white spruce transplants, that had been grown for 28 days in the greenhouse after removal from cold storage.     

Site preparation: Water relations and growth of newly planted jack pine and white spruce

Bareroot jack pine (Pinus banksiana Lamb.) and white spruce (Picea glauca (Moench) Voss) were planted near Elliot Lake, Ontario, on a boreal reforestation site. Site preparation treatments were mixed, mineral and undisturbed (i.e., control) soil. Seedling water relations and growth were examined during the first field season. During the first 28 days after planting, jack pine base (i.e., predawn) and minimum xylem water potential readings were more negative in the control site preparation treatment. White spruce, during the first 10 days, in all site preparation treatments had base and minimum xylem water potential readings more negative than –1.7 MPa. By day 28 base xylem water potentials of white spruce had increased to approximately –1.0 MPa in all site preparation treatments. As the growing season progressed, white spruce minimum xylem water potential readings ceased exceeding the measured turgor loss point first in the mixed followed by the mineral and then control site preparation treatment. Jack pine minimum xylem water potential readings, in all site preparation treatments, almost never exceeded the measured turgor loss point. Water stress and stomatal optimization integrals, day 28 and 125, for both species showed least water stress and greater stomatal optimization in the mixed, mineral and control site preparation treatments, respectively. Both species had less new root growth in the field during the first 28 days after planting compared to seedlings grown for 28 days in a greenhouse for root growth capacity testing. Root growth at 28 days and both shoot and root development at the end of the growing season, were greatest to least in mixed, mineral, and control site preparation treatments, respectively.     

Shoot water relations of mature black spruce families displaying a genotype x environment interaction in growth rate. II. Temporal trends and response to varying soil water conditions

Pressure-volume curves and shoot water potentials were determined for black spruce (Picea mariana (Mill.) BSP) trees from four full-sib families at the Petawawa Research Forest, Ontario, Canada. Trees were sampled from a dry site in 1992 and from the dry site and a wet site in 1993. Modulus of elasticity (epsilon), osmotic potential at turgor loss point (Psi(tlp)) and relative water at turgor loss point (RWC(tlp)) all decreased during the growing season. Osmotic potential at saturation (Psi(sat)) and turgor displayed no general temporal trend. Across a range of environmental conditions, Female 59 progeny had equal or lower Psi(sat), and higher or similar epsilon, mean turgor pressure (P(x)) and predawn turgor pressure (P(pd)) compared with Female 63 progeny. Osmotic potential at saturation decreased as water stress increased from mild to moderate and increased as water stress increased from moderate to severe. Stable genetic differences in Psi(sat) were maintained by the same rate of osmotic adjustment from low to moderate water stress. Modulus of elasticity and RWC(tlp) decreased with decreasing water availability, whereas Psi(tlp) showed no response. The combined effects of Psi(sat) and epsilon resulted in no change in P(pd) as water stress increased from low to moderate values, but turgor declined sharply as water stress increased from moderate to high values. We conclude that drought tolerance traits strongly influence the growth of these black spruce families across sites of varying water availability.     

Influence of drought stress and low irradiance on plant water relations and structural constituents in needles of Pinus ponderosa seedlings

The influence of low light on tolerance to prolonged drought was tested on unshaded and shaded seedlings of ponderosa pine (Pinus ponderosa var. scopulorum Dougl. ex Laws.). Unshaded seedlings of P. ponderosa var. ponderosa were also drought stressed to compare varietal responses to drought. The maximum irradiance received by shaded seedlings was 10% of full light. Seedlings were progressively drought stressed until predawn water potentials (Psi(x)) were -5.0 MPa. Relative water content (RWC) and the reciprocal of Psi(x) were analyzed by means of an unusual application of the pressure-volume relationship for determination of RWC of the apoplast (RWC(a)), osmotic potential at full turgor (Psi(oft)), and ratio of fully turgid weight to dry weight. Major varietal differences in drought response were in RWC(a) and needle cellulose content. The shaded seedlings showed tissue damage at relative water contents < 60%, and were killed by water deficits from which unshaded seedlings recovered. Correspondingly, shaded plants had significantly higher cell volume/cell mass ratio, Psi(oft), less cellulose in needle tissue, and lower RWC(a) than unshaded plants. These differences suggest that low irradiance restricts drought adaptation in ponderosa pine.     

Water relations and growth of loblolly pine seedlings planted under a shelterwood and in a clear-cut

We investigated the influence of shelterwood conditions on water relations and growth of loblolly pine (Pinus taeda L.) seedlings on two harsh sites in eastern Texas. Site I was harvested to provide four overstory density treatments (0, 2.3, 4.6 and 9.2 m(2) of residual basal area per ha). To quantify the effects of overstory competition, trenched and nontrenched subplots, each containing 25 one-year-old seedlings, were established within each overstory treatment plot, and predawn and midday water potentials (Psi(w)), seedling growth and survival were measured during the growing season. Leaf area and seedling biomass partitioning were measured at the end of the growing season. Site II was harvested to provide two overstory density treatments (0 and 6.9 m(2) ha(-1)) and planted with one-year-old loblolly pine seedlings. Seedling Psi(w), stomatal conductance (g(wv)), transpiration flux density (E), leaf area, height and survival were determined. On Site I, seedling Psi(w) increased with increasing overstory basal area, whereas trenching only substantially affected Psi(w) of seedlings in the 9.2 m(2) ha(-1) overstory treatment. Growth was not affected by overstory treatment or trenching. On Site II, Psi(w) and g(wv) were highest during the morning hours and lowest in the afternoon, whereas E peaked in the afternoon. Vapor pressure deficits and photosynthetic photon flux density were major factors in determining g(wv) differences between treatments. On individual days, the presence of an overstory increased Psi(w) and reduced both g(wv) and E. On Site II, leaf area was affected by overstory treatment throughout most of the study. We conclude that the presence of an overstory can have ameliorative effects on harsh sites at the western fringe of the loblolly pine natural range.     

Shoot water relations of mature black spruce families displaying a genotype x environment interaction in growth rate. I. Family and site effects over three growing seasons

Pressure-volume curves were determined for black spruce (Picea mariana (Mill.) BSP) trees from four full-sib families. During the first two years, trees were measured from a plantation on a dry site. In the third year, trees were sampled from the dry site and a wet site. Diurnal measurements of shoot water potential allowed in situ shoot turgor to be estimated in addition to standard water relations traits. Over all years, Female 59 progeny displayed lower osmotic potentials at saturation (Psi(sat)) than Female 63 progeny. Genetic differences in Psi(sat) were similar on both the dry and wet sites. Modulus of elasticity (epsilon) was greater for Female 59 progeny than for Female 63 progeny, producing a compensatory effect resulting in no genetic or site differences in osmotic potential at turgor loss point (Psi(tlp)) or relative water content at turgor loss point (RWC(tlp)). Mean and predawn shoot turgor pressures (P(x) and P(pd)) were higher for Female 59 progeny than for Female 63 progeny and higher at the wet site than the dry site. Genotype x environment trends were observed; compared to Female 63 progeny, Female 59 progeny displayed 9.8 and 5.1% higher P(pd) on the dry and wet sites, respectively, and 3.4 and 9.8% greater P(pd) values in wet and dry years, respectively. Tree volume growth showed no relationship to Psi(tlp) or RWC(tlp), but was correlated with Psi(sat) and P(x); however, the strongest correlation was with P(pd) (r = 0.90).     

Osmotic potential of several hardwood species as affected by manipulation of throughfall precipitation in an upland oak forest during a dry year

Components of dehydration tolerance, including osmotic potential at full turgor (Psi(pio)) and osmotic adjustment (lowering of Psi(pio)), of several deciduous species were investigated in a mature, upland oak forest in eastern Tennessee. Beginning July 1993, the trees were subjected to one of three throughfall precipitation treatments: ambient, ambient minus 33% (dry treatment), and ambient plus 33% (wet treatment). During the dry 1995 growing season, leaf water potentials of all species declined to between -2.5 and -3.1 MPa in the dry treatment. There was considerable variation in Psi(pio) among species (-1.0 to -2.0 MPa). Based on Psi(pio) values, American beech (Fagus grandifolia Ehrh.), dogwood (Cornus florida L.), and sugar maple (Acer saccharum Marsh.) were least dehydration tolerant, red maple (A. rubrum L.) was intermediate in tolerance, and white oak (Quercus alba L.) and chestnut oak (Quercus prinus L.) were most tolerant. During severe drought, overstory chestnut oak and understory dogwood, red maple and chestnut oak displayed osmotic adjustment (-0.12 to -0.20 MPa) in the dry treatment relative to the wet treatment. (No osmotic adjustment was evident in understory red maple and chestnut oak during the previous wet year.) Osmotic potential at full turgor was generally correlated with leaf water potential, with both declining over the growing season, especially in species that displayed osmotic adjustment. However, osmotic adjustment was not restricted to species considered dehydration tolerant; for example, dogwood typically maintained high Psi(pio) and displayed osmotic adjustment to drought, but had the highest mortality rates of the species studied. Understory saplings tended to have higher Psi(pio) than overstory trees when water availability was high, but Psi(pio) of understory trees declined to values observed for overstory trees during severe drought. We conclude that Psi(pio) varies among deciduous hardwood species and is dependent on canopy position and soil water potential in the rooting zone.     

Analysis of leaf water relations in leaves of two olive (Olea europaea) cultivars differing in tolerance to salinity

One-year-old rooted cuttings of olive (Olea europaea L. cvs. Frantoio and Leccino) were grown either hydroponically or in soil in a greenhouse. Plants were exposed to NaCl treatments (0, 100, and 200 mM) for 35 days, followed by 30 to 34 days of relief from salt stress to determine whether previously demonstrated genotypic differences in tolerance to salinity were related to water relations parameters. Exposure to high salt concentrations resulted in reductions in predawn water potential (Psi(w)), osmotic potential at full turgor (Psi(piFT)), osmotic potential at turgor loss point (Psi(piTLP)), and relative water content (RWC) in both cultivars, regardless of the growth substrate. Leaf Psi(w) and RWC returned to values similar to those of controls by the end of the relief period. The effect of salinity on Psi(pi) appeared earlier in Leccino than in Frantoio. Values for Psi(piFT) were -2.50, -2.87, and -3.16 MPa for the 0, 100, and 200 mM salt-treated Frantoio plants, respectively, and -2.23, -2.87, and -3.37 MPa for the corresponding Leccino plants. Recovery of Psi(pi) was complete for plants in the 100 mM salt treatment, but not for plants in the 200 mM salt treatment, which maintained an increased pressure potential (Psi(pi)) compared to control plants. Net solute accumulation was higher in Leccino, the salt-sensitive cultivar, than in Frantoio. In controls of both cultivars, cations contributed 39.9 to 42.0% of the total Psi(piFT), mannitol and glucose contributed 27.1 to 30.8%, and other soluble carbohydrates contributed 3.1 to 3.6%. The osmotic contribution of Na(+) increased from 0.1-2.1% for non-treated plants to 8.6-15.5% and 15.6-20.0% for the 100 mM and 200 mM salt-treated plants, respectively. The mannitol contribution to Psi(piFT) reached a maximum of 9.1% at the end of the salinization period. We conclude that differences between the two cultivars in leaf water relations reflect differences in the exclusion capacities for Na(+) and Cl(-) ions.     

Physiological responses of radiata pine roots to soil strength and soil water deficit

We investigated physiological responses of radiata pine (Pinus radiata D. Don) roots to soil strength and soil water deficit by measuring the osmotic potential (Psi(pi)) and yield turgor (Y) in the elongation zone of root segments of seedlings growing (i) in polyethylene glycol 4000-containing rooting solution of different water potentials (Psi(s)) and (ii) in soil of different soil strengths (Q) at the same soil matric potential (Psi(m)). Root elongation rate (Deltal/Deltat) decreased progressively with decreasing Psi(s) and was associated with decreased Psi(pi) and decreased turgor pressure (P). Osmotic adjustment occurred at Psi(s) < -0.2 MPa. Over a range in Psi(s) of -0.01 to -1.0 MPa, Psi(pi) fell 0.3 MPa whereas P fell 0.7 MPa. Mean Psi in the solution experiment was 0.37 MPa and did not differ significantly with Psi(s) (P = 0.10). Root elongation rate decreased exponentially as Q increased from 0 to 3.0 MPa, and was associated with an increase in P of 0.11 MPa as a consequence of Psi(pi) decreasing by the same amount. Mean Y in the soil experiment was 0.49 MPa and did not change significantly with Q (P = 0.87).     

Reduction in turgid water volume in jack pine,white spruce and black spruce in response to drought and paclobutrazol

Significant reductions in needle water content were observed in white spruce (Picea glauca (Moench) Voss), black spruce (Picea mariana (Mill) B.S.P.), and jack pine (Pinus banksiana Lamb.) seedlings in response to a 10-day drought, although turgor was apparently maintained. When the seedlings were re-watered after the drought, jack pine needles regained their original saturated volume, whereas white spruce and black spruce needles did not. Significant drought-induced reductions in turgor-loss volume (i.e., tissue volume at the point of turgor loss) were observed in shoots of all three species, especially jack pine. Repeated exposure to 7 days of drought or treatment with the cytochrome P(450) inhibitor, paclobutrazol ((2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-pentan-3-ol), reduced seedling height relative to that of untreated controls in all three species. The reductions in saturated and turgor-loss needle volumes in the paclobutrazol-treated seedlings were comparable with those of seedlings subjected to a 10-day drought. The treatment-induced reductions in shoot and needle water contents enabled seedlings to maintain turgor with tissue volumes close to, or below, the turgor-loss volume of untreated seedlings. Paclobutrazol-treated seedlings subsequently survived drought treatments that were lethal to untreated seedlings.     

Clonal and seasonal differences in leaf osmotic potential and organic solutes of five hybrid poplar clones grown under field conditions

Leaf osmotic potential at full turgor (Psi(pio)) and the major solutes that contribute to osmotic potential were characterized in five hybrid poplar clones of Populus trichocarpa Torr. & Gray x P. deltoides Bartr. (TD) and P. deltoides x P. nigra L. (DN), growing under field conditions at two sites in eastern Washington and Oregon, USA. Trees were drip irrigated with 46, 76 or 137 cm of supplemental irrigation during each growing season. Trees at Wallula, WA, which were in their third growing season in 1994, were sampled twice a year for two years (1994 and 1995), and trees at Boardman, OR, which were in their second growing season in 1994, were sampled once a year for three years (1994-1996). At Wallula, the TD and DN clones exhibited lower predawn leaf water potentials in the 46-cm treatment than in the 137-cm treatment (-1.2 versus -0.7 MPa) during a hot, dry period in July 1994. Clone TD had a lower Psi(pio) than Clone DN (-1.67 versus -1.56 MPa) during the same period and the difference was also evident in 1995 (-1.81 versus -1.72 MPa) when trees were in their fourth growing season. There was also a significant treatment effect on Psi(pio) in Clone TD, with trees in the 46-cm treatment having lower Psi(pio) than trees in the 137-cm treatment in July 1994. At Boardman, Psi(pio) was generally high with no treatment differences during the 1994-96 samplings. The TD clones had significantly lower Psi(pio) than the DN clones in 1994 (-1.44 versus -1.36 MPa) and 1996 (-1.72 versus -1.54 MPa), but there was no difference between clones in 1995 (-1.40 versus -1.43 MPa). In 1995, at Wallula, osmotic adjustment in Clone TD was largely accounted for by an increase in sucrose, which constituted 70% of total organic solutes. Although the total concentration of free primary amino acids in this clone was 28% higher in trees in the 46-cm treatment than in trees in the 137-cm treatment, amino acids constituted only a small fraction of the total solute pool. Sixty-two percent of total solutes were inorganic ions in Clone TD compared to 52% in Clone DN, and potassium was the main ion constituting about 30% of total solutes and 50% of total ions. However, the clonal difference in Psi(pio) was not fully accounted for by the difference in solute concentration. Osmotic potential at full turgor declined over the growing season and with age. We conclude that, because the extent of osmotic adjustment exhibited by these clones was small, other drought resistance mechanisms contributed to the clonal differences in field performance.     

Xylem cavitation and loss of hydraulic conductance in western hemlock following planting

Following planting, western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings experience water stress and declining xylem pressure potential (Psi(x)). Low Psi(x) can result in xylem cavitation and embolism formation, causing a decline in hydraulic conductance. This study focused on the relationship between Psi(x), xylem cavitation and transpiration (E) of newly planted seedlings. Leaf specific hydraulic conductance (k(AB)) declined from 0.56 to 0.09 mmol m(-2) s(-1) MPa(-1) over a 9-day period. Stomatal conductance (g(s)) declined from 143.5 to 39.15 mmol m(-2) s(-1) over the same period without an associated change in environmental conditions. A vulnerability profile indicated a 30% loss in hydraulic conductivity when seedlings experienced a Psi(x) between -2.5 and -3.0 MPa. A Psi(x) of -4.0 MPa led to a complete loss of conductivity. We conclude that following planting, western hemlock seedlings often experience Psi(x) values that are low enough to cause xylem cavitation and a decline in k(AB).     

Response of gas exchange to water stress in seedlings of woody angiosperms

Responses of net photosynthesis (A), leaf conductance to water vapor (g(wv)) and instantaneous water use efficiency (WUE) to decreasing leaf and soil water potentials (Psi(l), Psi(s)) were studied in three-month-old white oak (Quercus alba L.), post oak (Q. stellata Wangenh.), sugar maple (Acer saccharum Marsh.), and black walnut (Juglans nigra L.) seedlings. Quercus seedlings had the highest A and g(wv) when plants were well watered. As the soil was allowed to dry, both A and g(wv) decreased; however, trace amounts of A were observed at a Psi(l) as low as -2.9 MPa in Q. stellata and -2.6 MPa in Q. alba and A. saccharum. Photosynthesis was not measurable at Psi(l) lower than -2.2 MPa in J. nigra and water stress-induced leaflet senescence was observed in this species. Within each species, g(wv) showed a similar relationship to soil and leaf Psi, but the response to Psi(l) was shifted to more negative values by 1.2 to 1.6 MPa. As Psi(s) declined below -1 MPa, the difference between soil and leaf Psi diminished because of the suppression of transpiration. There was no indication that Psi(s) had a more direct influence on g(wv) than did Psi(l). Water use efficiency showed an initial increase as the soil dried, followed by a decline under severe water stress. Water use efficiency was highest in J. nigra, intermediate in Quercus species and lowest in A. saccharum. There was an evident relationship between gas exchange characteristics and natural distribution in these species, with the more xeric species showing higher A and g(wv) under both well-watered and water-stressed conditions. There was no trend toward increased efficiency of water use in the more xeric species.     

Relationship between nuclear DNA markers and physiological parameters in Sitka x interior spruce populations

Eight populations of Sitka spruce (Picea sitchensis (Bong.) Carr.) and interior spruce (Picea glauca (Moench) Voss x Picea engelmannii Parry ex. Engelm.) seedlings were sampled from a zone of Sitka-interior spruce introgression in British Columbia, Canada. Restriction fragment length polymorphisms of the nuclear ribosomal RNA genes (rDNA) were used to define species-specific hybridization patterns for the Sitka spruce and interior spruce populations. Hybridization was estimated from an index based on the relative abundance of polymorphic rDNA combining bands for each population. Sitka x interior hybrid seedlings had an index value for the relative abundance of interior spruce rDNA (Si-rDNA) ranging from 0.07 (Lower Nass; the most westerly collected source) to 0.95 (Bulkley Valley low-elevation seed orchard). During shoot elongation, osmotic potential at saturation (Psi(sat)) and turgor loss point (Psi(tlp)) increased, whereas total turgor (Psi(PTotal)) decreased. After bud set in the summer and throughout the fall, Psi(sat) and Psi(tlp) decreased, whereas Psi(PTotal) increased. At all times of year, populations with a higher Si-rDNA index had lower Psi(tlp) and Psi(sat) and higher Psi(PTotal) than populations with a lower Si-rDNA index. During the fall, Sitka x interior hybrid seedlings exhibited a seasonal decline in the temperature causing 50% needle electrolyte leakage (LT(50)) and in the critical temperature indicating the initial point of freezing injury. Seedlings with a higher Si-rDNA index had lower LT(50) and critical temperature values indicating greater freezing tolerance in the fall. Throughout most of the year, seedling population Si-rDNA index was related to the degree of drought and freezing tolerance.     

Physiological responses to water stress and waterlogging in Nothofagus species

Gas exchange and water relations were investigated in Nothofagus solandri var. cliffortioides (Hook. f.) Poole (mountain beech) and Nothofagus menziesii (Hook. f.) Oerst (silver beech) seedlings in response to water stress and waterlogging. At soil matric potentials (Psi(soil)) above -0.005 MPa, N. solandri had significantly higher photosynthetic rates (A), and stomatal and residual conductances (g(sw) and g(rc)), and lower predawn xylem water potentials (Psi(predawn)) than N. menziesii. The relative tolerance of plants to water stress was defined in terms of critical soil matric potential (Psi(cri)) and lethal xylem water potential (Psi(lethal)). The estimated values of Psi(cri) and Psi(lethal) were -1.2 and -7 MPa, respectively, for N. solandri, and -0.7 and -4 MPa, respectively, for N. menziesii. Photosynthesis was sustained to a xylem water potential (Psi(xylem)) of -7 MPa in N. solandri compared with -4 MPa in N. menziesii. Following rewatering, both A and Psi(xylem) recovered quickly in N. solandri, whereas the two variables recovered more slowly in N. menziesii. During the development of water stress, nonstomatal inhibition significantly affected A in both N. solandri and N. menziesii. Nothofagus menziesii was more susceptible to inhibition of A by waterlogging than N. solandri. However, the tolerance of N. solandri to severe waterlogging was also limited as a result of a failure to form adventitious roots, suggesting a lack of adaptation to these conditions. The differences in tolerance to water stress and waterlogging between the two species are consistent with the distribution patterns of N. solandri and N. menziesii in New Zealand.     

Winter water relations of New England conifers and factors influencing their upper elevational limits. I. Measurements

The upper elevational limits of tree species are thought to be controlled by abiotic factors such as temperature and the soil and atmospheric conditions affecting plant water status. We measured relative water contents (RWC), water potentials (Psi) and cuticular conductances (g(c)) of shoots of four conifer species-eastern hemlock (Tsuga canadensis (L.) Carr.), eastern white pine (Pinus strobus L.), red pine (P. resinosa Ait.) and red spruce (Picea rubens Sarg.)-during two winters on Mt. Ascutney, Vermont, USA. Some micrometeorological measurements are also reported. Eastern hemlock and white pine were studied near their upper elevational limits at a 640-m site, and red pine was studied near its upper elevational limit at 715 m. Red spruce was also studied at the 715-m site, which is in the middle of its elevational range on this mountain. There was no evidence of winter desiccation stress in any species. The observed distribution of seedlings suggested that the upper elevational limits on shade-intolerant eastern white pine and red pine are set by the absence of suitable seed beds after 100 years without fire. Eastern hemlock is able to reproduce in deep shade on organic substrates, but germination at high elevations may be restricted by low temperatures.     

Hydraulic conductance and soil water potential at the soil-root interface of Pinus pinaster seedlings inoculated with different dikaryons of Pisolithus sp

Seedlings of maritime pine (Pinus pinaster Ait.) were inoculated with different dikaryons of Pisolithus sp. from South Africa to determine the influence of extension of the extramatrical phase and diameter of the mycelial strands on water relations parameters including xylem water potential (Psi(x)), soil water potential at the soil-root interface (Psi(s)) and hydraulic conductance (L(p)) during and after a period of water stress. Seedlings inoculated with dikaryons having an extensive extramatrical phase and large diameter mycelial strands showed higher Psi(s) (-2 MPa) during severe water stress than seedlings inoculated with dikaryons producing fine hyphae and sparse extramatrical phases (-3.8 MPa). Seedlings inoculated with strand-forming dikaryons recovered faster from water stress than did non-inoculated seedlings or seedlings inoculated with non-strand-forming dikaryons. Architectural aspects of the extramatrical phase, including the presence of large diameter mycelial strands or fine hyphae, influenced the soil-root contact and the water relations of an inoculated host plant. When water stress was not limiting, the architecture of the extramatrical phase did not have a large effect on Psi(s). It is suggested that the architecture of the extramatrical phase influences the resistance to water flow through the soil-root interface and that large mycelial strands increase the water flow by bridging the gap between the soil and the root. These changes in physiology indicate that dikaryons can improve the survival of Pinus pinaster under dry conditions.     

Parameterization and testing of a coupled photosynthesis-stomatal conductance model for boreal trees

A coupled photosynthesis-stomatal conductance model was parameterized and tested with branches of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) trees growing in the Northern Study Area of the Boreal Ecosystem-Atmosphere Study (BOREAS) in Manitoba, Canada. Branch samples containing foliage of all age-classes were harvested from a lowland old black spruce (OBS) and an old jack pine (OJP) stand and the responses of photosynthesis (A(n)) and stomatal conductance (g(s)) to temperature, CO(2), light, and leaf-to-air vapor pressure difference (VPD) were determined under controlled laboratory conditions at the beginning, middle, and end of the growing season (Intensive Field Campaigns (IFC) 1, 2, and 3, respectively). The parameterized model was then tested against in situ field gas-exchange measurements in a young jack pine (YJP) and an upland black spruce (UBS) stand as well as in the OBS and OJP stands. Parameterization showed that Rubisco capacity (V(max)), apparent quantum yield (alpha') and Q(10) for sink limitation were the most crucial parameters for the photosynthesis sub-model and that V(max) varied among different measurement series in the laboratory. Verification of the model against the data used to parameterize it yielded correlation coefficients (r) of 0.97 and 0.93 for black spruce and jack pine, respectively, when IFC-specific parameters were used, and 0.77 and 0.87 when IFC-2 parameters were applied to all IFCs. For both measured and modeled g(s), the stomatal conductance sub-model, which linearly relates g(s) to (A(n)h(s))/c(s) (where h(s) and c(s) are relative humidity and CO(2) mole fraction at the leaf surface, respectively), had significantly steeper slopes and higher r values when only the VPD response data were used for parameterization than when all of the response data were used for parameterization. Testing the photosynthesis sub-model against upper canopy field data yielded poor results when laboratory estimates of V(max) were used. Use of the mean V(max) estimated for all upper canopy branches measured on a given day improved model performance for jack pine (from a nonsignificant correlation between measured and modeled A(n) to r = 0.45), but not for black spruce (r = 0.45 for both cases). However, when V(max) was estimated for each branch sample individually, the model accurately predicted the 23 to 137% diurnal variation in A(n) for all stands for both the upper and lower canopy. This was true both when all of the other parameters were IFC-specific (r = 0.93 and 0.92 for black spruce and jack pine, respectively) and when only mid-growing season (IFC-2) values were used (r = 0.92 for both species). Branch-specific V(max) estimates also permitted accurate prediction of field g(s) (r = 0.75 and 0.89 for black spruce and jack pine, respectively), although parameterization with all of the response data overestimated g(s) in the field, whereas parameterization with only the VPD response data provided unbiased predictions. Thus, after parameterization with the laboratory data, accurately modeling the range of A(n) and g(s) encountered in the field for both black spruce and jack pine was reduced to a single unknown parameter, V(max).     

Offsetting effects of reduced root hydraulic conductivity and osmotic adjustment following drought

Root hydraulic conductivity (L(p)) and leaf osmotic potential at full turgor (Psi(pi,o)) were measured in young, drought-stressed and nonstressed peach (Prunus persica (L.) Batsch), olive (Olea europaea L.), citrumelo (Poncirus trifoliata Raf. x Citrus paradisi Macf.) and pistachio (Pistachia integerrima L.). Drought stress caused a 2.5- to 4.2-fold reduction in L(p), depending on species, but Psi(pi,o) was reduced only in citrumelo and olive leaves by 0.34 and 1.4 MPa, respectively. No differences existed in L(p) among species for nonstressed plants. A simple model linking L(p) to osmotic adjustment through leaf water potential (Psi) quantified the offsetting effects of reduced L(p) and osmotic adjustment on the hypothetical turgor pressure difference between drought-stressed and nonstressed plants (DeltaPsi(p)). For olive, the 2.5-fold reduction in L(p) caused a linear decrease in DeltaPsi(p) such that the effect of osmotic adjustment was totally negated at Psi = -3.2 MPa. Thus, no stomatal closure would be required to maintain higher turgor in drought-stressed olive plants than in nonstressed plants over their typical diurnal range of Psi (-0.6 to -2.0 MPa). For citrumelo, osmotic adjustment was offset by reduced L(p) at Psi approximately -0.9 MPa. Unlike olive, stomatal closure would be necessary to maintain higher turgor in drought-stressed citrumelo plants than in nonstressed plants over their typical diurnal range of Psi (0 to -1.5 MPa). Regardless of species or the magnitude of osmotic adjustment, my analysis suggests that a drought-induced reduction in L(p) reduces or eliminates turgor maintenance through osmotic adjustment.