Water flow through junctions in Douglas-fir roots

Roots are important conduits for the redistribution of water within the rooting zone. Root systems are often highly branched, and water flow between regions undoubtedly involves passage through junctions between individual roots. This study considered junctions in the roots of Douglas-fir with regard to the resistances encountered by water flow through the xylem. Flow into the root branch distally along the main root encountered much greater resistance than flow into the branch and proximally along the main root (toward the plant stem). When the main root proximal to the junction was gradually shortened, the resistance to flow in the branch root and distally along the main root increased dramatically. Thus, flow in this manner appears to depend on lateral flow within the root over many centimetres proximal to the junction and not just within the direct connection at the junction. These results suggest that the hydraulic nature of junctions is an important aspect of hydraulic redistribution of water within the soil utilizing flow through roots.     

Native root xylem embolism and stomatal closure in stands of Douglas-fir and ponderosa pine: mitigation by hydraulic redistribution

Hydraulic redistribution (HR), the passive movement of water via roots from moist to drier portions of the soil, occurs in many ecosystems, influencing both plant and ecosystem-water use. We examined the effects of HR on root hydraulic functioning during drought in young and old-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] and ponderosa pine (Pinus ponderosa Dougl. Ex Laws) trees growing in four sites. During the 2002 growing season, in situ xylem embolism, water deficit and xylem vulnerability to embolism were measured on medium roots (2–4-mm diameter) collected at 20–30 cm depth. Soil water content and water potentials were monitored concurrently to determine the extent of HR. Additionally, the water potential and stomatal conductance (g_s) of upper canopy leaves were measured throughout the growing season. In the site with young Douglas-fir trees, root embolism increased from 20 to 55 percent loss of conductivity (PLC) as the dry season progressed. In young ponderosa pine, root embolism increased from 45 to 75 PLC. In contrast, roots of old-growth Douglas-fir and ponderosa pine trees never experienced more than 30 and 40 PLC, respectively. HR kept soil water potential at 20–30 cm depth above –0.5 MPa in the old-growth Douglas-fir site and –1.8 MPa in the old-growth ponderosa pine site, which significantly reduced loss of shallow root function. In the young ponderosa pine stand, where little HR occurred, the water potential in the upper soil layers fell to about –2.8 MPa, which severely impaired root functioning and limited recovery when the fall rains returned. In both species, daily maximum g_s decreased linearly with increasing root PLC, suggesting that root xylem embolism acted in concert with stomata to limit water loss, thereby maintaining minimum leaf water potential above critical values. HR appears to be an important mechanism for maintaining shallow root function during drought and preventing total stomatal closure.     

Influence of induced water stress in ponderosa pine on pine sawflies

Summary Three levels of water stress were induced on pole-size ponderosa pine (Pinus ponderosa) to determine the influence of plant moisture stress on oviposition, survival, and growth of two species of pine sawfly (Neodiprion fulviceps and N. autumnalis). It was found that water stress affected oviposition and subsequent egg survival but not larval development or survival. Stress had a negative effect on early season oviposition (N. fulviceps) and a positive effect on late season oviposition (N. autumnalis). Egg hatch was different between species and years and among treatment levels. Larval development, feeding, and survival were not affected by water stress. Overall, the effect of stress was not sufficient to explain population outbreaks of sawflies. Several hypotheses are presented as possible explanations for the observed experimental results.     

Sapwood water storage: its contribution to transpiration and effect upon water conductance through the stems of old-growth Douglas-fir

Abstract Enough water is stored in the sapwood of large Douglas-fir to significantly contribute to transpiration. Sapwood water content falls through the season, causing the wood's conductivity to fall. This leads to low leafwater potentials, stomatal closure, and reduced photosynthesis by the trees. The amount of water stored in the sapwood of Douglasfir 50-60 m tall, growing in the Cascade Mountains of Oregon, was estimated periodically over two seasons from measurements of sapwood relative water content (R_s). The relationship between R_s and volume of water contained in the sapwood was determined in the laboratory, and an equation describing the variation of relative conductivity (K) with R_s was derived from the literature. Stomatal conductance (k_s) and leaf water potentials were measured in the field. The relative conductivity of the sapwood was calculated from estimates of the flow rate through the tree and differences in water potential between dawn and the time of comparison. Flow rate was assumed to equal transpiration rate, calculated from the Penman-Monteith equation using measured k_s values. A sixfold decrease in K during the summer was attributed to changes in R_s. The maximum observed diurnal variation in K would require a change in R_S estimated at 25%. About 270 m³ ha^?1 (27 mm) of water were stored in sapwood, and 75% of that was in the stemwood. Withdrawal from this store reached 1.7 mm day^?1 on clear days after cloudy or rainy weather. Recharge could be almost as fast (up to 1.6 mm day^?1) after rain, but was very slow if the foliage remained wet.     

Path analysis of natural selection on plant chemistry: the xylem resin of ponderosa pine

We analyzed the pattern of correlations among fitness components, herbivory, and resin characteristics in a natural all-aged stand of ponderosa pine, to infer the strength and mechanism of natural selection on plant chemistry. Male and female cone production were monitored yearly for 15 years, and levels of herbivory for 9 years in 165 trees. Resin flow rate and monoterpene composition were determined for these same trees. Multiple regression of fitness components on resin characteristics showed significant associations consistent with directional selection for increased resin flow rates and increased proportions of α- and β-pinene, myrcene and terpinolene. However, negative correlations among monoterpene fractions of the resin constrained the overall selection. Selective herbivory by aphids approached statistical significance and monoterpenes showed some (non-significant) effect as deterrents against deer browse. Resin characteristics were not correlated with attack by cone insects or porcupines. However, the association between resin characteristics and fitness is significantly different from that predicted by the path coefficients involving herbivores. Therefore the hypothesis that these herbivores mediate selection on the resin is not supported by the observed pattern of correlations among resin characteristics, herbivory, growth and fecundity. In this population, most of the association between resin characteristics and fitness appears to be mediated by some other factor independent of attack by herbivore species present. Received: 18 March 1996/Accepted: 18 July 1996     

Relationship between growth rates and xylem hydraulic characteristics in young, mature and old-growth ponderosa pine trees

The first objective of the present study was to quantify the effects of tree age and stem position on specific conductivity (k_s), vulnerability to embolism and water storage capacity (capacitance) in trunks of young, mature and old‐growth ponderosa pine. The second objective was to determine relationships between hydraulic characteristics and radial and height growth rates to increase the understanding of possible tradeoffs. Within sapwood at all heights and in all ages of trees, outer sapwood had 25–60% higher k_s than inner sapwood. The water potential at which embolism started (air entry point) was 1.3 MPa lower in inner sapwood than outer sapwood within the mature trees, but there was no difference in the other trees. There was no significant difference in capacitances between the tops of the old growth trees, the mature trees and the young trees. Taking all data together, the capacitances increased sharply with an increase in k_s and an increase in vulnerability to embolism. The hydraulic characteristics of the three age classes were correlated with the height growth rate but not with the diameter growth rate. Within these age classes, high k_s was associated with the slowest yearly increase in sapwood area and with a low percentage of latewood, whereas high vulnerability to embolism and high capacitance were more closely associated with high height growth rates.     

Cavitation and water storage capacity in bole xylem segments of mature and young Douglas-fir trees

Hydraulic specific conductivity, vulnerability to cavitation and water storage capacity of Douglas-fir sapwood was determined for samples from six young (1.0-1.5 m tall) and six mature trees (41-45 m tall). Measurements on samples from young trees showedthere were no effects of two contrasting sample types (entire stem segments vs sectors chiseled out of entire stems) on any of the calculated hydraulic parameters, for vulnerability to cavitation and water storage capacity. Measurements on mature trees were made on wood from four heights on the bole and from two sapwood depths. Outer and inner sapwood at the base of the tree had higher water storage capacities and were more vulnerable to cavitation than was sapwood from the tree top. At every height, old trees were more vulnerable to cavitation than at 1.0 m from the ground in young trees. The water storage capacities showed three distinct phases at the base of the trunk. Young trees had similar water storage capacity (per unit volume of sapwood) to the topof the mature trees, which was lower than the water storage capacity throughout the rest of the bole xylem. The way in which capacitance was calculated (on a volumetric basis vs a relative water content basis) affected the conclusion one would draw at the low water potentials (<-3 MPa). Within a tree, we found an apparent trade-off between having both hydraulic specific conductivity and stem water storage, and vulnerability to cavitation.     

The flow of water into fruit trees. I. Resistances to water flow through roots and stems

Relative conductivity (K) to water in healthy apple trees ranged from maximum values of 18.2 cm³.100 s^-1.cm length.0.001 Pas.kPa^-1.cm^-2 xylem area, for major suberized roots to values of 1.6 for 1-yr-old twigs. The values for equivalent parts of healthy cherry trees were 26.3 and 3.3. Trees with roots affected by the larvae of the fruit tree root weevil (Leptopius squalidus) which causes either chronic growth decline or sudden wilting and death, had values as low as 1% of healthy trees, in those parts of the tree showing wilting and lack of growth. Water flow under pressure into the root systems of healthy apple trees increased linearly with increases in pressure from 200 to 800 kPa. Flows into dormant and active root systems respectively were 0.6 and 1.7 cm³.100 s^-1. 100 cm² root surface area. 100 kPa^-1.     

Influence of climate-driven shifts in biomass allocation on water transport and storage in ponderosa pine

Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (A_L/A_S) in desert versus montane trees could increase leaf-specific hydraulic conductance (K_L). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (J_S) and shoot water potential during the summer (June-July) and fall (August-September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and J_S. In the summer, mean maximum J_S was 31.80LJ.74 and 24.34Dž.05 g m^-2 s^-1 for desert and montane trees (a 30.6% difference), respectively. In the fall, J_S was 25.33NJ.52 and 16.36dž.64 g m^-2 s^-1 in desert and montane trees (a 54.8% difference), respectively. J_S was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher K_L than montane trees in the summer (2.41᎒^-5 versus 1.05᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001) and a 162% higher K_L in the fall (1.97᎒^-5 versus 0.75᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum g_C was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11 kg (11%) and 10.6 kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high K_L in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase K_L and not to increase the contribution of stored water to transpiration.     

Elevated atmospheric CO2 and soil N fertility effects on growth,mycorrhizal colonization,and xylem water potential of juvenile ponderosa pine in a field soil

Interactive effects of atmospheric CO₂ enrichment and soil N fertility on above- and below-ground development and water relations of juvenile ponderosa pine (Pinus ponderosa Dougl. ex Laws.) were examined. Open-top field chambers permitted creation of atmospheres with 700 µL L^-1, 525 µL L^-1, or ambient CO₂ concentrations. Seedlings were reared from seed in field soil with a total N concentration of approximately 900 µg g^-1 or in soil amended with sufficient (NH₄)₂SO₄ to increase total N by 100 µg g^-1 or 200 µg g^-1. The 525 µL L^-1 CO₂ treatment within the intermediate N treatment was excluded from the study. Following each of three consecutive growing seasons, whole seedlings of each combination of CO₂ and N treatment were harvested to permit assessment of shoot and root growth and ectomycorrhizal colonization. In the second and third growing seasons, drought cycles were imposed by withholding irrigation during which predawn and midday xylem water potential and soil water potential were measured. The first harvest revealed that shoot weight and coarse and fine root weights were increased by growth in elevated CO₂. Shoot and root volume and weights were increased by CO₂ enrichment at the second harvest, but growth stimulation by the 525 µL L^-1 CO₂ concentration exceeded that in 700 µL L^-1 CO₂ during the first two growing seasons. At the third harvest, above- and below-ground growth increases were largely confined to the 700 µL L^-1 CO₂ treatment, an effect accentuated by high soil N but evident in all N treatments. Ectomycorrhizal formation was reduced by elevated CO₂ after one growing season, but thereafter was not significantly affected by CO₂ and was unaffected by soil N throughout the study. Results of the xylem water potential measurements were variable, as water potentials in seedlings grown in elevated CO₂ were intermittently higher on some measurement days but lower on others than that of seedlings grown in the ambient atmosphere. These results suggest that elevated CO₂ exerts stimulatory effects on shoot and root growth of juvenile ponderosa pine under field conditions which are somewhat dependent on N availability, but that temporal variation may periodically result in a greater response to a moderate rise in atmospheric CO₂ than to a doubling of the current ambient concentration.     

Ectomycorrhizal communities of ponderosa pine and lodgepole pine in the south-central Oregon pumice zone

Forest ecosystems of the Pacific Northwest of the USA are changing as a result of climate change. Specifically, rise of global temperatures, decline of winter precipitation, earlier loss of snowpack, and increased summer drought are altering the range of Pinus contorta. Simultaneously, flux in environmental conditions within the historic P. contorta range may facilitate the encroachment of P. ponderosa into P. contorta territory. Furthermore, successful pine species migration may be constrained by the distribution or co-migration of ectomycorrhizal fungi (EMF). Knowledge of the linkages among soil fungal diversity, community structure, and environmental factors is critical to understanding the organization and stability of pine ecosystems. The objectives of this study were to establish a foundational knowledge of the EMF communities of P. ponderosa and P. contorta in the Deschutes National Forest, OR, USA, and to examine soil characteristics associated with community composition. We examined EMF root tips of P. ponderosa and P. contorta in soil cores and conducted soil chemistry analysis for P. ponderosa cores. Results indicate that Cenococcum geophilum, Rhizopogon salebrosus, and Inocybe flocculosa were dominant in both P. contorta and P. ponderosa soil cores. Rhizopogon spp. were ubiquitous in P. ponderosa cores. There was no significant difference in the species composition of EMF communities of P. ponderosa and P. contorta. Ordination analysis of P. ponderosa soils suggested that soil pH, plant-available phosphorus (Bray), total phosphorus (P), carbon (C), mineralizable nitrogen (N), ammonium (NH₄), and nitrate (NO₃) are driving EMF community composition in P. ponderosa stands. We found a significant linear relationship between EMF species richness and mineralizable N. In conclusion, P. ponderosa and P. contorta, within the Deschutes National Forest, share the same dominant EMF species, which implies that P. ponderosa may be able to successfully establish within the historic P. contorta range and dominant EMF assemblages may be conserved.     

Influence of watering and trenching ponderosa pine on a pine sawfly

Summary Neodiprion autumnalis (Smith) larvae were caged for two successive years on root-trenched, watered, and untreated ponderosa pine (Pinus ponderosa Doug. ex Laws.) to determine effects of host moisture stress on larval feeding. Levels of moisture stress (as measured by the Scholander pressure chamber) differed significantly among treatment levels during 1984 and 1985 larval feeding periods. Differences in larval feeding success were not detected in 1984. In 1985, however, larvae on trenched (stressed) trees clipped and rejected more foliage, consumed more needles, had lower pupal weights, lower survival, and a longer feeding period than larvae on watered or untreated trees. Frass production did not differ among treatment levels. The length of the feeding period was shorter for larvae on watered trees than for larvae on untreated trees, but other measures of feeding success did not significantly differ between watered and untreated trees.     

Location of the low temperature water flow barrier in stems

Experiments are described indicating the magnitude and location of the low temperature barrier to lateral water flow in stems of cotton (Gossypium hirsutum L. `Auburn 7-683'). Rehydration of wilted stem tissues was performed at 6 C and 32 C. Compared with the 32 C control, a 13-fold increase in the rehydration halftime was recorded at 6 C when water entered the secondary phloem tissues across the vascular cambium from the secondary xylem. However, only a 3-fold increase in the rehydration halftime occurred when water entered phloem tissues through the cortex, and most of this increase was due to the higher viscosity of water at the lower temperature. These results show that the cambial region of an intact cotton stem markedly resists the radial flow of water at lower temperatures. This resistance was not demonstated by other stem tissues.     

Glyoxysomes in megagamethophyte of germinating ponderosa pine seeds

Decoated ponderosa pine (Pinus ponderosa Laws) seeds contained 40% lipids, which were mainly stored in megagametophytic tissue and were utilized or converted to sugars via the glyoxylate cycle during germination. Mitochondria and glyoxysomes were isolated from the tissue by sucrose density gradient centrifugation at different stages of germination. It was found that isocitrate lyase, malate synthase, and catalase were mainly bound in glyoxysomes. Aconitase and fumarase were chiefly localized in mitochondria, whereas citrate synthase was common for both. Both organelles increased in quantity and specific activity of their respective marker enzymes with the advancement of germination. When the megagametophyte was exhausted at the end of germination, the quantity of these organelles and the activity of their marker enzymes decreased abruptly. At the stage of highest lipolysis, the isolated mitochondria and glyoxysomes were able to synthesize protein from labeled amino acids. Both organellar fractions contained RNA and DNA. Some degree of autonomy in glyoxysomes is indicated.     

Phosphorus transport to the xylem and its regulation by water flow

Summary The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of ³²P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the ³²P taken up by the plants was confined to the roots. Only about half of this ³²P was later translocated to shoots; there was very little translocation after 4 hours.Experiments after long-term labelling showed that only a small part of the total P in the root is readily translocated to shoots. This P appears to be in part of the symplast and contributes about 75% of the P transported to the xylem sap. The rest is presumably derived by leakage from vacuoles.A slow rate of water flow reduced both uptake into the symplast and the translocation to the shoots of P which had already been absorbed by the roots. This was conclusively demonstrated by giving a ³²P pulse before reducing the rate of water flow; ³²P not translocated to shoots was partly retained by the roots and partly lost to the external solution. Water flow also accelerates transport to the xylem of previously-absorbed P in excised roots.It is concluded that the major effect of water flow on phosphorus transport to shoots occurs after phosphorus uptake by the roots, probably during radial transport to the xylem.     

Seasonal and annual respiration of a ponderosa pine ecosystem

The net ecosystem exchange of CO₂ between forests and the atmosphere, measured by eddy covariance, is the small difference between two large fluxes of photosynthesis and respiration. Chamber measurements of soil surface CO₂ efflux (F_s), wood respiration (F_w) and foliage respiration (F_f) help identify the contributions of these individual components to net ecosystem exchange. Models developed from the chamber data also provide independent estimates of respiration costs. We measured CO₂ efflux with chambers periodically in 1996–97 in a ponderosa pine forest in Oregon, scaled these measurements to the ecosystem, and computed annual totals for respiration by component. We also compared estimated half-hourly ecosystem respiration at night (F_nc) with eddy covariance measurements. Mean foliage respiration normalized to 10 °C was 0.20 μmol m^–2 (hemi-leaf surface area) s^–1, and reached a maximum of 0.24 μmol m^–2 HSA s^–1 between days 162 and 208. Mean wood respiration normalized to 10 °C was 5.9 μmol m^–3 sapwood s^–1, with slightly higher rates in mid-summer, when growth occurs. There was no significant difference (P > 0.10) between wood respiration of young (45 years) and old trees (250 years). Soil surface respiration normalized to 10 °C ranged from 0.7 to 3.0 μmol m^–2 (ground) s^–1 from days 23 to 329, with the lowest rates in winter and highest rates in late spring. Annual CO₂ flux from soil surface, foliage and wood was 683, 157, and 54 g C m^–2 y^–1, with soil fluxes responsible for 76% of ecosystem respiration. The ratio of net primary production to gross primary production was 0.45, consistent with values for conifer sites in Oregon and Australia, but higher than values reported for boreal coniferous forests. Below-ground carbon allocation (root turnover and respiration, estimated as F_s– litterfall carbon) consumed 61% of GPP; high ratios such as this are typical of sites with more water and nutrient constraints. The chamber estimates were moderately correlated with change in CO₂ storage in the canopy (F_stor) on calm nights (friction velocity u* < 0.25 m s^–1; R² = 0.60); F_stor was not significantly different from summed chamber estimates. On windy nights (u* > 0.25 m s^–1), the sum of turbulent flux measured above the canopy by eddy covariance and F_stor was only weakly correlated with summed chamber estimates (R² = 0.14); the eddy covariance estimates were lower than chamber estimates by 50%.     

Ectomycorrhizal fungi associated with ponderosa pine and Douglas-fir: a comparison of species richness in native western North American forests and Patagonian plantations from Argentina

The putative ectomycorrhizal fungal species registered from sporocarps associated with ponderosa pine and Douglas-fir forests in their natural range distribution (i.e., western Canada, USA, and Mexico) and from plantations in south Argentina and other parts of the world are listed. One hundred and fifty seven taxa are reported for native ponderosa pine forests and 514 taxa for native Douglas-fir forests based on available literature and databases. A small group of genera comprises a high proportion of the species richness for native Douglas-fir (i.e., Cortinarius, Inocybe, and Russula), whereas in native ponderosa pine, the species richness is more evenly distributed among several genera. The comparison between ectomycorrhizal species richness associated with both trees in native forests and in Patagonia (Argentina) shows far fewer species in the latter, with 18 taxa for the ponderosa pine and 15 for the Douglas-fir. Epigeous species richness is clearly dominant in native Douglas-fir, whereas a more balanced relation epigeous/hypogeous richness is observed for native ponderosa pine; a similar trend was observed for Patagonian plantations. Most fungi in Patagonian Douglas-fir plantations have not been recorded in plantations elsewhere, except Suillus lakei and Thelephora terrestris, and only 56% of the fungal taxa recorded in Douglas-fir plantations around the world are known from native forests, the other taxa being new associations for this host, suggesting that new tree + ectomycorrhizal fungal taxa associations are favored in artificial situations as plantations.