Diurnal and seasonal variability in radial distribution of sap flux density: Implications for estimating stand transpiration

Daily and seasonal patterns in radial distribution of sap flux density were monitored in six trees differing in social position in a mixed coniferous stand dominated by silver fir (Abies alba Miller) and Norway spruce (Picea abies (L.) Karst) in the Alps of northeastern Italy. Radial distribution of sap flux was measured with arrays of 1-cm-long Granier probes. The radial profiles were either Gaussian or decreased monotonically toward the tree center, and seemed to be related to social position and crown distribution of the trees. The ratio between sap flux estimated with the most external sensor and the mean flux, weighted with the corresponding annulus areas, was used as a correction factor (CF) to express diurnal and seasonal radial variation in sap flow. During sunny days, the diurnal radial profile of sap flux changed with time and accumulated photosynthetic active radiation (PAR), with an increasing contribution of sap flux in the inner sapwood during the day. Seasonally, the contribution of sap flux in the inner xylem increased with daily cumulative PAR and the variation of CF was proportional to the tree diameter, ranging from 29% for suppressed trees up to 300% for dominant trees. Two models were developed, relating CF with PAR and tree diameter at breast height (DBH), to correct daily and seasonal estimates of whole-tree and stand sap flow obtained by assuming uniform sap flux density over the sapwood. If the variability in the radial profile of sap flux density was not accounted for, total stand transpiration would be overestimated by 32% during sunny days and 40% for the entire season.     

Radial distribution of sap flux density in trunks of a mature beech stand

In a mature beech stand located in north-eastern Germany, xylem sap flux measurements were continuously performed during the 2002–2004 growing seasons. Ten representative trunks were studied using heated thermal dissipation probes. The measurements aimed at identifying principles governing radial profiles of xylem flux in beech trunks. The measurements were taken up to a trunk depth of 132 mm. The sap flow density in the pericambial xylem was found to vary among trees of different diameters, but was not considerably smaller in suppressed trees. A model for the radial distribution of sap flux density was formulated relating trunk radius and sap flow density. The model takes into account different trunk diameter. About 90% of the sap flux was found to occur in the outer two fifths of the trunk. Using this model, an adequate estimate of transpiration can be achieved at tree and stand level, even when the sap flux measurements are restricted to the outer trunk sectors.     

Organic and inorganic sulfur transport in the xylem sap and the sulfur budget of Picea abies trees

Temporal changes in inorganic and organic sulfur compounds (sulfate, glutathione, cysteine, methionine) were analyzed in xylem sap of 40-year-old Norway spruce (Picea abies (L.) Karst.) trees growing on acidic soils at a healthy and a declining stand in the Fichtelgebirge (North Bavaria, Germany). Studies were carried out (1) to quantify glutathione (GSH) transport in the xylem of spruce, (2) to study the significance of reduced sulfur versus sulfate (SO(4) (2-)) transport in the xylem, and (3) to compare total sulfur (S) transport in the xylem with the amount of foliar uptake of SO(2) in an air-polluted environment. Glutathione was the main reduced S compound in the xylem ranging in concentration from 0.5 to 5 &mgr;mol l(-1). Concentrations of inorganic SO(4) (2-) in the xylem sap were up to 50 times higher than those of GSH ranging from 60 to 230 &mgr;mol l(-1). During the growing season, concentrations of all S compounds in the xylem were highest in May (up to 246 &mgr;mol l(-1)) and decreased during summer and fall (up to 21 &mgr;mol l(-1)). On average, SO(4) (2-) concentrations in xylem sap were 30% higher at the declining site compared with the healthy site. Diurnal changes in organic S compounds were significant for GSH and cysteine with high concentrations during the night and low concentrations during the day. Diurnal changes in inorganic concentrations were not significant. Xylem sap concentrations of SO(4) (2-) and cysteine were twice as high and GSH concentrations were tenfold higher in surface roots than in branches. At both sites, transport of organic S was low (up to 3% of total S) compared to transport of SO(4) (2-). Annual transport of total S in the xylem (SO(4) (2-) was the main component) ranged from 60 to 197 mmol tree(-1) year(-1) at the healthy site and from 123 to 239 mmol tree(-1) year(-1) at the declining site. Although gaseous uptake of SO(2) was estimated to be similar at both sites (38 mmol tree(-1) year(-1); Horn et al. 1989), the ratio between annual gaseous uptake of SO(2) and transport of S in the xylem was 1:4 and 1:5 at the healthy and declining sites, respectively.     

Assessing variation in the radial profile of sap flux density in Pinus species and its effect on daily water use

We monitored sap flux density (v) diurnally in nine mature southeastern pine (Pinus spp.) trees with a thermal dissipation probe that spanned the sapwood radius. We found the expected pattern of high v near the cambium and decreasing v with depth toward the center of the tree; however, the pattern was not constant within a day or between trees. Radial profiles of trees were steeper earlier in the day and became less steep later in the day. As a result, time-dependent changes in the shape of the radial profile of v were sometimes correlated with daily changes in evaporative demand. As the radial profile became less steep, the inner xylem contributed relatively more to total tree sap flow than it did earlier in the day. We present a 3-parameter Gaussian function that can be used to describe the radial distribution of v in trees. Parameters in the function represent depth in the xylem from the cambium, maximum v, depth in the xylem where maximum v occurs, and the rate of radial change in v with radial depth (beta). Values of beta varied significantly between trees and with time, and were sometimes correlated with air vapor pressure deficit (D). We hypothesize that this occurred during periods of high transpiration when the water potential gradient became great enough to move water in the inner sapwood despite its probable high hydraulic resistance. We examined discrepancies among estimates of daily water use based on single-point, two-point and multi-point (i.e., every 20 mm in the sapwood) measurements. When radial distribution of v was not considered, a single-point measurement resulted in errors as large as 154% in the estimate of daily water use relative to the estimate obtained from a multi-point measurement. Measuring v at two close sample points (10 and 30 mm) did not improve the estimate; however, estimates derived from v measured at two distant sample points (10 and 70 mm) significantly improved the estimate of daily water use, although errors were as great as 32% in individual trees. The variability in v with depth in the xylem, over time, and between trees indicates that measurements of the radial distribution of v are necessary to accurately estimate water flow in trees with large sapwood areas.     

Overstorey density influence on the height of Picea abies regeneration in northern Sweden

The study aimed specifically at investigating if canopy opennesswas a better predictor of the height growth of Norway spruce(Picea abies (L.) Karst.) advance regeneration than overstoreybasal area or overstorey standing volume. In 1990, a field experimentwith 3 x 2 factorial design and two replications (blocks) wasestablished in an uneven-aged Norway spruce forest. Plots hada net plot area of 30 x 30 m, each with a 10-m-wide treatedbuffer zone. Three overstorey density levels retained approximately15, 40 and 70 per cent of the pre-harvest overstorey standingvolume and were allotted to the plots. Two types of thinningthat harvested smaller trees or harvested larger trees wererandomly allocated to each pair of overstorey density plots.In mid-June 2000, canopy openness was estimated from hemisphericalphotographs taken at five marked points in the centre of eachof the plots at 0.9 m from ground to the top of the ‘fish-eye’camera lens. Regression results showed that canopy opennesswas a better predictor of height increments of spruce seedlings(0.1< height < 0.5 m), saplings (0.5 height < 2.0m), and small trees (height 2.0 m, diameter at 1.3 m height< 5 cm) than with overstorey basal area (m² ha^–1) oroverstorey standing volume (m³ ha^–1). The height incrementof the spruce advance regeneration was not significantly correlatedto stand basal area or to standing volume. Overstorey basalarea in the net plots was significantly negative (P 0.05) withmean canopy openness estimates, and the r² value was 0.40. Resultsindicated that basal area was not linearly related to canopyopenness as it increased, which might explain the lack of predictivepower of retained basal area on spruce regeneration height indense stands in boreal Sweden.     

Influence of initial spacing and tree class on the basic density of Picea abies

The effects of initial spacing and tree class on the basic density of Norway spruce were evaluated in a stand on a fertile site in southwestern Sweden. The basic density at 0 and 4 m above ground level was significantly higher for trees planted with an initial spacing of 1.5 m than for trees planted with wider initial spacings. A decrease in initial square spacing from 2.5 m to 1.5 m had little effect, of small practical importance, on the basic density. The density at 0 and 4 m above ground was significantly higher for the suppressed trees than for the intermediate and dominant ones, and for the intermediate trees compared with the dominant trees. Differences in basic density appeared to be due mainly to differences in growth‐ring width. Basic density decreased outwards from the pith to the bark at 1.3 and 4 m above ground.     

Estimating foliage biomass in Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) plots

Dynamic decomposition models are needed to estimate changes in the carbon stock of boreal soil because these changes are difficult to measure directly. An important aboveground carbon flux to the soil is foliage litterfall. To estimate this flux, both the amount and the turnover rate of the foliage biomass component must be known. Several methods for estimating foliage biomass of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.), including biomass equations and biomass expansion factors (BEFs), were compared with predicted foliage biomass based on forest inventory plot-level measurements. Measured foliage biomass was up-scaled from the branch-level to the plot-level by combining forest inventory variables (diameter, height, height at the crown base and crown base diameter) based on the assumptions of pipe model theory. Combining the foliage biomass: cross-sectional area ratio with the forest inventory variables provided accurate estimates of foliage biomass at the plot-level for plots in southern Finland. The results emphasize the need to test biomass equations with independent data, especially when the equations applied are based on neighboring regions.     

Canopy density in stands of Picea abies and Pinus sylvestris after different thinning methods

Altogether 82 plots (261 estimations) of Picea abies (L.) Karst, and 193 plots (360 estimations) of Pinus sylvestris (L.) stands were estimated by a vertical tube. The “crown free projection”, CFP, of stands thinned in three methods with different thinning grades was measured: unthinned, heavily and very heavily thinned, heavily thinned delayed first thinning, extra heavily thinned and thinned from the top. Basal area (m²ha^?1) density (stems ha^?1) and diameter sum (m ha^?1) were plotted against CFP. Basal area was the best practical measure of stand in this study. Generally Scots pine stands have higher CFP and the curves are steeper than in Norway spruce stands. Depending on the grade of thinning, heavily and very heavily thinned spruce stands, delayed first thinning included, have CFP values of 10–15% and stands thinned from the top, 20–40%, compared with 30–80% and 30–60% respectively in pine stands. Extra heavily thinned stands have the highest CFP, 20–80% in spruce and 50–90% in pine stands. The CFP levels after thinning are too high in pine stands for avoidance of sucker and sprout production of aspen and birch. In dense Norway spruce stands thinned from the top or heavily and very heavily thinned, the CFP values are low enough (≤30%) to diminish the production of suckers.     

Vertical and horizontal water redistribution in Norway spruce (Picea abies) roots in the Moravian Upland

Hydraulic redistribution (HR) by roots of large Norway spruce (Picea abies (L.) Karst.) trees was investigated by means of sap flow measurements made with the heat field deformation method. Irrigation was applied to a limited portion of the root system to steepen gradients of water potential in the soil and thus enhance rates of HR. On completion of the sap flow measurements, and to aid in their interpretation, the structure of the root system of seven of the investigated trees was exposed to a depth of 30 cm with a supersonic air-stream (air-spade). Before irrigation, vertical redistribution of water was observed in large coarse roots and some adjacent small lateral roots. Immediately after localized irrigation, horizontal redistribution of water from watered roots to dry roots via the stem base was demonstrated. The amount of horizontal distribution depended on the position of the receiving roots relative to the watered roots and the absorbing area of the watered root. No redistribution from watered roots via dry soil to roots of neighboring trees was detected. Responses of sap flow to localized irrigation were more pronounced in small lateral roots than in large branching roots where release and uptake of water are integrated. Sap flow measurements with multi-point sensors along radii in large lateral roots demonstrated water extraction from different soil horizons. We conclude that synchronous measurements of sap flow in both small and large lateral roots are needed to study water absorption and transport in tree root systems.     

Digital image analysis of radial shrinkage of fresh spruce (Picea abies L.) wood

Contact-free digital image analysis was performed of the radial shrinkage of fresh, fully saturated small spruce wood beams. An experimental test set-up was developed to ensure constant distance from the charge-coupled device camera to the sample surface as well as constant climate and light conditions during the whole experiment. Dimensional changes were observed immediately after the drying process began. An unexpected distinct effect could be observed which could not be explained by drying surface layers only. After a fast initial radial shrinkage a slowing down of the dimensional changes occurred at high mean moisture contents. A complete interruption of any dimensional changes followed. Finally, a recovery from shrinkage was even observed. It is assumed that strong negative pressure occurred in the fully saturated capillaries owing to dehydration which led to additional dimensional changes. As a consequence, the break of the water column and aeration in these capillaries finally resulted in a recovery period in the shrinkage rate due to the pressure release. After this effect, the dehydration was characterized by a phase of fast and almost linear shrinkage due to drying surface layers. Finally, the shrinkage slowed down to zero when reaching equilibrium moisture content.     

Influences of environmental factors on the radial profile of sap flux density in Fagus crenata growing at different elevations in the Naeba Mountains, Japan

Sap flux density was measured continuously during the 1999 and 2000 growing seasons by the heat dissipation method in natural Fagus crenata Blume (Japanese beech) forests growing between 550 and 1600 m on the northern slope of the Kagura Peak of the Naeba Mountains, Japan. Sap flux density decreased radially toward the inner xylem and the decrease was best expressed in relation to the number of annual rings from the cambium, or in relation to the relative depth between the cambium and the trunk center, rather than as a function of absolute depth. The relative influences of radiation, vapor pressure deficit and soil water on sap flux density during the growing season were similar for the outer and inner xylem, and at all sites. Measurements of soil water content and water potential at a depth of 0.25 m demonstrated that sap flux density responded similarly and sensitively to water potential changes in this soil layer, despite large differences in rooting depth at different elevations, localizing one important control point in the functioning of this forest ecosystem. Identification of the relative influences of radiation, vapor pressure deficit and drying of the upper soil layer on sap flux density provides a framework for in-depth analysis of the control of transpiration in Japanese beech forests. In addition, the finding that the same general controls are operating on sap flux density despite climate gradients and large differences in overall forest stand structure will enhance understanding of water use by forests along elevation gradients.     

Allozyme Variation of Picea abies in Poland

The aim of this study was to determine the levels and patterns of allozyme variation among 29 populations of Norway spruce [ Picea abies (L.) Karst.] from Poland. Thirteen investigated isoenzyme systems were encoded by 27 gene loci and, on average, 71% of the loci per population were polymorphic. The average and effective numbers of alleles per locus were 2.17 and 1.26, respectively, while the expected heterozygosity was 0.156. A relatively low allozyme differentiation among populations from north-eastern and southern Poland was observed ( F ST = 0.028, mean genetic distance D = 0.005). The results suggest that historical events and extensive gene flow played an important role in the distribution of the observed allozyme differentiation of Norway spruce in Poland.     

Acclimation to low irradiance in Picea abies: influences of past and present light climate on foliage structure and function

Leaf retention time increases with decreasing irradiance, providing an effective way of amortizing the costs of foliage construction over time. To elucidate the physiological mechanisms underlying this dependence, I studied needle life span, morphology, and concentrations of carbon, nitrogen and nonstructural carbohydrates along a gradient of relative irradiance in understory trees of Picea abies (L.) Karst. Maximum needle life span was greater in shaded trees than in sun-exposed trees. However, irrespective of irradiance, needles with maximum longevity were situated in the middle rather than the bottom of the canopy, suggesting that needle life span is determined by the irradiance to which needles are exposed during their primary growth. Morphology and chemistry of current-year needles were adapted to prevailing light conditions. Current-year needles exposed to high irradiances had greater packing of foliar biomass per unit area than shaded needles, whereas shaded needles maximized foliar area to capture more light. Nitrogen concentrations were higher in shaded needles than in sun-exposed needles. This nitrogen distribution pattern was related to the high nitrogen cost of light interception and was assumed to improve light absorptance per needle mass of shaded needles. In contrast, in both 1- and 2-year-old needles, morphology was independent of prevailing light conditions; however, needle nitrogen concentrations were adjusted toward more effective light interception in 2-year-old foliage but not in 1-year-old foliage, indicating that acclimation of sun-adapted needles to shading takes more than one year. At the same time, needle aging was accompanied by accumulation of nonstructural carbohydrates (NSC), and increasing concentrations of needle carbon, suggesting a shift in the balance between photosynthesis and photosynthate export. The accumulation of NSC and carbon resulted in a dilution of the concentrations of other needle chemicals and explained the decline in needle nitrogen concentrations with increasing age. Thus, although morphological inadequacy to low light availabilities may partly be compensated for by modifications in needle chemistry, age-related changes in needle stoichiometric composition progressively lessen the potential for acclimation to low irradiance. A conceptual model, advanced to explain how environmental factors and age-related changes in the activities of needle xylem and phloem transport affect needle longevity, predicted that adaptation of needle morphology to irradiance during the primary growth period largely determines the fate of needles during subsequent tree growth and development.     

Variation in the radial patterns of sap flux density in pubescent oak (Quercus pubescens) and its implications for tree and stand transpiration measurements

Radial variation in sap flux density across the sapwood was assessed by the heat field deformation method in several trees of Quercus pubescens Wild., a ring-porous species. Sapwood depths were delimited by identifying the point of zero flow in radial patterns of sap flow, yielding tree sapwood areas that were 1.5-2 times larger than assumed based on visual examinations of wood cores. The patterns of sap flow varied both among trees and diurnally. Rates of sap flow were higher close to the cambium, although there was a significant contribution from the inner sapwood, which was greater (up to 60% of total flow) during the early morning and late in the day. Accordingly, the normalized difference between outer and inner sapwood flow was stable during the middle of the day, but showed a general decline in the afternoon. The distribution of sap flux density across the sapwood allowed us to derive correction coefficients for single-point heat dissipation sap flow measurements. We used daytime-averaged coefficients that depended on the particular shape of the radial profile and ranged between 0.45 and 1.28. Stand transpiration calculated using the new method of estimating sapwood areas and the radial correction coefficients was similar to (Year 2003), or about 25% higher than (Year 2004), previous uncorrected values, and was 20-30% of reference evapotranspiration. We demonstrated how inaccuracies in determining sapwood depths and mean sap flux density across the sapwood of ring-porous species could affect tree and stand transpiration estimates.     

欧洲云杉、黑云杉、白云杉苗期对比试验

选择欧洲云杉3个种源、黑云杉3个种源、白云杉3个种源、青海云杉1个种源进行了不同种、种源的苗期生长性状变异试验。结果表明：云杉不同种、种源间苗期性状差异显著。4个种的1年生苗以苗高进行排序：欧洲云杉〉黑云杉〉白云杉〉青海云杉。欧洲云杉、黑云杉、白云杉各个种的不同种源间有很大的变异及选择潜力。因此在进行云杉引种时,应选择不同的种、种源,才能取得更好的效果。     

Microflora of sound-looking wood in Picea abies stems

Ascocoryne spp. (A. cylichnium and A. cf. sarcoides) were found in 48 % of 262 60-year-old Picea abies stems. They were isolated with increasing frequency towards the pith from the lower 2-3 m, more often from thicker than from thinner stems. Neobulgaria premnophila was present in 10 % of the stems, more evenly distributed, and with no difference in occurrence between thicker and thinner stems. Nectria fuckeliana was situated at random in 8 % of the trees.     

Lignin distribution in spruce (Picea abies) determined by mercurization with SEM-EDXA technique

Summary The lignin distribution between the middle lamella and the cell wall of spruce fibers has been determined by a new technique based on a mercurization of the lignin and a concomitant determination of mercury by the SEM-EDXA technique. The ratio of lignin in the middle lamella at the cell corners to the lignin in the secondary wall was 2.5±0.6 for latewood and 2.4±0.6 for earlywood. This gives a lignin content of 55–58% in the true middle lamella in the cell corners. The reactivity to mercuric acetate of different wood elements was determined in separate experiments. Fractions enriched in ray cells, middle lamella, and compression wood all reacted at the same rate as the whole wood; about one mole of mercury was incorporated per mole of lignin (C₉-unit).     

Element concentrations in the xylem sap of Picea abies (L.) Karst. seedlings extracted by various methods under different environmental conditions

We used a Scholander pressure chamber to assess the effects of various extraction methods under different environmental conditions on element concentrations in xylem sap of 3-year-old Picea abies (L.) Karst. seedlings. Sap from excised shoots contained higher element concentrations when extracted at low than at high over-pressures. When comparing plants differing in water status, we found that a high extraction over-pressure introduced a systematic error into the data. For example, in well-watered non-transpiring plants relative to unwatered transpiring plants, potassium concentrations were 70% higher in sap extracted at 0.1 MPa over-pressure, but only 10% higher in sap extracted at 1.0 MPa over-pressure. Moreover, treatment effects depended on the time of day when the sap was extracted. Increased water flux in transpiring plants relative to non-transpiring plants resulted in reduced xylem sap element concentrations when samples were collected after 9 h of transpiration, but not after 4 to 6 h of transpiration. Drought had little effect on xylem sap element concentrations, indicating that rates of element release into xylem conduits, element depletion by growing tissues, and water flow maintained a balance that may prevent nutrient stress during short-term drought.     

Effect of the span length of Granier-type thermal dissipation probes on sap flux density measurements

? Granier-type thermal dissipation sensors measure sap flux density (u) by using the temperature difference between the heater and the reference probe. To detect u correctly, heat must not be transferred to the reference probe by thermal conduction. The distance across which heat can be transferred by conduction is important for the span length of a sensor and spacing of a number of sensors.

? To validate span lengths and spacing of sensors, we used numerical simulations to calculate the potential distance across which heat can be transferred by conduction. We compared measurements with an original and a modified sensor for a Japanese red pine (Pinus densiflora) from December 2004 to May 2005. The span length of the original and the modified sensor is 15 and 4 cm, respectively.

? Numerical simulations showed that span length and spacing of Granier sensors should be more than 10 cm for trees in which u ceases for a few hours before the predawn period. The modified sensor underestimated u by 18–46% in winter (December–March) because its reference temperature was increased by heat transferred by conduction. The modified sensor measured u correctly in warm seasons, and only underestimated the annual amount of transpiration by 6%.

     

Transfer of the P-type of Heterobasidion annosum from old-growth stumps of Picea abies to Picea abies and Larix×eurolepis

The infection of Picea abies and Larix x eurolepis by Heterobasidion annosum was studied in felled trees in Sweden. Thinnings were carried out in two stands of L. x eurolepis, 15 and 20 years old, and in a 25-year-old stand of P. abies, established on a site heavily infected by H. annosum. The 15- and 20-year-old L. x eurolepis stands exhibited the greatest incidence of butt rot with infections amounting to 38 and 57%, respectively, of removed trees. The incidence of butt rot in the P. abies stand was only 5%. Heterobasidion annosum was the main butt rot causing fungus. All isolates of H. annosum were of the P-intersterility group. Transfer of H. annosum from old-growth P. abies stumps to trees felled in the thinning of the next generation was demonstrated by somatic incompatibility tests. However, the majority of infections could not be connected with decay already established in the previous generation. In L. c eurolepis, rot columns were frequent, including also several genotypes in the same stem, and typically positioned in the inner sapwood. The few infections of P. abies were situated in the heartwood.