In situ measurement of fine root water absorption in three temperate tree species—Temporal variability and control by soil and atmospheric factors

Miniature heat balance-sap flow gauges were used to measure water flows in small-diameter roots (3–4 mm) in the undisturbed soil of a mature beech–oak–spruce mixed stand. By relating sap flow to the surface area of all branch fine roots distal to the gauge, we were able to calculate real time water uptake rates per root surface area (J_s) for individual fine root systems of 0.5–1.0 m in length. Study aims were (i) to quantify root water uptake of mature trees under field conditions with respect to average rates, and diurnal and seasonal changes of J_s, and (ii) to investigate the relationship between uptake and soil moisture θ, atmospheric saturation deficit D, and radiation I. On most days, water uptake followed the diurnal course of D with a mid-day peak and low night flow. Neighbouring roots of the same species differed up to 10-fold in their daily totals of J_s (<100–2000 g m⁻² d⁻¹) indicating a large spatial heterogeneity in uptake. Beech, oak and spruce roots revealed different seasonal patterns of water uptake although they were extracting water from the same soil volume. Multiple regression analyses on the influence of D, I and θ on root water uptake showed that D was the single most influential environmental factor in beech and oak (variable selection in 77% and 79% of the investigated roots), whereas D was less important in spruce roots (50% variable selection). A comparison of root water uptake with synchronous leaf transpiration (porometer data) indicated that average water fluxes per surface area in the beech and oak trees were about 2.5 and 5.5 times smaller on the uptake side (roots) than on the loss side (leaves) given that all branch roots <2 mm were equally participating in uptake. Beech fine roots showed maximal uptake rates on mid-summer days in the range of 48–205 g m⁻² h⁻¹ (i.e. 0.7–3.2 mmol m⁻² s⁻¹), oak of 12–160 g m⁻² h⁻¹ (0.2–2.5 mmol m⁻² s⁻¹). Maximal transpiration rates ranged from 3 to 5 and from 5 to 6 mmol m⁻² s⁻¹ for sun canopy leaves of beech and oak, respectively. We conclude that instantaneous rates of root water uptake in beech, oak and spruce trees are above all controlled by atmospheric factors. The effects of different root conductivities, soil moisture, and soil hydraulic properties become increasingly important if time spans longer than a week are considered.     

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Summary Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day^–1 tree^–1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m² of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s^–1 that is equivalent to 112 mmol m^–2 s^–1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m^–2 s^–1 at the top and 150 mmol m^–2 s^–1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was –27.76±0.27 (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.This paper is dedicated to Prof. Dr. O.L. Lange on the occasion of his 65th birthday     

Water chemistry and nutrient budgets in an undisturbed evergreen rainforest of southern Chile

In a pristine evergreen rainforest of Nothofagus betuloides, located at the Cordillera de los Andes in southern Chile (41 °S), concentrations and fluxes of nutrients in bulk precipitation, cloud water, throughfall water, stemflow water, soil infiltration and percolation water and runoff water were measured. The main objectives of this study were to investigate canopy-soil-atmosphere interactions and to calculate input-output budgets. From May 1999 till April 2000, the experimental watershed received 8121 mm water (86% incident precipitation, 14% cloud water), of which the canopy intercepted 16%. Runoff water volume amounted 9527 mm. Bulk deposition of inorganic (DIN) and organic (DON) nitrogen amounted 3.6 kg ha^–1 year^–1 and 8.2 kg ha^–1 year^–1 respectively. Occult deposition (clouds + fog) contributes for 40% to the atmospheric nitrogen input (bulk + occult deposition) of the forest. An important part of the atmospheric ammonium deposition is retained within the canopy or converted to nitrate or organic nitrogen by epiphytic bacteria or lichens. Also the export of inorganic (0.9 kg ha^–1 year^–1) and organic (5.2 kg ha^–1 year^–1) nitrogen via runoff is lower than the input to the forest floor via throughfall and stemflow water (3.2 kg DIN ha–1 year^–1 and 5.6 kg DON ha^–1 year^–1). The low concentrations of NO ₃ ^– and NH ₄ ⁺ under the rooting depth suggest an effective biological immobilization by vegetation and soil microflora. Dry deposition and foliar leaching of base cations (K⁺, Ca²⁺, Mg²⁺) was estimated using a canopy budget model. Bulk deposition accounted for about 50% of the total atmospheric input. Calculated dry and occult deposition are both of equal value (about 25%). Foliar leaching of K⁺, Ca²⁺, and Mg²⁺ accounted for 45%, 38% and 6% of throughfall deposition respectively. On an annual basis, the experimental watershed was a net source for Na⁺, Ca²⁺ and Mg²⁺.     

Up-scaling to stand transpiration of an Asian temperate mixed-deciduous forest from single tree sapflow measurements

Species diversity in mixed forest stands is one of the factors that complicate up-scaling of transpiration from individual trees to stand level, since tree species are architecturally and functionally different. In this study, thermal dissipation probes were used to measure sap flow in five different tree species in a mixed-deciduous mountain forest in South Korea. Easily measurable tree characteristics that could serve to define individual tree water use among the different species were employed to scale up transpiration from single trees to stand level. Tree water use (TWU) was derived from sap flux density (SFD) and sapwood area (SA). Canopy transpiration E was scaled from TWU while canopy conductance (g _c) was computed from E and VPD. SFD, TWU and g _c were correlated with tree diameter at breast height (DBH) for all the five measured species (SFD: R ² = 0.21, P = 0.036; TWU: R ² = 0.83, P < 0.001; g _c: R ² = 0.63, P < 0.001). Maximum stand transpiration (E) during June, before the onset of the Asian monsoon rains, was estimated at 0.97 ± 0.12 mm per day. There was a good (R ² = 0.94, P < 0.0001) agreement between measured and estimated E using the relationship between TWU and DBH. Our study shows that using functional models that employ converging traits among species could help in estimating water use in mixed forest stands. Compared to SA, DBH is a better scalar for water use of mixed forest stands since it is non-destructive and easily obtainable.     

Deposition of ammonia to the forest floor under spruce and beech at the Höglwald site

Laboratory and field measurements of the flux of ammonia to forest floor canopies of spruce and beech stands at the Höglwald site in southern Bavaria are reported. Measurements were performed with an open chamber method. A linearity between ammonia concentration and ammonia flux from the atmosphere to the ground floor canopy was detected. Deposition of ammonia showed no saturation even at air concentrations up to 50 g NH₃ m^–3 air. Temperature, water content and the moss layer of the ground floor canopy had a minor influence on the deposition velocity in laboratory experiments. Deposition velocity of ammonia was higher to the spruce (1.3 cm s^–1), and limed spruce ground floor canopy (1.17 cm s^–1) compared to the beech stand (0.79 cm s^–1). In field studies, a diurnal course of the deposition velocity was detected with highest velocities in midday and minor during night times, but not in the climatic chamber. The flux of ammonia to the ground floor canopy was estimated of app. 10 kg N ha^–1 yr^–1 for the soil under spruce, 9 kg N ha^–1 yr^–1 for the limed spruce and 6 kg N ha^–1yr^–1 for the soil under beech. The fluxes are interpreted as fluxes from the atmosphere to the ground canopies of the stands.     

Stand fine root biomass and fine root morphology in old-growth beech forests as a function of precipitation and soil fertility

Only very limited information exists on the plasticity in size and structure of fine root systems, and fine root morphology of mature trees as a function of environmental variation. Six northwest German old-growth beech forests (Fagus sylvatica L.) differing in precipitation (520 – 1030 mm year^–1) and soil acidity/fertility (acidic infertile to basic fertile) were studied by soil coring for stand totals of fine root biomass (0–40 cm plus organic horizons), vertical and horizontal root distribution patterns, the fine root necromass/biomass ratio, and fine root morphology (root specific surface area, root tip frequency, and degree of mycorrhizal infection). Stand total of fine root biomass, and vertical and horizontal fine root distribution patterns were similar in beech stands on acidic infertile and basic fertile soils. In five of six stands, stand fine root biomass ranged between 320 and 470 g m^–2; fine root density showed an exponential decrease with soil depth in all profiles irrespective of soil type. An exceptionally small stand fine root biomass (<150 g m^–2) was found in the driest stand with 520 mm year^–1 of rainfall. In all stands, fine root morphological parameters changed markedly from the topsoil to the lower profile; differences in fine root morphology among the six stands, however, were remarkably small. Two parameters, the necromass/biomass ratio and fine root tip density (tips per soil volume), however, were both much higher in acidic than basic soils. We conclude that variation in soil acidity and fertility only weakly influences fine root system size and morphology of F. sylvatica, but affects root system structure and, probably, fine root mortality. It is hypothesized that high root tip densities in acidic infertile soils compensate for low nutrient supply rates, and large necromasses are a consequence of adverse soil chemical conditions. Data from a literature survey support the view that rainfall is another major environmental factor that influences the stand fine root biomass of F. sylvatica.     

Accession and cycling of elements in a coastal stand ofPinus radiata D. Don in New Zealand

Summary The accession and cycling of elements in a 14-year-old coastal stand ofPinus radiata D. Don was measured for one year. The element contents (mg m^–2 year^–1) of bulk precipitation and throughfall respectively were: NO₃–N 41, 12; NH₄–N 133, 154; organic-N 157, 396; Na 4420, 9700; K 387, 2900; Ca 351, 701; Mg 486, 1320. Of the increase in element content of rainwater beneath the forest canopy 20% (NH₄–N), 70% (organic-N), 3% (Na), 90% (K), 20% (Ca) and 30% (Mg) was attributed to leaching; the remainder to washing of aerosols filtered from the atmosphere by the vegetation. The canopy absorbed approximately 40 mg m^–2 year^–1 of NO₃–N. Litterfall was the major pathway for the above-ground biogeochemical cycle of N (93%), Ca (96%) and Mg (74%), and leaching was the major (73%) pathway for K.     

Whole-Plant Water use and Canopy Conductance of Cassava Under Iimited Available Soil Water and Varying Evaporative Demand

Cassava (Manihot esculenta Crantz), a perennial woody shrub, is known to be highly productive under favourable conditions and produce reasonably well under adverse conditions where other crops fail. Using constant heat sap flow sensors, sap flow density (F _d ) of cassava was monitored for 10 days in December 2002. Sap flow was highly correlated (R ² =0.72, P<0.05) to incoming solar radiation (R _s) than to other climatic factors. Using cross-correlation analysis, no time shift was detected between F _d and solar radiation, whereas vapour pressure deficit (VPD) lags F _d by 110 min. Solar radiation and VPD together explained 83% of diurnal variation in sap flow. Whole-plant transpiration ranged from 0.8 to 1.2 mm day⁻¹ and daily canopy conductance (g _c), computed based on the inverted Penman–Monteith model, varied between 0.7 and 2.1 mm s⁻¹ (mean = 1.4 ± 0.5 mm s⁻¹). For the measurement period, characterized by high evaporative demand coupled with low available soil water, transpiration accounted for 21% of the available energy and was only able to meet 24% of the atmospheric water demand. Average decoupling factor (Ω) of 0.05±0.02 estimated suggested that a 10% change in g _c may lead to more than 9% change in transpiration which further supports the notion that stomata play significant role in regulating cassava water use compared to other known mechanisms. Beyond light saturation (R _s >300 W m⁻²) and at higher VPD (>1.0 kPa), wind effects on the canopy transpiration under water stress condition were low, while VPD explains 94% of the observed variance in daily canopy conductance.     

基于树干液流及涡动相关技术的葡萄冠层蒸腾及蒸散发特征研究

针对西北干旱区绿洲经济作物葡萄树冠层蒸腾及蒸散发特征的相关问题,在甘肃省敦煌市南湖绿洲开展无核白葡萄树液流速率及蒸散发观测试验,采用基于热平衡原理的包裹式茎流计,详细分析了典型生长季7—9月份葡萄树蒸腾耗水规律,使用"单位叶面积上的平均液流速率SF×叶面积指数LAI"的方法,实现了从单株到林分冠层蒸腾的尺度扩展,并通过与涡动相关技术所测蒸散发数据对比,详细研究了葡萄地冠层蒸腾及蒸散发规律。结果表明:典型生长季中葡萄树液流速率日变化为单峰型曲线,日均耗水量从2.76 kg到10 kg不等,胸径越大的葡萄树日均耗水量越大;冠层蒸腾及蒸散发日变化曲线亦为单峰型,白天8:00—12:00与17:00—20:00期间,葡萄冠层蒸腾与蒸散发曲线均比较吻合,该时间段葡萄地蒸散发绝大部分来源于葡萄冠层蒸腾,而12:00—17:00之间由于午后太阳辐射强烈土壤蒸发量增加,葡萄蒸散发大于冠层蒸腾;典型生长季3个月中,葡萄冠层蒸腾量的变化范围在1.88—8.12 mm/d之间,日均冠层蒸腾量为6.12 mm/d,蒸散发在1.74 mm/d至10.78 mm/d之间,日均蒸散发量为7.13 mm/d;日均土壤蒸发量约为1.01 mm/d,只占总蒸散发量的14.2%,日均冠层蒸腾占日均蒸散发的比重达到85.8%,说明该生长阶段冠层蒸散发以作物蒸腾为主。     

Quercus ilex Transpiration as Affected by a Prolonged Drought Period

The effect of an extended drought (from 1992 to 1995) on water relations was assessed on evergreen oak (Quercus ilex L.) in a dehesa ecosystem (Seville, Southern Spain). Diurnal and seasonal transpiration patterns were analysed at leaf (porometry) and whole-tree level (sap flow), focusing on the relationship between tree transpiration rates (E_t) and potential evapotranspiration rates (PET). Daily maximum E_t varied over the year, becoming higher between May and August, and lower between November and April. Annual E_t (169 – 205 mm y^–1) accounted for less than 40 % of annual rainfall. The prolonged drought did not affect the water relations of the Q. ilex, mainly due to strong stomatal regulation avoiding the loss of water. Stomatal control was found in all seasons, although it was stronger in summer. This behaviour leads to low water consumption and low E_t/PET ratios throughout the year (0.05 to 0.27).     

Canopy transpiration in a chronosequence of Central Siberian pine forests

Tree transpiration was measured in 28, 67, 204 and 383‐y‐old uniform stands and in a multicohort stand (140–430 y) of Pinus sylvestris ssp. sibirica Lebed. in Central Siberia during August 1995. In addition transpiration of three codominant trees was monitored for two years in a 130‐y‐old stand. All stands established after fire. Leaf area index (LAI) ranged between 0.6 (28‐y‐old stand) and 1.6 for stands older than 67‐y. Stand xylem area at 1.3 m height increased from 4 cm² m^?2 (28‐y) to 11.5 cm² m^?2 (67‐y) and decreased again to 7 cm² m^?2 in old stands. Above‐ground living biomass increased from 1.5 kg dry weight m^?2 (28‐y) to 14 kg dry weight m^?2 (383‐y). Day‐to‐day variation of tree transpiration in summer was dependent on net radiation, vapour pressure deficit, and soil water stress. Tree‐to‐tree variation of xylem flux was small and increased with heterogeneity in canopy structure. Maximum rates of xylem flux density followed the course of net radiation from mid April when a constant level of maximum rates was reached until mid September when low temperatures and light strongly reduced flux density. Maximum sap flux density (60 g m^?2 s^?1) and canopy transpiration (1.5 mm d^?1) were reached in the 67‐y stand. Average canopy transpiration of all age classes was 0.72 ± 0.3 mm d^?1. Canopy transpiration (E) was not correlated with LAI but related to stand sapwood area SA (E = ? 0.02 + 1.15SA R²) which was determined by stand density and tree sapwood area.     

Biomass and production of a naturally regenerated oak forest in southern Korea

Biomass and production of two stands with Quercus variabilis Bl. as the dominant species (stands 1 and 3) and one with Q. mongolica Fisch. as the dominant species (stand 2) were investigated in southern Korea. Stands 1 and 3 naturally occurred on sites with southerly aspects while stand 2 naturally occurred on northerly aspects; stand ages were similar for the three stands (36–38 years old). Total above- and belowground biomass including understory vegetation (Mg ha^–1) was 108.4 for stand 1, 115.6 for stand 2, and 132.0 for stand 3, respectively. Understory vegetation constituted 17.4% of the total biomass in stand 1 but only 3.7–4.5% in stand 2 and stand 3. Roots constituted 20.1–24.6% of the biomass of the overstory vegetation. Although stand 3 showed the highest total biomass, net production was highest in stand 2 at 12.6 (Mg ha^–1 year^–1); net production levels for stands 1 and 3 were 11.7 and 11.1 (Mg ha^–1 year^–1), respectively. It appeared that the differences in site conditions related to aspect influenced the distribution of naturally regenerated oak species within a relatively small area and resulted in differences in biomass and production among the stands.     

Dynamics of leaf minerals,bdleaf area,and biomass from hardwoods intensively grown on a peat bog

Summary Water willow and grey alder were grown on a raised sphagnum bog in central Sweden. The stands were intensively treated by daily irrigation and fertilization during the growing period in order to improve site fertility. After a 2-year establishment period high production rates were achieved in willow stands, 0.8 kg stem dry weight m^–2 year^–1 on current plus one (C+1) year old shoots. In these stands the canopy was closed with a leaf area index (LAI) that peaked at approximately 7. The canopy in the alder stand did not close during the initial 3 years of growth and the measured production rate was relatively low, at approximately 0.4 kg dry weight m^–2 year ^–1 in the last year. The leaf nitrogen content was 3%–4% of dry weight during the summer and the other studied mineral elements were in almost optimal proportion to nitrogen. This was considered to be an effect of the intensive fertilization regime. Above-ground production close to maximum yield was attained in the prevailing conditions of soil, climate and biomass partitioning.     

The effects of reduced phosphorus and nitrogen loading on phytoplankton in Mondsee,Austria

Following restoration measures (ring canalization, treatment plant with phosphorus precipitation), phosphorus loading declined step-wise from 26.21 t year^–1 to 9.18 t year^–1 during the period 1979–1984 while P-retention increased from 48% to 78%. Phosphorus loading was poorely correlated with precipitation. Inorganic nitrogen load, largely NO₃– N, did not decline but was significantly correlated with precipitation (r = 0.95) throughout the investigation period (1978–1989).Total phosphorus loading reached acceptable levels in 1984 when compared to critical loading calculated according to Vollenweider (1976). Phosphorus input to the lake has remained at these levels in recent years.Average annual chlorophyll-a concentrations and biovolume of phytoplankton in the top 20 m layer of the lake decreased, in correspondence with the respective phosphorus concentrations, from eutrophic to mesotrophic levels. The decline was accompanied by a drastic reduction of blue-green algal populations, and especially of Oscillatoria rubescens D.C..     

A huge,undescribed soil ciliate (Protozoa: Ciliophora) diversity in natural forest stands of Central Europe

We investigated 12 natural forest stands in eastern Austria for soil ciliate diversity, viz., eight beech forests and two lowland and Pinus nigra forests each. The stands span a wide range of climatic (e.g., 543–1759 mm precipitation, 160–1035 m above sea-level) and abiotic (e.g., pH 4–7.4) factors. Samples were taken twice in autumn and late spring and analysed with the non-flooded Petri dish method. Species were identified in vivo, in silver preparations, and in the scanning electron microscope. A total of 233 species were found, of which 30 were undescribed, a surprising number showing our ignorance of soil ciliate diversity, even in Central Europe. Species number varied highly from 45 (acidic beech on silicate) to 120 (floodplain forest) and was strongly correlated with pH and overall habitat quality, as measured by climate, the C/P quotient (ratio of r-selected colpodean and k-selected polyhymenophorean ciliates), and the proportion of mycophagous ciliate species; multivariate analysis showed further important variables, viz., the general nutrient status (glucose, nitrogen, C/ N ratio) and microbial (urease) activity. The highest species number occurred in one of the two floodplain soils, supporting the intermediate disturbance hypothesis. The three main forest types could be clearly distinguished by their ciliate communities, using similarity indices and multidimensional scaling. Individual numbers varied highly from 135^–1 (lowland forest) to 10,925 ml^–1 (beech on silicate) soil percolate and showed, interestingly, a weak correlation with soil protozoan phospholipid fatty acids. Eight of the 30 new species found and a forgotten species, Arcuospathidium coemeterii (Kahl 1943) nov. comb., are described in detail, as examples of how species were recognized and soil protozoan diversity should be analyzed: Latispathidium truncatum bimicronucleatum, Protospathidium fusioplites, Erimophrya sylvatica, E. quadrinucleata, Paragonostomum simplex, Periholosticha paucicirrata, P. sylvatica, and Australocirrus zechmeisterae.     

Gas Exchange of Carrot Leaves in Response to Elevated CO2 Concentration

Short-term responses of four carrot (Daucus carota) cultivars: Cascade, Caro Choice (CC), Oranza, and Red Core Chantenay (RCC) to CO₂ concentrations (C _a) were studied in a controlled environment. Leaf net photosynthetic rate (P _N), intercellular CO₂ (C _i), stomatal conductance (g _s), and transpiration rate (E) were measured at C _a from 50 to 1 050 mol mol^–1. The cultivars responded similarly to C _a and did not differ in all the variables measured. The P _N increased with C _a until saturation at 650 mol mol^–1 (C _i= 350–400 mol mol^–1), thereafter P _N increased slightly. On average, increasing C _a from 350 to 650 and from 350 to 1 050 mol mol^–1 increased P _N by 43 and 52 %, respectively. The P _N vs. C _i curves were fitted to a non-rectangular hyperbola model. The cultivars did not differ in the parameters estimated from the model. Carboxylation efficiencies ranged from 68 to 91 mol m^–2 s^–1 and maximum P _N were 15.50, 13.52, 13.31, and 14.96 mol m^–2 s^–1 for Cascade, CC, Oranza, and RCC, respectively. Dark respiration rate varied from 2.80 mol m^–2 s^–1 for Oranza to 3.96 mol m^–2 s^–1 for Cascade and the CO₂ compensation concentration was between 42 and 46 mol mol^–1. The g _s and E increased to a peak at C _a= 350 mol mol^–1 and then decreased by 17 and 15 %, respectively when C _a was increased to 650 mol mol^–1. An increase from 350 to 1 050 mol mol^–1 reduced g _s and E by 53 and 47 %, respectively. Changes in g _s and P _N maintained the C _i:C _a ratio. The water use efficiency increased linearly with C _a due to increases in P _N in addition to the decline in E at high C _a. Hence CO₂ enrichment increases P _N and decreases g _s, and can improve carrot productivity and water conservation.     

Seasonal changes in apparent hydraulic conductance and their implications for water use of European beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) and sessile oak [<Emphasis Type="Italic">Quercus petraea</Emphasis> (Matt.) Liebl] in South Europe

The water status of Fagus sylvatica L. and Quercus petraea (Matt) Liebl. was analysed during a cycle of progressive natural drought in southern Europe. Predawn (Ψ_pd) and midday water potential were measured in transpiring (Ψ_leaf) and non-transpiring leaves (Ψ_xyl). Furthermore, photosynthesis (A), stomatal conductance to water vapour (g_s) and sap flow (F_d) were recorded on the same dates. Apparent leaf specific hydraulic conductance in the soil–plant–air continuum (K_h) and whole tree hydraulic conductance (K_hsf) were calculated by using the simple analogy of the Ohm’s law. K_h was estimated at different points in the pathway as the ratio between transpiration (E) in the uppermost canopy leaves at midday and the gradient of water potential in the different compartments of the continuum soil–roots–stem–branches–leaves. There was a progressive decrease in water potential measured on non-transpiring leaves at the base of tree crown in both species (Ψ^l_xyl) from the beginning of the growing season to the end of summer. A similar decrease was shown in shoot water potential (Ψ^u_xyl) at the uppermost canopy. Predawn water potential (Ψ_pd) was high in both species until late July (28 July); afterwards, a significant decrease was registered in F. sylvatica and Q. petraea with minimum values of −0.81±0.03 and −0.75±0.06 MPa, respectively, by 15 September. In both species, leaf specific hydraulic conductance in the overall continuum soil–plant–air (K_h) decreased progressively as water stress increases. Minimum values of K_h and K_hsf were recorded when Ψ_pd was lower. However, Q. petraea showed higher K_h than F. sylvatica for the same Ψ_pd. The decrease in K_h with water stress was mainly linked to its fall from the soil to the lowermost canopy (K_srs). Nevertheless, a significant resistance in the petiole–leaf lamina (K_pl) was also recorded because significant differences in all dates were found on Ψ between transpiring and non-transpiring leaves from the same shoot. The decline in K_h was followed by an increase in stomatal control of daily water losses through the decrease of stomatal conductance to water vapour (g_s) during the day. It promoted a seasonal increase in the stomatal limitation to carbon dioxide uptake for photosynthesis (A). These facts were more relevant in F. sylvatica, which had concurrently a higher decline in water use at the tree level than Q. petraea. The results showed a strong coupling in F. sylvatica and Q. petraea between processes at leaf and tree level. It may be hypothesised a role of specific hydraulic conductance not only in the regulation of water losses by transpiration but also of carbon uptake.     

Nitrogen concentration of cauliflower organs as determined by organ size,N supply,and radiation environment

Data from field experiments carried out in three consecutive years under contrasting N supply and radiation environment altered by artificial shading were used to identify (a) the relationship between N concentration and organ size under conditions of unrestricted N supply and (b) critical levels of soil nitrate (Nmin_crit), where nitrogen concentration of cauliflower organs begin to decline because of N limitations. The decline of N concentrations in cauliflower was analysed at different levels of morphological aggregation, i.e., the whole shoot level, the organ level (leaves, stem, and curd), and within different leaf groups within the canopy. Nmin_crit values (0–60 cm soil depth) for total nitrogen concentration of cauliflower organs leaves, stem and curd were estimated at 85, 93 and 28 kg N ha^–1, respectively. Within the canopy, Nmin_crit values for total N of leaves increased from the top to the bottom from 44 to 188 kg N ha^–1. Nmin_crit values for protein N in leaves from different layers of the canopy were much lower at around 30 kg N ha^–1, without a gradient within the canopy. It is discussed that these differences in Nmin_crit values are most likely a consequence of N redistribution associated with nitrogen deficiency. The decline of average shoot nitrogen concentrations, [Nm] (%N DM), with shoot dry matter, W _sh, (t ha^–1) under conditions of optimal N supply was [Nm]= 4.84 (±0.071) W _sh ^{–0.089(± 0.011)}, r ²=0.67 (±S.E.). The reduction of radiation intensity by artificial shading (60% of control) had no significant influence on total nitrogen concentrations of leaves and only a small influence on protein nitrogen concentrations in lower layers of the canopy. The leaf nitrate nitrogen fraction of nitrogen, f _nitr (–), within the canopy decreased linearly with increased average incident irradiance in different canopy layers (I _av, W PAR m^–2) (f _Nitr. = 0.2456(±0.0188) – 0.0023(±0.0004)I _av, r ² = 0.67.     

Biogeochemistry of trimethyllead and lead in a forested ecosystem in Germany

Lead compounds, especially ionic organolead compounds (OLC), are highly toxic and mobile pollutants strongly affecting many ecosystems. Soil pools and fluxes with precipitation, litterfall and runoff of trimethyllead (TML), one of the dominant ionic OLC in the environment, and Pb_total were investigated in a forested ecosystem in NE-Bavaria, Germany. In addition, ad/desorption of TML to soils was studied in batch experiments and its degradation in soils was investigated using long term incubations. Total soil storage in the catchment was 11.56 mg Pb ha^–1 for TML and 222 kg Pb ha^–1 for Pb_total. More than 90% of the soil storage of TML was found in the wetland soils of the catchment representing only 30% of the area. Most Pb_total (>90%) was found in the upland soils. In upland soils, TML was only detectable in the forest floor. The annual total deposition from the atmosphere, estimated as throughfall + litterfall fluxes, amounted to 3.7 mg Pb ha^–1 year^–1 for TML and 52 g Pb ha^–1 year^–1 for Pb_total. The contribution of litterfall was 1.5 and 32%, respectively. The concentrations of TML and Pb_total in wet precipitation were: fog > throughfall > bulk precipitation. The annual fluxes with runoff from the catchment was 0.5 mg Pb ha^–1 year^–1 for TML and 2.8 g Pb ha^–1 year^–1 for Pb_total. TML degraded rapidly in the forest floor (Oa horizon) with a half-life (t _1/2) of 33.5 days. The degradation of TML in Fen (t _1/2 = 421 days) and in the mineral soil (Bw-C horizon, t _1/2 = 612 days) was much slower. Emission of tetramethyllead from wetland soils was not observed during the 1 year incubation. The adsorption affinity of TML to different soils was Fen > Oa > A Bw-C. The ratio of total soil storages to the present annual input were 3.6 years for TML. TML and Pb_total are still deposited in remote areas even after the use of tetraalkyllead as additives has been terminated for years. The rates of deposition are, however, much lower than in the past. Forest soils act as a sink for deposited TML and Pbtotal. TML is accumulated mostly in wetland soils and seems to be stable under anoxic conditions for a long time. In upland soils, TML decomposes rapidly. Only small amounts of TML are transferred from soils into runoff.     

Recruitment,mortality and growth of mangrove (<Emphasis Type="Italic">Rhizophora</Emphasis> sp.) seedlings in Ulugan Bay,Palawan, Philippines

Propagule dispersal, establishment and recruitment to the sapling stage are critical steps in the life cycle of mangroves. Specific (i.e., per capita) rates of recruitment and mortality, and the growth rates of Rhizophora seedlings in three mangrove stands in Ulugan Bay (Palawan, Philippines) were estimated between March 1999 and February 2001. Recruitment and mortality were variable in space and time, with mortality exceeding rates of recruitment at all sites. The specific rates of seedling recruitment and mortality were higher in Buenavista (0.66 year^–1 and –1.67 year^–1) than in Umalagan (0.05 year^–1 and –0.33 year^–1) and Oyster Bay (0.13 year^–1 and –0.24 year^–1). The annual rate of production of internodes by the main stem was similar at the three sites (5.4–5.5 internodes year^–1), but the annual rate of elongation of the main stem was higher in Buenavista (10.6 cm year^–1) than in Oyster Bay (7.6 cm year^–1) and Umalagan (5.6 cm year^–1).