Cross validation of open-top chamber and eddy covariance measurements of ecosystem CO2 exchange in a Florida scrub-oak ecosystem

Simultaneous measurements of net ecosystem CO₂ exchange (NEE) were made in a Florida scrub‐oak ecosystem in August 1997 and then every month between April 2000 to July 2001, using open top chambers (NEE_O) and eddy covariance (NEE_E). This study provided a cross validation of these two different techniques for measuring NEE. Unique characteristics of the comparison were that the measurements were made simultaneously, in the same stand, with large replicated chambers enclosing a representative portion of the ecosystem (75 m², compared to approximately 1–2 ha measured by the eddy covariance system). The value of the comparison was greatest at night, when the microclimate was minimally affected by the chambers. For six of the 12 measurement periods, night NEE_O was not significantly different to night NEE_E, and for the other periods the maximum difference was 1.1 µ mol m ^{? 2}s ^{? 1}, with an average of 0.72 ± 0.09 µ mol m ^{? 2}s ^{? 1}. The comparison was more difficult during the photoperiod, because of differences between the microclimate inside and outside the chambers. During the photoperiod, air temperature (T_air) and air vapour pressure deficits (VPD) became progressively higher inside the chambers until mid‐afternoon. In the morning NEE_O was higher than NEE_E by about 26%, consistent with increased temperature inside the chambers. Over the mid‐day period and the afternoon, NEE_O was 8% higher that NEE_E, regardless of the large differences in microclimate. This study demonstrates both the uses and difficulties associated with attempting to cross validate NEE measurements made in chambers and using eddy covariance. The exercise was most useful at night when the chamber had a minimal effect on microclimate, and when the measurement of NEE is most difficult.     

Biophysical stratification of the Amazon basin

In field measurement programmes, stratified sampling can optimize sampling efficiency, but stratification is often undertaken subjectively, and is frequently based on a priori classification schemes such as those used for vegetation maps. In order to avoid the problems associated with a priori subjective schemes, we explore here an objective procedure, Regression Tree Analysis (RTA). RTA has previously been used in local-scale studies, but here we apply it to a very large study domain, namely the entire humid tropical zone of South America. The aim of the study was to develop an optimal sampling design in preparation for the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA). Co-registered spatially continuous fields of rainfall, temperature, photosynthetically active radiation (PAR), the normalized difference index (NDVI), an index of surface moisture, and other independent variables were used to predict three dependent variables, annual net radiation (Rn), latent heat (LE) and net primary production (NPP). Rather than simply dividing the study area based on differing levels of the three dependent variables, empirical models were developed using RTA to indicate how the relationships between these and possible forcing variables vary across the study area. For each variable long-term seasonal indices such as annual average, monthly minimum and amplitude were used to exclude effects of temporal phase differences between the hemispheres. The resulting hierarchical models revealed variations in the interdependence of the forcing variables throughout the study area and therefore provided a basis for a stratified sampling and identifying the most important variables to be collected in LBA for the Amazon basin as a whole as well as optimizing the sampling scheme for scaling up findings from the field scale to larger areas.     

Are correlations among foliar traits in ferns consistent with those in the seed plants?

Broad-based studies of gymnosperms and angiosperms reveal consistent and functionally significant correlations among foliar traits such as leaf mass per area (LMA), maximum photosynthetic rate (A(area)), foliar nitrogen (N(area)), foliar chlorophyll (Chl) and leaf longevity. To assess the generality of these relationships, we studied 20 fern species growing in the understorey of a temperate deciduous forest. We found that foliar N(area) increases with LMA, and that foliar N(area) and A(area) are positively correlated with one another, as are foliar N(area) and Chl. The ferns in general have very low LMA compared with most seed plants; A(area), N(area) and Chl are below median values for seed plants but are not extreme. Species with overwintering fronds have significantly higher LMA than species with fronds that senesce at the end of the growing season, as well as a significantly higher C : N ratio in frond tissue and relatively high foliar N on an areal basis. Correlations among foliar traits associated with gas exchange in these forest understorey ferns are in accordance with patterns reported for seed plants, suggesting a high degree of functional constraint on the interrelationships among key elements in foliar design.     

Effects of nitrogen supply on photosynthetic and anatomical changes in current-year needles of <Emphasis Type="Italic">Pinus koraiensis</Emphasis> seedlings grown under two irradiances

We investigated the responses of photon-saturated photosynthesis rate (P _sat) and its simultaneous acclimation of anatomy and nitrogen use patterns of current needles of Korean pine (Pinus koraiensis) seedlings grown under factorial combinations of two nitrogen levels and irradiances. Although N supply resulted in a significant increase of N content in needles under both irradiances, the increase of P _sat tended to be suppressed only in shade (S). The significant increase of P _sat in full sunlight (O) was associated with the increase of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and chlorophyll (Chl) contents. In contrast, small increase of Chl content and no increase of RuBPCO content were found in S (90 % cut of full irradiance), which would result in a small increase of P _sat. This result suggests that extra N is stocked in needles under shade for the growth in next season. With N supply, a significant decrease of specific leaf area (SLA) was detected only in O. This decrease of SLA was due to the increase of density of needle. Furthermore, the increase of needle density was not due to the increased number and size of mesophyll cells, but the increased density of each mesophyll cell. Therefore, although SLA changed in O, the change did not involve anatomical adaptation to use increased N effectively, at least observable by light microscopy. Hence, even though the SLA would change, N deposition will improve the photosynthetic capacity of Korean pine seedlings, not through the development of needle anatomy but through improvement of the allocation of N in both irradiances.     

Gas exchange and water relations of three <Emphasis Type="Italic">Vitis vinifera</Emphasis> L. cultivars growing under Mediterranean climate

Optical characteristics, contents of photosynthetic pigments, total soluble sugars, and starch, rates of gas exchange, chlorophyll (Chl) a fluorescence, and leaf water relations were analysed in three Vitis vinifera L. cultivars, Tinto Cão (TC), Touriga Nacional (TN), and Tinta Roriz (TR), grown in Mediterranean climate. Chl content was significantly lower in TC than in TN and TR leaves, while the Chl a/b ratio was higher. TR had the lowest net photosynthetic rate, stomatal conductance, and contents of soluble sugars and starch than TN and TC. In spite of low Chl content, TC showed the lowest photon absorbance and the highest photochemical efficiency of photosystem 2. TC had the lowest predawn and midday leaf water potential. The capability for osmotic adjustment was similar among cultivars and the calculated modulus of elasticity was higher in TC leaves. The typical lighter green leaves of TC seemed to be an adaptive strategy to high irradiance and air temperature associated to water stress.     

Effects of reduced irradiance on leaf morphology,photosynthetic capacity,and fruit yield in olive (<Emphasis Type="Italic">Olea europaea</Emphasis> L.)

One-year-old olive trees (cv. Koroneiki) were grown in plastic containers of 50 000 cm³ under full daylight and 30, 60, and 90 % shade for two years. The effects of shade on leaf morphology and anatomy, including stomatal density and chloroplast structure, net photosynthetic rate (P _N), stomatal conductance (g _s), and fruit yield were studied. Shade reduced leaf thickness due to the presence of only 1–2 palisade layers and reduced the length of palisade cells and spongy parenchyma. The number of thylakoids in grana as well as in stroma increased as shade increased, while the number of plastoglobuli decreased in proportion to the reduced photosynthetically active radiation (PAR). The higher the level of shade, the lower the stomatal and trichome density, leaf mass per area (ALM), g _s, and P _N. Shade of 30, 60, and 90 % reduced stomatal density by 7, 16, and 27 %, respectively, while the corresponding reduction in P _N was 21, 35, and 67 %. In contrast, chlorophyll a+b per fresh mass, and leaf width, length, and particularly area increased under the same shade levels (by 16, 33, and 81 % in leaf area). P _N reduction was due both to a decrease in PAR and to the morphological changes in leaves. The effect of shade was more severe on fruit yield per tree (32, 67, and 84 %) than on P _N indicating an effect on bud differentiation and fruit set. The olive tree adapts well to shade compared with other fruit trees by a small reduction in stomatal and trichome density, palisade parenchyma, and a significant increase in leaf area.     

Growth,biomass production,and assimilatory characters in <Emphasis Type="Italic">Cenchrus ciliaris</Emphasis> L. Under elevated CO<Subscript>2</Subscript> condition

The effect of elevated carbon dioxide (600±50 cm³ m⁻³; C₆₀₀) on growth performance, biomass production, and photosynthesis of Cenchrus ciliaris L. cv. 3108 was studied. This crop responded significantly by plant height, leaf length and width, and biomass production under C₆₀₀. Leaf area index increased triple fold in the crops grown in the open top chamber with C₆₀₀. The biomass production in term of fresh and dry biomass accumulation increased by 134.35 (fresh) and 193.34 (dry) % over the control (C₃₆₀) condition where the crops were grown for 20 d. The rate of photosynthesis and stomatal conductance increased by 24.51 and 46.33 %, respectively, in C₆₀₀ over C₃₆₀ plants. In comparison with C₃₆₀, the rate of transpiration decreased by 6.8 % under C₆₀₀. Long-term exposure (120 d) to C₆₀₀ enhanced photosynthetic water use efficiency by 34 %. Also the contents of chlorophylls a and b significantly increased in C₆₀₀. Thus C. ciliaris grown in C₆₀₀ throughout the crop season may produce more fodder in terms of green biomass.     

Response of photosynthetic machinery of field-grown kiwifruit under Mediterranean conditions during drought and re-watering

Groups of Actinidia deliciosa A. Chev. C.F. Liang et A.R. Ferguson var. deliciosa kiwifruit plants were subjected to soil water shortage (D), while other groups were well irrigated (I). Variations in chlorophyll (Chl) a fluorescence indices and leaf gas exchange were determined once plants were severely stressed (25 d after the beginning of the D-cycle). Daily maximum values of photosynthetic photon flux density (PPFD) were ca. 1 650 μmol(photon) m⁻² s⁻¹, while air temperatures peaked at 34.6 °C. High irradiance per se did not greatly affect the efficiency of photosystem (PS) 2, but predisposed its synergistic reduction by D co-occurrence. Fluorescence showed transient photodamage of PS2 with a complete recovery in the afternoon in both D and I plants. Upon re-watering the efficiency of PS2 was suboptimal (95 %) at day 2 after irrigation was reinitiated. At early morning of the day 5 of re-watering, photosynthesis and stomatal conductance recovered at about 95 and 80 % of I vines, respectively, indicating some after-stress effect on stomatal aperture. Once excessive photons reached PS2, the thermal dissipation of surplus excitation energy was the main strategy to save the photosynthetic apparatus and to optimize carbon fixation. The rather prompt recovery of both Chl a fluorescence indices and net photosynthetic rate during re-watering indicated that kiwifruit photosynthetic apparatus is prepared to cope with temporary water shortage under Mediterranean-type-climates.     

Physiological comparisons of true leaves and phyllodes in <Emphasis Type="Italic">Acacia mangium</Emphasis> seedlings

We found differences between true leaves (TL) and phyllodes (Ph) during ontogeny of Acacia mangium plants as reflected in chlorophyll (Chl) and carotenoid contents, gas exchange, Chl fluorescence, and growth. The production of TL enhanced the relative growth rate of the A. mangium seedlings, allowing the plants to accumulate enough dry biomass for later growth, while the production of thicker Ph in the later growth stage of A. mangium could help plants to cope with higher irradiance in their natural growth conditions.     

Photosynthetically versatile thin shade leaves: A paradox of irradiance-response curves

Thick sun leaves have a larger construction cost per unit leaf area than thin shade leaves. To re-evaluate the adaptive roles of sun and shade leaves, we compared the photosynthetic benefits relative to the construction cost of the leaves. We drew photosynthetically active radiation (PAR)-response curves using the leaf-mass-based photosynthetic rate to reflect the cost. The dark respiration rates of the sun and shade leaves of mulberry (Morus bombycis Koidzumi) seedlings did not differ significantly. At irradiances below 250 μmol m⁻² s⁻¹, the shade leaves tended to have a significantly larger net photosynthetic rate (P _N) than the sun leaves. At irradiances above 250 μmol m⁻² s⁻¹, the P _N did not differ significantly. The curves indicate that plants with thin shade leaves have a larger daily CO₂ assimilation rate per construction cost than those with thick sun leaves, even in an open habitat. These results are consistently explained by a simple model of PAR extinction in a leaf. We must target factors other than the effective assimilation when we consider the adaptive roles of thick sun leaves.