Contributions of climate, leaf area index and leaf physiology to variation in gross primary production of six coniferous forests across Europe: a model-based analysis.

Gross primary production (GPP) is the primary source of all carbon fluxes in the ecosystem. Understanding variation in this flux is vital to understanding variation in the carbon sink of forest ecosystems, and this would serve as input to forest production models. Using GPP derived from eddy-covariance (EC) measurements, it is now possible to determine the most important factor to scale GPP across sites. We use long-term EC measurements for six coniferous forest stands in Europe, for a total of 25 site-years, located on a gradient between southern France and northern Finland. Eddy-derived GPP varied threefold across the six sites, peak ecosystem leaf area index (LAI) (all-sided) varied from 4 to 22 m(2) m(-2) and mean annual temperature varied from -1 to 13 degrees C. A process-based model operating at a half-hourly time-step was parameterized with available information for each site, and explained 71-96% in variation between daily totals of GPP within site-years and 62% of annual total GPP across site-years. Using the parameterized model, we performed two simulation experiments: weather datasets were interchanged between sites, so that the model was used to predict GPP at some site using data from either a different year or a different site. The resulting bias in GPP prediction was related to several aggregated weather variables and was found to be closely related to the change in the effective temperature sum or mean annual temperature. High R(2)s resulted even when using weather datasets from unrelated sites, providing a cautionary note on the interpretation of R(2) in model comparisons. A second experiment interchanged stand-structure information between sites, and the resulting bias was strongly related to the difference in LAI, or the difference in integrated absorbed light. Across the six sites, variation in mean annual temperature had more effect on simulated GPP than the variation in LAI, but both were important determinants of GPP. A sensitivity analysis of leaf physiology parameters showed that the quantum yield was the most influential parameter on annual GPP, followed by a parameter controlling the seasonality of photosynthesis and photosynthetic capacity. Overall, the results are promising for the development of a parsimonious model of GPP.

Fruit development, not GPP, drives seasonal variation in NPP in a tropical palm plantation.

We monitored seasonal variations in net primary production (NPP), estimated by allometric equations from organ dimensions, gross primary production (GPP), estimated by the eddy covariance method, autotrophic respiration (R(a)), estimated by a model, and fruit production in a coconut (Cocos nucifera L.) plantation located in the sub-tropical South Pacific archipelago of Vanuatu. Net primary production of the vegetative compartments of the trees accumulated steadily throughout the year. Fruits accounted for 46% of tree NPP and showed large seasonal variations. On an annual basis, the sum of estimated NPP (16.1 Mg C ha(-1) year(-1)) and R(a) (24.0 Mg C ha(-1) year(-1)) for the ecosystem (coconut trees and herbaceous understory) closely matched GPP (39.0 Mg C ha(-1) year(-1)), suggesting adequate cross-validation of annual C budget methods. However, seasonal variations in NPP + R(a) were smaller than the seasonal variations in GPP, and maximum tree NPP occurred 6 months after the midsummer peak in GPP and solar radiation. We propose that this discrepancy reflects seasonal variation in the allocation of dry mass to carbon reserves and new plant tissue, thus affecting the allometric relationships used for estimating NPP.

Respiration as the main determinant of carbon balance in European forests.

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha(-1) yr(-1), with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.

Transpiration of a 64-year-old maritime pine stand in Portugal : 1. Seasonal course of water flux through maritime pine.

The transpiration, sap flow, stomatal conductance and water relations ofPinus pinaster were determined during spring and summer in a 64-year-old stand in Ribatejo (Portugal). The transpiration of the pine canopy was determined from sap flow or eddy covariance techniques. Canopy conductance values (g c) were estimated from inversion methods using eddy covariance or sap flow data, respectively, and from scaling-up methods using stomatal conductance values measured in the field and leaf area index (LAI) values. The transpiration was closely controlled by the stomatal conductance of pines (Ω was 0.05-0.15). For wet soil conditions, the various estimates ofg c showed reasonable agreement.g c peaked in the morning at 0.01 m×s-1, exhibited a midday depression and showed a secondary peak in late afternoon. This behaviour could be predicted simply on the basis of the stomatal sensitivity to air vapour pressure deficit. On a seasonal basis, monthly average values ofg c decreased from 4×10-3 m×s-1 in spring to 1.7×10-3 m·s-1 in late summer. Accordingly, the transpiration peaked at 3 mm×d-1 on wet soil in May. It decreased progressively during the summer drought to 0.8 mm×d-1 at the end of August. The minimal value of needle water potential was maintained at -1.9 MPa but predawn values decreased from -0.6 MPa in May to -0.9 MPa in July. It may have reached lower values in August. The amount of water stored in the trunk accounted for a 12% (10 kg×tree-1×day-1) of the daily transpiration in spring. The storage capacity of the canopy was within the same order of magnitude. The trunk storage increased to 25% (13 kg×tree-1×day-1) of the daily transpiration at the end of summer under drought conditions. The sap flow beneath the crown lagged accordingly behind transpiration with a time constant estimated between 26 min in spring and 40 min at the end of summer.

Transpiration of a 64-year old maritime pine stand in Portugal : 2. Evapotranspiration and canopy stomatal conductance measured by an eddy covariance technique.

The sensible and latent heat losses of a maritime pine stand (Pinus pinaster Aiton) and of its understorey were measured in Portugal, in conditions of mild water stress, using an eddy covariance technique (monodimensional sonic anemometer coupled with a thin thermocouple and a fast-response hygrometer) at two levels above and under the canopy: canopy exchanges were estimated by difference. This paper first discusses the corrections to be made to the raw fluxes, then shows that, over a 24-h interval, the energy balance closure was very satisfactory, as well as the comparison between estimations of canopy evaporation by eddy covariance and sap flow. Moreover, the phase shift between the two methods was small when sap flow was measured just at the base of the canopy. Canopy stomatal conductance, estimated with a flux-gradient model, increased very sharply at dawn, then decreased progressively in compensation for the increase in air saturation deficit.

Faecal water content and egg survival of goat gastro-intestinal strongyles under dry tropical conditions in Guadeloupe.

Faeces from naturally infected goats were deposited on a natural grassland during the dry season in Guadeloupe (French West Indies) at different times throughout the day. The grass was either 7 or 20-30 cm tall. After a period of between several hours and 7 days, the number of viable strongyle eggs and the faecal water content were measured. Faecal temperature was recorded continuously. Faecal temperature was greater than 40 degrees-45 degrees C at midday and dehydration was rapid between 8 a.m. and 2 p.m. Egg mortality was greater on short than on tall grass and higher in morning than in evening deposits. Minimal faecal water content during the first 36 h explained the 74%, 55% and 38% mortality rate for eggs of Oesophagostomum columbianum (OC), Haemonchus contortus (HC) and Trichostrongylus colubriformis (TC), respectively. In all, 5%-22% of the eggs of the latter species remained viable in a state of anhydrobiosis after 7 days on the ground. A delay of only 2 days between goat departure and irrigation would be sufficient to ensure that greater than 95% of O. columbianum and H. contortus eggs and 70% of T. colubriformis eggs are destroyed.

Effects of irrigation on appearance and survival of infective larvae of goat gastro-intestinal nematodes in Guadeloupe (French West Indies).

In Guadeloupe (French West Indies), faeces from naturally infected goats were deposited during the dry season on three plots, irrigated with long (plot A) or short herbage (B) and non-irrigated with long herbage (C). Microclimatic data and the evolution of L3 population size in faeces, on soil surface and on herbage were followed over a period of 26 days. The initial nematode egg population was comprised of 58% Haemonchus contortus (HC), 25% Trichostrongylus colubriformis (TC) and 17% Oesophagostomum columbianum (OC). Temperature and water content varied in time and space (soil, faeces, herbage) from homogeneous in A to very heterogeneous in C. In A and B, population dynamics were similar with higher values of maxima in A. Larval peaks occurred on day 9 after deposition in faeces in plot A: 23.1, 39.1 and 17.2 L3/100 eggs, respectively for HC, TC and OC; the same day in soil: 1.9, 0.6 and 3.1 L3/100 eggs. On day 26 it remained less than 1 L3/100 eggs in both soil and herbage for the three species. In C, only TC larvae were observed coming, after rain, from eggs in which hatching had been delayed. It was difficult to separate the respective effects of temperature and water content on the development of the eggs, but irrigation gave favourable conditions for all eggs to develop into larvae. Pasture rotation with 28-35 days of regrowth should minimize the increased risk of infection for the goats due to irrigation.

The effect of sudden solar exposure on thermophysiological parameters and on plasma prolactin and cortisol concentrations in male Creole goats.

Rectal temperature (RT), respiratory rhythm (RR), plasma cortisol and prolactin (PRL) levels and haematocrit were measured at noon in male Creole goats during their habituation to shade, during sudden exposure to sunlight and then while they were kept outdoors. Data on the microclimatic environment, especially black-globe temperature (Tg), were recorded. On the day (DO) the bucks were put in the sun, the increase of RT and RR, characteristic of circadian rhythm, was amplified (RT: 40.84 +/- 0,23 degrees C vs 39.48 +/- 0.19 degrees C P less than 0.001; RR: 98 +/- 22 vs 35 +/- 3 respirations/min; P less than 0.01). Plasma cortisol also increased (17.7 +/- 8.6 vs 5.57 +/- 1.7 ng/ml; P less than 0.05). After 24 h (D1), plasma prolactin concentration increased dramatically (923 +/- 653 vs 90 +/- 22 ng/ml; P less than 0.05) and haematocrit decreased (28.6 +/- 2.3 vs 33 +/- 2.3%; P less than 0.05). To clarify the simultaneous effects of time (t) and Tg, we developed a model Log-normal on t and linear on Tg for RR and PRL. A model decreasing exponentially with t and linear with Tg was developed for RT; the determination coefficients were R2 = 0.96, 0.75 and 0.59, respectively. About 3 weeks later, after the adaptation period, RT, RR and plasma PRL stabilized; RT and RR returned to shade values while the PRL level remained higher than the shade PRL level (248 +/- 109 vs 130 +/- 50 ng/ml; P less than 0.05). The 24-hour time-lag in prolactin hypersecretion and the role of prolactin in thermoregulation are discussed.

The influence of nutritional levels and shade structure on testicular growth and hourly variations of plasma LH and testosterone levels in young Creole bulls in a tropical environment.

Twelve Creole bulls 13.3 months of age were divided into three groups according to live weight. The first group received a low nutritional level and was housed in a shaded stable; the second group received a high nutritional level and was housed in the same stable as the first group; the third group received the same ration as the second group but was exposed to solar radiation. Live weight and testicular diameter were recorded fortnightly. After 65 days, the blood of the bulls was sampled every 20 min for 12 h. Plasma LH and testosterone were quantified by radioimmunoassay. There was no significant influence of solar radiation on the testicular growth or the hormonal levels. Underfeeding decreased testicular growth and the average plasma LH and testosterone levels. However, this action was effective only in decreasing the number of hormonal pulses per day (0.63 vs 2.55 LH pulses per day and 0.75 vs 2.50 testosterone pulses per day); it did not change the basal hormone levels. It is suggested that this decrease in LH pulsatility might be responsible for the slow testicular growth in the underfed bulls.