1,721,171 research outputs found
Notes on the host plant, larval features and life history of Cephalcia hartigii (Bremi) (Hym., Pamphiliidae).
No full textIndirect partitioning of soil respiration in a series of evergreen forest ecosystems
No full textA simple estimation of heterotrophic respiration can be obtained analytically as the y-intercept of the linear regression between soil-surface CO2 efflux and root biomass. In the present study, a development of this indirect methodology is presented by taking into consideration both the temporal variation and the spatial heterogeneity of heterotrophic respiration. For this purpose, soil CO2 efflux, soil carbon content and main
stand characteristics were estimated in seven evergreen forest ecosystems along an elevation gradient ranging from 250 to 1740 m. For each site and for each sampling date the measured soil CO2 efflux (RS) was predicted with the model RS = a x SC + b x RD±e, where SC is soil carbon content per unit area to a depth of 30 cm and RD is the root density of the 2–5 mm root class. Regressions with statistically significant
a and b coefficients allowed the indirect separation of the two components of soil CO2 efflux. Considering that the different sampling dates were characterized by different soil temperature, it was possible to investigate the temporal and thermal dependency of autotrophic and heterotrophic respiration. It was estimated that annual autotrophic respiration accounts for 16–58% of total soil CO2 efflux in the seven
different evergreen ecosystems. In addition, our observations show a decrease of annual autotrophic respiration at increasing availability of soil nitrogen
Experimental analysis of flux footprint for varying stability conditions in an alpine meadow
No full textEddy-covariance measurements, which were performed in the summer 2003 in an alpine grassland site, were used to determine
the footprint of CO2 turbulent fluxes and to investigate its dependence on stability. The analysis is based on the spatial variability of carbon sinks generated by the difference in the time of the cutting of two concentric portions of the footprint, located outside and inside the fence surrounding the eddy tower, at approximately 30 m distance. Due to this time difference, turbulent flux
measurements were performed both in homogeneous (BEFORE and AFTER cutting) and heterogeneous (BETWEEN cuts) condition of spatial sink distribution. The analysis is based exclusively on daytime measurements. Half-hourly records ranked in two stability classes were used to calculate light response functions separately for three 10-day periods (BEFORE, BETWEEN and AFTER cutting). The contribution of the area inside the fence to the total flux was determined for different photosynthetic photon flux density (PPFD) values considering the BETWEEN cuts light response curve as a weighted average of the BEFORE and AFTER ones. The weight of the BEFORE cutting light response curve has been analytically determined and corresponds to the flux fraction which originates from inside the fence. The experimental estimates of the relative importance of the area inside the fence produced values ranging from 30%, during stable conditions, and up to 80% during unstable conditions. These values derived from observations were later compared with the predictions of three analytical footprint models. One of the models systematically underestimated experimental observations, while observations concurred with the other two, particularly in moderately unstable
conditions
Main determinants of forest soil respiration along an elevation/temperature gradient in the Italian Alps
No full textThe main determinants of soil respiration were investigated in 11 forest types distributed along an altitudinal and thermal gradient in the southern Italian Alps
(altitudinal range 1520 m, range in mean annual temperature 7.8 1C). Soil respiration, soil carbon content and principal stand characteristics were measured with standardized methods. Soil CO2 fluxes were measured at each site every 15–20 days with a closed dynamic system (LI-COR 6400) using soil collars from spring 2000 to spring 2002. At the same time, soil temperature at a depth of 10 cm and soil water content (m3m-3) were measured at each collar. Soil samples were collected to a depth of 30 cm and stones, root content and bulk density were determined in order to obtain reliable estimates of carbon content per unit area (kgCm-2). Soil respiration and temperature data were fitted with a simple logistic model separately for each site, so that base respiration rates and mean annual soil respiration were estimated. Then the same regression model was applied to all sites simultaneously, with each model parameter being expressed as a linear function of site variables. The general model explained about 86% of the intersite variability of soil respiration. In particular, soil mean annual temperature explained the most of the variance of the model (0.41), followed by soil temperature interquartlile range (0.24), soil carbon content (0.16) and soil water content (0.05)
Trends and controls of inter-annual variability in the carbon budget of terrestrial ecosystems
Full text linkThe climate sensitivity of the terrestrial carbon budget will substantially affect the sign and strength of the land-climate feedbacks and the future climate trajectories. Current trends in the inter-annual variability of terrestrial carbon fluxes (IAV) may contribute to clarify the
relative role of physical and biological controls of ecosystem responses to climate change.
For this purpose we investigated how recent climate variability has impacted the carbon fluxes at long-term FLUXNET sites. Using a novel method, the IAV has been factored out in climate induced variability (physical control), variability due to changes in ecosystem
functioning (biological control) and the interaction of the two terms.
The relative control of the main climatic drivers (temperature, water availability) on the physical and biological sources of IAV has been investigated using both site level fluxes and global gridded products generated from the up-scaling of flux data. Results of this analysis highlight the fundamental role of precipitation trends on the pattern of IAV in the
last 30 years. Our findings on the spatial/temporal trends of IAV have been finally confirmed using the signal derived from the global network of atmospheric CO2
concentrations measurements
Drag coefficient and turbulence intensity in conifer canopies
No full textThe relationship between wind speed and drag coefficient was experimentally investigated in two mixed conifer forests of the Italian Alps. Drag coefficient was calculated from the mean momentum equation while canopy architecture was described using optical measurements of gap fraction. Two different formulations of the momentum equation were tested: the common one, where drag forces are expressed in terms of the squared mean velocity ( ̄u2), and an alternative one in which drag forces depend on the averaged product of instantaneous wind intensity and instantaneous longitudinal wind component |U|u. Values of drag coefficients computed according to the first approach, decrease with increasing wind speed in both sites and show different average values between sites (0.15 ± 0.14 and 0.34 ± 0.45). Drag coefficients obtained with the second approach do not show a clear dependence on wind intensity, and the mean drag coefficients of the two sites become more similar (0.09 ± 0.06 and 0.12 ± 0.12). According to these results the scaling based on |U|u seems more appropriate to characterise the physics of the phenomenon
Diffusione di alcune specie del genere Cephalcia Panzer in abetine del Trentino (Hymenoptera, Pamphiliidae)
No full textSources and patterns of the inter-annual variability of the terrestrial carbon budget
Full text linkLarge uncertainty in predicting future climate trajectories derives from the climate sensitivity of the terrestrial carbon balance and the sign and strength of the land-climate feedbacks. It is therefore of outmost importance to deepen our understanding on the ecosystem responses to climate variability. For this purpose we investigated how recent climate trends have impacted the terrestrial carbon balance using a combination of surface observations of atmospheric CO2 concentrations and ecosystem fluxes at long-term sites. Using a novel methodology, the interannual variability of the C fluxes (IAV) has been separated in climate induced variability, variability due to changes in ecosystem functioning and the interaction of the two. Results have been corroborated with a global scale analysis based on statistically up-scaled fluxes to highlight the role of temperature and precipitations on the spatial pattern of IAV in the last 30 years. Finally, the spatial/temporal trends of IAV have been confirmed using the signal derived from the global network of atmospheric CO2 concentrations measurements. This analysis highlights the importance of combined surface networks of CO2 fluxes and concentrations to support multi-scale analysis of the land-climate interactions and to address key questions on the future trajectories of the land biogeochemistr
Spatial Variability and Optimal Sampling Strategy of Soil Respiration
No full textSoil respiration is the second largest flux of carbon between terrestrial ecosystems and the atmosphere and is affecting climate sensitivity and vulnerability of the terrestrial carbon stock. Monitoring soil carbon dioxide efflux is a complex task, due to the high spatial and temporal variability of the fluxes. For this reason, more than 30 sampling points are required to attain reliable estimates of ecosystem soil respiration. However, the number of sampled points is often limited by labour, time and budget constraints. Stratified sampling is an alternative to random sampling as a method to reduce the number of sampling points when an effective proxy variable is available for the definition of the strata.
In order to evaluate different sampling strategies we tested, with a Monte Carlo simulation, the effectiveness of random and stratified samplings, using experimental data collected in three alpine ecosystems (two forests and one grassland). We evaluated an innovative method for defining the strata to be sampled. The method is based on an initial sampling of soil respiration from a large number of candidate points in order to account for the spatial variability. The minimum number of sampling points required to adequately represent soil respiration for the entire area were then selected using stratified statistical sampling.We show that this method is unbiased and that it reduces considerably the uncertainty in the sampling process compared to random sampling. The method was highly effective in the two forest ecosystems, characterized by a high spatial variability in soil respiration and by a high temporal correlation of the fluxes. On the contrary the method was not so effective in the grassland site, where fluxes have lower spatial variability and temporal correlation. However, the stratified sampling offered a consistent reduction of the error (%) of the estimated annual soil CO2 efflux in all the ecosystems. At the grassland ecosystem the average reduction of the error (%) of the annual CO2 efflux was about 12%, while at the forest ecosystems the average reductions were 55% and 57%, respectively.
# 2007 Elsevier B.V. All rights reserved.JRC.H.2 - Climate chang
Constraints on light interception efficiency due to shoot architecture in broad-leaved Nothofagus species
No full textShoot architecture may significantly alter mean quantum flux on foliage and thus, photosynthetic productivity. There is currently only limited information about plastic alterations in shoot structure caused by within-canopy variation in mean integrated irradiance (Qint) in broad-leaved trees. We studied leaf and shoot structure, and nitrogen and carbon content in late-successional, widely distributed, temperate, broad- leaved Nothofagus taxa to determine the architectural controls on light harvesting and photosynthetic performance. Nothofagus fusca (Hook. f.) Oersted has larger leaves and less densely leaved shoots than the N. solandri varieties. Nothofagus solandri var. solandri (Hook. f.) Oersted is characterized by rounder leaves that potentially have a larger overlap than the ovate-triangular leaves of N. solandri var. cliffortioides (Hook. f.) Poole. Leaf dry mass (MA) and nitrogen content (NA) per unit area increased with increasing Qint in all species, demonstrating enhanced investment of photosynthetic biomass in high light. Although MA differed between species at a common irradiance, there was a uniform relationship between NA and Qint across species. Leaf carbon content per dry mass and leaf dry mass to fresh mass ratio also scaled positively with irradiance, suggesting greater structural investments in high light. In all species, shoots became more horizontal and flatter at lower Qint, implying an enhanced use efficiency of direct irradiance in natural leaf positions. In contrast, irradiance effects on leaf aggregation varied among species. Across the data, leaf overlap or leaf area density was often greater at lower irradiances, possibly as a result of limited carbon availability for shoot axis extension growth. In N. fusca, leaves of which were more aggregated in high light, the shoot silhouette to total leaf area ratio (SS) declined strongly with increasing irradiance, demonstrating a lower light harvesting efficiency at high Qint. This effect was only moderate in N. solandri var. cliffortioides and SS was independent of Qint in N. solandri var. solandri. Although the efficiency of light interception at high irradiances was lowest in N. fusca, this species had the greatest nitrogen content per unit shoot silhouette area (2NA/SS), indicating superior shoot-level photosynthetic potential. These data collectively demonstrate that shoot architecture significantly affects light interception and photosynthesis in broad-leaved trees, and that structural carbon limitations may constrain leaf light harvesting efficiency at low irradiance
- …
