Radiation balance of an isolated holm oak tree (Quercus rotundifolia Lam.) in a mediterranean savannah-type woodland

In closed canopy forests the energy absorbed by the trees can be adequately estimated solely from the vertical radiation fluxes. However, in isolated or widely spaced trees this approach is no longer valid and radiation fluxes in all directions must be accounted for. An adequate estimate of the tree available energy is critical to model and calculate both interception losses and transpiration. Within a study where interception loss in a sparse evergreen oak woodland (montado) of Southern Portugal is evaluated and mod¬elled, the net amount of radiant energy absorbed by an isolated holm oak tree (Q) was measured under different radiation conditions. The measuring and calculating proce¬dure was based on the integration of the flux density of net radiation (Rn) at different points of a cylindrical surface (S) enclosing the tree crown. A set of 4 net radiome¬ters were used: one at a fixed position, on the top of the crown, and the remaining 3 mounted on a standing structure that could be moved around the tree to measure Rn fluxes through the inferior and lateral sides. Measurements of Q were made for 8 dif¬ferent days, during the first 3 months of 2006. Night time measurements of Rn were also done, but with the net radiometers at fixed positions around the tree. The meteoro¬logical conditions during the measurements included clear sky and cloudy days, some of which with light rain. Net radiation at the top of the crown accounted for about 72 % of the total energy absorbed by the tree, and this is reflected by the good linear fit between Q and Rn above the crown. Meteorological conditions seem to have some influence on this relationship, as suggested by the differences on the adjusted linear models when total, clear sky, cloudy or rainy data sets were used. The occurrence of rain tends to cause a slight increase in Q in comparison to dry conditions, for identical levels of Rn. Q also shows a strong linear response to solar radiation (Rs), given the dependence of net radiation upon short wave radiation. The same happens with the component of Q received by the top crown surface. However, energy absorbed lat¬erally is much less dependent on Rs, and the inferior component of Q is completely independent of solar radiation. Under conditions when rainfall interception is most likely to occur, i.e. cloudy/rainy days, the daily time-course of Q follows closely those of Rs and Rn, with a maximum of only 75 W m-2 (expressed per unit of leaf area). Similar maximum daily values were observed in other studies with different species but under similar weather conditions. During the night, net radiation should not have a significant spatial variability and Rn around the canopy should be relatively homo¬geneous. Accordingly, night time estimates of Q were obtained from measurements of Rn at fixed positions, which were considered representative of the Rn fluxes around the tree

Aerodynamic conductances in a sparse mixed oak woodland (Quercus rotundifolia Lam. and Quercus suber L.)

The study of heat and mass exchange between the vegetation and its local environment plays a central role in the analysis of plant-atmosphere interactions. These studies can be undertaken at different scales, ranging from individual leaves to isolated trees or even the canopy scale. In each of these cases, heat and mass fluxes depend on the use of adequate values of transfer conductances. Within a broader study on interception loss from a sparse cork and holm oak woodland (montado) of Southern Portugal, aerodynamic conductances were determined for the boundary layers of both leaves (LBL) and the entire canopy

A tree-based approach for modelling interception loss from evergreen oak mediterranean savannas

Evaporation of rainfall intercepted by tree canopies is usually an important part of the overall water balance of forested catchments and there have been many studies dedicated to measuring and modelling rainfall interception loss. These studies have mainly been conducted in dense forests; there have been few studies on the very sparse forests which are common in dry and semi-arid areas. Water resources are scarce in these areas making sparse forests particularly important. Methods for modelling interception loss are thus required to support sustainable water management in those areas. In very sparse forests, trees occur as widely spaced individuals rather than as a continuous forest canopy. We therefore suggest that interception loss for this vegetation type can be more adequately modelled if the overall forest evaporation is derived by scaling up the evaporation from individual trees. The evaporation rate for a single tree can be estimated using a simple Dalton-type diffusion equation for water vapour as long as its surface temperature is known. From theory, this temperature is shown to be dependent upon the available energy and windspeed. However, the surface temperature of a fully saturated tree crown, under rainy conditions, should approach the wet bulb temperature as the radiative energy input to the tree reduces to zero. This was experimentally confirmed from measurements of the radiation balance and surface temperature of an isolated tree crown. Thus, evaporation of intercepted rainfall can be estimated using an equation which only requires knowledge of the air dry and wet bulb temperatures and of the bulk tree-crown aerodynamic conductance. This was taken as the basis of a new approach for modelling interception loss from savanna-type woodland, i.e. by combining the Dalton-type equation with the Gash’s analytical model to estimate interception loss from isolated trees. This modelling approach was tested using data from two Mediterranean savanna-type oak woodlands in southern Portugal. For both sites, simulated interception loss agreed well with the observations indicating the adequacy of this new methodology for modelling interception loss by isolated trees in savanna-type ecosystems. Furthermore, the proposed approach is physically based and requires only a limited amount of data. Interception loss for the entire forest can be estimated by scaling up the evaporation from individual trees accounting for the number of trees per unit area

Rainfall interception modelling: is the wet bulb approach adequate to estimate mean evaporation rate from wet/saturated canopies in all forest types?

The Penman–Monteith equation has been widely used to estimate the maximum evaporation rate (E) from wet/saturated forest canopies, regardless of canopy cover fraction. Forests are then represented as a big leaf and interception loss considered essentially as a one-dimensional process. With increasing forest sparseness the assumptions behind this big leaf approach become questionable. In sparse forests it might be better to model E and interception loss at the tree level assuming that the individual tree crowns behave as wet bulbs (‘‘wet bulb approach”). In this study, and for five different forest types and climate conditions, interception loss measurements were compared to modelled values (Gash’s interception model) based on estimates of E by the Penman–Monteith and the wet bulb approaches. Results show that the wet bulb approach is a good, and less data demanding, alternative to estimate E when the forest canopy is fully ventilated (very sparse forests with a narrow canopy depth). When the canopy is not fully ventilated, the wet bulb approach requires a reduction of leaf area index to the upper, more ventilated parts of the canopy, needing data on the vertical leaf area distribution, which is seldom-available. In such cases, the Penman–Monteith approach seems preferable. Our data also show that canopy cover does not per se allow us to identify if a forest canopy is fully ventilated or not. New methodologies of sensitivity analyses applied to Gash’s model showed that a correct estimate of E is critical for the proper modelling of interception loss

Measurement of W Polarisation at LEP

The three different helicity states of W bosons produced in the reaction e+ e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to measure the polarisation of W bosons, and its dependence on the W boson production angle. The fraction of longitudinally polarised W bosons is measured to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and the second systematic, in agreement with the Standard Model expectation

Search for Anomalous Couplings in the Higgs Sector at LEP

Anomalous couplings of the Higgs boson are searched for through the processes e^+ e^- -> H gamma, e^+ e^- -> e^+ e^- H and e^+ e^- -> HZ. The mass range 70 GeV < m_H < 190 GeV is explored using 602 pb^-1 of integrated luminosity collected with the L3 detector at LEP at centre-of-mass energies sqrt(s)=189-209 GeV. The Higgs decay channels H -> ffbar, H -> gamma gamma, H -> Z\gamma and H -> WW^(*) are considered and no evidence is found for anomalous Higgs production or decay. Limits on the anomalous couplings d, db, Delta(g1z), Delta(kappa_gamma) and xi^2 are derived as well as limits on the H -> gamma gamma and H -> Z gamma decay rates

Measurement of W Polarisation at LEP

The three different helicity states of W bosons produced in the reaction e+ e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to measure the polarisation of W bosons, and its dependence on the W boson production angle. The fraction of longitudinally polarised W bosons is measured to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and the second systematic, in agreement with the Standard Model expectation

Neutral-Current Four-Fermion Production in e+e- Interactions at LEP

Neutral-current four-fermion production, e+e- -> ffff is studied in 0.7/fb of data collected with the L3 detector at LEP at centre-of-mass energies root(s)=183-209GeV. Four final states are considered: qqvv, qqll, llll and llvv, where l denotes either an electron or a muon. Their cross sections are measured and found to agree with the Standard Model predictions. In addition, the e+e- -> Zgamma* -> ffff process is studied and its total cross section at the average centre-of-mass energy 196.6GeV is found to be 0.29 +/- 0.05 +/- 0.03 pb, where the first uncertainty is statistical and the second systematic, in agreement with the Standard Model prediction of 0.22 pb. Finally, the mass spectra of the qqll final states are analysed to search for the possible production of a new neutral heavy particle, for which no evidence is found

Measurement of Exclusive rho+rho- Production in Mid-Virtuality Two-Photon Interactions and Study of the gamma gamma* -> rho rho Process at LEP

Exclusive rho+rho- production in two-photon collisions between a quasi-real photon, gamma, and a mid-virtuality photon, gamma*, is studied with data collected at LEP at centre-of-mass energies root(s)=183-209GeV with a total integrated luminosity of 684.8pb^-1. The cross section of the gamma gamma* -> rho+ rho- process is determined as a function of the photon virtuality, Q^2, and the two-photon centre-of-mass energy, W_gg, in the kinematic region: 0.2GeV^2 < Q^2 <0.85GeV^2 and 1.1GeV < W_gg < 3GeV. These results, together with previous L3 measurements of rho0 rho0 and rho+ rho- production, allow a study of the gamma gamma* -> rho rho process over the Q^2-region 0.2GeV^2 < Q^2 < 30 GeV^2

Bose-Einstein Correlations of Neutral and Charged Pions in Hadronic Z Decays

Bose-Einstein correlations of both neutral and like-sign charged pion pairs are measured in a sample of 2 million hadronic Z decays collected with the L3 detector at LEP. The analysis is performed in the four-momentum difference range 300 MeV < Q < 2 GeV. The radius of the neutral pion source is found to be smaller than that of charged pions. This result is in qualitative agreement with the string fragmentation model