J.S. David

Modelling interception loss for two sparse eucalypt and pine forests in central Portugal using reformulated Rutter and Gash analytical models

Abstract Gross rainfall, throughfall and stemflow were measured in Eucalyptus globulus Labill. and Pinus pinaster Ait. stands in central Portugal over two and a half years (from January 1992 to July 1994). The results show that the interception loss is higher in the pine stand (17% of gross rainfall) than in the eucalypt stand (11% of gross rainfall). Interception loss was also simulated by the Rutter model and Gashs original analytical model, but both models overestimated the interception loss from these Mediterranean sparse forests by 29–44%. To improve the description of the rainfall interception from sparse forests, the Rutter model was reformulated. This new version of the Rutter model and a previously reformulated version of Gashs analytical model greatly improved the accuracy of the simulation for both forests, resulting in estimates of the interception loss within 3% of the measured values.

Studies of the impact of forests on peak flows and baseflows: a European perspective

Most of the scientific studies of forest impacts on stream flows have been conducted in North America. Many were primarily concerned with felling effects. These have generally found forests to be associated with reducing both peak and low flows. Their results, however, may not necessarily be directly applicable to European forests due to differences in tree species, forest management, catchment physiography and climate. Forests are a major land cover in Europe, and there are plans to promote and further expand the area of European forests. The recent droughts and floods in different parts of Europe have heightened interest in the role of forests on river flow regimes, particularly flood peak and dry weather baseflows. This paper presents the hydrological results from 28 basins across Europe sampling a wide range of forest types, climate conditions and ground conditions. The aim was to determine if forestry can have significant impacts on stream flows and to identify particularly critical situations. The findings highlighted coniferous plantations on poorly drained soils in NW Europe and eucalyptus in Southern Europe as the situations where the most marked changes to flows are likely to occur. In contrast, other forest types, and changes in forest cover at a regional scale will be likely to have a relatively small effect on peak and low flows.

Transpiration of a 64-year old maritime pine stand in Portugal

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 (gc) 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 ofgc showed reasonable agreement.gc 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 ofgc 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 from a mature Eucalyptus globulus plantation in Portugal during a spring-summer period of progressively higher water deficit

Abstract The rates of transpiration from a mature Eucalyptus globulus Labill. stand in Portugal were evaluated during a drying period of the spring-summer 1994. Transpiration was measured by the Granier sap flow method and estimated by the Penman-Monteith model. During the experimental period daily transpiration varied between 3.64 and 0.50 mm day−1. For high-transpiration days, a good agreement was observed between Penman-Monteith estimates and sap flow measurements, both on a daily and on an hourly basis. However, for low-transpiration days, the Penman-Monteith model overestimated transpiration in comparison with the sap flow method. The diurnal variation of sap flow was then smoother and lagged behind the estimates of the Penman-Monteith model. E. globulus showed an efficient control of transpiration losses during dry periods through a progressive stomatal closure. As soil moisture deficit increased, the daily maximum stomatal conductance decreased from 0.46 to 0.14 cm s−1. The results also show that, on a seasonal basis, stomatal conductance and daily transpiration were mainly related to predawn leaf water potential and, thus, to soil moisture content.

Estimates and measurements of evaporation from wet, sparse pine forest in Portugal

Abstract Measurements of evaporation were made from a wet, sparse pine forest using the eddy correlation/energy balance method in saturated canopy conditions. The measurements were compared with the estimates of evaporation made using the Penman–Monteith equation in both the original and a sparse forest version of the Rutter model, where the evaporation is reduced in proportion to the canopy cover. The use of different formulations of the aerodynamic conductance was also investigated. The best agreement was found when the aerodynamic conductance for vapour flux was set equal to the measured conductance to momentum flux and then used in the sparse forest version of the Rutter model. This good agreement was a result of an accurate prediction of the aerodynamic conductance to vapour flux. There was poor agreement with observations when the estimated aerodynamic conductance for vapour included an empirical relationship between the roughness lengths for heat/vapour and momentum derived previously for dry conditions.

Drought-induced photosynthetic inhibition and autumn recovery in two Mediterranean oak species (Quercus ilex and Quercus suber)

Responses of leaf water relations and photosynthesis to summer drought and autumn rewetting were studied in two evergreen Mediterranean oak species, Quercus ilex spp. rotundifolia and Quercus suber. The predawn leaf water potential (Ψ(lPD)), stomatal conductance (gs) and photosynthetic rate (A) at ambient conditions were measured seasonally over a 3-year period. We also measured the photosynthetic response to light and to intercellular CO₂ (A/PPFD and A/C(i) response curves) under water stress (summer) and after recovery due to autumn rainfall. Photosynthetic parameters, Vc(max), J(max) and triose phosphate utilization (TPU) rate, were estimated using the Farquhar model. RuBisCo activity, leaf chlorophyll, leaf nitrogen concentration and leaf carbohydrate concentration were also measured. All measurements were performed in the spring leaves of the current year. In both species, the predawn leaf water potential, stomatal conductance and photosynthetic rate peaked in spring, progressively declined throughout the summer and recovered upon autumn rainfall. During the drought period, Q. ilex maintained a higher predawn leaf water potential and stomatal conductance than Q. suber. During this period, we found that photosynthesis was not only limited by stomatal closure, but was also downregulated as a consequence of a decrease in the maximum carboxylation rate (Vc(max)) and the light-saturated rate of photosynthetic electron transport (J(max)) in both species. The Vc(max) and J(max) increased after the first autumnal rains and this increase was related to RuBisCo activity, leaf nitrogen concentration and chlorophyll concentration. In addition, an increase in the TPU rate and in soluble leaf sugar concentration was observed in this period. The results obtained indicate a high resilience of the photosynthetic apparatus to summer drought as well as good recovery in the following autumn rains of these evergreen oak species.

Clearcutting effects on streamflow in coppiced Eucalyptus globulus stands in Portugal

Two Eucalyptus globulus catchments in Central Portugal were instrumented to gain some understanding of the hydrological effects of coppicing forest stands of this fast-growing species. Suspected increases in total flow, quick flow and delayed flow after clearcutting were tested statistically using the R-Student statistic for outlier detection. Under normal climatic conditions, significant annual flow increases were found only during the water-year immediately after clearcutting. However, when the water-year following harvesting was extremely dry, no flow increase occurred during that year although flow increases were found during the second water-year. Nevertheless, owing to the rapid regrowth of the stands, the hydrological effects of clearcutting were always short lived. Depending on the dominant runoff-generating processes, increases in total flow were due to increases in both the quick and delayed flow components or to increases only in the quick flow component.

Runoff generation in a plough-drained peat bog in southern Scotland

Abstract This paper reports on a study, using runoff plots and other equipment, of runoff generation in a peat bog in Scotland four years after it had been plough-drained for afforestation. In generally dry periods runoff responses were dominated by flow generated by rain falling directly onto the ditches. Only during very wet periods was this source of runoff less important than water draining into the ditches from the strips on either side. Flow from the strips to the ditches consisted entirely of groundwater. This occurred at a very low rate unless the water table was within 6–7 cm of the surface; then it could become as rapid as the rate at which rain landed on the area. The results emphasise the need to consider the ditches and the strips between them separately when seeking an understanding of runoff generation in densely plough-drained peatlands.

Hydraulic connectivity from roots to branches depicted through sap flow: analysis on a Quercus suber tree

The topology of the xylem network is likely to affect the transport of water, propagation of embolism and plant survival and growth. Few studies have been conducted on the hydraulics of the entire water pathway in trees. We evaluated the hydraulic connections from roots to branches in a mature Quercus suber L. tree, through sap flow responses upon branch severing. Sap flow was recorded in branches, stem and roots by the heat field deformation (HFD) method. Results showed that roots, except for the taproot, were hydraulically connected to all branches, but the rest of the tree (stem, branches and taproot) was highly sectored. In the large roots that showed an integrated response to branch severing, the outer xylem was preferentially connected to the same side branch and the inner xylem to the opposite branch. The hydraulic sectoriality in branches, stem and taproot may be regarded as an adaptive trait to water stress. The integrated hydraulic structure of roots is advantageous under patchy soil conditions, but may allow the spread of root diseases. The HFD sap flow method proved extremely useful to calculate xylem flux connectivity between different organs of a large tree, providing a comprehensive picture of its hydraulic architecture.

Root Function: In Situ Studies Through Sap Flow Research

Sap flow measured by the Heat Field Deformation technique, HFD, is sensitive to flow responses to small changes in water potential gradients within the tree hydraulic systems. When these changes occur abruptly, under experimental treatments (severing, localized irrigation, heavy loading), sap flow movement can be used as a marker to study root functionality, for example root ability to redistribute water and withstand heavy machinery pressure. Experiments also showed that a compensation mechanism may operate in trees, with a temporary increase in the absorbed water due to a preferential use of one part of the root system when another part is damaged or when a water source is lost. Long-term measurements of root sap flow allow distinguishing between water uptake from shallow and deep rooted trees, at different exposures at a forest edge and from healthy and infected trees. Root sap flow can be used as an indicator of tree stress or of the prevailing mechanisms used by trees to survive drought, under irrigation or rain-fed conditions.