The HartX period May 1992, seen against the background of twenty years of energy balance climatology at the Hartheim pine plantation

Summary The experimental site of the Department of Meteorology of Freiburg University at the Hartheim pine stand is first described. There, since 1973 long term measurements of net radiation and its components have been carried out. In addition we have been monitoring the different heat fluxes and components of the forest water budget.From May 11th to May 24th 1992 a special international and interdisciplinary observation period was organized in Hartheim (HartX 92). This took place in the frame of the international regional climatic project REKLIP (Regionales Klima Projekt). We then describe the permanent equipment and the special HartX installations. After that we show the climate of the region, in May 1992 and the weather during the HartX period. It was extraordinarily warm and the precipitation was much less than normal. The cloud cover was very small.We report the results of the radiation measurements (net radiation and its components). They are compared to the long standing measurements (1974–1988). Moreover the longstanding data of the components of the water budget (throughfall, canopy drip and stemflow, interception and transpiration) of the period 1978–1985 are dealt with. In addition we report the behaviour of the energy fluxes (soil-stand heat flux, turbulent sensible and latent heat fluxes) of the period 1974–1988. These estimations are compared to the conditions in May 1992 and the conditions during HartX 92.With 8 Figures     

The HartX-synthesis: An experimental approach to water and carbon exchange of a Scots pine plantation

Summary In May 1992 during the interdisciplinary measurement campaign HartX (Hartheim eXperiment), several independent estimates of stand water vapor flux were compared at a 12-m high Scots pine (Pinus silvestris) plantation on a flat fluvial terrace of the Rhine close to Freiburg, Germany. Weather during the HartX period was characterized by ten consecutive clear days with exceptionally high input of available energy for this time of year and with a slowly shifting diurnal pattern in atmospheric variables like vapor pressure deficit. Methods utilized to quantify components of stand water flux included porometry measurements on understory graminoid leaves and on pine needles and three different techniques for determining individual tree xylem sap flow. Micrometeorological methods included eddy covariance and eddy covariance energy balance techniques with six independent systems on two towers separated by 40 m. Additionally, Bowen ratio energy balance estimates of water flux were conducted and measurements of the gradients in water vapor, CO₂, and trace gases within and above the stand were carried out with an additional, portable 30 m high telescoping mast.Biologically-based estimates of overstory transpiration were obtained by up-scaling tree sap flow rates to stand level via cumulative sapwood area. Tree transpiration contributed between 2.2 and 2.6 mm/day to ET for a tree leaf area index (LAI) of 2.8. The pine stand had an understory dominated by sedge and grass species with overall average LAI of 1.5. Mechanistic canopy gas exchange models that quantify both water vapor and CO₂ exchange were applied to both understory and tree needle ecosystem compartments. Thus, the transpiration by graminoid species was estimated at approximately 20% of total stand ET. The modelled estimates for understory contribution to stand water flux compared well with micrometeorologically-based determinations. Maximum carbon gain was estimated from the canopy models at approximately 425 mmol/(m²day) for the tree needles and at 100 mmol/(m²day) for the understory. Carbon gain was suggested by the modelling analysis to remain relatively constant during the HartX period, while water use efficiency in carbon fixation increased with decreasing vapor pressure deficit. Biologically- and micrometeorologically-based estimates of stand water flux showed good general agreement with variation of up to 20% that reflects both errors due to the inherent assumptions associated with different methods as well as natural spatial variability in fluxes. The various methods support a reliable estimate of average ET from this homogeneous canopy during HartX of about 2.6 mm/day (a maximum of about 3.1 mm/day) with an insignificant decreasing trend in correlation with decreasing vapor pressure deficit and possibly soil moisture.Findings during HartX were embedded in local scale heterogeneity with greater roughness over the forest and much higher ET over the surrounding agricultural fields which results in weak but clearly existant circulation patterns. A variety of measurements were continued after the HartX campaign. They allow us to extend our findings for six months with changing environmental conditions, including shortage of soil moisture. Hydrological estimates of soil water extractions and micrometeorological estimates of ET by the one-propeller eddy covariance (OPEC) system were in very good agreement, supporting the use of this robust eddy covariance energy balance technique for long-term monitoring.With 5 Figures     

Comparisons of xylem sap flow and water vapour flux at the stand level and derivation of canopy conductance for Scots pine

Summary Simultaneous measurements of xylem sap flow and water vapour flux over a Scots pine (Pinus sylvestris) forest (Hartheim, Germany), were carried out during the Hartheim Experiment (HartX), an intensive observation campaign of the international programme REKLIP. Sap flow was measured every 30 min using both radial constant heating (Granier, 1985) and two types of Cermak sap flowmeters installed on 24 trees selected to cover a wide range of the diameter classes of the stand (min 8 cm; max 17.5 cm). Available energy was high during the observation period (5.5 to 6.9 mm.day^–1), and daily cumulated sap flow on a ground area basis varied between 2.0 and 2.7 mm day^–1 depending on climate conditions. Maximum hourly values of sap flow reached 0.33 mm h^–1, i.e., 230 W m^–2.Comparisons of sap flow with water vapour flux as measured with two OPEC (One Propeller Eddy Correlation, University of Arizona) systems showed a time lag between the two methods, sap flow lagging about 90 min behind vapour flux. After taking into account this time lag in the sap flow data set, a good agreement was found between both methods: sap flow = 0.745^* vapour flux,r ² = 0.86. The difference between the two estimates was due to understory transpiration.Canopy conductance (g _c ) was calculated from sap flow measurements using the reverse form of Penman-Monteith equation and climatic data measured 4 m above the canopy. Variations ofg _c were well correlated (r ² = 0.85) with global radiation (R) and vapour pressure deficit (vpd). The quantitative expression forg _c =f (R, vpd) was very similar to that previously found with maritime pine (Pinus pinaster) in the forest of Les Landes, South Western France.With 6 Figures     

Leaf-level gas exchange and scaling-up of forest understory carbon fixation rates with a “patch-scale” canopy model

Summary During the Hartheim experiment (HartX) 1992, conducted in the Upper Rhine Valley, Germany, we estimated water vapor flux from the understory by several methods as reported in Wedler et al. (this issue). We also examined the photosynthetic gas exchange of the dominant understory speciesBrachypodium pinnatum, Carex alba, andCarex flacca at the leaf level with an CO₂/H₂O porometer. A mechanisticallybased leaf gas exchange model was parameterized for these understory species and validated via the measured diurnal courses of carbon dioxide exchange. Leaf CO₂ gas exchange was scaled-up to patch- and then to stand-level utilizing the leaf gas exchange model as a component of the canopy light interception/energy balance model GAS-FLUX, and by further considering variation in vegetation patch-type distribution, patch-specific spatial structure, patch-type leaf area index, and microclimate beneath the tree canopy.At patch-level,C. alba exhibited the lowest net CO₂ uptake of ca. 75 mmol m^–2 d^–1 due to a low leaf-level photosynthetic capacity, whereas net CO₂ fixation ofB. pinnatum- andC. flacca-patches was approx. 178 and 184 mmol m^–2 d^–1, respectively. Highest CO₂ uptake was estimated for mixed patches whereB. pinnatum grew together with the sedge speciesC. alba orC. flacca. Scaling-up of leaf gas exchange to stand level resulted in an estimated average rate of total CO₂ fixation by the graminoid understory patches of approximately 93 mmol m^–2 d^–1 during the HartX period. The conservative gas exchange behavior ofC. alba at Hartheim and its apparent success in space capture seems to affect overall functioning of this pine forest ecosystem by limiting understory CO₂ uptake. The CO₂ uptake by the understory is approximately 20% of stand total CO₂ uptake. CO₂ uptake fluxes mirror the relative differences in water loss from the understory and crown layer during the HartX period. Comparative measurements indicate that understory vegetation in spruce and pine forests is not greatly different from that of other low-statured natural ecosystems such as tundra or marshes under high light conditions, although CO₂ capture by the understory at Hartheim is at the low extreme of the estimates, apparently due to the success ofC. alba. With 6 Figures     

The May–October energy budget of a Scots pine plantation at Hartheim,Germany

Summary This paper describes measurements of the Hartheim forest energy budget for the 157-day period of May 11 – Oct. 14, 1992. Data were collected as 30-min means. Energy available to the forest was measured with net radiometers and soil heat flux discs; sensible heat exchange between the canopy and atmosphere was measured with two One-Propeller Eddy Correlation (OPEC) systems, and latent energy (evapotranspiration orET) was determined as a residual in the surface energy balance equation. Net rediation, change in thermal storage, and sensible heat flux were verified by independent measurements during the Hartheim Experiment (HartX, May 11–12), and again during the HartX2 experiment over 20 days late in the summer (Sep. 10–29). Specifically, sensible heat estimates from the two adjacent OPEC sensor sets were in close agreement throughout the summer, and in excellent agreement with measurements of sonic eddy correlation systems in May and September. The eddy correlation/energy balance technique was observed to overestimate occurrence of dew, leading to an underestimate of dailyET of about 5%. After taking dew into account, estimates of OPECET totaled 358 mm over the 5.1-month period, which is in quite good agreement with an ET estimate of 328 mm from a hydrologic water balance. An observed decrease in forestET in July and August was clearly associated with low rainfall and increased soil water deficit. The OPEC system required only modest technical supervision, and generated a data yield of 99.5% over the period DOY 144–288. The documented verification and precision of this energy budget appears to be unmatched by any other long-term forest study reported to date.With 9 Figures     

Energy balance comparison of the Hartheim forest and an adjacent grassland site during the HartX experiment

Summary Energy balance components over a grassland surface were compared to those obtained above an adjacent, uniform Scots pine plantation during a five-day period of fine, sunny, spring weather. Soils were judged to contain ample water. Shortwave and total radiation flux densities were measured at both sites with pyranometers and total pyrradiometers. Soil heat flux densities were measured with heat flux plates at both sites, and additional storage changes were estimated for air and canopy at the forest site. The forest gained more shortwave energy than the grassland during daytime because of its lower albedo, but it lost more longwave radiation at night. The turbulent fluxes of sensible and latent energy were evaluated with the Bowen ratio energy balance (BREB) method at both sites. Temperature and humidity gradients were measured with fixed psychrometers at the grassland site, and with interchanging psychrometers at the forest site. Mean daily evapotranspiration (ET) averaged 2.26 mm over the five days for the Scots pine, or only 57 percent of the 3.94 mm measured at the grassland site. The mean Bowen ratios were 2.6 and 0.8, respectively.An error analysis was carried out for the BREB estimates of latent heat flux at the two sites. For a given error in latent heat flux and at a specified Bowen ratio the demands on accuracy of dry- and wet-bulb temperature gradients above the rough forest canopy was found to be 10 times higher than above the smoother grassland. If additionally the observed differences in transpiration rates between the two sites were taken into account, the precision for temperature gradient measurements above the slowly transpiring forest becomes fortyfold greater than required above the rapidly transpiring grass. At present, BREB precision requirements for gradients above rougher, drier canopies appear achievable only through use of specialized instrumentation, such as measurement systems that incorporate interchangeable psychrometers into their design.With 9 Figures     

Soil moisture variation and plant water stress at the Hartheim Scots pine plantation

Summary During two measurement campaigns in 1992 (the Hartheim Experiment HartX- and an additional experiment in autumn), measurements of soil moisture were carried out in aPinus sylvestris stand at Hartheim on the Oberrhein. Several methods were used to determine soil water status. They were compared in terms of suitability for estimating stand evapotranspiration (ET) via soil water depletion. Measurements of tree water potential suggested that conductance of the trees was affected by soil water depletion during the period of the HartX campaign in spring 1992. We interpret the observations to indicate a lesser influence of soil water availability on tree transpiration during the autumn experiment.Eddy correlation and xylem sapflow measurements provided reference ET values with which to compare the stand ET calculations based on soil moisture measurements. Profile measurements of soil moisture showed that particularly in springtime when the lower soil layers are saturated with water, the water transport from depths below the major rooting zone is a very important factor affecting evaluation of stand ET. Decreases in soil water storage may be determined best with permanently installed soil moisture sensors such as used in tensiometric or TDR measurements that provide high resolution of changes over time.With 8 Figures     

Penumbral and foliage distribution effects on Pinus sylvestris canopy gas exchange

Summary  Tree canopy water use and foliage net CO₂ uptake (NPP) were simulated for a 31-year-old Pinus sylvestris (Scots pine) plantation near Hartheim, in the Upper Rhine Valley, Germany with a mechanistically-based, three-dimensional stand gas-exchange model (STANDFLUX) for a ten-day period during spring 1992. STANDFLUX was formulated to include the effects of penumbra caused by the fine structure of the needles on light distribution within crowns. Good correspondence was found between simulated rates of tree canopy water use when including penumbral effects and eddy-covariance ET and sap flow transpiration measurements. Water use was 8–13% lower and NPP was 10–17% lower in simulations for the ten-day period when penumbral effects were not included. Simulated water use and CO₂ uptake were compared with similar outputs from a simplified layer canopy model (including or not including penumbra) which assumed horizontal homogeneity in canopy structure (GAS FLUX). Our results for the Pinus sylvestris stand indicate that penumbral effects were more important than the degree of model simplification with respect to foliage distribution (three-dimensional vs. layered structure) for estimating stand-level fluxes for these pines. Simulated maximum hourly NPP was similar to rates measured for other Pinus sylvestris stands using other methods. Predicted decreases in tree transpiration due to the modelled response of needle gas exchange to increasing vapour-pressure deficit agreed with measured changes in transpiration, and suggested that stomatal response may have been more important than decreasing soil water availability in controlling water flux to the atmosphere during this period. The overall results of the study demonstrate that current approaches in canopy modelling that separate light into sun versus shade intensities can be effective, but must be applied with caution when attempting to predict long-term water and carbon balances of forests. Received May 1, 1999 Revised November 9, 2000     

The available energy over a Scots Pine plantation: What's up for partitioning?

Summary Errors influencing the calculation of the available energy above a forest are discussed. The main emphasis is put on the investigation of the problems affecting the measurement of net radiation. This is done by utilizing the data set of a surface energy balance experiment which was conducted in and above a Scots Pine plantation from May 11 to 22, 1992. During that Hartheimer Experiment (HartX) there were redundant measurements of net radiation using five different radiometers of three different designs. The initially fair agreement between the net radiometer readings was considerably improved by introducing different responsivities for the shortand longwave range. The mean deviations to the relative net radiation after correction vary between — 1.4 and 1.2 Wm^–2 with standard deviations between ± 5.4 and 6.6 Wm^–2. The total error referring to the available energy is estimated to be up to ± 36 Wm^–2 (± 6%) around midday decreasing to 10 Wm^–2 during nighttime.With 3 Figures     

Estimates of water vapor flux and canopy conductance of Scots pine at the tree level utilizing different xylem sap flow methods

Summary During the Hartheim Experiment (HartX) 1992 conducted in the upper Rhine Valley, Germany, three different methods were used to measure sap flow in Scots pine trees via heating of water transported in the xylem: (1) constant heating applied radially in the sapwood (Granier-system-G), (2) constant heating of a stem segment (ermák-system-C), and (3) regulated variable heating of a stem segment that locally maintains a constant temperature gradient in the trunk (ermák/Schulze-system-CS). While the constant heating methods utilize changes in the induced temperature gradient to quantify sap flux, the CS-system estimates water flow from the variable power requirement to maintain a 2 or 3 degree Kelvin temperature gradient over a short distance between inserted electrodes and reference point. The C- and CS-systems assume that all transported water is encompassed and equally heated by the electrodes. In this case, flux rate is determined from temperature difference or energy input and the heat capacity of water. Active sapwood area need not be determined exactly. In contrast, the G-system requires an empirical calibration of the sensors that allows conversion of temperature difference into sap flow density. Estimates of sapwood area are used to calculate the total flux. All three methods assume that the natural fluctuation in temperature of the trunk near the point of insertion of heating and sensing elements is the same as that where reference thermocouples are inserted.Using all three systems, 24 trees were simultaneously monitored during the HartX campaign. Tree size within the stand ranged between 18 and 61 cm circumference at breast height, while sample trees ranged between 24 and 55 cm circumference. The smallest trees could only be measured by utilizing the G-system. Sap flow rates of individual trees measured at breast height increased rapidly in the morning along with increases in irradiance and vapor pressure deficit (D), decreased slowly during the course of the afternoon with continued increase inD, and decreased more slowly during the night.Ignoring potential effects introduced by the different methods, maximum flow rates of individual trees ranged between 0.5 and 2.5 kg H₂O h^–1 tree^–1 or 0.3 and 0.6 mm h^–1 related to projected crown area of trees and daily sums of sap flow for individual trees varied between 4.4 and 24 kg H₂O tree^–1 d^–1 or 1.1 and 6.0 mm d^–1. Maximum sap flow rates per sapwood area of trees varied least for the G-system (11–17 g cm^–2 h^–1) and was of similar magnitude as the C- (8–21 g cm^–2 h^–1) and CS-system (4–14 g cm^–2 h^–1).Regressions of total tree conductance (g _t ) derived from sap flow estimates demonstrated the same linear increase of conductance with increasing irradiance, however decrease of conductance with increasingD under non-limiting light conditions was different for the three systems with strongest reduction ofg _t measured with the CS-system followed by the C- and G-system. This led to different estimates of daily sap flow rates especially during the second part of the measurement period.Variation in sap flow rates is explained on the basis of variation in leaf area index of individual trees, heterogeneity in soil conditions, and methodological differences in sap flow measurements. Despite the highly uniform plantation forest at the scale of hectares, the heterogeneity in tree size and soil depth at the scale of square meters still make it difficult to appropriately and efficiently select sample trees and to scale-up water flux from individual trees to the stand level.With 5 Figures     

Monoterpene concentrations in and above a forest of scots pine

Diurnal and vertical ambient air measurements of the monoterpenes have been made in and above a Scots pine (Pinus sylvestris) forest of central Sweden, within the boreal northern coniferous biome. Sampling was done with Tenax TA, and analysis by GC and ion trap detection. Daytime mixing ratios were on the order of tenths of a ppbv from the forest floor to the top of the forest, and a factor of 2 or 3 lower above the forest. Mixing ratios at night were at the ppbv level, highest near the forest floor and the crown, and decreased with height above the forest. The highest total concentration observed was 8 ppbv inside the forest at 3 am (GMT). The average terpene composition was 3-carene 32%, -pinene 29%, limonene 18%, -pinene 10%, -phellandrene 7%, camphene 5%, and sabinene at less than 2%. The 3-carene/-pinene ratio varied with wind direction and speed, relative humidity, and wet/dry vegetation, but not with ozone or NO₂ concentration, solar radiation, or temperature. Variations in the observed terpene composition at the sampling site are mainly caused by the influence of other vegetation in the vicinity of the site. It would seem that wet Scots pine emits more 3-carene relative to -pinene than does dry pine.     

Effect of Secondary Circulations on the Surface–Atmosphere Exchange of Energy at an Isolated Semi-arid Forest

Afforestation in semi-arid regions can potentially enhance the global carbon sink by increasing the terrestrial biomass. However, the survival of planted forests under such extreme environmental conditions is not guaranteed a priori, and critically depends on the surface–atmosphere exchange of energy. We investigate the pine forest Yatir in Israel, an example of a man-made semi-arid ecosystem, by means of large-eddy simulations. We focus on the interaction between surface–atmosphere exchange and secondary circulations that couple the isolated forest to the surrounding shrubland. The large-eddy simulations feature a grid resolution that resolves the forest canopy in several layers, and are initialized by satellite data and Doppler lidar, eddy-covariance and radiosonde measurements. We perform three large-eddy simulations with different geostrophic wind speeds to investigate the influence of those wind speeds on the surface–atmosphere exchange. We reproduce the measured mean updrafts above the forest and mean downdrafts above the shrubland, which increase in strength with decreasing geostrophic wind speed. The largest updrafts emerge above the older, denser part of the forest, triggering secondary circulations. The spatial extent of these circulations does not cover the entire forest area, although we observe a reduced aerodynamic resistance in the regions of updraft. Our simulations indicate that the enhanced surface–atmosphere exchange of the Yatir forest is not sufficient to compensate for the increased net radiation, due to the lower albedo of the forest with respect to the surroundings, resulting in higher air temperatures inside the forest. However, the difference between the forest and shrubland temperatures decreases with increasing geostrophic wind speed due to reduction in the aerodynamic resistance.     

Model-based estimates of water loss from “patches” of the understory mosaic of the Hartheim Scots pine plantation

Summary During the Hartheim Experiment (HartX) 1992 conducted in the Upper Rhine Valley, Germany, we estimated water vapor flux from the understory and the forest floor by several methods. At the vegetation patch level, direct estimates were made with small weighing lysimeters, and water loss was scaled-up to the stand level based on vegetation patchtype distribution. At the leaf level, transpiration flux was determined with a CO₂/H₂O porometer for the dominant understory plant species,Brachypodium pinnatum, Carex alba, andCarex flacca. Measured leaf transpiration was scaled-up to patch level with a canopy light interception and leaf gas exchange model, and then to stand level as in the case of lysimeter data, but with further consideration of patchtype leaf area index (LAI). On two days, total understory latent heat flux was estimated by eddy correlation methods below the tree canopy.The understory vegetation was subdivided into five major patch-types which covered 62% of the ground area and resulted in a cumulative LAI of approx. 1.54 when averaged over total stand ground area and compared to the average tree canopy LAI of 2.8. The remaining 38% of ground area was unvegetated bare soil and/or covered by moss (mainly byScleropodium purum) or litter. The evapotranspiration from the understory and unvegetated areas equaled approx. 20% of total forest stand transpiration during the HartX period. The understory vegetation transpired about 0.4 mm d^–1 (13%) estimated over the period of May 13 to 21, whereas evaporation from moss and soil patches amounted 0.23 mm d^–1 (7.0%). On dry, sunny days, total water vapor flux below the tree canopy exceeded 0.66 mm d^–1. Using the transpiration rates derived from the GAS-FLUX model together with estimates of evaporation from moss and soil areas and a modified application of the Penman-Monteith equation, the average daily maximum conductance of the understory and the forest floor was 1.7 mm s^–1 as compared to 5.5 mm s^–1 for the tree canopy.With 6 Figures     

Climate change may already threaten Scots pine stands in the Swiss Alps

Summary Large numbers of Scots pine are dying in the dry inner-alpine valleys of the European Alps; in Switzerland, locally almost half the Scots pine (Pinus sylvestris L.) population has died since 1995. As Switzerlands temperature has increased at more than twice the global average in the 20^th century and as most of this increase has occurred during the last 20 years, we investigated possible relationships between the dying Scots pine and climatic parameters. We centred our studies in the upper Rhone valley.Our results show that the strong climatic warming that has occurred in recent years may well be the indirect cause of the mortality observed in these forests. Tree mortality was highest following the dry and hot year 1998, and tree defoliation, an indicator of tree vitality, showed a strong correlation with the previous years precipitation. While precipitation showed no clear significant trend over time, the number of warm days (mean>20°C, maximum>25°C) and potential evapotranspiration have significantly increased over the last 20 years.Higher temperatures favour pine wood nematodes and bark beetles, both of which are found at the study site, and increasing drought stress reduces tree resistance against pathogens. As these forests have in part protective functions, there is a need to better understand the mortality through interdisciplinary research and also to find means to change the species composition in order to establish tree species that are better able to withstand warmer temperatures.     

Scots Pine Growth Trends in Northwestern Kola Peninsula as an Indicator of Positive Changes in the Carbon Cycle

Growth trends of Scots pine (Pinus sylvestris) at its northernmost extent may be an indicator of changes in the carbon cycle of terrestrial forest ecosystems. Using a method which removed age trends from the data, a time-series analysis of annual radial increment in wood over the last few decades compared with the period of the last registered warming (maximum around 1930–40), revealed elevated growth of 78% for trees 0–20 years old, 56% for trees 21–40 years old, 21% for trees 41–60 years old, and 10% for trees more than 101 years old. Increments of trees in the61–80 and 81–100 years old age classes from the two periods were similar. The higher rate of growth in recent times occurred despite a decrease in temperature after about 1940 and significant air pollution. During the last century growth of Scots pine increased for trees in all age groups, except for trees in the 81–100 year old age class for which it was constant. The average rates of growth were estimated at 0.016 mm/year for trees in the 0–20 year age class, 0.012 mm/year for the 21–40 year age class, 0.005 mm/year for the 41–60 year age class, 0.008 mm/year for the 61–80 year age class and 0.006 mm/year for trees in the greater than 101 year age class. The growth trends were unstable over time and took place concurrent with increasing oscillations in radial increment. The most probable reasons for the marked increase in radial increment growth of Scots pine in this region are climate warming and higher levels of carbon dioxide. Together these may produce a synergistic effect on growth.     

Climate change and the incidence of a forest pest in Mediterranean ecosystems: can the North Atlantic Oscillation be used as a predictor?

Many forest pest species strongly depend on temperature in their population dynamics, so that rising temperatures worldwide as a consequence of climatic change are leading to increased frequencies and intensities of insect-pest outbreaks. In the Mediterranean area, the climatic conditions are strongly linked to the effects of the North Atlantic Oscillation (NAO). The aim of this work is to analyze the dynamics of the pine processionary moth (Thaumetopoea pityocampa), a severe pest of Pinus species in the Circunmediterranean, throughout a region of southern Spain, in relation to NAO indices. We related the percentage of forest plots with high defoliation by pine processionary moth each year with NAO values for the present and the three previous winters, using generalized linear models with a binomial error distribution. The time series is 16-year long, and we performed analyses for the whole database and for the five main pine species separately. We found a consistent relationship between the response variable and the NAO index. The relationship is stronger with pine species living at medium-high altitudes, such as Aleppo (P. halepensis), black (P. nigra), and Scots (Pinus sylvestris) pine, which show the higher defoliation intensities up to 3?years after a negative NAO phase. The results highlight, for the first time, the usefulness of using global drivers in order to understand the dynamics of pest outbreaks at a regional scale, and they open the window to the development of NAO-based predictive models as an early-warning signal of severe pest outbreaks.     

Profiles and simulated exchange of H2O,O3, NO2 between the atmosphere and the HartX Scots pine plantation

Summary Vertical profiles of H₂O, CO₂, O₃, NO and NO₂ were measured during the Hartheim Experiment (HartX) to develop and calibrate a multi-layer resistance model to estimate deposition and emission of the cited gaseous species. The meteorological and gas concentration data were obtained with a 30 m high telescopic mast with 7 gas inlets located at 5 m intervals and meteorological sensors at 5, 15 and 30 m above ground; a complete gas profile was obtained every 9 min 20 s. Measured profiles were influenced by several exchange processes, namely evapotranspiration, dewfall, assimilation of CO₂ in the tree crowns, soil respiration, deposition of NO₂ and O₃ to the soil and advection of NO_x from the nearby highway. Surprisingly, no decrease in O₃ concentration was observed in the crown layer during daytime, probably due to the relatively low density of foliage elements and strong turbulent mixing.The advantage of measuring in-canopy profiles is that turbulent exchange coefficients need not be estimated as a prerequisite to obtaining vertical flux estimates. In recent years, flux-gradient relationships in canopies have been subject to many criticisms. If fluxes are calculated at several heights considering only the transfers between the turbulent air and the interacting surfaces at a certain height, and those fluxes are then integrated vertically in a subsequent step, then exchange estimates (deposition or emission) can be obtained independent of turbulent exchange conditions.Typical estimated deposition velocities calculated for a 3-day period are between 4 and 10 mm/s for NO₂ and about 4–9 mm/s for O₃ (day and night values respectively). This leads to deposition rates of about 20–40 ng N/m²s for NO₂ and about 30–40 mg O₃/m² deposited daily under the conditions encountered during HartX. Sensitivity tests done with the best available and most realistic values for model parametrization have shown that sensitivity is large with respect to the soil and cuticula resistances as well as for gas-phase ozone destruction and that more research is required to describe the effectiveness of cuticula and soil in modifying sink characteristics for NO₂ and O₃.With 12 Figures     

基于森林资源清查的江西省森林贮碳功能研究

利用江西省1999--2003年森林资源二类清查资料,结合大岗山森林生态站的实测数据以及已公布的调查资料,运用材积源生物量法对江西省森林的碳储量和碳密度进行了估算和评价。结果表明,江西省不同类型森林乔木层碳密度,由大到小依次为硬阔林、针阔混交林、毛竹林、国外松林、杉木林、软阔林、灌木林、马尾松林和经济林,且碳密度随着林龄的增大而增大,随人口密度的增大而减小。森林碳密度土壤层最大,植被层次之,枯落物层最小。不同森林类型乔木层碳储量,由大到小依次为杉木林、硬阔林、马尾松林、毛竹林、灌木林、国外松林、经济林、针阔混交林、软阔林。从森林类型分布看,除杉木和国外松林外,其他森林类型天然林乔木层碳储量远大于人工林;从地理分布看,除南昌、萍乡、新余三市外,其余各市均是天然林乔木层碳储量远大于人工林。不同年龄森林乔木层碳储量,由大到小依次为中龄林、幼龄林、近熟林、成熟林、过熟林。不同森林碳储量由大到小依次为杉木林、马尾松林、硬阔林、灌木林、经济林、毛竹林、针阔混交林、国外松林和软阔林,南部和中西部要高于中东部和北部。江西省森林总碳储量为1．5Gt,占全国森林总碳储量的5．33％。     

Changed thinning regimes may increase carbon stock under climate change: A case study from a Finnish boreal forest

A physiological growth and yield model was applied for assessing the effects of forest management and climate change on the carbon (C) stocks in a forest management unit located in Finland. The aim was to outline an appropriate management strategy with regard to C stock in the ecosystem (C in trees and C in soil) and C in harvested timber. Simulations covered 100 years using three climate scenarios (current climate, ECHAM4 and HadCM2), five thinning regimes (based on current forest management recommendations for Finland) and one unthinned. Simulations were undertaken with ground true stand inventory data (1451 hectares) representing Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and silver birch (Betula pendula) stands. Regardless of the climate scenario, it was found that shifting from current practices to thinning regimes that allowed higher stocking of trees resulted in an increase of up to 11% in C in the forest ecosystem. It also increased the C in the timber yield by up to 14%. Compared to current climatic conditions, the mean increase over the thinning regimes in the total C stock in the forest ecosystem due to the climate change was a maximum of 1%; but the mean increase in total C in timber yield over thinning regimes was a maximum of 12%.     

Emissions of Oxygenated Volatile Organic Compounds from Plants Part II: Emissions of Saturated Aldehydes

Emissions of hexanal, heptanal, octanal, nonanal, and decanal from 6 different plant species were measured in continuously stirred tank reactors when the plants were exposed to ozone. Pathogen- and insect attack on plants also led to these emissions. The emission rates of individual aldehydes were related to each other implying a common mechanism for the emissions of these aldehydes. Furthermore, the emission pattern was similar in all cases indicating a similar emission mechanism for different plant species and different elicitors. Measurements with ozone exposed Scots pine plants (Pinus sylvestris L.) showed that the emission rates were dependent on temperature as well as on the ozone flux into the plants. The diurnal variation of aldehyde emissions from ozone exposed Scots pine were described quite well using a formalism including temperature and ozone flux as variables. Assuming the aldehyde emissions to be general for plants exposed to ozone, the global emissions were estimated to be in the range between 7 and 22 Tg/a. Because these emissions can be induced by other factors than ozone uptake alone this estimate may be a lower limit.Now atNow at