Barbara Köstner

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

SummaryTree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day−1 tree−1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m2 of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s−1 that is equivalent to 112 mmol m−2 s−1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m−2 s−1 at the top and 150 mmol m−2 s−1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was −27.76±0.27‰ (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.

Temporal and among-site variability of inherent water use efficiency at the ecosystem level

Half-hourly measurements of the net exchanges of carbon dioxide and water vapor between terrestrial ecosystems and the atmosphere provide estimates of gross primary production (GPP) and evapotranspiration (ET) at the ecosystem level and on daily to annual timescales. The ratio of these quantities represents ecosystem water use efficiency. Its multiplication with mean daylight vapor pressure deficit (VPD) leads to a quantity which we call “inherent water use efficiency” (IWUE*). The dependence of IWUE* on environmental conditions indicates possible adaptive adjustment of ecosystem physiology in response to a changing environment. IWUE* is analyzed for 43 sites across a range of plant functional types and climatic conditions. IWUE* increases during short-term moderate drought conditions. Mean annual IWUE* varied by a factor of 3 among all sites. This is partly explained by soil moisture at field capacity, particularly in deciduous broad-leaved forests. Canopy light interception sets the upper limits to canopy photosynthesis, and explains half the variance in annual IWUE* among herbaceous ecosystems and evergreen needle-leaved forests. Knowledge of IWUE* offers valuable improvement to the representation of carbon and water coupling in ecosystem process models

Evaporation, xylem sap flow, and tree transpiration in a New Zealand broad-leaved forest

Abstract Total evaporation ( E ), forest floor evaporation ( E f ), tree xylem sap flow ( F ), and environmental parameters were measured on 6 consecutive late-summer days under different weather conditions in a well-watered, temperate broad-leaved forest. Two tree species, Nothofagus fusca (Hook. f.) Oerst. (red beech) and N. menziesu (Hook. f.) Oerst. (silver beech), formed a vertically structured, complex canopy with a one-sided leaf area index of 7. The forest comprises 30–40 trees ha−1 of emergent red beech up to 36 m tall and 1.7 m diameter, above a mixed species canopy of 200 trees ha−1 about 20–30 m tall and 0.4 m average diameter, and approximately 900 trees ha−1 of sub-canopy, mostly silver beech m tall and 0.1 m average diameter. Agreement of E (determined by eddy covariance) and the difference between available energy and sensible heat flux densities was generally within 10% on half-hourly and daily bases. On clear days, the Bowen ratio obtained a broad plateau of about 1–2 for most of the time with much lower and even negative values around sunrise and sunset. Variable cloudiness caused substantial variation in available energy and the Bowen ratio. After rain, daytime Bowen ratios were somewhat lower and relatively constant at about 0.8 when the tree canopy was partially wet. Lysimeter measurements indicated that E f was a significant evaporation component and accounted for 10–20% of E , with rates up to 0.5 mm day−1. Agreement between F (measured by a xylem sap flow method in a representative 337 m2 plot of 14 trees) and tree canopy transpiration ( E − E f ) was reasonable, with an average disparity of order 10–20% or 0.3 ± 0.1 mm day −1 (standard deviation) when the tree canopy was dry. Within the plot, F typically varied by more than an order of magnitude. Tree social position, assessed by emergence of crown from the general canopy level, strongly affected an individuals contribution to plot sap flux density. About 50% of daily plot F emanated from only three emergent trees. Diurnal variation in coupling of the tree canopy to its aerial environment reflected changes in humidity and wind speed corresponding with changes in stomatal and aerodynamic conductances. Consequently, varying proportions of radiative and advective energy were involved in determining tree transpiration rate. Wet canopy evaporation rate was also examined because it significantly influences when transpiration takes place as rain falls on about 200 days of the year in the forest studied. Effects of leaf size and plant nutrition on tree transpiration and the partitioning of available energy are also discussed.

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

SummarySimultaneous 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,r2 = 0.86. The difference between the two estimates was due to understory transpiration.Canopy conductance (gc) was calculated from sap flow measurements using the reverse form of Penman-Monteith equation and climatic data measured 4 m above the canopy. Variations ofgc were well correlated (r2 = 0.85) with global radiation (R) and vapour pressure deficit (vpd). The quantitative expression forgc =f (R, vpd) was very similar to that previously found with maritime pine (Pinus pinaster) in the forest of Les Landes, South Western France.

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

SummaryDuring 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 (“Čermák-system”-C), and (3) regulated variable heating of a stem segment that locally maintains a constant temperature gradient in the trunk (“Čermák/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 (gt) 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 ofgt 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.

Above-ground woody carbon sequestration measured from tree rings is coherent with net ecosystem productivity at five eddy-covariance sites

• Attempts to combine biometric and eddy-covariance (EC) quantifications of carbon allocation to different storage pools in forests have been inconsistent and variably successful in the past. • We assessed above-ground biomass changes at five long-term EC forest stations based on tree-ring width and wood density measurements, together with multiple allometric models. Measurements were validated with site-specific biomass estimates and compared with the sum of monthly CO₂ fluxes between 1997 and 2009. • Biometric measurements and seasonal net ecosystem productivity (NEP) proved largely compatible and suggested that carbon sequestered between January and July is mainly used for volume increase, whereas that taken up between August and September supports a combination of cell wall thickening and storage. The inter-annual variability in above-ground woody carbon uptake was significantly linked with wood production at the sites, ranging between 110 and 370 g C m(-2) yr(-1) , thereby accounting for 10-25% of gross primary productivity (GPP), 15-32% of terrestrial ecosystem respiration (TER) and 25-80% of NEP. • The observed seasonal partitioning of carbon used to support different wood formation processes refines our knowledge on the dynamics and magnitude of carbon allocation in forests across the major European climatic zones. It may thus contribute, for example, to improved vegetation model parameterization and provides an enhanced framework to link tree-ring parameters with EC measurements.

Photosynthetic capacity, chloroplast pigments, and mineral content of the previous year's spruce needles with and without the new flush: analysis of the forest-decline phenomenon of needle bleaching

SummarySpruce (Picea abies) damage in the Fichtelgebirge (FRG) occurs as needle bleaching and a depression of CO2 assimilation. Such injury may primarily result from the direct, above-ground effects of air pollution or indirect, below-ground changes in mineral uptake.Typically, the new flush of spruce needles is green and exhibits high photosynthetic capacity. Mies and Zöttl concluded that the older foliage is damaged when nutrients are withdrawn to supply the current years needles. By removing the terminal buds of single branches in the spring, we produced an experimental set of the previous years needles with greater mineral reserves than the control needles. During the course of the growing period, the performance of the experimental needles, which lacked competition from the new flush, was compared to that of the control needles of the same age-class on intact branches with the new flush.Throughout the experiment, chloroplast pigments of a healthy control tree were not affected by the elimination of the new flush. However, the chlorophyll and carotenoid content as well as the photosynthetic capacity of the previous years needles on those branches of a heavily damaged tree where the new flush had been eliminated increased substantially. This increase was associated with an increase in minerals, which seemed to be deficient in the control needles with the new flush. Thus, in contrast to needles of the same age-class on intact branches with undisturbed new growth in the same atmospheric environment, the experimental needles escaped bleaching and a decrease in photosynthesis. It would seem that the bleaching and the loss in photosynthetic capacity typical of trees damaged by forest decline indirectly result from nutrient deficiencies through soil environment changes and/or root damage than directly from atmospheric pollutants.

How is water-use efficiency of terrestrial ecosystems distributed and changing on Earth?

A better understanding of ecosystem water-use efficiency (WUE) will help us improve ecosystem management for mitigation as well as adaption to global hydrological change. Here, long-term flux tower observations of productivity and evapotranspiration allow us to detect a consistent latitudinal trend in WUE, rising from the subtropics to the northern high-latitudes. The trend peaks at approximately 51°N, and then declines toward higher latitudes. These ground-based observations are consistent with global-scale estimates of WUE. Global analysis of WUE reveals existence of strong regional variations that correspond to global climate patterns. The latitudinal trends of global WUE for Earths major plant functional types reveal two peaks in the Northern Hemisphere not detected by ground-based measurements. One peak is located at 20° ~ 30°N and the other extends a little farther north than 51°N. Finally, long-term spatiotemporal trend analysis using satellite-based remote sensing data reveals that land-cover and land-use change in recent years has led to a decline in global WUE. Our study provides a new framework for global research on the interactions between carbon and water cycles as well as responses to natural and human impacts.

Quantifying ozone uptake at the canopy level of spruce, pine and larch trees at the alpine timberline: an approach based on sap flow measurement.

Micro-climatic and ambient ozone data were combined with measurements of sap flow through tree trunks in order to estimate whole-tree ozone uptake of adult Norway spruce (Picea abies), cembran pine (Pinus cembra), and European larch (Larix decidua) trees. Sap flow was monitored by means of the heat balance approach in two trees of each species during the growing season of 1998. In trees making up the stand canopy, the ozone uptake by evergreen foliages was significantly higher than by deciduous ones, when scaled to the ground area. However, if expressed per unit of whole-tree foliage area, ozone flux through the stomata into the needle mesophyll was 1.09, 1.18 and 1.40 nmol m(-2) s(-1) in Picea abies, Pinus cembra and Larix decidua, respectively. These fluxes are consistent with findings from measurements of needle gas exchange, published from the same species at the study site. It is concluded that the sap flow-based approach offers an inexpensive, spatially and temporally integrating way for estimating ozone uptake at the whole-tree and stand level, intrinsicly covering the effect of boundary layers on ozone flux.

Relating tree phenology with annual carbon fluxes at Tharandt forest

Within climate change research vegetation plays an important role in both indicating climate change by plant phenology and mitigating climate change by carbon sequestration. A 41-year study period of phenological observations at Tharandt International Phenological Garden and an 8-year study period of continuous flux measurements of carbon dioxide above Norway spruce (Picea abies L. [Karst.]) in the Tharandt forest were used to investigate long-term trends of phenological observations concurrent with climatic trends and to assess the importance of phenological changes for annual carbon gain of the forest. It could be shown that length of growing season determined from phenology was less variable than determined from temperature levels and significantly correlated with mean annual temperature. The slight increase in the length of growing season (LGS) (0.14 d/a) resulted from an earlier onset of spring (-0.32 d/a) and of autumn phases (-0.18 d/a). Obviously, this reflected regional climatic trends with significant temperature increase in spring and slight decrease in autumn. The pronounced effect in spring is also reflected by strongest correlation of annual carbon gain with the emergence of May shoot while the correlation with LGS was less significant. It was therefore concluded that earlier emergence of May shoot was more important for annual carbon gain (22.4 gC/m 2 /d gross uptake) than the increase in total LGS (17.1 gC/m 2 /d gross uptake). However, positive effects of premature phenological stages on carbon gain may be reduced by drought effects during summer as observed for the study year 2003.