Dr. Rudolf Häsler

Net ecosystem exchange (NEE) of two contrasting mountain forest types in Switzerland was measured by eddy covariance (EC) measurements at a montane mixed forest, the Lägeren forest, over 5 years (2005-2009), and at a subalpine coniferous forest, the Seehornwald in Davos, over 12 years (1997-2009). NEE was validated against annual carbon (C) storage estimates, based on biometric and soil respiration measurements as well as soil C modeling. Three different approaches were used: (1) calculation of net ecosystem production by quantifying C pools and fluxes, (2) assessment of change in wood biomass and soil C storage (ΔC), and (3) application of biomass expansion factors. Although biometric estimates were sensitive to assumptions made for each method applied, they agreed well with measured NEE. Comparing 5 years of EC measurements available at both sites during 2005 and 2009 revealed that NEE, gross primary production (GPP), and total ecosystem respiration (TER) were larger at the Lägeren forest compared to the Davos forest, whereas soil respiration and soil C sequestration were of similar magnitudes. Both sites showed similar annual trends for NEE, GPP and TER, but different seasonal courses, due to different responses to environmental conditions (temperature, soil moisture, and radiation). Differences in the magnitude as well as in the seasonality of ecosystem CO₂ exchange could mainly be attributed to tree phenology, productivity, and carbon allocation patterns, which are combined effects of tree type (broad-leaved vs. coniferous trees) and site-specific climatic conditions. Flux differences between the two mountain sites highlight the importance of considering the role of altitude in ecological studies and modeling.

Continuous stem radius changes (DR) include growth and water-related processes on the individual tree level. DR is assumed to provide carbon turnover information complementary to net ecosystem productivity (NEP) which integrates fluxes over the entire forest ecosystem. Here, we investigated the unexpectedly close relationship between NEP and DR and asked for causalities.
NEP (positive values indicate carbon sink) measured by eddy covariance over 11 yr was analysed at three time scales alongside automated point dendrometer DR data from a Swiss subalpine Norway spruce forest.
On annual and monthly scales, the remarkably close relationship between NEP and DR was positive, whereas on a half-hourly scale the relationship was negative. Gross primary production (GPP) had a similar explanatory power at shorter time scales, but was significantly less correlated with DR on an annual scale.
The causal explanation for the NEP-DR relationship is still fragmentary; however, it is partially attributable to the following: radial stem growth with a strong effect on monthly and annual increases in NEP and DR; frost-induced bark tissue dehydration with a parallel decrease in both measures on a monthly scale; and transpiration-induced DR shrinkage which is negatively correlated with assimilation and thus with NEP on a half-hourly scale.

At the alpine timberline, the growth of trees is hindered by many environmental factors. Besides low temperatures, relatively short vegetation periods, mechanical forces and biotic diseases, extreme wind can also reduce the growth of young trees or even kill them through abrasion.
In «Pru dal Vent» (windy pasture), an extremely windy area on the south side of the Bernina pass, in the Grisons, Switzerland (10° 2′ 2″ E, 46° 22′ 44″ N, 2210 m NN), wind speeds are a key factor affecting the growth and survival of young trees.
Young Swiss stone pines and European larches (Pinus cembra; Larix decidua) were planted in «Pru dal Vent» to test the effectiveness of different windbreaks against abrasion caused by very high winds, in addition to the high-altitude afforestation experiments at Stillberg Alp near Davos. From 1971 to 1995 experiments with windbreak walls of various sizes up to 2.5 m high were built. Their permeability varied, but all were treated with agents to reduce transpiration A meteorological station recorded the weather data. The antitranspirants appeared to have no positive effects. The lee-sides of the windbreaks accumulated extremely hard snow covers, which led to multiple fractures occurring in the young stems. As a result, most trees died.
In 1994 small windbreak devices, one for each tree, were put up, designed to be somewhat similar to those found in the luff of most of the few naturally growing trees in the surroundings. Again different shapes and forms of the shelters were tested including: small stone cairns, wooden shields of different shapes, and small topless boxes. The best results were observed with the stone cairns und with two small boards arranged in the shape of a V.
Since the small windbreaks were put up in 1994, the majority (roughly 96% of L. decidua and 70% of P. cembra) of the experimental trees behind them have survived. Some have even grown well. Thus this method using small shelters rather than larger windbreaks seems to be an effective way of protecting young trees).

We used local microclimatic conditions and twig sap flow rates to interpret midday stomatal closure in the canopies of two 250-year-old Norway spruce (Picea abies (L.) Karst.) trees at a subalpine site in the Swiss Alps (1650 m a.s.l.). Both trees showed midday stomatal closure on most clear summer days, despite the permanently wet soil. We used a modified Penman-Monteith formula to simulate potential transpiration of single twigs (ET_T) based on high-resolution temporal and spatial microclimate data obtained both inside and outside the crowns. Comparison of calculated ET_T values and measured twig sap flow rates enabled us to pinpoint the occurrence of midday stomatal closure and the microclimatic conditions present at that time.
We found that vapor pressure deficit (and for upper-crown twigs, ET_T) largely explained the timing of initial midday stomatal closure but gave no explanation for the different patterns of stomatal behavior after initial closure in upper- and lower-crown twigs. After the initial stomatal closure, upper-crown twigs maintained high transpiration rates by continuously regulating stomatal aperture, whereas stomatal aperture decreased rapidly in lower-crown twigs and did not increase later in the day. Midday stomatal closure in lower-crown twigs occurred on average 1 h later than in upper-crown twigs. However, the microclimate at the time of initial stomatal closure was similar at both crown locations except that lower-crown twigs received significantly less solar radiation than upper-crown twigs both at the time of initial stomatal closure and afterwards. High rates of sap flow in twigs did not always lead to stomatal closure and therefore could not explain the phenomenon. We conclude that stomatal conductance can be modeled accurately only when both local microclimatic conditions and tree water status are known. Further, we hypothesize that both the quantity and quality of light play an important role in the reopening of closed stomata during the day.
 

Wie reagiert ein subalpiner Fichtenwald auf die Klimaerwärmung? Wie viel Kohlendioxid kann ein Wald aus der Luft aufnehmen und speichern? Dies sind zwei der Fragen, auf die gegenwärtig im Seehornwald bei Davos Antworten gesucht werden.

Wachsen ausländische Gebirgsbaumarten schneller und sind sie widerstandsfähriger gegenüber Pilz- und Schneedruckschäden als unsere einheimisch Arven, Lärchen und Fichten? In Hochlagen-Aufforstungen, besonders in Kombination mit Lawinenverbauungen, sind immer wieder grössere Ausfälle der Pflanzen zu beobachten.

Testpflanzung mit Lärchen und Arven auf der Pru dal Vent (Alp Grüm, 2000 m ü. M.), die mit verschiedenen Windschutz-Elementen gegen die Hauptwindrichtung geschützt werden.
Im Zusammenhang mit den Hochlagenaufforstungsversuchen (1959 bis 1975) am Stillberg (Dischma, Davos, 2000-2230 m. ü. M.), wurden für eine erweiterte Fragestellung im Zusammenhang mit Windstandorten an der Waldgrenze zusätzlich verschiedene Testpflanzungen auf der Pru dal Vent (Windwiese) angelegt. (Nr. 85 auf Karte in Heftmitte).
Der meist schneearme Standort auf der Pru dal Vent bewirkt für die Pflanzen bei Windstärken von häufig 80 km/Std. und mehr und bei gefrorenem Boden enormen Stress. Eis- und Flugsandpartikel verursachen auf der windzugekehrten Seite oft starke Verletzungen an Rinde, Knospen und Nadeln. Bei sonnigem Wetter und gefrorenem Boden erleiden die schneefreien Pflanzen oft starke Trockenschäden (Frosttrocknis, Strahlungsschäden).

Internal water reserves in bark and foliage of trees contribute to transpiration (T) and play an essential role in optimizing water transport by buffering extreme peaks of water consumption. We examined patterns of stem shrinkage and their relationship to tree water dynamics. We measured fluctuations in root radius and stem radius at different stem heights, T of twigs at the top of the crown and sap flow velocities in stem sections of mature subalpine Norway spruce (Picea abies (L.) Karst.) trees over 2 years. The output of each sensor was coupled by physical functions to a mechanistic flow and storage model of tree water relations. The data verified the model-predicted lag in water storage depletion in response to the onset of transpiration and the lag increased with increasing distance from the crown periphery. Between the crown and stem base, the delay ranged from a few minutes to several hours, depending on microclimatic conditions and tree water status. Stem volume changes were proportional to the amount of water exchanged between the elastic tissues of the bark and the rigid xylem, indicating that the "peristaltic" wave of stem contraction along the flow path represented depletion of water stored in bark. On a daily basis, stems lost between 0.2 and 0.5% of their volume as a result of bark dehydration, corresponding to about 2 to 5 l of water. This water contributed directly to T. According to the model based on hydraulic principles, there are three main components underlying the dynamics of water storage depletion: flow resistance, storage capacities of needles and bark, and T of each tree section. The resistances and capacities were proportional to the response delay, whereas T in the lower parts of the tree was inversely proportional. The pattern of T within the crown depended on water intercepted by the branches. Because of these weather-dependent factors, there was no time constant for the response delay along the flow path. Nevertheless, the upper crown and the root section tended to have longer response delays per meter of flow path than the stem. The diurnal course of stem radius fluctuations represents the sum of all external and internal conditions affecting tree water relations; stem radius fluctuations, therefore, provide a sensitive measure of tree water status.

Internal water reserves are depleted and replenished daily, not only in succulent plants, but also in trees. The significance of these changes in tissue water storage for tree water relations was investigated by monitoring diurnal fluctuations in stem radius.
In 6-year-old potted Norway spruce (Picea abies (L.) Karst.) trees, whole-tree transpiration rate (T), sap flow at the stem base and fluctuations in stem radius were measured at 10-min intervals over eight successive weeks. The dynamics of diurnal water storage in relation to the daily course of water movement was simulated and the contribution of stored water to T quantified.
The finding that, in P. abies, the course of bark water content is linearly coupled to stem radius fluctuations provided the basis for linking stem radius changes to a functional flow and storage model for tree water relations. This model, which consists of physical functions only and is driven by a single input variable (T), accurately simulates the diurnal course of changes in stem radius and water storage of the tree crown and stem. It was concluded that fluctuations were mainly determined by the course of transpiration. The availability of soil water and the degree to which storage tissues were saturated were also factors affecting the diurnal course of stem radius changes. Internally stored water contributed to daily transpiration even in well-watered trees, indicating that stored water plays an important role not only during periods of drought, but whenever water transport occurs within the tree.
Needle and bark water reserves were most heavily depleted during transpiration. Together they supplied approximately 10% of daily T on sunny days, and up to 65% on cloudy days. On a daily basis, the crown (mainly needles) contributed approximately eight times more water to T than the stem (mainly bark). The depletion of the two storage pools and the water movements observed in the trees always occurred in the same sequence. In the morning, T first caused a depletion of the water stored in the crown. It then caused depletion of bark storage tissues at ever increasing distances from the needles. Up to 75% of the transpired water could be withdrawn from storage tissues when the increase in T reached a maximum.
 

Ozone (O₃) flux into Norway spruce (Picea abies) and cembran pine (Pinus cembra) needles was estimated under ambient conditions at six rural sites between 580 and 1950 m a.s.l. We also assessed age-related di€fferences in O₃ flux by examining changes in leaf conductance across the life span of Norway spruce. At the leaf level O₃ flux into the needles was eff€ectively controlled by stomatal conductance and, hence by factors such as temperature, irradiance and humidity, which control stomatal conductance. Seasonal variations in O₃ flux were mainly attributed to the course of the prevailing temperature. During the growing season, however, data have emphasised leaf-air vapour pressure di€erence as the environmental factor most likely to control stomatal conductance and O₃ flux into the needles. In the sun crown stomatal conductance averaged over the growing season decreased with increasing tree age from 42.0±‹3.5 mmol O₃ m^-2 s^-1 in 17-year-old trees to 7.1±‹1.0 mmol O₃ m^-2 s^-1 in 216-year-old trees, indicating that O₃ concentration in the substomatal cavities is higher in young than in old trees. Independent from tree age stomatal conductance and O₃ flux were approximately 50% lower in shade needles as compared to sun-exposed needles. Stomatal conductance was also greater in the current flush (24±‹5.6 mmol O₃ m^-2 s^-1) and in 1-year old needles (16±‹4 mmol O₃ m^-2 s^-1) than in older needle age classes (12±‹1 mmol O₃ m^-2 s^-1, averaged across the four older needle age classes). In trees similar in age (60-65 years old) average O₃ flux into sun needles increased from 0.55±‹0.36 nmol m^-2 s^-1 at the valley floor to 0.9 nmol m^-2 s^-1 in 1950 m a.s.l. Cumulative O₃ uptake during the vegetation period increased from 11.4±‹1.7 mol m^-2 in the valley to 14 mol m^-2 at the alpine timberline. Although stomatal conductance provides the principal limiting factor for O₃ flux, additional field research is necessary in order to improve our understanding concerning the quantitative 'physiological threshold dose' which internally can be active and can have adverse e€ects of O₃ on forest trees.

Temporal and spatial patterns of stem and root radius changes were continuously measured on mature, subalpine Norway spruce (Picea abies (L.) Karst.) over 2 years. In addition, freezing experiments with stem segments of saplings were carried out in a climate chamber. The dynamics of the radius fluctuations were analyzed in relation to temperature profiles of air, bark, and soil. We found that bark thickness sharply decreases by 1 mm and more within 2–3 days when the air temperature falls below −5°C. In contrast to the elastic tissues of the bark, the rigid xylem remains largely unaffected. This frost-induced shrinkage of the bark is up to 10 times larger than the measured amplitude of diurnal stem radius fluctuations in summer. During periods of rising air temperatures (>−12°C), the radius suddenly returns to its original size. We conclude, that the large stem and root radius changes in winter are related to changing bark water content, as is also the case during size fluctuations in summer. For a mature tree with a height of 25 m, the shrinkage of the entire stem bark is equivalent to approximately 20 l of water. A hypothesis is discussed which suggests a frost-induced transport of this water between bark and wood. It is based on the initial freezing of water in the xylem resulting in a water potential gradient between bark (solution) and wood (ice). The hypothesis suggests that the water transport between bark and wood is mainly determined by physical changes and that no biochemical transport energy or physiological control mechanisms are involved. As long as ice is initially formed in the xylem and not in the bark, this mechanism of bark dehydration comes into play and protects the living cells from frost damage.

Changes in the stem radius of young Norway spruce [Picea abies (L.) Karst.] were related to changes in stem water content in order to investigate the relationship between diurnal stem size fluctuations and internally stored water. Experiments were performed on living trees and on cut stem segments. The defoliated stem segments were dried under room conditions and weight (W), volume (V), and xylem water potential (Ψ_s) were continuously monitored for 95 h. Additionally, photos of cross-sections of fresh and air-dried stem segments were taken. For stem segments we found that the change in V was linearly correlated to the change in W as long as (Ψ_s) was >-2.3±0.3 MPa (phase transition point). Stem contraction occurred almost solely in the elastic tissues of the bark (cambium, phloem, and parenchyma), and the stem radius changes were closely coupled to bark water content. For living trees, it is therefore possible to estimate the daily contribution of "bark water" to transpiration from knowledge of the stem size and continuous measurements of the stem radius fluctuations. When Ψ_s reaches the phase-transition point, water is also withdrawn from the inelastic tissue of the stem (xylem), which - in the experiment with stem segments - was indicated by an increasing ratio between ΔV and ΔW. We assume that for Ψ_s below the transition point, air is sucked into the tracheids (cavitation) and water is also withdrawn from the xylem. Due to the fact that in living P. abies Ψ_s rarely falls below -2.3±0.3 MPa and the xylem size is almost unaffected by radius fluctuations, dendrometers are useful instruments with which to derive the diurnal changes in the bark water contents of Norway spruce trees.

Shoot and root elongation of young European larch (Larix decidua) growing in natural soil on E- and N-facing slopes (2180m a.s.l.; inclination 40°) at the Stillberg experimental area (near Davos, Switzerland) were measured during the vegetation periods 1988-1990.
Growth and mean growing period are greatly influenced by the microclimate. In spring, roots start growth 3 to 5 weeks earlier than the shoots; shoot elongation ceases at the end of August, whereas root growth continues until temperatures decrease. Shoot and root growth are correlated to air and soil temperature. Root growth increases faster with temperature than shoot growth.

Seasonal patterns of ozone (O₃) uptake in mature Norway spruce and cembran pine trees were estimated at six rural sites between 580 m and 1950 m a.s.l. Seasonal variations in O₃ uptake reflect variation in both ambient O₃ concentration and stomatal conductance. At all the study sites the annual course in the O₃ regime exhibited seasonal cycles with maxima between April and July and minima in winter. Seasonal variations in stomatal conductance and O₃ uptake were mainly attributed to the course of the prevailing temperature. Average O₃ uptake rates decreased with increasing tree age. However, in trees similar in age O₃ uptake increased with increasing altitude.

In situ water relations of a large subalpine Norway spruce (Picea abies) were analyzed by simultaneous measurements of sap flow at different crown positions. In the diurnal scale, transpiration varied greatly, both spatially and temporally. Over longer periods, however, different parts of the crown transpired in fairly constant proportions. The average estimated transpiration was about 3.5 times greater in the upper than in the lower half and decreased 1.6-fold from south to north. Water intercepted from rain, fog and dew buffered and significantly decreased the transpiration. The effect was strongest in those parts which were least coupled to the free atmosphere. The top of the clown seemed to experience a regular shortage of water shortly after starting transpiration, when it was forced to switch from internal reserves to sources in the soil. Further, lower branches then started transpiring, which may have led them to compete for the water. An enhanced nocturnal sap flow during warm and dry winds (Foehn) indicated that the tree also transpired at night. Shaded twigs had more capacity to intercept water externally than twigs in the sun. The significance of the crown structure for interaction with water in both liquid and vapour phases is discussed.

Two methods were used to measure the flow of sap through the xylem of a mature Norway spruce by balancing heat. One, the tissue heat balance (THB), had been designed for tree stems of large diameter and is based on the balance of heat within an internal compartment of tissue conducting the sap. The other, the stem heat balance (SHB), is applicable for stems of small diameter. It follows the principle of balancing heat within a section of the entire stem. Both methods were tested in the laboratory using simple experimental simulators of flow. In the field, comparative measurements were made within the crown of a mature Norway spruce. The THB was inaccurate mainly because (1) it failed to consider the dissipation of heat appropriately during measurements, and (2) assessing the temperature within the tissue of the stem in such a way that it represented the entire heated compartment was not practicable. The SHB yielded more realistic results. A procedure obtaining approximate results also by THB is proposed. It is a fundamental requirement of balancing heat that all its fluxes can be completely assessed. This is unlikely to be achieved for a compartment inside a stem as intended by THB. There the dissipation of heat is not traceable and varies as the flow of sap changes. An insulated section of an entire stem, however, is a compartment, which allows one to measure the SHB including all its major components. Thus, balancing heat appears to be suitable for quantifying the flow of sap in thin stems by SHB rather than in thick stems as intended by THB, unless complex calibrations are made.

An das Klima, das heisst an die durchschnittliche Witterung, ist die Vegetation eines Wuchsortes angepasst. Auch wenn gelegentlich beispielsweise durch Frost oder Wind, Schäden entstehen und Schwankungen in Artenzahl und -zusammensetzung auftreten, ist das Ökosystem als Ganzes bei gleichbleibendem Klima stabil. Extreme Witterungsereignisse können aber ganze Bestände, einzelne Bäume oder deren Teile zum Absterben bringen beziehungsweise sie schwächen.

Diurnal changes in the stem radius of a subalpine mature Norway spruce were measured simultaneously with the flow of sap in xylem. Matric potentials in the soil were > -35 kPa. The kinetics of the flow were closely related to the changes in the radius of the stem resulting from depletion of its extensible tissues. The radius of the stem oscillated daily and, fairly independently of this, fluctuated over several days. The daily shrinkage (ΔS_d) was correlated with the daily flow through its base (Q_d). When the crown transpired little and was nearly saturated during rainy days, ASd tended to increase relative to Q_d. Using a linear relation, the estimates of flow by ΔS_d deviated less than ± 10% from the values measured by heat balance, provided that the periods of calibration in their ratio of dry to rainy days were comparable to those estimated. If the two periods differed in this respect, the estimates of flow deviated up to 42%. A quadratic relation yielded estimates that depended less on weather. It reduced maximal deviations to ±22%. Since ΔS_d additionally may represent the time pattern of the daily transpiration better than Q_d, analysing changes in the radius of stems may supplement or partly replace measurements of sap flow in stems.

Diurnal fluctuations of stem radii and transpiration flows were recorded simultaneously at different crown levels of a subalpine Norway spruce. Daily oscillations of the stem radius at mid-crown level showed minimal amplitudes, and compared to those at the crown top a delay of more than 1 hour. Radius fluctuations over several days and the ratio F_base/F_mid-crown (daily xylem flow at 3.3 m/daily flow at 15.6 m) reflected depletion and replenishement of tree-internal water storage. On days of water storage depletion, the comparison of the daily xylem flow balances (F_base - F_mid-crown) with the changes of the stem volume suggests that water is withdrawn also from other organs than the stem. The relationships between diurnal amplitudes of radius variation and daily xylem flows of the corresponding crown levels showed a remarkable hysteresis at the stem base. Therefore, the time dynamics of daily amounts of tree transpiration are better represented by diurnal dendrographic amplitudes than by daily xylem flows at the stem base. The transpiration of lower crown layers was strongly influenced by crown water  interception.

Die Wechselwirkungen zwischen Witterung und Wald sind sehr vielfältig und komplex. Neben Strahlung, Temperatur und Luftfeuchtigkeit sind auch Niederschlag, Wind und Gewitter von grosser Bedeutung, wobei diese Faktoren nicht unabhängig voneinander wirken. Die bestehenden Wälder sind meist gut an die durchschnittlichen Witterungsbedingungen (Klima) des Standortes angepasst. Zu Schäden führen mehr oder weniger seltene extreme Witterungsereignisse. Am Beispiel von Temperatur und Niederschlag werden die komplizierten Zusammenhänge zwischen Witterung und Wald bis auf die Stufe pflanzenphysiologischer Vorgänge erläutert.