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R Zweifel

Publications and source records attributed to R Zweifel.

5 recordsLinked to original sources
PubMed2005

Modeling tree water deficit from microclimate: an approach to quantifying drought stress.

Tree water deficit estimated by measuring water-related changes in stem radius (DeltaW) was compared with tree water deficit estimated from the output of a simple, physiologically reasonable model (DeltaWE), with soil water potential (Psisoil) and atmospheric vapor pressure deficit (VPD) as inputs. Values of DeltaW were determined by monitoring stem radius changes with dendrometers and detrending the results for growth. We followed changes in DeltaW and DeltaWE in Pinus sylvestris L. and Quercus pubescens Willd. over 2 years at a dry site (2001-2002; Salgesch, Wallis) and in Picea abies (L.) Karst. for 1 year at a wet site (1998; Davos, Graubuenden) in the Swiss Alps. The seasonal courses of DeltaW in deciduous species and in conifers at the same site were similar and could be largely explained by variation in DeltaWE. This finding strongly suggests that DeltaW, despite the known species-specific differences in stomatal response to microclimate, is mainly explained by a combination of atmospheric and soil conditions. Consequently, we concluded that trees are unable to maintain any particular DeltaW. Either Psisoil or VPD alone provided poorer estimates of DeltaW than a model incorporating both factors. As a first approximation of DeltaWE, Psisoil can be weighted so that the negative mean Psisoil reaches 65 to 75% of the positive mean daytime VPD over a season (Q. pubescens: approximately 65%, P. abies: approximately 70%, P. sylvestris: approximately 75%). The differences in DeltaW among species can be partially explained by a different weighting of Psisoil against VPD. The DeltaW of P. sylvestris was more dependent on Psisoil than that of Q. pubescens, but less than that of P. abies, and was less dependent on VPD than that of P. abies and Q. pubescens. The model worked well for P. abies at the wet site and for Q. pubescens and P. sylvestris at the dry site, and may be useful for estimating water deficit in other tree species.

Climate↗
PubMed2002

Midday stomatal closure in Norway spruce--reactions in the upper and lower crown.

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 (1,650 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.

Biomass↗
PubMed2001

Link between diurnal stem radius changes and tree water relations.

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.

Circadian Rhythm↗
PubMed2001

Dynamics of water storage in mature subalpine Picea abies: temporal and spatial patterns of change in stem radius.

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.

Picea↗