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Abstract "Heft 24"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich

Heft 24

Klaus von Wilpert

Die Jahrringstruktur von Fichten in Abhängigkeit vom
Bodenwasserhaushalt auf Pseudogley und Parabraunerde
Ein Methodenkozept zur Erfassung
standortssezifischer Wasserstreßdisposition

Freiburg im Breisgau 1990

ISSN 0344-2691


Objective of the investigation
The dependance of tree ring growth on environmental factors has been a major question in forest science for many years. This question has gained attention in context with forest decline, air pollution and climate change.
Undoubtedly the water status of trees determines their growth decisively. Since tree ring width is influenced by many different environmental and endogenous factors, thus integrating them over time, tree ring width is not suitable for observation of short term changes in growth conditions.
In spruce ecosystems of cool temperate climates as Middle Europe, fluctuation in water availability is the most important variable for tree growth. The objective of the investigation presented is to find a monitoring criterium that represents growth reactions of mature spruce trees to short term changes of the water status or water stress. The growth reactions should not only be observed qualitatively, but their quantitative properties such as stress intensity should be interpretable. Additionally the observations should be extended to past environmental situations to gain a collective of observations that are large enough to characterise climatic variability. Therefore not actual physiological conditions of the trees should be observed, rather criteria which are records in
tree ring structure.

Material and methods
The investigation dealt with individual trees and not with entire stands. The trees were about 40 years old and 25 m high. The sites included, a Pseudogley and a Brown Soil Lessive, were quite different in their water relations.
Water status and tree ring structure were compared in the 10 years from 1976-1985. The water status was measuresd directly in a calibrating period (1985/86). For the preceeding years, the soil water data were reconstructed by a soil water simulation model. Input data for the model were basic weather data from the German weather survey (daily data for precipitation, air temperature, rel. air humidity and hours of sunshine).
The main calibrating steps were 1) the detailed description of the interception of precipitation in the crowns and 2) the iterative determination of vegetation characteristics such as the potential evapo-transpiration and the relation between potential and actual evapo-transpiration depending on soil water availability.

Measurement of actual waterstress
In order to observe actual waterstress during the calibration period, xylem water potentials were measured in the crown of the trees on one year old twigs. Indirectly, the water status of the trees was observed by the registration of changes of stem diameter with dendrographs. The knowledge of the xylem water potential was necessary for a physiological interpretation of the tree ring structure.

Recording and interpretation of the anatomical tree ring structure
The intraannual tree ring structure was measured by the time series of subsequent radial tracheid diameters in the tree ring during the 10 year study period.
The radial tracheid diameters are proportional to the mean turgor during their differentiation period. As the tracheids differentiate in a very short time, they represent a record of discrete turgor points with a very high time-resolutuion (1-3 days from tracheid to tracheid).
Paralleling between the time series of tracheid diameters and soil water potentials is only possible if the real time of tracheid growth is known. During the calibration period, the tracheid growth was observed by a marking method. This was the basis of an analytical description of the length of the growth period depending on air temperature, drought and daylength. The timeseries of tracheid diameters were indexed on the the normal tracheid development during the growth period, as earlywood tracheids are systematically larger than latewood tracheids.
After paralleling the tracheid- and soil water-timeseries it was possible to describe the relation between tracheid diameter and soil water potential by regression analysis.

The relation between soil water potential and tracheid diameters was very close and the
regression analysis was highly significant. There were few differences in tracheid reactions
between the two sites analysed. On the Pseudogley, extremely small tracheid-diameters
were more frequent than on the Lessive.    i
Through the measurement of xylem water potentials, the physiological interpretation of tracheid diameters was possible. The smallest tracheid diameters on the Pseudogley in 1976 corresponded with an xylem water potential of -3.5 MPa which is a critical level for lethal damage.
By interpretating the statistical distribution of tracheid diameters which represents site specific patterns of water stress intensities, it is possible to determine the risk of water stress.

Future studies and implications
The application and adaption of a water transport model on the forest situation would allow on both sites a simulation of spruce water status assuming altered climate conditions (caused for example by greenhouse warming). The close correlation between tracheid diameter and water status as demonstrated by this study indicates that tracheid diameters can be used as "stress monitoring characteristics". The studied relations allow a prognosis of the changes in site specific water stress factors in the sense of an early warning system.

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