Auswirkungen von Trockenheit und Entlaubung auf den Wasserhaushalt von Stiel- und Traubeneiche

Content Provider	Semantic Scholar
Author	Gieger, Thomas
Copyright Year	2002
Abstract	As indicated indirectly by tree ring analysis and climate records, defoliation in at least two consecutive years together with a dry summer is considered to be the most important combination of factors in oak decline in Northern Germany. The cause-effect relationship of the combined factors `drought stress` and `defoliation in two consecutive years` - single and in combination - has been tested under controlled conditions in saplings of Quercus robur and Q. petraea. To simulate the effect of insect defoliation, saplings were defoliated manually in spring of two consecutive years. In the first part of the investigation, drought was applied in the year after the last defoliation. In the second part, drought was applied in the year following the last defoliation, respectively. In the present work, measurements of water relations, gas exchange, biomass partitioning, wood anatomy as well as concentrations of N, chlorophyll and d13C in leaves and starch concentration in stem have been conducted. In addition, the results obtained from saplings were compared with those of 20-year-old trees on the ground of a tree nursery (defoliation as the only stress factor). 1. Drought stress led to a reduction in stomatal conductance and transpiration as well as to decreased water potentials. In addition, fine-root biomass increased after drought stress. 2. After defoliation, trees reacted with an increase in stomatal conductance and a higher transpiration per unit leaf area. This can be interpreted as a compensating response to the loss of assimilates caused by defoliation. After defoliation, the trees restored their canopy by the generation of leaves from dormant buds, but the total leaf area per plant still was lower than in the control trees. Fine-root biomass was reduced in the year of the second defoliation as well as in the year thereafter. Late-wood width decreased after defoliation, however, this had no impact on the hydraulic conductance. 3. Limitations of the water relations were observed only in response to the combined effect of the stressors. Drought stress in the year of the last defoliation led to an increased loss of hydraulic conductance as well as to a strongly reduced hydraulic.conductance from soil to leaf in Q. petraea. In the year after the last defoliation, alterations of the biomass partitioning towards the leaves were observed. The trees restored their leaf biomass, but still exhibited a reduced fine-root biomass, especially Q. petraea. However, the combination of both stressors did not result in a more severe impairment of the water relations than drought stress alone. 4. Q. petraea is known to be well adapted to drought stress. However, its productivity generally is lower than that of Q. robur. Therefore, a defoliation induced loss of assimilates, and a subsequent decrease in fine-root production, renders Q. petraea more susceptible to drought stress than Q. robur. This is obvious by more unfavourable water relations in Q. petraea upon drought stress. Generally, Q. petraea was more sensitive to the combination of the stressors than Q. robur. Differences in the response of water relations to defoliation between saplings and 20- year-old Q. robur have not been detected. 5. A mechanistical cause-effect relationship between defoliation-induced late-wood reduction and a limitation of the water conductance in the stem could not be confirmed. However, the obtained results are not sufficient to completely reject the hypothesis. 6. Instead, indications of a reduction in plant performance due to a defoliation induced reduction in fine-root production and subsequent reduction in the water uptake capacity were found. This combination of factors can be assumed to be crucial in oak decline. Additional biotic and abiotic factors (e.g. hydromorphic soils) can be stress intensifying.
File Format	PDF HTM / HTML
Alternate Webpage(s)	https://ediss.uni-goettingen.de/bitstream/handle/11858/00-1735-0000-0006-ABF8-0/gieger.pdf?sequence=1
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article