Effects of defoliation and drought stress on biomass partitioning and water relations of Quercus robur and Quercus petraea

2002 | journal article. A publication with affiliation to the University of Göttingen.

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​Effects of defoliation and drought stress on biomass partitioning and water relations of Quercus robur and Quercus petraea​
Gieger, T. & Thomas, F. M.​ (2002) 
Basic and Applied Ecology3(2) pp. 171​-181​.​ DOI: https://doi.org/10.1078/1439-1791-00091 

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Gieger, T.; Thomas, Frank M.
To test the isolated and combined effects of drought and defoliation on biomass partitioning and water relations, four-year-old saplings of Quercus robur L. and Q. petraea (Matt.) Liebl. were investigated under controlled conditions. It was hypothesized that defoliation leads to a reduction of late wood and that, as a consequence, subsequent drought stress will more rapidly lead to desiccation damage due to the loss of reserves in water transport capacity. Alternatively, defoliation could have a negative effect on water relations via a growth reduction of the fine-root system, thereby diminishing the tree's capacity for water uptake. Four treatments were applied to each species: (1) control C, (2) defoliation in two consecutive years De, (3) drought stress in the second year Dr, (4) a combination of (2) and (3) DeDr. Drought-stressed trees showed a decrease of predawn and midday water potentials and of maximum stomatal conductance (g(sm)). Defoliated trees exhibited a higher g(sm), compared to non-defoliated trees even under drought-stress conditions. At the end of the drought period the daily sum of transpiration per unit leaf area (E-d) was only reduced in Dr, but not in DeDr trees, and was even increased in De trees of Q. robur. Fine-root mass, leaf mass, leaf area and specific leaf area decreased due to defoliation. Drought stress also led to a reduction of leaf mass in Q. petraea as well as to reduced leaf/fine-root ratio (Dr and DeDr trees) in both species. Although a reduction of late wood was found in DeDr trees of Q. robur and in the interannual comparison of De and DeDr twigs of Q. petraea, no restrictions of transpiration or hydraulic conductance could be observed in both species. Therefore, this finding supports the alternative hypothesis, i.e. a possible impairment of the tree's water relation caused by a reduction of the fine-root biomass due to defoliation. A combination of both stress factors impairs the morphological acclimatization to drought, i.e. an increased formation of fine-root biomass, and, partly, the physiological avoidance of dehydration, i.e. a reduction in stomatal conductance. Therefore, it is concluded that, in contrast to an isolated occurence of the stressors, their combined action - depending on the time of occurrence of the stressors - will ultimately result in tree damage due to desiccation.
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Urban & Fischer Verlag
Basic and Applied Ecology 



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