The effects of slightly elevated ozone concentrations and mild drought stress on the physiology and growth of Norway spruce, Picea abies (L.) Karst. and beech, Fagus sylvatica L., in open-top chambers

Thiec, D. le; Dixon, M.; Garrec, J.P.

New Phytologist 128(4): 671-678

1994


ISSN/ISBN: 0028-646X
DOI: 10.1111/j.1469-8137.1994.tb04031.x
Accession: 002713920

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Abstract
Two clones of 8-yr-old trees of Norway spruce and beech, planted directly into soil and in open-top chambers, were exposed to elevated ozone concentrations and subjected to mild soil drought. When ambient ozone was increased by 50 p.p.b., shoot extension of both Norway spruce clones was significantly reduced. Mild drought stress began in July 1993 and following soil-drying associated with a spell of fine weather, drought remained relatively constant for over a month. During this period, gas exchange parameters were regularly monitored between 7.00 and 9.00 GMT and on two occasions followed throughout the day. Both clones responded to mild drought by reducing stomatal conductance and photosynthesis. There were no apparent interactions between drought and ozone. Reaction of beech was more complicated. Ozone caused a significant decrease in stomatal conductance and photosynthesis in well-watered treatments. All ozone-drought stress treated trees had greater stomatal conductance than non-drought stressed equivalents in the morning, but the situation was reversed as the day went on. As peak ozone concentrations occur in the afternoon, it is hypothesized that over a period of time drought stressed trees are less affected by ozone as they receive a lower cumulative dose. Coupled with low values of vapour pressure deficit occurring in the morning, ozone-treated, drought-stressed trees can support relatively higher rates of photosynthesis than well-watered equivalents. Chlorophyll contents of beech lent support to this hypothesis, as ozone associated reductions were less marked in drought stressed trees.

The effects of slightly elevated ozone concentrations and mild drought stress on the physiology and growth of Norway spruce, Picea abies (L.) Karst. and beech, Fagus sylvatica L., in open-top chambers