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Water Economy of beech in temperate mixed stands: Fading away or going on under stress?

Final Activity Report Summary - WETMISTFAGUS (Water Economy of beech in Temperate MIxed STands: Fading Away or Going on Under Stress?)

Due to recent forest management, most of the natural occupation areas of beech, i.e. fagus sylvatica L., are now dominated by planted conifers. One of the aims of current forest policy is to reverse this process, favouring the natural regeneration of beech. However, it remains necessary to increase the efforts to characterise the eco-physiology of this species, particularly regarding water relations, in order to define the most adequate management criteria. In this context, the analysis of carbon and oxygen stable isotopes, namely d13C and d18O, in different plant fractions can be a useful ecophysiological tool, combining the ability to integrate physiological information in time with relative simplicity.

The main objectives of the project WETMISTFAGUS were to:

1. calibrate models to estimate water and carbon balance from the stable isotopes d13C and d18O in beech and competing species
2. characterise fractionation processes determining d13C and d18O in these species and
3. assess the competitive ability of beech.

The major scientific project outcomes were the following:

1. effect of water availability on leaf water evaporative enrichment, related to objectives one and two. In order to assess the effect of water availability on a crucial parameter of leaf water enrichment models, the effective path length L, we performed experiments under controlled conditions with beech and oak seedlings. These experiments showed, for the first time, that this parameter was strongly affected by changes in water availability (slightly drought stressed plants and resulted in an approximately fourfold increase in L) and that changes in the tortuosity factor, and not changes in the leaf water content, were responsible for the changes in L. These findings were relevant to the use of water isotopes in the study of water balance, since a variable L could prevent a direct relationship between leaf water enrichment and transpiration rates.
2. competition for water resources between beech and oak, related to all three project objectives. To study the water uptake patterns of beech and oak, a field campaign in a mixed stand near Freiburg was performed. Although the results from the last season were still under process, this study showed that beech and oak had different water uptake patterns, the latter exploiting deeper soil layers during dry episodes. According to the lack of stomatal limitation of transpiration, however, there was no evidence of a competitive disadvantage for beech. A labelling experiment with beech and oak seedlings, grown either as monoculture or mixed together in pots, confirmed that the uptake patterns of these species were complementary; indeed, the optimum water use was found in mixed pots.
3. tracking the environmental signal from leaves to tree-rings in beech and oak. During two growing seasons and in two drought-exposed beech stands, both pure and mixed with oak, we studied the seasonal pattern of transpiration along with stable isotopes from leaves to tree-rings. Apart from the conclusions regarding competition between beech and oak, these studies revealed that the environmental signal of d13C was transferred from leaves to phloem and tree rings, but post-photosynthetic fractionation caused an offset and storage and transport effects on the d13C of tree-ring varied between species and probably with environmental conditions. On the other hand, the strength of the leaf-water signal in d18O became diluted from leaf water to phloem and tree-rings, and, although its occurrence was strongly influenced by source water, the oxygen exchange with stem water during cellulose synthesis varied over the growing season.

These findings indicated that tree rings, with adequate physiological information, could be used to characterise time integrated water balance, although further research on the source of variability of stable isotopes in tree-rings was necessary.