Abstract In a water-exclusion experiment, five different ecotypes of beech (Fagus sylvatica L.; representing re- gions of different environmental and climatic conditions in Baden-Württemberg, Germany) were subjected to drought conditions of different severity between July and September of two consecutive years. Drought stress as characterised by the water content and the pre-dawn wa- ter potential of the leaves was related to the degree of mycorrhization, the type of ectomycorrhiza, and the physiological properties of individual fungus/plant inter- actions at the fine roots of different beech ecotypes. Our data show that decreased soil water availability did not significantly change either the degree of fungal coloni- sation of beech roots (measured by the amount of ergos- terol) or the number of ectomycorrhizal types per root system. Drought did, however, have an influence on the composition of the ectomycorrhizal community, and dif- ferent mycorrhizal types responded to drought different- ly in terms of their patterns of occurrence/abundance. While the abundance of the dominant mycorrhizal types, formed with Byssocorticium atrovirens and Lactarius subdulcis, was not affected, drought increased the abun- dance of mycorrhiza formed between beech and Xero- comus chrysenteron. A detailed analysis of plant and fungal carbohydrates in mycorrhizas indicated that dif- ferent drought intensities led to distinguishable respons- es. In plants exhibiting a pre-dawn water potential of down to –1.96 MPa, drought caused the accumulation of sucrose, glucose and fructose, and of fungus-specific compounds such as mannitol and arabitol in mycorrhizal roots at the expense of, e.g. trehalose. The accumulation of sugar alcohols, which constitute compatible solutes known to counteract drought stress, was species-specific. Mycorrhizas with X. chrysenteron formed large amounts of arabitol, while those with L. subdulcis accumulated mannitol. Sustained partitioning of carbon towards the mycorrhizal fungi under drought was also reflected by an increase of nitrogen storage in the fungal vacuoles. In treatments where the pre-dawn water potential reached values of as low as –2.4 MPa, such alterations were no longer found. In such plants, the starch and soluble sug- ars content was generally reduced, which also resulted in a lack of increase in protective, fungus-specific sugar alcohols. In summary, the data show that, within certain limits, an increase in drought causes a shift in plant/fungus communities. The shift in the pattern of fungus-specific compounds could possibly be used as a sensitive measure of physiological stress imposed on this symbiosis. Keywords Beech ecotypes · Ectomycorrhiza diversity · Fungal carbohydrates · Drought resistance · Nitrogen storage Introduction Beech (Fagus sylvatica) covers more than 20% of the forest surface in south-western Germany (Baden-Würt- temberg) and is one of the most important tree species in this area (Anonymous 1996). Different ecotypes, adapted to local environments, have evolved naturally (Müller- Stark 1997). In central Europe, the optimal conditions for the growth of beech are 8°C annual mean tempera- ture and more than 800 mm mean annual precipitation. Water availability is one of the main limiting factors for beech productivity and vitality (Ebert 1996). Increasing average temperatures and decreasing total precipitation thus threaten the growth of beech. Due to the large con- tact area between fungal hyphae and soil particles, my- corrhiza formation can improve water availability for the host plant. This is documented by the work of Duddridge L. Shi · M. Guttenberger · R. Hampp ( ✉ ) Physiologische Ökologie der Pflanzen, Botanisches Institut, Universität Tübingen, Auf der Morgenstelle 1, 72076, Tübingen, Germany e-mail: ruediger.hampp@uni-tuebingen.de Tel.: +49-7071-2976155, Fax: +49-7071-295635 I. Kottke Spezielle Botanik/Mykologie, Botanisches Institut, Universität Tübingen, Auf der Morgenstelle 1, 72076, Tübingen, Germany Mycorrhiza (2002) 12:303–311 DOI 10.1007/s00572-002-0197-2 ORIGINAL PAPER Lanbo Shi · Martin Guttenberger · Ingrid Kottke Rüdiger Hampp The effect of drought on mycorrhizas of beech (Fagus sylvatica L.): changes in community structure, and the content of carbohydrates and nitrogen storage bodies of the fungi Received: 12 February 2002 / Accepted: 5 July 2002 / Published online: 2 August 2002 © Springer-Verlag 2002