Summary Within the scope of a research project on the condition of roots of declining oaks (Quercus robur, Quercus petraea), samples of fine roots and surrounding soil, specimens of stripe cankers on the stem base, and samples of stream water were examined for the presence of Phytophthora species using both baiting methods and selective agar media. At 27 sites in Germany (Bavaria, Rheinland‐Pfalz, Schleswig‐Holstein), Switzerland, Hungary, Italy and Slovenia the following species were isolated (mainly from soil): Phytophthora citricola, Phytophthora cactorum, Phytophthora cambivora, Phytophthora gon‐apodyides, Phytophthora undulata, a species with affinity to Phytophthora drechsleri, and two additional species with close affinity to the Phytophthora cactorum group. Moreover, Pythium group P, Pythium anandrum, Pythium chamaehyphon, and many other Pythium species that have not yet been identified could be recovered. In a soil infestation test most isolates induced dieback of long root tips and necrotic lesions in the root cortex and at the root collar of Quercus robur seedlings. All Phytophthora species tested and Pythium group P caused cortical necrosis after stem inoculation of young Quercus robur trees. It could be shown in vitro that Phytophthora gonapodyides and Pythium group P were able to produce a wilting toxin. Nitrogen input and climatic changes are discussed as predisposing factors for root damage observed in the field.

A survey was made on the occurrence of soilborne Phytophthora species in 35 oak stands on a range of geologically different sites in Bavaria. The most widespread species were P. quercina, P. cambivora and P. citricola. Seven other Phytophthora species were isolated infrequently. The fine root systems of 106 healthy and 111 declining mature trees of Quercus robur and Q. petraea were intensively investigated. The results indicate that, depending on the site conditions, at least two different complex diseases are referred to under the name`oak decline'. On sites with a mean soil pH (CaCl 2 ) > 3´5 and sandy-loamy to clayey soil texture Phytophthora spp. were commonly isolated from rhizosphere soil, and highly significant correlations existed between crown transparency and various root parameters. Oaks with P. quercina or other Phytophthora spp. in their rhizosphere had markedly higher levels of fine root damage than oaks without Phytophthora spp., and were subject to a relative risk of severe crown symptoms of 2´1 and 2´8, respectively. In contrast, in stands with sandy to sandy-loamy soils and a mean soil pH < 3´9, Phytophthora spp. were not found. In these stands, correlations between crown transparency and various root parameters were either less significant or not significant. It is concluded that Phytophthora species are strongly involved in oak decline on sandy-loamy to clayey sites with a mean soil-pH (CaCl 2 ) > 3´5.

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A concept is presented for analysing and quantifying the competitiveness of woody plants. Efficiency ratios are defined that relate resource investments and gains to above‐ and belowground space sequestration. Such efficiencies are exemplified in comparisons between young and mature trees of European beech (Fagus sylvatica) and Norway spruce (Picea abies), resulting in remarkable consistencies across plant age and species. The demonstration extends to assessments of the inter‐ and intra‐specific competition between young beech and spruce plants in a phytotron study, using CO2/O3 regimes as disturbants for examining the sensitivity of the efficiency ratios. In mixed plantations, the growth of spruce was enhanced and that of beech reduced as compared with the corresponding monocultures. This effect related to a high efficiency of spruce in aboveground space sequestration per resource investment, whereas the efficiency in carbon gain (return per unit of crown volume) was of minor importance. In contrast, beech displayed advantages in belowground space sequestration. However, competitiveness of young trees was hardly affected by the applied CO2/O3 treatments. The concept proves to be adequate for quantitatively comparing competitiveness across species, age and growth conditions. One further efficiency ratio, i.e., volume‐related respiratory and transpiratory costs for sustaining functionality in resource sequestration, deserves attention in achieving a mechanistic understanding of competitive behaviour, in particular, in woody plants.