The aim of the present study was to determine how internal diversity of oak-hornbeam forest modifies the course and effects of infestation of small-flowered balsam (Impatiens parviflora) with rust Puccinia komarovii. The study investigated the effect of the disease on the demography of the population, and the habit and biomass allocation of the infested specimens, as well as the initiation of non-specific defense mechanisms. It was shown that: 1) the percentage of infected specimens was independent of the undergrowth-coverage rate and of the I. parviflora density; 2) infected populations differ from healthy ones in the seasonal dynamics of abundance changes; 3) high mortality is observed as early as May and the first part of June, i.e. it pertains to specimens, which did not begin reproduction; 4) the presence of infection and its intensity stimulate the growth of the stem and the hypocotyl; 5) fresh weight of infected specimens is by almost 30% lower in comparison to the weight of uninfected plants; the weight of badly infected plants decreases by 20% in comparison to the less severely infected balsam plants; the weight of generative organs drops most significantly, even by over 50%; 6) biomass allocation does not fluctuate considerably; 7) reproduction effort expressed by the ratio of the weight of fruits (or only seeds) to the total biomass shows a distinct downward trend; 8) the environmental factor does not affect the force of defense mechanisms, the strength of biochemical response of diseased plants depends on the degree of their infestation

This study investigated the extend to which the snail Columella edentula is more strongly associated with the small balsam Impatiens parviflora than with other plants in the herb layer of an oak-hornbeam forest, and to interpret the character of the interaction Impatiens parviflora -Columella edentula.Numbers of C. edentula and rates of colonization were compared on various plant species under natural and laboratory conditions. Seasonal variation in snail abundance on I. parviflora was observed on permanent plots. The leaf injuries caused by C. edentula were localized in respect of the morphological and anatomical structure of leaves.The results show that I. parviflora is one of the plant species of the herb layer that are most abundantly colonized by this snail in oak-hornbeam forest. Snail finds a plant particularly suitable as a place for resting. The most favoured attachment site is on the underside of the leaf, along the midrib, which provides the highest and relatively stable humidity, as well as protection from direct sunlight and predators. I. parviflora is also a food for the snails, but they do not eat these fragments of leaves where calcium carbonate is accumulated.