Irena Axmanová

Environmental factors influencing herb layer productivity in Central European oak forests: insights from soil and biomass analyses and a phytometer experiment

Habitat productivity and vegetation biomass are important factors affecting species diversity and ecosystem function, but factors determining productivity are still insufficiently known, especially in the forest herb layer. These factors are difficult to identify because different methods often yield different results. We sampled the herb layer biomass and assessed soil nutrients, moisture and light availability in 100xa0m2 plots in Czech oak forests. Habitat productivity was estimated independently from nutrient content in the soil, herb layer biomass and using a bioassay experiment (growing phytometer plants of Raphanus sativus under standardised conditions in soil samples taken from forest plots). The generalised linear model for herb layer biomass showed it to increase with light, soil phosphorus and moisture availability, but only 10.7% of its variation was explained by these factors. The phytometer biomass increased mainly with soil pH and phosphorus availability; together with soil C/N ratio these factors explained 56.1% of the phytometer biomass variability. Combined evidence based on different approaches indicates that canopy shading and soil phosphorus tend to be the most important factors influencing the herb layer productivity of the studied oak forests.

Exceptionally poor land snail fauna of central Yakutia (NE Russia): climatic and habitat determinants of species richness

There is an ongoing debate on the causes of the latitudinal diversity gradient, but diversity decline towards high latitudes is poorly documented for many invertebrate taxa. Therefore, we sampled land snail assemblages at 79 sites and in various habitat types in central Yakutia, a region with extremely continental, cool and dry climate. We tested whether habitats lacking suitable shelters for winter survival harbour less species than those with vegetation cover that softens climatic extremes. Both local species diversity and regional species diversity were extremely low: 13 species were recorded in total with an average of 1.4 species per site. While the majority of grassland sites were without snails (26 of 34 sites), forest sites supported at least one snail species in most cases (38 of 45 sites). Within grasslands, snail occurrences were associated with a higher herb-layer biomass. Numbers of snail species correlated with the amount of available calcium only in forests, in which species accumulation towards more favourable habitats was possible due to softening of climate harshness. As minute snails are known to be effective passive dispersers and the study area was not glaciated during the last glacial stage, there was certainly enough time for colonization of all favourable habitats. Our results suggest climatically driven limitations of both local and regional land snail diversity in central Yakutia. We conclude that the hypothesis of climate harshness remains the most probable explanation of a sharp drop in land snail diversity in high-latitude areas with cold climate.

Plant species richness–productivity relationships in a low-productive boreal region

Local species richness–productivity (SR–P) relationship is usually reported as unimodal if long productivity gradients are sampled. However, it tends to be monotonically increasing in low-productive environments due to the decreasing part of the SR–P curve being truncated. Previous work indicated that this can hold true for forest herb layers, because of an upper bound on productivity caused mainly by canopy shading. Here, we ask whether the same pattern exists in a region with an upper bound on productivity caused by a harsh climate. We sampled herbaceous vegetation of boreal forests and grasslands in a low-productive region of central Yakutia (NE Siberia) with dry and winter-cool continental climate. We collected data on species composition, herb-layer productivity (aboveground herbaceous biomass), soil chemistry and light availability. We applied regression models to discriminate between monotonically increasing, decreasing and unimodal responses of herb-layer species richness to measured variables and analysed trends in the species-pool size and beta diversity along the productivity gradient. Our expectation of the monotonically increasing SR–P relationship was confirmed for neither forest herb layers nor grasslands. In the forest herb layers, no relationship was detected. In grasslands, the relationship was unimodal with species richness decline starting at much lower productivity levels than in more productive temperate grasslands. Potential causes for this decline are either limitation of local species richness by the species pool, which contains few species adapted to more productive habitats, or competitive exclusion, which can become an important control of species richness under lower levels of productivity than is the case in temperate grasslands.

The type of nutrient limitation affects the plant species richness–productivity relationship: evidence from dry grasslands across Eurasia

The species richness-productivity relationship is one of the most debated patterns in ecology. Species coexistence theory suggests that it could be tightly linked to the type of nutrient limitation (no limitation, single-nutrient limitation, colimitation by several nutrients). Yet, the effects of nutrient limitation on the species richness-productivity relationship have been rarely studied at the regional and continental scales. Combining the predictions of the humped-back model and the niche dimension hypothesis, we hypothesized that an increase in plant species richness with the number of different limiting nutrients is detectable only at higher productivity levels, at which competition for nutrients is more intense. Therefore, we expected the shape of the diversity-productivity relationship to differ between sites colimited by nitrogen (N) and phosphorus (P), sites limited by a single nutrient (either N or P), and sites not limited by any of these nutrients. To test this hypothesis, we used species richness data collected in 10 m x 10 m plots at 694 temperate dry grassland sites across eight regions in northern Eurasia. Productivity ranged from 10 to similar to 500 g/m(2) of above-ground standing biomass. The type of nutrient limitation was identified by critical nutrient ratios alone and their combination with critical nutrient concentrations measured in the plant tissue. Relationships were analysed using generalized linear and mixed-effect models. In line with our expectations, species richness of Eurasian temperate dry grasslands increased more steeply and peaked higher under higher productivity levels at N&P-colimited sites. When nutrient limitation was assessed by both ratios and concentrations, species richness at N&P-colimited sites continued to increase monotonically until the maximum productivity sampled in this study. In contrast, at sites with a single-nutrient limitation or no limitation, the peak in species richness was lower and occurred at a lower productivity of about 300-400 g/m(2). Synthesis. We provide the first evidence that the species richness-productivity relationship may depend on the type of nutrient limitation as predicted by the species coexistence theory. To generalize these findings, the role of nutrient limitation needs to be tested in other ecosystems, including more productive plant communities.