Quantification of neighbourhood-dependent plant growth by Bayesian hierarchical modelling

1. The effects of neighbours on the growth of individual plants are fundamental to dynamics in plant populations and can be described by means of mathematical functions, so-called competition kernels, in formal spatiotemporal models. Little is known about the form and components such functions should have. 2. We evaluate some properties of kernel functions using data on the growth of Arabidopsis thaliana plants in replicated, even-aged stands of many individuals. Because of the essential non-independence of plant growth in stands, we employed a Bayesian hierarchical modelling approach to estimate values and uncertainties of kernel parameters in location-dependent models of interacting plants. 3. During the experiment plant size and a simple measure of neighbourhood crowding became strongly correlated, plants tending to be small where local crowding was intense, indicating that local competition was an important process in the growth of the plants. 4. Competitive interactions between plants of different sizes were strongly asymmetric, the larger individual acquiring a disproportionately greater share of resources. Competition increased with plant size and attenuated rapidly at distances of a few centimetres, but the exact shape of the attenuation function was less important. 5. Kernel functions with the same kind of structural features were similar in their predictive ability. However, a simple zone-of-influence model, based on overlap of pairs of individuals, with competition favouring the larger individual, was arguably the most parsimonious. 6. Neighbourhood competition in stands of even-aged plants may be successfully captured with relatively simple kernel functions. The results should inform and enhance the formal theory of spatiotemporal plant population and community dynamics. Bayesian hierarchical modelling is a powerful tool with which to analyse complex, spatially dependent data, and has potential as a widely applicable statistical approach for plant ecology

Holocene climatic and environmental changes inferred from midge records (Diptera : Chironomidae, Chaoboridae, Ceratopogonidae) at Lake Berkut, southern Kola Peninsula, Russia

A radiocarbon-dated sediment sequence from Lake Berkut in the southern part of the Kola Peninsula, northwest Russia, was investigated by means of midge analysis in order to reconstruct the Holocene climatic and environmental history of the region. Past mean July air temperatures at the study site and hypolimnetic oxygen contents of the lake water were inferred from chironomid-based transfer functions. The early Holocene (c. 10 100 - 8400 cal. yr BP) is characterized by summer temperatures and hypolimnetic oxygen concentrations broadly similar to present-day conditions. The midge records give evidence of a lake- level fall at c. 7000 cal. yr BP, resulting in weakened thermal stratification of the lake water, and improvement of the hypolimnetic oxygen conditions. After c. 4000 cal. yr BP midge assemblages suggest a lake- level rise related to increased effective moisture. A secondary mid-Holocene climatic optimum with inferred mean July air temperatures of c. 12.1 degrees C was recorded between 6000 and 4400 cal. yr BP, followed by minimum values (c. 11.3 degrees C) between 3500 and 1500 cal. yr BP, accompanied by the onset of natural acidification of the lake. The late-Holocene midge assemblages suggest weakly acidic conditions, low hypolimnetic oxygen contents and a general warming trend during the last c. 1500 years, although paludification and natural acidification may have influenced the temperature reconstruction at this stage. However, the modern chironomid-inferred mean July air temperature (12.5 degrees C) is consistent with meteorological data from a nearby station. In general, the midge records reflect relatively warm and moist conditions at Lake Berkut before c. 7000 cal. yr BP, a relatively warm and dry climate at c. 7000 - 4000 cal. yr BP, and a shift to cooler and wetter climatic conditions in the region after c. 4000 cal. yr BP