Search

Aim: Ecological niche models (ENMs) typically require point locations of species' occurrence as input data. Where exact locations are not available, geographical centroids of the respective administrational spatial units (ASUs) are often used as a substitute. We investigated how the use of ASU centroids in ENMs affects model performance, what role the size of ASUs plays, and what effects different grain sizes of explanatory variables have. Location: Europe. Major taxa studied: Virtual species. Methods: We set up a two‐factorial study design with artificial ASUs of three different sizes and environmental data of four commonly used grain sizes, repeated over three study regions. To control other factors that may affect ENM performance, we created a virtual species with a known response to environmental variables, precise and even sampling and a known spatial distribution. We ran a series of Maxent models for the virtual species based on centroids and precise occurrence locations under varying ASU and grain sizes. Results: The use of ASU centroids introduces a value frequency mismatch of the explanatory variables between centroids and true occurrence locations, and it has a negative effect on ENM performance. Value frequency mismatch, negative effect on ENM performance and over‐prediction of the species' range all increase with ASU size. The effect of grain size of environmental data, on the contrary, was small in comparison. Main conclusions: ENMs built upon ASU centroids can suffer considerably from the introduced error. For ASUs that are sufficiently small or show low spatial heterogeneity of explanatory variables, ASU centroids can still be a viable and convenient surrogate for precise occurrence locations. When possible, however, central tendency values (median, mean) that represent the whole ASU rather than just a single point location need to be considered. [ABSTRACT FROM AUTHOR]

Aim Biodiversity loss and climate-driven ecosystem modification are leading to substantial changes in forest structure and function. However, the effects of diversity on demographic responses to the environment are poorly understood. We tested the diversity hypothesis (measured through functional diversity) and the mass ratio hypothesis (measured through functional identity) in relation to tree growth, tree mortality and sapling abundance. We sought to determine whether functional diversity underlies demographic responses to environmental variation in European forests. Location Europe (Spain, Germany, Wallonia, Finland and Sweden). Methods We used data from five European national forest inventories from boreal to Mediterranean biomes ( c. 700,000 trees in 54,000 plots and 143 tree species) and the main forest types across Europe (i.e. from needle-leaved evergreen forests to broad-leaved deciduous forests). For each forest type, we applied maximum likelihood techniques to quantify the relative importance of stand structure, climate and diversity (i.e. functional diversity and functional identity) as determinants of growth, mortality and sapling abundance. We also tested whether demographic responses to environmental conditions (including stand density, evapotranspiration and temperature anomalies) varied with functional diversity. Results Our results suggest that functional diversity has a positive effect on sapling abundance and growth rates in forests across Europe, while no effect was observed on tree mortality. Functional identity has a strong effect on mortality and sapling abundance, with greater mortality rates in forests dominated by needle-leaved individuals and a greater abundance of saplings in forests dominated by broad-leaved individuals. Furthermore, we observed that functional diversity modified the effects of stand density on demographic responses in Mediterranean forests and the influence of evapotranspiration and temperature anomalies in forests widely distributed across Europe. Main conclusion Our results suggest that functional diversity may play a key role in forest dynamics through complementarity mechanisms, as well as by modulating demographic responses to environmental variation. [ABSTRACT FROM AUTHOR]