a b s t r a c tMaximum entropy modeling (Maxent) is a widely used algorithm for predicting species distributions across space and time. Properly assessing the uncertainty in such predictions is non-trivial and requires validation with independent datasets. Notably, model complexity (number of model parameters) remains a major concern in relation to overfitting and, hence, transferability of Maxent models. An emerging approach is to validate the cross-temporal transferability of model predictions using paleoecological data. In this study, we assess the effect of model complexity on the performance of Maxent projections across time using two European plant species (Alnus glutinosa (L.) Gaertn. and Corylus avellana L.) with an extensive late Quaternary fossil record in Spain as a study case. We fit 110 models with different levels of complexity under present time and tested model performance using AUC (area under the receiver operating characteristic curve) and AICc (corrected Akaike Information Criterion) through the standard procedure of randomly partitioning current occurrence data. We then compared these results to an independent validation by projecting the models to mid-Holocene (6000 years before present) climatic conditions in Spain to assess their ability to predict fossil pollen presence-absence and abundance. We find that calibrating Maxent models with default settings result in the generation of overly complex models. While model performance increased with model complexity when predicting current distributions, it was higher with intermediate complexity when predicting mid-Holocene distributions. Hence, models of intermediate complexity resulted in the best trade-off to predict species distributions across time. Reliable temporal model transferability is especially relevant for forecasting species distributions under future climate change. Consequently, species-specific model tuning should be used to find the best modeling settings to control for complexity, notably with paleoecological data to independently validate model projections. For cross-temporal projections of species distributions for which paleoecological data is not available, models of intermediate complexity should be selected.

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Aim To reconstruct the historical biogeography of Pinus sylvestris in the Cantabrian Range (Iberian Peninsula) during the Holocene, and to consider the interactions between vegetation dynamics, climate change and the role of man in the present-day distribution of the species.Location The study site is a mire (1300 m a.s.l.) at Vega de Viejos, on a south-facing slope of the western Cantabrian Range, Spain. The region's present-day landscape is almost treeless, with the exception of some patches of Quercus pyrenaica and a few copses of Salix and Betula along stream banks.Methods Tree macrofossils from Vega de Viejos were studied by transmitted light and dark-field reflection microscopy; strobili were subjected to comparative morphological analyses. Two Pinus macrofossils were dated by conventional 14 C methods. ResultsThe taxonomic accuracy achieved in the identification of the macrofossils provided new information regarding the Holocene history of Pinus sylvestris in this territory. Ninety-five cones of this species were identified; in fact, more than 80% of the 36 identified wood remains were of Pinus gr. sylvestris. Radiocarbon dating revealed that the forest to which the fossils belonged was present until at least 2170 ± 50 yr bp -its disappearance was therefore relatively recent.Main conclusions Pinus sylvestris suffered long-term isolation, and after the Würm glacial period tended to migrate towards the east. In western Iberia, a temperate climate and autogenic succession favoured broadleaved taxa at the expense of Pinus. Late Holocene human disturbances may have further accelerated the decline of P. sylvestris; in the Cantabrian Range, only a few stands on southern slopes have persisted until the present day. The history of the capercaillie (Tetrao urogallus), a bird characteristic of pure or mixed Palaeartic coniferous forests, was almost certainly affected by the demise of these forests in this area. Cantabrian capercaillies are the only members of this species that live in purely deciduous forests, perhaps a recent adaptation to the regional extinction of pines. Today's P. sylvestris and capercaillie populations are now highly fragmented and their future, given the predictions of global climate change, is uncertain.