Laura Camila Pacheco Riaño , Fride Høistad Schei, Suzette G. A. Flantua, John-Arvid Grytnes

Aim: Current global warming is driving changes in biological assemblages by increasing the number of thermophilic species while reducing the number of cold-adapted species, leading to thermophilization of these assemblages. However, there is increasing evidence that thermophilization might not keep pace with global warming, resulting in thermal lags. Here, we quantify the magnitude of thermal lags of plant assemblages in Norway during the last century and assess how their spatio-temporal variation is related to variables associated with temperature-change velocity, topographic heterogeneity, and habitat type.
Location: Norway
Time period: 1905 - 2007
Major taxa studied: Vascular plants
Methods: We inferred floristic temperature from 16,351 plant assemblages and calculated floristic temperature anomaly (difference between floristic temperature and baseline temperature) and thermal lag index (difference between reconstructed floristic temperature and observed climatic temperature) from 1905 until 2007. Using generalized least square models, we analyzed how the variation in observed lags since 1980 is related to temperature-change velocity (measured as magnitude, rate of temperature change, and distance to past analogous thermal conditions), topographic heterogeneity, and habitat type (forest vs non-forest).
Results: The floristic temperature anomaly increases overall during the study period. However, thermophilization falls behind temperature change, causing a constantly increasing lag for the same period. The thermal lag index increases most strongly in the period after 1980, when it is best explained by variables related to temperature-change velocity. We also find a higher lag in non-forested areas, while no relationship is detected between the degree of thermal lag and fine-scale topographic heterogeneity.
Main conclusions: The thermal lag of plant assemblages has increased as global warming outpaces thermophilization responses. The current lag is associated with different dimensions of temperature-change velocity at a broad landscape scale, suggesting specifically that limited migration is an important contributor to the observed lags.