Tree species identity shapes the relationship between canopy cover and herb‐layer species in temperate forests

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Abstract The composition of canopy tree species is known to impact the distribution of herb‐layer species through various factors, such as light availability, soil structure and composition. However, relationships between these strata have been rarely demonstrated empirically across a wide range of species and large spatial areas, which could help understand and spatially predict herb‐layer species distribution patterns. We examined the response of 85 herb‐layer species occurrence (presence/absence) distribution to the cover of 105 tree species across 7255 sites in mainland France. By accounting for site characteristics and canopy diversity, we ensured that associations between herbaceous and tree species were not a reflection of these factors. We then predicted the occurrence distribution of herbaceous species by considering these environmental factors and comparing them with the addition of individual tree species cover. Our models revealed that tree species identity cover relationships are important for all herb‐layer species and with some strong associations between these two strata. While all herb‐layer species had increased explanatory and predictive model performance from the information added by tree species identity cover, non‐forest and open vegetation species showed the greatest increase. Non‐forest generalist species are those with the most negative associations with these canopy tree species. Models also highlighted that tree species identity cover cannot be fully replaced by unidimensional functional tree groups based on trophic, dynamic, hydric or life form classification. Our study emphasizes that incorporating tree species identity composition can enhance the prediction of herb‐layer species occurrence distribution at new sites. Consequently, we provide an empirical demonstration of improved prediction leveraging on species associations, which synthesizes various factors challenging to measure in situ. This knowledge could guide future experiments to explore mechanisms involved, especially for the high‐magnitude associations detected. Synthesis. Our study is an empirical demonstration of the importance of tree species identity cover in shaping the distribution of the herb layer, surpassing the effects of tree diversity alone. By considering the significance of tree species identity cover, we can inform forest management and conservation measures, improve spatial prediction of herb‐layer species distribution and better understand herb layer and tree species relationships.