Phenological turnover matters when making trait-based predictions of plant-pollinator interactions

1. Understanding the processes determining species interactions is key to predicting and safeguarding ecological networks under rapid environmental change. One approach to estimating interactions is to use morphologies of taxa interacting across trophic levels to reveal suites of traits they are more likely to interact with (i.e. a trait niche). 2. Previous work studying these morphological trait niches has typically used interactions between species that are pooled in space and time. However, species assemblages, and the traits of individuals within species, can change across even small landscapes over a season, leading to morphological trait space being dynamically reshaped. Therefore, it is unclear how morphological trait turnover affects our inferences of trait niches, and our ability to answer this is in part limited by a lack of individual-level trait data. 3. Here, we directly address this by studying a montane Arctic plant-pollinator community over five growing seasons (>1,300 hours of fieldwork). Specifically, we linked every recorded plant-bumblebee interaction with the traits of the bee individual involved (n = 1,150), to investigate 1) whether plant taxa (n = 10) exhibited bee trait niches by interacting with specific regions of multidimensional trait space of visiting bumblebees, and 2) how our inference of these trait niches was affected by considering bumblebee trait turnover and plant taxon turnover. 4. When not considering turnover (interactions in space and time are pooled), plant taxa demonstrated bee trait niches. However, next we considered how bee trait space is reshaped over the elevational and seasonal gradient (especially with the emergence of different castes), and how this reshaping co-occurs with different spatiotemporal ranges of the plant taxa. From this we found the degree to which plant taxa exhibited trait niches declined significantly, and that seasonal reshaping of bee trait space was the primary driver of this trend. 5. Overall, in highly dynamic systems, like the Arctic, overlooking community turnover could mask and even overestimate the ability of morphology to explain interactions. Hence, determining how morphological traits of individual interaction partners are in phenological synchrony at localised scales will be fundamental to understanding the role morphology plays in underpinning plant-pollinator interactions.