Spring Forward: Reproductive Phenology of the Holoparasite Lathraea squamaria (Orobanchaceae).

Lathraea is a peculiar genus of holoparasitic plants in the Orobanchaceae. In addition to their unusual early development, plants in this genus remain below ground during most of their life cycle, deriving nutrients from the roots of various deciduous trees. In Lathraea squamaria, known as common toothwort, plants can persist underground for up to a decade before initiating flowering aboveground. To assess the effects of climate variability on the reproductive phenology and seed output of this species, we conducted a 14-year population monitoring study. Our data show that the average onset of flower anthesis and seed dispersal have shifted -0.4 and -0.3 days/year over time, respectively. This resulted in these phenophases stating 5 days (anthesis) and 9 days (seed release) earlier in 2021 compared to 2007. Nevertheless, these phenological changes were not significantly correlated with local temperature and precipitation, suggesting that developmental timing in L. squamaria may be more influenced by host-derived physiological cues. Indeed, early flowering has also been reported by one of the most common host species in the region, Carpinus betulus, the European hornbeam. Earlier flowering of common toothwort may also lead to temporal mismatches with pollinators, such as bumblebees. These findings underscore the importance of host-parasite synchrony in understanding the ecological resilience of holoparasitic plants under changing environmental conditions.