Plant phenology, the study of repeated plant lifecycle events, is known to play a key role in ecosystem function, species distributions, nutrient cycling and many other ecosystem processes. While many studies have focused on the timing of phenological events of deciduous angiosperm trees, the ecological implications of the sequence of these events for individual species or genera has received comparatively less attention. In the present study we compiled a database of 32 traits for 72 angiosperm tree species native to the Great Lakes region of North America in order to a) assess the phylogenetic pattern of flower and leaf emergence sequence (FLS), b) test and compare four hypotheses suggested to explain the phylogenetic pattern of FLS (i.e., those related to cold adaptation, water availability, pollination and dispersal, and seed mass), and c) investigate and identify additional traits associated with FLS, including environmental tolerances and floristic/structural traits.
Of the species assessed, 29% were found to flower prior to leaf emergence, and FLS was found to be conserved in 35 of 37 genera. At the species level, FLS was phylogenetically clustered (Fritz’s D = -0.09), while at the genus level there was no evidence of clustering (Fritz’s D = 1.3). Flowering month, minimum precipitation tolerance, and seed mass were each significantly related to FLS (p < 0.05) at the species level. Random forest models indicate that, at the species level, fruit type was the most important variable related to FLS. Fruit types of species that flower prior to leaf emergence included samaras, capsules, legumes and berries. 78% (14 of 18) of samara-producing species flowered before leaf emergence, representing four genera. At the genus level, the latitude of the northern range edge was also found to be important in explaining FLS. Shared structural traits amongst trees that flower prior to leaf emergence suggests that FLS may prove to be an important way to categorize and identify species with high reproductive vulnerability in future climates. Generally, findings suggest the sequence of phenological events is linked to different ecological strategies that need further study especially within the context of global change.