Drivers of phenological transitions in the seedling life stage

Plant functional ecology research has primarily focused on juvenile and
adult plants even though regeneration from seed can be the most
consequential life-history bottleneck with cascading influence on later
stages of growth and reproduction. Understanding relationships among
phenology, morphology, and growth-related functional traits has improved
our knowledge of plant life-history strategies and adaptive responses to
changing climate. However, whether relationships among phenological and
morpho-physiological traits exist during plant regeneration is unknown. We
also lack understanding of the relative importance of these relationships
compared with those of regeneration phenology with other factors like
plant phylogeny, geographic location, and whether a species is native or
non-native to the location.  To better understand these gaps in
knowledge, we evaluated three phenological traits (days to germination,
first and third true leaves) and six morpho-physiological traits (seed
mass, relative growth rate, root elongation rate, root:shoot ratio,
specific leaf area, seedling C:N) associated with regeneration for 131
forb species from six globally distributed grasslands. 
Morpho-physiological traits showed several significant correlations with
phenological traits. Boosted regression trees revealed that their relative
importance in predicting phenological traits varied among the three
phenological stages (34-51%). Interestingly, the relative importance of
morpho-physiological traits on the phenological stages was comparable to
that of phylogeny (36-46%). In general, species with faster phenologies
produced seedlings that grew faster. The influence of geographic location
on phenological traits was strongest at germination (29%) and decreased
(8-15%) at later phenological stages. Native vs non-native origin had
little to no impact (0-2%) on regeneration phenology.  Strong
relationships between days to germination and geographic location indicate
signatures of local adaptation in the earliest life stages. Similar
morpho-physiological trait values between native and non-native forbs
imply that trait matching may be essential for non-native establishment.
While associations between phenological and morpho-physiological traits
during regeneration have not been previously recognized, our results
suggest that these are complex and variable across plant regeneration.
Better understanding of these associations and their variation across
plant life stages may help capture species shifts with ongoing climate
change and be used to develop novel approaches to seed-based restoration.