Who’s on first?: Comparisons of above- and belowground phenological variation among native and invasive annuals
Phenological variation among coexisting species is an often overlooked and yet critically important factor in community assemblage and invasibility. Variation in community diversity can result solely from differences in the order of establishment or onset of growth. In plant communities early phenologies (e.g. the timing of germination, flowering, etc.) can lead to seasonal priority effects. That is, early root and shoot growth can usurp resources, thereby inhibiting establishment, growth and reproduction of species with later activities through resource preemption. Within habitat comparisons have shown that invasive species tend to have earlier growth activity relative to natives. However, the majority of studies limit quantification to one phenophase (flowering) and neglect important belowground dynamics. In the present study we sought to quantify above- and belowground activity of native and invasive annuals and asked whether these patterns were indicative of priority effects underlying the competitive superiority of invasive species. Phenological state, density, relative growth rates and cover were quantified aboveground. Root growth dynamics by depth were quantified via an automated mini-rhizotron imaging system.
Over the first year of observation phenological data was collected for 18 species - 4 invasive and 14 native. As predicted, invasive species displayed earlier activity (F = 202.7, p < 0.001), covered a greater percentage of ground earlier (F = 5.8, p < 0.001) and tended to dieback as natives continued to increase later in the season. In contrast to expectations, natives displayed faster relative growth rates and greater survivorship than invasives. Belowground observations did not reveal any significant patterns of rooting dynamics in depth or time. This is surprising since literature on soil resource acquisition by invasive species indicate use of a greater amount of resources, faster than natives. Our data indicate that priority effects might be in play. However, the discrepancy between above- and belowground activities is intriguing and deserves further investigation.