The timing of plant growth (phenology) impacts carbon fluxes, resource use, and interspecific interactions. Aboveground phenology is one of the strongest indicators of climate change because leaves generally grow earlier in the year when temperatures are warmer. In contrast, controls on the timing of root growth are poorly understood. Current climate models represent root phenology as a synchronous portion of aboveground growth, but root and shoot growth may be asynchronous. Roots are an important component of total plant phenology because they can comprise up to 75% of total plant biomass and contribute considerably to carbon cycling.
We reviewed 65 studies of root phenology and found that only 29% examined root phenology quantitatively. In order to rigorously examine factors that influence root phenology, we quantitatively examined two long-term datasets. In these datasets, we tracked the timing of grape root production over five years in Fredonia, New York under a humid, continental climate and over three years in Oakville, California, under a Mediterranean-type climate. In both climates, we examined whether belowground phenology related to the timing of aboveground growth. We also compared interannual variation between above- and belowground phenology and determined the influence of environmental factors on key root phenological phases.
In both New York and California, root phenology was not tightly linked to aboveground phenological periods. Additionally, root phenology was more variable between years than aboveground phenology and was less related to temperature. The timing of peak root growth was influenced by temperature, but controls on spring root initiation and fall root growth cessation were less clear. These findings suggest that belowground phenology does not directly track aboveground phenology, due to higher interannual variability of roots than shoots and the implication that root initiation and cessation are controlled by different factors than shoot growth. If aboveground grape phenology is altered by air temperature and root phenology is not, global warming may further uncouple above- and belowground phenology, increasing inaccuracies in current climate models.