Are above and belowground phenology in sync?

Abstract:

Globally, root production accounts for 30-70% of terrestrial net primary productivity and influences decomposition via root production and turnover, carbon (C) allocation to mycorrhizal fungi and root exudation. As recognized aboveground, the timing of phenological events affects terrestrial C balance, yet there is no parallel understanding for belowground phenology. The objective of this study is to use meta-analysis to identify broad patterns in the phenology of root production and its relationship to temperature, soil moisture, and aboveground phenology. Synthesizing 87 observations of whole plant phenology from 40 studies, we found that on average root growth occurs 25±8 days after shoot growth but that the offset between the peak in root and shoot growth varies >200 days across biomes (boreal, temperate, Mediterranean, and subtropical). Growth form also affected phenology, with deciduous trees more synchronous than evergreen trees. This and the temperature differential between air and soil in spring may explain the relatively early shoot compared to root growth in boreal biomes. Root and shoot growth are positively correlated with median monthly temperature and mean monthly precipitation in boreal, temperate, and subtropical biomes. However, a temperature hysteresis in these biomes leads to the hypothesis that internal controls over C allocation to roots are an equally, if not more, important driver of phenology. In addition, a lack of correlation with temperature or precipitation in the Mediterranean biome implies that other mechanisms are driving phenology. The specific mechanism(s) are as yet unclear but are likely mediated by some combination of photosynthate supply, hormonal signaling, and growth form.