What is the relationship between above and belowground phenology?: A meta-analysis and case study

Background/Question/Methods

Roots are hidden from view and heterogeneously distributed making them difficult to study in situ. As a result, the causes and timing of root production are not well understood. Researchers have long assumed that above and belowground phenology is synchronous; for example, most parameterizations of belowground carbon allocation in terrestrial biosphere models are based on allometry and represent a fixed fraction of net C uptake.  However, using results from meta-analysis as well as empirical data from Harvard Forest, we show that synchronous root and shoot growth is the exception rather than the rule. We collected root and shoot phenology measurements from studies across four biomes (boreal, temperate, Mediterranean, and subtropical). General patterns of root phenology varied widely with up to six growth peaks in one growing season. In the majority of cases maximum shoot production occurred before root production. To explore the range of phenological relationships within woody plants in the temperate biome, we compared above and belowground phenology in three common northeastern tree species, Quercus rubra, Tsuga canadensis, and Fraxinus americana. Greenness index, fine root production, respiration, and exudation were measured beginning in April 2012 through October 2013 at the Harvard Forest in Petersham, MA, USA. 

Results/Conclusions

In the meta-analysis, root growth was earlier in study locations with higher median annual temperature and mean annual precipitation (F_1,85=10.64, p=0.0016). Dominant growth form also influenced offset; conifer root growth peaked on average 44.3±11.8 days later than deciduous tree species (F_1,32=7.52, p=0.009). At Harvard Forest, greenness index peaked in late May and early June with one clear maximum growth period. In contrast, root growth was characterized by multiple production peaks. Q. rubra and F. americana root growth peaked in early and mid-summer, while T. canadensis root growth peaked later in the growing season, resulting in greater asynchrony in the phenology of coniferous T. canadensis. Of the study species, Q. rubra allocated the most C to root growth, though it allocated the lowest fraction of GPP to roots in the form of growth, respiration and exudation. Timing of belowground C allocation to fine roots may be regulated by climate factors as well as endogenous factors such as growth form and tradeoffs in C allocated between plant organs. Plant roots supply substrate to microbial communities via root turnover and exudation, hence their production feeds back to other plant and soil processes that affect ecosystem C fluxes.