Aboveground net primary production (ANPP) and its controls have been thoroughly studied and there is general agreement that precipitation is one of the major drivers of ANPP and its influence decreases from deserts to grasslands and forests as ecosystems become limited by other resources as nutrients or light. On the contrary, belowground net primary production (BNPP) has been less frequently studied because of methodological difficulties. Our rudimentary understanding of the controls of BNPP represents a major void in our understanding of the functioning of ecosystems because BNPP represents the major input of material into the soil. Moreover, predictions of the effects of climate change on carbon storage in these ecosystems hinge on our understanding of the BNPP drivers. Not only are the major controls of BNPP uncertain but its relationship with ANPP is still unclear. Are BNPP and ANPP responding in synchrony to major drivers of ecosystem functioning across biomes? Or, specifically, does water availability affect ANPP and BNPP in similar ways? Or does the effect of water availability on BNPP offset the ANPP response? Few studies have explored the effect of precipitation on the fraction of total primary production allocated belowground. Our work explores hypotheses regarding how changes in water availability affect the fraction of belowground net productivity relative to total production and how the effect of water availability changes across spatial gradients of long-term mean precipitation from deserts to tropical forests. In order to test these hypotheses, we synthesized complementary datasets from field and satellite observations. We estimated belowground productivity by subtracting aboveground productivity from total net primary production from 80 sites worldwide. We developed statistical models that explained patterns of productivity responses to precipitation for different biomes.
Results/Conclusions
ANPP increased with mean annual precipitation across sites (F1,77=119.9, P<0.001) but at a higher rate than BNPP (F1,77=7.475, P=0.007). The fraction of NPP allocated to belowground decreased with mean precipitation (F1,77=7.475, P=0.007) and did not change among biomes indicating a somewhat universal productivity partitioning trend. Different biomes showed similar ANPP responses but different BNPP responses to precipitation where grasslands, shrublands and savannas showed a positive BNPP response whereas tropical and temperate forests showed a negative BNPP response. Our results highlight the relative importance of precipitation versus other resources in the allocation of total NPP between above and belowground. This study contributes to a better understanding of the functioning of ecosystems and their ability to sequester carbon.