Plants alter their vertical root distribution rather than biomass allocation in response to changing precipitation

Abstract

Elucidating the variation of allocation pattern of ecosystem net primary productivity (NPP) and its underlying mechanisms is critically important for understanding the changes of aboveground and belowground ecosystem functions. Under optimal partitioning theory, plants should allocate more NPP to the organ that acquires the most limiting resource, and this expectation has been widely used to explain and predict NPP allocation under changing precipitation. However, confirmatory evidence for this theory has mostly come from observed spatial variation in the relationship between precipitation and NPP allocation across ecosystems, rather than directly from the influences of changing precipitation on NPP allocation within systems. We performed a 6‐yr five‐level precipitation manipulation experiment in a semiarid steppe to test whether changes in NPP allocation can be explained by the optimal partitioning theory, and how water requirement of plant community is maintained if NPP allocation is unaltered. The 30 precipitation levels (5 levels × 6 yr) were divided into dry, nominal, and wet precipitation ranges, relative to historical precipitation variation over the past six decades. We found that NPP in both aboveground (ANPP) and belowground (BNPP) increased nonlinearly as precipitation increased, while the allocation of NPP to BNPP (fBNPP) showed a concave quadratic relationship with precipitation. The declined fBNPP as precipitation increased in the dry range supported the optimal partitioning theory. However, in the nominal range, NPP allocation was not influenced by the changed precipitation; instead, BNPP was distributed more in the surface soil horizon (0–10 cm) as precipitation increased, and conversely more in the deeper soil layers (10–30 cm) as precipitation decreased. This response in root foraging appears to be a strategy to satisfy plant water requirements and partially explains the stable NPP allocation patterns. Overall, our results suggest that plants can adjust their vertical BNPP distribution in response to drought stress, and that only under extreme drought does the optimal partitioning theory strictly apply, highlighting the context dependency of the adaption and growth of plants under changing precipitation.

The importance of extreme rainfall events and their timing in a semi‐arid grassland

“…In fact, compared to the Ambient plots, the treatment plots had 80% greater deep root biomass, but only 36% greater shallow root biomass (a non-significant increase). In contrast, a study in the Mongolian semi-arid steppe found that increased precipitation stimulated more shallow than deep root production (Zhang et al, 2019).…”

“…However, our results suggest the opposite-elevated soil moisture from a deluge disproportionally increased deep root production. This difference could be due to the pattern of precipitation addition- Zhang et al (2019) increased the size of each ambient precipitation event throughout the growing season (resulting in many smaller rainfall additions), whereas we added a single large event. A deluge may result in deeper water infiltration into the soil, stimulating deep root production to access this water source, whereas smaller rain events remain in the upper soil layers, stimulating shallow root production (Sala et al, 1992).…”

Distinct fine‐root responses to precipitation changes in herbaceous and woody plants: a meta‐analysis

“…Woody shallow roots showed negative responses to precipitation reduction, which was similar to herbaceous root responses, whereas responses of whole root systems were nearly neutral (Figs b, b). These results suggest that only sampling shallow roots can overestimate root responses of woody plants to reduced precipitation, as plants can shift their root distribution to deeper layers in response to drought stress (López et al , ; Zhang et al , ). Also, in the additional analysis with pot experiment data, the average response ratio of woody fine‐root biomass in precipitation reduction was negative, whereas that in increased precipitation was not significantly different from zero (Fig.…”

Nitrogen Fertilizer Levels Affect the Growth and Quality Parameters of Astragalus mongolica

“…Therefore, the values of above-ground indexes gradually decrease, while the values of underground indexes gradually accumulate. Under optimal partitioning theory, plants should allocate more energy and nutrients to the organ(s) that require(s) the most limiting resources [38].Therefore, in the present study, the root biomass increased sharply from August and constantly accumulated until the later harvest stage (October).…”