Above and below ground traits vary in response to soil moisture availability and plant competition

Functional traits have been used to explain plant responses to changes in water availability. Our understanding of plant responses to biotic and abiotic drivers is largely based on above-ground plant traits. Due to the limited consideration of subterranean plant characteristics, a general overview of plant water use strategies remains elusive.

In a study published in Annals of BotanyResearchers at the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences attempted to study and compare the response of above- and below-ground traits and patterns of biomass allocation to soil moisture and plant competition treatments.

Researchers selected seedlings from five tropical tree species and grew them in a greenhouse under a soil moisture gradient (low (drought), medium, and high (well-watered) humidity) with and without intraspecific competition for 16 weeks. At harvest, they measured nine above-ground and five below-ground characteristics of all seedlings based on standard protocols.

They found that above- and below-ground traits exhibit heterogeneous responses and conflicting correlations to soil moisture availability and plant competition. In response to the soil moisture gradient, aboveground traits were found to be consistent with the foliar economy spectrum, while belowground traits were mismatched with the root economy spectrum. Specific leaf area was positively correlated with specific root length, while negatively correlated with average root diameter across moisture levels. However, leaf dry matter content unexpectedly correlated positively with both specific root length and root branching index.

Intraspecific competition affected both above- and below-ground traits, but interacted with soil moisture to affect only below-ground traits. However, the allocation of biomass to above- and below-ground plant organs across the soil moisture gradient supports the functional equilibrium theory, in which relatively larger proportions of biomass were allocated to organs that capture limited water resources for growth.

“Our results indicate that the response of below-ground traits to intraspecific competition and soil moisture conditions of plants may not be inferred from above-ground traits. The plants can employ a range of ecological strategies in response to different environmental changes,” said XTBG’s Yang Jie.