Changes in above- versus belowground biomass distribution in permafrost regions in response to climate warming

Abstract

Permafrost regions contain approximately half of the carbon stored in land ecosystems and have warmed at least twice as much as any other biome. This warming has influenced vegetation activity, leading to changes in plant composition, physiology, and biomass storage in aboveground and belowground components, ultimately impacting ecosystem carbon balance. Yet, little is known about the causes and magnitude of long-term changes in the above- to belowground biomass ratio of plants (η). Here, we analyzed η values using 3,013 plots and 26,337 species-specific measurements across eight sites on the Tibetan Plateau from 1995 to 2021. Our analysis revealed distinct temporal trends in η for three vegetation types: a 17% increase in alpine wetlands, and a decrease of 26% and 48% in alpine meadows and alpine steppes, respectively. These trends were primarily driven by temperature-induced growth preferences rather than shifts in plant species composition. Our findings indicate that in wetter ecosystems, climate warming promotes aboveground plant growth, while in drier ecosystems, such as alpine meadows and alpine steppes, plants allocate more biomass belowground. Furthermore, we observed a threefold strengthening of the warming effect on η over the past 27 y. Soil moisture was found to modulate the sensitivity of η to soil temperature in alpine meadows and alpine steppes, but not in alpine wetlands. Our results contribute to a better understanding of the processes driving the response of biomass distribution to climate warming, which is crucial for predicting the future carbon trajectory of permafrost ecosystems and climate feedback.

Why we need long-term monitoring to understand ecosystem change

“… The findings of Yun et al ( 8 ) can be simplified as over the past 27 y in ecosystems with an abundance of soil moisture (wetlands) plants allocated proportionally more resources toward above-ground structures, while in ecosystems with limited soil moisture (meadows and steppes) plants allocated proportionally more resources toward below ground structures. Note in these cold-dominated ecosystems of the Tibetan Plateau approximately two-thirds of the biomass is below ground (illustration by Judy C. Hollister).…”

“…Often, researchers assume that changes in belowground biomass either correspond with changes in aboveground biomass or that it follows changes in species composition; for example, an increase in the cover of woody plant would result in more biomass aboveground because woody plants have a higher above to belowground biomass ratio ( 13 ). The findings of Yun et al ( 8 ) show that the changes in the ratio of above to belowground biomass were primarily explained by changes within a species and only partially by changes in the abundance of species in the plots. In other words, these long-lived plants changed their allocation of resources toward above-ground biomass where water was plentiful or toward belowground biomass where water was limited.…”

“…What is less understood, and much more difficult to track, is how plants may be changing their belowground biomass ( 6 , 7 ). In PNAS, Yun et al ( 8 ) document changes in above and belowground biomass over 27 y across what many call the third pole, the Tibetan Plateau. They found, in wetlands, aboveground biomass increased over time and the increase was strongly linked with regional warming.…”