Decadal biomass increment in early secondary succession woody ecosystems is increased by CO2 enrichment

Abstract

Increasing atmospheric CO2 stimulates photosynthesis which can increase net primary production (NPP), but at longer timescales may not necessarily increase plant biomass. Here we analyse the four decade-long CO2-enrichment experiments in woody ecosystems that measured total NPP and biomass. CO2 enrichment increased biomass increment by 1.05 ± 0.26 kg C m−2 over a full decade, a 29.1 ± 11.7% stimulation of biomass gain in these early-secondary-succession temperate ecosystems. This response is predictable by combining the CO2 response of NPP (0.16 ± 0.03 kg C m−2 y−1) and the CO2-independent, linear slope between biomass increment and cumulative NPP (0.55 ± 0.17). An ensemble of terrestrial ecosystem models fail to predict both terms correctly. Allocation to wood was a driver of across-site, and across-model, response variability and together with CO2-independence of biomass retention highlights the value of understanding drivers of wood allocation under ambient conditions to correctly interpret and predict CO2 responses.

Understanding the uncertainty in global forest carbon turnover

“…Reduced rates of mortality due to elevated atmospheric CO2 concentration (H7b) are conceptually included in five of the TBMs through the growth efficiency process (Table 3) and is evident in the overall response for two of them (Table 5). Such behaviour 540 follows well-established leaf-level responses of photosynthesis and water-use efficiency to atmospheric CO2 concentration, and is supported by detailed stand-level modelling (Liu et al, 2017), but is hard to verify with observations in mature trees (Walker et al, 2019). If trees expend their extra NPP on growing proportionally larger, thereby increasing their respiration demands, then the positive effect of enhanced NPP could be offset.…”

The fate of carbon in a mature forest under carbon dioxide enrichment

“…Whether mature forests will remain carbon sinks into the future is of critical importance for aspirations to limit climate warming to no more than 1.5 °C above pre-industrial levels 13 . Free-Air CO 2 Enrichment (FACE) experiments provide an opportunity to determine the capacity of ecosystems to sequester carbon under the higher atmospheric CO 2 concentrations expected in the future 3,4,5, 7, 8, 10, 11 . Evidence gathered from the four first generation forest FACE experiments, which all measured responses of rapidly-growing young forest plantations, has generally indicated a strong CO 2 fertilization effect on biomass growth 3, 4 .…”

“…Free-Air CO 2 Enrichment (FACE) experiments provide an opportunity to determine the capacity of ecosystems to sequester carbon under the higher atmospheric CO 2 concentrations expected in the future 3,4,5, 7, 8, 10, 11 . Evidence gathered from the four first generation forest FACE experiments, which all measured responses of rapidly-growing young forest plantations, has generally indicated a strong CO 2 fertilization effect on biomass growth 3, 4 . This CO 2 fertilization effect has been hypothesized to be one of the largest drivers of the terrestrial carbon sink and its acceleration in recent decades 14 , potentially accounting for up to 60% of present-day terrestrial carbon sequestration 2 .…”

“… AbstractAtmospheric carbon dioxide enrichment (eCO 2 ) can enhance plant carbon uptake and growth 1,2,3,4,5 , thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO 2 concentration 6 . While evidence gathered from young aggrading forests has generally indicated a strong CO 2 fertilization effect on biomass growth 3,4,5 , it is unclear whether mature forests respond to eCO 2 in a similar way.…”

“… AbstractAtmospheric carbon dioxide enrichment (eCO 2 ) can enhance plant carbon uptake and growth 1,2,3,4,5 , thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO 2 concentration 6 . While evidence gathered from young aggrading forests has generally indicated a strong CO 2 fertilization effect on biomass growth 3,4,5 , it is unclear whether mature forests respond to eCO 2 in a similar way. In mature trees and forest stands 7,8,9,10 , photosynthetic uptake has been found to increase under eCO 2 without any apparent accompanying growth response, leaving an open question about the fate of additional carbon fixed under eCO 2 4, 5, 7,8,9,10,11 .…”