Aboveground sink strength in forests controls the allocation of carbon below ground and its [CO2]-induced enhancement

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Abstract
The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO2concentration ([CO2]) determines the enhancement in C sequestration, yet no clear partitioning rules exist. Here, we used first principles and published data from four free-air CO2enrichment (FACE) experiments on forest tree species to conceptualize the total allocation of C to below ground (TBCA) under current [CO2] and to predict the likely effect of elevated [CO2]. We show that at a FACE site where leaf area index (L) ofPinus taedaL. was altered through nitrogen fertilization, ice-storm damage, and droughts, changes inL, reflecting the aboveground sink for net primary productivity, were accompanied by opposite changes in TBCA. A similar pattern emerged when data were combined from the four FACE experiments, using leaf area duration (LD) to account for differences in growing-season length. Moreover, elevated [CO2]-induced enhancement of TBCA in the combined data decreased from ≈50% (700 g C m−2y−1) at the lowestLDto ≈30% (200 g C m−2y−1) at the highestLD. The consistency of the trend in TBCA withLand its response to [CO2] across the sites provides a norm for predictions of ecosystem C cycling, and is particularly useful for models that useLto estimate components of the terrestrial C balance.