Aboveground sink strength in forests controls the allocation of carbon below ground and its [CO ₂ ]-induced enhancement

Abstract

The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO 2 concentration ([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 CO2 enrichment (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) of Pinus taeda L. was altered through nitrogen fertilization, ice-storm damage, and droughts, changes in L, 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 Ϸ50% (700 g C m ؊2 y ؊1 ) at the lowest LD to Ϸ30% (200 g C m ؊2 y ؊1 ) at the highest LD. The consistency of the trend in TBCA with L and its response to [CO2] across the sites provides a norm for predictions of ecosystem C cycling, and is particularly useful for models that use L to estimate components of the terrestrial C balance.aboveground net primary production ͉ free-air CO 2 enrichment ͉ leaf area index ͉ nitrogen fertilization ͉ soil respiration I n terrestrial ecosystems, the largest and most recalcitrant carbon (C) pools are found in soils (1). Thus, assessing long-term C sequestration potential of these ecosystems requires understanding of the processes that control the dynamics of soil C. The buildup of soil C is controlled in part by the input of aboveground litter and the allocation of C below ground. Belowground C allocation by plants supports root production, respiration, rhizodeposition, and mychorrhizal fungi (2). Only a small fraction of C allocated below ground is retained by soils in recalcitrant pools (3). However, because primary productivity in forests is large, even a small change in the total belowground C allocation (TBCA), e.g., under elevated atmospheric CO 2 concentration ([CO 2 ]), can alter terrestrial C storage.In forest ecosystems, TBCA, i.e., the flux of C belowground, has been shown to be comparable with or greater than, the aboveground net primary productivity (ANPP) (4). Yet the controls of TBCA are poorly understood, leading to unreliable estimates of the soil C dynamics under current climatic and atmospheric conditions. The reliability of estimates decreases further when predictions are made for global change scenarios because few data are available from long-term ecosystem-level CO 2 enrichment experiments (4). Here, we combine new and published data from free-air CO 2 enrichment (FACE) experiments and show that, when canopy leaf area index (L) is known, reasonable predictions of TBCA can be made under both current and future climate scenarios.C allocation is commonly quantified as the partitioning...

Climate change impacts on growth and carbon ­balance of forests in Central Europe

“…Despite many simplifications, such as keeping inherently dynamic parameters constant (e.g. the partitioning of C allocation between aboveground and belowground compartments, or stand mortality) (Palmroth et al 2006), process-based models deliver broad sets of output variables that forest managers may need to predict future forest development (Pretzsch 2009). Mäkelä & Valentine (2001) argue that stand-growth models can give correct answers for the wrong reasons, and that statistical calibration of such models should therefore be avoided whenever possible.…”

Ecosystem effects of CO<sub>2</sub> concentration: evidence from past climates

“…Allocation of carbon to fine roots is increased at high c i , suggesting an adaptive response to an overall increased N demand (e.g. Palmroth et al, 2006). Provided that N demands are met, increased c a should lead to increased NPP, and reduced c a should lead to reduced NPP, although changing allocation patterns might limit the magnitude of this response.…”

Abundance and community structure of ammonia-oxidizing bacteria and archaea in a temperate forest ecosystem under ten-years elevated CO2

“…Previous studies at the Duke free-air CO 2 enrichment (FACE) site indicated that both elevated CO 2 and N fertilization altered soil microbial community composition based on phospholipid fatty acid (PLFA) profiling (Feng et al, 2010), whereas another study suggested that the bacterial community composition was more influenced by spatial variations than the CO 2 enrichment (Ge et al, 2010). Increased allocation of C (carbohydrates) into the soil under elevated CO 2 (Palmroth et al, 2006) and enhanced soil N availability with N fertilization have led to the shifts in the abundance and composition of soil microbial communities . Reports on the responses of AOB to elevated CO 2 are controversial in the literature.…”

“…Yet, a four-year exposure to CO 2 in a FACE experiment site decreased the abundance of archaea but did not affect the population size of soil bacteria under trembling aspen (Lesaulnier et al, 2008). It was believed that elevated CO 2 increased plant photosynthetic rate and productivity, and the increase in the above-ground photosynthesis will result in greater below-ground C allocation via root exudation and rhizodeposition (Hungate et al, 1997;Jackson et al, 2009;Palmroth et al, 2006;Phillips et al, 2006), which can directly stimulate changes in the size and community structure of soil microorganisms (Drigo et al, 2009). However, no significant difference in DOC between ambient CO 2 and elevated CO 2 was observed in this study which was partly in consistent with the study of Jackson et al (2009), who found that no significant change of DOC between ambient and elevated CO 2 condition in the O horizon of soil.…”