The decadal state of the terrestrial carbon cycle: Global retrievals of terrestrial carbon allocation, pools, and residence times

Abstract

The terrestrial carbon cycle is currently the least constrained component of the global carbon budget. Large uncertainties stem from a poor understanding of plant carbon allocation, stocks, residence times, and carbon use efficiency. Imposing observational constraints on the terrestrial carbon cycle and its processes is, therefore, necessary to better understand its current state and predict its future state. We combine a diagnostic ecosystem carbon model with satellite observations of leaf area and biomass (where and when available) and soil carbon data to retrieve the first global estimates, to our knowledge, of carbon cycle state and process variables at a 1°× 1°r esolution; retrieved variables are independent from the plant functional type and steady-state paradigms. Our results reveal global emergent relationships in the spatial distribution of key carbon cycle states and processes. Live biomass and dead organic carbon residence times exhibit contrasting spatial features (r = 0.3). Allocation to structural carbon is highest in the wet tropics (85-88%) in contrast to higher latitudes (73-82%), where allocation shifts toward photosynthetic carbon. Carbon use efficiency is lowest (0.42-0.44) in the wet tropics. We find an emergent global correlation between retrievals of leaf mass per leaf area and leaf lifespan (r = 0.64-0.80) that matches independent trait studies. We show that conventional land cover types cannot adequately describe the spatial variability of key carbon states and processes (multiple correlation median = 0.41). This mismatch has strong implications for the prediction of terrestrial carbon dynamics, which are currently based on globally applied parameters linked to land cover or plant functional types.carbon cycle | biomass | soil carbon | allocation | residence time T he terrestrial carbon (C) cycle remains the least constrained component of the global C budget (1). In contrast to a relatively stable increase of the ocean CO 2 sink from 0.9 to 2.7 Pg C y −1 over the past 40 y, terrestrial CO 2 uptake has been found to vary between a net 4.1-Pg C y −1 sink to a 0.4-Pg C y −1 source, and accounts for a majority of the interannual variability in atmospheric CO 2 growth. The complex response of terrestrial ecosystem CO 2 exchanges to short-and long-term changes in temperature, water availability, nutrient availability, and rising atmospheric CO 2 (2-6) remains highly uncertain in C cycle model projections (7). As a result, there are large gaps in our understanding of terrestrial C dynamics, including the magnitude and residence times of the major ecosystem C pools (8, 9) and rates of autotrophic respiration (10). Moreover, the impact of climatic extremes on C cycling, such as recent Amazon droughts (11), highlights the importance of understanding the terrestrial C cycle sensitivity to climate variability. To understand terrestrial CO 2 exchanges in the past, present, and future, we need to better constrain current dynamics of ecosystem C cycling from regional to global scales.C uptake, allocati...

Biomass turnover time in terrestrial ecosystems halved by land use

“…Complementary to previous attempts to understand determinants of patterns of C turnover time 1,8 , which did not explicitly analyse land use as a covariate, we here aim to explore the role of land use as a determinant of rates and patterns of τ b . By adopting an approach that has proved useful in quantifying land-use effects on ecosystem properties such as net primary production (NPP) 15,20,21 , we here compare τ b of the potential and the actual vegetation.…”

“…Biomass turnover time (τ b ) is a critical parameter of the global carbon cycle and a key vegetation property 1,3,5 . τ b is a SC pot = NPPpot / SC act NPP act (1) decisive parameter for the elemental composition (stoichiometry) of ecosystems, critically influencing the accumulation and availability of chemical elements in ecosystems, rendering τ b a key factor for plant growth dynamics 17 and a crucial determinant of fluxes between terrestrial vegetation and the atmosphere [3][4][5][6][7] .…”

“…τ b is a SC pot = NPPpot / SC act NPP act (1) decisive parameter for the elemental composition (stoichiometry) of ecosystems, critically influencing the accumulation and availability of chemical elements in ecosystems, rendering τ b a key factor for plant growth dynamics 17 and a crucial determinant of fluxes between terrestrial vegetation and the atmosphere [3][4][5][6][7] . Because τ b is an ecosystem property that emerges from the interplay between climate, soil, vegetation type, the chemical composition of the atmosphere, precipitation, and land use, it is highly variable across space and time 8,9 .…”

“…However, patterns and determinants of the variability of τ b are poorly understood 5,9,18 . In particular, the inability of land-cover or plant functional type classifications, which form the basis of many carbon cycle models 8,10 , to comprehensively represent the variability of τ b induces massive uncertainties in predictions of future global carbon cycle dynamics 1,5,8,[10][11][12][13]19 . Thus, improving the understanding of covariates for τ b is central to understanding the biosphere's responses to a changing climate.…”

“…A significant proportion of the variability of τ b within each type of land cover is potentially causedbesides climatic covariates 1,8 -by management effects. Inclusion of robust land management information, including its impacts on soil processes, is key for assessing the fundamental trade-offs between carbon stocks and carbon turnover related to different biomes and land-use systems, and thereby better understand land-atmosphere fluxes of carbon.…”