Changes In Carbon Use Efficiency Research Articles

Changes in global carbon use efficiency in the 21st century and the potential controlling factors

Extensive studies have demonstrated the spatiotemporal changes in carbon use efficiency (CUE) and its driving factors over the past three decades. However, how the global CUE will change and to what extent the CUE is affected by the dominant factor in this century is still unclear. Herein, based on CMIP6 model outputs, we estimated the situation and change trends of CUE in baseline (1982–2014) and future (2015–2100), and identified the controlling factor of CUE variation by boosted regression tree. Further, we predicted the CUE-controlling factor sensitivity (S value, referring to higher/lower controlling factor producing more/less CUE) and its variation under four representative pathways, and revealed the relationship between S value and social economy. Results showed decreased CUE at the end of the 21st century, especially in the SSP5-8.5, its decline rate of CUE is 1.2 × 10−2 ± 5.2 × 10−4/decade, which is 10 times higher than that in the SSP1-2.6. Spatially, 56.9%, 74.5%, 83.1%, and 88.6% of the global land will exhibit a decreased CUE under the SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios, and primarily concentrates at the middle-high latitudes of the Northern Hemisphere (30°–60° N). Except in Africa, temperature is the controlling factor for CUE variation, and the S value decreases over time, indicating an enhanced inhibitory effect of temperature on CUE. The turning time of S value change will advance with increases in CO2 emission, presenting prolonged high-temperature stress of vegetation ecosystem under high-emission scenarios. A threshold effect can be found between S value change and precipitation, and the precipitation threshold is higher under the SSP5-8.5 scenario. The negative effect of temperature on CUE is attenuated by economic development and population control but this effect diminishes with rising CO2 concentrations; in the future, developing clean energy and formulating population management policies can be used to enhance the carbon sink ability of the global ecosystem.

Altered activities of extracellular soil enzymes by the interacting global environmental changes.

Soil enzymes are crucial in mediating ecosystems' responses to environmental drivers, so that the comprehension of their sensitivity to drivers of global change can help make predictions of future scenarios and design tailored interventions of biomanipulation. Drivers of global change usually act in combination of two or more, and indirect effects of one driver acting through modification of another one often occur, yet most of both manipulative and meta-analysis studies available tend to focus on the direct effect of one single driver on the activity of specific soil enzymes. One of the biggest challenges is, therefore, represented by the difficulty in assessing the interactions between different drivers, due to the complexity of disentangling the single direct effects from the indirect and combined ones. In this review, after elucidating the general mechanisms of soil enzyme production and activity regulation, we display the state-of-the-art knowledge on direct, indirect and combined effects of the main drivers of global change on soil enzyme activities, identify gaps in knowledge and challenges from research, plus we analyse how this can reverberate in the future of biomanipulation techniques for the improvement of ecosystem services. We conclude that qualitative but not quantitative outcomes can be predicted for some interactions such as warming + drought or warming + CO2 , while for other ones, the results are controversial: future basic research will have to center on this holistic approach. A general trend toward the overall increase of soil enzyme activities and acceleration of biogeochemical cycles will persist, until an inflection will be caused by factors such as future shifts in microbial communities and changes in carbon use efficiency. Applied research will develop toward the refinement of "in situ" analytical systems for the study of soil enzyme activities and the support of bioengineering for the better tailoring of interventions of biomanipulation.

The linkages between photosynthesis, productivity, growth and biomass in lowland Amazonian forests

Understanding the relationship between photosynthesis, net primary productivity and growth in forest ecosystems is key to understanding how these ecosystems will respond to global anthropogenic change, yet the linkages among these components are rarely explored in detail. We provide the first comprehensive description of the productivity, respiration and carbon allocation of contrasting lowland Amazonian forests spanning gradients in seasonal water deficit and soil fertility. Using the largest data set assembled to date, ten sites in three countries all studied with a standardized methodology, we find that (i) gross primary productivity (GPP) has a simple relationship with seasonal water deficit, but that (ii) site-to-site variations in GPP have little power in explaining site-to-site spatial variations in net primary productivity (NPP) or growth because of concomitant changes in carbon use efficiency (CUE), and conversely, the woody growth rate of a tropical forest is a very poor proxy for its productivity. Moreover, (iii) spatial patterns of biomass are much more driven by patterns of residence times (i.e. tree mortality rates) than by spatial variation in productivity or tree growth. Current theory and models of tropical forest carbon cycling under projected scenarios of global atmospheric change can benefit from advancing beyond a focus on GPP. By improving our understanding of poorly understood processes such as CUE, NPP allocation and biomass turnover times, we can provide more complete and mechanistic approaches to linking climate and tropical forest carbon cycling.

Theoretical analysis of change in forest carbon use efficiency with stand development: A case study on Hinoki Cypress ( Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) plantation from the seedling stage

Changes in carbon use efficiency (CUE), which is defined as the ratio of net primary production (NPP) to gross primary production (GPP), were estimated for the aerial parts of the Hinoki Cypress ( Chamaecyparis obtusa (Sieb. et Zucc.) Endl.) with respect to stand development. The analysis incorporated previously published data from the early stages of stand development, namely the seedling stages of the cypress. For this analysis, a simple mathematical model to assess the changes in CUE was developed by incorporating data on physiological variables and mass of woody species. The CUE tended to increase with increases in the aboveground biomass of the stand, and then decreased gradually despite increases in the aboveground biomass. The CUE-value (0.28, 0.39) of the seedling stage was lower than that (0.33–0.58) of the young or mature trees. To examine the effect of physiological variables and mass on CUE, the ratios of the specific respiration rate to the specific photosynthetic rate ( r/ a) and the leaf biomass to the aboveground biomass or leaf mass ratio ( y L / y T ) were calculated. The low value of CUE at the seedling stage was due to the high ratio of specific respiration rate to specific photosynthetic rate r/ a, but was not due to the high value of the leaf mass ratio y L / y T . In addition, the decline in CUE associated with older stages of stand development was due to the decreasing changes in y L / y T , and the r/ a ratio did not influence the change in CUE.

Mathematical analysis of change in forest carbon use efficiency with stand development: A case study on Abies veitchii Lindl.

Changes in carbon use efficiency (CUE), which is defined as the ratio of net primary production (NPP) to gross primary production (GPP), were analyzed for Abies veitchii Lindl. forests with respect to stand development by developing a simple mathematical model incorporating data on physiological variables and leaf mass ratio. A decrease in CUE with stand development was successfully expressed as a function of stand biomass ( y) based on the following three assumptions: (1) a power-law relationship between mean respiration and mean individual tree mass, (2) a power-functional relationship between mean gross primary production and mean individual tree mass, and (3) self-thinning relationship between stand biomass and density. Based on this model, a parameter of CUE– y relationship was defined, and it was clarified that CUE decrease with stand development is caused not by the ratio of specific respiration rate to specific gross photosynthetic rate, but by leaf mass ratio. Since CUE is high in young forests, helpful information on selecting woody species when planting seedlings was provided from the viewpoints of reducing CO 2 in the atmosphere and global warming.

Night temperature has a minimal effect on respiration and growth in rapidly growing plants.

Carbon gain depends on efficient photosynthesis and adequate respiration. The effect of temperature on photosynthetic efficiency is well understood. In contrast, the temperature response of respiration is based almost entirely on short-term (hours) measurements in mature organisms to develop Q(10) values for maintenance and whole-plant respiration. These Q(10) values are then used to extrapolate across whole life cycles to predict the influence of temperature on plant growth. In this study, night temperature in young, rapidly growing plant communities was altered from 17 to 34 degrees C for up to 20 d. Day temperature was maintained at 25 degrees C. CO(2) gas-exchange was continuously monitored in ten separate chambers to quantify the effect of night-temperature on respiration, photosynthesis and the efficiency of carbon gain (carbon use efficiency). Respiration increased only 20-46 % for each 10 degrees C rise in temperature (total respiratory Q(10) of between 1.2 to about 1.5). This change resulted in only a 2-12 % change in carbon use efficiency, and there was no effect on cumulative carbon gain or dry mass. No acclimation of respiration was observed after 20 d of treatment. These findings indicate that whole-plant respiration of rapidly growing plants has a small sensitivity to temperature, and that the sensitivity does not change among the species tested, even after 20 d of treatment. Finally, the results support respiration models that separate respiration into growth and maintenance components.

Long‐term carbon budget of the above‐ground parts of a young hinoki cypress (Chamaecyparis obtusa) stand

The carbon budget of the above‐ground parts of a young hinoki (Chamaecyparis obtusa) stand was analyzed over a 4‐year period to evaluate trends in changes in carbon use efficiency and growth conversion (biosynthetic) efficiency with stand development. Litter production of the stand was estimated from the stem cross‐sectional area at the crown base. A biomass increment was estimated using the stem volume of individual trees in the stand, measured at monthly intervals. Net production, estimated from litter production and the biomass increment, was 7.40, 8.44, 8.45 and 8.29 Mg C ha−1 year−1 for Years I–IV, respectively. The respiration rate of the entire above‐ground parts of selected sample trees were measured at monthly intervals using the enclosed whole‐tree method. The Q10 value of respiration decreased with increasing air temperature. Respiration rate was partitioned into growth and maintenance components using a two‐component functional model. The maintenance respiration coefficient increased in the following order: winter, spring, autumn and summer. The maintenance respiration coefficient also decreased with either stand development or age for all seasons. The growth respiration coefficient, which did not vary with stand development, was 0.69 ± 0.08 (mean ± SE), 0.61 ± 0.03, 0.54 ± 0.03 and 0.67 ± 0.07 g C g−1 C for winter, spring, summer and autumn, respectively. The growth conversion efficiency of the stand was 0.76, 0.72, 0.72 and 0.75 for Years I–IV, respectively. Carbon use efficiency was estimated to be 0.58, 0.57, 0.54 and 0.53 for Years I–IV, respectively. The hypothesis that respiration reduces productivity in old stands could not be validated for this hinoki stand.