Forest stand and canopy development unaltered by 12 years of CO2 enrichment

Increased leaf area index and efficiency drive enhanced production under elevated atmospheric [CO2 ] in a pine-dominated stand showing no progressive nitrogen limitation

Enhancement of net primary production (NPP) in forests as atmospheric [CO2 ] increases is likely limited by the availability of other growth resources. The Duke Free Air CO2 Enrichment (FACE) experiment was located on a moderate-fertility site in the southeastern US, in a loblolly pine (Pinus taeda L.) plantation with broadleaved species growing mostly in mid-canopy and understory. Duke FACE ran from 1994 to 2010 and combined elevated [CO2 ] (eCO2 ) with nitrogen (N) additions. We assessed the spatial and temporal variation of NPP response using a dataset that includes previously unpublished data from 6 years of the replicated CO2  × N experiment and extends to 2 years beyond the termination of enrichment. Averaged over time (1997-2010), NPP of pine and broadleaved species were 38% and 52% higher under eCO2 compared to ambient conditions. Furthermore, there was no evidence of a decline in enhancement over time in any plot regardless of its native site quality. The relation between spatial variation in the response and native site quality was suggested but inconclusive. Nitrogen amendments under eCO2 , in turn, resulted in an additional 11% increase in pine NPP. For pine, the eCO2 -induced increase in NPP was similar above- and belowground and was driven by both increased leaf area index (L) and production efficiency (PE = NPP/L). For broadleaved species, coarse-root biomass production was more than 200% higher under eCO2 and accounted for the entire production response, driven by increased PE. Notably, the fraction of annual NPP retained in total living biomass was higher under eCO2 , reflecting a slight shift in allocation fraction to woody mass and a lower mortality rate. Our findings also imply that tree growth may not have been only N-limited, but perhaps constrained by the availability of other nutrients. The observed sustained NPP enhancement, even without N-additions, demonstrates no progressive N limitation.

Modelling the impact of forest management and CO2-fertilisation on growth and demography in a Sitka spruce plantation

Afforestation and reforestation to meet 'Net Zero' emissions targets are considered a necessary policy by many countries. Their potential benefits are usually assessed through forest carbon and growth models. The implementation of vegetation demography gives scope to represent forest management and other size-dependent processes within land surface models (LSMs). In this paper, we evaluate the impact of including management within an LSM that represents demography, using both in-situ and reanalysis climate drivers at a mature, upland Sitka spruce plantation in Northumberland, UK. We compare historical simulations with fixed and variable CO2 concentrations, and with and without tree thinning implemented. Simulations are evaluated against the observed vegetation structure and carbon fluxes. Including thinning and the impact of increasing CO2 concentration ('CO2 fertilisation') gave more realistic estimates of stand-structure and physical characteristics. Historical CO2 fertilisation had a noticeable effect on the Gross Primary Productivity seasonal-diurnal cycle and contributed to approximately 7% higher stand biomass by 2018. The net effect of both processes resulted in a decrease of tree density and biomass, but an increase in tree height and leaf area index.

Urban CO2 imprints on carbon isotope and growth of Chinese pine in the Beijing metropolitan region

Rapid urbanization has occurred globally and resulted in increasing CO₂ emissions from urban areas. Compared to natural forests, urban forests are subject to higher atmospheric CO₂ concentrations in view of strong urban-periurban-rural gradients of CO₂ emissions. However, relevant insights in the CO₂-associated urban imprints on the physiology and growth of regional forests remain lacking. By sampling foliage and tree rings of Chinese pine (Pinus tabuliformis) in the Beijing metropolitan region, China, we explored whether and how urban CO₂ emissions affect stable carbon isotope ratios (δ¹³C) and tree growth spatially and/or temporally. The results indicate a significant decrease in foliar δ¹³C values towards the urban center and this pattern was mainly explained by the urban-periurban-rural gradients of CO₂ emissions as surrogated by trunk road density. Tree-ring δ¹³C values showed a significant decrease over last four decades and this trend was mainly explained by rising levels of CO₂ and secondarily mediated by the variations of aridity index during growing season. Moreover, annual basal area increment of Chinese pine was significantly accelerated during last two decades, being mainly driven by increasing CO₂ emissions and secondarily mediated by climate variations. These findings reveal significant CO₂-associated imprints of urbanization on plant growth and provide empirical evidences of significant CO₂-induced alteration of carbon cycles in urban forests.

Meta-analysis shows forest soil CO2 effluxes are dependent on the disturbance regime and biome type

Forest soil CO₂ efflux (FCO₂ ) is a crucial process in global carbon cycling; however, how FCO₂ responds to disturbance regimes in different forest biomes is poorly understood. We quantified the effects of disturbance regimes on FCO₂ across boreal, temperate, tropical and Mediterranean forests based on 1240 observations from 380 studies. Globally, climatic perturbations such as elevated CO₂ concentration, warming and increased precipitation increase FCO₂ by 13% to 25%. FCO₂ is increased by forest conversion to grassland and elevated carbon input by forest management practices but reduced by decreased carbon input, fire and acid rain. Disturbance also changes soil temperature and water content, which in turn affect the direction and magnitude of disturbance influences on FCO₂ . FCO₂ is disturbance- and biome-type dependent and such effects should be incorporated into earth system models to improve the projection of the feedback between the terrestrial C cycle and climate change.

Plant biomass responses to elevated CO2 are mediated by phosphorus uptake

Elevated atmospheric CO₂ concentrations [CO₂] potentially alter carbon (C) and phosphorus (P) cycles in terrestrial ecosystems. Although numerous field experiments and a few meta-analyses have been conducted, it is still largely unclear how the P cycle affects plant biomass responses under elevated [CO₂] globally. Here, we conducted a global synthesis by analyzing 111 studies on the responses of above- and belowground P cycling to elevated [CO₂], to examine how changes in the P cycle affect the plant biomass response to elevated [CO₂]. Our results show that elevated [CO₂] significantly increased plant aboveground biomass (+13 %), stem biomass (+4 %), leaf biomass (+11 %), belowground biomass (+12 %), and the root: shoot ratio (+7 %). Effects of elevated [CO₂] on aboveground biomass, belowground biomass, and root: shoot ratio were best explained by plant P uptake. In addition, elevated [CO₂]-induced changes in the aboveground P pool, leaf P pool, and leaf P concentration were modulated by ecological drivers, such as ΔCO₂, experimental duration, and aridity index. Our findings highlight the importance of plant P uptake for both above- and belowground plant biomass responses under elevated [CO₂], which should be considered in future biosphere models to improve predictions of terrestrial carbon-climate feedbacks.

Structural and Functional Responses of the Heteromorphic Leaves of Different Tree Heights on Populus euphratica Oliv. to Different Soil Moisture Conditions

Populus euphratica Oliv., a pioneer species of desert riparian forest, is characterized heterophylly. To understand the adaptation strategies of the heteromorphic leaves of P. euphratica to soil drought, we assessed the structural and functional characteristics of the heteromorphic leaves at different heights in suitable soil moisture conditions (groundwater depth 1.5 m) and drought conditions (groundwater depth 5 m), which include morphology, anatomical structure, photosynthetic capacity, water use efficiency, osmotic adjustment capacity, and endogenous hormones. These results indicate that leaf area, leaf thickness, fence tissue, palisade-to-sea ratio, main vein xylem area, vessel area, net photosynthetic rate, transpiration rate, and proline, MDA, IAA, GA3, and ZR contents showed a positive correlation with the tree height under the two soil moisture conditions, but leaf shape index, leaf water potential (LWP), and ABA content showed a decreasing trend. In addition, the main vein vascular bundle area, main vein xylem area, and contents of malondialdehyde, ABA, GA3, and IAA were significantly greater under soil drought conditions than normal soil water content. Under soil drought stress, the heteromorphic leaves of P. euphratica showed more investment in anatomical structure and greater water use efficiency, proline, and hormone contents, and synergistic changes to maintain high photosynthetic efficiency. This is an adaptation strategy to water stress caused by soil drought and tree height changes.