Recent temperature increases have elicited strong phenological shifts in temperate tree species, with subsequent effects on photosynthesis. Here, we assess the impact of advanced leaf flushing in a ...winter warming experiment on the current year’s senescence and next year’s leaf flushing dates in two common tree species: Quercus robur L. and Fagus sylvatica L. Results suggest that earlier leaf flushing translated into earlier senescence, thereby partially offsetting the lengthening of the growing season. Moreover, saplings that were warmed in winter–spring 2009–2010 still exhibited earlier leaf flushing in 2011, even though the saplings had been exposed to similar ambient conditions for almost 1 y. Interestingly, for both species similar trends were found in mature trees using a long-term series of phenological records gathered from various locations in Europe. We hypothesize that this long-term legacy effect is related to an advancement of the endormancy phase (chilling phase) in response to the earlier autumnal senescence. Given the importance of phenology in plant and ecosystem functioning, and the prediction of more frequent extremely warm winters, our observations and postulated underlying mechanisms should be tested in other species.
Attempts to combine biometric and eddy-covariance (EC) quantifications of carbon allocation to different storage pools in forests have been inconsistent and variably successful in the past.
We ...assessed above-ground biomass changes at five long-term EC forest stations based on tree-ring width and wood density measurements, together with multiple allometric models. Measurements were validated with site-specific biomass estimates and compared with the sum of monthly CO2 fluxes between 1997 and 2009.
Biometric measurements and seasonal net ecosystem productivity (NEP) proved largely compatible and suggested that carbon sequestered between January and July is mainly used for volume increase, whereas that taken up between August and September supports a combination of cell wall thickening and storage. The inter-annual variability in above-ground woody carbon uptake was significantly linked with wood production at the sites, ranging between 110 and 370 g Cm−2 yr−1, thereby accounting for 10–25% of gross primary productivity (GPP), 15–32% of terrestrial ecosystem respiration (TER) and 25–80% of NEP.
The observed seasonal partitioning of carbon used to support different wood formation processes refines our knowledge on the dynamics and magnitude of carbon allocation in forests across the major European climatic zones. It may thus contribute, for example, to improved vegetation model parameterization and provides an enhanced framework to link tree-ring parameters with EC measurements.
The traditional view of forest dynamics originated by Kira and Shidei Kira T, Shidei T (1967) Jap J Ecol 17:70–87 and Odum Odum EP (1969) Science 164(3877):262–270 suggests a decline in net primary ...productivity (NPP) in aging forests due to stabilized gross primary productivity (GPP) and continuously increased autotrophic respiration (R ₐ). The validity of these trends in GPP and R ₐ is, however, very difficult to test because of the lack of long-term ecosystem-scale field observations of both GPP and R ₐ. Ryan and colleagues Ryan MG, Binkley D, Fownes JH (1997) Ad Ecol Res 27:213–262 have proposed an alternative hypothesis drawn from site-specific results that aboveground respiration and belowground allocation decreased in aging forests. Here, we analyzed data from a recently assembled global database of carbon fluxes and show that the classical view of the mechanisms underlying the age-driven decline in forest NPP is incorrect and thus support Ryan’s alternative hypothesis. Our results substantiate the age-driven decline in NPP, but in contrast to the traditional view, both GPP and R ₐ decline in aging boreal and temperate forests. We find that the decline in NPP in aging forests is primarily driven by GPP, which decreases more rapidly with increasing age than R ₐ does, but the ratio of NPP/GPP remains approximately constant within a biome. Our analytical models describing forest succession suggest that dynamic forest ecosystem models that follow the traditional paradigm need to be revisited.
Thermal acclimation of soil organic matter (SOM) decomposition is frequently observed and has often been attributed to substrate depletion under warming, but other mechanisms, such as changes in ...microbial community structure and functioning, have received less attention. In order to determine whether shifts in microbial community structure and functioning are involved in thermal acclimation of SOM decomposition, a laboratory incubation experiment was conducted using an artificial forest soil. Samples were first subjected to different temperatures of 5, 15, and 25 °C during a 72-day pre-incubation period and then half of the microcosms from each pre-incubation temperature were incubated at 5 or 25 °C for a period of 11 days. Substantial thermal acclimation of SOM decomposition was observed, with the SOM decomposition in soils pre-incubated at higher temperatures being less sensitive to temperature. Along with the reduced temperature sensitivity in response to warming, significant changes in microbial community PLFAs, microbial biomass carbon (MBC), and the potential activities of 11 enzymes were also observed. Nevertheless, shifts in microbial community PLFAs and particular enzyme activities provided the most explanatory power for the decreased temperature sensitivity with warming, as revealed by a multivariate regression analysis. The microbial community structure shifts were mainly manifested as an increase in the relative abundance of Gram-positive bacteria and decreases in the relative abundances of Gram-negative bacteria and fungi. Microbial communities pre-incubated under lower temperatures experienced greater shifts in their structure. Substrate depletion did not occur in this short-term incubation experiment, since neither total organic carbon (TOC) nor dissolved organic carbon (DOC) decreased with increasing temperature. Our results suggest that shifts in microbial community structure and functioning may underlie the thermal acclimation of SOM decomposition and should be taken into account when predicting the response of soil CO2 efflux to global warming.
•Obvious thermal acclimation occurred rapidly in the short-term incubation experiment.•The acclimation was strongly related to shifts in soil microbial community structure.•Enzymes activities especially urease also had explanatory power to the acclimation.•The depletion of DOC was not detected therefore could not explain the acclimation.
Background and aims
Soil organic carbon (SOC) accrual is central to the discussion on active atmospheric CO
2
removal strategies, but it requires a clear understanding of the mechanisms driving new ...SOC formation. SOC is formed from the decomposition of above- and below-ground plant inputs, yet their accurate quantification remains a major challenge. In the current study the in-growth soil core and δ
13
C methods were combined to quantify the net C input from single or combined above- and below-ground parts of a poplar (
Populus
) bioenergy plantation.
Methods
Three different mesh sizes were used for the in-growth cores in four different treatments designed to estimate the relative contributions of above-ground litter fall, fine roots and mycorrhizae (C3 input from the poplar) to the new C formation in cores filled with a C4 soil, by applying a mass balance equation.
Results
Soil C formation was higher in the treatments with roots as compared to the treatments with above-ground C inputs only (29 vs 16 g C m
−2
), despite the disproportionally lower root inputs as compared to above-ground C inputs (34 vs 175 g C m
−2
y
−1
). Soil C formation from different sources (above- and below-ground) was additive; i.e. the observed soil C formation in the combined treatment was the sum of those in the single soil C input treatments. As a result, below-ground C-inputs had a high conversion efficiency to SOC of 76%. Above-ground plant inputs had a very low SOC formation efficiency of 9%, and were presumably mainly mineralized by microorganisms.
Conclusion
The higher soil C accumulation rates from below-ground C inputs are particularly important in bioenergy plantations where the above-ground biomass is frequently removed for biomass and energy production.
Plant invasion is an emerging driver of global change worldwide. We aimed to disentangle its impacts on plant–soil nutrient concentrations. We conducted a meta‐analysis of 215 peer‐reviewed articles ...and 1233 observations. Invasive plant species had globally higher N and P concentrations in photosynthetic tissues but not in foliar litter, in comparison with their native competitors. Invasive plants were also associated with higher soil C and N stocks and N, P, and K availabilities. The differences in N and P concentrations in photosynthetic tissues and in soil total C and N, soil N, P, and K availabilities between invasive and native species decreased when the environment was richer in nutrient resources. The results thus suggested higher nutrient resorption efficiencies in invasive than in native species in nutrient‐poor environments. There were differences in soil total N concentrations but not in total P concentrations, indicating that the differences associated to invasive plants were related with biological processes, not with geochemical processes. The results suggest that invasiveness is not only a driver of changes in ecosystem species composition but that it is also associated with significant changes in plant–soil elemental composition and stoichiometry.
Leaves of fast‐growing, woody bioenergy crops often emit volatile organic compounds (VOC). Some reactive VOC (especially isoprene) play a key role in climate forcing and may negatively affect local ...air quality. We monitored the seasonal exchange of VOC using the eddy covariance technique in a ‘coppiced’ poplar plantation. The complex interactions of VOC fluxes with climatic and physiological variables were also explored by using an artificial neural network (Self Organizing Map). Isoprene and methanol were the most abundant VOC emitted by the plantation. Rapid development of the canopy (and thus of the leaf area index, LAI) was associated with high methanol emissions and high rates of gross primary production (GPP) since the beginning of the growing season, while the onset of isoprene emission was delayed. The highest emissions of isoprene, and of isoprene photo‐oxidation products (Methyl Vinyl Ketone and Methacrolein, iₒₓ), occurred on the hottest and sunniest days, when GPP and evapotranspiration were highest, and formaldehyde was significantly deposited. Canopy senescence enhanced the exchange of oxygenated VOC. The accuracy of methanol and isoprene emission simulations with the Model of Emissions of Gases and Aerosols from Nature increased by applying a function to modify their basal emission factors, accounting for seasonality of GPP or LAI.
Clay is generally considered an important stabiliser that reduces the rate of decomposition of organic matter (OM) in soils. However, several recent studies have shown trends contradicting this ...widely held view, emphasising our poor understanding of the mechanisms underlying the clay effects on OM decomposition. Here, an incubation experiment was conducted using artificial soils differing in clay content (0, 5, and 50%) at different temperatures (5, 15, and 25 °C) to determine the effects of clay content, temperature and their interaction on fresh OM decomposition. CO2 efflux was measured throughout the experiment. Phospholipid fatty acids (PLFAs), enzyme activities, microbial biomass carbon (MBC), and dissolved organic carbon (DOC) were also measured at the end of the pre-incubation and incubation periods in order to follow changes in microbial community structure, functioning, and substrate availability. The results showed that higher clay contents promoted OM decomposition probably by increasing substrate availability and by sustaining a greater microbial biomass, albeit with a different community structure and with higher activities of most of the extracellular enzymes assayed. Higher clay content induced increases in the PLFA contents of all bacterial functional groups relative to fungal PLFA content. However, clay content did not change the temperature sensitivity (Q10) of OM decomposition. The higher substrate availability in the high clay artificial soils sustained more soil microbial biomass, resulting in a different community structure and different functioning. The higher microbial biomass, as well as the changed community structure and functions, accelerated OM decomposition. From these observations, an alternative pathway to understanding the effects of clay on OM decomposition is proposed, in which clay may not only accelerate the decomposition of organic materials in soils but also facilitate the SOM accumulation as microbial products in the long term. Our results highlight the importance of clay content as a control over OM decomposition and greater attention is required to elucidate the underlying mechanisms.
•High clay content accelerated OM decomposition in the artificial soils.•High clay soils offered more substrates and fed greater microbial biomass.•Varied clay contents led to differed microbial community structure and functioning.•Clay content did not change Q10 of OM decomposition in the artificial soils.
•A two-year warming experiment was conducted using saplings of three deciduous tree species (oak, beech and birch).•The temperature sensitivity of leaf unfolding to six different warming levels was ...quantified.•Different warming levels elicited different levels of advancement in leaf unfolding.•A significant sigmoid correlation between chilling and forcing was found.•Warming did not result in further advancement of leaf unfolding in oak and beech, when it exceeded a threshold of +4°C.
Phenological events are sensitive to climate change, and a clear advancement of spring leaf phenology has been well documented. However, the observed phenological trends cannot be directly used to forecast future changes because the underlying mechanisms are still unclear. We conducted a two-year experimental warming experiment using one-meter-high saplings of birch (Betula pendula L.), oak (Quercus robur L.) and beech (Fagus sylvatica L.). The main objective of this study was to quantify the temperature sensitivity of leaf unfolding to six different warming levels. The results suggested that leaf unfolding dates were significantly advanced with warming for all species in both experimental years. However, different warming treatments elicited different leaf unfolding responses. At low and moderate warming (<+4°C), the sensitivity of leaf unfolding to temperature (changes in days per °C) increased with warming in all three studied species. For birch, the temperature sensitivity continued to increase with additional warming. For oak and beech, in contrast, the temperature sensitivity dramatically decreased when warming exceeded +4°C. Our findings confirm that continued and intense warming may not result in further advances of leaf unfolding in the late successional species oak and beech, if the warming exceeds a certain threshold leading to insufficient chilling temperatures for dormancy release.
The global distribution of the optimum air temperature for ecosystem-level gross primary productivity (Formula: see text) is poorly understood, despite its importance for ecosystem carbon uptake ...under future warming. We provide empirical evidence for the existence of such an optimum, using measurements of in situ eddy covariance and satellite-derived proxies, and report its global distribution. Formula: see text is consistently lower than the physiological optimum temperature of leaf-level photosynthetic capacity, which typically exceeds 30 °C. The global average Formula: see text is estimated to be 23 ± 6 °C, with warmer regions having higher Formula: see text values than colder regions. In tropical forests in particular, Formula: see text is close to growing-season air temperature and is projected to fall below it under all scenarios of future climate, suggesting a limited safe operating space for these ecosystems under future warming.
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