Twenty chambers for measurement of soil CO2 efflux were compared against known CO2 fluxes ranging from 0.32 to 10.01 mumol CO2 m(-2) s(-1) and generated by a specially developed calibration tank. ...Chambers were tested on fine and coarse homogeneous quartz sand with particle sizes of 0.05-0.2 and 0.6 mm, respectively. The effect of soil moisture on chamber measurements was tested by wetting the fine quartz sand to about 25% volumetric water content. Non-steady-state through-flow chambers either underestimated or overestimated fluxes from -21 to +33% depending on the type of chamber and the method of mixing air within the chamber's headspace. However, when results of all systems tested were averaged, fluxes were within 4% of references. Non-steady-state non-through-flow chambers underestimated or overestimated fluxes from -35 to +6%. On average, the underestimation was about 13-14% on fine sand and 4% on coarse sand. When the length of the measurement period was increased, the underestimation increased due to the rising concentration within the chamber headspace, which reduced the diffusion gradient within the soil. Steady-state through-flow chambers worked almost equally well in all sand types used in this study. They overestimated the fluxes on average by 2-4%. Overall, the reliability of the chambers was not related to the measurement principle per se. Even the same chambers, with different collar designs, showed highly variable results. The mixing of air within the chamber can be a major source of error. Excessive turbulence inside the chamber can cause mass flow of CO2 from the soil into the chamber. The chamber headspace concentration also affects the flux by altering the concentration gradient between the soil and the chamber.
It is well known that microbial-mediated soil respiration, the major source of CO sub(2) from terrestrial ecosystems, is sensitive to temperature. Here, we hypothesize that some mechanisms, such as ...acclimation of microbial respiration to temperature and/or regulation by plant fresh C inputs of the temperature sensitivity of decomposition of soil organic matter (SOM), should be taken into account to predict soil respiration correctly. Specifically, two hypotheses were tested: (1) under warm conditions, temperature sensitivity (Q sub(10)) and basal rates of microbial-mediated soil respiration (Bs sub(20,) respiration at a given temperature) would be primarily subjected to presence/absence of plant fresh C inputs; and (2) under cold conditions, where labile C depletion occurred more slowly, microbial-mediated soil respiration could adjust its optimal temperatures to colder temperatures (acclimation), resulting in a net increase of respiration rates for a given temperature (Bs sub(20)). For this purpose, intact soil cores from an oak savanna ecosystem were incubated with sufficient water supply at two contrasting temperatures (10 and 30C) during 140days. To study temperature sensitivity of soil respiration, short-term temperature cycles (from 5 to 40C at 8h steps) were applied periodically to the soils. Our results confirmed both hypotheses. Under warm conditions ANCOVA and likelihood ratio tests confirmed that both Q sub(10) and Bs sub(20) decreased significantly during the incubation. Further addition of glucose at the end of the incubation period increased Bs sub(20) and Q sub(10) to initial values. The observed decrease in temperature sensitivity (Q sub(10)) in absence of labile C disagrees with the broadly accepted fact that temperature sensitivity of the process increases as quality of the substrate decreases. Our experiment also shows that after 2months of incubation cold-incubated soils doubled the rates of respiration at cold temperatures causing a strong increase in basal respiration rates (Bs sub(20)). This suggest that microbial community may have up-regulated their metabolism at cold conditions (cold-acclimation), which also disagrees with most observations to date. The manuscript discusses those two apparent contradictions: the decrease in temperature sensitivity in absence of labile C and the increase in microbial-mediated soil respiration rates at cold temperatures. While this is only a case study, the trends observed could open the controversy over the validity of current soil respiration models.
Abstract
The temperature sensitivity of soil respiration (SR) is often estimated from the seasonal changes in the flux relative to those in soil temperature, and subsequently used in models to ...interpolate or predict soil fluxes. However, temperature sensitivities derived from seasonal changes in SR (from here on denoted seasonal Q
10
) may not solely reflect the temperature sensitivity of SR, because seasonal changes in SR can also be affected by other seasonally fluctuating conditions and processes. In this manuscript, we present a case study of how the seasonal Q
10
of SR can be decoupled from the temperature sensitivity of SR. In a mixed temperate forest, we measured SR under vegetations with different leaf strategies: pure evergreen, pure deciduous, and mixed. Seasonal Q
10
was much higher under deciduous than under evergreen canopies. However, at a shorter time scale, both vegetation types exhibited very similar Q
10
values, indicating that the large differences in seasonal Q
10
do not represent differences in the temperature sensitivity of the soil metabolism. The seasonal Q
10
depends strongly on the amplitude of the seasonal changes in SR (SR
s
), which, under the particular climatic and edaphic conditions of our forest study site, were significantly larger in deciduous forest. In turn, SR
s
was positively correlated with the seasonal changes in leaf area index (LAI
s
), a measure of the deciduousness of the vegetation. Thus, in this temperate maritime forest, seasonal Q
10
of SR was strongly influenced by the deciduousness of the vegetation. We conclude that the large differences in seasonal Q
10
were not entirely due to different temperature sensitivities, but also to different seasonal patterns of plant activity in the evergreen and deciduous plants of this site. Some coniferous forests may be more seasonal than the one we studied, and the deciduous–evergreen differences observed here may not be broadly applicable, but this case study demonstrates that variation of plant phenological process can significantly contribute to the seasonality of SR, and, hence, calculated Q
10
values. Where this occurs, the seasonal Q
10
value for SR does not accurately represent temperature sensitivity. Because the strong seasonal correlation between SR and temperature does not necessarily imply a causal relationship, Q
10
values derived form annual patterns of SR should be used with caution when predicting future responses of SR to climatic change.
Scots pine (Pinus sylvestris L.) is one of the most widely distributed trees on Earth. Despite its huge ecological plasticity, many studies show that its capacity to resist drought is being overcome ...in several regions, particularly at the southern limit of its distribution in the Mediterranean basin. This paper summarizes recent work on the direct and indirect effects of drought on Scots pine in the context of climate change. More specifically, the following aspects are addressed: (1) what are the ecophysiological characteristics that explain the vulnerability of Scots pine to drought?; (2) what environmental factors determine the growth patterns of Scots pine and how have these factors varied over the last decades?; (3) what environmental factors explain the spatial variability in the demographic rates of this species (growth, mortality, recruitment) at different scales?; and (4) what are the likely impacts of more frequent droughts and forest fires? Overall, the results currently available suggest that the mid term viability of a substantial part of the Scots pine populations in the Iberian Peninsula is at risk if climate change projections become true. We conclude exploring to what degree the previous information can be used to identify the more vulnerable individuals or populations, and how could forest management be used to modulate the expected impacts.
The temperature sensitivity of soil respiration (SR) is often estimated from the seasonal changes in the flux relative to those in soil temperature, and subsequently used in models to interpolate or ...predict soil fluxes. However, temperature sensitivities derived from seasonal changes in SR (from here on denoted seasonal Q sub(10)) may not solely reflect the temperature sensitivity of SR, because seasonal changes in SR can also be affected by other seasonally fluctuating conditions and processes. In this manuscript, we present a case study of how the seasonal Q sub(10) of SR can be decoupled from the temperature sensitivity of SR. In a mixed temperate forest, we measured SR under vegetations with different leaf strategies: pure evergreen, pure deciduous, and mixed. Seasonal Q sub(10) was much higher under deciduous than under evergreen canopies. However, at a shorter time scale, both vegetation types exhibited very similar Q sub(10) values, indicating that the large differences in seasonal Q sub(10) do not represent differences in the temperature sensitivity of the soil metabolism. The seasonal Q sub(10) depends strongly on the amplitude of the seasonal changes in SR (SR sub(s)), which, under the particular climatic and edaphic conditions of our forest study site, were significantly larger in deciduous forest. In turn, SR sub(s) was positively correlated with the seasonal changes in leaf area index (LAI sub(s)), a measure of the deciduousness of the vegetation. Thus, in this temperate maritime forest, seasonal Q sub(10) of SR was strongly influenced by the deciduousness of the vegetation. We conclude that the large differences in seasonal Q sub(10) were not entirely due to different temperature sensitivities, but also to different seasonal patterns of plant activity in the evergreen and deciduous plants of this site. Some coniferous forests may be more seasonal than the one we studied, and the deciduous-evergreen differences observed here may not be broadly applicable, but this case study demonstrates that variation of plant phenological process can significantly contribute to the seasonality of SR, and, hence, calculated Q sub(10) values. Where this occurs, the seasonal Q sub(10) value for SR does not accurately represent temperature sensitivity. Because the strong seasonal correlation between SR and temperature does not necessarily imply a causal relationship, Q sub(10) values derived form annual patterns of SR should be used with caution when predicting future responses of SR to climatic change.
At a mixed patchy coniferous/deciduous forest in Brasschaat, Belgium, eddy correlation measurements of carbon dioxide flux were made above the forest canopy over a 5-yr period. The data were used to ...calculate the annual net ecosystem exchange. The CO sub(2) vertical flux was calculated as the covariance between fluctuations in the vertical wind speed and CO sub(2) concentrations. To estimate CO sub(2) storage in the air layer below the eddy correlation measurement height, measurements also included a profile of CO sub(2) concentrations at levels of 10, 24, 32, and 40 m above the ground. While net uptake fluxes were measured during the day, even in winter, net uptake on a daily basis only occurred from the beginning of April until the end of September, leading to a maximum length of the growing season of about six months. Net ecosystem exchange estimates were made using different gap filling methods, which showed that the uncertainty was relatively large. Overall, the forest was found to be a consistent net source of CO sub(2) of 110 g C/m super(2)/yr.
Tree‐ring data has been widely used to inform about tree growth responses to drought at the individual scale, but less is known about how tree growth sensitivity to drought scales up driving changes ...in forest dynamics. Here, we related tree‐ring growth chronologies and stand‐level forest changes in basal area from two independent data sets to test if tree‐ring responses to drought match stand forest dynamics (stand basal area growth, ingrowth, and mortality). We assessed if tree growth and changes in forest basal area covary as a function of spatial scale and tree taxa (gymnosperm or angiosperm). To this end, we compared a tree‐ring network with stand data from the Spanish National Forest Inventory. We focused on the cumulative impact of drought on tree growth and demography in the period 1981–2005. Drought years were identified by the Standardized Precipitation Evapotranspiration Index, and their impacts on tree growth by quantifying tree‐ring width reductions. We hypothesized that forests with greater drought impacts on tree growth will also show reduced stand basal area growth and ingrowth and enhanced mortality. This is expected to occur in forests dominated by gymnosperms on drought‐prone regions. Cumulative growth reductions during dry years were higher in forests dominated by gymnosperms and presented a greater magnitude and spatial autocorrelation than for angiosperms. Cumulative drought‐induced tree growth reductions and changes in forest basal area were related, but initial stand density and basal area were the main factors driving changes in basal area. In drought‐prone gymnosperm forests, we observed that sites with greater growth reductions had lower stand basal area growth and greater mortality. Consequently, stand basal area, forest growth, and ingrowth in regions with large drought impacts was significantly lower than in regions less impacted by drought. Tree growth sensitivity to drought can be used as a predictor of gymnosperm demographic rates in terms of stand basal area growth and ingrowth at regional scales, but further studies may try to disentangle how initial stand density modulates such relationships. Drought‐induced growth reductions and their cumulative impacts have strong potential to be used as early‐warning indicators of regional forest vulnerability.
The relatively poor simulation of the below-ground processes is a severe drawback for many ecosystem models, especially when predicting responses to climate change and management. For a meaningful ...estimation of ecosystem production and the cycling of water, energy, nutrients and carbon, the integration of soil processes and the exchanges at the surface is crucial. It is increasingly recognized that soil biota play an important role in soil organic carbon and nutrient cycling, shaping soil structure and hydrological properties through their activity, and in water and nutrient uptake by plants through mycorrhizal processes. In this article, we review the main soil biological actors (microbiota, fauna and roots) and their effects on soil functioning. We review to what extent they have been included in soil models and propose which of them could be included in ecosystem models. We show that the model representation of the soil food web, the impact of soil ecosystem engineers on soil structure and the related effects on hydrology and soil organic matter (SOM) stabilization are key issues in improving ecosystem-scale soil representation in models. Finally, we describe a new core model concept (KEYLINK) that integrates insights from SOM models, structural models and food web models to simulate the living soil at an ecosystem scale.
Soil respiration (SR) constitutes the largest flux of CO2 from terrestrial ecosystems to the atmosphere. However, there still exist considerable uncertainties as to its actual magnitude, as well as ...its spatial and interannual variability. Based on a reanalysis and synthesis of 80 site-years for 57 forests, plantations, savannas, shrublands and grasslands from boreal to tropical climates we present evidence that total annual SR is closely related to SR at mean annual soil temperature (SRMAT), irrespective of the type of ecosystem and biome. This is theoretically expected for non water-limited ecosystems within most of the globally occurring range of annual temperature variability and sensitivity (Q10). We further show that for seasonally dry sites where annual precipitation (P) is lower than potential evapotranspiration (PET), annual SR can be predicted from wet season SRMAT corrected for a factor related to P/PET. Our finding indicates that it can be sufficient to measure SRMAT for obtaining a well constrained estimate of its annual total. This should substantially increase our capacity for assessing the spatial distribution of soil CO2 emissions across ecosystems, landscapes and regions, and thereby contribute to improving the spatial resolution of a major component of the global carbon cycle. (Résumé d'auteur)
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