Component respiration, ecosystem respiration and net primary production of a mature black spruce forest in northern Quebec

Estimation of CO2 flux components over northern hemisphere forest ecosystems by using random forest method through temporal and spatial data scanning procedures

Modeling CO₂ flux components is an important task in ecosystem analysis and terrestrial studies. Net ecosystem exchange (NEE), ecosystem respiration (R), and gross primary production (GPP) are three CO₂ flux components. Despite the ecosystem land cover characteristics, climatic factors can make considerable impact on quantity and mechanism of these components. Nevertheless, such climatic factors are not available in most of the areas, especially in developing regions. Therefore, obtaining the models that can exempt using locally recorded variables would be of great importance. A modeling study was carried out here to simulate CO₂ flux components using soft computing-based random forest (RF) model in both local and external (spatial) scales, assessed by k-fold validation procedure. Data from 11 sites located in three forest ecosystems, e.g. deciduous broad leaf (DBF), evergreen needle leaf (ENF), and mixed forest (MF), were used to simulate the flux components. The obtained results showed that the temperature-related parameters (e.g., air and soil temperature, vapor pressure deficit) along with the net radiation play key role in determining the flux components in all studied ecosystems. It was confirmed that a chronologic scan of the available patterns is needed for a thorough assessment of the performance accuracy of the local models. The external models provided promising results when compared with the locally trained models. This is a very great step forward in estimating CO₂ flux components under data scarcity conditions.

Community-level respiration of prokaryotic microbes may rise with global warming

Understanding how the metabolic rates of prokaryotes respond to temperature is fundamental to our understanding of how ecosystem functioning will be altered by climate change, as these micro-organisms are major contributors to global carbon efflux. Ecological metabolic theory suggests that species living at higher temperatures evolve higher growth rates than those in cooler niches due to thermodynamic constraints. Here, using a global prokaryotic dataset, we find that maximal growth rate at thermal optimum increases with temperature for mesophiles (temperature optima [Formula: see text]C), but not thermophiles ([Formula: see text]C). Furthermore, short-term (within-day) thermal responses of prokaryotic metabolic rates are typically more sensitive to warming than those of eukaryotes. Because climatic warming will mostly impact ecosystems in the mesophilic temperature range, we conclude that as microbial communities adapt to higher temperatures, their metabolic rates and therefore, biomass-specific CO[Formula: see text] production, will inevitably rise. Using a mathematical model, we illustrate the potential global impacts of these findings.

Is NPP proportional to GPP? Waring's hypothesis 20 years on

Gross primary production (GPP) is partitioned to autotrophic respiration (Ra) and net primary production (NPP), the latter being used to build plant tissues and synthesize non-structural and secondary compounds. Waring et al. (1998; Net primary production of forests: a constant fraction of gross primary production? Tree Physiol 18:129-134) suggested that a NPP:GPP ratio of 0.47 ± 0.04 (SD) is universal across biomes, tree species and stand ages. Representing NPP in models as a fixed fraction of GPP, they argued, would be both simpler and more accurate than trying to simulate Ra mechanistically. This paper reviews progress in understanding the NPP:GPP ratio in forests during the 20 years since the Waring et al. paper. Research has confirmed the existence of pervasive acclimation mechanisms that tend to stabilize the NPP:GPP ratio and indicates that Ra should not be modelled independently of GPP. Nonetheless, studies indicate that the value of this ratio is influenced by environmental factors, stand age and management. The average NPP:GPP ratio in over 200 studies, representing different biomes, species and forest stand ages, was found to be 0.46, consistent with the central value that Waring et al. proposed but with a much larger standard deviation (±0.12) and a total range (0.22-0.79) that is too large to be disregarded.

Environmental controls on light inhibition of respiration and leaf and canopy daytime carbon exchange in a temperate deciduous forest

Uncertainty in the estimation of daytime ecosystem carbon cycling due to the light inhibition of leaf respiration and photorespiration, and how these small fluxes vary through the growing season in the field, remains a confounding element in calculations of gross primary productivity and ecosystem respiration. Our study focuses on how phenology, short-term temperature changes and canopy position influence leaf-level carbon exchange in Quercus rubra L. (red oak) at Harvard Forest in central Massachusetts, USA. Using leaf measurements and eddy covariance, we also quantify the effect of light inhibition on estimates of daytime respiration at leaf and ecosystem scales. Measured rates of leaf respiration in the light and dark were highest in the early growing season and declined in response to 10-day prior air temperatures (P < 0.01), evidence of within-season thermal acclimation. Leaf respiration was significantly inhibited by light (27.1 ± 2.82% inhibited across all measurements), and this inhibition varied with the month of measurement; greater inhibition was observed in mid-summer leaves compared with early- and late-season leaves. Increases in measurement temperature led to higher rates of respiration and photorespiration, though with a less pronounced positive effect on photosynthesis; as a result, carbon-use efficiency declined with increasing leaf temperature. Over the growing season when we account for seasonally variable light inhibition and basal respiration rates, our modeling approaches found a cumulative 12.9% reduction of leaf-level respiration and a 12.8% reduction of canopy leaf respiration, resulting in a 3.7% decrease in total ecosystem respiration compared with estimates that do not account for light inhibition in leaves. Our study sheds light on the environmental controls of the light inhibition of daytime leaf respiration and how integrating this phenomenon and other small fluxes can reduce uncertainty in current and future projections of terrestrial carbon cycling.

Can Future CO2 Concentrations Mitigate the Negative Effects of High Temperature and Longer Droughts on Forest Growth?

(1) Background: Climate change may subject forests to climate conditions to which they are not adapted. Elevated temperatures can potentially reduce net photosynthesis by increasing respiration rates and increasingly long droughts dramatically increase morbidity. While CO2 enrichment enhances productivity, it is not clear to what extent CO2 enrichment can offset the negative effects of elevated temperatures and longer droughts; (2) Methods: We used a mechanistic landscape model to conduct controlled simulation experiments manipulating CO2 concentration, temperature, drought length and soil water capacity; (3) Results: We found that elevated CO2 stimulates productivity such that it dwarfs the negative effect caused by elevated temperature. Energy reserves were not as strongly mitigated by elevated CO2, and the mortality of less competitive cohorts increased. Drought length had a surprisingly small effect on productivity measures, but longer droughts increased the risk of mortality; (4) Conclusions: Elevated CO2 compensated for the negative effect of longer droughts in terms of productivity measures, but not survival measures.

Aboveground autotrophic respiration in a Spanish black pine forest: Comparison of scaling methods to improve component partitioning

Total wood CO₂ efflux (R_w) varies vertically within individual trees, and leaves experience large variations in foliar respiration (R_f) rates over their life spans and during daily periods. Therefore, accurate sampling approaches are required to improve aboveground autotrophic respiration (R_Aa) estimations in stand-scale carbon cycling studies. We scaled-up R_w (comprising stem and branch CO₂ efflux; ES and EB, respectively) and R_f from biometric and flux-chamber measurements taken between 2011 and 2013 in a Spanish black pine (Pinus nigra Arn. ssp. salzmannii) forest at an unburnt (UB) site and a low burn-severity (LS) site. We measured seasonal ES at breast height (1.30m) on 9 trees at each site, which was also vertically examined on 5 of those trees. We also measured seasonal R_f in current- and previous-year needles on 3 trees at each site, and quantified R_f variations in darkness and light. Finally, we compared complex and simple scale-up methods which did or did not account for the vertical variation in R_w and the effects of leaf ageing and light inhibition on R_f, respectively. The simple methods underestimated the annual stand-level stem, branch, and total wood respiration ≈35%, 55%, and 41%, respectively, and overestimated annual stand-level whole-canopy foliage respiration ≈43% at both sites. Both methods provided similar annual stand-level R_Aa estimates, although the complex methods improved estimations of the relative contribution of R_Aa components. Thus, based on the complex methods the mean annual R_Aa at the stand-level was 4.53±0.25 and 4.45±0.12MgCha^-1year^-1 at the UB and LS sites, respectively. Our data also confirmed that the low-severity fire did not alter the R_Aa rates. Collectively, this study reveals that complex approaches, applicable in other forest ecosystems, enhance the accuracy of partitioning R_Aa sources by reducing the error in scaling-up in chamber-based measurements.

Negative impacts of high temperatures on growth of black spruce forests intensify with the anticipated climate warming

An increasing number of studies conclude that water limitations and heat stress may hinder the capacity of black spruce (Picea mariana (Mill.) B.S.P.) trees, a dominant species of Canada's boreal forests, to grow and assimilate atmospheric carbon. However, there is currently no scientific consensus on the future of these forests over the next century in the context of widespread climate warming. The large spatial extent of black spruce forests across the Canadian boreal forest and associated variability in climate, demography, and site conditions pose challenges for projecting future climate change responses. Here we provide an evaluation of the impacts of climate warming and drying, as well as increasing [CO2 ], on the aboveground productivity of black spruce forests across Canada south of 60°N for the period 1971 to 2100. We use a new extensive network of tree-ring data obtained from Canada's National Forest Inventory, spatially explicit simulations of net primary productivity (NPP) and its drivers, and multivariate statistical modeling. We found that soil water availability is a significant driver of black spruce interannual variability in productivity across broad areas of the western to eastern Canadian boreal forest. Interannual variability in productivity was also found to be driven by autotrophic respiration in the warmest regions. In most regions, the impacts of soil water availability and respiration on interannual variability in productivity occurred during the phase of carbohydrate accumulation the year preceding tree-ring formation. Results from projections suggest an increase in the importance of soil water availability and respiration as limiting factors on NPP over the next century due to warming, but this response may vary to the extent that other factors such as carbon dioxide fertilization, and respiration acclimation to high temperature, contribute to dampening these limitations.

Forest ecosystem respiration estimated from eddy covariance and chamber measurements under high turbulence and substantial tree mortality from bark beetles

Eddy covariance nighttime fluxes are uncertain due to potential measurement biases. Many studies report eddy covariance nighttime flux lower than flux from extrapolated chamber measurements, despite corrections for low turbulence. We compared eddy covariance and chamber estimates of ecosystem respiration at the GLEES Ameriflux site over seven growing seasons under high turbulence [summer night mean friction velocity (u*) = 0.7 m s(-1)], during which bark beetles killed or infested 85% of the aboveground respiring biomass. Chamber-based estimates of ecosystem respiration during the growth season, developed from foliage, wood, and soil CO2 efflux measurements, declined 35% after 85% of the forest basal area had been killed or impaired by bark beetles (from 7.1 ± 0.22 μmol m(-2) s(-1) in 2005 to 4.6 ± 0.16 μmol m(-2) s(-1) in 2011). Soil efflux remained at ~3.3 μmol m(-2) s(-1) throughout the mortality, while the loss of live wood and foliage and their respiration drove the decline of the chamber estimate. Eddy covariance estimates of fluxes at night remained constant over the same period, ~3.0 μmol m(-2) s(-1) for both 2005 (intact forest) and 2011 (85% basal area killed or impaired). Eddy covariance fluxes were lower than chamber estimates of ecosystem respiration (60% lower in 2005, and 32% in 2011), but the mean night estimates from the two techniques were correlated within a year (r(2) from 0.18 to 0.60). The difference between the two techniques was not the result of inadequate turbulence, because the results were robust to a u* filter of >0.7 m s(-1). The decline in the average seasonal difference between the two techniques was strongly correlated with overstory leaf area (r(2) = 0.92). The discrepancy between methods of respiration estimation should be resolved to have confidence in ecosystem carbon flux estimates.

Vertical gradients and seasonal variation in stem CO2 efflux within a Norway spruce stand

Stem CO2 efflux is known to vary seasonally and vertically along tree stems. However, annual tree- and stand-scale efflux estimates are commonly based on measurements made only a few times a year, during daytime and at breast height. In this study, the effect of these simplifying assumptions on annual efflux estimates and their influence on the estimates of the importance of stems in stand-scale carbon cycling are evaluated. In order to assess the strength of seasonal, diurnal and along-stem variability in CO2 efflux, half-hourly measurements were carried out at three heights on three mature Norway spruce (Picea abies (L.) Karst.) trees over a period of 3 years. Making the common assumption of breast height efflux rates being representative of the entire stem was found to result in underestimations of 10-17% in the annual tree-scale CO2 efflux. Upscaling using only daytime measurements from breast height increased the underestimation to 15-20%. Furthermore, the results show that the strength of the vertical gradient varies seasonally, being strongest in the early summer and non-existent during the cool months. The observed seasonality in the vertical CO2 efflux gradient could not be explained by variation in stem temperature, temperature response of the CO2 efflux (Q10), outer-bark permeability, CO2 transport in the xylem or CO2 release from the phloem. However, the estimated CO2 concentration immediately beneath the bark was considerably higher in the upper stem during the main period of diameter growth, coinciding with the strongest vertical efflux gradient. These results suggest that higher growth rates in the upper stem are the main cause for the observed vertical variation in the stem CO2 effluxes. Furthermore, the results indicate that accounting for the vertical efflux variation is essential for assessments of the importance of stems in stand-scale carbon cycling.

State of the interface between conservation and physiology: a bibliometric analysis

Contemporary conservation science benefits from the perspectives of a variety of different disciplines, including a recent synergy with physiology, an interface known as 'conservation physiology'. To evaluate the degree of interaction between conservation and animal/plant physiology, we conducted three bibliometric analyses. We first pursued the use of the term 'conservation physiology' since its first definition in 2006 to determine how frequently it has been used and in which publications. Secondly, we evaluated the occurrence of conservation terms in animal and plant physiology journals, physiological terms in conservation journals, and a combination of terms in ecology journals. Thirdly, we explored trends in a subset of conservation physiology articles published between 2006 and 2012. We identified a surge in the use of the term 'conservation physiology' in 2012, after only a slow increase in usage between 2006 and 2011. Conservation journals tend to have been significantly more active in publishing conservation physiology than animal physiology, plant physiology or ecology journals. However, we found evidence that ecology and animal physiology journals began to incorporate more conservation physiology after 2006, while conservation- and plant physiology-themed journals did not. Among 299 conservation physiology articles that we identified, vertebrate taxa have been over-represented in conservation physiology compared with their relative taxonomic abundance, invertebrate taxa have been under-represented, and plants have been represented in proportion to their relative taxonomic abundance; however, those findings are reasonably consistent with publication trends in conservation biology. Diffuse distribution of conservation physiology papers throughout the literature may have been a barrier to the growth of the subdiscipline when the interface was emerging. The introduction of the focused journal Conservation Physiology in 2013 may address that deficiency. Moreover, development of a unifying framework could help to aggregate knowledge and attract potential contributors by highlighting and facilitating access to and application of conservation physiology.