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Temperature, as a measure of thermal motion, is a significant parameter characterizing a cold atomic ensemble optical quantum memory. In a cold gas, storage lifetime strongly depends on its temperature and is associated with the spin wave decoherence. Here we experimentally demonstrate a new spin wave thermometry method relying on this direct dependence. The short-wavelength spin waves resulting from the counter-propagating configuration of the control and the probe laser beams make this thermometry highly suitable for probing in situ the atomic motion in elongated clouds as the ones used in quantum memories. Our technique is realized with comparable precision for memories that rely on electromagnetically induced transparency as well as far-detuned Raman storage.

Photosynthetically carbon (C) allocation largely determines yield and C sequestration of agroecosystem. However, how C allocation of crops responses to climate change at the long-time scale is poorly defined. Combining thirteen years of eddy covariance and inventory measurements, we comprehensively investigated C allocation mechanism in a winter-wheat and summer-maize double cropping field. Significantly increased gross primary production (GPP) was benefited from CO2 fertilization, and 35% of increased GPP transferred to strengthening C sequestration. However, elevated temperature and drying surface soil moisture stimulated the partitioning of GPP to autotrophic respiration, resulted in conservative net primary production and grain yield. Maize faced a greater risk of C loss and yield reduction than wheat to warming and drying. By synthesizing published long-term data of agroecosystems, we further highlight that the GPP partitioning cannot be simply predicted by allometric theory, particularly for grains, which should be considered in predicting C budget and crop yield.

Belowground litter decomposition represents an important source of the limiting nutrients nitrogen (N) and phosphorus (P) to forest soils. However, the rates and drivers of nutrient immobilization and release from root litter remain poorly understood. To address this gap, we conducted a seven-year field decomposition experiment using roots from three species, across five diameter classes (up to 20 mm) in a temperate forest. All roots released P to the surrounding soil within the first year of decomposition but immobilized N for much longer, particularly coarse roots. Long-term N release was mainly related to initial nutrient stoichiometry of the substrate, whereas P release was better predicted by substrate carbon (C) quality and decomposition rate. Initial root nutrients well predicted the difference between long-term N versus P release. Our results highlight the fact that N and P dynamics should be considered separately when modeling nutrient release during root decomposition.

Changes in precipitation regimes have significant effects on soil carbon (C) cycles; however, these effects may vary in arid versus humid areas. Additionally, the corresponding details of soil C cycles in response to altered precipitation regimes have not been well documented. Here, a meta-analysis was performed using 845 pairwise observations (control vs. increased or decreased precipitation) from 214 published articles to quantify the responses of the input process of exogenous C, the contents of various forms of C in soil, and the soil-atmosphere C fluxes relative to increased or decreased precipitation. The results showed that the effects of altered precipitation regimes did not differ between rainfall and snowfall. Increased precipitation significantly enhanced the soil C inputs, pools and outputs by 18.17%, 18.50%, and 21.04%, respectively, while decreased precipitation led to a significant decline in these soil C parameters by 10.18%, 9.96%, and 17.98%, respectively. The effects of increased precipitation on soil C cycles were more significant in arid areas (where mean annual precipitation, MAP

Senesced-leaf nutrient concentrations vary significantly among coexisting plant species reflecting different leaf nutrient use strategies. However, interspecific variation in senesced-leaf nutrients and its driving factors are not well understood. Here, we aimed to determine interspecific variation and its driving factors in senesced-leaf nutrients. We explored interspecific variation in carbon (C), nitrogen (N) and phosphorus (P) concentrations in newly fallen leaves of 46 coexisting temperate deciduous woody species across the Maoershan Forest Ecosystem Research Station, Northeast China. The relative importance of 10 biotic factors (i.e. mycorrhiza type, N-fixing type, growth form, shade tolerance, laminar texture, coloring degree, coloring type, peak leaf-coloration date, peak leaf-fall date and end leaf-fall date) was quantified with the random forest model. N and P concentrations varied 4- and 9-fold among species, respectively. The high mean N (15.38 mg g−1) and P (1.24 mg g−1) concentrations suggested a weak N and P limitation in the studied forest. Functional groups had only significant effects on specific nutrients and their ratios. P concentration and N:P were negatively correlated with peak and end leaf-fall dates for the ectomycorrhiza species group. Brighter-colored leaves (red > brown > yellow > yellow-green > green) tended to have lower N and P concentrations and higher C:N and C:P than darker-colored leaves. The random forest model showed that autumn coloration and leaf-fall phenology contributed 80% to the total explanation of nutrient variability among species. The results increase our understanding of the variability in senesced-leaf nutrients as a strategy of woody plant nutrition in temperate forests.

Background Vegetation indices (VIs) by remote sensing are widely used as simple proxies of the gross primary production (GPP) of vegetation, but their performances in capturing the inter-annual variation (IAV) in GPP remain uncertain. Methods We evaluated the performances of various VIs in tracking the IAV in GPP estimated by eddy covariance in a temperate deciduous forest of Northeast China. The VIs assessed included the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI), and the near-infrared reflectance of vegetation (NIRv) obtained from tower-radiometers (broadband) and the Moderate Resolution Imaging Spectroradiometer (MODIS), respectively. Results We found that 25%–35% amplitude of the broadband EVI tracked the start of growing season derived by GPP (R2: 0.56–0.60, bias < 4 d), while 45% (or 50%) amplitudes of broadband (or MODIS) NDVI represented the end of growing season estimated by GPP (R2: 0.58–0.67, bias < 3 d). However, all the VIs failed to characterize the summer peaks of GPP. The growing-season integrals but not averaged values of the broadband NDVI, MODIS NIRv and EVI were robust surrogates of the IAV in GPP (R2: 0.40–0.67). Conclusion These findings illustrate that specific VIs are effective only to capture the GPP phenology but not the GPP peak, while the integral VIs have the potential to mirror the IAV in GPP.

Litter decomposition is a fundamental process of biogeochemical cycles and particularly sensitive to global change. However, the overall effects of warming, elevated carbon dioxide and changed precipitation regime on litter decomposition are not well studied. To assess the effects of these three common global change factors on litter decomposition, we performed a meta-analysis using 366 pairwise observations from 103 published articles. We quantified the responses of litter decomposition rate to the effects of warming, elevated CO2, and changed precipitation regime (increased and decreased). At the global scale, warming and precipitation addition significantly stimulated litter decomposition rate by an average of 4.20% and 11.72%, respectively. In contrast, elevated CO2 and precipitation removal showed significant negative effects on litter decomposition rate (-2.99% and − 12.60%). In addition, study type, plant functional traits, and climate were consistent moderators. These results indicate that warming, elevated CO2, and changed precipitation regime have significantly affected litter decomposition, but the direction and magnitude of the effects of different factors varied, and were also differently mediated by moderator variables. Global cycles of carbon and nutrients via the litter decomposition process can be substantially affected by global change. However, the combined effects of these global change factors on litter decomposition and the different effects between the arid and humid areas cannot be addressed due to the lack of data, indicating the need of more focus on multi-factor manipulative experiments in a wider range of study sites.

In this paper, the initial boundary value problem for a nonlocal semilinear pseudo-parabolic equation is investigated, which was introduced to model phenomena in population dynamics and biological sciences where the total mass of a chemical or an organism is conserved. The existence, uniqueness and asymptotic behavior of the global solution and the blowup phenomena of solution with subcritical initial energy are established. Then these results are extended parallelly to the critical initial energy. Further the blowup phenomena of solution with supercritical initial energy is proved, but the existence, uniqueness and asymptotic behavior of the global solution with supercritical initial energy are still open.

Essential for building quantum networks over remote independent nodes, the indistinguishability of photons has been extensively studied by observing the coincidence dip in the Hong-Ou-Mandel interferometer. However, indistinguishability is not limited to the same type of bosons. For the first time, we hereby observe quantum interference between flying photons and a single quantum of stored atomic coherence (magnon) in an atom-light beam splitter interface. We demonstrate that the Hermiticity of this interface determines the type of quantum interference between photons and magnons. Consequently, not only the bunching behavior that characterizes bosons is observed, but counterintuitively, fermionlike antibunching as well. The hybrid nature of the demonstrated magnon-photon quantum interface can be applied to versatile quantum memory platforms, and can lead to fundamentally different photon distributions from those occurring in boson sampling., Comment: 15 pages (6 pages for main text, 9 pages for supplemental material), 9 figures (4 figures for main text, 5 figures for supplemental material), 1 table (1 table for supplemental material)

We report an atom-light beamsplitter-type interface, in which flying photons and stored atomic coherence can be transformed reversibly. The degree of Hermiticity of this interface can alter the atom-photon quantum interference from bosonic to fermion-like.

For the nonlinear Kirchhoff-type wave system with logarithmic nonlinearities and weak dissipation the global well-posedness of initial boundary value problem is analyzed. Focusing on the interplay between Kirchhoff terms and logarithmic sources, we investigate the Kirchhoff system controlled by logarithmic forces thus amplifying the difficulties in blow up analysis which is the primary scenario of interest. By employing potential well method and concavity method, we obtain several results related to the sufficient conditions posed on subcritical initial energy and critical initial energy, which is used to classify initial data for global existence and finite time blow up. Finally, via careful analysis involving the unstable invariant set under supercritical initial energy, we are able to show an affirmative result that the solution blows up in finite time when initial data satisfy some suitable assumptions.

The leaf litter collection method is often used as a baseline for assessing leaf area index (LAI) measurements in forests by optical methods, but different protocols for measuring specific leaf area (SLA) may introduce biases in stand-level LAI estimates, which have not been quantified yet. To develop a standardized protocol for measuring SLA of leaf litter for estimating LAI via litterfall collection, we quantified the relative errors (RE) in the SLA measurements introduced by six protocols commonly-used in the literature with 46 deciduous woody species in a temperate forest, and assessed the stand-level LAI estimates derived from the SLA with eight permanent plots in the forest. The mean leaf area shrinkage from the litter collection across the species varied from 7.31 – 25.41% among the protocols, while the mean leaf mass loss varied from 2.07 – 24.68%. Compared with the reference, the mean RE of SLA introduced by the rest five protocols varied from −25.59 – 7.18%, and the subsequent RE of the stand-level LAI varied from −32.43 – 8.97%. We recommended a practical and reliable protocol for measuring SLA from leaf litter, i.e., weighing the dry mass first, then measuring the leaf area after rehydration with wet-filter-papers, and finally correcting for the leaf area shrinkage relative to the green leaf. We also provided general models for correcting for the SLA biases due to area shrinkage and mass loss of senesced leaves in temperate forests. Our findings will extend the direct measurements of LAI and improve the estimation of carbon and nutrient budgets in forest ecosystems.

We establish a new finite time blowup theorem for the solution to the initial boundary value problem of a class of semilinear pseudo-parabolic equations at high initial energy level. And we also estimate the upper bound of the blowup time.

Carbon dioxide (CO 2 ) fluxes between terrestrial ecosystems and the atmosphere are primarily measured with eddy covariance (EC), biometric, and chamber methods However, it is unclear why the estimates of the CO 2 -fluxes, when measured using different methods, converge at some sites but diverge at others. We synthesized a novel global dataset of forest CO 2 -fluxes to evaluate the consistency between EC and biometric or chamber methods for quantifying the CO 2 budget in forest ecosystems. The EC approach, compared with the other two methods, overestimated net ecosystem production (NEP) by 25% (0.52 Mg C ha −1 yr −1 ), and underestimated ecosystem respiration (Re) by 10% (1.39 Mg C ha −1 yr −1 ) and gross primary production by 3% (0.48 Mg C ha −1 yr −1 ). The differences between EC and the other methods were greater at the sites with complex topography and dense canopy than at the sites with flat topography and open canopy. Forest age also influenced the differences mainly through changes in leaf area index. Open-path EC system induced large positive bias in the NEP estimated by EC, presumably due to its surface-heating effect. These results suggest that EC method likely produce biased estimates of NEP and Re in forest ecosystems. A global extrapolation suggests that the differences in the forest CO 2 -fluxes measured with different methods be consistent with the global overestimation of NEP and underestimation of Re by EC method. Accounting for these differences would substantially improve our estimates of the forest carbon budget. The uncertainties involved in each method were also discussed. To reduce uncertainty in quantifying both local and global carbon budgets, we recommend cross-validation of forest CO 2 -fluxes measured by different methods with more accurate measurements and careful data processing strategies.

Canopy litterfall is an important component of net primary productivity (NPP) in forests, but the climatic and biotic controls on its interannual fluctuation remain poorly understood. In this study, we quantified the interannual variation in and the influencing factors of the canopy litterfall with 45 litter traps in nine plots for 12 years (2008 − 2019) in a temperate deciduous broad-leaved forest in Northeast China. The coefficients of interannual variation in the total (7.3%) and leaf (5.6%) litterfall were substantially lower than those in the basal area (BA) increment (21.0%) and gross primary productivity (10.2%). Climatic (i.e., air temperature, thermal time, precipitation, and radiation) and biotic (i.e., BA and species richness) factors jointly controlled the annual production of litterfall for individual plots and species. Significant climate legacy effects were detected in all components of the litterfall, with the strongest in woody tissue component that contributed to 58.9% of the total litterfall variability. The most important factor driving the variability of the total litterfall and its non-leaf components was climate legacy effect (67.2% to 86.6%), while those for the leaf litterfall were BA (40.4%) and species richness (38.3%). We conclude that high species richness of the deciduous forest increases the temporal stability of canopy productivity, and leaf productivity is much more stable than wood productivity as indicated by BA increament. These findings provide insight into spatiotemporal dynamics in the litterfall and canopy NPP in temperate deciduous forests.

In this paper we deal with the initial boundary value problem for two classes of reaction diffusion systems with two source terms in bounded domain. Under some assumptions on the exponents and the initial data, applying the comparison principle, the maximum principle and the supersolution-subsolution method, we prove the global existence and blow up of solutions. We also establish some upper blow up rates.

Near-surface remote sensing is an effective tool for in situ monitoring of canopy phenology, but the uncertainties involved in sensor-types and their deployments are rarely explored. We comprehensively compared three types of sensor (i.e., digital camera, spectroradiometer, and routine radiometer) at different inclination- and azimuth-angles in monitoring canopy phenology of a temperate deciduous forest in Northeast China for three years. The results showed that the greater contribution of understory advanced the middle of spring (MOS) for large inclination-angle of camera and spectroradiometer. The length of growing season estimated by camera from the east direction extended 11 d than that from the north direction in 2015 due to the spatial heterogeneity, but there was no significant difference in 2016 and 2018.The difference infield of view of sensors caused the MOS and the middle of fall, estimated by camera, to lag a week behind those by spectroradiometer and routine radiometer. Overall, the effect of azimuth-angle was greater than that of inclination-angle or sensor-type. Our assessments of the sensor types and their deployments are critical for the long-term accurate monitoring of phenology at the site scale and the regional/global-integration of canopy phenology data.

Regular polytopes (RPs) are an extension of 2D (two-dimensional) regular polygons and 3D regular polyhedra in n-dimensional ( n &ge, 4 ) space. The high abstraction and perfect symmetry are their most prominent features. The traditional projections only show vertex and edge information. Although such projections can preserve the highest degree of symmetry of the RPs, they can not transmit their metric or topological information. Based on the generalized stereographic projection, this paper establishes visualization methods for 5D RPs, which can preserve symmetries and convey general metric and topological data. It is a general strategy that can be extended to visualize n-dimensional RPs ( n &gt, 5 ).

Litterfall collection is a non-destructive direct method to estimate forest leaf area index (LAI) and validate indirect LAI products. However, the potential errors associated with the variation in specific leaf area (SLA) are rarely explored. Here, we measured the SLA of leaf litter for each tree species in a temperate deciduous forest using the litterfall collection method from 2012 to 2018, and assessed the spatial and temporal variation in SLA and its consequence on LAI estimates. The results showed that the spatial and temporal variation in SLA for the 10 major species across the seven years ranged from 0.8% to 24.3%, with the variation across the nine permanent plots (20 m × 30 m) being higher than that within plot (five 1-m2 traps), and interannual higher than seasonal. Across the 63 plot-years, the 10 simplified SLA sampling protocols introduced the errors in LAI by −12.4% to 22.2% relative to the reference protocol (sampling leaves from each trap at each collection date for the major species). Applying the SLA obtained from one trap in each plot, one plot, at the leaf fall peak for each year, or for a single year induced the errors in LAI by –3.7% to 2.9%, −6.4% to 14.7%, −9.2% to 4.7%, and −8.1% to 8.6%, respectively. Considering the trade-off between inter-plot and interannual variation, we recommend sampling from one-trap for each plot at the leaf-fall peak for measuring SLA and quantifying the spatial pattern of LAI, and sampling each species in the typical plot for measuring SLA and monitoring the temporal fluctuation in LAI. The tight relationship between the MODIS and ground reference LAI across the seven years (R2 = 0.77) validated the use of MODIS LAI to study the long-term change in forest LAI. These findings help to establish a standardized protocol for long-term accurately measuring forest LAI with the litterfall collection.

We consider the well-posedness of solution of the initial boundary value problem to the fourth order wave equation with the strong and weak damping terms, and the logarithmic strain term, which was introduced to describe many complex physical processes. The local solution is obtained with the help of the Galerkin method and the contraction mapping principle. The global solution and the blowup solution in infinite time under sub-critical initial energy are also established, and then these results are extended in parallel to the critical initial energy. Finally, the infinite time blowup of solution is proved at the arbitrary positive initial energy.

The interannual variability (IAV) of CO2 fluxes of temperate forests in continental East Asia was rarely explored via direct measurements. To explore the IAV of net ecosystem exchange of CO2 (NEE) and its driving factors, we continuously measured the CO2 fluxes of a temperate deciduous broadleaved forest at the Maoershan site, northeast China with a continental monsoon climate, by eddy covariance over 11 years (2008 – 2018). The means of NEE, gross primary production (GPP), and ecosystem respiration (Re) were −157 ± 64 (mean ± standard deviation), 1356 ± 148, and 1200 ± 138 g C m−2 yr−1, respectively. For annual fluxes, the IAV of NEE was jointly controlled by the length of net CO2 uptake period and the summer peak of net CO2 uptake, whereas those of GPP and Re were predominated by the summer peaks (GPPmax and Remax). Meanwhile, the dominant role of GPPmax on the IAV of GPP was attributed to the leaf-level photosynthetic capacity rather than the maximum leaf area index. However, the environmental factors showed weak impacts on NEE, largely because of the offset between the positive responses of GPP and Re to the spring and autumn soil water content, respectively. For particular seasons, phenology was the dominant biotic driver of the spring and autumn NEE, whereas spring precipitation and autumn photosynthetically active radiation were the main environmental drivers of the spring and autumn NEE, respectively. Overall, this work filled the gap of the relatively long-term CO2 fluxes measured in the temperate forests of continental East Asia. Taking the influences of ecosystem- and leaf-level physiology, canopy structure, and phenology, together with their environmental interactions, on the CO2 fluxes into account will improve the understanding and prediction of the forest carbon dynanics.

The CO 2 storage ( F s ) measurement can contribute significantly to the estimation of the net ecosystem exchange of CO 2 (NEE) especially in tall-canopy forest ecosystems. The F s is often measured with a profile of CO 2 concentration or with an eddy covariance system at the tower top, but few studies investigated potential errors in the F s measurements. We assessed the errors in F s relevant to the vertical distribution of sampling levels and window sizes of averaging time of CO 2 mixing ratio and their effects on NEE in a temperate deciduous forest site in Northeast China using the standardized major axis method. The CO 2 storage per unit height typically decreased with the height increasing, suggesting that the below-canopy layer need a higher spatial resolution. CO 2 storage could be underestimated by up to one third based only on the tower-top measurement. The uncertainty (standard deviation) of the F s decreased with the length of the averaging time window increasing. However, taking time averaging of CO 2 mixing ratio caused significant underestimate of F s , and consequently led to significant underestimates of CO 2 uptake and release at a 30-min time scale. Our results highlight that appropriately combining spatial resolution and temporal resolution (response time for a whole sampling of all levels) is essential to improving the F s estimates with the existing CO 2 profile systems in forest ecosystems. Since the systematic bias and random error in F s estimate with a single profile system are irreconcilable, there is an urgent need to develop a fast response planar-averaging profile or measure the instantaneous vertical-mean concentration to improve the accuracy of F s measurement.

Despite the economic and ecological importance of key carbon (C) traits (concentration, sequestration, and allocation to different tree components), there are few studies on their variation among populations of a tree species and the possible adaptive significance of these traits. We investigated differences in the C traits among 16 white birch populations grown in two provenance trials in northeastern China. Effects of provenance were significant for most combinations of C trait, tree portion, and trial site. Substantial differences among provenances were found in (1) C concentration in the bole wood and in bark, in fine roots, and at the whole tree scale; (2) C amount sequestered in each studied tree component and in the whole tree; and (3) ratios of C partitioning to bole wood, bole bark, leaves, and existing fine roots. Mostly, significant variations in C concentration and partitioning ratios were independent of provenance volume growth, while those in C amount were strongly positively correlated with volume growth. Summer climate at the provenance origin was the best predictor, of C concentration and allocation but clinal patterns in these traits were found for only some tree components. The C sequestration showed a clinal pattern following the mean annual temperature at the provenance origin. While this suggests an adaptive nature of C traits in white birch, site conditions and/or tree density act strongly on expression of these traits. Implications of this study to C budgeting, trading C credits, and to genetic selection are discussed. Our results may interest ecologists studying dynamics of ecosystems in times of climate change.

Leaf nutrient resorption helps plants minimize nutrient loss via litterfall. The commonly-used parameter to express the resorption, nutrient resorption efficiency (NuRE, the percentage of the leaf nutrient pool withdrawn before abscission), can be compromised by leaf mass loss (LML) and leaf area shrinkage (LAS) when they are not correctly accounted. However, the variations in LML and LAS and their consequences on real NuRE are still poorly understood. To explore the drivers of LML and LAS and their impact on NuRE, we conducted a comprehensive measurement on LML and LAS concomitant with NuRE for 46 deciduous woody species in a temperate forest. The LML and LAS ranged from –3.13% to 32.21% and 0.65% to 18.07%, respectively. On average, ignoring LML (LAS) underestimated the nitrogen and phosphorus resorption efficiencies by 6.93% and 7.39% (4.66% and 4.57%), respectively. The differences in LML and LAS among species and regions should be considered in NuRE studies. The LAS was significantly influenced by plant functional groups and leaf structural traits, while the LML was only weakly related to leaf-fall date. Green leaf area and thickness shrinkages could be used to model LAS. We finally provided suggestions for recovering the historical data and improving future measurements of NuRE.

This paper is devoted to the blowup phenomena for a class of fourth-order wave equation with nonlinear strain term. First, for the model equation with strain structure, we prove the finite time blowup of solution for the critical initial energy, i.e., E ( 0 ) = d . Then, the finite time blowup and the upper bound of the blowup time are established for the sup-critical initial energy, i.e., E ( 0 ) > 0 .

A sufficient condition on initial profile ensuring high initial energy finite time blowup along with the upper bound of the corresponding blowup time for the classical semilinear parabolic equations involving initial boundary value problem is established in this paper.

Autumn (i.e., leaf-fall) phenology plays an important role in regulating the canopy duration and is often monitored using near-surface and satellite remote sensing techniques, but the measurements have rarely been validated by the ground observation. The objectives of this study were to: (1) evaluate the performance of radiometer-based broadband vegetation index (VIB) and Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation index (VIM) in monitoring the autumn phenology of a Chinese temperate deciduous forest with decadal (2008 – 2018) leaf-litterfall measurements, and (2) explore the feasibility of using VIB to validate the VIM products for monitoring the autumn phenology. We found that the seasonal and interannual trends of both VIB and VIM agreed well with those of the leaf-litterfall (correlation coefficients r > 0.93). The broadband normalized difference vegetation index (NDVIB) best tracked the interannual variation in the end-of-season (EOS) among the six metrics of the VIB investigated, while the MODIS enhanced vegetation index (EVIM) did so among the six metrics of the VIM, with the corresponding determination coefficient (R2) of 0.66 and 0.44 with the EOS estimated by the leaf-litterfall. The EOS estimated by VIB and VIM was 0 – 11 d earlier than that by the leaf-litterfall. Comparing the six metrics of VIM with the corresponding ones of VIB, the EOS derived from the NDVI had the closest correlation with each other (R2 = 0.67). Conclusively, our study validated the remote-sensed leaf-fall phenology with decadal ground measurements, and suggested that radiometer and MODIS could effectively track the autumn phenology in temperate deciduous forests and the leaf-litterfall collection could be used as a complementary approach.

Accurate estimation of seasonal leaf area index (LAI) variations is essential for predicting forest growth, but rapid and reliable methods for obtaining such estimates have rarely been reported. In this study, direct measurements of LAI seasonal variations in deciduous broadleaf forests in China were made through leaf seasonality observations in the leaf-out season and litter collection in the leaf-fall season. Meanwhile, indirect LAI measurements were made using a digital hemispherical photography (DHP) method. Our objectives were to explore the relationship between direct and indirect LAI measurements and to recommend a rapid and reliable method to determine the seasonal variation of LAI in forests. To achieve these objectives, we first evaluated seasonal variations of the biases due to key factors (woody materials, clumping effects and incorrect automatic exposure) known to influence the estimation of LAI by DHP. The results showed that the biases due to these factors exhibited different seasonal variation patterns, and the total contribution of these factors could explain 72% of the difference between direct LAI and DHP LAI throughout the entire growing season. Second, linear regression models between direct and DHP LAI were first constructed for each 10-day period as well as the entire growing season. Significance tests were made to the differences among the models for different dates, and models for estimating LAI based on DHP in each date were aggregated to 4 periods with R 2 and RMSE values of 0.91 and 0.22, 0.79 and 0.29, 0.81 and 0.14, 0.97 and 0.14, respectively. There was no significant difference between direct LAI and estimated LAI using the four models in each aggregated period ( p

Optical methods have been widely used to estimate seasonal changes of the leaf area index (LAI) in forest stands because they are convenient and effective; however, their accuracy in deciduous broadleaf forests has rarely been evaluated. We estimate the seasonal changes in the LAI by combining periodic observations of leaf area variation with litter collection (LAIdir) in deciduous broadleaf forests and use these estimates to evaluate the accuracy of optical LAI measurements made using digital hemispherical photography (DHP). We also propose a method to correct DHP-derived LAI (LAIDHP) values for seasonal changes in major factors that influence the determination of LAI, including the woody to total area ratio (α), the element clumping index (ΩE, using three different methods), and the photographic exposure setting (E). Before these corrections were made, LAIDHP underestimates LAIdir by 14%–55% from 21 May to 1 October but overestimates it by 78% on 12 May and by 226% on 11 October. Although pronounced differences are observed between LAIdir and LAIDHP, they are significantly correlated (R2 = 0.85, RMSE = 0.32, P < 0.001). After considering seasonal changes in α, ΩE, and E, the accuracy of LAIDHP improves markedly, with a mean difference between the corrected LAIDHP and LAIdir of DHP is useful and effective for estimating seasonal LAI variation in deciduous broadleaf forests.

The thermally driven wind over mountainous terrains challenges the estimation of CO2 exchange between forests and the atmosphere when using the eddy covariance technique. In this study, the wind regimes were investigated in a temperate deciduous forested valley at the Maoershan site, Northeast China. The wind direction above the canopy was preferentially up-valley in the daytime and down-valley in the nighttime, corresponding to the diurnal patterns of above-canopy temperature gradient and stability parameter. In both leaf-on and -off nighttime, a down-valley flow with a maximum velocity of 1~3 m∙s−1 was often developed at 42 m above the ground (2.3-fold of the canopy height). However, the below-canopy prevailing wind was down-slope in the night, contrast to the below-canopy temperature lapse and unstable conditions. This substantial directional shear illustrated shallow slope winds were superimposed on larger-scale valley winds. As a consequence, the valley-wind component becomes stronger with increasing height, indicating a clear confluence of drainage flow to the valley center. In the daytime, the below-canopy wind was predominated down-slope due to the temperature inversion and stable conditions in the leaf-on season, and was mainly up-valley or down-slope in the leaf-off season. The isolation of momentum flux and radiation by the dense canopy played a key role in the formation of the below-canopy unaligned wind and inverse stability. Significant lateral kinematic momentum fluxes were detected due to the directional shear. These findings suggested a significant interaction between slope and valley winds at this site. The frequent vertical convergence / divergence above the canopy and horizontal divergence/convergence below the canopy in the nighttime / daytime is likely to induce significant advections of trace gases and energy flux.