Different hydraulic strategies under drought stress between Fraxinus mandshurica and Larix gmelinii seedlings

Persistent and severe drought induced by global climate change causes tree mortality mainly due to the hydraulic imbalance of conduit systems,but the magnitude of injury may be species dependent.A water-exclusion experiment was carried out on seedlings of two tree species with distinct characteristics,i.e.,Fraxinus mandshurica and Larix gmelinii to examine hydraulic responses of leaf,stem,and root to drought stress.The two species displayed different hydraulic strategies and related traits in response to drought stress.L.gmelinii reduced its leaf hydraulic conductance by quick stomatal closure and a slow decline in leaf water potential,with a more isohydric stomatal regulation to maintain its water status.In contrast,F.mandshurica was more anisohydric with a negative stomatal safety margin,exhibiting strong resistance to embolism in stem and leafstem segmentation of hydraulic vulnerability to preserve the hydraulic integrity of stem.These differences in hydraulic behaviors and traits between the two species in response to drought stress provide a potential mechanism for their coexistence in temperate forests,including which in the forest modeling would improve our prediction of tree growth and distribution under future climate change.

Population structure and body condition of White-crested Elaenia(Elaenia albiceps)in relation to habitat in a modiffed Neotropical forest landscape

Despite the widely accepted view that planted forests are valuable habitat for wildlife species,there is a lack of empirical evidence showing the suitability of this novel habitat,especially in relation to plantations of exotic tree species.Furthermore,little is known about the effects of forest management on the ecology and dynamics of wild populations in the Neotropics.During the breeding season,the migrant flycatcher White-crested Elaenia is the most abundant bird species in forested landscapes in Chile.For several years we have studied different aspects of these artificial forests in the coastal range of south-central Chile as habitat for the species,particularly in contrast to the native forest.In general,our results indicate that plantation forests offer a poorer quality habitat,where the density of the species tends to be lower than in the native forest,although a significant edge effect suggests that its quality can be mitigated by planting as a matrix mosaic with native forest.Furthermore,compared to native forests,populations in plantations contained a higher proportion of younger adult individuals maintaining larger territories,and showing poorer body condition.We suggest that these differences could also have an impact on the birds'longevity and survival.

Filling the“vertical gap”between canopy tree species and understory shrub species:biomass allometric equations for subcanopy tree species

Subcanopy tree species are an important component of temperate secondary forests.However,their biomass equations are rarely reported,which forms a“vertical gap”between canopy tree species and understory shrub species.In this study,we destructively sampled six common subcanopy species(Syringa reticulate var.amurensis(Rupr.)Pringle,Padus racemosa(Lam.)Gilib.,Acer ginnala Maxim.,Malus baccata(Linn.)Borkh.,Rhamnus davurica Pall.,and Maackia amurensis Rupr.et Maxim.)to establish biomass equations in a temperate forest of Northeast China.The mixed-species and species-specifi c biomass allometric equations were well fi tted against diameter at breast height(DBH).Adding tree height(H)as the second predictor increased the R^(2)of the models compared with the DBH-only models by–1%to+3%.The R^(2)of DBH-only and DBH-H equations for the total biomass of mixed-species were 0.985 and 0.986,respectively.On average,the biomass allocation proportions for the six species were in the order of stem(45.5%)>branch(30.1%)>belowground(19.5%)>foliage(4.9%),with a mean root:shoot ratio of 0.24.Biomass allocation to each specifi c component diff ered among species,which aff ected the performance of the mixed-species model for particular biomass component.When estimating the biomass of subcanopy species using the equations for canopy species(e.g.,Betula platyphylla Suk.,Ulmus davidiana var.japonica(Rehd.)Nakai,and Acer mono Maxim.),the errors in individual biomass estimation increased with tree size(up to 68.8%at 30 cm DBH),and the errors in stand biomass estimation(up to 19.2%)increased with increasing percentage of basal area shared by subcanopy species.The errors caused by selecting such inappropriate models could be removed by multiplying adjustment factors,which were usually power functions of DBH for biomass components.These results provide methodological support for accurate biomass estimation in temperate China and useful guidelines for biomass estimation for subcanopy species in other regions,which can help to improve estimates of forest biomass and carbon stocks.

Eff ect of prescribed burning on the small-scale spatial heterogeneity of soil microbial biomass in Pinus koraiensis and Quercus mongolica forests of China

Prescribed burning can alter soil microbial activity and spatially redistribute soil nutrient elements.However,no systematic,in-depth studies have investigated the impact of prescribed burning on the spatial patterns of soil microbial biomass in temperate forest ecosystems in Northeast China.The present study investigated the impacts of prescribed burning on the small-scale spatial heterogeneity of microbial biomass carbon(MBC)and microbial biomass nitrogen(MBN)in the upper(0–10 cm)and lower(10–20 cm)soil layers in Pinus koraiensis and Quercus mongolica forests and explored the factors that infl uence spatial variations of these variables after prescribed burning.Our results showed that,MBC declined by approximately 30%in the 10–20 cm soil layer in the Q.mongolica forest,where there were no signifi cant eff ects on the soil MBC and MBN contents of the P.koraiensis forest(p>0.05)after prescribed burning.Compared to the MBC of the Q.mongolica forest before the prescribed burn,MBC spatial dependence in the upper and lower soil layers was approximately 7%and 2%higher,respectively.After the prescribed burn,MBN spatial dependence in the upper and lower soil layers in the P.koraiensis forest was approximately 1%and 13%lower,respectively,than that before the burn,and the MBC spatial variability in the 0–10 cm soil layer in the two forest types was explained by the soil moisture content(SMC),whereas the MBN spatial variability in the 0–10 cm soil layer in the two forests was explained by the soil pH and nitrate nitrogen(NO_(3)^(–)-N),respectively.In the lower soil layer(10–20 cm)of the Q.mongolica forest,elevation and ammonium nitrogen(NH 4+-N)were the main factors aff ecting the spatial variability of MBC and MBN,respectively.In the 10–20 cm soil layer of the P.koraiensis forest,NO_(3)^(–)-N and slope were the main factors aff ecting the spatial variability of MBC and MBN,respectively,after the burn.The spatial distributions of MBC and MBN in the two forests were largely structured with higher spatial autocorrelation(relative structural variance C/[C 0+C]>0.75).However,the factors infl uencing the spatial variability of MBC and MBN in the two forest types were not consistent between the upper and lower soil layers with prescribed burning.These fi ndings have important implications for developing sustainable management and conservation policies for forest ecosystems.

Tree sapling vitality and recovery following the unprecedented 2018 drought in central Europe

Background:Ongoing climate change is anticipated to increase the frequency and intensity of drought events,thereby affecting forest recovery dynamics and elevating tree mortality.The drought of 2018,with its exceptional intensity and duration,had a significant adverse impact on tree species throughout Central Europe.However,our understanding of the resistance to and recovery of young trees from drought stress remains limited.Here,we examined the recovery patterns of native deciduous tree sapling species following the 2018 drought,and explored the impact of soil depth,understory vegetation,and litter cover on this recovery.Methods:A total of 1,149 saplings of seven deciduous tree species were monitored in the understory of old-growth forests in Northern Bavaria,Central Germany.The vitality of the saplings was recorded from 2018 to 2021 on 170 plots.Results:Fagus sylvatica was the most drought-resistant species,followed by Betula pendula,Acer pseudoplatanus,Quercus spp.,Corylus avellana,Carpinus betulus,and Sorbus aucuparia.Although the drought conditions persisted one year later,all species recovered significantly from the 2018 drought,albeit with a slight decrease in vitality by 2021.In 2018,the drought exhibited a more pronounced adverse effect on saplings in deciduous forests compared to mixed and coniferous forests.Conversely,sapling recovery in coniferous and mixed forests exceeded that observed in deciduous forests in 2019.The pivotal factors influencing sapling resilience to drought were forest types,soil depth,and understory vegetation,whereas litter and forest canopy cover had a negative impact.Conclusion:Long-term responses of tree species to drought can be best discerned through continuous health monitoring.These findings demonstrate the natural regeneration potential of deciduous species in the context of climate change.Selective tree species planting,soil management practices,and promoting understory diversity should be considered when implementing adaptive management strategies to enhance forest resilience to drought events.

Trees species’ dispersal mode and habitat heterogeneity shape negative density dependence in a temperate forest

Conspecific negative density dependence(CNDD)is a potentially important mechanism in maintaining species diversity.While previous evidence showed habitat heterogeneity and species’dispersal modes affect the strength of CNDD at early life stages of trees(e.g.,seedlings),it remains unclear how they affect the strength of CNDD at later life stages.We examined the degree of spatial aggregation between saplings and trees for species dispersed by wind and gravity in four topographic habitats within a 25-ha temperate forest dynamic plot in the Qinling Mountains of central China.We used the replicated spatial point pattern(RSPP)analysis and bivariate paircorrelation function(PCF)to detect the spatial distribution of saplings around trees at two scales,15 and 50 m,respectively.Although the signal was not apparent across the whole study region(or 25-ha),it is distinct on isolated areas with specific characteristics,suggesting that these characteristics could be important factors in CNDD.Further,we found that the gravity-dispersed tree species experienced CNDD across habitats,while for wind-dispersed species CNDD was found in gully,terrace and low-ridge habitats.Our study suggests that neglecting the habitat heterogeneity and dispersal mode can distort the signal of CNDD and community assembly in temperate forests.

Do forest health threats affect upland oak regeneration and recruitment?Advance reproduction is a key co-morbidity

We analyzed national forest inventory data collected from circa 2000–2018 across 37 states in the eastern United States to better understand the influence of forest health related canopy disturbances on the regeneration and recruitment dynamics of upland oaks(Quercus).We found low levels of oak recruitment across all disturbance types examined but limited evidence of any direct effects from the type of disturbance on the population of regenerating oaks.The general lack of differences in oak regeneration response between forest health disturbances and disturbances caused by harvested or non-disturbed plots does not indicate that the effects of forest health disturbances were benign,however.Instead,low level of oak recruitment across all disturbance types highlights the pervasiveness of the trend of shifting composition in once oak-dominated forests where oak is absent or sparse in the regeneration layer.Our results show that oak recruitment was higher when oak was present as advance reproduction prior to disturbance from any cause examined.Collectively,these results lead us to conclude that the widespread inadequacy of oak advance reproduction in mature oak-dominated forests is the prevailing threat to oak forest health and sustainability.We suggest the status of advance reproduction be treated as a co-morbidity when weighing the risk and potential outcomes from other threats to upland oak forests in the eastern United States.

Effect of nitrogen content on the microstructure and mechanical properties of titanium-bearing nonquenched and tempered steel after hot forging

The microstructure and mechanical properties of titanium(Ti)-bearing medium-carbon nonquenched and tempered steel with different nitrogen content before and after hot forging were investigated through smelting,forging,and laboratory tests.The results show that the grain size of nonquenched and tempered steel was gradually refined,and the ferrite content gradually increased with an increase in nitrogen content.The grain size of the material with low nitrogen content increased abnormally,and its impact properties significantly decreased after hot forging.The grain size of nonquenched and tempered steel with higher nitrogen content was slightly larger than that before forging,and the tensile and yield strength increased,but the impact toughness was not significantly reduced.The Ti-bearing nonquenched and tempered steel showed better strength and toughness after hot forging with the addition of 0.010%0.015%nitrogen.

Effect of quenching and tempering processes on properties and microstructure of G115 steel

The novel martensitic heat-resistant steel G115 was designed for thick-section boiler components of ultra-supercritical(USC) power plants at 630-650 ℃.The impact of the quenching and tempering processes on the properties and microstructure of G115 steel was explored.The samples that were quenched and tempered twice had a higher tensile strength at room temperature and 650 ℃,and the impact energy was significantly improved.The strength and impact energy increased in proportion to the increase in the first quenching temperature.The microstructure differences between the single and double quenched and tempered samples were examined using metallographic microscopy and scanning electron microscopy.The grain size of the double quenched and tempered samples was finer than that of the single quenched and tempered samples, and the tempered martensite lath is more visible, as are the carbides and other precipitates, which are finer and more uniformly distributed.As the first quenching temperature increased, the grains became coarser but more uniform.

Instrumented oscillographic study on impact toughness of an axle steel DZ2 with different tempering temperatures

Compared with the conventional Charpy impact test method,the oscillographic impact test can help in the behavioral analysis of materials during the fracture process.In this study,the trade-off relationship between the strength and toughness of a DZ2 axle steel at various tempering temperatures and the cause of the improvement in impact toughness was evaluated.The tempering process dramatically influenced carbide precipitation behavior,which resulted in different aspect ratios of carbides.Impact toughness improved along with the rise in tempering temperature mainly due to the increase in energy required in impact crack propagation.The characteristics of the impact crack propagation process were studied through a comprehensive analysis of stress distribution,oscilloscopic impact statistics,fracture morphology,and carbide morphology.The poor impact toughness of low-tempering-temperature specimens was attributed to the increased number of stress concentration points caused by carbide morphology in the small plastic zone during the propagation process,which resulted in a mixed distribution of brittle and ductile fractures on the fracture surface.

Grouping tree species to estimate basal area increment in temperate multispecies forests in Durango,Mexico

Multispecies forests have received increased scientific attention,driven by the hypothesis that biodiversity improves ecological resilience.However,a greater species diversity presents challenges for forest management and research.Our study aims to develop basal area growth models for tree species cohorts.The analysis is based on a dataset of 423 permanent plots(2,500 m^(2))located in temperate forests in Durango,Mexico.First,we define tree species cohorts based on individual and neighborhood-based variables using a combination of principal component and cluster analyses.Then,we estimate the basal area increment of each cohort through the generalized additive model to describe the effect of tree size,competition,stand density and site quality.The principal component and cluster analyses assign a total of 37 tree species to eight cohorts that differed primarily with regard to the distribution of tree size and vertical position within the community.The generalized additive models provide satisfactory estimates of tree growth for the species cohorts,explaining between 19 and 53 percent of the total variation of basal area increment,and highlight the following results:i)most cohorts show a"rise-and-fall"effect of tree size on tree growth;ii)surprisingly,the competition index"basal area of larger trees"had showed a positive effect in four of the eight cohorts;iii)stand density had a negative effect on basal area increment,though the effect was minor in medium-and high-density stands,and iv)basal area growth was positively correlated with site quality except for an oak cohort.The developed species cohorts and growth models provide insight into their particular ecological features and growth patterns that may support the development of sustainable management strategies for temperate multispecies forests.

Abiotic and biotic drivers of species diversity in understory layers of cold temperate coniferous forests in North China

Understory plants are important components of forest ecosystems and play a crucial role in regulating community structures,function realization,and community succession.However,little is known about how abiotic and biotic drivers affect the diversity of understory species in cold temperate coniferous forests in the semiarid climate region of North China.We hypothesized that(1)topographic factors are important environmental factors affecting the distribution and variation of understory strata,and(2)different understory strata respond differently to environmental factors;shrubs may be significantly affected by the overstory stratum,and herbs may be more affected by surface soil conditions.To test these hypotheses,we used the boosted regression tree method to analyze abiotic and biotic environmental factors that influence understory species diversity,using data from 280 subplots across 56 sites in cold temperate coniferous forests of North China.Elevation and slope aspect were the dominant and indirect abiotic drivers affecting understory species diversity,and individual tree size inequality(DBH variation)was the dominant biotic driver of understory species diversity;soil water content was the main edaphic factors affecting herb layers.Elevation,slope aspect,and DBH variation accounted for 36.4,14.5,and 12.1%,respectively,of shrub stratum diversity.Shrub diversity decreased with elevation within the range of altitude of this study,but increased with DBH variation;shrub diversity was highest on north-oriented slopes.The strongest factor affecting herb stratum species diversity was slope aspect,accounting for 25.9%of the diversity,followed by elevation(15.7%),slope(12.2%),and soil water content(10.3%).The highest herb diversity was found on southeast-oriented slopes and the lowest on northeast-oriented slopes;herb diversity decreased with elevation and soil water content,but increased with slope.The results of the study provide a reference for scientific management and biodiversity protection in cold temperate coniferous forests of North China.

Litter decomposition and C and N dynamics as affected by N additions in a semi-arid temperate steppe, Inner Mongolia of China

Litter decomposition is the fundamental process in nutrient cycling and soil carbon（C） sequestration in terrestrial ecosystems. The global-wide increase in nitrogen（N） inputs is expected to alter litter decomposition and,ultimately, affect ecosystem C storage and nutrient status. Temperate grassland ecosystems in China are usually N-deficient and particularly sensitive to the changes in exogenous N additions. In this paper, we conducted a 1,200-day in situ experiment in a typical semi-arid temperate steppe in Inner Mongolia to investigate the litter decomposition as well as the dynamics of litter C and N concentrations under three N addition levels（low N with 50 kg N/（hm2？a）（LN）, medium N with 100 kg N/（hm2？a）（MN）, and high N with 200 kg N/（hm2？a）（HN）） and three N addition forms（ammonium-N-based with 100 kg N/（hm2？a） as ammonium sulfate（AS）, nitrate-N-based with 100 kg N/（hm2？a） as sodium nitrate（SN）, and mixed-N-based with 100 kg N/（hm2？a） as calcium ammonium nitrate（CAN）） compared to control with no N addition（CK）. The results indicated that the litter mass remaining in all N treatments exhibited a similar decomposition pattern： fast decomposition within the initial 120 days, followed by a relatively slow decomposition in the remaining observation period（120–1,200 days）. The decomposition pattern in each treatment was fitted well in two split-phase models, namely, a single exponential decay model in phase I（〈398 days） and a linear decay function in phase II（≥398 days）. The three N addition levels exerted insignificant effects on litter decomposition in the early stages（〈398 days, phase I; P〉0.05）. However, MN and HN treatments inhibited litter mass loss after 398 and 746 days, respectively（P〈0.05）. AS and SN treatments exerted similar effects on litter mass remaining during the entire decomposition period（P〉0.05）. The effects of these two N addition forms differed greatly from those of CAN after 746 and 1,053 days, respectively（P〈0.05）. During the decomposition period, N concentrations in the decomposing litter increased whereas C concentrations decreased, which also led to an exponential decrease in litter C：N ratios in all treatments. No significant effects were induced by N addition levels and forms on litter C and N concentrations（P〉0.05）. Our results indicated that exogenous N additions could exhibit neutral or inhibitory effects on litter decomposition, and the inhibitory effects of N additions on litter decomposition in the final decay stages are not caused by the changes in the chemical qualities of the litter, such as endogenous N and C concentrations. These results will provide an important data basis for the simulation and prediction of C cycle processes in future N-deposition scenarios.

Precipitation behavior and martensite lath coarsening during tempering of T/P92 ferritic heat-resistant steel

Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facili- tates the formation of final tempered martensite under serving conditions. In this study, we have gained deeper insights on the mechanism underlying the microstructural evolution during tempering treatment, including the precipitation of carbides and the coarsening of martensite laths, as systematically analyzed by optical microscopy, transmission electron microscopy, and high-resolution transmission electron mi- croscopy. The chemical composition of the precipitates was analyzed using energy dispersive X-ray spectroscopy. Results indicate the for- mation of M3C （cementite） precipitates under normalized conditions. However, they tend to dissolve within a short time of tempering, owing to their low thermal stability. This phenomenon was substantiated by X-ray diffraction analysis. Besides, we could observe the precipitation of fine carbonitrides （MX） along the dislocations. The mechanism of carbon diffusion controlled growth of M23C6 can be expressed by the Zener＇s equation. The movement of Y-junctions was determined to be the fundamental mechanism underlying the martensite lath coarsening process. Vickers hardness was estimated to determine their mechanical properties. Based on the comprehensive analysis of both the micro- structural evolution and hardness variation, the process of tempering can be separated into three steps.

Effects of Si on the stability of retained austenite and temper embrittlement of ultrahigh strength steels

Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of retained austenite and the formation of reversed austenite.

Influence of climate warming and nitrogen deposition on soil phosphorus composition and phosphorus availability in a temperate grassland,China

Climate warming and nitrogen （N） deposition change ecosystem processes, structure, and functioning whereas the phosphorus （P） composition and availability directly influence the ecosystem structure under condi- tions of N deposition. In our study, four treatments were designed, including a control, diurnal warming （DW）, N deposition （ND）, and combined warming and N deposition （WN）. The effects of DW, ND, and WN on P composition were studied by 3~p nuclear magnetic resonance （3~p NMR） spectroscopy in a temperate grassland region of China. The results showed that the N deposition decreased the soil pH and total N （TN） concentration but increased the soil OIsen-P concentration. The solution-state 31p NMR analysis showed that the DW, ND and WN treatments slightly decreased the proportion of orthophosphate and increased that of the monoesters. An absence of myo-inositol phosphate in the DW, ND and WN treatments was observed compared with the control. Furthermore, the DW, ND and WN treatments significantly decreased the recovery of soil P in the NaOH-EDTA solution by 17%-20%. The principal component analysis found that the soil pH was positively correlated with the P recovery in the NaOH-EDTA solution. Therefore, the decreased soil P recovery in the DW and ND treatments might be caused by an indirect influence on the soil pH. Additionally, the soil moisture content was the key factor limiting the available P. The positive correlation of total carbon （TC） and TN with the soil P composition indicated the influence of climate warming and N deposition on the biological processes in the soil P cycling.

Control Method for Steel Strip Roughness in Two-stand Temper Mill Rolling

How to control surface roughness of steel strip in a narrow range for a long time has become an important question because surface roughness would significantly influence the appearance of the products. However, there are few effective solutions to solve the problem currently. In this paper, considering both asperities of work roll pressing in and squeezing the steel strip, two asperity contact models including squeezing model and pressing in model in a two-stand temper mill rolling are established by using finite element method（FEM）. The simulation investigates the influences of multiple process parameters, such as work roll surface roughness, roll radius and roll force on the surface roughness of steel strip. The simulation results indicate that work rolls surface roughness and roll force play important roles in the products; furthermore, the effect of roll force in the first stand is opposite to the second. According to the analysis, a control method for steel strip surface roughness in a narrow range for a long time is proposed, which applies higher work roll roughness in the first stand and lower roll roughness in the second to make the steel strip roughness in a required narrow range. In the later stage of the production, decreasing the roll force in the first stand and increasing the roll force in the second stand guarantee the steel strip roughness relatively stable in a long time. The following experimental measurements on the surface topography and roughness of the steel strips during the whole process are also conducted. The results validate the simulation conclusions and prove the effect of the control method. The application of the proposed method in the steel strip production shows excellent performance including long service life of work roll and high finished product rate.

Tempering microstructure and mechanical properties of pipeline steel X80

The tempering microstructure and mechanical properties of X80 steel used for heating-bent pipe were analyzed. The results show that the microstructure of X80 steel tempered at 550 ℃ and 600 ℃ is bainitic ferrite (BF)+granular bainite (GB), and partial ferrite laths in BF merge and broaden. The interior sub-lath boundary of some GB begins to disappear due to merging, the M/A constituent (a mixture of martensite plus retained austenite) in GB is orbicular. At the two tempering temperatures the tested X80 steel shows a certain degree of tempering stability. After being tempered at 650 ℃, the microstructure of X80 steel is GB+quasi-polygonal ferrite(QF), and the original BF laths have merged to form smaller GB crystal grains. The reason is that the steel shows better match of strength and toughness. After being tempered at 700 ℃ , the microstructure of X80 steel is composed mainly of QF, which can improve the plasticity but decline severely the yield strength of X80, and the M/A constituent assembles and grows up at the grain boundary of QF, resulting in excellent lower low-temperature toughness of X80.

Land cover changes and fragmentation in mountain neotropical ecosystems of Oaxaca, Mexico under community forest management

Changes in land cover have a direct impact on forest ecosystem goods and services. In this study, changes in land cover in Sierra de Juarez–Oaxaca ecosystems were estimated using a consistent processing of Landsat images and OBIA methodology. Additionally, landscape analyses using FRAGSTAT were conducted. In 2014, Sierra de Juarez–Oaxaca was covered by approximately 84% of forests, mainly pine-oak and cloud forests. After extensive deforestation until 2001, this trend was reversed and the forest cover surface area in 2014 was slightly higher than in 1979. The comparison of the landscape structure of the forested and agricultural lands suggests an increase in habitat heterogeneity. However, interspersion and juxtaposition indices, showing the patch shape by patch area and perimeter, were similar throughout the study period(1979–2014). Social and economic drivers can explain this situation: namely, community organization, forest enterprises, payment for ecosystem services programs, and changes of agricultural activity. Communities in the Sierra of Oaxaca have reforested degraded lands, created community forest enterprises, and preserved the forest under conservation schemes like those proposed by the Mexican payment for ecosystem services programs. However, their sustainable management faces internal challenges and has become highly dependent on political and institutional decisions beyond their control.

Carbon,nitrogen and phosphorus stoichiometry in Pinus tabulaeformis forest ecosystems in warm temperate Shanxi Province,north China

Although carbon(C), nitrogen(N), and phosphorous(P) stoichiometric ratios are considered good indicators of nutrient excess/limitation and thus of ecosystem health, few reports have discussed the trends and the reciprocal effects of C:N:P stoichiometry in plant–litter–soil systems. The present study analyzed C:N:P ratios in four age groups of Chinese pine, Pinus tabulaeformis Carr., forests in Shanxi Province, China: plantation young forests(AY,<20 year-old); plantation middle-aged forests(AM, 21–30 year-old); natural young forests(NY,<30 year-old); and natural middle-aged forests(NM,31–50 year-old). The average C:N:P ratios calculated for tree, shrub, and herbaceous leaves, litter, and soil(0–100 cm) were generally higher in NY followed by NM,AM, and AY. C:N and C:P ratios were higher in litter than in leaves and soils, and reached higher values in the litter and leaves of young forests than in middle-aged forests;however, C:N and C:P ratios were higher in soils of middle-aged forests than in young forests. N:P ratios were higher in leaves than in litter and soils regardless of stand age; the consistent N:P<14 values found in all forests indicated N limitations. With plant leaves, C:P ratios were highest in trees, followed by herbs and shrubs, indicating a higher efficiency in tree leaf formation. C:N ratios decreased with increasing soil depth, whereas there was no trend for C:P and N:P ratios. C:N:P stoichiometry of forest foliage did not exhibit a consistent variation according to stand age. Research on the relationships between N:P, and P, N nutrient limits and the characteristics of vegetation nutrient adaptation need to be continued.