Partitioning tree water usage into storage and transpiration in a mixed forest

Background:Water migration and use are important processes in trees.However,it is possible to overestimate transpiration by equating the water absorbed through the plant roots to that diffused back to the atmosphere through stomatal transpiration.Therefore,it is necessary to quantify the water transpired and stored in plants.Method:The δ^(2)H/δ^(18)O technique and heat ratio method were used to explore the water usage of coniferous and broad-leaved tree species,including the proportions of water used for transpiration and water storage.Results:Platycladus orientalis and Quercus variabilis had strong plasticity in their water usage from different sources.Platycladus orientalis primarily used groundwater(30.5%)and the 60-100-cm soil layer(21.6%)throughout the experimental period and was sensitive to precipitation,absorbing water from the 0-20-cm layer(26.6%)during the rainy season.Quercus variabilis absorbed water from all sources(15.7%-36.5%)except from the 40-60-cm soil layer during the dry season.In addition,it did not change its water source but increased its groundwater uptake during the rainy season.The annual mean water fluxes of P.orientalis and Q.variabilis were 374.69 and 469.50 mm·year−1,with 93.49% and 93.91% of the water used for transpiration,respectively.However,nocturnal sap flow in P.orientalis and Q.variabilis was mainly used for water storage in the trunk rather than transpiration,which effectively alleviated drought stress and facilitated the transport of nutrients.Conclusions:The water stored in both species comprised 6%-7% of the total water fluxes and,therefore,should be considered in water balance models.

Variation in water supply leads to different responses of tree growth to warming

Background:Global climate change,characterized by changes in precipitation,prolonged growing seasons,and warming-induced water deficits,is putting increased pressure on forest ecosystems globally.Understanding the impact of climate change on drought-prone forests is a key objective in assessing forest responses to climate change.Methods:In this study,we assessed tree growth trends and changes in physiological activity under climate change based on measurements of tree ring and stable isotopes.Additionally,structural equation models were used to identify the climate drivers influencing tree growth for the period 1957–2016.Results:We found that the mean basal area increment decreased first and then increased,while the water use efficiency showed a steady increase.The effects of climate warming on tree growth switched from negative to positive in the period 1957–2016.Adequate water supply,especially snowmelt water available in the early critical period,combined with an earlier arrival of the growing season,allowed to be the key to the reversal of the effects of warming on temperature forests.The analysis of structural equation models(SEM)also demonstrated that the growth response of Pinus tabuliformis to the observed temperature increase was closely related to the increase in water availability.Conclusions:Our study indicates that warming is not the direct cause of forest decline,but does indeed exacerbate droughts,which generally cause forest declines.Water availability at the beginning of the growing season might be critical in the adaptation to rising temperatures in Asia.Temperate forests may be better able to withstand rising temperatures if they have sufficient water,with boosted growth even possible during periods of rising temperatures,thus forming stronger carbon sinks.

Separating component parts of soil respiration under Robinia pseudoacacia plantation in the Taihang Mountains,China

Partitioning the respiratory components of soil surface CO2 efflux is important in understanding carbon turnover and in identifying the soil carbon sink/source function in response to land-use change. The sensitivities of soil respiration components on changing climate patterns are currently not fully understood. We used trench and isotopic methods to separate total soil respiration into autotrophic （RA） and heterotrophic components （RH）. This study was undertaken on a Robinia pseudoacacia L. plantation in the southern Taihang Mountains, China. The fractionation of soil ^13CO2 was analyzed by comparing the δ^13C of soil CO2 extracted from buried steel tubes with results from Gas Vapor Probe Kits at a depth of 50 cm.at the preliminary test （2.03‰）. The results showed that the contribution of autotrophic respiration （fRA） increased with increasing soil depth.The contribution of heterotrophic respiration （fR/4） declined with increasing soil depth. The contribution of autotrophic respiration was similar whether estimated by the trench method （fRA, 23.50%） or by the isotopic method in which a difference in value of ^13C between soil and plant prevailed in the natural state （RC, 21.03%）. The experimental error produced by the trench method was insignificant as compared with that produced by the isotopic method, providing a technical basis for further investigations.

Response of forestland soil water content to heavy rainfall on Beijing Mountain, northern China

Continuous recording of precipitation and soil water content（SWC）, especially during long periods of torrential rainfall, has proven challenging. Over a 16 h period spanning 21-22 July, 2012, Beijing experienced historic rainfall that totaled 164.4 mm. We used large lysimeter technology in four forested plots to record precipitation and variation in SWC at 10-min intervals to quantify the response of forestland SWC to heavy rainfall in a semi-arid area. Mean,maximum and minimum rainfall intensities were 23.4, 46.8and 12.0 mm/h, respectively. Rainfall was concentrated in 2-6 mm bursts that accounted for 67.32 % of the total rainfall event. Soil moisture conditions in this region are strongly dependent on patterns of precipitation. Water infiltration into 20, 40, 60, 80, 100, 120 and 160 cm soil layers required 1, 5,20, 37, 46, 52 and 61 mm of precipitation, respectively, and to fully saturate these soil layers required 80, 120, 140, 150, 180,200 and 220 mm of precipitation, respectively.

Photosynthetic stimulation of saplings by the interaction of CO2 and water stress

Climate change necessitates research into interactions between elevated carbon dioxide(CO2)concentrations and drought on plant photosynthetic physiology.This study describes the physiological properties of Platycladus orientalis(Chinese thuja)and Quercus variabilis(Chinese cork oak)saplings cultivated through orthogonal treatments of four CO2 concentrations combined with five soil volumetric water contents(SWC).It highlights the differences between the interactive effects from the treatments.Water stress had little effect on photosynthetic traits until the soil volumetric water contents exceeded 70–80 or 100%.Similar variations in carbon-13 isotope abundance(δ^13C)of water soluble compounds(δ^13CWSC)extracted from leaves of two species have been observed.Whether soil volumetric water contents exceeded or fell below the water threshold values(70–80%of field capacity for P.orientalis and 100%of field capacity for Q.variabilis),instantaneous water use efficiency decreased.Elevated carbon dioxide could increase iWUE and enhance drought tolerance,depending on stimulating net photosynthetic rates and declining stomatal conductance and transpiration rates.Augmenting either drought,excess water,or ambient carbon dioxide could alleviate the physiological inhibition caused by the stresses described above.

The anatomical structure of woody plants in arid habitats is closely related to nonstructural carbohydrates storage

Background:The high temperatures and drought conditions associated with global climate change have led to the widespread death of trees and forests,which has generated research interest in the impact of non-structural carbohydrates(NSC)reserves on woody plant survival in changing environments.The anatomical characteristics of plants can affect the scale and variability of NSC reserves,and linking the anatomical characteristics of trees to NSC concentration can help answer questions about the potential role of NSC in the repair of xylem embolism during drought.Results:The anatomical characteristics,NSC concentrations and gas exchange parameters were determined in 16 common afforestation tree species in typical arid limestone habitats in rocky mountain regions.We found that the anatomical structures varied greatly among the tree species studied,and a branch xylem hydraulic efficiency-safety tradeoff was not observed.NSC concentration was significantly and positively correlated with variables including mean vessel diameter(MVD),maximum vessel diameter(XVD),potential hydraulic conductivity(Kp),vessel wall thickness(VWT),ratio of palisade tissue thickness to sponge tissue thickness(PT/ST),net photosynthetic rate(Pn),water use efficiency(WUE)and axial parenchyma cell area,and negatively with sponge tissue thickness(ST)and lower epidermis thickness(LET)of leaves.Among the four latent variables,mechanical strength was inversely related to NSC concentration,whilst hydraulic efficiency,leaf carbon fixation,and the embolism repair and storage capacity all had a positive effect.Conclusions:Our study revealed that tree species with larger vessel diameters,thicker vessel walls,more abundant axial parenchyma,and higher PT/ST have higher NSC storage in arid habitats in northern China.Varying xylem anatomical characteristics and leaf anatomical characteristics among different tree species lead to differentiated water transport and photosynthetic processes,thereby regulating the NSC concentrations.A higher NSC concentration may enhance the embolism repair capacity of plants and play an important role in maintaining hydraulic integrity in arid habitats.

Study on Daily ET Based on SEBAL Model in a Typical Watershed of the Loess Plateau of China

Under the double impact of global climate change and human intervention, surface evapotranspiration (ET), as an important part of hydrological cycle, is affecting watershed land-use planning and economic development. The Loess Plateau is mostly arid and semi-arid areas and its ecological environment is fragile. Drought and soil erosion not only reflects the regional climate change, but also human social activities and ecosystems. In particular, these processes are directly related to the thermal and water gradients exchange in the soil-vegetation-atmosphere system. Luoyugou watershed, a typical Loess Plateau watershed, is selected for this study. Estimating ET through SEBAL (Surface Energy Balance Algorithm for Land) model demonstrates that the SEBAL model in the watershed is more applicable. At the same time, ET seasonal variation is got, combined with the corresponding periods of the land-use changes for analysis. The results show that ET in the Northwest of the basin is higher than the East on the spatial distribution, especially in the high vegetation coverage area. Land use changes significantly in the watershed over the past 20 years, mainly transferring from sloping terraces to the terraces and woodland. ET significantly increased area accounts for 47.6% of the total area in the valley, however, conspicuously reduced area accounts for 13.2%. The daily ET of different land use types had significant differences with the ET of water land and wood land was the largest. It is important that this study on the ET distribution can aid policy and decision makers for land use planning and environmental construction in Luoyugou watershed.

Influence of physiological and environmental factors on the diurnal variation in emissions of biogenic volatile compounds from Pinus tabuliformis

Biological volatile organic compounds(BVOCs) have a large influence on atmospheric environmental quality, climate change and the carbon cycle. This study assesses the composition and diurnal variation in emission rates of BVOCs from Pinus tabuliformis, using an enclosure technique. Environmental parameters(temperature and light intensity) and physiological parameters(net photosynthetic rate, P_n; stomatal conductance, g_s; intercellular CO_2 concentration, C_i; and transpiration rate, T_r) that may affect emission behavior were continuously monitored. The 10 most abundant compound groups emitted by P. tabuliformis were classified by gas chromatography–mass spectrometry. The dominant monoterpenoid compounds emitted were α-pinene, β-myrcene, α-farnesene and limonene. The diurnal emission rate of BVOCs changed with temperature and light intensity, with dynamic analysis of BVOCs emissions revealing that their emission rates were more affected by temperature than light. The variation in monoterpene emission rates was consistent with estimates of P_n, g_s and T_r. Basal emission rates(at 30 °C,) of the main BVOCs ranged from 0.006 to 0.273 μg^(-1)/(hr g),while the basal ER standardization coefficients ranged from 0.049 to 0.144 °C^(-1). Overall, these results provide a detailed reference for the effective selection and configuration of tree species to effectively prevent and control atmospheric pollution.

Relationship between types of urban forest and PM_(2.5) capture at three growth stages of leaves

Particulate matter diameter ≤ 2.5 μm（PM2.5） causes direct harm to human health. Finding forms of urban forest systems that with the ability to reduce the amount of particulate matter in air effectively is the aim of this study. Five commonly cultivated kinds of urban forest types were studied in Beijing city at three stages of leaf growth. Results show that the urban forest system is capable of storing and capturing dust from the air. The types of shrubs and broadleaf trees that have the ability to capture PM2.5from the air are most effective when leaves have fully developed. In the leafless season, the conifer and mixed tree types are the most effective in removing dust from the air. For all kinds of forest types and stages of leaf growth, the PM2.5concentration is highest in the morning but lower in the afternoon and evening. Grassland cannot control particles suspended in the air,but can reduce dust pollution caused by dust from the ground blown by the wind back into the air.

National assessment of soil erosion and its spatial patterns in China

The spatial patterns of soil erosion(SE)are an important part of ecological security patterns and critical to erosion control.We assessed the SE and its spatial distribution in China based on geographic information system(GIS)and spatial data sets using the Universal Soil Loss Equation(USLE).The soil erosion area(SEA)and soil erosion amount(SEM)totaled 173.06 million ha and 8.87 billion Mg,respectively,with an average soil erosion rate(SER)of 9.39 Mg·ha^(-1)·yr^(-1).Slight erosion dominated from the aspect of SEA,whereas extreme erosion contributed the most in terms of SEM.Spatial heterogeneity in soil erosion was obvious in China,with heavily eroded areas mainly concentrated in the Loess Plateau,the Three Gorges reservoir area,and the hot,dry valley of the Jinsha River.Regionally,the provinces of Tibet,Sichuan,Yunnan,Xinjiang,Inner Mongolia,Gansu,Shaanxi,Shanxi,Guizhou,and Guangxi,and the basins of the Yangtze River,Yellow River,and southwestern rivers made a large contribution to the SEA and SEM.Geographically,soil erosion increased,then decreased with increasing slope and elevation.Slopes of 15-25°and 8-15°and elevations of 1000-2000 m were the most seriously eroded.Cropland and grassland ecosystems were major sources of SE,with their SEA and SEM accounting for 64.44%and 77.96%of the total.This study revealed the current situation and spatial characteristics of SE in China on the national scale,which can serve as a scientific basis for regional SE control and decision-making policy.

Effects of forest cover type and ratio changes on runoff and its components

Changes in forest cover can affect not only the total runoff from a watershed,but also the runoff components(e.g.,surface runoff,interflow,groundwater flow).In this study,based on the WetSpa model simulation method and the recursive digital filtering(RDF)method,the Banchengzi watershed in the mountainous region of Beijing,China,was selected to investigate how changes in forest cover type and cover percentage affect total runoff,surface runoff,interflow,and groundwater flow through scenario settings.Our results show that the difference between the WetSpa model and the RDF method for separating runoff components is small,with only 4.7%and 0.4%difference between the calibration and validation periods.Total runoff in different forest types followed the order shrub forest>coniferous forest>mixed forest>broadleaf forest.Regarding runoff components,the proportions of baseflow(sum of interflow and groundwater flow)to total runoff were 61.1%and 60.8%for broadleaf and mixed forests,which was significantly higher than those of 53.0%and 43.1%for coniferous and shrub forests.However,the proportion of shrub forest baseflow was high in wet years,and that of broadleaf forest baseflow was high in normal and dry years.The proportions of interflow and groundwater flow from various forest cover types to total runoff continued to increase with increasing forest cover rate.Our results have important implications for the implementation of afforestation projects and forest conservation programs,contributing to water resource regulation and ecosystem protection in watersheds.