Forest Transpiration Research Articles

Evapotranspiration from the understory of a tropical dry deciduous forest in Cambodia

The water balance of tropical dry deciduous forests is less well understood than some other forest ecosystems. To help close this knowledge gap, we separately measured the evapotranspiration from the whole ecosystem (ETW), transpiration (TR) and interception loss (IL) from overstory trees, and evapotranspiration from the understory vegetation (ETU) in a tropical dry deciduous forest in Cambodia. It was found that ETW was equivalent to 73.7% of rainfall (P) at the annual scale. In the dry season, ETW corresponded to 120.1% of P, which indicates the utilization of soil water replenished during the wet season. The sum of transpiration estimated by the thermal dissipation (TD) method with the original coefficient (TRG), IL, and ETU was smaller than ETW, except for the middle of the dry season, due to an underestimation of TRG. Although recently reported calibration coefficients can reasonably correct TRG, future calibrations of the TD method are highly recommended for the precise evaluation of single-tree-scale transpiration in tropical dry forests. The annual contribution of the understory vegetation to ETW (ETU/ETW) was 34.6%, leading to the conclusion that the understory vegetation cannot be ignored when trying to gain a comprehensive understanding of the hydrologic cycle in tropical dry forests. The seasonal variations in ETU/ETW were mainly controlled by the leaf area index (LAI) of overstory trees, resulting from the overall stability of ETW and decreasing trend of ETU with increasing LAI in the wet season, with the opposite holding true in the dry season, i.e., decreasing ETW with the decline of LAI and less variations of ETU. Thus, LAI influenced both the seasonality and the annual contribution in ETU/ETW, exerting a notable influence on hydrological cycling in this forest.

The Variation in Water Consumption by Transpiration of Qinghai Spruce among Canopy Layers in the Qilian Mountains, Northwestern China

It is important for integrated forest-water management to develop a better understanding of the variation of tree transpiration among different canopy layers in the forests and its response to soil moisture and weather conditions. The results will provide insights into water consumption by trees occupying different social positions of the forests. In the present study, an experiment was conducted in the Qilian Mountains, northwest China, and 13 trees, i.e., 4–5 trees from each one of dominant (the relative tree height (HR) > 1.65), subdominant (1.25 < HR ≤ 1.65) and intermediate-suppressed (HR ≤ 1.25) layers) were chosen as sample trees in a pure Qinghai spruce (Picea crassifolia Kom.) forest stand. The sap flux density of sample trees, soil moisture of main root zone (0 to 60 cm) and meteorological conditions in open field were observed simultaneously from July to October of 2015 and 2016. The results showed that (1) The mean daily stand transpiration for the study period in 2015 and 2016 (July–October), was 0.408 and 0.313 mm·day−1, and the cumulative stand transpiration was 54.84 and 40.97 mm, accounting for 24.14% (227.2 mm) and 16.39% (249.9 mm) of the total precipitation over the same periods, respectively. (2) The transpiration varied greatly among canopy layers, and the transpiration of the dominant and codominant layers was the main contributors to the stand transpiration, contributing 86.05% and 81.28% of the stand transpiration, respectively, in 2015 and 2016. (3) The stand transpiration was strongly affected by potential evapotranspiration (PET) and volumetric soil moisture (VSM). However, the transpiration of trees from the dominant and codominant layers was more sensitive to PET changes and that from the intermediate-suppressed layer was more susceptible to soil drought. This implied that in dry period, such as in drought events, the dominant and codominant trees would transpire more water, while the intermediate-suppressed trees almost stopped transpiration. These remind us that the canopy structure was the essential factor affecting single-tree and forest transpiration in the dryland areas.

Inconsistent Responses of Transpiration of Different Canopy Layers to Simulated Canopy and Understory N Depositions in a Low‐Subtropical Evergreen Broadleaf Forest

AbstractQuantifying transpiration and its response to meteorological factors at a stand scale and different canopy layers of forest is important for understanding the hydrological impact of nitrogen (N) deposition on local water balance. For this purpose, sap flows were measured in three upper (Schima superba, Castanea henryi, and Machilus chinensis) and three lower (Symplocos ramosissima, Ilex ficoidea, and Schefflera octophylla) canopy species based on a manipulative experimental platform with canopy (CN) and understory (UN) N additions in a low‐subtropical evergreen broadleaf forest. Each application included concentrations of 0, 25, and 50 kg·ha−1·a−1 N. Results showed that N addition obviously influenced the sap flux density and such influence varied among different species and between canopy layers. Upper canopy species contributed the most transpiration for the whole forest, and CN25 greatly affected the transpiration in both upper and lower canopy layers. Daily water use (Qd) increased logarithmically with daytime average photosynthetically active radiation and daily average vapor pressure deficit. An appropriate canopy N addition (CN25) promoted the Qd and its sensitivity to micrometeorological factors, whereas UN had a minimal effect. These results suggest a substantial difference in response of transpiration to the N addition ways. CN had more effective influence on the canopy transpiration. Thus, UN could not fully reflect the effects of increased N deposition on the canopy‐associated transpiration, and the response of forest transpiration to N deposition needs to be analyzed beyond species and upscaled to the canopy level in favor of meaningful assessments of N deposition.

Editorial: Biographical Memoirs , Volume 68

Editorial: <i>Biographical Memoirs</i> , Volume 68

Sparse Pinus Tabuliformis Stands Have Higher Canopy Transpiration Than Dense Stands Three Decades After Thinning

Hydrological effects of forest thinning have been studied at small watershed scales using the paired watershed approach since the 1920s. However, how forest transpiration, a critical component of evapotranspiration, changes decades after thinning is not well understood despite its importance for modifying drought resilience of forest ecosystem under climate change. In a semi-arid mountainous area of northern China, we measured growing season sap flow of Pinus tabuliformis, a widely planted afforestation species, in 44-year-old monoculture plantation stands with low (983 stems ha−1), medium (1688 stems ha−1), and high (2160 stems ha−1) density. Three decades after thinning, diameters at breast height (DBH) were larger in sparse stands than in dense stands. While its relation with sapwood area was density independent, the accompanying high sapwood area at the tree level for sparse stands resulted the highest stand sapwood area in the medium density stand (33.26 m2 ha−1), rather than in the high density stand (29.84 m2 ha−1). Similar to short-term studies, sparse stands demonstrated higher sensitivity to climatic fluctuations and drought depressions than dense ones. Nevertheless, stand density had no effect on the isohydric strategy of Pinus tabuliformis. Contrary to the positive relation between stand density and stand canopy transpiration soon after thinning, sparse stands exhibited higher growing season canopy transpiration than dense stands three decades later. In the dry year 2014, these density differences were relatively most pronounced, with July-September transpiration totals of 56.7 mm, 31.1 mm, and 22.1 mm in the low, medium, and high density stands, respectively. Our findings highlighted that stand density was not an appropriate indicator of thinned forest transpiration over long time scales. Interactions between soil droughts and thinning on forest transpiration need to be further clarified, especially in longer periods of time.

半干旱区城市环境下油松林分蒸腾特征及其影响因子

PDF HTML阅读 XML下载 导出引用 引用提醒 半干旱区城市环境下油松林分蒸腾特征及其影响因子 DOI: 10.5846/stxb201811262571 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家科技支撑计划项目（2015BAD07B06-4） The stand transpiration characteristics of Pinus tabulaeformis and its influential factors in a semi-arid urban environment Author: Affiliation: Fund Project: The research and demonstration on the cultivation and management technology system of water-saving urban landscape forests in Inner Mongolia and Beijing 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:在城市环境下，由于不透水地面面积的增加，土壤-植物-大气之间水汽循环减弱，水汽调节能力差，因而研究城市树木蒸腾对环境因子的响应对于城市进行合理的水汽调节具有重要意义。于2017年生长季，在内蒙古呼和浩特市区树木园内选择58年生油松（Pinus tabulaeformis Carr.）作为研究树种，采用热扩散法测定其树干液流，并同步监测气象因子和土壤含水量变化，利用彭曼公式计算冠层气孔导度。结果表明：（1）生长季内，油松林分蒸腾存在明显日、月变化，晴天天气下林分蒸腾日变化呈单峰曲线，月林分蒸腾量5月最大，其次是7月、8月、6月和9月，分别为20.96、19.89、18.09、17.25 mm和7.49 mm。（2）油松林分蒸腾与饱和水汽压差、太阳总辐射、土壤含水量和风速均存在极显著相关关系（P<0.01），太阳总辐射、饱和水汽压差和土壤含水量是影响林分蒸腾的主要环境因子（R2=0.47、R2=0.31和R2=0.16），风速对林分蒸腾的影响程度最小（R2=0.12）；不同降雨量对林分蒸腾的影响作用不同，10 mm以上的日降雨量对油松林分蒸腾作用明显。（3）除环境因子外，油松叶片气孔通过响应环境变化控制蒸腾作用，当饱和水汽压差<1.5 kPa时，叶片气孔对饱和水汽压差的响应更敏感；当太阳总辐射<250 W/m2时，叶片气孔对蒸腾起促进作用，超过该阈值，叶片气孔关闭从而抑制树木蒸腾。 Abstract:The water vapor cycle between soil-plant-atmosphere is weakened and its ability for regulation is poor due to the increase in impervious surface areas in urban environments. Therefore, it is of great significance to study the transpiration response of urban trees to environmental factors amidst reasonable water vapor regulation in urban areas. In the growing season of 2017, 58-year-old Pinus tabulaeformis Carr. was selected as a research tree species for sap flow measurement, by thermal diffusion, at the Hohhot arboretum, Inner Mongolia. We measured meteorological factors and soil water content simultaneously and estimated canopy stomatal conductance using the Penman Formula. The results showed that (1) stand transpiration of P. tabulaeformis had obvious daily and monthly changes in the growing season. The diurnal stand transpiration variation graph showed a single peak curve on sunny days. Monthly stand transpiration reached the maximum in May, followed by July, August, June and September, which was 20.96, 19.89, 18.09, 17.25 mm and 7.49 mm, respectively. (2) There were extreme significant correlations between stand transpiration and vapor pressure deficits, global radiation, soil water content, and wind speed (P<0.01). Global radiation, vapor pressure deficit, and soil water content were the main environmental factors affecting forest transpiration (R2=0.47, R2=0.31 and R2=0.16), and wind speed had the smallest impact on forest transpiration (R2=0.12). Different rainfall events had varied effects on stand transpiration, but a significant effect when daily rainfall exceeded 10 mm. (3) In addition to environmental factors, leaf stomata of P. tabulaeformis controlled transpiration in response to environmental factors. Leaf stomata were more sensitive to vapor pressure deficits when it was < 1.5 kPa, whereas, global radiation, when < 250 W/m2, promoted transpiration. When the threshold was exceeded, leaf stomata would close to suppress tree transpiration. 参考文献 相似文献 引证文献

Paul Gordon Jarvis. 23 May 1935—5 February 2013

Paul Gordon Jarvis was a widely known and well-respected plant ecologist and physiologist, who pioneered the scientific analysis of the exchange of water and carbon dioxide between forests and the atmosphere, and laid the foundations for decades of study on the interplay between forests and the climate system. He was one of the first to measure directly the photosynthesis and transpiration of forests, and leading from this, his analysis of the relationships between the physiology of plants and the weather has informed and inspired a generation of young scientists. In particular, he was one of the first to address the linkage between knowledge gained at the microscopic scale of stomata and the landscape scale of forests, and the implications of that linkage for the climate system.

How afforestation affects the water cycle in drylands: A process-based comparative analysis.

The world's largest afforestation programs implemented by China made a great contribution to the global "greening up." These programs have received worldwide attention due to its contribution toward achieving the United Nations Sustainable Development Goals. However, emerging studies have suggested that these campaigns, when not properly implemented, resulted in unintended ecological and water security concerns at the regional scale. While mounting evidence shows that afforestation causes substantial reduction in water yield at the watershed scale, process-based studies on how forest plantations alter the partitioning of rainwater and affect water balance components in natural vegetation are still lacking at the plot scale. This lack of science-based data prevents a comprehensive understanding of forest-related ecosystem services such as soil conservation and water supply under climate change. The present study represents the first "Paired Plot" study of the water balance of afforestation on the Loess Plateau. We investigate the effects of forest structure and environmental factors on the full water cycle in a typical multilayer plantation forest composed of black locust, one of the most popular tree species for plantations worldwide. We measure the ecohydrological components of a black locust versus natural grassland on adjacent sites. The startling finding of this study is that, contrary to the general belief, the understory-instead of the overstory-was the main water consumer in this plantation. Moreover, there is a strict physiological regulation of forest transpiration. In contrast to grassland, annual seepage under the forest was minor in years with an average rainfall. We conclude that global long-term greening efforts in drylands require careful ecohydrologic evaluation so that green and blue water trade-offs are properly addressed. This is especially important for reforestation-based watershed land management, that aims at carbon sequestration in mitigating climate change while maintaining regional water security, to be effective on a large scale.

Dominance-caused differences in transpiration of trees in a Karst broadleaved mixed forest

Estimating stand transpiration of natural forests using traditional methods through up-scaling of sap flux density from sample trees based on stand sapwood area only is difficult because of the complexity of species, ages, and hierarchical structure of natural forests. To improve stand transpiration estimation, we developed an up-scaling method by considering the tree dominance effect based on the assumption that individual tree transpiration is affected by crown dominance and species, in addition to factors previously considered such as meteorological conditions, sapwood area, and soil moisture. In this study, the meteorological factors, soil moisture, and sap flux density of 15 sample trees of different species and dominance in a natural evergreen and deciduous broadleaved mixed forest were simultaneously monitored from March 2012 to February 2014 in the Karst mountain region in southwestern China. After establishing a single tree transpiration model which considers the effects of dominance and species, an up-scaling method was explored to estimate stand transpiration. The results show that the transpiration intensity increased exponentially with increasing tree dominance. The contribution to annual stand transpiration from a few dominant trees (5.4% of trees, 28.2% of basal area) was up to 65.0%. The corresponding contribution was 16.2% from sub-dominant trees (7.6% of trees, 16.2% of basal area) and 22.8% from middle- and lower-layer trees (87.0% of trees, 55.6% of basal area). The variation of individual tree transpiration was mainly (97.9%) explained by tree dominance, but very weakly by tree species. The estimated annual stand transpiration was 300.2 mm when using the newly developed method which considers tree dominance, 52.5 mm (14.9%) lower than the estimation (352.7 mm) of traditional method which considers only the sapwood area effect, and 8.5 mm (2.7%) lower than the estimation (308.6 mm) which considers the effects of both species composition and sapwood area. The main tree characteristics affecting stand transpiration are tree size (sapwood area) and dominance. Consideration of tree dominance will significantly improve stand transpiration estimation and provide a more solid basis for guiding integrated forest–water management at stand scale.

Precipitation mediates sap flux sensitivity to evaporative demand in the neotropics.

Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remains elusive. With a progressively warming climate, atmospheric evaporative demand [i.e., vapor pressure deficit (VPD)] will be increasingly important for plant functioning, becoming the major control of plant water use in the twenty-first century. Using measurements in 34 tree species at seven sites across a precipitation gradient in the neotropics, we determined how the maximum sap flux velocity (vmax) and the VPD threshold at which vmax is reached (VPDmax) vary with precipitation regime [mean annual precipitation (MAP); seasonal drought intensity (PDRY)] and two functional traits related to foliar and wood economics spectra [leaf mass per area (LMA); wood specific gravity (WSG)]. We show that, even though vmax is highly variable within sites, it follows a negative trend in response to increasing MAP and PDRY across sites. LMA and WSG exerted little effect on vmax and VPDmax, suggesting that these widely used functional traits provide limited explanatory power of dynamic plant responses to environmental variation within hyper-diverse forests. This study demonstrates that long-term precipitation plays an important role in the sap flux response of humid tropical forests to VPD. Our findings suggest that under higher evaporative demand, trees growing in wetter environments in humid tropical regions may be subjected to reduced water exchange with the atmosphere relative to trees growing in drier climates.

A process-based numerical approach to estimate forest groundwater consumption in flatland petrocalcic soils

Abstract Grasslands are extensively distributed in flatland areas around the world, such as the Pampas in South America. It is one of the most economically productive landscapes and, as in other regions, they are being replaced by forests at increasing rates. Soil salinization emerges as a negative consequence associated with water deficit and forest groundwater consumption (∼250–500 mm/yr, in this region). An assessment of forest groundwater consumption is crucial for risk evaluation of soil salinization on flatland environments. For this aim, numerical modeling based on physical/biological processes and atmospheric boundary conditions was successfully applied in monitored grassland and afforested plots. Modeling results suggested a partial hydraulic disconnection between forest and phreatic aquifer due to the presence of petrocalcic horizons. Forest transpiration estimates were approximately 13% of total groundwater usage. Forest water consumption was then restricted to that soil portions above the petrocalcic horizons. Estimated forest transpiration rates (∼723 mm/yr) were similar to and even exceeded those reported in salinized sites with similar features. However, the risk of salinization of these soils was unlikely, because forest transpiration was restricted to the upper soil portions filled with fresh rainwater. The petrocalcic horizon retained water and prevented both deep drainage and, indirectly, soil salinization.

Transpiration of Moso bamboo in southern China is influenced by ramet age, phenology, and drought

Forest transpiration is coupled with environmental variables and has a profound effect on regional water cycle and water balance. Understanding how the biotic and abiotic environments influence transpiration is vital in understanding forest water use strategies, and for evaluating forest feedback to climate change. Moso bamboo (Phyllostachys edulis), a woody bamboo species, is rapidly spreading in the southern parts of China and must have a significant influence on ecosystem processes here. This study examined culm sap flow in P. edulis in southern China to identify key factors that determine its transpiration at individual and stand scale. Sap flux density was higher in the 1–2 years old ramets than those >3 years old, during the entire growing season. Daily accumulated sap flux density and its sensitivity to the physical environment (air vapor pressure deficit-VPD and solar radiation) during the non-shooting period were approximately 2 and 3 times higher than during the shooting-leafing period for 1–2 and >3 years old culms, respectively. During the non-shooting period, daily maximum and accumulated sap flux density of the 1–2 years old ramets had no correlation with soil water content, while that of >3 years old ramets declined linearly with decreasing soil water content (SWC). Stand transpiration remained low, and was not affected by changes in SWC during the shooting-leafing period, while that of non-shooting period was linearly correlated with SWC. The results showed that transpiration of Moso bamboo was interactively influenced by culm age, phenological stages and soil drought. We demonstrate the role of ramet age and phenology on bamboo stand transpiration and how it responds to soil drought. Our study reveals the special water use strategies in Moso bamboo forests.

Transpiration and growth of young African mahogany plants subject to different water regimes.

Timber production has been prominent in the Brazil scenario to minimize deforestation. Thus, technical information is necessary to define the productive process of the African mahogany in the Midwest region of Brazil, especially with regard to its hydric parameters. Recent studies, reported in the literature, have shown that irrigation improves the performance of young African mahogany plants in the field. Sap flow measurement can be used to estimate transpiration of perennial plants and to determine their water demand. This study evaluated the influence of two water regimes on the transpiration and growth of an African mahogany forest after irrigation has ceased. Moreover, this study also characterizes the seasonal patterns of transpiration and growth of African mahogany under these conditions. African mahogany plants with 2.5years of age were cultivated in Bonfinopolis-GO and evaluated for 2years. Treatments were IT-irrigated until 2years of age-and NIT-non-irrigated. Plant height (PH), breast height diameter (DBH), trunk volume (TRV), leaf area (LA), leaf dry matter (LDM), and transpiration (T) were monitored by heat dissipation probe (HDP) between Oct/2014 and Oct/2015. Higher growth in LA, DBH, and LDM were observed in IT. However, increase in PH and TRV was similar in both treatments. The mean annual T was similar between treatments (15.0Lm-2month-1). The highest T was recorded in October/2014 (IT = 33.0Lm-2month-1) and July/2015 (NIT = 20.5Lm-2month-1). The greater LA and water deficit blades DEF > 30mm promoted lower transpiration in the irrigated plants. Irrigation maintained plant growth in PH, DBH, and LA in the third year, even after irrigation has ceased. However, non-irrigated plants were similar in TRV (0.065m3) and transpiration rates (≈ 15Lm-2month-1). Winter transpiration (11.3Lm-2month-1) was lower than in summer (18.8Lm-2month-1) for irrigated plants and similar for non-irrigated plants (≈ 14Lm-2month-1). Based on that, in order to maintain the homogeneity of the plants, the irrigation in the first 2years of cultivation is recommended, and also, the sap flow measures presented satisfactory results regarding the determinations of the water needs of African mahogany.

Transpiration and subsurface controls of streamflow recession characteristics

AbstractIn headwater catchments, streamflow recedes between periods of rainfall at a predictable rate generally defined by a power–law relationship relating streamflow decay to streamflow. Research over the last four decades has applied this relationship to predictions of water resource availability as well as estimations of basin‐wide physiographic characteristics and ecohydrologic conditions. However, the interaction of biophysical processes giving rise to the form of these power–law relationships remains poorly understood, and recent investigations into the variability of streamflow recession characteristics between discrete events have alternatively suggested evapotranspiration, water table elevation, and stream network contraction as dominant factors, without consensus. To assess potential temporal variability and interactions in the mechanism(s) driving streamflow recession, we combine long‐term observational data from a headwater stream in the southern Appalachian Mountains with state and flux conditions from a process‐based ecohydrologic model. Streamflow recession characteristics are nonunique and vary systematically with seasonal fluctuations in both rates of transpiration and watershed wetness conditions, such that transpiration dominates recession signals in the early growing season and diminishes in effect as the water table elevation progressively drops below and decouples with the root zone with topographic position. As a result of this decoupling, there exists a seasonal hysteretic relationship between streamflow decay and both evapotranspiration and watershed wetness conditions. Results indicate that for portions of the year, forest transpiration may actively compete with subsurface drainage for the same water resource that supplies streamflow, though for extended time periods, these processes exploit distinct water stores. Our analysis raises concerns about the efficacy of assessing humid headwater systems using traditional recession analysis, with recession curve parameters treated as static features of the watershed, and we provide novel alternatives for evaluating interacting biological and geophysical drivers of streamflow recession.

Determining the independent impact of soil water on forest transpiration: A case study of a black locust plantation in the Loess Plateau, China

Transpiration (Tr) is influenced by environmental factors, vegetation properties, and anthropogenic management. The effect of environmental factors on Tr are taking place from two aspects: evaporative demand (i.e., potential evapotranspiration, PET) and water supply (i.e., soil water). Soil water is one of the most important factors that limit plant transpiration in terrestrial ecosystems, especially in semi-arid and arid regions. Investigating the relationship between Tr and soil water is crucial for an improved understanding of plant survival strategies and predicting hydrological cycles and water resources under climate change. Although the relationships of soil water and Tr have been widely studied, the independent effects of soil water on Tr are difficult to separate because soil water and PET occur concurrently under natural conditions. This study carried out field observations of sapflow density, meteorological factors and soil water in a black locust (Robinia pseudoacacia) plantation in the semi-arid Loess Plateau of China. This information was used to develop a model integrating the daily PET and relative extractable soil water (REW, an index that represents the available soil water): Tr = (0.27 × PET − 0.02 × PET2 − 0.32) × (1 − e−5.70×REW). The model fitted the measured data well (R2 = 0.65 and RMSE = 0.06 mm day−1). We found that the daily Tr increased as the REW increased under varying PET levels. Additionally, the independent effects of soil water on Tr were analysed using the factorial experiment analysis method. The REW was manipulated and PET varied naturally during the measurements to separate the independent effects of the REW on Tr. The results showed that Tr increased with the REW during the study period at a rate of 0.53 mm day−1 per 0.1 REW when the REW < 0.4 and 0.09 mm day−1 per 0.1 REW beyond the threshold (REW = 0.4). Tr increased by 27.3 mm (43.7%) compared to the controlled Tr (observed PET and lowest REW) due to the effects of increasing REW (ranging from 0.14 ∼ 0.81) during the study period (growing seasons of 2015 and 2016). Furthermore, canopy-level stomatal conductance was calculated using a simplified inverted Penman-Monteith equation. The reference canopy-level stomatal conductance (Gsref), which represents the Gs capacity and its sensitivity to soil water, showed an increasing response with increasing REW, which is the possible reason for the Tr dynamics with the REW. Our study provided an improved understanding of the soil water-Tr relationship and predicted effects of climate change on tree water use. Although the method required abundant field observation data, we have provided a feasible method of accurately quantifying the independent contribution of soil water on forest Tr.

Drought Differentially Affects Growth, Transpiration, and Water Use Efficiency of Mixed and Monospecific Planted Forests

Drought conditions may have differential impacts on growth, transpiration, and water use efficiency (WUE) in mixed species and monospecific planted forests. Understanding the resistance (i.e., the capacity to maintain processes unchanged) of different tree species to drought, and how resistance is affected by complementary interactions within species mixtures, is particularly important in the seasonally dry tropics where projected increases in the frequency and severity of drought threaten tree planting efforts and water resources. Complementary interactions between species may lead to more resistant stands if complementarity leads to greater buffering capacity during drought. We examined growth, transpiration, and WUE of mixtures and monocultures of Terminalia amazonia (J.F. Gmel.) Exell and Dalbergia retusa Hemsl. before and during a prolonged drought using intensive measurements of tree sap flow and growth. Tree sapwood area growth was highest for T. amazonia in mixtures during normal (6.78 ± 4.08 mm2 yr−1) and drought (7.12 ± 4.85 mm2 yr−1) conditions compared to the other treatments. However, stand sapwood area growth was greatest for T. amazonia monocultures, followed by mixtures, and finally, D. retusa monocultures. There was a significant decrease in stand transpiration during drought for both mixtures and T. amazonia monocultures, while Dalbergia retusa monocultures were most water use efficient at both the tree and stand level. Treatments showed different levels of resistance to drought, with D. retusa monocultures being the most resistant, with non-significant changes of growth and transpiration before and during drought. Combining species with complementary traits and avoiding combinations where one species dominates the other, may maximize complementary interactions and reduce competitive interactions, leading to greater resistance to drought conditions.

Partitioning the causes of spatiotemporal variation in the sunny day sap flux density of a larch plantation on a hillslope in northwest China

AimsTree sap flux density, as the basis for estimating forest transpiration, exhibits high spatiotemporal variation on hillslopes. This is largely due to the variation in the canopy leaf area index (LAI), potential evapotranspiration (PET), and volumetric soil moisture (VSM) that are induced by the slope position and shaded terrain effects. To improve the estimation of the daily sap flux density (Js) of sloped forests, a new approach is required that will extrapolate the Js measured at given slope position to other slope positions by considering both effects on the influencing factors. MethodThis study was carried out on a southeast-facing hillslope of a Larix principis-rupprechtti plantation in NW China. The Js was monitored during the growing season of 2016 on three plots located at the foot-slope (FS), mid-slope (MS) and upper-slope (US). To exclude interferences from fluctuating weather conditions, only data from 16 sunny days were used. The dynamic optimal Js was applied to quantify the Js response to changes in the PET, LAI and VSM induced by the slope position and shaded terrain effects. ResultThe mean daily Js (ml⋅cm−2⋅min−1) of 16 sunny days differed among slope positions as MS (0.059) > FS (0.050) > US (0.041). The strong shaded terrain effect at FS was verified by the markedly delayed starting time of sap flow at the FS when compared to that at the US and MS under a weak shaded terrain effect. The strong shaded terrain effect at the FS induced a PET reduction which decreased with the rising DOY until the end of July and then increased, this effect at FS also induced an LAI reduction that varied with the DOY inversely to PET. The mean Js (ml⋅cm−2⋅min−1) of 16 sunny days was reduced by 0.063 at US and 0.040 at MS mainly due to the slope position effect, and was reduced by 0.051 at the FS due to the joint impacts of a strong shaded terrain effect (0.0505) and a slope position effect (0.0005). When extrapolating the Js measured at the MS to estimate the Js at the US, the consideration of the slope position effect lowered the estimation error to 7.8% markedly decreased from 50.6% when not considering any effect. However, the Js estimation error at the FS increased from 25% to 87% when considering only the slope position effect, but decreased to 11.2% when considering both the shaded terrain effect and the slope position effect. ConclusionThe substantial spatiotemporal variation in Js exists on hillslopes due to the varying effects of the slope position and the shaded terrain. The rising slope position leads to a decrease of PET, LAI, VSM and consequently, Js, while the strong shaded terrain effect at the lower slope (e.g., SF in this study) leads to a decrease in LAI, PET and Js. Thus, it is necessary to consider the slope position and shaded terrain effects, as these variables can greatly improve the Js estimation accuracy when scaling up from measured values at one slope position to other slope positions and even the entire hillslope.

Effects of social position and competition on tree transpiration of a natural mixed forest in Chongqing, China

Key message This study explored the influence of social position and competition on tree transpiration using spatial structure parameters and competition indices.

Riparian forest transpiration under the current and projected Mediterranean climate: Effects on soil water and nitrate uptake

AbstractVegetation plays a key role in riparian area functioning by controlling water and nitrate (N─NO3−) transfers to streams. We investigated how spatial heterogeneity modifies the influence of vegetation transpiration on soil water and N─NO3− balances in the vadose soil of a Mediterranean riparian forest. On the basis of field data, we simulated water flow and N─NO3− transport in three riparian zones (i.e., near‐stream, intermediate, and hillslope) using HYDRUS‐1D model. We investigated spatiotemporal patterns across the riparian area over a 3‐year period and future years using an IPCC/CMIP5 climate projection for the Mediterranean region. Potential evapotranspiration was partitioned between evaporation and transpiration to estimate transpiration rates at the area. Denitrification in the forest was negligible, thus N─NO3− removal was only considered through plant uptake. For the three riparian zones, the model successfully predicted field soil moisture (θ). The near‐stream zone exchanged larger volumes of water and supported higher θ and transpiration rates (666 ± 75 mm) than the other two riparian zones. Total water fluxes, θ, and transpiration rates decreased near the intermediate (536 ± 46 mm transpired) and hillslope zones (406 ± 26 mm transpired), suggesting that water availability was restricted due to deeper groundwater. Transpiration strongly decreased θ and soil N─NO3− in the hillslope and intermediate zones. Our climate projections highlight the importance of groundwater availability and indicate that soil N─NO3− would be expected to increase due to changes in plant‐root uptake. Lower water availability in the hillslope zone may reduce the effectiveness of N─NO3− removal in the riparian area, increasing the risk of excess N─NO3− leaching into the stream.

Modelling tropical forest responses to drought and El Niño with a stomatal optimization model based on xylem hydraulics.

The current generation of dynamic global vegetation models (DGVMs) lacks a mechanistic representation of vegetation responses to soil drought, impairing their ability to accurately predict Earth system responses to future climate scenarios and climatic anomalies, such as El Niño events. We propose a simple numerical approach to model plant responses to drought coupling stomatal optimality theory and plant hydraulics that can be used in dynamic global vegetation models (DGVMs). The model is validated against stand-scale forest transpiration (E) observations from a long-term soil drought experiment and used to predict the response of three Amazonian forest sites to climatic anomalies during the twentieth century. We show that our stomatal optimization model produces realistic stomatal responses to environmental conditions and can accurately simulate how tropical forest E responds to seasonal, and even long-term soil drought. Our model predicts a stronger cumulative effect of climatic anomalies in Amazon forest sites exposed to soil drought during El Niño years than can be captured by alternative empirical drought representation schemes. The contrasting responses between our model and empirical drought factors highlight the utility of hydraulically-based stomatal optimization models to represent vegetation responses to drought and climatic anomalies in DGVMs.This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.