Lei Ouyang

Tree Species with Photosynthetic Stems Have Greater Nighttime Sap Flux

An increasing body of evidence has shown that nighttime sap flux occurs in most plants, but the physiological implications and regulatory mechanism are poorly known. The significance of corticular photosynthesis has received much attention during the last decade, however, the knowledge of the relationship between corticular photosynthesis and nocturnal stem sap flow is limited at present. In this study, we divided seven tree species into two groups according to different photosynthetic capabilities: trees of species with (Castanopsis hystrix, Michelia macclurei, Eucalyptus citriodora, and Eucalyptus grandis × urophylla) and without (Castanopsis fissa, Schima superba, and Acacia auriculiformis) photosynthetic stems, and the sap flux (Js) and chlorophyll fluorescence parameters for these species were measured. One-way ANOVA analysis showed that the Fv/Fm (Maximum photochemical quantum yield of PSII) and ΦPSII (effective photochemical quantum yield of PSII) values were lower in non-photosynthetic stem species compared to photosynthetic stem species. The linear regression analysis showed that Js,d (daytime sap flux) and Js,n (nighttime sap flux) of non-photosynthetic stem species was 87.7 and 60.9% of the stem photosynthetic species. Furthermore, for a given daytime transpiration water loss, total nighttime sap flux was higher in species with photosynthetic stems (SlopeSMA = 2.680) than in non-photosynthetic stems species (SlopeSMA = 1.943). These results mean that stem corticular photosynthesis has a possible effect on the nighttime water flow, highlighting the important eco-physiological relationship between nighttime sap flux and corticular photosynthesis.

Maximised photosynthetic capacity and decreased hydraulic failure risk during aging in the clump bamboo, Bambusa chungii

To assess the water use of a clumped bamboo species, we investigated water use, physiological responses and structural changes related to culm aging in the clumped bamboo species Bambusa chungii McClure. Anisohydric behaviour was characterised by the independent stomatal conductance (gs) to leaf water potential (ΨL), and the low stomatal sensitivity (-m) in the young (0.52) and mature groups together with the aged group (0.41): gs was negatively related to intercellular CO2 (Ci), especially during the dry season (R2=0.62). Hydraulic conductivity (ks) decreased by 57.9 and 58.8% in the mature and aged groups. This was accompanied by a leaf area (AL) decrease of 55.7 and 63.7% and a water transport path (h) reduction of 8.5 and 23.3% to maintain the hydraulic safety. The net photosynthetic rate (An) was similar among the three age groups even during the dry season when water deficits occurred. The observed increase in chlorophyll content (5.3% greater for the mature group) and stomata density (7.4 and 8.1% greater for the mature and aged groups) with age might compensation the reduced whole culm carbon assimilation caused by decreased AL. Physiological and structural regulation contributes to the fitness of B. chungii.

Stomatal uptake of O3 in a Schima superba plantation in subtropical China derived from sap flow measurements.

Canopy stomatal ozone (O3) flux (Fst,O3) in a plantation of Schima superba, an ecologically and economically important evergreen pioneer tree species in subtropical China, was quantified based on sap flow measurements during a 2-year period. Mean Fst,O3 and accumulated Fst,O3 (AFst0) were significantly higher in wet seasons from April to September (4.62 nmol m(-2) s(-1) and 35.37 mmol m(-2), respectively) than in dry seasons from October to March (3.90 nmol m(-2) s(-1) and 24.15 mmol m(-1), respectively), yet comparable between the 2 years of the experiment, being 4.23 nmol m(-2) s(-1) and 58.23 mmol m(-2) in April 2013-March 2014 and 4.29 nmol m(-2) s(-1) and 60.80 mmol m(-2) in April 2014-March 2015, respectively. At the diurnal scale, Fst,O3 generally peaked in the early to middle afternoon hours (13:00-15:00), while the maximum stomatal conductance (Gst,O3) typically occurred in the middle to late morning hours (09:00-11:00). Monthly integrated AFst0 reached the maximum in July, although accumulated O3 exposure (SUM0) was highest in October. Seasonally or yearly, the accumulated O3 doses, either exposure-based or flux-based, notably exceeded the currently adopted critical thresholds for the protection of forest trees. These results, on the one hand, demonstrated the decoupling between the stomatal uptake of O3 and its environmental exposure level; on the other hand, indicated the potential O3 risk for S. superba in the experimental site. Therefore, the present study endorses the use of sap flow measurements as a feasible tool for estimating Fst,O3, and the transition from the exposure-based toward flux-based metrics for assessing O3 risk for forest trees. Further studies are urgently needed to relate stomatal O3 uptake doses with tree growth reductions for an improved understanding of O3 effects on trees under natural conditions.

Hydraulic Balance of a Eucalyptus urophylla Plantation in Response to Periodic Drought in Low Subtropical China

A clear understanding of hydraulic regulation in cultivated plants is crucial for addressing challenges to forest water cycling due to climate changes in low subtropical China. Experiments were conducted to determine the hydrologic balance of a Eucalyptus urophylla plantation in response to periodic drought. Trees displayed lower stomatal conductance (GS) and leaf water potentials (ΨL) during the dry periods. A decrease of 22.4% was found for the maximum reference GS (GS at D = 1 kPa; GSref-max). Accordingly, specific hydraulic conductivity (ks) decreased by 45.3 – 65.6% from the wet to the dry season, depending on the tree size. Fairly stable leaf stomatal conductance (gs) with decreasing ΨL (ΨL < -1.6 MPa) contributed to the high water-use efficiency (WUE) of this Eucalyptus species. Additionally, the lower stomatal sensitivity (-m = 0.53) in the dry season might also be responsible for the high WUE, since we found an anisohydric behavior that was associated with photosynthetically active radiation (Q0). Larger trees were found to use water more efficiently than small trees, due to the higher sensitivity of ks to decreasing ΨL. This was also verified by the decreasing leaf carbon isotope discrimination (Δ13C) with increasing tree diameter. However, further studies are needed to determine the universality of these results for other Eucalyptus species in this region.

Responses of sap flux and intrinsic water use efficiency to canopy and understory nitrogen addition in a temperate broadleaved deciduous forest

Increasing atmospheric nitrogen (N) deposition could profoundly impact structure and functioning of forest ecosystems. Therefore, we conducted a two-year (2014-2015) experiment to assess the responses of tree sap flux density (Js) and intrinsic water use efficiency (WUEi) of dominant tree species (Liquidambar formosana, Quercus acutissima and Quercus variabilis) to increased N deposition at a manipulative experiment with canopy and understory N addition in a deciduous broadleaved forest. Five treatments were administered including N addition of 25 kg ha-1 year-1 and 50 kg ha-1 year-1 onto canopy (C25 and C50) and understory (U25 and U50), and control treatment (CK, without N addition). Our results showed neither canopy nor understory N addition had an impact on leaf N content and C:N ratio (P > 0.05). Due to the distinct influencing ways, canopy and understory N addition generated different impacts on Js and WUEi of the dominant tree species. Canopy N addition increased WUEi of Q. variabilis, whereas understory addition treatment had a minimal impact on WUEi. Both N additions did not exert impacts on WUEi of L. formosana and Q. acutissima. Canopy N addition exerted negative impacts on Js and its sensitivity to micrometeorological factors of Q. acutissima and Q. variabilis in 2014, while understory addition showed no effect. Neither canopy nor understory N addition had an influence on Js of L. formosana in 2014. Probably owing to the increased soil acidification as the experiment proceeded, Js of L. formosana and Q. variabilis was decreased by understory N addition while canopy addition had a minimal effect in 2015. Thus, the traditional understory addition approach could not fully reflect the effects of increased N deposition on the canopy-associated transpiration process indicated by the different responses of Js and WUEi to canopy and understory N addition, and exaggerated its influences induced by the variation of soil chemical properties.

Water transport of native and exotic tree species in relation to xylem anatomical characteristics in low subtropical China

Aims Exotic fast‐growing tree species have been commonly planted as pioneer species to facilitate ecological restoration in South China. Their growth and resource utilization behavior related to intrinsic physiology and structural properties have profound influences on forest ecosystem. However, the contrastive research focusing on water utilization features along with xylem anatomical properties between native and exotic species is scarce in South China. The objective of this study is to investigate the sapwood anatomical characteristics and water utilization conditions of native and exotic fast‐growing species, and to elucidate the relationship between sap‐flux density and conduit features. Methods We measured sap‐flux density, conduit length, diameter and density of four native species (Schima superba, Michelia macclurei, Castanopsis hystrix and Castanopsis fissa) and four exotic species (Eucalyptus citriodora, Eucalyptus urophylla × grandis, Acacia auriculaeformis and Acacia mangium). Sap flux density was measured based on the Graniers thermal dissipation probe method. The whole‐tree water transport was quantified by multiplying sap‐flux density by sapwood area. The measurements of conduit characteristics were conducted by using segregation and slice method. Important Findings Sapwood area increased with the growing diameter at breast height (DBH) as a power function. Native species had a larger water‐conducting tissue area than exotic species at the same DBH value when trees grew to a size with a certain value of DBH. The conduit diameter of exotic species was significantly larger than that of native species. Conversely, native species, such as S. superba and M. macclurei, had longer conduit length and higher conduit density than other tree species. Based on a physiological interpretation of the measured conduit characteristics, native tree species developed a safe water transport system while exotic fast‐growing tree species come into being an efficient system instead. Water transport increased with the growing DBH as a power function, and the exponent for native species (1.60) was higher than that for exotic species (1.22). Under the combined impact of sap‐flux density and sapwood area, native species presented a larger water transport at a larger DBH value, indicating that growth advantage of exotic fast‐growing species might weaken as DBH increased.

The sap flow-based assessment of atmospheric trace gas uptake by three forest types in subtropical China on different timescales

Assessing the uptake of trace gases by forests contributes to understanding the mechanisms of gas exchange between vegetation and the atmosphere and to evaluating the potential risk of these pollutant gases to forests. In this study, the multi-timescale characteristics of the stomatal uptake of NO, NO2, SO2 and O3 by Schima superba, Eucalyptus citriodora and Acacia auriculiformis were investigated by continuous sap flow measurements for a 3-year period. The peak canopy stomatal conductance (GC) for these three species appeared between 9:00 and 12:00, which was jointly regulated by the vapour pressure deficit (VPD) and photosynthetically active radiation (PAR). Additionally, annual and seasonal variations in the stomatal uptake of trace gases for these three tree species suggested that there was a combination effect between canopy stomatal conductance and ambient concentration on the uptake of trace gases. Furthermore, the result demonstrated that the trace gas absorption capacities among these three forest types followed the order of S. superba > E. citriodora > A. auriculiformis. The findings of this study have theoretical significance and application value in assessing air purification and the risk of harm to forests in Southern China.

Stand-scale transpiration of a Eucalyptus urophylla × Eucalyptus grandis plantation and its potential hydrological implication

Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, Guangdong Provincial Key Laboratory of Applied Botany, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China Chinese Academy of Meteorological Sciences, Beijing 100086, China Chinese Academy of Sciences, Guangxi Institute of Botany, Guilin 541006, China Correspondence Ping Zhao, Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, Guangdong Provincial Key Laboratory of Applied Botany, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China. Email: zhaoping@scib.ac.cn

The tree height-related spatial variances of tree sap flux density and its scale-up to stand transpiration in a subtropical evergreen broadleaf forest: spatial variances of tree sap flux density and its scale-up

College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, China Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China University of Chinese Academy of Sciences, Beijing 100049, China 4 Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China Correspondence Zhenzhen Zhang, College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, China. Email: zhangzhen@zjnu.cn Funding information National Natural Science Foundation of China, Grant/Award Numbers: 41630752, 41701226 and 41030638

Biotic- and abiotic-driven variations of the night-time sap flux of three co-occurring tree species in a low subtropical secondary broadleaf forest

Abstract Although several studies on the night-time water use of different plant species have been reported, comparative studies under the same climatic conditions of a region are scarce. This study aimed to analyse the inter- and intraspecific variations in night-time water use in relation to environmental factors and to tree morphological features to understand and elucidate the possible underlying mechanisms. The sap flow of three co-occurring tree species in a low subtropical secondary broadleaf forest in South China was monitored using Granier-style sap flux sensors. All examined environmental factors except wind speed exerted significant influence on the daytime sap flows of Schima superba, Castanopsis hystrix and Michelia macclurei, but the impacts of all factors, including wind speed, on the night-time sap flux were trivial. These results indicated that sap flow was mainly used for water recharge at night. The morphological features of the trees, except tree height, significantly affected the daytime water use, but no morphological features significantly affected the night-time water use. We found that night-time water recharge was strongly affected by the maximum flux density. A principal component analysis showed that there were more intraspecific than interspecific variations in water transport. The results also revealed that the night-time water use and the percentage of night/day (Qn/Qd) of photosynthetic stem species (C. hystrix and M. macclurei) were greater than those of non-photosynthetic stem species (S. superba).