Liwei Zhu

Temporal Variation in Sap-Flux-Scaled Transpiration and Cooling Effect of a Subtropical Schima superba Plantation in the Urban Area of Guangzhou

Abstract Agriculture could suffer the water stress induced by climate change. Because climate warming affects global hydrological cycles, it is vital to explore the effect of tree transpiration, as an important component of terrestrial evapotranspiration, on the environment. Thermal dissipation probes were used to measure xylem sap flux density of a Schima superba plantation in the urban area of Guangzhou City, South China. Stand transpiration was calculated by mean sap flux density times total sapwood area. The occurrence of the maximum sap flux density on the daily scale was later in wet season than in dry season. The peak of daily sap flux density was the highest of 59 g m −2 s −1 in July and August, and the lowest of 28 g m −2 s −1 in December. In the two periods (November 2007-October 2008 and November 2008-October 2009), the stand transpiration reached 263.2 and 291.6 mm, respectively. During our study period, stand transpiration in wet season (from April to September) could account for about 58.5 and 53.8% of the annual transpiration, respectively. Heat energy absorbed by tree transpiration averaged 1.4×10 8 and 1.6×10 8 kJ per month in this Schima superba plantation with the area of 2 885 m 2 , and temperature was reduced by 4.3 and 4.7°C s −1 per 10 m 3 air.

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.

Applying Time Series Models to Estimate Time Lags between Sap Flux and Micro-Meteorological Factors

ABSTRACT Sap flux (Ft) measurements are extensively used to scale-up canopy transpiration and conductance, but time lag between sap flux and canopy transpiration is a problem. As canopy transpiration is nearly synchronous with micrometeorological drivers, better understanding of the lag relationships between Ft and micrometeorological drivers and soil water conditions would benefit the up-scaling of canopy transpiration from sap flux. Time series modeling at different spatial and temporal scales can identify and incorporate time lag effects, as well as multiple variables affecting transpiration and the interactions between them. SARIMAX and GARCH hybrid models were used to capture seasonality and autoregressive conditional heteroscedasticity effects. Two univariate hybrid models were designed to measure vapor pressure deficit (VPDt) and photosynthetic active radiation (PARt), and one multivariate hybrid modelwas used in each season. Sap flow lagged behind canopy transpiration by 0–10 min in the dry season and 10–20 min in the wet season. VPDt had a stronger influence on transpiration than PARt . Only the interaction between VPDt and PARt in the wet season was observed. This study extends the application of time series modeling to the prediction of sap flow dynamics.

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.

Ecophysiological responses of two closely related Magnoliaceae genera to seasonal changes in subtropical China

Aims Plants use a variety of hydraulic strategies to adapt to seasonal drought that differ by species and environmental conditions. The early‐diverging Magnoliaceae family includes two closely related genera with contrasting leaf habits, Yulania (deciduous) and Michelia (evergreen), which naturally inhabit temperate and tropical regions, respectively. Here, we evaluate the hydraulic strategy of species from both genera that have been ex situ conserved in a subtropical region to determine how they respond to the novel cool‐dry season climatic pattern. Methods We measured ecophysiological traits in five Michelia and five Yulania species conserved in the South China Botanical Garden in both wet and dry season conditions and monitored the whole‐year sap flow for four of these species. Important Findings We found that Magnoliaceae species that have been ex situ conserved in a subtropical climate did not suffer from excessive water stress due to the mild drought conditions of the dry season and the ecophysiological adjustments the species made to avoid this stress, which differed by leaf habit. Specifically, deciduous species completely shed their leaves during the dry season, while evergreen species decreased their turgor loss points, dry mass based photosynthetic rates, stomatal conductance and specific leaf areas (SLAs) compared to wet season measurements. In comparing the two distinct leaf habits during the wet season, the leathery‐leaved evergreen species had higher leaf hydraulic conductance and leaf to sapwood area ratios than the papery‐leaved deciduous species, while the deciduous species had greater hydraulic conductivity calculated on both a stem and leaf area basis, dry mass based photosynthetic rates, leaf nutrients, SLAs and stomatal sizes than the evergreen species. Interestingly, species from both genera maintained similar sap flow in the wet season. Both photosynthetically active radiation and vapour pressure deficit affected the diurnal patterns of sap flow in the wet season, while only vapour pressure deficit played a dominant role in the dry season. This study reveals contrasting hydraulic strategies in Yulania and Michelia species under subtropical seasonal conditions, and suggests that these ecophysiological adjustments might be affected more by leaf habit than seasonality, thus reflecting the divergent evolution of the two closely related genera. Furthermore, we show that Magnoliaceae species that are ex situ conserved in a subtropical climate are hydraulically sound, a finding that will inform future conservation efforts of this ancient family under the threat of climatic change.

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.