Zeng Xp

The impact of drought on sap flow of cooccurring Liquidambar formosana Hance and Quercus variabilis Blume in a temperate forest, Central China

Diffuse- and ring-porous species with different vessel structures exhibited contrasting water use strategies for adapting to water stress. The sap flow rates of two tree species (diffuse-porous: Liquidambar formosana Hance; and ring-porous: Quercus variabilis Blume) under different environmental conditions were monitored in a temperate forest in the south of Henan Province, Central China. The mechanisms underlying the contrasting water use strategies in response to drought stress were explored by analysing the correlation of sap flow per unit sapwood area (Fd) with vapour pressure deficit and the contribution of nocturnal Fd to total water use under different soil water contents. The results showed that the Fd of Q.xa0variabilis decreased under drought conditions, whereas that of L.xa0formosana increased. Under drought stress, stronger stomatal control was shown for both tree species. Stomatal regulation and changes of leaf area jointly led to constant sap flow per unit leaf area (JL) across the seasons for Q.xa0variabilis. The seasonal variations in the ratio of nocturnal mean Fd to total Fd were consistent with those in the daytime mean JL for both tree species. Nocturnal sap flow played a critical role in the increased water use of diffuse-porous species, L.xa0formosana, under drought conditions. Our study highlights the importance of nocturnal sap flow for water use by forest species. This finding provides insights for future studies of the water cycle in forest ecosystems and demonstrates that such studies should take into account the nocturnal physiological processes of plants.

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.

Responses of Tree Transpiration and Growth to Seasonal Rainfall Redistribution in a Subtropical Evergreen Broad-Leaved Forest

Precipitation changes such as more frequent drought and altered precipitation seasonality may impose substantial impacts on the structure and functioning of forest ecosystems. A better understanding of tree responses to precipitation changes can provide fundamental information for the conservation and management of forests under future climate regimes. We conducted a 2-year seasonal rainfall redistribution experiment to assess the responses of tree transpiration and growth to manipulated precipitation changes in a subtropical evergreen broad-leaved forest. Three precipitation treatments were administered including a drier dry season and wetter wet season treatment (DD), an extended dry season and wetter wet season treatment (ED), and an ambient control treatment, with the total amount of annual rainfall being kept the same among the three treatments. Our results showed that the DD and ED treatments reduced daily transpiration of Schima superba by 8–16 and 13–25%, respectively. The ED treatment also reduced the DBH increment of larger S. superba individuals. In contrast, neither treatment showed obvious effects on the transpiration and DBH increment of another dominant species Michelia macclurei. However, the transpiration of both species showed clear inter-annual differences between the 2xa0years with contrasting annual rainfall (2094 vs 1582xa0mm). S. superba had a lower transpiration-to-precipitation ratio (T/P) compared to M. macclurei and showed decreased sensitivities to total solar radiation and vapor pressure deficit under the DD and ED treatments. These results indicate the deep-rooted S. superba may be suppressed with a lower ability to obtain water and assimilate carbon compared to the shallow-rooted M. macclurei under the precipitation seasonality changes, which could potentially cause shifts in species dominance within the forest community.

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