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Climate change causes the global redistribution of precipitation, yet little is known about the effects of the changes in precipitation intensity and frequency on the seedlings of wood trees in warm temperate forests. In this study, we focused on the effects of variability in both the intensity and frequency of water supply on the physiological traits, biomass, and growth of an important plantation wood species, Robinia pseudoacacia. In the greenhouse, we exposed R. pseudoacacia seedlings to three rainfall intensity and three rainfall frequency treatments. The results from the 62-day experiment revealed that lower rainfall intensity and frequency significantly reduced the photosynthetic performance, growth, and biomass of the tree seedlings. In lower rainfall intensity and frequency conditions, the seedlings had improved water absorption and utilization by increasing the water use efficiency and root shoot ratio, and reduced water consumption by defoliating the compound leaves of the lower crown. More importantly, we found that R. pseudoacacia seedlings were more sensitive to rainfall frequency than to rainfall intensity. Therefore, our results suggest that increasing the irrigation water, especially irrigation frequency, could better facilitate the survival and growth of R. pseudoacacia seedlings and eventually promote the process of vegetation restoration in the future global climate change context. [ABSTRACT FROM AUTHOR]

Key message: Defoliation significantly affected biomass allocation of Robinia pseudoacacia L. and Sophora japonica L., but leaf physiology readjusted to control levels at the end of the experiment. Considering carbon or sink limitation and relative height growth rate, defoliated R. pseudoacacia grew faster than S. japonica under well-watered conditions, while defoliated S. japonica and R. pseudoacacia had similar performance under drought conditions. Context: Climate change may result in increases of both drought intensity and insect survival, thereby affecting both exotic and native trees in warm temperate forests. Aims: In this study, we examined the interaction effects of defoliation and drought on an exotic species Robinia pseudoacacia and a native species Sophora japonica in a warm temperate area, to provide a theoretical basis for predicting the distribution and dynamics of the two species under future climate change. Methods: In a greenhouse, both species were exposed to three soil moisture (75%, 55%, and 35% of field capacity) and three defoliation treatments (no defoliation, 50% defoliation, and 100% defoliation). Leaf physiology, biomass, and non-structural carbohydrate were determined. Results: Leaf physiology of defoliated trees did not differ from controls trees, but defoliated seedlings allocated relatively more resources to the leaves at the end of the experiment. In well-watered conditions, defoliated R. pseudoacacia was not carbon or sink limited and defoliated S. japonica was carbon limited, while defoliated individuals of the two species were sink limited under drought. Defoliated R. pseudoacacia grow more rapidly than S. japonica in well-watered conditions. Defoliated R. pseudoacacia had a similar growth rate to S. japonica in drought. Conclusions: Defoliation clearly affects biomass allocation of the two species, but not leaf physiology. Considering the carbon or sink limitation, the growth of S. japonica and R. pseudoacacia may be limited by future global climate change scenarios. [ABSTRACT FROM AUTHOR]

Seedlings in regenerating layer are frequently attacked by herbivorous insects, while the combined effects of defoliation and shading are not fully understood. In the present study, two Leguminosae species (Robinia pseudoacacia and Amorpha fruticosa) were selected to study their responses to combined light and defoliation treatments. In a greenhouse experiment, light treatments (L+, 88% vs L−, 8% full sunlight) and defoliation treatments (CK, without defoliation vs DE, defoliation 50% of the upper crown) were applied at the same time. The seedlings' physiological and growth traits were determined at 1, 10, 30, and 70 days after the combined treatment. Our results showed that the effects of defoliation on growth and carbon allocation under high light treatments in both species were mainly concentrated in the early stage (days 1–10). R. pseudoacacia can achieve growth recovery within 10 days after defoliation, while A. fruticosa needs 30 days. Seedlings increased SLA and total chlorophyll concentration to improve light capture efficiency under low light treatments in both species, at the expense of reduced leaf thickness and leaf lignin concentration. The negative effects of defoliation treatment on plant growth and non-structural carbohydrates (NSCs) concentration in low light treatment were significantly higher than that in high light treatment after recovery for 70 days in R. pseudoacacia , suggesting sufficient production of carbohydrate would be crucial for seedling growth after defoliation. Plant growth was more sensitive to defoliation and low light stress than photosynthesis, resulting in NSCs accumulating during the early period of treatment. These results illustrated that although seedlings could adjust their resource allocation strategy and carbon dynamics in response to combined defoliation and light treatments, individuals grown in low light conditions will be more suppressed by defoliation. Our results indicate that we should pay more attention to understory seedlings' regeneration under the pressure of herbivorous insects. [ABSTRACT FROM AUTHOR]

Changing precipitation patterns have aggravated the existing uneven water distribution, leading to the alternation of drought and rewatering. Based on this variation, we studied species, namely, Robinia pseudoacacia and Quercus acutissima , with different root forms and water regulation strategy to determine physiological responses to repeated drought-rewatering under different planting methods. Growth, physiological, and hydraulic traits were measured using pure and mixed planting seedlings that were subjected to drought, repeated drought-rewatering (i.e., treatments), and well-irrigated seedlings (i.e., control). Drought had negative effects on plant functional traits, such as significantly decreased xylem water potential (Ψmd), net photosynthetic rate (AP), and then height and basal diameter growth were slowed down, while plant species could form stress imprint and adopt compensatory mechanism after repeated drought-rewatering. Mixed planting of the two tree species prolonged the desiccation time during drought, slowed down Ψmd and AP decreasing, and after rewatering, plant functional traits could recover faster than pure planting. Our results demonstrate that repeated drought-rewatering could make plant species form stress imprint and adopt compensatory mechanism, while mixed planting could weaken the inhibition of drought and finally improve the overall drought resistance; this mechanism may provide a theoretical basis for afforestation and vegetation restoration in the warm temperate zone under rising uneven spatiotemporal water distribution. [ABSTRACT FROM AUTHOR]

Plant size influences plant responses to combined environmental factors under climate change. However, their roles in plant ecophysiological responses are not fully understood. Two rapidly growing Leguminosae species (Robinia pseudoacacia and Amorpha fruticosa) were used to examine plant responses to combined drought and defoliation treatments (two levels of both treatments). Both 1.5 month-old seedlings and 3 month-old seedlings were grown in a greenhouse, and seedling growth, leaf gas exchanges, stem hydraulics, and concentrations of non-structural carbohydrates were determined after 60 days of treatment. Our results indicated defoliation had no significant effect on plant height, basal diameter, and total biomass whatever plant sizes and species. Under the low water availability treatment, the defoliated seedlings significantly increased by 24% in stem water potential compared with non-defoliated seedlings in large R. pseudoacacia. Compared with the high water availability in large non-defoliated R. pseudoacacia seedlings, the low water availability significantly reduced by 26% in stem starch concentration to maintain the stem soluble sugar concentration stable, but not in small R. pseudoacacia seedlings. We also found a negative correlation between leaf and root soluble sugar concentration under low water availability in A. fruticosa. The results demonstrate defoliation could relieve the effect of low water availability in large seedlings. Large seedlings had more compensatory mechanisms in response to defoliation and drought treatments than small seedlings, thus species with large carbon reserves are more recommended for vegetation restoration under combined drought and defoliation conditions. Future studies with more species are crucial for obtaining more rigorous conclusions. [ABSTRACT FROM AUTHOR]

Robinia pseudoacacia L. is a legume species that is widely used in afforestation, which has high N2 fixation capacity and rapid growth rate. Both nitrogen (N) supply and phenology affect plant growth, photosynthesis and leaf senescence. The aim of this study was to determine how N supply affects N2 fixation, leaf photosynthesis and senescence of R. pseudoacacia at different phenological stages. Seedlings of R. pseudoacacia were supplied with different levels of 15N-labelled NH4NO3 solution, with seedlings of Sophora japonica Linn. as reference plants to calculate the percentage of N derived from the atmospheric N2 (%Ndfa). Compared with plants supplied with a high N level, those with a low N supply had a higher %Ndfa at an early developmental stage. Nitrogen fixation compensated the effect of a low N supply on plant growth in R. pseudoacacia. A high N supply decreased biomass allocation to lateral roots and nodules, and increased the relative growth rate of plant height as well as specific leaf area. The eighth mature compound leaf of R. pseudoacacia tended to have a higher net photosynthetic rate than the fourth leaf, and the leaves still maintained a moderate photosynthetic rate in early autumn. Plants tended to allocate more biomass to leaves at an early developmental stage and to stems and roots at a later developmental stage (3 months old). The N level did not affect leaf photosynthesis at different phenological stages, primarily due to (i) a high %Ndfa under low N supply at early growing stage, and a similar high %Ndfa under all N supplies at a late growing stage, and (ii) the delayed greening phenotype of expanding leaves to save nutrients for mature leaves. [ABSTRACT FROM AUTHOR]

Precise and accurate estimation of key hydraulic points of plants is conducive to mastering the hydraulic status of plants under drought stress. This is crucial to grasping the hydraulic status before the dieback period to predict and prevent forest mortality. We tested three key points and compared the experimental results to the calculated results by applying two methods. Saplings (n = 180) of Robinia pseudoacacia L. were separated into nine treatments according to the duration of the drought and rewatering. We established the hydraulic vulnerability curve and measured the stem water potential and loss of conductivity to determine the key points. We then compared the differences between the calculated [differential method (DM) and traditional method (TM)] and experimental results to identify the validity of the calculation method. From the drought-rewatering experiment, the calculated results from the DM can be an accurate estimation of the experimental results, whereas the TM overestimated them. Our results defined the hydraulic status of each period of plants. By combining the experimental and calculated results, we divided the hydraulic vulnerability curve into four parts. This will generate more comprehensive and accurate methods for future research. [ABSTRACT FROM AUTHOR]

Plants may maintain long-term xylem function via efficiency-safety tradeoff and segmentation. Most studies focus on the growing season and community level. We studied species with different efficiency-safety tradeoff strategies, Quercus acutissima , Robinia pseudoacacia , Vitex negundo var. heterophylla , and Rhus typhina , to determine the seasonality of this mechanism. We separated their branches into perennial shoots and terminal twigs and monitored their midday water potential (Ψmd), relative water content (RWC), stem-specific hydraulic conductivity (Ks), loss of 12, 50, and 88% of maximum efficiency (i.e., P12, P50, P88) for 2 years. There were no correlations between water relations (Ψmd, RWC, Ks) and embolism resistance traits (P12, P50, P88) but they significantly differed between the perennial shoots and terminal twigs. All species had weak annual hydraulic efficiency-safety tradeoff but strong segmentation between the perennial shoots and the terminal twigs. R. pseudoacacia used a high-efficiency, low-safety strategy, whereas R. typhina used a high-safety, low-efficiency strategy. Q. acutissima and V. negundo var. heterophylla alternated these strategies. This mechanism provides a potential basis for habitat partitioning and niche divergence in the changing warm temperate zone environment. [ABSTRACT FROM AUTHOR]

Plant responses to drought and their subsequent rehydration can provide evidence for forest dynamics within the context of climate change. In this study, the seedlings of two native species (Vitex negundo var. heterophylla, Quercus acutissima) and two exotic species (Robinia pseudoacacia, Amorpha fruticosa) to China were selected in a greenhouse experiment. The gas exchange, stem hydraulic parameters, plant osmoprotectant contents and antioxidant activities of the seedlings that were subjected to sustained drought and rehydration (test group) as well as those of well‐irrigated seedlings (control group) were measured. The two native species exhibited a greater degree of isohydry with drought because they limited the stomatal opening timely from the onset of the drought. However, the two exotic species showed a more 'water spender'‐like strategy with R. pseudoacacia showing anisohydric responses and A. fruticosa showing isohydrodynamic responses to drought. Severe drought significantly decreased the leaf gas exchange rates and hydraulic properties, whereas the instantaneous water use efficiency and osmoprotectant contents increased markedly. Most of the physiological parameters recovered rapidly after mild drought rehydration, but the water potential and/or supply of nonstructural carbohydrates did not recover after severe drought rehydration. The results demonstrate that the xylem hydraulic conductivity and shoot water potential jointly play a crucial role in the drought recovery of woody plants. In brief, the native species may play a dominant role in the future in warm‐temperate forests because they employ a better balance between carbon gain and water loss than the alien species under extreme drought conditions. [ABSTRACT FROM AUTHOR]

Background and aims: Increasing atmospheric nitrogen (N) deposition and biological invasion have become major concerns with global environmental change. This study aimed to determine the effects of an exotic species on a native one under increasing N deposition. Methods: We conducted a greenhouse experiment in which the exotic species Robinia pseudoacacia and the native species Quercus acutissima were grown in mixture and monocultures under four levels of simulated N deposition (0, 3, 6, 12 g m year). After 12 weeks of treatment, plant growth, leaf physiological traits and soil chemical properties were determined. Results: With its strong capability for nutrient absorption and carbon assimilation, R. pseudoacacia dominated in competition. R. pseudoacacia reduced the growth of Q. acutissima, but the relative competition index decreased with increasing N deposition. At the end of the experiment, the soil available phosphorus (P) in mixture was significantly lower than that in the monoculture of Q. acutissima, while the soil available N in the two cultivations did not show obvious differences. Conclusions: Increased N deposition alleviated the competitive effects of R. pseudoacacia on Q. acutissima. In the future, besides N, increased P availability should also be considered in the interaction between the two species. [ABSTRACT FROM AUTHOR]

A greenhouse experiment was conducted in which two leguminous species commonly used in the Yellow River Delta for vegetation restoration, Robinia pseudoacacia and Amorpha fruticosa, were subjected to five salt treatments: 0, 50, 100, 150, and 200 mmol L−1. We aimed to determine which of the two species would be better suited for growth in a saline environment, and whether the acclimation capacity to salinity resulted from an inherently higher phenotypic plasticity. The results showed that salinity affected most growth and biomass parameters but had no effects on most leaf traits and physiological parameters of the two species. Height, relative growth rate of crown area, root biomass, and leaf mass ratio of R. pseudoacacia were reduced by higher salinity, while A. fruticosa was not affected. Chlorophyll a-to-chlorophyll b ratio and total antioxidative capacity of A. fruticosa increased with higher salinity, whereas those of R. pseudoacacia remained unchanged. Root mass ratio and vitamin C concentration of both species were not affected by salinity, whereas vitamin C concentration of A. fruticosa was higher than that of R. pseudoacacia. The root-to-shoot ratio of A. fruticosa was higher than that of R. pseudoacacia in most salt treatments. Of all leaf traits, only leaf area differed between treatments. R. pseudoacacia generally exhibited a greater plasticity than A. fruticosa in response to salinity, but A. fruticosa was more resistant to the higher salinities than R. pseudoacacia, and was thus a better candidate for vegetation restoration in saline areas. [ABSTRACT FROM AUTHOR]

and carbon-related measurements can help elucidate drought-induced plant mortality. To study drought mortality mechanisms, seedlings of two woody species, including the anisohydric Robinia pseudoacacia and isohydric Quercus acutissima , were cultivated in a greenhouse and subjected to intense drought by withholding water and mild drought by adding half of the amount of daily water lost. Patterns of leaf and root gas exchange, leaf surface areas, growth, leaf and stem hydraulics, and carbohydrate dynamics were determined in drought-stressed and control seedlings. We detected a complete loss of hydraulic conductivity and partial depletion of total nonstructural carbohydrates contents (TNC) in the dead seedlings. We also found that intense drought triggered a more rapid decrease in plant water potential and a faster drop in net photosynthesis below zero, and a greater TNC loss in dead seedlings than mild drought. Additionally, anisohydric R. pseudoacacia suffered a rapider death than the isohydric Q. acutissima. Based on these findings, we propose that hydraulic conductivity loss and carbon limitation jointly contributed to drought-induced death, while the relative contributions could be altered by drought intensity. We thus believe that it is important to illustrate the mechanistic relationships between stress intensity and carbon-hydraulics coupling in the context of isohydric vs. anisohydric hydraulic strategies. • There were complete conductivity loss and partial C depletion in dead plants. • Intense drought also resulted in a smaller C loss and in a rapid conductivity loss when compared with mild drought. • Anisohydric R. pseudoacacia suffered higher risk of hydraulic failure, and a rapid mortality than isohydric Q. acutissima. [ABSTRACT FROM AUTHOR]