Weak Tradeoff and Strong Segmentation Among Plant Hydraulic Traits During Seasonal Variation in Four Woody Species

Abstract

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.

Quantifying Key Points of Hydraulic Vulnerability Curves From Drought-Rewatering Experiment Using Differential Method

“…Notably, some researches indicated that the “air-injection” method may be prone to artifacts if the maximum length of the xylem vessels is not considered when preparing the samples for conducting the measurements (Ennajeh et al, 2011 ). However, this research was based on 4-month-old saplings, it is impossible to have a long conduit like a tree, according to the shape of our HVC ( Figure 2 ) and previous researches (Zhu et al, 2018 ; Li et al, 2019 , 2020 ; Liu et al, 2020 ), we convince that the 30-cm-long segments had no open vessels, so that the “air-injection” method did not cause experimental artifacts in this research. Moreover, the “air-injection” method can accurately control the stem water potential, and it can improve the precision of the HVC (Sergent et al, 2020 ).…”

“…Notably, some researches indicated that the "air-injection" method may be prone to artifacts if the maximum length of the xylem vessels is not considered when preparing the samples for conducting the measurements (Ennajeh et al, 2011). However, this research was based on 4-month-old saplings, it is impossible to have a long conduit like a tree, according to the shape of our HVC (Figure 2) and previous researches (Zhu et al, 2018;Li et al, 2019Liu et al, 2020), we convince that the 30-cm-long segments had no open vessels, so that the "air-FIGURE 4 | Differences among: CK, D 3 , D 8 (A); D 12 U, R 2 U to REU (C); D 12 L, R 2 L to REL (E) for the stem water potential (Ψ ). The loss of conductivity (LC) differences among CK, D 3 , D 8 (B); D 12 U, R 2 U to REU (D); and D 12 L, R 2 L to REL (F).…”

“…After the K m measurement, the segments were fixed in double-sleeved air-injection chambers (1505D-EXP, PMS Instrument Co, Albany, OR, USA). K s was then measured after exposing the segments to progressively increased air-injection pressures that range from 0.00 to 4.00 MPa, at 0.20 MPa steps, and then 4.50, 5.00, and 6.00 MPa, according to the characteristics of the curve and our previous research (Liu et al, 2020 ). The air-injection pressure remained constant at each injection pressure level using a gas pressure regulator for 5 min.…”

Physiological Responses of Robinia pseudoacacia and Quercus acutissima Seedlings to Repeated Drought-Rewatering Under Different Planting Methods

“…Subsequently, the samples were transported promptly to the laboratory with the crowns covered with black plastic bags. All leaves and bark were removed, and each segment of R. pseudoacacia was 30 cm long, while that of Q. acutissima was 20 cm long, to ensure there would never be open vessels ( Liu et al, 2020 ). The segments were connected to a hydraulic conductivity measurement system that contained degassed, filtered 20.0 mmol L –1 KCl solution.…”

“…This phenomenon has aggravated the existing spatiotemporal uneven water distribution in the warm temperate zone ( Luan et al, 2011 ; Corlett, 2016 ), leading to the alternation of drought and rewatering. Severe drought may lead to plant hydraulic failure, which has been invoked as the most direct and critical mechanism causing tree dieback and forest mortality ( Martinez-Vilalta and Pinol, 2002 ; Nardini et al, 2013 ; O’Grady et al, 2013 ; Liu et al, 2020 ). After drought, the pace of recovery differs among physiological processes; leaf water potential typically recovers within few days even few hours upon rewatering, while leaf gas exchange-related variables lag ( Ruehr et al, 2019 ), leading to plant growth traits slow recovery ( Li et al, 2019 , 2020 ).…”