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全球气候变化改变降雨量、降雨频率(天、周、月模式)和降雨强度,未来严重干旱发生的可能性增大[1-3]。近年,突然的和大范围的森林死亡、衰败被认为与干旱和温度升高有关[4];而森林和林地植物死亡致快速地和大尺度地生态系统结构和功能的改变[5]。水力失衡和碳饥饿假说是目前解释干旱导致树木死亡的主要机制[6]。水力失衡假说是指减少的土壤水分供应和高的蒸发需求导致木质部导管和根系产生空穴化,植物长距离水分运输受限,使植物发生不可逆的干化现象[7]。非结构性碳(NSC)是植物的光合产物,主要是可溶性糖和淀粉,其累加值为总非结构性碳(TNC)。TNC为植物生长繁殖和新陈代谢提供能量,对抵抗干扰和干扰后的恢复也有重要作用[8]。碳饥饿假说认为,长期的干旱胁迫会打破植物碳摄取与碳支出的平衡,当包括光合作用、可动员的TNC储备和水解供应的NSC不能抵消包括呼吸、生长和防御所需的NSC的时候,NSC的浓度会降低,当NSC降低到一定程度时,植物会因碳饥饿而死亡[8-10]。很多植物为了阻止水力失衡的发生关闭了气孔,但气孔关闭的代价是光合碳固定减少[7]。碳饥饿和水力失衡之间可能存在交互作用,因为栓塞修复可能需要利用木质部薄壁细胞内的糖降低栓塞导管的渗透势,进而对栓塞进行修复[11];而水分的有效性也影响NSC的生产和运输[8],并且在抵抗干旱过程中,NSC对渗透调节和维持细胞膨压等方面也有作用[12]。水力失衡与碳饥饿在树木死亡过程中哪一个起主要作用还不清楚,并且缺乏实例证据,特别是国内还未见这方面的报道,且国外已经报道的研究也大多采用盆栽控水方法[12-13],缺少森林原位的研究。
2013年4月笔者在河南省西南部的宝天曼锐齿栎(Quercus aliena var. acuteserrata Maxim.)天然次生林内建立了截雨样地(减少的是林内穿透雨),以伴生种三桠乌药(Lindera obtusiloba Bl. Mus. Bot.)为研究对象,探讨降雨减少对三桠乌药生理生态的影响,从水和碳的角度,探讨三桠乌药顶端枯死的原因,确定截雨处理后三桠乌药是否出现适应性的变化。
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2013年4月在森林原位建立3块20 m×20 m的降雨减少(截雨)样地,3块20 m×20 m的对照样地。截雨样地通过拦截近地位置的穿透雨实现样地降雨减少,拦截率约为50%。每块截雨样地均由约160块(0.5 m×3 m)透明减水膜组成,薄膜安装在距地面约1.5~2.5 m高的不锈钢架上,装置所截雨水由导水槽导出样地之外。每块样地四周均设有约70 cm深的隔水板,防止样地内外的土壤水分交换。截雨样地距离宝天曼生态站约4 km。
截雨样地共有三桠乌药21棵(样地内树木大部分为锐齿栎),胸径为4.11±0.46 cm,对照样地共有三桠乌药38棵,胸径为4.3±0.33 cm,2个样地三桠乌药的胸径差异不显著,且2个样地的三桠乌药树高一般都不超过5 m。每次测定时每个样地随机采样6棵。
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降雨数据用临近的一个气象站采集,样地土壤湿度用Em50 Series Data Collection System (Decagon Devices, Inc., USA)进行测定,测定值为土壤体积含水量,每个样地2个探头,随机布设,频率为30 min,测定深度为5 cm。
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2014年8月2日用压力室(model 1000; PMS Instruments Co., USA)测定截雨样地和对照样地的枝条凌晨水势和中午水势,凌晨水势测定时间为5:30—6:30,中午水势测定时间为12:30—13:00。为防止测定栓塞(导水损失率)时产生人为栓塞和了解三桠乌药木质部导管特性,在进行栓塞测定之前,用注入空气法[14]测定三桠乌药的最大导管长度,约为60 cm。采样时保证测定凌晨栓塞和中午栓塞的枝条长度大于60 cm,约1 m,采样时间与水势测定时间相同,采样后用湿浴巾小心包裹,马上带到宝天曼生态站实验室,用低压液流系统[14]测定导水损失率和枝条导水率。所测枝条的所有叶片都用叶面积仪(Li-3000C; Li-Cor Inc., USA)测定叶面积,枝条末端的边材面积与枝条总叶面积的比值为Huber值,并选取成熟展开叶片计算平均叶面积。冲洗栓塞后的枝条最大导水率与所测茎段的边材面积的比值为边材比导率,最大导水率与茎段以上所有叶片总面积的比值为叶片比导率。测定栓塞的茎段用FAA固定液(乙醇∶乙酸∶甲醛∶水=10∶1∶2∶7)带回实验室,制作半薄切片,用Axio Imager A1光学显微镜(Carl Zeiss Inc., Oberkochen, Germany)观察导管并拍照,当茎段大于2年生,只测定最外面2圈年轮内的导管,截雨样地与对照样地的三桠乌药均选取20个以上的观察视野统计导管直径和导管密度。
为方便三桠乌药栓塞脆弱性曲线测定,均是雨后凌晨在生态站周围取样,采用传统的干燥法[14]进行测定。曲线用如下公式拟合:
$ PLC = \frac{{100}}{{1 + \exp (a(\psi - b))}} $
式中:PLC是导水损失率(%),ψ为对应的水势(MPa)。导水损失率50%所对应的水势值为P50,实测的中午水势平均值与P50的差值为平均水力安全边际,而实测到的水势最低值与P50的差值为最低水力安全边际。
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用LI-6400XT型便携式光合仪(Li-Cor, Lincoln, USA)测定光合生理参数,包括净光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)。设定CO2浓度为400 μmol·mol-1,叶室温度25℃,光合有效辐射(PAR)为1 500 μmol·m-2·s-1,选择晴朗天气测定。2014年8月3日上午9:00—11:00,用高枝剪剪下枝条后马上测定,待数据稳定后再记录。
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测定凌晨水势时采样,带到实验室并把枝条的韧皮部和木质部分离,105℃杀青30 min,75℃烘干72 h,烘干样品研磨过100目筛。用蒽酮-硫酸法[15]测定叶片、韧皮部、木质部3个器官的可溶性糖和淀粉浓度,可溶性糖和淀粉浓度的和即为总非结构性碳(TNC)浓度。
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数据用SPSS 17.0软件进行统计分析。各指标均用独立样本t检验分析截雨样地与对照样地的差异显著性,当P<0.05时认为差异显著。文中数据均为平均值±标准误差,且全部为6个重复。采用Sigmaplot 12.5软件作图。
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由图 1可知:2014年7月19日有一场强降雨,但在7月20日至8月8日间,降雨量非常小,单日最大降雨量只有5.6 mm,20 d的总降雨量也只有19.6 mm;之后一直到8月末,虽然有几场小降雨,但是截雨样地的土壤湿度一直降低且低于对照样地,而对照样地波动较大,即在远离大的降雨事件后,截雨样地的土壤水分明显低于对照。
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本试验在2014年8月初进行测定,此时有几天连续的晴朗天气。8月末样地调查时发现,截雨样地的21棵三桠乌药有不同程度的顶梢枯死,而对照样地的38棵三桠乌药只有5棵发生顶梢枯死。
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从表 1可知:截雨样地三桠乌药的凌晨水势和中午水势显著低于对照,且中午水势平均值为-1.61 MPa,最小值为-1.85 MPa;三桠乌药凌晨的栓塞与对照差异不显著,但中午的栓塞显著高于对照,且均值高达81.3%,实测最大值达88.3%。
表 1 对照样地和截雨样地三桠乌药的凌晨和中午水势、栓塞
Table 1. Pre-dawn and midday water potential, and pre-dawnand midday PLC of Lindera obtusiloba in control plots and rainfall reduced plots
处理Treatment 水势Water potential/MPa 栓塞Embolism/% 凌晨Pre-dawn 中午Midday 凌晨Pre-dawn 中午Midday 对照CK -0.39±0.029a -1.15±0.089a 36.6±5.91a 57.8±1.24b 截雨Rainfall reduction -0.51±0.034b -1.61±0.083b 49.6±5.24a 81.3±2.33a 注:表中同列不同小写字母表示差异显著(P<0.05),下同。
Note: Different lowercases in the same column denote significant differences at p<0.05, the same below. -
由三桠乌药的栓塞脆弱性曲线得到其栓塞50%的水势值(P50)为-1.43 MPa。由图 2可知:截雨样地三桠乌药的平均水力安全边际和最低水力安全边际都低于对照,且都是负值,而对照都是正值。
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截雨样地三桠乌药的净光合速率(Pn)、气孔导度(Gs)和蒸腾速率(Tr)均显著低于对照(表 2),而截雨样地三桠乌药叶片、韧皮部、木质部的可溶性糖、淀粉和总非结构性碳浓度均与对照差异不显著(图 3)。
表 2 对照样地和截雨样地三桠乌药的气体交换参数
Table 2. The gas exchange parameters of Lindera obtusiloba in control plots and rainfall reduced plots
处理
Treatment净光合速率(Pn)
Net photosynthetic rate/(μmol·m-2·s-1)气孔导度(Gs)
Stomatal conductance/(mol·m-2·s-1)蒸腾速率(Tr)
Transpiration rate/(mmol·m-2·s-1)对照CK 6.73±0.29a 0.187±0.009a 1.22±0.053a 截雨Rainfall reduction 5.03±0.43b 0.118±0.017b 0.72±0.079b -
由表 3可知:截雨样地三桠乌药的单叶叶面积显著低于对照,而Huber值显著大于对照;截雨样地三桠乌药的边材比导率、叶片比导率和导管直径均显著低于对照,而导管密度显著大于对照。
表 3 对照样地和截雨样地三桠乌药的水力结构参数
Table 3. The hydraulic parameters of Lindera obtusiloba in control plots and rainfall reduced plots
处理Treatment 对照CK 截雨Rainfall reduction 单叶叶面积Single leaf size/cm2 34.02±1.32a 29.43±1.09b Huber值Huber value/×10-4(mm2·cm-2) 86.76±3.79b 102.78±6.15a 边材比导率Sapwood specific conductivity/(kg·m-1·s-1·MPa-1) 2.31±0.203a 1.07±0.396b 叶片比导率Leaf specific conductivity/×10-4(kg·m-1·s-1·MPa-1) 1.99±0.18a 1.00±0.37b 导管密度Vessel density/(N·mm-1) 133.96±6.14b 202.83±10.78a 导管直径Vessel diameter/μm 30.59±0.32a 25.77±0.29b
宝天曼三桠乌药对降雨减少后的生理生态响应
Ecophysiological Responses of Lindera obtusiloba to Rainfall Reduction in Baotianman Nature Reserve
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摘要:
目的 在宝天曼森林原位建立降水减少(截雨)样地后,样地内的三桠乌药出现了顶端枯死,本研究从水和碳的角度,探讨三桠乌药顶端枯死的原因。 方法 2013年4月在宝天曼锐齿栎林原位建立了3块截雨样地,在2014年生长季干旱时,采用压力室、低压液流系统和蒽酮硫酸法测定了三桠乌药的水力系统特征和非结构性碳等指标。 结果 显示:(1)三桠乌药在截雨处理后出现了顶端枯死;三桠乌药最大导管长度约为60 cm,栓塞50%时的水势(P50)为-1.43 MPa,其木质部栓塞脆弱性较大。在截雨处理1年后生长季较干旱时,净光合速率、凌晨水势、中午水势显著低于对照,中午栓塞显著高于对照,且水力安全边际为负值。(2)三桠乌药气孔导度、蒸腾速率、叶面积、导管直径、边材比导率、叶片比导率显著下降,Huber值、导管密度显著升高;而叶片、韧皮部、木质部3个器官的可溶性糖、淀粉、总非结构性碳与对照差异不显著。 结论 水力失衡是三桠乌药顶端枯死的主要原因,而蒸腾面积、水分输导系统等的变化表明三桠乌药在缺水环境下进行了一定的适应性调节,但这些调节不足以使三桠乌药在缺水情况下避免水力失衡而导致的顶端枯死。 Abstract:Objective The objective of this study is to investigate the reason why Lindera obtusiloba Bl.Mus.Bot.in Baotianman Mountain showed dieback based on the perspective of water and carbon. Method Three rainfall-intercepted plots were established at the forest in Baotianman Mountain in April 2013. The hydraulic characteristics and nonstructural carbohydrate of L. obtusiloba were measured during the drought period of August, 2014. Result (1) The rainfall reduction caused a lot of shoot dieback of L. obtusiloba. The maximum vessel length of L. obtusiloba was about 60 cm and the water potential causing 50% loss of hydraulic conductivity (P50) was-1.43 MPa, and thus, the xylem vulnerability to cavitation was very large. After being treated with the interception of rainfall for more than one year, in comparison with the control, the net photosynthetic rate, pre-dawn water potential and midday water potential significantly decreased, the percentage loss of hydraulic conductivity significantly increased in the dry period, and terribly its hydraulic safety margin was negative. (2) In addition, the stomatal conductance, transpiration rate, leaf area, vessel diameter, sapwood specific conductivity and leaf specific hydraulic conductivity significantly decreased, the Huber value and the vessel density significantly increased compared with the control. However, there were no significant differences in the soluble sugar, starch and total nonstructural carbohydrate of leaf, phloem and xylem between the rainfall-intercepted trees and the control. Conclusion The results indicate that L. obtusiloba dieback in the rainfall-intercepted plots is attributed to hydraulic failure rather than carbon starvation, and the variation of transpiration area and hydraulic conduction system after treated with rainfall interception indicate that L. obtusiloba has to some degree drought acclimation performances in drought circumstance, but the adaptive adjustment is not enough to prevent L. obtusiloba from dieback. -
表 1 对照样地和截雨样地三桠乌药的凌晨和中午水势、栓塞
Table 1. Pre-dawn and midday water potential, and pre-dawnand midday PLC of Lindera obtusiloba in control plots and rainfall reduced plots
处理Treatment 水势Water potential/MPa 栓塞Embolism/% 凌晨Pre-dawn 中午Midday 凌晨Pre-dawn 中午Midday 对照CK -0.39±0.029a -1.15±0.089a 36.6±5.91a 57.8±1.24b 截雨Rainfall reduction -0.51±0.034b -1.61±0.083b 49.6±5.24a 81.3±2.33a 注:表中同列不同小写字母表示差异显著(P<0.05),下同。
Note: Different lowercases in the same column denote significant differences at p<0.05, the same below.表 2 对照样地和截雨样地三桠乌药的气体交换参数
Table 2. The gas exchange parameters of Lindera obtusiloba in control plots and rainfall reduced plots
处理
Treatment净光合速率(Pn)
Net photosynthetic rate/(μmol·m-2·s-1)气孔导度(Gs)
Stomatal conductance/(mol·m-2·s-1)蒸腾速率(Tr)
Transpiration rate/(mmol·m-2·s-1)对照CK 6.73±0.29a 0.187±0.009a 1.22±0.053a 截雨Rainfall reduction 5.03±0.43b 0.118±0.017b 0.72±0.079b 表 3 对照样地和截雨样地三桠乌药的水力结构参数
Table 3. The hydraulic parameters of Lindera obtusiloba in control plots and rainfall reduced plots
处理Treatment 对照CK 截雨Rainfall reduction 单叶叶面积Single leaf size/cm2 34.02±1.32a 29.43±1.09b Huber值Huber value/×10-4(mm2·cm-2) 86.76±3.79b 102.78±6.15a 边材比导率Sapwood specific conductivity/(kg·m-1·s-1·MPa-1) 2.31±0.203a 1.07±0.396b 叶片比导率Leaf specific conductivity/×10-4(kg·m-1·s-1·MPa-1) 1.99±0.18a 1.00±0.37b 导管密度Vessel density/(N·mm-1) 133.96±6.14b 202.83±10.78a 导管直径Vessel diameter/μm 30.59±0.32a 25.77±0.29b -
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