浙江天童常绿阔叶林不同演替阶段木本植物的水力结构特征

水力结构是植物应对环境形成的与水分运输相关的形态策略.探索不同演替阶段和群落不同高度层植物的水力结构特征, 有助于理解植物的水分运输和利用策略.该研究以浙江天童常绿阔叶林演替前中后期群落的上层木(占据林冠层的树种)和下层木(灌木层物种)为对象, 测定了演替共有种(至少存在于两个演替阶段的物种)和更替种(仅存在于某一演替阶段的物种)的枝边材比导率,叶比导率和胡伯尔值, 以及边材疏导面积,末端枝总叶面积和枝条水势, 分析植物水力结构在群落上层木和下层木间以及在演替阶段间的差异, 及其与枝叶性状的相关关系.结果显示: (1)上层木植物边材比导率和叶比导率显著高于下层木植物(p < 0.05); (2)上层木和下层木的边材比导率与叶比导率在演替阶段间均无显著差异(p > 0.05); 上层木的胡伯尔值在演替阶段间无显著差异, 下层木的胡伯尔值随演替显著下降(p < 0.05); (3)上层木共有种仅边材比导率随演替进行显著降低(p < 0.05), 更替种的3个水力结构参数在演替阶段间无显著差异; 下层木共有种水力结构参数在演替阶段间无明显差异, 更替种仅胡伯尔值随演替减小(p < 0.05); (4)植物边材比导率与枝疏导面积和末端枝所支撑的总叶面积显著正相关(p < 0.01), 胡伯尔值与枝条水势及末端枝总叶面积显著负相关(p < 0.01).以上结果表明: 天童常绿阔叶林演替各阶段上层木比下层木具有更大的输水能力和效率; 随着演替进行, 上层木与下层木的共有种和更替种边材比导率的相反变化表明上层木水力结构的变化可能由微生境变化引起, 而下层木水力结构特征的变化可能由物种更替造成.     

中国热带珊瑚岛优势木本植物抗风桐和草海桐的水分适应策略解析

植物水力性状能够反映植物对不同水分条件的适应能力，研究热带珊瑚岛特殊生境下优势植物的水力功能特征对深入理解热带珊瑚岛植物的水分适应策略，从而选择热带珊瑚岛植被构建和恢复的适生物种具有重要意义。该研究以中国热带珊瑚岛生境中2种优势适生木本植物：抗风桐(Pisonia grandis)和草海桐(Scaevola sericea)为研究对象，比较了其叶片和枝条的水力性状，并分析了其水分适应策略。结果表明, 抗风桐的叶片栓塞抗性、枝条边材比导水率和叶片膨压丧失点显著高于草海桐，而枝条栓塞抗性、叶片导水率、边材密度和叶面积边材面积比均显著低于草海桐。抗风桐的叶片具有比枝条更强的抗栓塞能力，对水分胁迫敏感，但同时选择以高效的枝干水分运输来满足叶片高蒸腾需求的充足供水。草海桐枝条与叶片则存在水力脆弱性分区，在面临水分胁迫时叶片充当“安全阀”以保证枝干木质部的水力安全。抗风桐与草海桐能够通过协调叶片与枝条水力性状采取不同的水分适应策略，从而更好地适应热带珊瑚岛的特殊生境。     

润楠属植物叶片和枝条水力特征的协同关系

以润楠属(Machilus) 7种植物成年个体为材料,对其进行生理指标测定,并对它们的叶片水分供需关系以及木质部纹孔特征和导水效率之间的关联进行分析。结果显示,润楠属7种植物相比原始被子植物具有更高的叶脉密度(VD),叶脉密度为9.8～14.1 mm/mm~2;气孔密度(SD)与叶脉密度呈显著正相关,说明叶片水分供需存在协同关系;气孔密度与气孔大小(GLC)呈负相关;气孔越大的叶片其膨压丧失点(TLP)的绝对值越低。枝条边材比导率(Ks)较低,为0.13～1.87 kg·m~(-1)·s~(-1)·MPa~(-1),且种间差异较大。叶脉和气孔密度均与边材比导率呈正相关。边材比导率与纹孔膜面积、纹孔口面积以及纹孔口长短轴比例相关性不显著。研究结果表明润楠属植物虽然叶脉密度较高,且木质部水分供应和叶片结构具有协同关系,但木质部解剖结构较为原始,导管多具梯形穿孔板,导水效率低,只能适应比较湿润的生境。     

沙柳和柠条茎水力学特性对模拟降雨改变的响应

利用模拟降雨控制试验(对照、降雨增加45%和减少50%),研究了黄土高原水蚀风蚀交错带典型灌木沙柳和柠条茎水力学特性对模拟降雨改变的响应,以揭示两种灌木对未来降雨改变的适应性.结果表明: 沙柳茎比导水率(K_s)、比叶导水率(K_l)和Huber值对增水有显著响应,而对干旱无显著响应;柠条黎明前和正午叶水势、水分传输效率(K_s及K_l)对干旱有显著响应,但对增水无响应.两种灌木不同处理间抵抗栓塞能力无显著差异,沙柳不同处理间正午原位栓塞程度亦无显著差异,而柠条干旱处理正午原位栓塞程度显著增大.沙柳增水处理导管直径和导管面积占边材面积的比例显著增加,干旱导致沙柳导管密度显著增大,水力直径变小;柠条增水处理的木质部结构无明显改变,干旱导致其导管密度和木材密度显著增加.说明增水提升了沙柳的水力功能,而长期干旱显著降低了柠条水力功能,预测在未来气候旱化条件下,柠条的水力适应性可能不如沙柳.     

云南哀牢山6种常绿阔叶树木质部解剖特征的轴向和径向变化

West、Brown和Enquist提出的树木水分传导的分形网络模型(简称WBE模型)认为,树木连续分枝之间的导管或管胞直径按照一定的比率均匀变细,其总的水力阻力与水分传导的路径长度无关,从而使不同部位叶片获得基本相当的水分供应。该模型对树木高生长的水力限制假说提出了置疑。为了验证WBE模型中树木导管或管胞均匀变细的假说,该文研究了云南哀牢山中湿性常绿阔叶林中6种常绿阔叶树, 腾冲栲(Castanopsis wattii)、景东石砾(Lithocarpus chintungensis)、木果石砾(L. xylocarpus)、长尾青冈(Cyclobalanopsis stewardiana)、滇木荷(Schima noronhae)和舟柄茶(Hartia sinensis)木质部解剖特征随树高和年龄的变化。对这6个树种共14株样木进行了不同高度树干圆盘和边材生长轮取样,样木的高度为15~25 m,按照常规木材解剖的处理和分析方法,在显微镜下测定木材切片的导管直径和密度等特征。结果表明:在14株样木中,有4株树木导管直径随树木高度增加呈线性减小, 1株没有明显变化,其它9株树木导管直径在树冠以下的树干部分变化幅度较小或没有明显变化,而从树冠基部往上直到树木顶端导管直径显著减小。同一植株随着高度的增加,导管密度增加并且在树冠内增加更显著。有三分之一的树木导管占边材面积的比例随树高增加没有明显变化,其余树木导管占边材面积比在树冠以上有所减小。多数树木理论比导率在树冠以下没有明显变化而在树冠基部往上显著降低。在从髓芯开始往外的20~40个年轮范围内导管直径增加显著,但大部分植株导管直径在40个年轮后趋于稳定。不同高度圆盘导管直径随形成层发育时间的变化呈相似的趋势,并且相同发育年龄的导管直径没有明显差异。该文的研究结果说明,导管直径的轴向和径向变化一定程度上补偿了水分运输阻力随树木个体增大而增加的缺陷,但是6种常绿阔叶树树干的导管基本不按一定比率均匀变细,不支持WBE模型。     

长白山阔叶树种木质部环孔和散孔结构特征的分化导致其水力学性状的显著差异

木质部的解剖结构特征对树木水分传输功能有重要的影响,阔叶树种木质部环孔和散孔结构特征的分化,很可能导致两个功能类群在水力学结构上存在显著差异,但是有关两个功能类群间细致的水力学性状的对比研究还较少,二者整枝水平的导水率及纹孔水平的细致结构差异尚未见报道.本试验以长白山阔叶红松林常见的3个环孔材和4个散孔材乔木树种为研究对象,对比了两个功能类群树种的整枝导水率(k_shoot)、枝条木质部栓塞脆弱性(p_50)等重要水力学相关生理功能特征,并分析了两个功能类群间的木质部组织水平和纹孔水平上的解剖结构特征差异.结果表明:与茎段导水率差异一致,环孔材树种的整枝导水率也显著高于散孔材,但枝条木质部气穴化抵抗力显著弱于散孔材,二者的差异反映了整枝水平上木质部导水效率和安全性之间的权衡关系,与两个功能类群的水力学生理特征存在显著差异一致,二者在最大导管长度、导管直径、纹孔开口面积、纹孔开口比例等光学和扫描电镜观测解剖结构特征上都存在显著差异;木质部解剖特征(组织水平、纹孔水平)和k_shoot、p_50等生理特征间,以及木质部不同解剖特征之间存在显著的相关,且两个功能类群遵循相同的规律,反映了木质部结构对水分传输功能的重要影响,而导水率和气穴化抵抗力对木质部对立的结构要求,体现了树木水分传输系统构建的生物物理局限性.     

土壤干旱胁迫对樟树（Cinnamomum camphora (L.) Presl）苗木水力结构特征的影响

植物水力结构特征通常用导水率(Kh)、比导率(Ks)、叶比导率(LSC)、胡伯尔值(Hv)等参数来表征.设置了3种土壤干旱胁迫强度和对照共4种处理,每一处理5个重复,采用"冲洗法"对喀斯特地区造林树种两年生樟树(Cinnamomum camphora (L.) Presl)苗木的水力结构相关参数进行测定.结果表明:樟树苗木在正常水分和不同强度土壤干旱胁迫条件下,其茎段导水率、比导率、叶比导率、胡伯尔值均与茎段直径呈正相关关系,并可以用不同类型函数进行拟合.随着干旱胁迫强度的加剧,不同处理的导水率、比导率、叶比导率降低,胡伯尔值在不同处理间的差异不显著.这说明较粗的茎段,其单位横截面积的导管投入与使用效率明显高于较细茎段,且供给末端叶片水分所需要的压力梯度小于较细茎段.结果为进一步探讨土壤干旱对樟树苗木水力结构作用机理及在石漠化山地造林的适应性提供理论依据.     

不同林龄刺槐解剖结构及生理特性

全球变暖导致干旱强度增大、频率增加，探究黄土高原不同林龄刺槐林对干旱的响应机制对于理解该区人工林生产力稳定性具有重要意义。本研究以黄土高原半干旱区刺槐幼龄林、中龄林和成熟林为对象，探究不同林龄木质部解剖特征、水力特征和非结构性碳水化合物含量及其之间的相关关系。结果表明：幼龄林、中龄林的导管直径、纹孔总面积、纹孔面积、附着物面积和附着比例均显著大于成熟林，而纹孔密度显著小于成熟林。导管直径、纹孔面积和附着物面积与导水率之间存在显著相关性。幼龄、中龄和成熟林枝条导水率分别为2.30、2.12和0.76 kg·m^-1·s^-1·MPa^-1,栓塞程度分别为54.5%、53.8%和45.1%。导水率与可溶性糖和淀粉含量之间存在显著相关性。幼龄、中龄和成熟林枝条木质部可溶性糖含量分别为4.9%、4.2%和3.8%。成熟林刺槐木质部生长力下降，形成具有较多较小纹孔的小导管，在维持水力安全的同时降低了水分运输效率，组织内维持生长代谢的非结构性碳水化合物含量显著降低。本研究解释了不同林龄刺槐人工林对干旱的响应机制，为黄土高原不同林龄刺槐人工...     

陕北沙地3种典型灌木根木质部解剖结构及水力特性

比较了陕北沙地沙柳、柠条和沙棘3种典型灌木不同土壤深度(0～20和30～50 cm)根木质部解剖结构和水力特性.结果表明： 沙柳具有较高的叶水势,根木质部导管平均最大直径(d_max)、平均最小直径(d_min)、平均导管面积(A_lum)以及导管面积占木质部面积比例(A_ves/A_xyl)显著高于柠条和沙棘,根导管密度(VD)与沙棘相当但显著高于柠条;沙柳根的比导水率分别为柠条和沙棘的5.0和2.8倍;沙柳根栓塞脆弱性指数与柠条根相当,但显著高于沙棘根.表明沙柳属耗水型水分利用策略,而柠条和沙棘属节水型水分利用策略,且柠条更耐旱.3种灌木在2个土层深度的d_max、d_min和A_lum无显著差异,但30～50 cm土层根VD和A_ves/A_xyl显著高于表层;30～50 cm土层根比导水率显著高于表层根,但脆弱性指数小于表层根,深层根具有高的水分传输效率和低的水力脆弱性.     

不同冲洗液对毛白杨和油松枝条水力导度和抵抗空穴化能力测定值的影响

植物通过木质部管道系统进行水分运输, 木质部的水分运输效率和抗空穴化能力等水力结构特征对于植物物种的分布、抗逆能力等方面起关键性作用。目前, 国内外学者一般采用“冲洗法”进行木质部水力结构研究, 然而在该方法中使用的不同冲洗溶质可能对植物木质部水力结构等产生较大影响, 因此该文研究了3种溶质的冲洗溶液对毛白杨(Populus tomentosa)和油松(Pinus tabulaeformis)枝条的水力导度和抵抗空穴化能力的影响。实验结果表明: 相对于去离子水, 用0.01 mol·L^-1的草酸和0.03 mol·L^-1KCl溶液作为冲洗溶液, 均导致毛白杨木质部导管和油松管胞的水力导度测定值的增大。KCl导致毛白杨和油松木质部抵抗空穴化能力测定值的提高, 草酸导致杨树抵抗空穴化能力测定值增强, 但导致油松抗空穴化能力显著(p<0.01)减弱。小枝水平上, 毛白杨和油松的水分运输效率和抗空穴化能力之间没有显著相关性。另外, 在截枝实验中发现, 毛白杨小枝木质部水力导度随长度增加变化不大, 而油松枝条的木质部水力导度有逐渐增大的趋势。以上的实验结果表明不同溶质下毛白杨和油松枝条的木质部水力导度和抵抗空穴化能力不同, 草酸和KCl可能对木质部管道系统及纹孔处的果胶等产生作用, 从而使毛白杨和油松的水力结构发生变化。毛白杨与油松水力结构在去离子水、草酸和KCl的作用下的不同结果及两物种截枝试验下水力导度的不同变化趋势表明, 导管运输系统和管胞运输系统可能具有不同的水分运输影响因素。     

Hydraulic architecture of evergreen broad-leaved woody plants at different successional stages in Tiantong National Forest Park,Zhejiang Province,China

Aims Hydraulic architecture is a morphological strategy in plants to transport water in coping with environmental conditions. Change of hydraulic architecture for plants occupying different canopy layers within community and for the same plant at different successional stages reflect existence and adaptation in plant's water transportation strategies. The objective of this study was to examine how hydraulic architecture varies with canopy layers within a community and with forest succession.Methods The study site is located in Tiantong National Forest Park, Zhejiang Province, China. Hydraulic architectural traits studied include sapwood-specific hydraulic conductivity, leaf-specific hydraulic conductivity, Huber value, sapwood channel area of twigs, total leaf area per terminal twig, and water potential of twigs. We measured those traits for species that occur in multiple successional stages (we called it "overlapping species") and for species that occur only in one successional stage (we called it "turnover species") along a successional series of evergreen broadleaved forests. For a given species, we sampled both overstory and understory trees. Hydraulic architectural traits between overstory and understory trees in the same community and at successional stages were compared. Pearson correlation was used to exam the relationship between hydraulic architectural traits and the twig/leaf traits.Important findings Sapwood-specific hydraulic conductivities and leaf-specific hydraulic conductivities were significantly higher in overstory trees than those in understory trees, but did not significantly differ from successional stages. Huber value decreased significantly for understory trees, but did not change for overstory trees through forest successional stages. For overstory trees, a trend of decreasing sapwood-specific hydraulic conductivity was observed for overlapping species but not for turnover species with successional stages. In contrast, for understory trees, a trend of decreasing Huber values was observed for turner species but not for overlapping species with successional stages. Across tree species, sapwood-specific hydraulic conductivity was positively correlated with sapwood channel area and total leaf area per terminal twig size. Huber value was negatively correlated to water potential of twigs and total leaf area per terminal twig size. These results suggest that water transportation capacity and efficiency are higher in overstory trees than in understory trees across successional stages in evergreen broadleaved forests in Tiantong region. The contrasting trends of sapwood-specific hydraulic conductivity between overlapping species and turnover species indicate that shift of microenvironment conditions might lead to changes of hydraulic architecture in overstory trees, whereas species replacement might result in changes of hydraulic architecture in understory trees.     

Hydraulic properties and photosynthetic rates in co-occurring lianas and trees in a seasonal tropical rainforest in southwestern China

In this study, we examined wood anatomy, hydraulic properties, photosynthetic rate, and water status and osmotic regulation in three liana species and three tree species co-occurring in a seasonal tropical rain forest. Our results showed that the three liana species had larger vessel diameter, lower sapwood density, and consequently higher branch sapwood specific hydraulic conductivity (K _S) than the three tree species. Across species, K _S was positively correlated with leaf nitrogen concentration and maximum net CO₂ assimilation rate. However, it was also positively correlated with xylem water potential at 50% loss of hydraulic conductivity, indicating a trade-off between hydraulic efficiency and safety. Compared to the tree species, the liana species had higher predawn leaf water potential and lower osmotic adjustment in the dry season. The combination of more efficient water transport, higher photosynthetic rates, and their ability to access to more reliable water source at deeper soil layers in the dry season in the lianas should contribute to their fast growth.     

锡林浩特多年生草本根部导管性状与植株生态策略的相关特征

干旱半干旱区植物的木质部输水系统对维持植物生长发育有重要作用。以中国干旱半干旱区的草本植物为研究对象,旨在探究草本物种根系导管解剖结构与植物生长之间的关系。用石蜡切片法,将在锡林浩特草原采集的草本物种的主根样品制作成切片,得到固定面积内导管解剖结构参量(导管数量、导管分数、平均导管面积、平均水力传导率和水力直径),然后用逐步回归法和Pearson相关分析各导管解剖性状与植物生长特征(年龄、生长速率和平均高度)之间的关系。结果发现(1)生长速率与导管数量(R=-0.494,P<0.01)和导管分数(R=-0.255,P<0.05)显著负相关,与平均导管面积(R=0.274,P<0.05)、平均水力传导率(R=0.263,P<0.05)和水力直径(R=0.245,P<0.05)显著正相关,表明生长快的草本具有大而少的导管,需要较高的水分传输能力,而生长慢的草本具有小而多的导管,水力安全性较高;(2)植株的高度与导管数量(R=-0.354,P<0.01)显著负相关,与平均导管面积(R=0.293,P<0.05)、平均水力传导率(R=0.289,P&l...     

Distribution of vessel size,vessel density and xylem conducting efficiency within a crown of silver birch (<Emphasis Type="Italic">Betula pendula</Emphasis>)

Spatial patterns in vessel diameter, vessel density and xylem conducting efficiency within a crown were examined in closed-canopy trees of silver birch (Betula pendula). The variation in anatomical and hydraulic characteristics of branches was considered from three perspectives: vertically within a crown (lower, middle and upper crown), radially along main branches (proximal, middle and distal part), and with respect to branch orders (first-, second- and third-order branches). Hydraulically weighted mean diameter of vessels (D _h) and theoretical specific conductivity of the xylem (k _t) exhibited no vertical trend within the tree crown, whereas leaf-specific conductivity of the xylem (LSC_t) decreased acropetally. Variation in LSC_t was governed by sapwood area to leaf area ratio (Huber value) rather than by changes in xylem anatomy. The acropetal increase in soil-to-leaf conductance (G _T) within the birch canopy is attributable to longer path length within the lower-crown branches and higher hydraulic resistance of the shade leaves. D _h, k _t and LSC_t decreased, while vessel density (VD) and relative area of vessel lumina (VA) increased distally along main branches. A strong negative relationship between vessel diameter and VD implies a trade-off between hydraulic efficiency and mechanical stability of xylem. D _h and VD combined explained 85.4% of the total variation of k _t in the regression model applied to the whole data set. Xylem in fast-growing branches (primary branches) had greater area of vessel lumina per unit cross-sectional area of sapwood, resulting in a positive relationship between branch radial growth rate and k _t. D _h, k _t and LSC_t decreased, whereas VD increased with increasing branch order. This pattern promotes the hydraulic dominance of primary branches over the secondary branches and their dominance over tertiary branches. In this way crown architecture contributes to preferential water flow along the main axes, potentially providing better water supply for the branch apical bud and foliage located in the outer, better-insolated part of the crown.     

Sapwood hydraulic conductivity and leaf area – sapwood area relationships following thinning of a Eucalyptus nitens plantation

This study tested the hypothesis that the relationship between leaf area (projected, or one‐sided) and sapwood area in Eucalyptus nitens (Deane and Maiden) Maiden is affected by thinning treatment. However, no difference in the relationship between leaf area and sapwood area was found 8 years after thinning. This result suggests that a single regression equation can be used to predict the leaf area of trees in thinned and unthinned stands. The relationship was non‐linear, implying a causal relationship between growth rate and the ratio of leaf area to sapwood area (A_l : A_s). Sapwood hydraulic conductivity increased by approximately 100% from breast height to crown base, whereas sapwood area decreased by 19%. This implies that the efficiency of water transport through the sapwood increased by 60% along this length. This conclusion is supported by the A_l : A_s relationships which showed that the sapwood area at crown base supported, on average, close to 60% more leaf area than a similar amount of sapwood area at breast height. That large trees in this study had greater hydraulic conductivity and higher A_l : A_s lends support to the argument that resource capture, and hence growth rate, influence sapwood hydraulic conductivity.     

Stem and leaf hydraulics of congeneric tree species from adjacent tropical savanna and forest ecosystems

Leaf and stem functional traits related to plant water relations were studied for six congeneric species pairs, each composed of one tree species typical of savanna habitats and another typical of adjacent forest habitats, to determine whether there were intrinsic differences in plant hydraulics between these two functional types. Only individuals growing in savanna habitats were studied. Most stem traits, including wood density, the xylem water potential at 50% loss of hydraulic conductivity, sapwood area specific conductivity, and leaf area specific conductivity did not differ significantly between savanna and forest species. However, maximum leaf hydraulic conductance (K _leaf) and leaf capacitance tended to be higher in savanna species. Predawn leaf water potential and leaf mass per area were also higher in savanna species in all congeneric pairs. Hydraulic vulnerability curves of stems and leaves indicated that leaves were more vulnerable to drought-induced cavitation than terminal branches regardless of genus. The midday K _leaf values estimated from leaf vulnerability curves were very low implying that daily embolism repair may occur in leaves. An electric circuit analog model predicted that, compared to forest species, savanna species took longer for their leaf water potentials to drop from predawn values to values corresponding to 50% loss of K _leaf or to the turgor loss points, suggesting that savanna species were more buffered from changes in leaf water potential. The results of this study suggest that the relative success of savanna over forest species in savanna is related in part to their ability to cope with drought, which is determined more by leaf than by stem hydraulic traits. Variation among genera accounted for a large proportion of the total variance in most traits, which indicates that, despite different selective pressures in savanna and forest habitats, phylogeny has a stronger effect than habitat in determining most hydraulic traits.     

Impacts of long‐term precipitation manipulation on hydraulic architecture and xylem anatomy of piñon and juniper in Southwest USA

Hydraulic architecture imposes a fundamental control on water transport, underpinning plant productivity, and survival. The extent to which hydraulic architecture of mature trees acclimates to chronic drought is poorly understood, limiting accuracy in predictions of forest responses to future droughts. We measured seasonal shoot hydraulic performance for multiple years to assess xylem acclimation in mature piñon (Pinus edulis ) and juniper (Juniperus monosperma ) after 3+ years of precipitation manipulation. Our treatments consisted of water addition (+20% ambient precipitation), partial precipitation‐exclusion (?45% ambient precipitation), and exclusion‐structure control. Supplemental watering elevated leaf water potential, sapwood‐area specific hydraulic conductivity, and leaf‐area specific hydraulic conductivity relative to precipitation exclusion. Shifts in allocation of leaf area to sapwood area enhanced differences between irrigated and droughted K _L in piñon but not juniper. Piñon and juniper achieved similar K _L under ambient conditions, but juniper matched or outperformed piñon in all physiological measurements under both increased and decreased precipitation treatments. Embolism vulnerability and xylem anatomy were unaffected by treatments in either species. Absence of significant acclimation combined with inferior performance for both hydraulic transport and safety suggests piñon has greater risk of local extirpation if aridity increases as predicted in the southwestern USA.     

Relationship between growth rates and xylem hydraulic characteristics in young, mature and old-growth ponderosa pine trees

The first objective of the present study was to quantify the effects of tree age and stem position on specific conductivity (k_s), vulnerability to embolism and water storage capacity (capacitance) in trunks of young, mature and old‐growth ponderosa pine. The second objective was to determine relationships between hydraulic characteristics and radial and height growth rates to increase the understanding of possible tradeoffs. Within sapwood at all heights and in all ages of trees, outer sapwood had 25–60% higher k_s than inner sapwood. The water potential at which embolism started (air entry point) was 1.3 MPa lower in inner sapwood than outer sapwood within the mature trees, but there was no difference in the other trees. There was no significant difference in capacitances between the tops of the old growth trees, the mature trees and the young trees. Taking all data together, the capacitances increased sharply with an increase in k_s and an increase in vulnerability to embolism. The hydraulic characteristics of the three age classes were correlated with the height growth rate but not with the diameter growth rate. Within these age classes, high k_s was associated with the slowest yearly increase in sapwood area and with a low percentage of latewood, whereas high vulnerability to embolism and high capacitance were more closely associated with high height growth rates.     

元宝枫苗木的水力结构特征

在温室条件下,控制不同干旱梯度,用改良的冲洗法测定了4年生元宝枫苗木的水力结构参数．研究表明,随着小枝水势的降低,水力结构各参数随茎段功能木质部直径的变化可以用不同的方程来模拟;导水率的大小受茎段所在区域的影响,限速区的导水率明显低于非限速区,限速区的存在对苗木个体的生存竞争有利．导水率、比导率和叶比导率都和功能木质部直径和小枝水势呈明显的正相关．较粗茎段的叶比导率远高于多次分枝的未端细小分枝,有利于苗木在干旱时保存那些光合积累较大的器官．在落叶之前,相同直径枝条的胡伯尔值随小枝水势的变化很小,说明苗木水分胁迫主要源于木质部空穴和栓塞．