【作者】 王林；

【导师】 万贤崇；

【作者基本信息】 中国林业科学研究院 ， 生态学， 2013， 博士

【摘要】 刺槐（Robinia pseudoacacia）、栓皮栎（Quercus variabilis）、侧柏（Platycladus orientalis）是华北石质山区的主要造林树种，对维持华北石质山区脆弱的生态环境起着重要的作用，本研究以不同生长环境（不同密度、不同坡向、不同土层厚度、混交）下这三个树种为研究对象，通过检测树木生长，年轮δ¹³C值、水分生理指标、气体交换、叶片形态指标以及土壤水分状况来研究这三个树种在脆弱的生态环境下的响应，探讨在该地区土壤对不同树木的水分承载能力，进一步求证近年来华北石质山区刺槐死亡与水分因素的关系，并试图分析华北石质山区长期干旱导致刺槐死亡机理。研究结果表明：1、应对林分密度的变化，三个树种均在生长一定年限以后低密度年轮宽度高于高密度，刺槐为5年左右，侧柏和栓皮栎为10年左右。但在水分利用效率上有不同的规律，高密度刺槐生长5年以后δ¹³C值显著高于低密度林分，而侧柏与刺槐相反，低密度林分δ¹³C值更高，栓皮栎在两种密度之间δ¹³C值没有显著性差异。土壤和植物水分状况、气体交换、叶片δ¹³C值等指标也表明在旱季高密度刺槐受到水分胁迫较低密度更重，高密度侧柏水分状况好于低密度林分，栓皮栎在两种密度之间没有显著差别。研究发现低密度侧柏的树冠体积更大，这加大了树冠蒸腾量，可能是导致低密度侧柏水分状况更差的主要原因。以上结果表明刺槐抗旱能力较差，高密度刺槐受到水分胁迫更重，这可能是导致高密度刺槐生长减慢的一个重要原因，侧柏表现出较强的抗旱能力，高密度林分虽有较低的年轮和树冠生长，但它们的水分供应状况比低密度林分的更好，栓皮栎水分利用效率没有受到林分密度影响可能与栓皮栎树冠以及叶面积的自身调节能力有关。2、对不同坡向栓皮栎、侧柏的生长和水分状况研究结果显示：阴坡栓皮栎和侧柏年轮和年增加的截面积显著大于阳坡，两个树种都表现出阳坡叶片δ¹³C值显著（p<0.05）大于阴坡。在白天，阳坡气温高于阴坡、相对湿度低于阴坡、饱和蒸汽压匮缺高于阴坡。相关分析显示旱季降雨量和栓皮栎的树干截面积年增长量显著相关（p<0.05），而旱季的降雨量和侧柏的截面积年生长没有显著关系。以上结果表明：干旱条件更严重的影响到栓皮栎生长和叶片水分利用效率，反映出这两种树木耐旱能力的差异，以及应对干旱策略的不同。华北石质山区的土壤储水能力低，阳坡较大的蒸散加剧了干旱对树木生长的不利影响。3、对侧柏-刺槐混交林的研究结果表明：混交刺槐在前5年年轮宽度与纯林刺槐没有显著差异，而在此后，混交刺槐年轮宽度显著低于纯林，而混交侧柏在5年以后年轮宽度显著高于纯林，刺槐在前5年快速生长，5年以后生长速度显著下降，而侧柏维持相对稳定的生长速度。混交刺槐5年以后年轮δ¹³C值更高。同时混交侧柏δ¹³C值在几乎整个生长过程中低于纯林，特别是在前5年和16-25年，两者之间有显著差异。旱季土壤含水量为：刺槐纯林＞混交林＞侧柏纯林，同时混交刺槐旱季凌晨水势、最大光合速率、气孔导度均低于刺槐纯林，水分利用效率高于纯林，混交侧柏凌晨水势、最大光合速率、气孔导度均高于侧柏纯林，水分利用效率低于侧柏纯林。以上结果表明：随着树木的生长，树木之间的竞争更加激烈，侧柏有着比刺槐更强的抗旱能力和水分竞争能力，导致混交刺槐受到的干旱胁迫趋于严重，更重的水分胁迫使得刺槐生长变慢，表现出受胁迫的症状。混交侧柏由于竞争优势水分状况比侧柏纯林好，后期生长速度较纯林快。对水分状况的差别响应是干旱半干旱地区驱动树种演替的主要动力。4、对不同土层厚度刺槐的水分状况和生长进行研究表明：随着土层厚度减小，旱季土壤含水量下降、凌晨枝条水势越低；气孔导度和最大光合速率都减小，而瞬时水分利用效率增加；在雨季上述指标均差异不显著，旱季土壤含水量只有雨季的60%左右。随着土层变薄，刺槐叶片δ¹³C增高，叶面积减小，比叶重增加；刺槐树高和胸径减小。以上结果表明：刺槐在不同季节下的水分状况综合反映出土壤的供水能力，土层浅薄导致的土壤水分植被承载力不足致使刺槐在旱季受到较严重的水分胁迫，这可能是刺槐出现衰败的重要原因。5、濒死刺槐与健康刺槐比较，濒死刺槐在生长10年以后其年轮宽度和截面积增量均显著低于健康刺槐，濒死刺槐的年轮δ¹³C值在整个生命周期均高于健康刺槐，濒死刺槐在最近10年导管直径显著低于健康刺槐，健康刺槐的早材导管直径在整个生命周期持续增加，而濒死刺槐在最后10年导管直径减小。濒死刺槐拥有更小的单叶面积和单小枝叶面积。濒死刺槐由于土层厚度更低，旱季土壤含水量显著低于健康刺槐样地，在旱季，濒死刺槐旱季凌晨水势显著低于健康刺槐，最大光合速率只有健康刺槐的1/3，气孔导度只有正常刺槐的1/4，水分利用效率为健康刺槐的2倍。濒死刺槐的小枝最大导水率只有健康刺槐的1/3，叶面积比导水率显著高于健康刺槐。濒死刺槐当年生小枝的导管直径只有正常刺槐的2/3，导管密度是正常刺槐的2倍。以上结果表明：濒死刺槐由于水分状况较差导致长期受到较重的水分胁迫，长期严重的水分胁迫导致刺槐光合面积和光合能力下降，光合产物合成受到抑制可能是最终导致刺槐死亡的主要原因。刺槐应对长期干旱胁迫的可塑性反应表现为：叶面积变小，光合能力降低，导水策略趋于更保守，导水能力下降，这些可塑性反应对保护刺槐水力学性状有利，但进一步限制了刺槐的光合能力，这些抗旱策略优先保护水力学性状而非碳摄取，优先应对短期干旱而对长期干旱不利，加重了植物受到的碳胁迫，最终会导致刺槐死亡。更多还原

【Abstract】 Robinia pseudoacacia, Quercus variabilis and Platycladus orientalis are widely used asafforestation tree species in the rocky mountains of northern China, and they play an importantrole in ecological restoration in this region where the ecological environment are fragile withthin soil depth and remarkable precipitation seasonality due to monsoonal influence. Three treespecies growing on different environment （different density, different slopes, different soilthickness and mixed forest or pure stand） were used to study the effects of water status on treegrowth. The tree-ring width and the corresponding area increment were measured to analyzetree growth. The δ¹³C value of tree-rings was measured to research water balance of the trees.The pre-dawn water potential, soil water content, gas exchange, leaf δ¹³C value, and leafmorphology in the dry season and the wet season were measured to investigate the currentwater status and the influence on plants. The results are as follows:1. Responding to different stand density: the ring width of three tree species growing inlow stand density was wider than that of the corresponding species in high stand density after acertain period of years’ growth, approximately5years for R. pseudoacacia, and10years for Q.variabilis and P. orientalis. However there was difference in water use efficiency among thethree species. R. pseudoacacia in the high density forest had significantly greater tree-ring δ¹³Cvalues than that in the low density forest, P. orientalis in the high density forest had smallertree-ring δ¹³C values than that in the low density, and there were no difference in tree-ring δ¹³Cvalues of Q. variabilis between the high density and the low density forest. The current soilmoisture, pre-dawn twig water potential and gas exchange also indicated R. pseudoacacia inthe high density suffered severer water stress than trees in the low density forest, while P.orientalis in the high density was better hydrated than trees in the low density forest. Furtheranalysis showed that the crown volume of the low density P. orientalis trees was bigger thanthat of the higher density’s, and the bigger crown volume that increase the canopy transpiration would be the main reason for worse hydrated in low density forest. Those results indicated thatR. pseudoacacia was a relatively poor drought-resistant species, and the high-density R.pseudoacacia suffered severer water stress, which might be one of important reasons for theslower growth in the high density. In contrast, P. orientalis has stronger drought resistancecapacity, although the high density stand trees had smaller growth in ring width and crown, thewater use efficiency was higher than the low density stand. For Q. variabilis, the stand densitydid not affected the moisture condition, which might be associated with crown adjustmentability.2. The results of growth and water condition on different slopes of Q. variabilis and P.orientalis showed that the tree-ring width of the two species on shady slopes was significantlygreater （p<0.01） than that of on sunny slopes. The corresponding basal area increment on theshady slopes was also significantly （p<0.01） greater than that of on sunny slopes. The δ¹³Cvalues of the two species on sunny slopes were significantly （p<0.05） greater than that of onshady slopes. In day time, especially at noon, there were higher air temperature, lower relativehumidity, and greater vapor pressure deficit （VPD） on sunny slopes than those on shady slopes.Correlation analyses displayed that the precipitation in dry season was significantly （p<0.01）correlated with the annual basal area increment of Q. variabilis, especially on the sunny slope.However, the precipitation in dry season was not significantly correlated to the basal areaincrement of P. orientalis. Those results indicated that drought condition more severelyinhibited growth and water use efficiency of Q. variabilis than that of P. orientalis, reflectingthat there existed variations in tolerance and responsive strategy to drought stress. With shallowsoil thickness and low capacity of water storage, the higher evapotranspiration on the sunnyslopes exacerbated the adverse effects of drought on tree growth.3. The results of growth and water condition in R. pseudoacacia-P. orientalis mixed forestshowed that there was no difference in ring width increment of R. pseudoacacia in the initial5years between the pure and mixed forests. R. pseudoacacia in the mixed forest had lower5-year tree-ring width than that of pure trees after5years, and had higher δ¹³C of the tree-ring than that of pure trees after5years. There was no difference in ring width increment of P.orientalis in the initial10years between the pure and mixed forests. The mixed P. orientalishad higher tree-ring width increment than that of pure trees after10years. The tree-ring δ¹³C ofP. orientalis in the mixed forest was significantly lower than that in the pure forest in almostthe whole life time, especially for the intial5years and the16th-20thyears. In the dry season thesoil moisture was: R. pseudoacacia pure forest＞mixed forest＞P. orientalis pure forest, and R.pseudoacacia in the mixed forest had significantly lower pre-dawn twig water potential, netphotosynthetic rate and stomatal conductance than that in the pure forest. P. orientalis in themixed forest had significantly higher pre-dawn twig water potential, net photosynthetic rateand stomatal conductance than that in the pure forest. The results revealed: P. orientalis hadstronger tolerance to drought stress than R. pseudoacacia. The severer water stress in the dryseason might be the main reason for the slower growth and decline of R. pseudoacacia in themixed forest. Differential tolerance in responses to drought stress would be the main drivingforce for species succession in the arid and semi-arid area.4. The effects of soil thickness on water relations and growth in R. pseudoacacia: in dryseason, the pre-dawn twig water potential and soil moisture significantly declined withdecreased soil thickness, and the net photosynthetic rate and stomatal conductance of R.pseudoacacia also decreased. But there was no significant difference in the above-mentionedindexes in wet season. The average soil moisture in dry season was only60%as much as thatin wet season. With decrease in soil thickness, the δ¹³C value increased, the area of leafdecreased, and lamina mass per unit area increased. As a result, the height and diameter atbreast height significantly decreased with decrease in soil thickness. The results revealed thatthe water status of R. pseudoacacia in the different humidity seasons comprehensivelyreflected water supply capacity of the soil, and the thin soil thickness was the main cause forthe decline of R. pseudoacacia forests due to the low carrying capacity of vegetation, causedby the inadequate water storage in the dry season. 5. The comparison between dying and healthy R. pseudoacacia trees showed that therewas no difference in the tree-ring width between the healthy and dying trees in the initial10years （1976-1985）, however after then the tree-ring width of healthy trees was significantlywider than that of dying trees in the last25years （1986-2010）. Dying trees had a significantlyhigher δ¹³C than healthy trees （P<0.05） in both tree-ring and leaf. Dying trees had lowerphotosynthetic rate, higher WUEi, smaller and thicker leaves, lower maximal hydraulicsconductivity, larger Huber value in annual twigs, however the dying trees had higher securityof hydraulic structure in both annual twigs and trunk. Those results indicated that limitation ofcarbon uptake caused by low stomatal conductance and smaller leaf area in chronic water stresswould be the main cause for the low growth rate of R. pseudoacacia forests. On the other hand,R. pseudoacacia expressed to certain degree plasticity in performance under drought conditions,which facilitated itself to maintain the hydraulic structure and to avoid failure of hydraulicstructure at the expense of carbon uptake of dying trees. Plasticity performances in long-termwater stress caused carbon starvation, which might be the main cause of the death of R.pseudoacacia.更多还原