Responses Of Different Tree Species To Water Restriction In The Rocky Mountains Of Notrhern China

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.