Characteristics Of Stand Structure And Hvdrological Function Of Damaged Masson Pine Forests In The Acid Rain Region Of Chongqing

Until now, the effect of acid deposition on forest ecosystem has not been removed, but theglobal climate change with increasingly frequent extreme drought events brings newenvironmental challenge for the damaged forests in acid rain region. Understanding theeco-hydrological processes and functional responses in a changing environment becomes anew hot spot for managing the eco-environment and water resources in21stcentury. In order tocomprehensively and further understand the health changes of forests damaged by acid rainunder the background of climate change, to enrich the theory of forest eco-hydrology, and alsoto provide theoretical guidance and policy-making support for the evaluation and managementon eco-hydrological services of damaged forests, the stand structure of damaged Masson pine(Pinus massoniana) forests (40-60a) were investigated, and its hydrological processes werepermanently monitored in the acid rain region of Tieshanping, Chongqing. Further, the studyresults were compared with the stand structure and hydrological function of healthy (Massonpine) forests reported in literature. Then, the change of stand structure of Masson pine forestsand the eco-hydrological characteristics caused by acid rain were identified. The mainconclusions are as follows:1. Remarkable changes of stand structure of damaged Masson pine forestsThe tree canopy layer of studied stands was mainly composed of Masson pines, but nearlyno Masson pine was found in the underwood layer. The tree species in the stand were fewer(only4-9), and biodiversity (Simposon) index was low and varied from0.23to0.69. Thebroad-leaves tree species (Cinnamomum camphora, Schima superba, Phoebe zhennan, et al)were mainly distributed in the underwood layer, but they begin to access the retarded tree layer,and their competition with the currently dominant Masson pines became to be intensive. Thecanopy density of Masson pine stand was as high as0.87. However, the high defoliation causedby acid rain led to a sparse canopy, which can not restrict the existence of shrub layer and herb layer. The coverage of shrub layer was63.95%and the coverage of herb layer was48.63%.However, the shrub layer affected the distribution of herb layer significantly. Compared withhealthy Masson pine stand, the root biomass of damaged Masson pine stand was decreased andits depth distribution became shallower (mainly with0-40cm), especially the fine root biomasswas decreased and mainly distributed in the soil layer of0-10cm and decreased rapidly withincreasing soil depth.2. Smaller needle size and slower litter-fall decomposition of damaged Masson pineforestsCompared with healthy Masson pine stands, the dead needles and green needle inlitter-fall of damaged stand were smaller, with a length of15.10cm and12.83cm, respectively;their weight of single needle was also lower, with the value of0.024g and0.019g, respectively;and the specific leaf area (SLA) of dead needles was higher (35.12cm2/g). This means that theneedles of damaged Masson pine stands became smaller and thinner. The canopy LAI ofdamaged Masson pines was as low as only1.25, much lower than that of healthy Masson pinestands (of3.76-3.94) located in other places. The LAI of the damaged Masson pine standsdecreased slowly from the beginning of a year until to April-May, and then gradually increaseduntil September-October, afterwards it decreased again gradually. This was an integrated resultof needle growth and litter-fall rhythm. The decomposition rate of un-decomposed litter waslower than that of other healthy Masson pine stands, with an annual quality loss rate of35.62%,a half-decomposition period of1.3years and a fully-decomposition (95%) period of l2.5years.3. Worse soil hydrological properties of damaged Masson pine forestsThe soil hydrological properties of0-10cm soil layer were the best in the damagedMasson pine stands; and they were moderate in the10-20cm soil layer and worse in deeper soillayers. The non-capillary porosity and infiltration rate decreased sharply with increasing soildepth; as a contrast, the non-capillary porosity in healthy Masson pine stands at other placesdecreased slowly with increasing soil depth. The water-retention functions of the soil ofdamaged Masson pine stands were pretty much the same with broadleaved-coniferous mixed forest, but significant worse than the broad-leaf forests and better than the bamboo forests inthe same study site. Compared with the healthy Masson pine stands of other places, the soilhydrological properties of damaged Masson pine stands were obviously worse, presenting by ahigher bulk density of each soil layer, lower infiltration rate and water-holding capacity of soil,and smaller porosity (including the capillary, non-capillary and total porosity).4. Increased annual litter-fall with less needles and more twigs and mainly concentrated indry-summer periodCompared with healthy or less damaged Masson pine stands, the annual litter-fall amountof damaged Masson pine stands was increased, but with changed composition, representing bya decreased needles and increased twigs. The order of weight percentage of litter-fallcomponents was: needles> twigs> debris> bark and cones> broad leaves. The variation oflitter-fall became quite sensitive to drought stress. The amount of all components of litter-fall,but except the broad leaves, was affected by the soil moisture, especially in the soil layers of30-40cm and40-50cm. Because of the drought stress, the pattern of litter-fall of damagedMasson pine stands has significantly changed, with more distributed in the dry-summer period.However, the litter-fall rhythm of broad-leaves tree species was not changed (by maintainingthe two litter-fall peaks in April-May and November-December), this shows that broad-leavestree species have a stronger adaptability than Masson pine in the acid rain region.5. The health decline of damaged Masson pines related with water deficit was caused bythe reduced water-absorbing abilityAn obvious patio-temporal variation of soil moisture was observed in the damagedMasson pine stands. Compared with other forest types at the same study sites, such as thebroadleaved-coniferous mixed forest, broad-leaf forest and bamboo forest, the soil moisture ofdamaged Masson pine stands was higher as a result of less canopy evapotranspiration and itwas pretty much the same of bamboo forest. The field capacity (or the capillary porosity) and82%of field capacity (or80%of capillary porosity) are two important thresholds of soilmoisture to affect the health condition of damaged Masson pines; the soil moisture lower than these thresholds will lead to a high defoliation. This was mainly caused by a relative soildrought due to the declined capacity of absorbing water as the result of the quantity reductionof fine root and its shallower distribution. This was confirmed by the observed relationbetween sap flow density of a nearly-dead Masson pine and soil moisture. Therefore, thedamaged Masson pines become very easily to suffer from water deficit stress, especially in thedry-summer period, regardless if the annual precipitation is abundant, normal or less thanaverage. This means that the water deficit has become a new stress besides the acidificationstress affecting the forest health condition.6. Reduced needles and fine-roots caused lower transpiration of damaged Masson pinesThe difference of sap flow density between individual sample trees of Masson pine wasmore directly affected by their difference in leaf area index, rather than by their dominancedetermined by their relative height in canopy. For example, when the dominant tree of No.3had a low LAI of0.68, its monthly sap flow density was lower than that of the co-dominanttree of No.4which has a higher LAI of1.46; but the monthly sap flow density of tree No.3washigher after its LAI surpassed the tree No.4. Generally, damaged Masson pine suffered fromhigh needle defoliation and this resulted to a reduced LAI of canopy. This caused a reduction oftree transpiration. Therefore, the soil moisture of damaged Masson pine stands was the highestamong the four forest types invested in the same study site. The monthly transpiration variedfrom10.19to48.89mm for the damaged Masson pine stands, with an average of27.24mm. Ithad a seasonal variation of firstly increase and then decrease. It reaches a peak of about40mm/month in the period of July to August. The tree transpiration is closely related with soilmoisture, so the lower water-availability caused by the reduced fine roots is also an importantreason of the lower transpiration of damaged Masson pine stands.7. Characteristics of hydrological processes and functions in damaged Masson pine standsThe damaged Masson pine stands presented a sparse canopy and lower canopyinterception capacity of1.54mm. This led to a higher through-fall ratio (84.66%), lower stemflow ratio (0.26%) and canopy interception ratio (15.07%) and the critical rainfall depth (5mm) to form stem flow. The humus quantity on the floor of damaged Masson pine stands was11.06t/hm2; the separately measured water-holding capacity of un-decomposed, half-decomposedhumus layer and the total humus layer was195.54%,214.48%and197.61%, respectively; thecorresponding water-holding capacity in depth was0.71,0.82and1.46mm, respectively. Allof them were lower than those of healthy Masson pine stands in other places (15.7-32.20t/hm2,203-277%,3.40-5.12mm). This shows that the hydrological functions of humus layer indamaged Masson pine stands was declined. The ratio of evapotranspiration in damagedMasson pine stands was reduced (to67.38%) during the study, while the runoff ratio wasincreased (to33.51%).