The Mechanisms Of Hydraulic Failure And Carbon Starvation Resulting In Tree Mortality Under Different Conditions

The increase of forest mortality at global scale is a hot topic with the climate change. The main cause of tree mortality was presumed to be that trees could not maintain the carbon-water balance. The physiological mechanisms of hydraulic failure and carbon starvation for explaining tree death were studied under different environmental conditions or relationships, including different rainfall levels, insect outbreaks and tree phenology, as well as irradiation and tree size. We analyzed the stomata behavior of three speciesDendrobenthamia japonica(DC.) Fang var. chinensis(Osborn.) Fang, Lindera obtusiloba Bl. Mus. Bot., Sorbus alnifolia(Sieb. et Zucc.) K. Koch, with different isohydric-anisohydric strategies after rainfall reduction, and the xylem cavitation vulnerability, and the variations of water and carbon of the three species, and the dieback mechanism of L. obtusiloba. In addition, we investigated the reasons why Schima superba Gardn. et Champ. suffered much more from the pest attacks than Engelhardia roxburghiana, Wall. even though being subjected to similar insects attack in Dinghu Mountains. Moreover we studied effects of irradiation and tree size on the carbon-water balances in Quercus aliena var. acuteserrata forest of Baotianman. This study aims to understand effects of water-carbon balances on plant growth and survival, and forecast forest succession. The main findings are as follows:1 、 D. japonica and S. alnifolia were on opposite ends of the isohydry-anisohydry functional spectrum, and L. obtusiloba was between the two species. Rainfall-intercepted D. japonica conserved water through closing stoma, which caused its water potential was highest in this three species, but its net photosynthetic rate and total nonstructural carbohydrate(NSC) reduced. Whereas, rainfall-intercepted S. alnifolia diffused its water with opening stoma, so its water potential was lowest in this three species. But its gas exchange was easy to proceed, so most easily resulting in NSC surplus in this three species. That was to say, the midday water potential of anisohydric S. alnifolia was lower than isohydric D. japonica, but its NSC accumulation was less restricted.2 、 When rainfall reduced in Baotianman, anisohydric S. alnifolia and isohydric D. japonica could adapt to the water reduced environment. But L. obtusiloba’s stomata behavior was between the two species, and hydraulic failure caused its dieback. Cavitation-resistant ability plays an important role in determining tree mortality. Cavitation vulnerability of L. obtusiloba was largest in the three species. So after rainfall reduced, L. obtusiloba’s shoots occured dieback because of hydraulic failure. This study proved that hydraulic failure not only depended on stomata behavior, but also on xylem cavitation vulnerability.3、There was not interaction between seasonal drought and insect pest in Dinghushan, and reduced NSC reserves resulting from phenology cycle difference affected the injury extent of pest. During the canopy rebuilding phase of the tree phenology cycle(February) of S. superba, the NSC concentrations significantly decreased in S. superba leaf-, stem xylem-, stem phloemand root tissues. In contrast, another canopy species, E. roxburghiana, has a different phenology cycle from S. superba. Its annual leaf renewal phase occurs in the month of May. Insects outbroke(March-April) just after S. superba renewed its leaves, and before E. roxburghiana renewed its leaves. A retrospective analysis based on ring growth rates in mature trees showed that insect(T. quadraria) outbreaks reduced radial growth in S. superba and did not in E. roxburghiana. And artificial defoliation, which simulated an insect pest attack saplings, showed that defoliation severely inhibited both leaf and shoot growth and prevented the restoration of NSC in the storage tissues of S. superba roots and xylem for ten months. These effects led to high tree mortality rates. Although defoliation reduced leaf- and shoot growth rates in E. roxburghiana as well, the extent of the reduction was far less than that seen in S. superba. The NSC of the defoliated E. roxburghiana almost recovered to control levels within ten months. And all E. roxburghiana saplings survived. Thus, the NSC restoration and injury extent of pest was closely related. When taking the phenology cycles into account, it was easy to understand that the defoliation hindered Scs growth rates, NSC restoration and survival rates more serious than Enr. The water relation measurements in this study did not demonstrate that dry season caused more water deficit in S. superba.4、Adult Q. aliena tree canopy top,canopy bottom, sapling in understory and sapling in gap were used as the study materials. The hydraulic limitation in adult tree canopy top was obvious, which caused the net photosynthetic rate, stomatal conductance of adult tree canopy top were significantly lower than saplings in gap. Compared to saplings in gap, the NSC of adult tree canopy top leaf and phloem accumulated resulting from a decrease in growth caused by hydraulic limitation, and frequent embolism repairing consumed much NSC in adult tree canopy top xylem. The light availability of adult tree canopy top and saplings in gap was much higher, and their photosynthetic capacity were significantly higher than the shaded canopy bottom and saplings in understory. The photosynthetic capacity of adult tree canopy bottom was also higher than saplings in understory. So the irradiation was the main factor for modulating the photosynthesis of Q. aliena.5、Illumination intensity in understory was extremely lower than that in forest outside. The shaded environment significantly reduced the photosynthetic capacity and carbon fixation of saplings in understory, and their NSC concentrations were significantly lower than adult trees and saplings in gap. The NSC status tended to caused saplings in understory die by carbon starvation, which explained Q. aliena forest could not regenerate in Baotianman. Besides, the potted shading experiment showed that the dry matter yield and NSC content of Q. aliena seedlings in shaded treatments were significantly lower than CK. Different light environment played significant effects on photosynthetic capacity of Q. aliena seedlings. Simulated sunfleck treatments significantly advanced the photosynthetic capacity of shaded seedlings. And the NSC content of seedlings in simulated sunfleck treatments was significantly higher than the seedlings always in shaded treatments. So sunfleck relieved the the negative influences of carbon reserves of Q. aliena seedlings caused by shade. Q. aliena seedlings were difficult to survive in serious shaded environment, but high density sunfleck should play some role in maintaining positive net carbon balance of Q. aliena seedlings in understory.