Ecophysiological Adaptation Of Juniperus Przewalskii And Picea Crassifolia To The Different Slopes

Qilian juniper (Juniperus przewalskii Kom.) and Qinghai spruce (Picea crassifolia Kom.) which are dominant tree species in alpine ecosystems in the northeastern Qinghai-Tibetan Plateau distribute at dry and infertile sunny slopes, and moist and fertile shaded slopes, respectively. Thus, the slopes aspect is the most important topographic factor to restrict the distribution of these two species. To identify the underlying adaptation mechanisms of these two species in the different slope aspects, in this thesis, we investigated the differences of ecophysiological traits and adaptive strategies in these two species by field and controlled experiments. These knowledges will explain the adaptive mechanisms of plants for distributing in the various slope aspects and improve the understanding of the different responses of these species to future climate changes.The mixed forest of Qilian juniper and Qinghai spruce rarely occurs in the same region except in the Qinghainan Mountans. To explore the differences of ecophysiological traits and influences of slope aspect in foliar traits between two species in the field, we measured variations in foliar stable carbon isotope composition (δ13C), leaf mass per unit area (LMA), foliar nitrogen use efficiency (NUE) and leaf water content (LWC). Firstly, Qilian juniper had significantly higher foliar 813C, LMA, NUE and lower LWC than Qinghai spruce. Secondly, Qinghai spruce was more vulnerable to slope aspect than Qilian juniper. Needles of west-facing slope in Qinghai spruce had remarkably higher foliarδ13C and LMA, and lower LWC than the ones of east-facing slope, whereas all foliar traits showed no significant changes between two slope aspects in Qilian juniper. These results suggested that two species employed different water-use and nitrogen-use strategies even if they distributed in the same geographical region.For further understanding the key role of water conditions in these adaptive mechanisms of different slopes, we designed the drought stress experiment. Here, the ratio of shoot to root growth (R/S), foliar relative water content (RWC), foliar stable carbon isotope composition (δ13C), foliar nitrogen use efficiency (NUE), foliar content of malonaldehyde (MDA), foliar activity of superoxide dismutase (SOD), net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (E) and instantaneous water use efficiency (WUEi) were measured in different water regimes. In both species, the foliarδ13C, NUE, WUEi, MDA, SOD increased, and the foliar RWC, Pn, Gs, E decreased wih drought stress. But, the patterns in the allocation of assimilation (i.e., R/S) were different, which the R/S of Qinghai spruce was decreased, inversely, the one of Qilian juniper was increased. By comparing responses of two species to drought stress, we found that Qinghai spruce significantly decreased in foliar RWC, Pn, Gs, E and increased in foliarδ13C, NUE, WUEi, MDA, SOD than Qilian juniper. In addtion, whether individuals under the drought stress or not, the foliarδ13C and NUE of Qilian juniper were observably higher than the ones of Qinghai spruce, inversely, the foliar RWC of Qilian juniper was observably lower than the one of Qinghai spruce. These further confirmed that two species employed different water-use strategies, and responses of two species to drought stress had significant difference. I.e., Qinghai spruce employed prodigal water-use strategies and was more sensitive to drought stress, and greater impact of drought stress. Qilian juniper employed conservative water-use strategies and had stronger drought tolerance, and resistance to drought stress was significantly higher than that of Qinghai spruce.Combining the two experimental results, key conclusions can be drawn from here:firstly, adaptation mechanisms of two species had significant difference to different slope aspect habitats. Qilian juniper and Qinghai spruce employed different water-use and nitrogen-use strategies, even if they distributed in the same geographical region. Secondly, Qilian juniper, as tree species mainly distributing sunny slope, employed a conservative water-use and nitrogen-use strategy, whereas Qinghai spruce, mainly distributing shaded slope tree species, employed a prodigal water-use and nitrogen-use strategy. Hence, Qilian juniper more adapted to dry and infertile conditions than Qinghai spruce. Thirdly, the difference of water-use strategy was likely one of the main reasons to cause significant habitat divergence between Qinghai spruce and Qilian juniper. According to our results, we can predicate that if climate become more and more drought in the future, Qilian juniper will have an advantage over Qinghai spruce in the aspect of growth and distribution, whereas the growth and distribution of Qinghai spruce could be restricted.