| Global warming and human activities have caused more and more serious impact on forest ecosystems, and aggravated the pressure exposed on forest vegetations. Plants could adapt to adverse environmental conditions by phenotypic plasticity. Stress tolerance to environmental factors and response strategies vary among species, which will have far-reaching influence on forest community and succession. Therefore, the relationship of plants and environment has always been one of the focuses of ecologist. This paper selected several dominant species in Nnorth China forests of Quercus variabilis, Q. mongolica, Q. variabilis, A. truncation and A. buergerianum as the research objects to discusses the influence of different soil moisture, lighting environment and nitrogen (N) supply on their growth and eco-physiological characteristics, as well as the similarities and differences in responses and adaptation strategies between different species and genera to environmental change. This paper includes four parts, the first three parts discusses effects of soil moisture, interactive effects of soil moisture and irradiance, and interactive effects of soil moisture and N deposition on ecophysiological characteristics of Quercus and Acer species, especially compares differences in drought-coping strategy between maple and oak species. The last part compares differences in the morphological plasticity and adaptive mechanism between the oak and maple plants in response to increased soil N fertilizer and light conditions. The results of this paper can help us to understand possible responses and change trend of the dominant species in North China faced with changes of environment conditions and under forest succession, the environmental adaptive mechanisms and niche differentiation of different species, and to provide scientific data and theoretical support for vegetation restoration and the development and management of regional ecological environment.Firstly, a greenhouse simulation experiment was developed to study different physiological and ecological response of Q. variabilis, Q. aliena, and A. truncatum seedlings to different soil moisture contents, in terms of the growth characteristics, physiological changes and morphological modification, etc. The results showed that with loss of soil water availability, seedling biomass of all three species appeared a first increasing then reducing trend. The biomass of Q. aliena and Q. variabilis peaked in mild drought level, while A. truncatum in moderate drought. Two oak species adapted to adverse soil moisture conditions by changing the biomass allocation proportion among different organs. Under moderate drought treatment, the underground biomass allocation increased, while under severe drought stress, seedling root biomass reduced. Biomass allocation of A. truncatum unchanged with with water availability. In addition, both severe drought and high water conditions damaged the activity of photosynthetic artifacts in two oaks, reduced fluorescence parameters, led to photoinhibition, and reduced photosynthetic rate. However, A. truncatum exhibited reduced fluorescence parameters only under high water conditions. Judging from physiological characterization, A. truncatum had stronger water stress tolerance, but was more sensitive to plenty water than the oak species. Under limited water condition, three species could increase water use efficiency through reducing blade stretching rate, average leaf area and leaf shape index. In a word, different plasticity levels for both the whole plant growth and among leaf traits determined the drought adaptability and strategyof different species.Secondlly, a greenhouse experiment was conducted to study the adaptive mechanisms of Q. aliena seedlings in response to varied light and soil water levels. Netural plastic film and black shading net were used to simulate the light environment of forest gap and understory. Plant traits related to leaf physiology, morphology, anatomy, and biomass production were determined. The results showed that plant size had significant effects on leaf area, leaf number, shoot height, basal diameter and crown area. After excluding the influence of plant size, water treatment had stronger effects on plants compared to light levels, and their interaction effect was significant. The limited water supply significantly inhibited leaf photosynthetic rate and the fluorescence efficiency under high light. However, leaves submitted to moderate drought stress showed enhanced fluorescence activity under shade condition. Grand plasticity of leaf physiology and growth was the highest, followed by biomass allocation and leaf morphology, and lastly anatomy, and this ranking did not change as resources considered. Among the variables, leaf petiole length, chlorophyll content and leaf area could be selected as candidates for estimation of species’plasticity to water, light and their interaction, respectively. Therefore, our results suggested that there was a hierarchy existing among traits plasticity in Q. aliena, and supported the above-ground facilitation hypothesis that shade could alleviate the adverse effect of drought.Then, in a greenhouse, ammonium nitrate (NH4NO3) solution was added into pot soil to simulate N deposition levels, and different soil water content was realized throught soil drying. The plasticity response of Q. variabilis and Q. mongolica seedlings to combined nitrogen (N) deposition and drought stress was evaluated, and their performance in natural niche overlaps was predicted. Plant traits associated with growth, biomass production, leaf physiology, and morphology were determined. Results showed that drought stress inhibited seedling performance, altered leaf morphology, and decreased fluorescence parameters in both species. By contrast increased N supply had beneficial effects on the nutritional status and activity of the photosystem II (PSII) complex. The two species showed similar responses to drought stress. Contrary to the effects in Q. mongolica, N deposition promoted leaf N concentration, PSII activity, leaf chlorophyll contents, and final growth of Q. variabilis under well-watered conditions. Thus, Q. variabilis was more sensitive to N deposition than Q. mongolica. However, excessive N supply (20 g N m-2 year-1) did not exert any positive effects on the two species. Among the observed plasticity of the plant traits, plant growth was the most plastic, and leaf morphology was the least plastic. Therefore, drought stress played a primary role at the whole-plant level, but N supply significantly alleviated the adverse effects of drought stress on plant physiology. A critical N deposition load around 20 g N m-2 year-1 may exist for oak seedlings, which may more adversely affect Q. variabilis than Q. mongolica.Lastly, A greenhouse experiment was conducted employing three sunlight levels and two soil nutrition levels to test the responsive strategies of two native oak species (Quercus variabilis and Q. mongolica) and two native maple species(Acer truncatum and A. buergerianum). Seed mass, whole-plant structure, leaf gas exchange, leaf chlorophyll fluorescence, leaf nutrition content, and leaf morphology were then measured. Light availability significantly changed plant architecture (seedling height and crown area). In the maple species, the net photosynthetic rate and fluorescence quantum yield increased under HL, but the leaf length to leaf petiole length ratio (Ll/LPL) was not significantly affected. In the oak species, the different light levels did not affect shoot growth, increased the net photosynthetic rate and fluorescence parameters under ML, and elevated Ll/LPL under LL. Nitrogen supply increased the Ll/LPL of maple seedlings but decreased that of oak seedlings. Light and soil nitrogen nutrition exerted combined effects on Q. variabilis and A. buergerianum seedlings. The positive effects of ML on the biomass of Q. variabilis disappeared under nitrogen supply. Nitrogen supply significantly increased the net photosynthetic rate of Q. variabilis and A. buergerianum under HL but decreased the same parameter under ML or LL. Maple seedlings were more plastic to heterogeneous light conditions than the shade-tolerant oak seedlings, but the difference in response to nitrogen was mainly species dependent and not genus dependent. Q. variabilis and A. buergerianum were susceptible to high nitrogen input, which may be disadvantageous for their adaptation to nitrogen changes in the environment. Significant differences among species in response to light and nitrogen availability may determine forest succession and composition.In conclusion, this study found that light, soil moisture and soil N content could interact with each other. The oak species of Q. aliena showed adaptive responses to changes of light and soil moisture availability. There was a plasticity hierarchy among different traits. Plasticity evaluation for the species to a certain environmental factor can be found through the selection of particular characteristics. At the same time, there were also differences in environmental responses and adaptability between different species within one genus. The sensitivity of Q. variabilis and Q. mongolica to drought and N deposition was inconsistent that the former one had stronger plasticity, but more sensitivive to excess soil N content than the latter one. It was estimated that the critical N deposition value of oak plants could be around 20 g N m2 year"1, which had more serious consequences on Q. variabilis. For the oak and maple species of Q. variabilis, Q. aliena, and A truncatum, they all had good self-regulation and drought tolerance which makes them be the dominant species in warm temperate forest. The adaptation strategy is different between plants of the two oak and maple genera in response to light availability and soil N content, leading to their differentiation in ecological niche selection and succession. |
PDF Full Text Request