中国暖温带辽东栎林主要树种共存机制的研究

物种共存是植物群落生态学的一个重要问题。对物种共存的研究可以促进人们对生态系统结构与功能的认识，并为生物多样性保护及恢复提供理论指导。在本论文中，通过对辽东栎－棘皮桦林主要树种空间格局及空间关系、竞争作用和繁殖策略的研究，初步确定本地区辽东栎林主要树种共存的机制。 空间点格局分析表明辽东栎、棘皮桦和五角枫的幼苗、幼树和大树均呈聚集分布。辽东栎幼苗和幼树的分布与同种大树个体正相关，与棘皮桦的大树个体负相关，而与死树个体无显著的相关关系。这些结果说明种子散播和萌生繁殖过程影响辽东栎树木个体的空间格局。棘皮桦幼苗和幼树的分布在小的空间尺度上与同种活树和死树个体正相关，而与其它树种的活树及死树个体的分布呈负相关，说明萌生是该物种繁殖的一个重要途径。五角枫的幼苗和幼树呈聚集分布，并与同种大树个体的分布有关，说明其种子有限的散播范围影响其分布。辽东栎大树个体的分布与棘皮桦大树个体在小尺度上呈负相关，表明这两个树种之间可能存在竞争关系。 离散模型（the diffusion model）和生长动态模型（the growth dynamics model）分析表明在一些树种之间存在单向的种间竞争，例如，黄花柳对白桦和棘皮桦的竞争作用，棘皮桦对辽东栎，辽东栎对五角枫，和五角枫对山杨的竞争作用。这些单向竞争主要是林冠层的大树个体对林下层小个体的竞争。在这些树种中没有发现种内竞争。但是，各树种的生长率（G (t, x)）与生长率方差（ ）呈直线相关，而且样地中树木的死亡率较低，说明树种间的竞争强度很小。因此我们推测非对称的种间竞争在塑造该森林群落结构和物种共存过程中的作用不大。 五角枫、辽东栎、糠椴和山杨的幼苗和幼树在森林的林下层占优势，密度和出现频率均较高，可以形成稳定的幼苗库。五角枫、辽东栎和糠椴的幼苗大小级结构呈倒J形，山杨幼苗的大小结构呈单峰形。棘皮桦幼苗很少，但幼树相对较多，说明该树种可以形成幼树库。辽东栎幼苗在林窗中密度较大，而幼树的密度与林窗的出现与否无明显的相关关系，说明林窗的形成对新更新苗有显著影响，而林窗内的幼树可能是由林窗形成前已经建立的幼苗发展而来。林冠下的辽东栎幼苗多数为萌生起源，寿命较长，一旦上层大树死亡，可以迅速生长，进入林冠层。因此，林冠层下的幼苗库和林窗更新对辽东栎种群动态至关重要。五角枫幼苗和幼树的耐荫性或避荫性较强，可以形成巨大的长期存在的幼苗库和幼树库，以维持其种群的更新。糠椴和山杨的幼苗，主要起源于根的萌蘖。由于母树提供营养，幼苗生长速度较其它三个树种快。但它们对土壤湿度和营养的要求较高，这限制了它们很难占据其它生境。 虽然，在所研究的处于不同演替阶段的群落中，树种的组成有所不同，但辽东栎、大叶白腊和五角枫的幼苗均非常丰富，可以形成幼苗库。辽东栎的幼苗库大部分为萌生苗，而大叶白腊的幼苗库主要由实生苗构成。大叶白腊的幼苗种群中小个体丰富，它们生长速度很慢，在林下采取一种“坐－等”（sit-and-wait）的策略。在三个树种中，五角枫幼苗主枝的伸长生长率最高，而且生长率与幼苗的基径和生境的光照强度正相关。 通过以上研究，我们初步得出结论：竞争作用在研究的辽东栎－棘皮桦林主要树种共存中起的作用很小;形成稳定长久的幼苗库是这些树种的主要繁殖策略。因此，理解这些树种共存机制的关键在于这些树种幼苗库或幼树库的不同起源，及幼苗和幼树对生境因子的不同生态响应所创造的丰富的更新生态位。

Morphological Adaptations to Drought and Reproductive Strategy of the Moss Syntrichia caninervis in the Gurbantunggut Desert, China

The biological soil crusts (BSCs) in the Gurbantunggut Desert, the largest fixed and semi-fixed desert in China, feature moss-dominated BSCs, which play an indispensable role in sand fixation. Syntrichia caninervis Mitt. (S. caninervis) serves as one of the most common species in BSCs in the desert. In this study we examined the morphological structure of S. caninervis from leafy gametophyte to protonema using light and scanning electron microscopy (SEM). We also examined the relationships between the morphological structure of S. caninervis and environmental factors. We found that: (1) this moss species is commonly tufted on the sand surface, and its leaves are folded upwards and twisted around the stem under dry conditions; (2) the cells on both upper and lower leaf surfaces have C-shaped dark papillae, which may reflect sunlight to reduce the damage from high temperature; (3) the leaf costa is excurrent, forming an awn with forked teeth; and (4) the protonema cells are small and thickset with thick cell walls and the cytoplasm is highly concentrated with a small vacuole. In addition, we also found that the protonema cells always form pouches on the tip of the mother cells during the process of cell polarization. Our results suggest that S. caninervis has, through its life cycle, several morphological and structural characteristics to adapt to dry environmental conditions. These morphological features of S. caninervis may also be found in other deserts in the world due to the world-wide distribution of the species.

Two-Point Closure Strategy in the Mapping Closure Approximation Approach

A two-point closure strategy in mapping closure approximation (MCA) approach is developed for the evolution of the probability density function (PDF) of a scalar advected by stochastic velocity fields. The MCA approach is based on multipoint statistics. We formulate a MCA modeled system using the one-point PDFs and two-point correlations. The MCA models can describe both the evolution of the PDF shape and the rate at which the PDF evolves.

A new strategy of enhancing two-photon absorption in benzothiazole-containing fluorene molecules: Introduction of auxiliary donor-acceptor effects

A series of benzothiazole-containing fluorene molecules have been designed and their one- and two-photon absorption properties have been investigated theoretically by using the ZINDO method. The effects of electron-excessive/deficient heterocyclic bridges as auxiliary donors (auxD)/acceptors (auxA) on TPA cross-sections were studied. The results show that the molecules with D-pi-auxA-A, D-aux D-pi-A, or D-auxD-pi-auxA-A structure types have large TPA cross-section, which can be a valuable strategy in the design of two-photon absorption materials. Also, a linear relationship between the first hyperpolarizability and the TPA cross-section is observed. (c) 2006 Elsevier B.V. All rights reserved.