BENTHIC DIATOM DISTRIBUTION IN A PENNSYLVANIA STREAM: ROLE OF pH AND NUTRIENTS1

Buffalo Creek is in a forested watershed in eastern Pennsylvania and is relatively acid in upstream reaches (pH～6), becoming alkaline downstream (pH～8). Temperature, nitrogen (NO₃-N) and phosphorus (O-PO₄) increase significantly downstream whereas N/P declines. Nutrient-diffusing substrata were deployed in triplicate at an upstream and downstream site. Six treatments included two concentrations of nitrate, two concentrations of phosphate, nitrogen + phosphate, and a control. Substrata were collected after 18 days, scraped and analyzed for accrual of chlorophyll a and algal community structure. Chlorophyll a and algal biovolume were greatest downstream across all nutrient treatments. At the community level, accrual appeared to be limited by phosphorus at upstream sites. Downstream accrual also may have been phosphorus-limited, but the results were equivocal. Benthic algae on all treatments at both sites were ～96% diatoms. Minimal overlap in species composition was observed between upstream and downstream sites. Of the 75 species of diatoms encountered in the study, 58 species did not occur at the upstream site and 10 species did not occur at the downstream site. The upstream site was depauperate in species and dominated by Eunotia exigua (Bréb. ex Kütz.) Rabh., which showed a positive response to phosphorus and accounted for over 50% of the biomass across treatments. The downstream site showed a four-fold increase in species richness. Communities at this site contained some species that appeared to be phosphorus-limited, e.g. Melosira varians Ag., and others that seemed to be nitrogen-limited, e.g. Diatoma vulgare Bory and Navicula seminulum Grun. We conclude that extreme conditions upstream (low pH, high N/P) result in a species-poor community dominated by acidophilous phosphorus-limited diatoms. Increases in downstream nutrients and pH result in a relatively rich and diverse community.     

北京东灵山地区辽东栎林种群空间分布分形分析

通过植物个体的坐标点与模拟冠幅的分形维数的分析,对辽东栎林内不同种群沿海拔梯度变化的空间分布格局和种群动态进行了比较和讨论。结果表明:模拟冠幅分形分析方法更适于分析具有大小不等、复杂多样的冠幅的植物个体空间分布格局。随着海拔的升高,不同种群的空间占据能力也随之变化。辽东栎种群的空间分布占绝对优势地位,分形维数可达1.9811;五角枫种群则逐步下降,分形维数最低为0.1170。低海拔山坡六道木种群空间占据能力较强,高海拔山坡照山白种群有较大的空间占据能力。在一定的环境条件下,不同的种群可能具有相近的空间占据能力,但对乔木层与灌木层来说,相同的分形维数的内涵是不同的。冠幅的分形维数作为表征植物种群空间占据能力的工具,是种群动态分析和种群分布格局研究的的重要指标之一。     

兴安落叶松种群格局的分形特征——关联维数

种群格局关联维数揭示出个体空间关联的尺度变化规律,表明种群个体的空间相关程度。采用关联维数对大兴安岭兴安落叶松种群格局的研究表明,各类兴安落叶松林中兴安落叶松种群格局具有较高的关联维数（接近２）,个体空间关联程度较高,其次序为杜鹃－兴安落叶松林（１．７４６）＞草类－兴安落叶松林（１．７４０）＞越桔－兴安落叶松林（１．５５０）＞杜香－兴安落叶松林（１．４６８）。兴安落叶松－白桦林中兴安落叶松种群格局的关联维数较小（＜１．５１２,远离２）,个体间关联较弱,处于劣势伴生地位。通过将各天然森林类型与兴安落叶松人工林比较显示,个体空间相关程度由高至低的次序为兴安落叶松人工林（１．７６２）＞兴安落叶松天然林（１．６２６）＞兴安落叶松－白桦林（１．４３４）,揭示出兴安落叶松种群在不同森林类型中个体空间关联的尺度变化的差异。文中还对综合运用各种分形维数揭示种群格局尺度变化特征的问题进行了讨论。     

THE EVOLVING GENETIC HISTORY OF A POPULATION OF LATHYRUS SYLVESTRIS: EVIDENCE FROM TEMPORAL AND SPATIAL GENETIC STRUCTURE

We analyze patterns of genetic microdifferentiation within a natural population of Lathyrus sylvestris, a perennial herb with both sexual reproduction and clonal growth. In a population from the northern foothills of the Pyrénées in southwestern France, a combined demographic and genetic investigation enabled the study not only of spatial genetic structure of the population, but also of the history of the population's spatial genetic structure over time. Excavation of all individuals allowed identification of clonemates. Age of each individual was determined by counting annual growth rings in the taproot, a method tested with individuals of known age planted in experimental gardens. Each individual was mapped, and genotypes of all individuals were determined using allozyme markers for a number of polymorphic loci. Distribution patterns and spatial genetic structure, both for all individuals and for different age classes, were analyzed using spatial autocorrelation statistics (Geary's Index, Moran's Index). Patterns of gene flow within the population were also studied using F-statistics and tests for random associations of alleles. Because age, allele frequencies, and location were known for each individual, it was possible to study how spatial genetic structure changed over time. Results from all these diverse approaches are consistent with one another, and clearly show the following: (1) founder effects, with the study transect being first colonized by individuals at either end of the transect that were homozygous for different alleles at one marker locus; (2) a difference in spatial distribution of individuals originated from sexual reproduction (seedlings) and from clonal growth (connected individuals); (3) restricted gene flow, due to inbreeding among related, clumped individuals; and (4) increase in heterozygote deficit within the youngest cohort of individuals. The results indicate that genetic differentiation in time was much less marked than differentiation in space. Nevertheless, the results revealed that the studied population is experiencing demographic and genetic variation in time, suggesting that it is not at equilibrium. On the one hand, spatial structuring is becoming less marked due to the recombination of founder genotypes; on the other hand, as establishment of new individuals increases, a new spatial structure emerges due to mating between relatives.