The following 10 curves were chosen as the models for species-area curves: S = b + aA (1) S= b+alnA (2) S=(b+alnA)c (3) S=aln(A+1) (4) S=aln(bA+1) (5) S= aAb (6) S=aA/(1+bA) (7) S=c/(1+ae-bA) (8) S=c-ae-bA (9) S=a(1-e-bA) (10) The algorithms were given to calculate the initial values of the parameters in the 7 nonlinear models, and Gauss-Newton and Marquardt algorithms were used to solve the nonlinear problems. Four indices were chosen to assess the fittness of the models, which are residual standard error (RSE), correlation index (CRI), average of absolute deviation (AAD), and average of absolute relative deviation(AARD). The results show that :l) the algorithms of calculating the initial values of the parameters in the 7 nonlinear models were suitable, and the solutions from nonlinear least squares were better than those from linear least squares according to the 4 assessment indices; 2) the fittness of the 10 models were high because 71.5% of the 200 values of CRI were greater than 0.9 and 89% were greater than 0.8. The calculation results of curves (3) and (9) were the best two; the next three were (5), (6) ,and (2) ;and the worst two were (1) and (10); 3) rank correlation analyses show that there existed extremely strong positive rank correlations between any two of the three indices of RSE, AAD, and AARD.

An effort has been made to investigate the characteristics of herbage production and water use of perennial ryegrass population under different types and levels of drought stress in field conditions. The experimental material was cv. AberElan, and was grown in self-designed soil columns in May 1995. The columns were packed with soils taken from the field. The soil water content was monitored by regular weighing of the column and was maintained within a predetermined range by controlling watering frequency and amount of water applied each time. Three types of drought stress were imposed to the columns, i, e. , cyclic, sustained and progressive, each of which was further divided into mild, moderate and severe levels, and a well-watered column was used as control. Under cyclic and sustained types of drought stress, the effect of moderate stress on dry matter yield was similar to that of mild stress and was not significantly different from the control, while severe stress reduced dry matter yield; under progressive type of drought stress, however, mild stress did not affect dry matter yield, but moderate and severe stresses caused a reduction in dry matter yield. For each type of drought stress ,dry matter content of herbage was negatively associated with the dry matter yield. There was a linear relationship between dry matter yield and water loss from soil columns. The moderate stress of cyclic type tended to increase water use efficiency(WUE)of the population whereas under sustained and progressive types of stress the response of the population WUE to drought stress varied with time in the growing season.

Diurnal variation curve of net photosynthetic rate (Pn) of flag leaf of winter wheat in farmland of Tibet plateau was flat one or with one single peak, having hardly midday depression of Pn. Daily maximum of Pn was near to approach that observed in the east plain of China. The peak of the daily total amount of Pn is about 66%～96% of that in the plain. In Tibet plateau, the environmental conditions which enable the flag leaf of the plant to have Pn higher than 20μmol CO2·m-2·s-1 were the following : PFD was higher than 2000μmol·m-2·s-1, Ta was 25～29℃, ambient CO2 density was higher than 0.41mg·dm-3. Soil surface temperature was 18～23℃ ,and underground temperature (5cm depth) was 15～19℃. Probability at which suchlike factors combination occurs was not high. Ambient CO2 density was about one third lower than that in the plain. It restricted the daily total amount of Pn. Flag leaf of the plant in Tibet plateau had daily maximum of Pn approach similar to that in the plain, but its daily total amount of Pn was lower than that in the plain. Photosynthetic capabilities were high, but sunlight utilization efficiency was low.

Young trees of Cathaya argyrophylla grew well in terms of height growth in partial shade condition while less height growth under overshade condition, leading to death finally. The growth process of C. argyrophylla can be devided into 4 stages. The flouring DBH growth was obvious between the age of 30 to 80 years. 60 percent of the aboveground biomass of a sample Cathaya tree was allocated to the portion of below the half tree height. The trunk weighted over 2/3 of the total aboveground biomass. The aboveground biomass of C. argyrophylla population greatly differed among community types and varied from 33000 to 117000 kg·hm-2.

This paper deals with the plant species and the function of the evergreen broad-leaved forest floor in Tiantong. The results are as follows: there were 44 families, 78 genera, 114 species of vascular plants. The Pteridaphyta, especially the Diplopterygium glaucum, took the most advantage, being the most dominant species. The cosmopolitan families and hemicryptophyte occupied the highest percentage of the herbs. The evergreen species accounted for 84.6%. And the genera of the Asian and North American discontinuous distribution were not discovered. The seedling of trees and shrubs accounted for 51 species. 80% of the plant individuals belonged to 4～6 species. There were 24 species of liana in the forest floor. 25% of them were herbaceous species, 42% were deciduous. On the ground, between bryophytes (22 species) cover and litter exhibited mosaic distribution pattern. In late-successional communities the species of bryophytes became more and its cover were higher than early stages. D. glaucum and its litter established a very complex layer ,which obstructed the forest litter fall to the ground. It reduced near-ground light intensity and made it below seedling light composition point. It inhibited the natural regeneration of the forest. The litter obstructed the seeds to the duff, only about 17%～22.3% of the Castanopsis fargesii seeds could penetrate litter and got into the duff. In winter, about 98.2% of the ground C. fargesii seeds were consumed or transported by rodents. Among litter fall about 28.2% of the seeds went rotten. Because of deficiency of soil water, the seeds of litter surface regeneration rate and survival rate were lower than the duff. The naturally regenerated seedlings of C. fargesii were mainly found in the "herb gaps".

Canopy distribution of precipitation and its controlling factors in two deciduous broad-leaved forests were measured and analyzed during the growing season in 1996. For the Quercus liaotungensis forest, the stemflow, throughfall, interception were 66.7mm, 539.9mm and 117.6mm,amounting to 9.2% ,74.5% and 16.3% of the precipitation of 723.6mm,respectively. For the deciduous broad-leaved mixed forest, the stemflow, throughfall and interception were 32.8mm(4.5%) ,555.9mm (76.8%) and 134.9(18.6%),respectively. The stemflow was positively related to the total precipitation and precipitation during the initial 24 hours ,while throughfall was positively related to the precipitation and maximum rainfall density.

Based on field survey, the species composition, flora characteristics, biodiversity of the Davidia involucrata community were investigated along with its appearance ,structure ,dynamics, and regeneration of D. involucrata population. The results suggest that: 1 ) The flora of D. involucrata community was rich, ancient, and prominently endemic;2) The community appearance was dominated by the deciduous macro- and mega-high species and evergreen mega- and microhigh species. The vertical structure of the community is complex;3) The population structure of D. involucrata changed from developing to stable type from the front to middle stage of the community succession;4) The probability of the sexual regeneration of D. involucrata comes down along with the increasing of its importance values in the community.