Study On The Population Characters And Community Dynamics Of Larix Chinensis

Larix chinensis is one of typical endemic species in China,only distributed in alpine and sub-alpine belt in Mt.Qinling,Shaanxi Province of China.It is also one of endangered species in Chinese Plant red Data Book.It is the only tree that forms the pure forest which has special functions to conserve water,firm mountain stone and maintain water and soil at the timberline area.Mountain peaks covered in ice and rocks,steep slopes,wild torrents,and fragile soil and vegetation cover characterize the topography, where the environmental risks are high and natural disasters(e.g.,flood,erosion,rock and land slides,avalanches etc.) are frequent.The sub-alpine vegetation belt is a very important buffer zone limiting potential threats in mountainous areas,and undisturbed sub-alpine forests can efficiently reduce the risks of natural disasters.However,this vegetation belt is also a sensitive area.Assessing and analyzing population characters and community dynamics are therefore prerequisites for understanding ecological processes and restoration of natural forests.The population characters and community dynamics of L. chinensis populations along the altitudinal gradient on the southern slope of Taibai Mountains were studied in this paper.The forest was divided into three transects,i.e., low-altitude(2900～3 000 m),mid-altitude(3100～3200 m) and high-altitude(3300～3400 m).Primary results of this paper were summarized as following:1 Age structure studies along an altitudinal gradient would be helpful in understanding the limiting factors on the regeneration of natural forests.L.chinensis individuals were defined as trees,saplings,or seedlings by their heights.Age of each individual was estimated by the age-height relationships of seedlings or saplings and the age-DBH(diameter at breast height) relationships of big trees.The age structures differed across altitude classes.The age structure in low-altitude transect and in mid-altitude was closed to bell-shaped,which was characterized by the dominance of adult trees. Multi-modal age distribution was found in the high altitude transect,and was caused by lack of young seedlings and saplings.This suggested that different limiting factors played important roles in shaping the age structure and forest regeneration at different altitudes. In the low altitude,density dependent interspecific and gap disturbance was probably the most important limiting factor.In the mid-altitudinal transect,density dependent intraspecific competition between trees and gap disturbance likely controlled regeneration of L.chinensis.We propose that limiting climatical factors,e.g.temperature,play an important role in determining the age structure of L.chinensis populations in high-altitude areas.2 Alpine timberlines are ecotones highly sensitive to disturbances and enviromental changes that have become a major focus of global climate change research.To explore the survival status of L.chinensis populations,one plot(100 m×100 m) were investigated and analyzed.Time-specific life tables or vertical life table curves of survival-mortality based and hazard based age structures were drawn.The results showed that:①The survival curve of the population tended to be the type between DeeveyⅡandⅢ;②There existed two peaks of mortality in the lifespan,one was adult stage(70～80 years) and the other in the old stage(180～190 years);③Most individuals of different age class had a clumped distribution at different scales.With the population age increased,the distribution pattern had a trend from clustering to random,and the clustering intensity decreased somewhat.3 In order to explore the functions of gaps on forest dynamics and biodiversity protection in the sub-alpine coniferous forest,the survey of gaps and community had been carried out in Taibai Mountain Nature Reserve.Natural disturbance regimes of gaps,and tree species regeneration responses to gaps were studied.The results showed that:①In low-altitude and in mid-altitude transect,the gap size class distributions were also similar and showed a strong positive skewness with a few large and many small gaps;gaps＜100 m~2 were most frequent and gaps＞150 m~2 were rare.In high-altitude,most of the gap size class distributions were large.②There were significant differences in the size and number of seedlings among the different sized gaps and understorey.③In low-altitude transect and in mid-altitude,the regeneration of L.chinensis in the different sized gaps were as follows:medium＞small＞large＞understorey;the regeneration of Abies fargesii in the different sized gaps were as follows:small＞medium＞large＞understorey.In the high-altitude,small gap was benefit for the regeneration of L.chinensis.Common tree species in a sub-alpine coniferous forest in Taibai Mountains exhibited different gap regeneration behaviors.Shade-tolerant A.fargesii vigorously regenerates in gaps from advance regenerations.L.chinensis may also regenerate in gaps from new individuals,but regeneration was infrequent or absent.Because L.chinensis usually requires both a canopy opening and exposed mineral soil surfaces for seedling establishment.Therefore,the lower frequency middle gap may explain the absence of the L.chinensis regeneration.4 The experiments were made with the seeding of L.chinensis.The results showed that:①It was studied that daily change of chlorophyll fluorescence of L.chinensis by PAM-2100.The results indicated that in sunny day,F_v/F_m,Yield and qP daily change of photosynthesis decreased and there is the lowest photosynthesis at 14:00.The non-photochemical quenching rate(qN) increased significantly.It was proved that the PSⅡwas damaged at the high temperature.②At high temperature(＞25℃) and low temperature(＜0℃),the maximum quantum yield(F_v/F_m) of photosystemⅡ(PSⅡ) sharp declined with the increasing of time.5 According to the forest-growth-cycle theory,forest communities are dynamic, mosaic systems composed of patches in different developmental phases.Based on an investigation in a sample of L.chinensis forest types on Taibai Mountain,China,four distinct growth phases or patch types were recognized and patterns of patch mosaics and changes in species diversity were studied.Diurnal changes in light and temperature regimes in different patch types of the forest growth cycle were measured and analyzed. Our results were as follows:①The percentages of different patch types within the sampled forest community in low-altitude,mid-altitude and high-altitude,were for the gap phase(32.3%,40.3%and 48.5%),building phase(24.5%,34.0%and 32.0%),mature phase(23.5%,17.2%and 12.3%) and degenerate phase(19.7%,8.5%and 7.2%);②The change of densities(number of individuals/ha) of trees in the forest growth cycle was mainly caused by the larger sized trees;③The changes of species diversity with the forest growth cycle showed a wave-like pattern;④The average DBH,average height,average basal area at DBH,and average volume of each individual and stand volume all increased with the forest cycle process;⑤The environmental factors changed significantly,both diurnally and with progress of the forest cycle.Light intensity and soil surface temperature changed more in the gap phase than the other three phases.A framework to explain the mechanism for maintaining biological diversity within the sub-alpine coniferous forest is described.From our study,we hypothesize the following as a framework to explain the mechanism for maintaining biological diversity within the Taibai subalpine coniferous forest:Disturbance factors(e.g.,wind,snow and pest) acting on the forest canopy and trees causing tree deaths.Gaps of different sizes forming after the death of canopy trees. Restoration and regeneration process of trees in gaps,initiating the process of the forest growth cycle.Variation in ecological factors in patches of different developmental phases, having selective effects for different species groups and different sized trees.Varying regeneration and recruitment densities of different species groups and different sizes in different patch types.Variations in growth response for each species group in different patch types within a forest community,and all species satisfying their life history requirements in the patch mosaics within the community.In a word,some measures should be taken to protect L.chinensis.In all,there were lack of young seedling and sapling in the forest.According to the heliophytic and xerophytic characteristics of L.chinensis,different management activities should be taken according to the population characters and community dynamics in different altitude,so as to promote optimal environmental conditions for seedling recruitment in the rare L. chinensis.We should foster the sapling and cut down the higher trees reasonably to reduce the competition intensity.We can offer space and abundant light to the sapling by producing gap,cutting non-target species,clearing herbs,or felling the longer boughs of higher trees.Of course,there are many problems needing our research.For the growth of seedlings,the temperature should be 5～20℃.The temperature is too high or too low, temperatures can affect the seedling of L.chinensis growth.As a result,seedlings should be protection(such as adding a cover,etc.) to prevent extreme temperatures caused by damage to the seedlings.