Several Of Evergreen Broad-leaved Forest Tree Seedlings Response And Adapt To Different Light Environments

Mountain Jinyun, situated in middle subtropics in China, developed typical subtropics humid evergreen broad-leaved forest. Several seedlings of dominant trees from different successive stages in subtropical evergreen broad-leaved forest of Mt. Jinyun had been sampled to measure physiological parameters of growth, photosynthesis, evaporation, stoma conductance to investigate their responses and adaptation to different light environments (open land, gap, understory). The selected species were Camptotheca acuminate, Gordonia acuminate, Sloanea leptocarpa belonging to deciduous broad-leaved species, evergreen broad-leaved species and evergreen broad-leaved shade-tolerant species respectively. The results are as follows:(1) Gaps favored the height growth of seedlings of shade-tolerant Sloanea leptocarpa. while the seedlings of Gordonia acuminate showed maximum increment in height growth in open land and gap. But the seedlings of deciduous broad-leaved Camptotheca acuminate exhibited the biggest increment of height growth, which was relevant to species characteristics. The stem diameter of the 3 seedlings in gaps and open lands were bigger than that in the understory, but without significant differences.(2) In the understory, Gordonia acuminate grew very slowly through the whole growing season while Sloanea leptocarpa and Camptotheca acuminate had too low survival rates to grow normally. Though the total biomass of Gordonia acuminate Sloanea leptocarpa and Camptotheca acuminate was bigger in open land than that in gap, there was no significant difference through the whole growing season. As light and temperature increased in a growing season, three seedlings in open land allocated more biomass to underground root resulting in an increase in root mass ratio to absorb more water and nutrients to response to light stress and enhance survival capacity. In gap, Gordonia acuminate increased leaf biomass ratio to optimize the acquisition of light. Shade-tolerant Sloanea leptocarpa having increasing leaf biomass allocation in summer had changed to increase the stem allocation as temperature went down in autumn while Camptotheca acuminate enhanced the leaf and stem mass ratios to optimize the acquisition of light.(3 ) In summer, the increment of Gordonia acuminate leaf area and number was slow in open land resulting from that the stress of intense light induced seedling defoliation and the restraint on leaf area growth. Because of low radiation and temperature the leaf number and area increased quicker in autumn than in summer. In gaps the increase in seedling leaf number and area washigher than in open land while only the leaf area was higher in Sloanea leptocarpa. the increment of Sloanea leptocarpa leaf number increase swiftly in open land, the Camptotheca acuminate leaf number increased in both open land and gap while average leaf area was higher in gap than in open land. After autumn, due to defoliation, leaf number and average leaf area decreased remarkably and growth stopped. The chlorophyll content of three seedlings was following the sequence: understory> gap>open land. Chlorophyl a/b in Gordonia acuminate changes greatest in different light environment compared with that in Sloanea leptocarpa and Camptotheca acuminate, and this is one reason that why Gordonia acuminate grows better than the other two species in the understory .(4) In growing season, the photosynthetic rates of 3 seedlings in the understory were low due to low radiation. The net photosynthetic rate of Gordonia acuminate was higher than that of shade-tolerant Sloanea leptocarpa resulting in successful growth of Gordonia acuminate* In open land, there exited a remark midday photosynthetic depression while stoma conductance and transpiration rate increased to prevent photoinhibitiono The fluctuation of photosynthetic rate in Sloanea leptocarpa was not significant while the increase in stoma conductance ,respiration rate and CO2 absorbing had been found but leaf temperature decreased. With high photosynthetic rate, Camptotheca acuminate, its Diurnal variation of stoma conductance and respiration rate were in accordance with that of photosynthesis. The midday photosynthetic depression of three seedlings in gaps was not prominent, which was relevant to lower PAR compared to open land. The reason why there was no significant difference in seedling growth between gap and open land was that no remarkable difference in the diurnal average net photosynthetic rate had been found among Gordonia acuminate,Sloanea leptocarpa,Camptotheca acuminate.(5) High net photosynthetic rate and apparent quanta efficency, low rate of respriration and light compensation point were favorable for Gordonia acuminate growing in the understory. The light compensation point of Camptotheca acuminate (30 umol-m"2-s"') and Sloanea leptocarpa (39 umol-nfV ) were higher than that of Gordonia acuminate, which was one of reasons for unrealized growth. The light compensation point of the three seedlings was ranged as the sequence: open land>gap>understory. But there was no significant difference between gap and open land. In open land, the maximal net photosynthetic rate sequence was: Camptotheca acuminate (the biggest value can reach 9.98 umol-m"2-s'')> Gordonia acuminate > Sloanea leptocarpa. And thatwas why Camptotheca acuminate had the biggest growth rate. In gap, the maximal net photosynthetic rate sequence was: Gordonia acuminate (8.92umol-m"2-s"') > Camptotheca acuminate (7.32 umol-m'2-s"') > Sloanea leptocarpa (4.64 umol-m'^s'1). Light compensation point didn't decrease with light intensity, which may be relevant to the time of light adaptation.Further studies were in need.