Temporal and spatial distribution of photosynthetically active radiation flux of coniferous and broadleaved mixed forests at the Dinghushan Mountain, low subtropical China

PAR(photosynthetically active radiation) is an important ecological factor which controls the energy transforming and matter production.It is very helpful to understand the physio-ecological factors and photosynthesis to research the temporal and spatial characteristics of PAR flux of the ecosystem.The temporal and spatial distribution of PAR of the coniferous and broadleaved mixed forests at the Dinghushan Mountain,low subtropical China,was analyzed in 2003 in this paper.The results were as the followings:the intensity of daily PAR reduced and the variation coefficients(CV) increased,when PAR came into the ecosystem.The curves of monthly total PAR for each forest layer presented a single peak in a year.The maximum and minimum PAR values of each layer appeared in July and March respectively.The diurnal changes of average PAR were just like single peak types both in rainy season and dry season.The PAR above the canopy(PAR₁) and that in the first arbor layer(PAR₂) presented a normal peak type curve,but it was not with the third arbor layer(PAR₃) and forth arbor layer(PAR₄).The average PAR values in dry season were about 68.99%-71.48% of those in wet season.The CV values of daily PAR flux for each layer were very similar in dry season,but those of PAR₃ and PAR₄ became dramatically higher in wet season.The annual PAR₁,PAR₂,PAR₃ and PAR₄ were 1 842.27,1 374.37,471.38 and 288.39 MJ/m² respectively,about 40.07%,29.89%,10.25% and 6.27% of downwelling shortwave solar radiation(R_Sd) respectively.The RPFD(relative photosynthesis photon flux density) of PAR₂,PAR₃ and PAR₄ were 74.6%,23.6% and 15.7% respectively.An obviously significant linear correlation existed between the daily and monthly PAR₁ and R_Sd.The coefficients of R_Sd were less than the usual values,which ranges from 0.44 to 0.50.