Photosynthetic Productivity Of Crown Layer In Larix Kaempferi Plantation

The forest canopy is the main functional interface for exchanging between trees and atmospheric environment.It is also the major site for tree photosynthesizing.Crown photosynthetic productivity determined by photosynthesis and respiration is the original energy which could promote and support the formation of forest.It is important to research photosynthesis and respiration in different crown layers for revealing synthetic process of tree productivity.In this study,3-,8-and 20-year-old Larix kaempferi plantations in Xiaolong mountain forest area of Gansu province were selected as the studied objects.To compare temporal and spatial changes in physiological,morphological and photosynthetic characteristics of the needles and respiration rates of branches and stems,nutrient content,chlorophyll content(Chl(a+b)),specific leaf area(SLA)and photosynthetic response curves,diurnal changes of photosynthesis in different crown layers of the sampling trees were measured in growing season in 2014 and 2015,and respiration rates of branches and stems were measured from October in 2014 to September in 2015.The main environmental factors affecting photosynthesis and optimal environmental factors of the photosynthesis in different crown layers were determined by analyzing the relationships between photosynthetic parameters and environmental factors.Photosynthetic net primary productivity was estimated by light response curve,photosynthetic active radiation(PAR)and leaf area on the base of crown gradients.Respiratory loss in different crown layers were estimated by branch surface area,air temperature and the relationships between respiration rate and air temperature.The height of functional crown was determined by means of the contribution rates of needles in different crown layers to photosynthetic productivity of the whole crown,which would provide important reference for artificial pruning and the maintenance of photosynthetic productivity in the crown.The main conclusions from this study were as follows:(1)There were apparent temporal and spatial changes in physiological and morphological characteristics of the needles,but there were some differences in temporal and spatial changes in needle nutrient contents among different tree ages.With the change of season,there was an increasing trend in non-structural carbohydrates(NSC)and a decreasing trend in SLA.However,Chl(a+b)contents showed increasing trend first and then decreased with seasonal change,and reached their maximum values in June.These results suggested that the utilization efficiency of light energy of the needles reached the highest at mid-growing season,and also indicated that the accumulation of NSC in the needles increased gradually during the late period of growing season,which could result in an increase in the diffusion resistance of the mesophyll.With the increasing crown,there was an increasing trend in NSC and chlorophyll a/b(Chla/b),but a decreasing trend in Chl(a+b)and SLA.These results suggested that photosynthesis and the utilization efficiency of high light intensity of the needles decreased with crown depth,but the utilization efficiency of low light intensity,absorptance and light capture of the needles increased with the decreasing crown.The ratio of nitrogen and phosphorus(N:P)were less than 14 in Larix kaempferi and less than 3 in forestland soil which indicated that the growth of Larix kaempferi was mainly limited by N in the study area.N increased first and then decreased with season processing in 3-and 20-year-old trees.On the contrary,it decreased first and then increased in 8-year-old trees.There was greater N in the middle crown than in the lower crown in 3-year-old trees,while N in the lower crown had maximum values in 8-and 20-year-old trees.The differences in temporal and spatial changes of N among different tree ages indicated that the utilization efficiency of nutrient was different with the varying developmental stages in Larix kaempferi.(2)There were apparent temporal and spatial changes in photosynthetic characteristics and photosynthetic net primary productivity.There were no significant differences in photosynthetic response parameters among different orientations.Maximum photosynthetic rate(Pmax),net photosynthetic rate(Pn)and stomatal conductance(gs)increased with the increasing crown,while there were maximum values in the middle crown and minimum values in the lower crown for maximum carboxylation rate(Vcmax).There was an increasing trend first and then a decreasing trend for Pmax,and the values of Pmax reached maximum around in July.There was a decreasing trend first and then an increasing trend for Vcmax in 8-and 20-year-old trees,and the values of Vcmax attained minimum around in July.The seasonal variation in Pn varied with developmental time.There was an increasing trend first and then a decreasing trend for Pn in 20-year-old trees.In contrast,there was a decreasing trend first and then an increasing trend in 8-year-old trees.The maximal and minimal values of photosynthetic net primary productivity were in the middle and lower crown layers,respectively.The results suggested that the needles in the middle crown had greatest contribution to the growth of Larix kaempferi,and the needles in the lower crown had least contribution.With the change of season,photosynthetic net primary productivity began to increase at early growing season,and reached its maximum at mid-growing season,and then decreased gradually.(3)There were apparent spatial changes in the optimal environmental factors affecting Pn and gs.Environmental factors affecting Pn and gs were determined based on the methods of multiple linear regression and path analysis.PAR was the main environmental factors affecting Pn,while vapor pressure deficit(VPD)was the main environmental factor affecting gs.There were 33.0%~81.5% of the total variation in Pn and 59.8%~77.8% of the total variation in gs were explained by environmental factors,respectively.The vapor pressure deficit thresholds(VPDT)and the optimal air temperature(Taopt)of Pn and gs were determined by means of the methods of boundary-line analysis.The VPDT of Pn and gs showed an increasing trend with crown depth.However,the Taopt of Pn and gs were greater in the middle and lower crown layers than in the upper crown.The values of VPDT varied from 0.53 to 2.54 kPa,and the values of Taopt varied from 25.39 to 27.92? for Pn within the crown.The values of VPDT varied from 0.88 to 2.18 kPa,and the values of Taopt varied from 25.84 to 27.62? for gs within the crown.(4)There were apparent temporal and spatial changes in respiration rates and respiratory loss of branches and stems.There was an increasing trend first and then a decreasing trend in respiration rates and respiratory loss of branches and stems with seasonal change,and they attained maximum in the mid-growing season and minimum in the dormant season.Branch respiration rates decreased with crown depth,and the values of respiration rate in the upper crown were significantly greater than in the middle and lower crown layers.The respiratory loss of branches increased with the decreasing crown in 3-year-old trees,while the respiratory loss of branches showed maximal values in the middle crown and minimal values in the upper crown.The results indicated that the spatial variation of the respiratory loss was related to spatial distribution of the branches.The contribution which the maintenance respiration rate of branches accounted for the total respiration rate increased with the decreasing crown.However,the contribution which the maintenance respiration loss of branches accounted for the total respiratory loss were greater in the middle and lower crown layers than in the upper crown.The results suggested that the photosynthate which were consumed to provide basic energy for metabolism of the branches were greater in the middle and lower crown layers than in the upper crown,while the photosynthate which were consumed to promote growth were less than in the upper crown.Branch respiration rates decreased with the increasing tree age,while the contribution which the maintenance respiration rate of branches accounted for the total respiration rate increased with the increasing tree age.The growth respiration rate and maintenance respiration rate of stems was almost unaffected by tree age.Under the comprehensive effect of photosynthesis and respiration,the above-ground photosynthetic net primary productivity of individual tree were 2703.22,2355.69 and 2456.64 gC·a-1 in 3-,8-and 20-year-old trees,and they accounted for 82.4%,84.5% and 65% of their photosynthetic net primary productivity,respectively.The results indicated that there is fast-growing biological characteristic at early ages in Larix kaempferi.Respiration rates of the branches and stems were mainly affected by air temperature.56%~82% of the variation in branch respiration rate and 81%~84% of the variation in branch respiration rate were explained by air temperature.(5)The spatial distribution pattern of photosynthetic net primary productivity were identified by researching leaf area and photosynthetic characteristics in different crown heights.Both photosynthetic net primary productivity and leaf area showed an increasing trend first and then a decreasing trend with increasing crown,and they attained maximum at the relative crown height 41%~52%.The maximal leaf area and photosynthetic net primary productivity were 2.74m2 and 266.5mgC·h-1,respectively.The results suggested that needles in the middle crown provided more photosynthate for Larix kaempferi growth than in other crown layers.The proportions of leaf area and photosynthetic net primary productivity to the whole crown were approximately equal at the relative crown height 31%~41%.According to the equal proportions of needle area and their productivity to the whole crown,the upper 2/3 crown and 1/3 crown were defined as functional crown and nonfunctional crown,respectively.To maintain higher photosynthetic net primary productivity of Larix kaempferi plantation,it was suggested that artificial pruning should be carried out in time after canopy closure.The branches which had low photosynthetic productivity and great maintenance respiration loss in the lower 1/3 crown should be pruned,which was beneficial to reduce the consumption of nonfunctional crown to photosynthetic productivity.