Spatial Differentiation,Influencing Factors,and Simulation And Prediction Of Pinus Massoniana Productivity At The Regional Scale

Since the industrial revolution,the global CO₂ concentration has risen sharply,and climate warming has brought serious impacts on the development,production and life of human society.Forest is an important part of terrestrial ecosystem and the most important carbon pool.It plays an irreplaceable role in regulating global carbon balance and mitigating the rise of greenhouse gas concentration.Forest productivity is one of the important indicators for assessing the carbon sequestration capacity of forests,and it is affected by many biological and abiotic factors.In the context of global climate change,understanding the response of forest productivity to various factors at various time and space scales is of great significance for forest carbon cycle research and the formulation of carbon sequestration management policies.In this paper,Pinus massoniana Lamb.,an important plantation tree species in southern China,was taken as the study object.Taking the“rule exploration-actual measurement verification-model driving-scenario prediction”as the study idea,the regional-scale database of stem,branch,leaf,root,aboveground organs and tree layer productivity and the dendrochronology-based“growth-climate”database of P.massoniana were established based on a large number of published literatures.The spatial differentiation and influencing factors of P.massoniana productivity in latitude,longitude and altitude were discussed based on the regional-scale productivity database,combined with soil data and meteorological interpolation data with a spatial resolution of 1 km.The seasonal climatic factors affecting the radial growth of P.massoniana and their spatial differentiation were analyzed based on“growth-climate”relationships,and the spatial differentiation characteristics and formation reasons of P.massoniana productivity were revealed.According to the results of meta-analysis,the typical field plots for key influencing factors of P.massoniana productivity were investigated,and the effects of temperature,precipitation,stand density and stand age on P.massoniana productivity were analyzed and verified.The 3-PG productivity model was parameterized for P.massoniana forests on a regional scale by using literature databases and field measured data,and the response of P.massoniana productivity to different climate scenarios in the future was simulated,to provide scientific basis for the scientific management and carbon sink management of P.massoniana forests.The main conclusions of this study are as follows:（1）Meta-analysis based on the regional-scale productivity database of P.massoniana showed that the net primary productivity of all organs showed a large range of variation,the stem productivity ranged from 0.29 to 8.71 t·hm^-2·a^-1,the branch productivity ranged from 0.10 to 2.11 t·hm^-2·a^-1,the leaf productivity ranged from 0.02 to 2.31 t·hm^-2·a^-1,the root productivity ranged from 0.07 to 2.11 t·hm^-2·a^-1,the productivity of aboveground organs（stem,branch,and leaf）ranged from 0.88 to 10.81 t·hm^-2·a^-1,the productivity of tree layer ranged from 1.04 to 13.13 t·hm^-2·a^-1.The average productivity of each organ from large to small was as follows:stem(3.51 t·hm^-2·a^-1)>root(0.81 t·hm^-2·a^-1)>branch(0.69t·hm^-2·a^-1)>leaf(0.34 t·hm^-2·a^-1).The productivity of all organs of P.massoniana decreased significantly with the increase of latitude,but there was no obvious spatial differentiation in longitude and altitude.（2）The productivity of P.massoniana was affected by climate conditions,soil properties and stand characteristics at the regional scale,and the mechanism and effect of these factors on the productivity of various organs were different.In general,the mean annual temperature（MAT）,mean annual precipitation（MAP）,stand age,stand density,soil available phosphorus and organic matter content all showed a positive correlation with P.massoniana productivity,while the mean high temperature from July to August had a negative correlation with P.massoniana productivity.There was also a negative correlation between leaf productivity and forest age.The results of variance partitioning analysis showed that stand characteristics and climatic conditions had a greater impact on P.massoniana productivity than soil properties.The spatial variability of soil physical and chemical properties in the study area was relatively complex,and only the soil available phosphorus,p H and bulk density showed a slight variation in latitude.The spatial variability of precipitation was relatively small,and the precipitation can basically meet the needs of P.massoniana growth.It was worth noting that the spatial variation range of mean high temperature from July to August,which was significantly negatively correlated with P.massoniana productivity,was small（27.94～33.45℃）,indicating that the growth of P.massoniana in the whole study area could be inhibited by high temperature in summer.At the same time,the MAT and low temperature in December increased significantly with the decrease of latitude,the growing season of P.massoniana in the south subtropics was longer than that in the middle and north subtropics,therefore showing higher productivity.It can be seen that temperature was a key factor explaining the latitude differentiation of P.massoniana productivity.（3）The dendrochronology-based analysis of the influence of seasonal climate on P.massoniana growth showed that January–March temperature(T_1–3),June–August temperature(T_6–8),and July–September precipitation(P_7–9)were the key climate signals influencing P.massoniana productivity during the current and previous year of its growth.T_1–3 and P_7–9 generally enhanced tree growth in most of the sampling sites,whereas T_6–8lead to a decrease in tree growth.P.massoniana growth was mainly affected by the temperature and precipitation in the current year of its growth,but was less affected by the“lag effect”of climatic conditions in the previous year.The temperature seasonality,temperature annual range,and mean temperature of driest quarter had significant effects on the formation of“growth-climate”relationship of P.massoniana.The distribution of sampling plots with significant positive correlation,significant negative correlation,and insignificant relationship between the tree-ring width of P.massoniana and T_6–8showed no obvious spatial differentiation in latitude,longitude or altitude,therefore,the analysis of the influence of seasonal climate on P.massoniana growth also reflected that the inhibitory effect of high temperature in summer on P.massoniana productivity exist in the entire tree species distribution area.（4）The results of field survey and analysis based on the key influencing factors of P.massoniana productivity（temperature,precipitation,stand density and forest age）showed that on different temperature gradients with the same precipitation level,P.massoniana productivity in descending order was as follows:P.massoniana forests in Pingxiang,Guangxi（MAT:22.30℃）>P.massoniana forests in Huitong,Hunan（MAT:17.50℃）>P.massoniana forests in Enshi,Hubei（MAT:11.16℃）,P.massoniana productivity increased with the increase of MAT of sampling site;on different precipitation gradients with the same temperature level,P.massoniana productivity in descending order was as follows:P.massoniana forests in Huitong,Hunan（MAP:1349.71 mm）>P.massoniana forests in Qianyanzhou,Jiangxi（MAP:1617.34 mm）>P.massoniana forests in Taizishan,Hubei（MAP:1116.65 mm）.The sample sites in the medium precipitation gradient（Huitong）had the highest productivity,while the productivity in Qianyanzhou with high MAP and Taizishan with low MAP were both low,and the difference between them was not significant.With the increase in stand density,the height,diameter at breast height,and the biomass of each organ of the individual tree of P.massoniana decreased significantly,but the stand productivity increased.The biomass of individual tree of P.massoniana increased with the increase of forest age.There was no significant difference in productivity between middle-aged forest and near-mature forest,and they were both lower than the productivity of overmature forest.The actually measured results of the field survey were generally consistent with the results of meta-analysis,reflecting the effects of climatic conditions and stand characteristics on P.massoniana productivity,and also confirmed that temperature played an important role in the spatial heterogeneity of P.massoniana productivity in latitude.However,various soil physical and chemical indexes did not show a regular variation trend between the study sites and soil layers,and their influence on the P.massoniana productivity was also different from the results obtained by the meta-analysis,therefore the effect of soil properties on the P.massoniana productivity was complex and uncertain.（5）After parameterizing the 3-PG productivity model for P.massoniana forests at the regional scale based on the data of literature database and field survey,the model was used to simulate the productivity of the stem,branch,leaf,root,aboveground organs and tree layers of P.massoniana forests in all typical sites.The results showed that there was a high correlation between the simulated and measured values of the productivity of tree layer and aboveground organs of P.massoniana,with R² of 0.77 and 0.70,RMSE of 0.85 and 0.79t·hm^-2·a^-1,respectively,indicating that the 3-PG model could well simulate the productivity of tree layer and aboveground organs at various study sites.However,the simulation accuracy of leaf and root productivity was not high,and the productivity of these two organs was usually underestimated,which was corresponding to the overestimation of stem and branch productivity.Although the model simulated the productivity of the tree layer and aboveground organs of P.massoniana well,the biomass allocation was affected by many factors,such as temperature,precipitation,light,and soil conditions.The ecological environment in the distribution area of P.massoniana is complex and varied greatly,so it was difficult to accurately simulate the productivity of various organs of P.massoniana in multiple sampling sites by setting the biomass allocation parameters of the model uniformly on the regional scale.（6）Taking the climate conditions from 1981 to 2010 as the benchmark,3-PG model was used to simulate the response of tree layer productivity of P.massoniana to the climate change of atmospheric CO₂,temperature,and precipitation from 2081 to 2100.The response of P.massoniana productivity to climate scenarios was generally consistent in different study sites:P.massoniana productivity decreased with the increase of temperature,and increased with the increase of CO₂ concentration and precipitation,but precipitation had little effect on P.massoniana productivity.Under the scenarios of CO₂ concentration,temperature and precipitation changes,the increase in CO₂ concentration and precipitation had a greater positive effect on P.massoniana growth than the negative effect caused by the increase in temperature,therefore P.massoniana productivity generally showed an upward trend on the whole.However,the P.massoniana productivity in Pingxiang,Guangxi,was greatly affected by the negative effect of temperature rise,and the simulation value of productivity under the representative concentration path of high emission scenario（RCP 8.5）was lower than that under the benchmark climate.The negative response was different from the positive response of P.massoniana productivity to the climate scenario in the other four sites in middle and north subtropical regions.