Drivers Of Forest Productivity Of Pinus Massoniana Plantation In South Tropical Zones

Forest productivity is not only the basic content of forest structure and function,but also an important index to evaluate forest quality.Scientific and reasonable management measures can effectively improve the productivity of forests.Exploring the impact factors on forest productivity plays an important role in forest management.Based on 53 permanent sample plots and 20 typical permanent sample plots of Pinus massoniana plantation in the Tropical Forestry Experimental center,Chinese Academy of Forestry in South Subtropical Zone,the dynamic changes of stand basal area growth(forest productivity)and soil nutrient elements under different forest age stages,density grades and tree species composition types were compared and analyzed.In addition,in order to provide theoretical basis for forest management of Pinus massoniana in South subtropical zone,the stand basal area growth model introducing different factors(age,density,species composition,soil nutrient elements)were constructed respectively.The main results were listed as follows:(1)Forest age had a significant effect on productivity of Pinus massoniana Plantation in South Subtropical Zone.The average DBH.total stand basal area,average individual volume and stand volume of Pinus massoniana Plantation at different forest age stages were significantly different(p<0.05),and the average DBH,total stand basal area,average individual volume and stand volume increased with the increase of forest age.Stand volume per annual increment and growth rate decreased with the increase of forest age,and the decline was more obvious during the transition from middle-aged forest to near-mature forest.On this basis,the basal area growth model of Pinus massoniana forest with fixed effect of age stage was constructed.The results showed that with the increase of forest age,the values of parameter a corresponding to different forest age stages showed a trend of increasing and then decreasing.The values of parameter a at near-mature stage were the largest,which indicated that the productivity of Pinus massoniana Plantation in the study area increased with the increase of forest age and reached the maximum at near-mature stage.(2)Stand density had a significant effect on productivity of Pinus massoniana plantation.Average DBH and average individual volume decreased with the increase of density,while total basal area and stand volume increased with the increase of density.For the middle-aged forest of Pinus massoniana,stand volume per annual increment(PAI)and growth rate(Pt)of the stand under medium density(750-1100 trees*hm-2)were the largest,which were 18.716m3*hm-2*a-1 and 11.97%respectively.For the near-mature forest of Pinus massoniana,stand volume per annual increment under different density grades was not significant.Incoporating the different density indices to stand basal area growth model,it showed that the incorporating the stand density index(SDI)improved stand basal area model performance.On this basis,the basal area growth model of Pinus massoniana forest with fixed density grade effect was constructed.The results showed that the value of parameter a increased with the increase of stand density index(SDI),which indicated that the productivity of Pinus massoniana Plantation increased with the increase of stand density.(3)The composition of tree species had an important influence on the productivity of Pinus massoniana plantation.Stand volume per annual increment and growth rate of stand volume and the entrance volume per unit area of different tree species(A1-A4)were larger than those of the control pure forest.Among them,the forest growth in the transformation mode A2 was bigger than other transformation modes in all aspects,while the loss of the control pure forest was much bigger than that of each transformation mode.At the level of individual tree,the average DBH growth,average height growth and volume growth of individual tree in each transformation mode were larger than those in the control pure forest,and the transformation mode A2 was larger than other transformation modes in all aspects.The transformation mode A2(replanting Quercus griffithii and Erythrophleum fordii under forest)had the best effect in the short term.On this basis,the basal area growth model of Pinus massoniana forest with fixed effect of species composition type was constructed.The results showed that the maximum value of parameter a was found in the pure forest of Pinus massoniana,followed by the mixed forest of Pinus massoniana and Quercus griffithii,and the mixed forest of Pinus massoniana and other broad-leaved tree species(such as Castanopsis hystrix,Manglietia glauca,etc.).(4)The effects of different soil nutrient elements on the productivity of Pinus massoniana plantation were also different,and the pH value decreased at all forest age stages;the content of organic matter decreased significantly at the middle age stage;the content of total nitrogen and available nitrogen increased basically at all forest age stages;the content of total phosphorus increased or remained stable at all forest age stages,and the content of available phosphorus decreased significantly at all forest age stages.The content of total potassium increased in all stages except mature forests,and the content of available potassium decreased significantly in middle-aged forests but changed little in other stages.In order to explore the relationship between productivity and soil nutrient elements,Pearson correlation analysis and stepwise regression analysis were used to analyze the relationship between productivity and soil nutrient elements.Pearson correlation analysis showed that the stand productivity of Pinus massoniana was significantly negatively correlated with pH value,negatively correlated with total potassium content and available phosphorus content,and positively correlated with alkali-hydrolyzed nitrogen content.The stepwise regression analysis showed that pH value,total nitrogen content,alkali-hydrolyzed nitrogen content and available phosphorus content had significant effects.On this basis,the basal area growth model of Pinus massoniana Stand containing soil nutrients was constructed by re-parameterization method.The results showed that pH value,total nitrogen content and available phosphorus content were negatively correlated with basal area growth,while alkali-hydrolyzed nitrogen content was positively correlated with basal area growth.