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50-YEAR SUITABILITY EVALUATION ON PINUS DENSIFLORA FOREST PLANTATION IN MOUNT TA, SHANDONG, CHINA

In this study, with a 50-year-old Pinus densiflora forest plantation on Mount Ta as the experimental group and 50-year-old secondary forest as the control group, diameter distribution and species diversity of Pinus densiflora and secondary forests were assessed using quadrat method to analyze 4 species diversity indices and 7 soil nutrient indices, evaluate the suitability of Pinus densiflora forest plantation in Mount Ta, and provide a reference for Pinus densiflora forest plantation and plant community succession in Mount Ta even extending to Yimeng mountain area. The results indicate that: (1) Evaluated in terms of individual and population, Mount Ta is a suitable area for Pinus densiflora forest, but Pinus densiflora seedlings and saplings are significantly scarce, which will make it difficult for population to renew. (2) In terms of 4 species diversity indexes of arborous layer and shrub layer in Mount Ta, Pinus densiflora forests do not significantly different from secondary forests, while 4 species diversity indices of field layer are significantly higher in Pinus densiflora than in secondary forests. The soil contents of organic matter, total nitrogen, phosphorus and potassium and available nitrogen, phosphorus and potassium are not significantly different between Pinus densiflora and secondary forests in Mount Ta. Evaluated from the perspective of community and ecosystem, Mount Ta is a suitable area for Pinus densiflora. (3) Pinus densiflora forest plantation or long-term enclosure in this area can promote vegetation recovery.


Introduction
Biodiversity and species coexistence have always been regarded as a core research topic in ecology (Maurer and Rosenzweig, 1996;Boeye et al., 2014;Thomas et al., 2018;Parvinen et al., 2020). Forest development status and potential are limited, subject to restriction of regional ecological conservation and biodiversity restoration (Hilmers et al., 2018). At present, the majority by forest resources in China are various secondary forests and plantations, attracting much attention from many scholars (Gao and Wang, 2018). Soil provides the basic condition and material basis for the forest ecosystem, which determines and restricts forestry production and forest health (Cantidio and Souza, 2019).
Pinus densiflora is naturally distributed in narrow and long intermittent manner mainly from Changbai Mountain to Yuntai Mountain across Liaodong Peninsula and Shandong Peninsula (Yin et al., 2018;Kim et al., 2019). Photophilous and cold-resistant Pinus densiflora grows fast, which is the main choice of afforestation tree species in the neutral to acid soil mountains in the distribution area (Zhang et al., 2018;Gao et al., 2020). The Pinus densiflora in Mount Ta, Shandong mainly derives from artificial afforestation in the late 1950s to the mid-1970s followed by long-term enclosure, accounting for about 30% of Mount Ta forest area, while Wayward Pines and secondary forest account for 50% and 15%, respectively. Some scholars have conducted systematic surveys on the vegetation status of Mount Ta in 1959 (Zhou, 1962) and 2009 (Gao et al., 2011), but there has been no report on suitability evaluation of Pinus densiflora forest plantation. In this study, we chose Pinus densiflora as the experimental group and secondary forest as the control group, and diameter distribution and species types of Pinus densiflora and secondary forest were assessed using quadrat method to analyze 4 species diversity indices. 7 soil nutrient indices, definitely judge suitability of Pinus densiflora forest plantation in Mount Ta, and provide a reference for Pinus densiflora forest plantation and plant community succession in Mount Ta and even Yimeng mountain area.
Scientific hypotheses were made in this study: (1) Predicted from individual and population perspective, Mount Ta is a suitable distribution area for Pinus densiflora. (2) Predicted from the community and ecosystem perspective, the 4 species diversity indices and 7 soil nutrient parameters will not be significantly lower in Pinus densiflora of Mount Ta than in the local secondary forest.

Overview of the study area
Mount Ta is located in southeastern Shandong (35°1′-36°0′N, 117°4′-118°2′E), with an area of 204 km 2 , an average annual temperature of 13.4℃, an average annual precipitation of 900 mm, and a forest coverage rate of 85% (Gao et al., 2011), which is now a world geopark and a national forest park.

Field investigation
The forest community surveyed in this study is a middle-aged forest, which is in a state of natural succession in long-term (50 years) enclosure. Five repeated plots of Pinus densiflora and secondary forest were set. The plots are close to each other in similar ecological environment. The quadrat was set to: 20 m × 30 m. For arborous layer and shrub layer, species with DBH ≥ 5 cm and < 5 cm, quantity and DBH were measured (

Diameter structure and arbor species types
Expanded species Quercus variabilis, Robinia pseudoacacia and Acer truncatum, a total of three species (

Soil nutrients
The seven soil nutrient parameters of Pinus densiflora and secondary forest in Mount Ta include: soil organic matter, total nitrogen, hydrolyzable nitrogen and available potassium content: that of Pinus densiflora > secondary forest (p > 0.05), soil total phosphorus, total potassium and available phosphorus content: that of Pinus densiflora < Secondary forest (p > 0.05) (see Table 2). forest, but Pinus densiflora seedlings and saplings are significantly scarce, which will make it difficult for population to renew.
(2) In terms of 4 species diversity indexes of arborous layer and shrub layer in Mount Ta, Pinus densiflora has no significant difference from secondary forests, while 4 species diversity indexes of field layer are significantly higher in Pinus densiflora than in secondary forests. The soil organic matter, total nitrogen, phosphorus and potassium and available nitrogen, phosphorus and potassium contents in soil are not significantly different between Pinus densiflora and secondary forests in Mount Ta. Evaluated from the perspective of community and ecosystem, Mount Ta is a suitable area for Pinus densiflora.
(3) From the 1950s to the 1970s, Pinus densiflora was planted on a large scale during Mount Ta forest vegetation recovery. After 50 years of enclosure and natural succession, the original Pinus densiflora still exists on a large scale, but secondary native tree species like Quercus variabilis have been restored to a large extent, which have successfully infiltrated into Pinus densiflora forest, and some areas have even formed forests, indicating that Pinus densiflora forest plantation or long-term enclosure in this area can promote vegetation recovery.