2.1. Study area
Wuzhishan City is a county-level city under the jurisdiction of the Hainan Province in south China. Located in the south-central part of Hainan Island, Wuzhishan City is an inland city as well as a major city and transportation hub on Hainan Island (Fig. 1.). Central Hainan is the traditional home of the ethnic minorities. The geographical coordinates are 109°19 ' E and 18°38 ' N, covering an area of about 1144 km2. According to the latest census held in 2020 (The seventh census,in China), the permanent population of Wuzhishan City was 112,269, with a population density of 98 persons/km2. In 2020, the city achieved a GDP of 3.41 million Yuan, with a per capita GDP of 25,595 Yuan (in June 15, 2020, the exchange rate was US$1.00 = Chinese currency Renminbi 7.0902 Yuan).
Wuzhishan City is a famous "jade mountain city", named after the namesake Wuzhishan Mountain, the highest peak on Hainan Island. There are scenic spots such as the Wuzhishan Tropical Rainforest, Hainan Ethnic Museum, tropical Botanical Garden and other scenic spots serving as tourist attractions. Wuzhishan City has a mild tropical monsoon climate characteristic of the mountain and island geography (Duan, 2018). By monthly average temperature, January is the coolest at 18.4 ºC and July is the warmest at 26.2 ºC. The winter is not too cold and the summer is not too hot. On an annual average basis, the temperature is 22.4 ºC, the rainfall is 1690 mm, and the relative humidity is 84%. It is seldom affected by cold waves and occasionally by typhoons.
The city’s solid geology is mainly composed of medium and acidic extrusive volcanic rocks. The continual deposition and decomposition of organic litter have built up the humus content to render the soil relatively fertile and suitable for tropical and subtropical plant growth (Liu and Wang,2016). The city’s main vegetation was originally mountainous evergreen broad-leaved forest dominated by tropical and subtropical families, mainly Lauraceae, Hamamelidaceae and others (Meng and Du,2016). A small number of temperate families and genera could be found mainly at high altitudes. For example, Carpinus turczaninowii from Betulaceae, Acer laurinum from Sapindaceae, and Rhododendron from Ericaceae.
2.2. Data gleaned from the literature
The basic properties OVTs were acquired from the pertinent references. We searched the literature written in English, and in Chinese in national journals, reports and government documents. Some data were captured from the Technical regulation for surveying of old and notable trees (State Forestry Administration of China, 2016), which helped to grading by OVT age and assessing growth potential.
2.3. Field methods
The OVTs and site data were mainly collected by field investigations from June to October 2021. The morphological characteristics of OVTs were used to determine their species classification against the Flora of China. The data comprised the species identity, quantity, tree locations with coordinates, growth potential, tree age, tree height, trunk girth, crown width and tree management department.
OVT structural characteristics comprise tree height, trunk girth and crown width. We measured tree height using a laser rangefinder (Contour XLR, Lasercraft Inc., Norcross, GA, USA) aimed at the base and tallest point on crown of the tree, respectively, at a distance of no less than 10 m from the target tree. We measured the trunk girth at 1.3 m above the ground using a breast size ruler (to an accuracy of 1 mm). We used the projection method and then measured crown width in the east-west and north-south directions with a tape measure (1 cm accuracy) and calculated the average value.
The growth potential of OVT comprised normal, weak, endangered and dead. We assessed the growth potential of the OVTs with the help of the classification scheme in the technical regulation for surveying of old and notable trees (State Forestry Administration of China, 2016). Tree assessments were conducted during the active growing season from March to September using a set of standard visual criteria, and it assigned to four classes: (1) Normal: Attaining the normal growth of the species, leaf retention rate above 90%, no or a small number of dead shoots, trunk and main branches free from pests and diseases, and overall indication of normal growth. (2) Weak: Leaf retention rate below 90%, some dead shoot, trunk and main branches infected slightly by pests and diseases, and indicators of weak growth. (3) Endangered: Growth of new shoots not obvious, normal leaf retention rate below 50%, many dead shoots, trunk and main branches with obvious diseases and insect pests, and the growth is seriously dampened. (4) Dead: Foliage extensively yellow, wilted, or dropped, dead trunk main branches, and tree showing no living signs (dead tree).
We extracted an incremental wood core of 0.6 cm in diameter from a 1.3 m high trunk and determined the age of the tree via wood cores. The core samples were analyzed using the LINTAB Series 6 Professional Pack linear tree-ring measuring stage (Rinntech, Heidelberg, Germany). The tree age measurements were supplemented by field visits to conduct interviews with local elderly people to collect corroborative oral-history evidence.
Data on GPS positioning of the OVTs were acquired by the latest GNSS system double star positioning technology (Yang and Kang, 2017). Professional equipment Pro XRT (Trimble Pro XRT, Trimble, Sunnyvale, California, USA) was selected for the reference station to collect precise geographical coordinate data, which were uploaded to the local network to provide accurate differential correction data information. In the field, the coordinates with a precision of 50 cm could be obtained by connecting to the network for real-time determination. If the GPS signal was seriously blocked by buildings and other factors, the mobile station Pro XRT2(Trimble Pro XRT2, Trimble, Sunnyvale, California, USA) was used to collect location data to significantly improve the positioning speed.
2.4. Data analysis
We adopted the criteria used in the technical regulation for surveying of old and notable trees (State Forestry Administration of China, 2016) to divide the OVTs into three age grades (Grade 1 over 500 years, Grade 2 300‒499 years, and Grade 3 100‒299 years). The relative dominance of each tree species was determined by a species’ tree frequency in relation to the total OVTs frequency. MS Excel 2010 was used to conduct some of the data analyses and computations.
The dominance index is a measure of the distribution of individuals among species in a community. It can be used in the study of OVT resources to determine the dominant species of OVT, and the formula is (Jiang, 2021):
![](data:image/png;base64,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)
(1)
In the formula (1): N represents the total number of individuals of all species in seven townships in Wuzhishan city, ni represents the total number of individuals of species i, and fi represents the frequency of occurrence of this species at each township. When Y > 0.02, the species was the dominant species in the community.
From the perspective of data morphology, OVTs belong to point-like elements. Usually, the spatial distribution types of point-like elements are uniform distribution, random distribution and aggregated distribution, which can be discriminated by the Morisita index in ecology. The Morisita index indicates the spatial distribution characteristics of individuals, i.e., the degree of concentration of individuals within a group (distribution mode), and its formula is (Jiang, 2021):
(2)
In the formula (2): N is the number of townships in Wuzhishan City, x is the total number of individuals of this species of OVTs, ni is the number of individuals of this species of OVTs in the ith township. I < 1 indicates a uniform distribution, I = 1 indicates a random distribution, and I > 1 indicates an aggregation distribution.
We downloaded the WorldClim version two (Fick et al., 2017) from its website. It contains the standard (19) WorldClim bioclimatic variables from WorldClim version two, as well as solar radiation, wind speed and water vapor pressure.. The elevation data of WorldClim 2.1 had a resolution of 30 seconds. Using ArcGIS 10.8 (ESRI, Redlands, CA, USA), the Wuzhishan City was divided into a grid of 5 km*5 km, and the OVT locations were recorded as points by latitude and longitude (Fig. 1). The OVT frequency in each grid by Grades 1, 2 and 3 were enumerated. The average tree height, trunk girth, and crown width of OVT in each grid were also calculated. We extracted WorldClim data from each grid and calculated Z-cores to normalize their values. Using R 4.1.1, General Linear Model (GLM) analysis was computed to explore the relationships between OVT frequency, average tree height, trunk girth, and crown width in each grid and the standardized natural factors. The GLM model fitness was tested by the Akaike Information Criterion (AIC) to identify the optimal model with the minimum AIC value. The correlations between OVT frequency, average tree height, trunk girth, and crown width and natural factors were evaluated.