Climate Change Effects on Southern Subtropical and Tropical Tree Species in Ganzhou City, China

Aims: To confirm climate warming changes that have taken place in Gannan Arboretum and Ganzhou City by the expect success of introduced tropical tree species as a response to climatic warming scenarios. Study Design: Cross-sectional study. Study Site and Duration: Survey and observational sites were located in 1) Gannan Arboretum and 2) Ganzhou City, Jiangxi Province, China. The survey and observations of introduced tree species were conducted from October, 2009, to November, 2010. Additional growth records were obtained for a period prior to and including the 1970s to assist in the investigation. Methodology: 1) The meteorological data (1951–2009) record was used to analyse climatic change patterns. 2) Growth and development of introduced tree species were Research Article British Journal of Environment & Climate Change, 2(2): 163-179, 2012 164 examined to evaluate effects of introduction. 3) Climate zone attributes of introduced tree species were determined in order to establish place of origin. Effects of introduced tree species were coupled with climatic change scenario data to analyze associative relationships. Results: 1) A trend in climate warming has been evident in Ganzhou City since the1950s. Based on metrological records from 1951 to 2009, a steady rise in annual average temperature has occurred in the region, increasing from 0.2oC to 0.3oC each 20 year period. This trend has been especially evident by changes to average temperature during the coldest month (January) as well as changes in annual minimum temperatures. 2) According to observations and measurements, 39 introduced tropical tree species have successfully established themselves in the Ganzhou region through natural domestication. Among these, 24 tree species were considered basically successful with normal growth patterns while the remaining 15 species achieved preliminary success. 3) Stem analysis data on the five primary introduced tree species indicate that growth patterns were normal and vigorous, suggesting that these tropical tree species have successfully established themselves. It was therefore determined that the successful northward migration of the 39 tree species investigated was chiefly the result of a climate warming trend taking place in the region.


INTRODUCTION
Temperatures around the world have increased by 0.74ºC in the last 100 years (from 1906 to 2005) according to the Intergovernmental Panel on Climate Change (IPCC, 2007), resulting in global warming, which today is a concept generally accepted among scientists and state leaders. Although a tendency towards warming is more apparent in high latitudinal regions, similar conditions also occur in lower latitudes. Based on a number of reports, temperatures have been increasing at a rate of 0.26ºC/10a in tropical regions , and the boundary of the tropics are broadening towards the Polar Regions . A persistent change in climatic conditions and atmospheric composition may lead to a change in the global distribution patterns of species (Walther et al., 2002). This may even result in an increased risk in species extinction (Pereira et al, 2010). For example, IPCC (2007) suggested that 20% to 30% of all species would be at an increased risk of extinction if the rise in the global average warming trend exceeded 1.5ºC to 2.5ºC, and if it were to exceed an approximate threshold of 3.5ºC, 40% to 70% of all species may become endangered.
Species migration may be an adaptive mechanism of habitat tracking by which species can escape extinction (Woodall et al., 2009). Based on recent forest inventory data from North America collected by the United States Department of Agriculture (USDA), northward migration speed of monitored tree species was 100km100a -1 when compared against a distribution map generated in 1971 of 30 tree species (Woodall et al., 2009). Unfortunately, investigating and measuring forests at a species level is difficult when utilizing the general forest inventory of China and is impossible when trying to determine and track tree species distribution changes based on the current forest inventory of southern China. Furthermore, the natural migration of tree species has always been impeded and interrupted by the fragmentation of most forest ecosystems, resulting in difficulties in obtaining reliable data from permanent monitoring plots on the natural migration of tree species. The Anthropocene (Crutzen, 2002) has resulted in maximum changes to tree species distribution patterns. It may therefore be reasonable as well as practicable to utilize cultivated plants instead of natural plants to monitor and provide evidence of climate change.
As far as it can be determined, most literature related to introduced species has concerned itself with the negative impacts to ecological environments. Vitousek et al. (1977) pointed out that biological invasions represent an anthropogenic breakdown of regional distinctiveness of Earth's flora and fauna. Moreover, well-documented examples are available in literature concerning how invading species that degrade human health and welfare also alter the structure and function of undisturbed ecosystems and/or threaten native biodiversity. More than 2,000 alien species have established themselves in the continental United States of America, of which many resulted in various degrees of ecologic and economic degradation (Simberloff, 2000). For example, Robinia pseudoacccia (Hunter, 1996a;Peloquin and Hiebert, 1999) and Ailanthus altissima (Hunter, 1996b) can greatly interfere with forest restoration processes, and Paulownia tomentosa typically outcompetes native tree species in eastern forests in the United States of America (Johnson, 1996), having displaced native pines in a number of national parks. Sapium sebiferum invades rainforests, having displaced native forests in the United States of America through self-seeding (Bruce et al.;Miller, 1997) and Ligustrum sinense prevents bottomland hardwood (pine forests) regeneration (Miller 1997). However, few reports on the negative impacts of introducing alien tree species to terrestrial regions of China exist even though more than 1,000 alien species have been introduced into the country.
Since being founded in 1976, the Gannan Arboretum has from 1978 onwards introduced a large number of alien tree species, most of which originated in South China (Guangdong, Guangxi, Fujian and Yunan) while a few were native to southern Jiangxi forests (Huang et al., 2010). A total of 39 tree species attributable to tropical regions were observed and measured for this study (excluding introduced tree species determined unsuccessful), of which 31 species were from the Gannan Arboretum and eight species were selected from gardens and residential areas within Ganzhou City. In all, 24 tree species were classified as fundamentally successful in introduction and 15 were classified as preliminarily successful in introduction (Table 2). Based on the growth rate and development of these introduced tree species, this study attempted to conduct a preliminary summation and evaluation on the effects of all introduced species as well as to couple these effects with regional climate data from 1951 to 2009 in order to analyze relationships between introduced tree species and climate change in the region. According to an index review of literature, little research on the subject exists with the exception of one PhD dissertation (Song, 2007) that discussed the phenology and growth of introduced tropical plant species and their response to climate change. Phytogeographical principles are the fundamental basis of plant introduction and distribution (Wang et al., 2005), for which biotic and/or floristic homogenization is the important determining factor (McKinney, 2004;Olden, 2006;Olden and Poff, 2003;Olden and Rooney, 2006). However, floristic may in fact be more closely linked to plant introduction as it relates to introduced species that share a place of origin and possess close phytogeographical affinity with the flora of target areas. This may potentially be attributable to their high success rates. Determining initial distribution patterns of introduced tree species and key ecological factors related to original place may therefore be fundamental to the success or failure of introduced species.

Natural Survey
Situated in the upper region of the Gan River, Ganzhou City comprises an area of 394,000 ha, constituting 23.6% of the total area of Jiangxi Province. Its approximate geographical coordinate is from latitude 24º29' N to 27º09' N and from longitude 113º54' E to 116º38' E ( Fig. 1A, 1B). Landform type is hill-basin surrounded by a series of mountain ranges: Yu Mountains to the north, Wanyangshan and Zhuguangshan to the west, Wuyishan to the east as well as Dayuling and Jiulianshan that constitute the eastern Nanling mountain ranges, located in the southern region of Ganzhou City (Fig.1A, 1B). With a distance of only 360 km from Taiwan Strait, Ganzhou City is impacted by monsoon climate conditions. The "local warm habitat" of Ganzhou City area covers an approximate length from 200 km to 300 km from north to south and a similar width from east to west, constituting the geographical basis for the migration and distribution of South China flora and the expect introductory success of species from South China. Gannan Arboretum is the primary site where observations and surveys were carried out for this study. It is located in Ganzhou's Shangyu County, belonging to the hilly moist climatic region south of Changjiang River, situated within the midsubtropical zone of China (Cheng et al., 2010). According to Ganzhou City meteorological records, the annual average maximum temperature of Gannan Arboretum is 19.5ºC, with a maximum of 29.5ºC in July and a minimum of 8.2ºC in January. The annual average minimum temperature is −1.5ºC; the annual accumulated temperature is 6189ºC ≥ 10ºC; and the annual frost free period is >290 days. Total precipitation is 1444.3 mm.

Vegetation and Flora
Native forests of Ganzhou City area belong to the southern evergreen broadleaf subzone of the mid-subtropics for which Castanopsis fordii, C. lamontii, Altingia chinensis,  (Wu et al., 1983) or from boreal flora to Paleotropical flora (Good, 1964). According to earlier investigations (Yao et al., 2002;Huang et al., 2011), numerous "typical" tropical families and genera are distributed throughout the region but rarely found near its northern boundaries. 55.9% genera of tropical areal-type and 41.8% genera of temperate areal-type among 889 genera of seed plants were recorded within the Jiulian mountains (located in the southernmost region) (Liu et al., 2002). Among the tropical areal-type plants, it was found that tropical Asian areal-type plants (Wu et al., 2006) had closer phytogeographical affinity with Ganzhou area flora, for which most genera comprised dominant elements of its evergreen broadleaf forests. As stated above, there are strong reasons to believe that Ganzhou City area is the northernmost frontier of South China flora. This is probably due to the fact that the potential success rate would likely be higher for tropical tree species introduced from South China under a scenario of a warming climate.

Weather Data
Meteorological data used in this study were taken from China's Weather Bureau, including the consecutive annual average temperature (from 1951 to 2009) of Ganzhou City, the average temperature in January, the average temperature in July, annual minimum temperature, and annual precipitation. In order to analyze climate change accurately, relevant climate dynamic curve and trend line graphs were generated based on the original weather data from the aforementioned source.

Growth and Developmental Observations of Introduced Tree Species
Introduced tree species investigated for this study came from two sources: Gannan Arboretum (Shangyou Xian) and from gardens and residential areas situated within the Ganzhou City. For this study, tropical tree species that originated from South China were the primary items of investigation, and observations and measurements taken from introduced tree species focused on the following items: site and time of introduction, plant size (stem number or area), tree height and diameter at breast height (DBH), flower and fruit behaviour, introduction evaluation, etc. For a number of cultivated tree species with larger plant sizes (such as Illicium verum, Mytilaria laoensis, Canarium album, Reevesia thyrsoidia, and Podocarps wallichiana), plots were established to carry out further investigation in order to better understand growth processes. This included measuring each stem in the plot and the selection of one stem as an analytical tree sample by which to carry out stem analysis.

Climate Zone Attributes of Introduced Tree Species
Introduced tree species originated from various climatic zones and regions (Xi et al. 1984;Cheng et al. 2010). It became necessary to classify them into respective zone types according to their original distribution area. The following is a list of the classification zone

Analysis of Climate Change
Weather records from Ganzhou City between 1951 and 2009 can be divided into four separate 10 year intervals (1950s, 1970s, 1990s and 2000s) for the convenience of comparing differences and changes among timeframes. The following results were ascertained (  1951-1960) 1970s (1970-1979) 1990s (1990-1999) 2000s (2000-2009) 1950s, 1970s, 1990s, and 2000s were calculated by the total value of average temperatures of the coldest month (January) from 1951 to 1960, 1970 to 1979, 1990 to 1999, and 2000 to 2009, divided by 10. Other calculation methods, including average temperatures of the hottest month (July), annual minimum temperatures, and annual average precipitation, were carried out in the same manner.
Climate change in the twenty-first century has varied from extreme cold to extreme hot. As shown in Fig. 2 Fig. 5 show a slight rise in the variance curve and variance trend line of average temperatures during the coldest month of the year (January) as well the variance curve and variance trend line of average temperatures during the hottest month of the year (July). In addition (as shown in Fig. 6), a trend towards a slight drop in both the variance curve and the trend line of annual precipitation took place from 1951 to 2009. There is evidence that suggests that the trend in climate change may be heading towards hotter and drier conditions for the region.

Fig. 6. Variance curve and its annual precipitation trend line (1951-2009)
The origin of data and how data was calculated (as shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5 and Fig. 6) are as follows: All meteorological data was supplied by the China Meteorological Data Sharing System (MDS), and climatic data pertaining to standard month (IM) and standard year (IY) was downloaded from its website, including data related to annual average temperatures, annual minimum temperatures, annual average temperatures of the coldest month (January), annual average temperatures of the hottest month (July), and annual precipitation, wherein all data refers to the entire study period . This data were the basis on which the two-dimensional graphs were generated and variance curves and tread lines were drawn.

Outcome of Tropical and Subtropical Trees Species Introduction
The outcome of tropical and southern subtropical trees species introduction into the Gannan Arboretum as well as into Ganzhou City gardens and residential areas are listed and summarized in

Mitilaria laoensis
Mitilaria laoensis, a marginal tropical zone to Chinese southern subtropical zone tree species (d) was distributed throughout China's southern provinces, that is Guangxi, Guangdong and Yunan as well as northern Vietnam and Laos. The stem sample used for analysis was obtained from a 22 year old tree of 17.2 m height, 23.2 cm DBH, and 0.33 m 3 volume. Results of the analysis indicated rapid growth in height and DBH. Moreover, height (Fig. 7A), DBH, and volume growth curves (Figs. 7B and C) exhibited a vigorous rising trend. Forest structure and dynamics of the species were stable with more natural seedling production occurring under the forest canopy, confirming that the introductory experiment of this species was successful.

Illicium verum
The Illicum verum stand (a: South China southern subtropical zone) planted in 1980 originated from the south and southwest of Guangxi Province. According to the survey carried out on the 20×30 m plot, an average height of 10.0 m was measured for the 102 stems with an average DBH of 12.4 cm and an average volume of 133.6 m 3  ha -1 . The stand produced approximately 250kg0.1 ha -1 of fruit. The sample taken showed that the stem was 30 years old, 10.8 m height, 16.0 cm DBH and 0.11 m 3 volumes. Fig. 7 illustrates a normal rising tendency for all growth curves (height growth curve (A), DBH growth curve (B), and volume growth curve (C)). Fruit production commenced at the age of 15 years (1986). According to the criteria established by this study, the introduction of Illicum verum has been successful.

Reevesia thyrsoidia
Reevesia thyrsoidia belongs to the marginal tropical zone to Chinese southern subtropical zone(d). Its original area of distribution was in the Chinese provinces of Guangdong, Guangxi, and Fujian as well as Vietnam and Cambodia. It has rarely been seen naturally occurring in native forests of Ganzhou City. The sample stem (retrieved as a coppiced stem) was collected from a 24 year old plantation forest, 20 m height, 18.4 cm DBH and 0.26 m 3 volumes. Figures 7A, 7B & 7C show that height growth, DBH growth and volume growth occurred rapidly. Growth curves suggest a vigorous growth tendency.

Podocarps wallichiana
Podocarps wallichiana, whose original area of distribution was in Xihuangbanna (Yunnan Province, China) and Vietnam and India, belongs to the mid-tropical and southern subtropical zone (e) (delet "mid-tropical -Chinese southern subtropical zone"). Sample age was 20 years old, 17.9 m height, 16.5 cm DBH, and 0.22 m 3 volume. Growth curves were identical to the aforementioned tree species, indicating it grew rapidly and exhibited a trend towards vigorous growth.

Canarium album
Canarium album was originally distributed throughout southern China and into Vietnam. It belongs to the marginal tropical to Chinese southern subtropical zone (d). Sample age (retrieved as a coppiced stem) was nine years old, 15.2 m height, 17.4 cm DBH, and 0.157 m 3 volumes. Growth curves (such as it was for Podocarps wallichiana) were identical to the aforementioned tree species, indicating it grew rapidly and exhibited a rising trend for all growth indexes. Stem analysis results from all five species indicated that the growth rate of these tropical and southern subtropical species was rapid and vigorous. The outcome of these introduced tree species suggests that the overall natural condition of Ganzhou City is suitable for the growth of thermophilous tree species introduced from tropical and southern subtropical regions. It also serves as confirmation to the climate warming taking place in the region.

CONCLUSION
Since the late 1970s, the introduction of tree species from tropical and southern subtropical regions into Gannan Arboretum and Ganzhou City gardens has become increasingly successful. Among the 39 introduced tree species, 24 are listed as basically successful and 15 are considered preliminarily successful. Other introduced and cultivated tree species that exhibited a lower success rate may be the result of issues relating to plantation technology and mismanagement. Introduction success are likely attributable to three factors: 1) It has been established through the existing meteorological records that the climate of the local region has been experiencing an increasingly warming trend since the 1950s, and this trend has been especially prevalent since the 1970s. Since most introductory experiments were initiated in the late 1970s, during a time when a concurrence of tree species introduction and climate warming was taking place, it is reasonable to attribute introduction success to climate warming.
2) The effectiveness of the local warm habitat likely played a role in the introductory success of tree species from tropical and southern subtropical zones.
3) The potential ecological niche of tree species can be extended by means of a change in natural conditions (Wang et al., 2005). Abundant intrinsic tropical floristic behaviour has taken place in Ganzhou. In addition, more new tropical floristic behaviour from introduced tree species has been added to this region in recent years. It can therefore be safely concluded that Gannan Arboretum may become "the South China in the northern part of Nanling" under the impact of global warming.