Assessing climatic suitability of Pinus radiata (D. Don) for summer rainfall environment of southwest China

Forest Ecology and Management 234 (2006) 199–208 www.elsevier.com/locate/foreco

Assessing climatic suitability of Pinus radiata (D. Don) for summer rainfall environment of southwest China Hong Yan a, Huiquan Bi b,*, Rongwei Li c, Robert Eldridge b, Zhongxing Wu d, Yun Li e, Jack Simpson b b

a Research Institute of Forestry, Chinese Academy of Forestry, Beijing 100091, PR China Forest Resources Research, New South Wales Department of Primary Industries, P.O. Box 100, Beecroft, NSW 2119, Australia c Sichuan Forestry Academy, Chengdu 610081, Sichuan Province, PR China d Aba Forest Research Institute, 71 Jiaochang Street, Wenchuan 623000, Sichuan Province, PR China e Department of Information Management, Beijing Forestry University, Beijing 100083, PR China

Received 30 December 2005; received in revised form 4 July 2006; accepted 4 July 2006

Abstract Originating from a very restricted natural distribution Pinus radiata has become one of the most widely planted exotic pine species in the world, particularly in winter and uniform rainfall environments of the Southern Hemisphere. This paper describes a new climatic profile for the species that identifies summer rainfall areas in southwest China where the species may be suitable for environmental planting on degraded lands to reduce soil erosion. The new climatic profile delineates the climatic requirements of P. radiata through six climatic factors. It includes the absolute minimum temperature as a measure of frost risk in the continental climatic environment and also has lower temperature and rainfall limits than profiles previously developed for commercial plantations. Digital elevation models are developed at both regional and national scales to provide a surrogate of the three-dimensional geographic space of the target area for the spatial interpolation of climatic data. Areas with climatic conditions that match the new climatic profile are mapped using ArcInfo GIS. A chi-square statistic is used to evaluate the influence of each climate variable in the profile in determining the spatial limit of the mapped area. At the national scale, a climatically suitable area of more than 266,000 km2 across three provinces in southwest China is identified. Mean maximum temperature of the hottest month and the length of dry season appear to be the major factors limiting the spatial extent of matched areas at this broad scale. The results of climate matching for the Minjiang dry valley area in particular correspond well with the growth performance of experimental plantings in the field. At this regional scale, mean annual precipitation and mean minimum temperature of the coldest month are the major factors constraining the spatial extent of climatically suitable areas. The mapped areas can help define the working limits and serve as indicative zones for environmental plantings of P. radiata aimed at reducing soil erosion in southwest China. They will also enhance our understanding of the fundamental climatic niche and the potential geographical range of P. radiata. # 2006 Elsevier B.V. All rights reserved. Keywords: Pinus radiata; Tree introduction; Climate matching; Environmental plantings; Soil erosion

1. Introduction Pinus radiata (D. Don) is native to a Californian coastal environment under an essentially Mediterranean climate with a dry summer and a cool, wet winter. Its natural occurrence is now limited to five small separate populations, three along the central coast of California and two on the Mexican islands of Cedros and Guadalupe off the coast of Baja California (Lavery and Mead, 1998; Rogers, 2002). The three mainland

* Corresponding author. Tel.: +61 2 9872 0168; fax: +61 2 9871 6941. E-mail address: [email protected] (H. Bi). 0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2006.07.009

populations fall within a climatic region that is moderated by low-lying fogs rolling in from the Pacific Ocean, resulting in mild winters, rarity of frost, prevalence of summer fog, and moderate summer temperatures (Lindsay, 1937; Forde, 1966; Rogers, 2002). The two island populations grow under a climate that has much less rainfall and greater temperature extremes than the mainland stands (Libby et al., 1968; Rogers, 2002). The presence of fog is also a climatic feature that plays a critical role in the growth, survival and distribution of P. radiata on the islands. The extant natural populations designate a narrow realised climatic niche of P. radiata. From a restricted native range, P. radiata has been introduced to many parts of the world over the last 150 years

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and has now become the most widely planted exotic pine species in the world (Lavery and Mead, 1998). The success of P. radiata as a major plantation species, especially in the Southern Hemisphere, indicates that the fundamental climatic niche is much broader than its realised niche in its natural range. Webb et al. (1984) described the climatic niche of P. radiata through a climatic profile that consisted of six climatic variables. This profile does not seem to provide an adequate coverage of the fundamental climatic niche of P. radiata since it identifies no suitable locations for the species in New Zealand where there are more than 1.4 million ha of P. radiata plantations (Booth, 1990; Lavery and Mead, 1998). To correct this significant omission error, Booth (1990) modified this profile after a bioclimatic analysis of Australian plantations. When applied on a global scale, the improved climatic profile identified most major commercial plantation areas of P. radiata in the world. This climatic profile was further refined by Jovanovic and Booth (2002) to include climatic conditions of additional plantation sites with higher rainfall in Tasmania and with a longer dry season in Western Australia. The Webb et al. (1984), Booth (1990) and Jovanovic and Booth (2002) climatic profiles all specified winter or uniform rainfall distribution as a requirement for climatically suitable conditions. P. radiata was introduced into the summer rainfall environment of China in the 1970s and 1980s when seeds became available from a seed exchange program with other countries. Seedlings were planted in several species introduction trials across the warm temperate and subtropical climate zones (Wu, 1983; Sun and Huang, 1987; Xu, 1987; Yin and Deng, 1992; Pang et al., 1995; Yang et al., 2000; Tang et al., 2004a). The reported results of early growth were mixed: some were poor and others promising. Seedlings at many sites did not grow up to trees. Such failures largely went unreported but were known to forest researchers in China. Damp summer heat and its induced disease and pest attacks were considered to be the major causes of failure (Wu, 1983). These results led to a general view that P. radiata was not suitable to most areas in China due to its typical continental climate with summer rainfall. One exception is the dry river valley area in Aba prefecture of Sichuan Province in southwest China, where P. radiata was first planted in the early 1990s (Bi et al., 2003; Wu et al., 2005). This area lies along the upper reaches of the Minjiang River, one of the four principal tributaries of the Yangtze River. High and steep mountain peaks towering 1500–3500 m above the deep river valleys are the prominent features of the local topography. Much of this area was covered by forests 600–700 years ago (Shi and Yong, 2001; Fan and Zhang, 2002; Sun et al., 2005; Ye et al., 2002). Repeated disturbances in the distant past and destructive exploitation of the forest resources in more recent times have degraded a large part of this area (Li, 1990; Bao and Wang, 2000; Fan and Zhang, 2002; Ye et al., 2002). Now arid climate and the easily erodible soils on steep and often unstable slopes form a vulnerable arid and semi-arid dry river valley ecosystem covering a total area of more than 150,000 ha. The degraded ecosystem has long passed the threshold of irreversibility in terms of its structure and

function (cf. Aronson et al., 1993; Liu et al., 2003). Native vegetation is limited in coverage and diversity, and difficult to re-establish on the slopes that have been deforested and severely degraded by repeated human disturbances through history (Winkler, 1996; Mykra¨ and Salo, 2000). Soil erosion, landslides and debris flows threaten the livelihood of surrounding communities already impoverished by the degraded lands, damage roads and bridges and often block traffic for hours even days in the more severe cases (Xie et al., 2004; He et al., 2005). The estimated sediment delivery from this area is between 1000 and 5000 t km
2 year
1 (Lu et al., 2003). From a sub-catchment area of 30,661 km2 gauged at Pengshan, about 10.3 Mt of sediment is carried away by the Minjiang river each year on average (Higgitt and Lu, 2001). The sediment delivery poses a problem to the newly completed 156-m-high Zipingpu hydro-electric dam at the lower end of the dry river valley and the Three Gorges Dam further down stream (Fan et al., 2006). To reduce the extent and magnitude of soil erosion and the consequent social economic impacts, this area has been identified by the Chinese government for afforestation. Over the past 20 years a number of native as well as exotic species have been tested in planting trials in this area. Among these species P. radiata had the lowest mortality and the best growth rates during early establishment (Bi et al., 2003), while native forest species were difficult to establish. Subsequently, several experimental plantations of P. radiata with a total area of more than 120 ha had been established by year 2000 (Bi et al., 2003). An expansion of the planting is being planned and implemented by the provincial and prefectural governments to cover much of the degraded area as a part of the shelterbelt forests development program along the upper reaches of Yangtze river (Anonymous, 1995; Li, 2004). Compared with countries where commercial plantations of P. radiata are successfully grown, the climatic conditions at the planting sites in Aba are largely within the range of climate required by P. radiata, apart from summer rather than winter or uniform rainfall seasonality and the minimum temperature in winter (Bi et al., 2003). The low temperature extremes render this area unsuitable for P. radiata according to the climatic profiles developed by Webb et al. (1984), Booth (1990) and Jovanovic and Booth (2002) for commercial plantations. However, climatic requirements for successful commercial plantations are not necessarily the same as those for environmental plantings with the objective of reducing soil erosion and those for species survival within its native range (Rogers, 2002). To identify climatically suitable areas for environmental plantings in southwest China, the climatic profile based on successful commercial plantations will have to be modified. The objective of this paper is to identify climatically suitable areas for environmental plantings of P. radiata in southwest China in general and in the Minjiang dry river valley area in particular. Since soil and water conservation rather than timber yield are of the primary concern for environmental plantings in this region, the climatic profiles of Booth (1990) and Jovanovic and Booth (2002) are modified to include lower temperature

H. Yan et al. / Forest Ecology and Management 234 (2006) 199–208

and rainfall extremes based on the climatic conditions of the native range of P. radiata and on that of the already established plantation sites in the dry river valley area. Data from meteorological stations for climatic variables in the climatic profile are interpolated spatially for the target areas for climate matching at each grid cell according to the climatic requirements of P. radiata. The suitable areas indicate the potential for environmental plantings of P. radiata in southwest China and they will also add to our understanding of the fundamental climatic niche and the potential geographical range of P. radiata. 2. Methods 2.1. Constructing a climatic profile for environmental plantings of P. radiata The climatic profile of a species describes its climatic requirements or tolerances through a number of climatic factors. It usually takes the form of a multivariate summary of the rainfall regime and the lower and upper limits of the climatic factors that are thought to be of biological significance for its survival and growth (e.g., Jovanovic and Booth, 2002). Six climatic factors (mean annual rainfall, rainfall regime, dry season length, mean maximum temperature of hottest month, mean minimum temperature of coldest month, mean annual temperature) have been used to develop the climatic profiles for P. radiata (Webb et al., 1984; Booth, 1990; Jovanovic and Booth, 2002). These climatic profiles are based on the climatic

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conditions at a large number of ex situ plantation sites of P. radiata that share a common winter rainfall regime (Table 1). Therefore, they describe the climatic requirements for successful commercial plantations, but do not necessarily represent the climatic tolerances of P. radiata. According to these climatic profiles, none of the already established plantation sites in Aba is suitable for P. radiata because of the rainfall seasonality and the extreme minimum temperature in winter. Even the climatic conditions of its native range are deemed unsuitable for P. radiata because it has not proved commercially successful under these conditions (Booth, 1990). Since soil and water conservation rather than commercial timber yield are of the primary concern for the environmental plantings in Aba (Bi et al., 2003), a new climatic profile is needed to better match the climatic requirements of P. radiata. For this purpose, the climatic conditions of its native habitats as well as that of ex situ plantation sites need to be taken into account. Compared with climatic requirements for commercial plantations, the realised climatic niche of P. radiata in its native range is narrower, but has lower precipitation and temperature extremes. Meteorological data relating to the areas of natural P. radiata forest on the mainland show that mean annual precipitation ranges from about 400 to 800 mm, with a minimum annual rainfall of 200 mm and maximum annual rainfall about 1000 mm (Lindsay, 1937; Lavery and Mead, 1998; Rogers, 2002). The annual precipitation of Guadalupe and Cedros islands is even lower, only about 150 mm, and trees there rely on crown interception of sea fogs for additional

Table 1 Climatic tolerances of P. radiata in its natural range and some major production regions (adapted from Lewis and Ferguson, 1993). Climatic conditions for the planted sites in the dry river valley area of Aba prefecture are also listed for comparison Region

Mean annual precipitation (mm)

Mean annual temperature (8C)

Mean temperature of coldest month (8C)

Mean temperature of hottest month (8C)

Absolute minimum temperature (8C)

Natural habitats

420–700

13–15

10–11

16–18


7

Australia Bathurst Tumut Mt Gambier

650–950 800–1300 650–800

11–13 11–14 13–14

0.4–0.6 0.5–0.8 4–6

24–28 25–30 25–28


7 to
9
7 to
10
2 to
4

1300–1500 1300 700–850 960–1000

10–13 10.5 11–12 8–10

7–9 4.6 5–6 3–5

11–19 15.7 15–17 13–15


6 to
9
9.4
5 to –7
9 to
10

906–1095

9–12

10–13

20–24


3

Chile Coastal cordillera Northern part Central part Southern part (Valdivia)

450–950 1300 2349

13–15 12–13 11.9

10–12 8–10 7.7

16–18 16–17 17.0

6–8 4–6 4.5

Central valley Central part Southern part (Temura)

1000–1300 1403

13–14 11.9

7–8 7.7

20–21 17.0

3–5 4.1

490–590

15–18


3.4 to
0.7

25–28


8.6 to
11.6

New Zealand Kaingaroa Nelson (Golden downs) Canterbury Southland South Africa Cape Province

China Aba, Sichuan

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moisture supply (Libby et al., 1968; Lavery and Mead, 1998; Rogers, 2002). The islands also have lower temperature extremes than the mainland areas. Moderately severe winter conditions sometimes with frost and heavy snow falls are not rare event on Guadalupe Island (Libby et al., 1968). The climatic conditions in the various P. radiata plantation regions of the world present a broader fundamental climatic niche than the realised one in its natural range. The annual range of temperature represented by the difference of mean monthly temperature between the hottest and coldest months is 6–7 8C in the natural habitats (Table 1). Outside the native range, it varies from 6 8C in Chile to more than 25 8C in Australia. The extreme minimum temperature is about
7 8C in the natural habitats, while it is about
10 8C in New Zealand and
11.6 8C in Aba, China. Successful commercial plantation in Southern Hemisphere is generally limited to the temperate zone and those subtropical regions where there is limited incidence of damp-heat in summer, or where there is small incidence of low temperature extremes in winter and untimely frost (Lewis and Ferguson, 1993). Extremes of minimum temperatures, rather than average minima, are the major reason for the species not finding a niche in most Northern Hemisphere countries within the suitable latitudinal range with adequate rainfall (Lavery and Mead, 1998). China has a typical continental climate dominated by monsoons. The advance and retreat of the monsoons account to a large degree for the timing of seasonal change in temperature and rainfall throughout the country. Northern winds coming from high latitude areas in winter are cold and dry, while southern winds from the ocean at low latitude in summer are warmer and moister. This monsoonal climate brings about a clear seasonal difference in precipitation and a large seasonal range of temperature over most of the country. Within China, the differences in latitude, longitude and altitude give rise to variations in climatic conditions between regions. A large portion of southwest China is mountainous since it is located in the mountain ranges of the eastern Himalayas. Due to the mountainous topography, climate variations are more pronounced with complex spatial patterns. The high elevation areas have a characteristically vertical environmental gradient and climate zones, with high mountain summits constantly under snow caps and lower mountain sides under a temperate climate. The dry river valley in Aba prefecture goes through a mountainous area within latitudes between 318 and 328N and

longitudes between 1008 and 1018E, where elevation ranges from 1000 m at the floor of the lower stretch of the valley to above 4000 m at the highest mountain summits. This area is administrated by three adjacent counties: Wenchuan, Li Xian and Mao Xian. It has a warm temperate climate with a reduced influence of monsoons due to its enclosed landform. Huge variations in microclimate exist in this area because of the range of elevation and diversity of topography. The arid and semi-arid environment that characterises the dry river valley is largely confined to areas with elevation below 2000 m, where the mean annual evaporation is more than three times of the mean annual rainfall. The lower stretch of the dry river valley in Wenchuan has a mean annual rainfall above 500 mm, a mean annual temperature of about 14 8C and about 260 frost-free days. The middle stretch in Li Xian is the most arid, and has a mean annual rainfall of about 370 mm, a mean annual temperature of about 13 8C and 255 frost-free days. The upper stretch in Mao Xian is the coldest. It has a mean annual rainfall of about 500 mm, a mean annual temperature of about 11 8C and 216 frost-free days. The extreme minimum temperature recorded in the county town is
11.6 8C. Over the entire course of the dry river valley, extreme minimum temperatures and moisture supply seem to be the limiting climatic factors for P. radiata. Therefore, the climatic profiles of Booth (1990) and Jovanovic and Booth (2002) are modified to include lower rainfall extremes and absolute minimum temperature as a limiting climatic factor for environmental plantings of P. radiata in southwest China and the dry river valley area (Table 2). The absolute minimum temperature has been recognised as a useful measure of frost risk also in other climate mapping programs (Booth et al., 1994, 2002). Since damp heat in summer is detrimental to the healthy growth of P. radiata (Lewis and Ferguson, 1993; Lavery and Mead, 1998), it would also be desirable to include humidity during the summer months as a climatic variable in the profile. However, climatic data incorporating humidity are not available. 2.2. Mapping climatically suitable areas for environmental planting of P. radiata Because of the complex landscape in southwest China, there is a high degree of spatial dependency of climate on topography. To estimate climatic conditions at any given location within this region, climate data from sparsely scattered and irregularly distributed meteorological stations have to be

Table 2 Comparative climatic profiles of Pinus radiata Author

Mean annual precipitation (mm)

Months with rainfall < 40 mm

Mean annual temperature (8C)

Mean minimum temperature of coldest month (8C)

Mean maximum temperature of hottest month (8C)

Webb et al. (1984) Booth (1990) Jovanovic and Booth (2002) This paper

650–1600 650–1600 650–1800

2–3 0–3 0–5

11–18 11–18 11–18

2–12
2 to 12
2 to 12

20–30 20–30 18–30

550–1600

0–5

11–18


2 to 12

20–30

Absolute minimum temperature (8C)


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interpolated. As the accuracy of climate interpolation depends critically on the calibration of spatially varying dependencies of climatic variables on topography, the underlying topographic surface has to be derived from digital elevation models (DEM) first. The digital elevation models not only provide a surrogate of three-dimensional geographic space but also determine the spatial resolution of interpolated climatic variables. Therefore, the choice of scale in DEM determines the resolution of the derived topographic surface and the accuracy of the interpolated climatic variables over it (Albani et al., 2004), which in turn affects the accuracy of predictive mapping of a species’ suitable climate space (Pearson and Dawson, 2003; Guisan and Thuiller, 2005). Two digital elevation models with different levels of resolution were used for climate interpolation at the national and regional scales. The DEM at national scale was regularly spaced at 0.18 (approximately 10 km), while the regional DEM had a finer resolution at 250 m for more detailed climate estimation of the dry river valley area. The topographic surface derived from each DEM was checked carefully to detect and remove the unreasonable sinks and peaks by using a hydrological function of geographic information system that made the DEMs more reliable for interpolation (Yan et al., 2005). Computer software (ANUSPLIN) developed by Hutchinson (1997) was used to fit climatic surfaces for each of the climatic variables in Table 2. The surfaces at national scale were developed previously using mean climate data in period of 1971–2000 from the Chinese national meteorological network (Yan et al., 2005). This dataset did not include regional climate data from Aba prefecture. Additional climatic data from meteorological stations of Aba were used for regional climate interpolation. Thin plate smoothing spline surfaces were fitted for each climatic variable to derive climatic surfaces at regional scale. The summary statistics of the derived climate surfaces indicated the mean errors of interpolation for temperature variables were generally below 0.7 8C. Such a magnitude of error of interpolation was considered acceptable for our purpose of climate matching. Therefore, two climate datasets at national and regional scales were generated in regular spacing grid format by coupling climatic surfaces with given DEMs. ArcInfo GIS was used to interrogate the two climate datasets according to the climatic requirements of P. radiata profiled for the environmental plantings in Table 2. The program used upper and lower limits of each climatic factor as input to evaluate the suitability of climate of each grid cell systematically. The number of climatic variables that fall within the range of climatic requirements of P. radiata was recorded for each cell and used to evaluate its climatic suitability. Cells with more climatic factors falling within the range of climatic requirements were deemed more suitable. Cells with all six climatic factors satisfying the climatic requirements of P. radaita are recognised as suitable and those with five as potentially suitable. This procedure enabled the identification of areas that matched the climatic requirements of P. radiata as profiled in Table 2.

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2.3. Identifying climatic factors influencing the spatial limit of the mapped area Since the procedure of climate matching gives equal weight to all climatic variables, the influence of each variable in determining the spatial limit of the mapped area cannot be isolated and identified. To evaluate such influence, the variables in the climatic profile are eliminated one at a time to allow the matched area to expand until it is limited by other climatic variables. The elimination of those climate variables that have a significant influence in determining the spatial limit of the mapped area will result in a substantial spatial expansion, while the release of other variables that have little influence will result in less change. A chi-square statistic is then calculated to evaluate the change in spatial expansion for each climate variable as follows: x2 ¼

ðN 0
N 1 Þ2 N1

where N0 is the number of matched pixels when the climate variable is eliminated and N1 is the number of matched pixels using all climate variables in the profile. This approach has been adopted by Cole and Arundel (2005) and Arundel (2005) to establish climatic limiters of plant species’ distributions. 3. Results The dry river valley in Aba prefecture locates in a mountainous area of transition between the edge of Sichuan basin and Qinghai-Tibet plateau. The topographic surface derived from the digital elevation model reveals more spatial detail about topographical features of this area than topographical maps (Fig. 1). It shows that a rapid rise of elevation from less than 1000 m at lowland areas to above 4000 m at the mountain peaks creates an extremely rugged terrain characterised by high mountains and deep valleys. The Min river flows through this area from northeast to southwest passing Mao Xian and Wenchuan, and the Zagunao river, one of the principal tributaries of Minjiang river, flows mostly eastward passing Li Xian. The steep and often unstable slopes on both sides of the river valley support little vegetation due to the arid and semi-arid environment induced by forest clearing during the last few hundred years, and the south-facing slopes are relatively drier. The climatic surfaces derived from climate interpolation provide data to develop climatic patterns at national and regional scales. The dry river valley area has a typical continental climate. As elsewhere in China, there is a large seasonal range of temperature due to the continental climate dominated by monsoons. The mean minimum temperature of the coldest month is as low as
3.4 8C, while the extreme minimum temperature was
11.6 8C. In contrast to the winter or uniform rainfall climatic pattern of the natural distribution of P. radiata and the major plantation regions in Southern Hemisphere countries, the dry river valley area has a synchronised summer rainfall pattern (Fig. 2). Precipitation

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Fig. 1. A topographical map of Aba prefecture and its location within Sichuan Province of China.

Fig. 2. Monthly pattern of mean precipitation (triangles), mean maximum temperature (squares) and mean minimum (diamonds) throughout a year in three comparative P. radiata growing regions.

Fig. 3. Areas potentially suitable for environmental plantings of P. radiata in southwest China as identified by climate matching at a national scale.

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Table 3 Rank of climatic variables in constraining extension of potential distribution of Pinus radiata Climatic variable

Mean maximum temperature of the hottest month Months with rainfall < 40 mm Mean annual precipitation Absolute minimum temperature Mean annual temperature Mean minimum temperature of coldest month

Chi-square statistic National scale

Regional scale

967768 41124 6982 1120 1103 139

0 0 4955 0 4 3576

Fig. 4. Areas potentially suitable for environmental plantings of P. radiata in the dry river valley area of Aba as identified by climate matching at a regional scale. The colour codes indicate the number of climatic factors that satisfy the climatic requirements of P. radaita.

increases with temperature in the first 6 months of the year and decreases with temperature in the latter half of the year, and so about 80% of the annual rainfall occurs in summer. The dry season lasts about 5 months from late autumn over winter to spring, longer than that of major P. radiata plantation regions in other parts of the world. However, the dry season is over winter when trees stop growing. When mean minimum temperature is above 10 8C, monthly rainfall is always more than 40 mm, providing better moisture conditions for tree growth than other P. radiata production regions during a dry season. Results from climate matching at national scale indicate that central Yunnan, southwest Guizhou and areas around the Sichuan basin are the regions within southwest China that are most likely to meet the climatic requirements of P. radiata for environmental planting at a national scale (Fig. 3). The total area identified as suitable is more than 266,000 km2 across the three provinces. Mean maximum temperature of the hottest month and the length of the dry season are the most influential factors limiting the spatial extent of climatic suitability for P. radiata in southwest China (Table 3). Most of the dry river valley area in Aba prefecture was identified as climatically unsuitable at this broad scale (Fig. 3). However, when the spatial resolution of climatic data is refined from 10 km to 250 m and additional climate data from Aba prefecture are used for climate matching at a regional scale, a total of 26,000 ha over the dry river valley area is identified as potentially suitable for environmental plantings of P. radiata (Fig. 4). The elevation

of most of these areas is below 1500 m (Fig. 4). The area lying between 1500 and 2000 m elevation is approximately 63,000 ha where only four climatic factors are within the climatic requirements of P. radatia. The major limiting factors are the mean annual rainfall and the mean minimum temperature of the coldest month associated with the rapid decline of temperature with elevation in most parts of the Aba prefecture. 4. Discussion A common belief among many forestry professionals is that P. radiata is not suitable to the warm temperate and subtropical summer rainfall climate in China. This belief is based on previously failed attempts at introducing P. radiata to China and also on climate matching of P. radiata as a plantation species at a global scale by Booth (1990). This study demonstrates that this view may need revision at least for environmental plantings aimed at reducing soil erosion. The central region of Yunnan Province, the southwest of Guizhou Province, the areas around the Sichuan basin and the dry river valley area of Aba prefecture are identified to be the regions within southwest China that are most likely to meet the climatic tolerances of P. radiata. The northwest of Hubei Province was not identified as climatically suitable, although it was suggested by some to introduce P. radiata there (Tang et al., 2004b). The results of

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climate matching for the dry river valley area in particular correspond well with the growth performance of experimental plantings in the field. Over the past 15 years, trial plantings of P. radiata with a total area of over 120 ha have been established over a range of sites: at elevations of 800–2000 m, slope of 10–308, rainfall of 400–800 mm, evapotranspiration of 1000–1800 mm, mean annual temperature of 8–13.2 8C, extreme maximum temperature of 32 8C, extreme minimum temperature of
11.6 8C and soil with pH of 6.0–8.0 (Bi et al., 2003). The stocking has been set at more than 3000 trees/ha for all areas. During winter in January 2005 the dry river valley area experienced the coldest spell in 30 years, and the absolute minimum temperature reached
11.9 8C and lasted for 4 days. Apart from sporadic death of the newly planted seedlings at a provenance experiment site in Mao Xian, no mortality was observed among the established trees. In a 6-year-old stand at elevation 1000 m in Wenchuan, the average DBH (diameter overbark at breast height of 1.3 m) reached 5.2 cm and the average tree height reached 4.1 m. A similar growth performance was observed for a site at elevation 2000 m in Li Xian. The oldest stand was at Xiao Gou, a site at elevation 1750 m in Mao Xian. When a growth plot of 34 trees was measured at age 14, the average DBH was about 17.3 cm and the average tree height was about 10 m (Wu, personal communication). The growth of P. radiata at the sites in the dry river valley area appear to be better than that on poor sites in its native habitats where tree height in mature stands was observed to be in the range of 12–22 m (Lindsay, 1937). By commercial standards, the growth rates are low. For commercial plantations, site index (defined as mean height of 75 tallest trees/ha at age 20) less than 20 m is deemed poor in productivity (Lewis and Ferguson, 1993). The comparatively low growth rates are caused partly by the stocking rates being too high particularly in a semi-arid environment. However, this growth performance is still much superior to that of the native trees of both coniferous and broad leaf species including Cupressus chengiana, P. tabulaeformis, Ailanthus altissima, Pistacia chinensis, Prunus armeniaca and Gleditsia sinensis, as shown by planting trials of Aba Forest Research Institute. The best performing native species was the native conifer, P. tabulaeformis and its average diameter did not reach 6 cm and average tree height was below 3 m at age 9–10 years (Bi et al., 2003). Mean total stand biomass of 5-year-old P. radiata plantations were found to be 19.5 t/ha, about 12 times greater than that of plantations of native conifer P. tabulaeformis and C. chengiana at the same age (Pan et al., 2005). The comparative growth performance of P. radiata makes it the species of choice for environmental plantings over many degraded lands to reduce soil erosion in southwest China (Wu et al., 2005). In a similar case, P. radiata was also found to be a species capable of growing in eroded soils over much of the degraded land in Chile where native forest species of economic importance are difficult to establish (Toro and Gessel, 1999). There are generally two types of error associated with bioclimatic modelling to identify a species’ climatic space and to predict its distribution: false negatives, i.e., omission error and false positives, i.e., commission error (Iverson and Prasad,

1998; Anderson et al., 2003; Guisan and Thuiller, 2005). In the case of climate matching for tree species introduction, an omission error may potentially lead to a loss of planting opportunity for it fails to identify where a species can grow, while a commission error may eventually lead to a waste of effort since it predicts species success where it cannot grow. These errors are inevitable in predictive mapping because of the complexity of the natural system and the limitations of climate modelling that is based on correlation between observed species distributions and current climate variables (Pearson and Dawson, 2003). However, the error rates are dependent upon the scale, data quality and resolution in the analysis because there is a hierarchy of factors operating at different scales in determining a species’ suitable climate space (Willis and Whittaker, 2002; Pearson and Dawson, 2003; Guisan and Thuiller, 2005). At global and continental scales, climate is the dominant factor, while at regional and landscape scales, topography become increasingly important (Pearson and Dawson, 2003). The climate matching of P. radiata at a global scale by Booth (1990) and that at national scale for China in this paper were both performed with coarse data resolutions and data density, and both showed omission errors. This study demonstrates that the choice of scale, resolution of DEM, frequency and quality of climate station data have to be appropriate to avoid such omission errors in climate matching for any area with such topographical complexity as the dry river valley area of Aba. A winter or uniform rainfall regime is the common climatic feature of the natural habitats of P. radiata and most major production regions in the Southern Hemisphere summarised in Lewis and Ferguson (1993). For instance, plantations with a summer rainfall regime constituted only less than 2% of the total area of P. radiata plantation in Australia and nil in New Zealand (Turner et al., 2001). The temperate maritime climate to which P. radiata as an exotic is well suited in the major productions regions incorporates a relatively dry summer (Lavery, 1986). A winter or uniform rainfall regime is also a common climatic factor in the three climatic profiles constructed by Webb et al. (1984), Booth (1990) and Jovanovic and Booth (2002). The lack of a strongly four seasons Mediterranean climate has been recognised as having a deleterious effect on P. radiata in terms of the development of pathogen loads (Lavery and Mead, 1998). The identified areas for P. radiata in southwest China have a continental climate with a typical summer rainfall regime (Fig. 3). However, humidity is not high in the summer season especially in the dry river valley area of Aba. So these areas are not constrained by damp summer heat that has caused pathogen problems in Africa and South America (Lavery and Mead, 1998) and has limited the use of P. radiata in the subtropical, summer rainfall area of northern NSW and southern Queensland in Australia (Wright and Marks, 1970). Our results suggest that the climatic niche of P. radiata should not completely exclude the summer rainfall regime that dominates the temperate climate in China. Thus, a relatively dry summer may need to be added to the criteria for winter and uniform rainfall regime in the climatic profile for P. radiata.

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Our climatic profile for the environmental plantings of P. radiata in Table 2 is within its climate space as we currently understand, although with lower rainfall extremes and absolute minimum temperature as a limiting climatic factor than that specified in other climatic profiles by Webb et al. (1984), Booth (1990) and Jovanovic and Booth (2002). Guidelines for site selection for commercial afforestation with P. radiata usually include a minimum annual rainfall in the range of 600–700 mm (Lavery, 1986). Mean annual rainfall in most major P. radiata productions regions is above 650 mm. However, there are some areas of P. radiata with less than 600 mm mean annual rainfall in the northern part of the coastal cordillera of Chile (Table 1). Besides large-scale commercial forestry, P. radiata has been used in areas with less than 600 mm of mean annual rainfall for farm forestry in Tasmania (Private Forests Tasmania, 2004) and with less than 500 mm for land rehabilitation in the wheat belt of Western Australia (Bell and Adams, 2004). The rainfall in these areas is comparable with the lower mean annual rainfall limit of 550 mm in our climatic profile. Among the major production regions of P. radiata, the absolute minimum temperature ranges from
9 to
10 8C in parts of Australia and New Zealand (Table 1). Field experiment showed that frost tolerance of P. radiata seedlings ranged from
6 8C in the summer to
14 8C in the winter (Menzies and Chavasse, 1982). The absolute minimum temperature of
11 8C in our climatic profile is well within the range of observed frost tolerance of P. radiata seedlings and established plantations. Identifying the suitable areas for a tree through climate matching is only the first step in species introduction. Within the identified climatic niche, growth response will also vary along gradients of other environmental factors. Since damp heat in summer is detrimental to the health of P. radiata (Lewis and Ferguson, 1993; Lavery and Mead, 1998), humidity in the hottest summer months may place an additional constraint on the climatic suitability of P. radiata in southwest China. In addition to suitable climatic conditions, the successful establishment and growth of P. radiata in the mapped areas of China will depend upon edaphic factors. A predictive model of growth response along environmental gradients within the identified climatic niche will further aid the evaluation of potential planting sites. Such a model can be developed by adapting the approach to modelling species distribution and species response to environmental gradients in the ecological literature (e.g., Austin, 1992, 2002; Austin et al., 1994). To do so, a substantial amount of growth data, currently not available, will be required. Therefore, it is necessary to establish field plots in the scattered plantations in Aba to accumulate growth data over time. At present the mapped areas can only help define the working limits and serve as indicative zones for the environmental plantings of P. radiata in China. Site-specific evaluation of humidity in the hottest summer months and edaphic factors is still needed for planning planting programs within the mapped area. Current plantings in the dry river valley area in Aba provide no information at the provenance level, although there is a large amount of genetic variation in P. radiata in terms of growth rate, form, disease resistance, drought tolerance and resistance to frost

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damage (Eldridge, 1997). Improved provenance-site matching will require more detailed information on both tree growth and site conditions. The provenance experiments currently undertaken over three sites in the dry river valley area will hopefully provide data in the future for more detailed climate and site mapping for environmental plantings of P. radiata in China. Acknowledgements This work is a part of the project ‘‘Assessment of the potential of P. radiata for ecological restoration of the Yangtze river catchment in Aba Prefecture, Sichuan, China’’ sponsored by the Australian Centre for International Agricultural Research (ACIAR), State Forests of New South Wales, Australia, Chinese Academy of Forestry and Department of Forestry, Sichuan Province, China. References Anderson, R.P., Lew, D., Peterson, A.T., 2003. Evaluating predictive models of species’ distributions: criteria for selecting optimal models. Ecol. Model. 162, 211–232. Anonymous, 1995. Forestry Action Plan for China’s Agenda 21. Ministry of Forestry, Beijing, People’s Republic of China, 110 pp. Albani, M., Klinkenberg, B., Andison, D.W., Kimmins, J.P., 2004. The choice of window size in approximating topographic surfaces from digital elevation models. Int. J. Geogr. Inf. Sci. 18, 577–593. Aronson, J., Floret, C., Le Flo’h, E., Ovaller, C., Pontanier, R., 1993. Restoration and rehabilitation of degraded ecosystems in arid and semi-arid lands I. A view from the south. Restoration Ecol. 1, 8–17. Arundel, S.T., 2005. Using spatial models to establish climatic limiters of plant species’ distributions. Ecol. Model. 182, 159–181. Austin, M.P., 1992. Modelling the environmental niche of plants: implications for plant community response to elevated CO2 levels. Aust. J. Bot. 40, 615– 630. Austin, M.P., 2002. Spatial prediction of species distribution: an interface between ecological theory and statistical modelling. Ecol. Model. 157, 101–118. Austin, M.P., Nicholls, A.O., Doherty, M.D., Meyers, J.A., 1994. Determining species response functions to an environmental gradient by means of a bfunction. J. Veg. Sci. 5, 215–228. Bell, T.L., Adams, M.A., 2004. Ecophysiology of ectomycorrhizal fungi associated with Pinus spp. in low rainfall areas of Western Australia. Plant Ecol. 171, 35–52. Bao, W.K., Wang, C.M., 2000. Degradation mechanism of mountain ecosystem at the dry valley in the upper reaches of the Minjiang River. J. Mount. Sci. 18, 57–62 (in Chinese with English title and abstract). Bi, H.Q., Simpson, J., Li, R.W., Yan, H., Wu, Z.X., Cai, S.M., Eldridge, R., 2003. Introduction of Pinus radiata for afforestation: a review with reference to Aba. J. For. Res. 14, 217–222. Booth, T.H., 1990. Mapping regions climatically suitable for particular tree species at the global scale. For. Ecol. Manage. 36, 47–60. Booth, T.H., Jovanovic, T., New, M., 2002. A new world mapping program to assist species selection. For. Ecol. Manage. 163, 111–117. Booth, T.H., Jovanovic, T., Yan, H., 1994. Climatic analysis methods to assist choice of Australian species and provenances for frost-affected areas in China. In: Brown, A.G. (Ed.), Australian Tree Species Research in China, Australian Centre for International Agricultural Research, Canberra, Australia, ACIAR Proceedings No. 48, pp. 26–31. Cole, K.L., Arundel, S.T., 2005. Modeling the Climatic Requirements for Southwestern Plant Species. In: Starratt, S., Bloomquist, N. (Eds.), Proceedings of the Twenty First Annual Pacific Climate Workshop. Technical Report 77. State of California, Interagency Ecological Program for the San Francisco Estuary.

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