Abstract

Davidia involucrata is a rare endangered species belonging to the paleo-tropical flora of the Tertiary period, endemic to China, and has an important protection and ornamental value. However, the natural populations and distribution areas of this ancient plant have been decreased sharply, because it suffers much disturbance from human activity in many regions. In this paper, data on Davidia involucrata forest in its most natural distribution areas were collected; the percentages of geographical areal types of seed-plant genera (Sensu Wu 1991) were summed; the community types were summarized using Braun-Blanquet methodology; and the geographical differentiation among communities was analyzed by detrended canonical correspondence analysis. The analysis of geographical types shows that temperate species are dominant and that tropical species also make up a high percentage. This indicates that Davidia involucrata forest is mainly temperate vegetation. In total, 16 communities were recognized by classification, and these communities were usually dominated by deciduous broad-leaved species, sometimes mixed with some evergreen species. The shrub layer of Davidia involucrata forest is generally occupied by bamboos. The geographical patterns among the communities are influenced mainly by temperature and moisture availability, and the forest is most likely to grow in warm, humid habitats.

Keywords: China, Community, Classification, Phytogeographical Areal Type, Davidia involucrata

1. Introduction

Davidia involucrata is a deciduous broad-leaved tree species in the monotypic family Davidiaceae [1] . It occurred widely in many areas of the world in the late Tertiary, but natural Davidia involucrata has now survived only in mountain areas of mainly south-central China (Sichuan, Hubei, Hunan, Guizhou, Yunnan, Chongqing city, and southernmost Gansu and Shaanxi), which were influenced only weakly by the Quaternary glaciations [1] -[3] . This species is well known as a Tertiary relict plant and “botanic living fossil”; it also has small but beautiful flowers with two pure white bracts [4] [5] . Since Davidia involucrata was introduced from China to Europe and North America in 1904, it has gradually become a popular ornamental tree species in parks and larger gardens [6] -[8] . However, the natural distribution range of Davidia involucrata forests is being reduced rapidly, because these suffer much disturbance from human activity in many regions [9] [10] . Therefore, protection for the natural populations and forests of Davidia involucrata is needed.

Since the 1970s, Davidia involucrata has been widely studied in many fields such as anatomy, genetics, biology, ecology, introduction cultivation [11] -[14] , etc. Synecological study of Davidia involucrata forest has also been carried out, and community types in some areas have been classified [15] -[18] . However, these studies of Davidia involucrata forest are still local and incomplete, and synthetic, systematic phytosociological reviews are rarer.

Wu classified the Chinese genera of seed plants into 15 areal types according to their contemporary geographical distribution in the world [19] . The geographical areal types include 6 tropical types, 7 temperate types, 1 cosmopolitan type and 1 type for endemics to China. Areal types represent a concept described and is widely used in Chinese vegetation literature, because the proportions of tropical types and temperate types can indicate the forest property. In this paper, we try to summarize community types of Davidia involucrata forest, using the Braun-Blanquet methodology, and to describe characteristics of the communities and analyze their different geographical distributions. Our goal is to provide synthetic, systematic phytosociological data for protecting this relic species resource in China.

2. Material and Methods

2.1. Study Area

The study sites comprise most of the distribution areas of natural Davidia involucrata forests in south-central China, from 102˚25'E to 111˚11'E and 26˚57'N to 31˚30'N. Usually, the climate of the study area is hot in summer and cold in winter, the duration of sunshine is from 1200 h to 1400 h, the mean annual temperature is 8˚C - 16˚C, the mean temperature of the coldest month (January) is 0.4˚C - 3.6˚C, and the mean temperature of the warmest month (July) is 18.4˚C - 22.5˚C. Also, the study areas are rainy and foggy, the mean annual rainfall varies from 600 mm to 2600 mm, and the average relative humidity is more than 80%. The altitude range of the Davidia involucrata forests is from 600 m to 3200 m above sea level. The main rock type is sandstone in some areas and slate in other areas. The predominant soil types in the study regions are mountain yellow soil and mountain yellow-brown soil.

2.2. Data Collection

As a rare endangered species, Davidia involucrata occurs in relatively few extant forests, but the range of its geographical distribution is wider. In this research, vegetation data were obtained from relevés made by the authors according to Braun-Blanquet methodology [20] and from published literature (see Table 1, Figure 1). All constituent species of Davidia involucrata forests were recorded in detail in our relevés, but the data collected from literature did not include all species appearing in herb layer. In Hubei, only some relevés followed the full-floristic methodology. Therefore, the omitted species were not included in this study.

2.3. Analysis

The floristic composition of Davidia involucrata forest was classified according to the geographical areal types of Chinese genera established by Wu [19] , and the numbers and proportions of seed plants (including species and genera) in each geographical areal type were computed. Also, in order to explore the geographical properties of Davidia involucrata forest, we summarized the 15 geographical areal types into four types (i.e. cosmopolitan, tropical, temperate and Chinese endemic), and counted their numbers and proportions.

The original data were classified (qualitatively and quantitatively) according to species abundance, at each site, by the tabular comparison method [34] , and the number of tree species was used to determine the types of Davidia involucrata communities. The herb species were not used to distinguish community types, because

many herbs in samples from published literature had been omitted by the authors. For the communities with fewer than 5 samples, the constancy values of constituent species are represented by the number of samples in which each species was present; for those with 5 or more samples, the constancy values of the species are represented by the percent frequency classes for the samples in which the species was present (i.e. r: 0% - 5%, +: 6% - 10%, I: 11% - 20%, II: 21% - 40%, III: 41% - 60%, IV: 61% - 80%, V: 81% - 100%). All samples, from field data and literature, are summarized in a synoptic table (Table 2).

Note: the other species were omitted.

Detrended canonical correspondence analysis (DCCA, Canoco for Windows 4.5) was used to identify patterns of environmental variation among communities. The values of seven environmental factors were determined for all communities, namely longitude (LO), latitude (LA), altitude (AL), aspect (AS, on an eight-point scale: 1 = N; 2 = NE; 3 = NW; 4 = E; 5 = W; 6 = SE; 7 = SW; 8 = S), slope (SL), average annual temperature (AT) and average annual precipitation (AP). Square root-transformed percentage frequencies were used as the input species data. The importance values for species were calculated using the following formula [35] :

where IV_ijk is the importance value of species i, O_ijk is percent cover of species i in layer k of community j with a total of n species, and L_jk is the projected cover (in percent) of layer k of community j.

3. Results

3.1. Plant Floristic Characteristics

In this study, a total of 394 species was recorded in Davidia involucrata forest, including 382 species of seed plants, 10 fern species and 2 moss species. The seed plants belong to 88 families and 213 genera. The families with more than 10 species are Lauraceae (7 genera and 30 species), Rosaceae (10 genera and 25 species), Saxifragaceae (12 genera and 24 species), Liliaceae (12 genera and 20 species), Caprifoliaceae (5 genera and 19 species), Fagaceae (5 genera and 14 species), Aceraceae (2 genera and 14 species) and Gramineae (11 genera and 13 species).

The statistical results for the geographical areal types show that, except for the Central Asian type, all the other 14 types appeared in Davidia involucrata forest (Table 3). The plant genera belonging to the north-temperate distribution type showed the highest proportion (23.47%) of all distribution types. Other areal types with

more genera are the East Asian type (14.55%), East Asia-North America disjuncts (12.68%), Chinese endemics (10.80%) and pantropical (10.33%). Temperate genera are most common (55.39% of all genera) followed by tropical genera (25.83%), cosmopolitan genera 10.80% and genera endemic to China 7.98%. North-temperate species were most numerous (126 species), followed by East Asia-North America disjuncts (48 species), East Asian species (39 species), and pantropic species (38 species). Thus, Davidia involucrata forest has the properties of temperate forests but is associated closely with tropical floristics. In addition, most genera contain more than one species, the average being 1.8. The ratio of species to genera among “tropical Asia-tropical America disjuncts” is 4.2, indicating that interspecific differentiation among the constituent species of Davidia involucrata forest is significant.

3.2. Community Description

The 72 samples of Davidia involucrata forest were summarized as 16 communities, and the characteristics of these communities are quite different (see Table 2).

1) Yushania suijiangensis-Davidia involucrata community (Community A)

The stands of this community appear on north-facing slopes at an average altitude of 1855 m in Yiliang County of northeastern Yunnan [21] . This community is characterized mainly by species such as Yushania suijiangensis, Sycopsis triplinervia, Acanthopanax evodiaefolius var. gracilis, Polystichum longipaleatum and Aulacolepis treutleri. Stratification of this community shows four layers, and mean cover of the canopy is about 60%. The tree layer reaches 10 - 17 m and is dominated by Davidia involucrata, Acer oliverianum, Tetracentron sinense, Rehderodendron macrocarpum, Enkianthus chinensis, Acanthopanax evodiaefolius var. gracilis, etc. The shrub layer attains 1.4 m, is dominated by Yushania suijiangensis, and there are relatively few individuals of the companion species. The herb layer has a mean cover of 60% and is often occupied by some shade-tolerant species such as Aulacolepis treutleri and Polystichum longipaleatum.

2) Chimonobambusa tumidissinoda-Davidia involucrata community (Community B)

Stands of this community are situated on slight slopes at altitude from 1530 m to 2039 m in northeastern Yunnan [21] . The differential species of this community include Chimonobambusa tumidissinoda, Camellia pitardii, Athyrium iseanum, Elatostema nasutum and Allantodia crenata. This community consists of four layers, and the cover of tree layer reaches 60% - 85%. The tree layer attains 10 - 25 m and is constituted mainly by Davidia involucrata, Carpinus fangiana and some evergreen trees such as Castanopsis platyacantha, Camellia pitardii, Symplocos sumuntia and Symplocos lucida. The shrub layer attains 3 m and dominated mainly by Qiongzhuea tumidissinoda. The herb layer is affected by the upper layers and has cover ranging from 3% to 85%, with predominant species Elatostema nasutum, Impatiens scutisepala, Athyrium iseanum and Allantodia spp.

3) Rhododendron ririei-Davidia involucrata community (Community C)

This community is located on a 20˚ northwest-facing slope at an average altitude of 1740 m, on Emei-Shan mountain of Sichuan [22] . The differential species include Padus napaulensis, Machilus longipedicellata, Cornus wilsoniana, Camellia yunnanensis, Styrax roseus, Rhododendron ririei, Cunninghamia lanceolata, Acer robustum, Prunus sericea and Litsea elongata. Stratification of this community can be divided into three tree layers, one shrub layer and one herb layer. The canopy reaches an average height of 22.5 m, with an average cover of 75%. The main species of the tree layers are Davidia involucrata, Tetracentron sinense, Litsea elongata, Juglans cathayensis, Prunus napaulensis, Machilus longipedicellata and Castanopsis platyacantha. The shrub layer attains 2.5 m with a cover of 70%, and is composed by the single species Sinarundinaria fangiana. The herb layer reaches 0.4 m, and is dominated by Pilea sinofasciata. The species composition of this community is relatively rich, and 70 species were identified.

4) Fargesia spathacea-Davidia involucrata community (Community D)

The stands of this community occur at altitude 1620 m to 2010 m, on steep slopes (25˚ to 37˚), in the Labahe nature reserve of Sichuan [23] . The differential species are Lindera limprichtii, Fargesia spathacea, Parathelypteris glanduligera, Salix clolia and Litsea populifolia. The structure of this shorter forest generally consists of only three layers, but the cover of the tree layer attains 70% - 85%. The tree layer reaches 7.2 m, and the most constant species are Davidia involucrata, Lindera limprichtii, Viburnum betulifolium, Phoebe sheareri, Litsea populifolia, Cornus controversa, Salix clolia, Juglans cathayensis and Betula albo-sinensis. The shrub layer includes few individuals and is usually composed by Lindera limprichtii, Juglans cathayensis, Fargesia spathacea and Cornus controversa. The herb layer has an average height of 0.25 m, and the main species are Parathelypteris glanduligera, Arthraxon hispidus, Actinidia chinensis, Akebia trifoliata, Cyperus sp. and Smilax sp. The average stand richness reaches 42 species.

5) Maddenia wilsonii-Davidia involucrata community (Community E)

Sites of this community are on gentle slopes of 15˚ to 30˚ facing northeast, southeast and northwest, at altitude 2000 m to 2100 m, within the area of Chengdu city of Sichuan [24] . The diagnostic species of this community are Maddenia wilsonii, Hydrangea xanthoneura, Acer maximowiczianum, Acer davidii, Pilea martini, Litsea moupinensis var. szechuanica, Sinarundinaria fangiana, Ligularia dentata, Padus obtusata, Schisandra rubriflora, Hydrangea anomala and Chrysosplenium davidianum. Stratification usually shows four layers. The upper canopy attains 18 m, and is dominated by Davidia involucrata, rarely mixed with Cryptomeria fortunei and Tilia nobilis. The understory tree layer reaches 12 m, and the main species include Maddenia wilsonii, Hydrangea xanthoneura, Acer maximowiczianum, Litsea moupinensis var. szechuanica, Meliosma cuneifolia, Acer davidii and Ilex pernyi. The height of the shrub layer is 1 - 3 m, and the predominant species are Viburnum betulifolium, Symplocos lucida, Hydrangea anomala, Schisandra rubriflora, Philadelphus delavayi, and Smilax spp. The herb layer attains 0.8 m with a cover of 65% - 80%, and the main species are Sinarundinaria fangiana, Pilea marthnii, Ligularia dentata, Chrysosplenium davidianum, Polystichum hecatopteron, etc. The average number of species per sample was 48.

6) Cerasus dielsiana-Davidia involucrata community (Community F)

This relatively tall forest community is situated at altitude 1500 m to 1800 m in Wolong nature reserve of Sichuan [25] . The diagnostic species of this community are Cerasus dielsiana, Rubus chingii, Adiantum capillus-veneris, Mahonia fortunei, Arisaema erubescens, Hemarthria compressa, Paris thibetica, Ranunculus ficariifolius, Paeonia lactiflora, Polygonatum odoratum, Polystichum tsus-simense, Pilea plataniflora and Ophiopogon bodinieri. Stratification of the community can be divided into three tree layers, one shrub layer and two herb layers. The upper canopy reaches 25 m and consists of Davidia involucrata and Juglans cathayensis; the upper understory tree layer reaches 15 - 20 m and is dominated by Davidia involucrata and Cornus controversum; and the lower tree layer is 5 - 15 m and is composed by Davidia involucrata, Juglans cathayensis, Cornus controversum, Symplocos botryantha, Phoebe neurantha, Ilex pernyi, Lindera pulcherrima var. hemsleyana, etc. The individuals of the shrub layer are fewer, and the main species are Davidia involucrata, Phoebe neurantha, Rubus palmatus, Symplocos botryantha, Ilex pernyi, Acer oliverianum, etc. The herb layer attains 0.6 m, and the dominant species are Oxalis griffithii, Polystichum tsus-simense, Ophiopogon bodinieri, etc. The average number of species per sample reaches 67.

7) Pterocarya insignis-Davidia involucrata community (Community G)

This open community occurs on south-facing slopes of 22˚ to 28˚ at altitudes of 1525 m to 1670 m in the Wolong nature reserve of Sichuan [26] . The community can be distinguished by the species Pterocarya insignis, Symplocos anomala and Aristolochia moupinensis. Stratification consists of four layers, with an upper canopy at 12 - 16 m but cover of only 10% - 20%; the main canopy species are Davidia involucrata, Davidia involucrata var. vilmoriniana, Phoebe neurantha, Aesculus wilsonii, Pterocarya insignis, Acer oliverianum, Cyclobalanopsis oxyodon and Lindera pulcherrima var. hemsleyana. The lower canopy attains 7 - 12 m with cover of 65% - 70%, and the dominant species are Symplocos lucida and Lindera pulcherrima var. hemsleyana, sometimes mixed with Tetracentron sinense and Cercidiphyllum japonicum. The shrub layer (5 m) has cover of 10% - 15%, and the main species are Ilex pernyi, Clerodendrum trichotomum, Viburnum oliganthum, Viburnum cordifolium, Acanthopanax henryi, Helwingia japonica, Mahonia bealii, Philadelphus incanus and Fargesia robusta. The cover of the herb layer (0.8 m) is 80% - 85%, and the common species include Smilax china, Aristolochia moupinensis, Senecio scandens, Polygonum suffultum, Polygonatum verticillatum, Astilbe rivularis var. myriantha, Smilax menispermoidea and Actinidia kolomikta. The average number of species obtained is 53 per sample.

8) Ilex fargesii-Davidia involucrata community (Community H)

Sites of this forest community are on northwest and north facing slopes of 22˚ to 35˚, at altitude from 1500 m to 1620 m, in the Wolong nature reserve [18] . The diagnostic species are Ilex fargesii and Litsea coreana var. sinensis. Stratification of this community includes three tree layers, and the cover ranges from 75% to 90%. The upper canopy layer reaches 14 - 20 m and consists of Davidia involucrata, Cercidiphyllum japonicum, Tetracentron sinense, Padus grayana, Juglans cathayensis, Alniphyllum fortunei, Kalopanax septemlobus, Euptelea pleiospermum, etc. The height of the main tree understory ranges from 6 m to 10 m, and the main trees are Cyclobalanopsis oxyodon, Lindera communis, Ilex fargesii, Litsea coreana var. sinensis, and Cladrastis wilsonii. The common species of the lowest tree layer are Gmelina hainanensis, Cornus officinalis, Viburnum erosum, Ilex pernyi, etc. The shrub layer has an average cover of 15%, and the main species are Viburnum erosum, Ilex pernyi, Litsea sericea, Gmelina hainanensis, Cornus officinalis, Salix microphyta, Kalopanax septemlobus, etc. The herb layer has higher cover degree (60% - 80%), and the dominant species are Dysosma versipellis, Paris polyphylla, Codonopsis convolvulacea var. forrestii, and Trillium tschonoskii. The community has an average of 60 species per sample.

9) Betula fargesii-Davidia involucrata community (Community I)

This short but species-rich forest community occurs on steep slopes (30˚ to 35˚) facing southeast, at 1400 altitude, in Hupingshan nature reserve of Hunan [27] . The number of constituent species reaches 65. The diagnostic species of this community include Betula fargesii, Zelkova schneideriana, Litsea pedunculata, Sycopsis sinensis, Celtis sinensis, Alangium platanifolium, Meliosma flexuosa, Lonicera chrysantha, Smilax davidiana, and Beesia calthifolia. There are generally only three layers. The tree layer attains 75% to 90% cover with an average height of 15 m, and the dominant tree layer species are Davidia involucrata, Tetracentron sinense, Betula fargesii, Zelkova schneideriana, Litsea pedunculata, Sycopsis sinensis, Celtis sinensis, Dipteronia sinensis, Cyclobalanopsis multinervis, and Cornus officinalis. The shrub layer usually includes species such as Alangium platanifolium, Meliosma flexuosa, Schizophragma integrifolium, Smilax davidiana, Lonicera chrysantha, Ficus heteromorpha, Padus napaulensis, Fargesia spathacea, and Davidia involucrata. The herb layer has an average cover of 50%, and the dominant species of are Sedum amplibracteatum, Beesia calthifolia, Sanicula lamelligera, Chrysosplenium grossophyllum, Hemiboea subcapitata and Celastrus orbiculatus.

10) Chimonobambusa quadrangularis-Davidia involucrata community (Community J)

This short wooded community is situated at altitude ranging from 1570 m to 1700 m in the Kuankuoshui forest area of Guizhou [28] . The diagnostic species are Chimonobambusa quadrangularis, Sassafras tzumu, Morus mongolica, Yulania sprengerii, Eurya japonica, and Rubus irenaeus. Stratification shows usually four layers with two tree sub-layers. The upper canopy reaches 10 - 15 m and is dominated by Davidia involucrata, mixed with Pterostyrax psilophylla, Toxicodendron succedaneum, Cyclobalanopsis multinervis, Magnolia sprengeri, Aesculus wilsonii, Sassafras tzumu, Fagus longipetiolata, etc. The lower tree layer attains 3 - 8 m, and the main species include Litsea euosma, Weigela japonica var. sinica, Morus mongolica, Eurya japonica, Ilex chinensis, Rhododendron stamineum, Lindera glauca, etc. The shrub layer (1 m) is a dense cover (80% - 90%) of dominant Chimonobambusa quadrangularis. Under the bamboo in the herb layer has a sparse cover of only 10% - 15%, with common species Lysimachia congetiflora, Impatiens siculifer, Meehania fargesii, Chrysosplenium davidianum, Cyrtomium macrophyllum, Chrysosplenium macrophyllum, Galium asperuloides var. hoffmeisteri, etc.

11) Chimonobambusa utilis-Davidia involucrata community (Community K)

This forest community is found on a steep northeast-facing slope of 26˚ at a higher altitude of 1940 m to 1960 m on the Baiqing karst platform of Guizhou [29] . This community can be distinguished by Chimonobambusa utilis, Corylus heterophylla var. sutchuenensis, Sorbus caloneura and Neolitsea confertifolia. Stratification involves four layers, with two tree sub-layers. The height of upper canopy layer is 12 - 18 m with cover of 60% - 70%, and the main species are Davidia involucrata, Acer oliverianum, Tetracentron sinense, and Corylus heterophylla var. sutchuenensis. The lower tree layer has 6 - 12 m in height and consists of Davidia involucrata, Acer oliverianum, Prunus padus, Sorbus caloneura, Neolitsea confertifolia, Cornus controversum, Acer henryi, etc. The shrub layer is dominated by Chimonobambusa utilis and has a few tree saplings. The herb layer (0.3 m) has fewer species, mainly belonging to the Pteridaceae, Urticaceae, Rosaceae, Violaceae and Rubiaceae.

12) Styrax hypoglauca-Davidia involucrata community (Community L)

The stands of this forest community are located at altitudes ranging from 1300 m to 1500 m on the FanjingShan (mountain) of Guizhou [30] . The diagnostic species are Styrax hypoglauca, Weigela japonica var. sinica, Rubus peltatus and Cornus capitata. This community usually has four layers. The canopy layer is 15 - 25 m in height with a cover of about 70%, and its common species include Davidia involucrata, Styrax hypoglauca, Liquidambar acalycina, Pterostyrax psilophyllus, Tetracentron sinense, Aesculus wilsonii, Acer palmatum, Betula insignis, Acer davidii, Acer sinense, Juglans cathayensis, Carpinus fangiana, Fagus longipetiolata, Castanopsis chunii, Cyclobalanopsis oxyodon, etc. The lower tree layer consists of Camellia cuspidata, Symplocos stellaris, Camellia pitardii, Daphniphyllum macropodum, Cornus capitata, etc. The shrub layer reaches 3 m and its cover is 30% - 60%. The main species of the shrub layer are Lindera glauca, Lindera fragrans, Euonymus alatus, Rubus peltatus, Mahonia bealii, Weigela japonica, Callicarpa cathayana, Stachyurus chinensis, Dichroa febrifuga, Sambucus williamsii, Hydrangea chinensis, Ilex pernyi, Aucuba chinensis, and Rhododendron spp. The cover of the herb layer is less than 30%, and the predominant species are Dysosma versipellis, Panax transitoxinus, Arisaema rhizomatum, Arthraxon hispidus, Reineckia carnea, Rohdea japonica, Paris polyphylla and Oxalis griffithii. The average number obtained was 42 species per sample.

13) Fargesia robusta-Davidia involucrata community (Community M)

The stands of this forest community are on steep slopes (55˚ to 64˚) at altitude from 1600 m to 1730 m, facing northeast, northwest and southwest, in the Baiguo forest farm of Wuxi County near Chongqing [31] . The diagnostic species are Fargesia robusta, Taxus wallichiana var. chinensis, Rhododendron stamineum, Primula hupehensis, Dryopteris labordei, Platycarya strobilacea and Akebia quinata. These forests have an average cover of 85% and can be divided into five layers, including three tree sub-layers, one shrub layer and one herb layer. The canopy layer attains 25 m and consists of Davidia involucrata, Tetracentron sinense, Cercidiphyllum japonicum and a few individuals of Platycarya strobilacea. The middle tree layer reaches 10 - 25 m, and the main species include Davidia involucrata, Tetracentron sinense, Cercidiphyllum japonicum, Torreya fargesii, Carpinus fargesiana, Taxus wallichiana var. chinensis, Acer palmatum and Acer wilsonii. The lower tree layer is 10 m in height, and the predominant species are Acer palmatum, Platycarya strobilacea, Davidia involucrata and Euptelea pleiospermum. The cover of the shrub layer reaches 20% with a height of 1 - 3 m, and species such as Fargesia robusta, Carpinus fargesiana, Alangium chinense and Rhododendron stamineum are often found in the shrub layer. The herb layer attains 0.5 m, and the dominant species are Dryopteris labordei, Cyrtomium fortunei, Primula hupehensis, Carex henryi, and Dysosma versipellis. The mean number of species in a sample is 41.

14) Meliosma veitchiorum-Davidia involucrata community (Community N)

This community can be found at altitude 1400 m to 1600 m in Hunan [32] . The diagnostic species are Meliosma veitchiorum, Photinia glabra, Ribes tenue, Ribes glaciale, Lonicera gynochlamydea, Sinofranchetia chinensis, Pileostegia viburnoides, Sanicula orthacantha, Astilbe chinensis, Toona ciliata and Cycolocarya paliurus. Stratification of this community involves four layers, and usually the tree layer and herb layer are developed well. The canopy layer consists of Davidia involucrata, Tetracentron sinense, Cercidiphyllum japonicum, Meliosma veitchiorum, Toona ciliata, Pterostyrax psilophyllus, etc. The dominant species of the lower tree layer include Dipteronia sinensis, Acer henryi, Euptelea pleiospermum, Photinia glabra, Staphylea holocarpa, Euonymus oblongifolius, Illicium henryi, Camellia pitardii, etc. The main species of the shrub layer are Ribes glaciale, Ribes tenue, Stephanandra chinensis, Neolitsea confertifolia, Viburnum dilatatum, Elaeagnus difficilis, Symplocos sumuntia, Abelia macrotera, Lonicera gynochlamydea, Lonicera maackii, Sambucus williamsii, etc. The predominant species of the herb layer are Astilbe chinensis, Chrysosplenium griffithii, Chrysosplenium macrophyllum, Saxifraga stolonifera, Tiarella polyphylla, Thalictrum acutifolium, Oxalis griffithii, Elatostema involucratum, Hemiboea henryi, Sanicula orthacantha, etc. The average number of species is 60.

15) Viburnum rhytidophyllum-Davidia involucrata community (Community O)

Relevés for this forst community were obtained in the Houhe national nature reserve of Hubei. The stands are located on steeper slopes (22˚ to 42˚) between 980 m and 1427 m elevation, facing southeast and northeast. This community is characterized by Liriope graminifolia, Elaeagnus pungens, Carex henryi, Chloranthus serratus, Ophiorrhiza japonica, Machilus ichangensis, Viburnum rhytidophyllum, Phoebe faberi, Machilus thunbergii, Zanthoxylum dissitum var. dissitum, Litsea pungens, Rubus corchorifolius, Euonymus alatus, Aster ageratoides, Epimedium sagittatum, Cypripedium formosanum, Arisaema bockii, Polystichum tripterpon, Campylandra chinensis, Dryopteris enneaphylla, Urtica fissa, Polygonatum humile, Asarum sieboldii, Globba racemosa and Parthenocissus tricuspidata. Stands can be divided into four layers. The canopy reaches 17 - 26 m with an average cover of 65%, and the predominant species are Davidia involucrata, Pterocarya hupehensis, Phoebe sheareri, Cyclobalanopsis oxyodon, Fagus longipetiolata, Phoebe neurantha and Machilus ichangensis. The height of lower tree layer is 9 - 17 m, and the cover ranges from 20% to 40%. The main species of the lower tree layer include Davidia involucrata, Elaeagnus lanceolata, Meliosma cuneifolia, Acer mono, Phoebe faberi, Machilus thunbergii, Dipteronia sinensis, Aesculus wilsonii, Cornus controversa, Litsea pungens, Acer oliverianum, etc. The shrub layer is 2 - 3 m in height and attains 15% - 35% in cover. The dominant species of the shrub layer are Viburnum rhytidophyllum, Zanthoxylum dissitum var. dissitum, Rubus corchorifolius, Sabia japonica, Viburnum dilatatum, Camellia cuspidata, Ficus heteromorpha, Euonymus alatus, etc. The cover of the herb layer ranges from 40% to 60%, with an average height of 0.5 m, and the predominant herb species include Chloranthus serratus, Chrysosplenium macrophyllum, Liriope graminifolia, Elatostema involucratum, Carex henryi, Epimedium sagittatum, Cyrtomium fortunei, Hemiboea henryi, Aster ageratoides, Cypripedium formosanum, Arisaema sikokianum var. serratum, Polystichum tripterpon, Dryopteris enneaphylla, Urtica fissa, Polygonatum humile, Asarum sieboldii, Globba racemosa, and Parthenocissus tricuspidata. The species composition is relatively rich, with an average of 63 species per relevé.

16) Arisaema heterophyllum-Davidia involucrata community (Community P)

The samples of this forest community are at altitudes from 1200 m to 1700 m in Changyang County of western Hubei [33] . The differential species are Arisaema heterophyllum, Paris polyphylla, Camellia cuspidata, Saxifraga stolonifera, Paeonia obovata, Epimedium brevicornu [sic] and Lithocarpus henryi. Stratification can be divided into four layers. The canopy layer is 17 - 20 m in height and is dominated by Davidia involucrata, mixed with Tetracentron sinense, Davidia involucrata var. vilmoriniana, Cerasus conradinae, Pterostyrax psilophyllus, Cyclobalanopsis glauca, Pterocarya hupehensis, and Phoebe sheareri. The lower tree layer reaches 6 - 11 and is dominated by Davidia involucrata, with companion species Ilex chinensis, Cornus controversum, Lithocarpus henryi, Acer palmatum, Litsea coreana var. sinensis, Carpinus turczaninowii, and Quercus fabri. The shrub layer attains 2.5 m and has an average cover of 30%. The main species of the shrub layer include Camellia cuspidata, Symplocos sumuntia, Padus grayana, Eurya nitida, Cerasus pseudocerasus, Symplocos paniculata and Illicium henryi. The herb layer reaches 0.8m, and its cover ranges from 40% to 60%, with predominant species Arisaema heterophyllum, Saxifraga stolonifera, Paeonia obovata, Epimedium brevicornu, Paris polyphylla, Mahonia bealii, Matteuccia orientalis, Smilax china, Akebia trifoliata, Rodgersia podophylla, and Tiarella polyphylla. The average species number was 43 per sample.

3.3. Phytogeographical Differentiation

The results of detrended canonical correspondence analysis show that the first three axes are correlated with environmental conditions included in the analysis. The species-environment correlation coefficients for these three axes are 0.979, 0.963 and 0.939, respectively (Table 4).

The results of a Monte Carlo test show that seven environmental variables are related significantly to the first axis (P < 0.05) and all canonical axes (P < 0.005). The first two axes explained 42.5% of the total variance, and the correlation coefficient between the two axes is lower (r = 0.006). Therefore, the general pattern of environmental variation among communities is expressed most clearly by the combination of the first two axes. The first axis is most closely connected with the slope (r = −0.670), latitude (r = −0.567) and average annual temperature (r = 0.402), indicating effective micro-environmental temperatures and micro-topography. The second axis is most closely correlated with average annual precipitation (r = −0.630) and aspect (r = 0.582), reflecting overall humidity and water availability.

Figure 2 shows that the distribution of the communities along the two axes is uneven. The communities A, B, K, D, E, O, I and M are strongly influenced by the first axis, and the communities C, J, P, N, L, D, H, G and F

are significantly affected by the second axis. Especially, the communities A, B and K are more likely to occur in warmer areas at low latitude (south of 28.1˚N); the communities M, I and O more frequently occupy habitats on shady steep slopes (more than 32˚); the communities F and G are more likely located in sunny, dry environments; and the communities C, J, P, N and J are found in more humid environments. However, many communities were scattered along the second axis, indicating that the forests adapt more to habitats with intermediate temperature, although the temperature tolerance range is wider.

4. Discussions

This study analyzed Davidia involucrata forests, which occur from 27˚35'N to 31˚25'N and from 102˚17'E to 110˚20'E in south-central China. The 72 total samples of Davidia involucrata forest were summarized into 16 communities. Davidia involucrata communities are predominantly made up of deciduous broad-leaved trees, mixed with some evergreen broad-leaved species belonging to the genera of Castanopsis, Lithocarpus, Machilus, Cyclobalanopsis, Ilex, Camellia, etc. Davidia involucrata forest is usually differentiated from other deciduous forests by the species Davidia involucrata, Tetracentron sinense, Cornus controversa, Cercidiphyllum japonicum, Juglans cathayensis, Aesculus chinensis var. wilsonii, Pterostyrax psilophyllus, Padus avium, Acer henryi, Acer oliverianum, Davidia involucrata var. vilmoriniana, Acer palmatum, Euptelea pleiospermum, Acer flabellatum, Meliosma cuneifolia, Chrysosplenium macrophyllum, Padus grayana, Staphylea holocarpa, Ficus heteromorpha, Dipteronia sinensis, Hemiboea henryi, Schizophragma integrifolium, Litsea mollis and Tapiscia yunnanensis. Also, the shrub layer of Davidia involucrata forest is usually dominated by different bamboos, just like Fagus forest [36] .

Plants belonging to the temperate element are dominant, but species belonging to the tropical element also occur in large proportions, indicating that Davidia involucrata forest has the features of temperate forests but is also connected closely with tropical floristics. The general pattern of geographical variation among the communities is explained clearly by the temperature and water availability (including air humidity), and the forest is more likely to grow in warm, moist habitats.

Acknowledgements

We would like to thank E.O. Box who patiently checked our manuscript and made valuable suggestions.

References