Fagus dominance in Chinese montane forests : natural regeneration of Fagus lucida and Fagus hayatae var. pashanica

Fagus species are important components of certain mesic temperate forests in the Northern Hemisphere. Of eleven Fagus species distinguished, five are found in China. Chinese beeches are restricted to the mountains of southern China. In the montane zones of the northern subtropics beeches (Fagus engleriana in the north, and Fagus hayatae var. pashanica in the northwest) often predominate in the forests: they are mixed with several other deciduous broadleaved tree species. In these forests the understorey is rather sparse. In the montane zones of the middle and southern subtropics beeches (Fagus lucida at higher altitudes and Fagus longipetiolata at lower altitudes) dominate in many forests and are mixed mainly with evergreen broadleaved trees. There, in contrast, the forest understorey is often very dense and consists of evergreen trees or shrubs and bamboos. Bamboos often cover the forest floor throughout the range of Chinese beech forests.

All Chinese beech forests are natural forests, and they are rich in plant and animal species, many of which are rare or endangered. Mixed beech forests are major forest communities in many nature reserves or national parks established in subtropical China, and are important in biodiversity conservation. Because they are located on mountain slopes and ridges, Chinese beech forests are also important in the prevention of soil erosion and in the maintenance of the stability of watersheds. Beech yields good-quality hardwood timber, and has been suggested as being useful for afforestation in mountainous areas of subtropical China.

In the forests where Chinese beeches co-occur with evergreen broadleaved trees, adult beeches are common, whereas juvenile beeches are rare. This gave rise to the following research questions:

1. Will beech dominance in the forest canopy in these mixed forests be maintained, or are the current beech-dominated forests a stage in a longer successional cycle or series?

2. If beech remains dominant in the canopy, then where do juvenile beeches establish themselves, and how do they grow up and reach the canopy? How does beech differ from co-dominant tree species in these aspects?

The regeneration processes of trees in natural forests are regulated by many abiotic and biotic factors, e.g. climate, disturbances, species composition, architecture of the forests, and site heterogeneity. The temperament, growth performance and architecture of trees largely influence their success in regeneration. Therefore, the four aims of this study were:

1. To quantify climatic heat and moisture ranges in Chinese beech habitats (Chapter 2).

2. To determine the relation between macro-climate (temperature, precipitation and major climatic hazards) and species composition of Chinese beech forests (Chapter 3).

3. To identify the relations between topography and beech regeneration and between species composition and beech regeneration in two forests, representatives of the Chinese beech forests in the south and in the north of the beech range (Chapter 4).

4. To compare juvenile growth and architectural strategies of beech and other shade-tolerant canopy or subcanopy species in relation to the light environment (Chapter 5).

In chapter 2, data from herbaria, from literature and personal observations in ten beech sites are combined, and the occurrence and geographical range of each Chinese beech species are described and mapped. Using climatic data from 40 weather stations located in the montane zones of the beech range, the heat and atmospheric moisture ranges in the beech habitats were characterized using Thornthwaite's (1948) moisture index and annual potential evapotranspiration and Kira's (1945) Warmth and Coldness indices. To identify the climatic factors that limit beech distribution, the heat and moisture ranges in beech habitats were compared with those in the surrounding areas outside the beech range and in the lowlands below the beech range, using data from 111 weather stations in the surrounding areas and from the literature (Chang 1989; Fang & Yoda 1990). Also, the heat and moisture ranges in Chinese beech habitats were compared with those in beech habitats elsewhere in the world.

Chinese beeches occur in southern China (22.3-34.3° N; 101-121.5° E), mainly in the montane zones between 700 and 2500 m altitude. Fagus longipetiolata, Fagus lucida and Fagus engleriana have wide geographical ranges. Fagus longipetiolata has the widest range and occurs as far south as the tropical mountains in southeastern Yunnan province of China and in northern Vietnam. It occasionally occurs below 700 in in altitude (400 to 500 m) in lower parts of mountains in the eastern and central areas. Fagus engleriana is mainly found in the northern part of the whole Fagus range and is multi-stemmed. Fagus lucida is somewhat concentrated in the central area of that range. Fagus chienii and Fagus hayatae occur locally.

Chinese beeches experience a mean annual temperature ranging from about 5 to 17 °C, a Kira's Warmth Index of 40-140 °C, ^. month, and annual potential evapotranspiration of 450-840 mm. They enjoy a humid to perhumid climate, Thornthwaite's moisture index (Im) lying between 26 and 320. They are found in areas with an annual precipitation of between 740 and 2800 mm. largely concentrated in the growing season. Chinese beech forests mainly lie in perhumid climates (Im over 100), where water is sufficiently supplied throughout the year from precipitation. The heat requirements of Chinese and foreign beeches largely coincide. However, the warmth extremes in the American and Chinese beech habitats are larger than in the European and Japanese beech habitats. The conditions in the montane Chinese beech habitats are more humid than those in lowland beech habitats in Europe and North America. Northwards, Chinese beeches are limited by moisture deficits. But the climate of the area near the Bohai Bay in northeastern China is still within the climatic range of beech habitats. Southwards and at lower altitudes, high temperature and relatively low moisture are unfavourable to beech. However, like other beeches, Chinese beeches tolerate a certain water deficit occurring occasionally during the growing season. The failure of Fagus species to compete with evergreen broadleaved trees probably largely determines their southern geographical and lower altitudinal limits.

In Chapter 3 the trends of occurrences of climatic hazards in the Chinese beech range are described using climatic data of eleven montane weather stations; and relations between climatic parameters (heat, moisture, hazards) and species diversity of Chinese beech forests were characterized. The climatic hazards studied are storms, heavy rains, freezing rains, freezing fogs, snow, thunderstorms, and hailstorms. They influence the development of the beech forests. Using the SURFER software (the Kriging method), the mean number of days per annum on which these hazards occur were interpolated and extrapolated for eight beech forests. The Shannon-Wiener diversity index, the number of woody species with individuals taller than 5 m the relative importance of light-demanding trees and the relative importance of deciduous broadleaved trees were analysed in ten combined forest sample units (between 0.12 and 0.23 ha in area) in the eight beech forests, by combining the number of individuals and the basal area of each tree species represented by individuals taller than 5 m in each unit. These indices in the forest units smaller or larger than 0.20 ha were converted according to 0.20 ha. The relations between these indices and climatic parameters (temperature, annual precipitation and frequencies of major climatic hazards) were ascertained by multiple linear regression. For the analysis, factor analysis (method: PCA) was applied to extract common factors from some interrelated climatic parameters.

Storms, heavy rains, freezing rains, freezing fogs and snow storms can cause forest canopies of Chinese beech forests to open up at the levels of forest patches and forest mosaics. Storms and heavy rains cause the forest canopies to open up throughout the beech forest range. Storms and heavy rains are more frequent in the east than in the west. Freezing rains, freezing fogs and snow storms are most frequent and intense in the centre and the northeast. These cold hazards are rare in the southernmost parts of the beech range. Lightning and hailstorms seem to have minor impacts on Chinese beech forests.

The species diversity of Chinese beech forests increases towards warmer sites. To a lesser degree, it also increases towards coastal areas that experience more frequent storms and heavy rains. The importance of evergreen broadleaved trees increases towards the sites with less snowfall. Ice storms (freezing rains and freezing fogs) and snow storms favour beeches but not evergreen broadleaved trees nor fast-growing deciduous trees in the canopy dominance, because beech is resistant to these cold hazards, thanks to its deciduous habit and dense wood. The amount of annual precipitation was not related to any diversity index of the beech forests.

In Chapter 4, using transect sampling (from ridge to valley bottom), the regeneration sites and population structures of Fagus hayatae var. pashanica and Fagus lucida are characterized and compared with those of other main species in two beech forests: one in the north (Daba) and one in the south (Miao'ershan) of the Chinese Fagus range. Besides the beech species, the species compared are two deciduous broadleaved species Carpinus cordata var. chinensis and Quercus aliena var. acuteserrata in Daba, and are three evergreen broadleaved species Castanopsis lamontii, Lithocarpus hancei and Manglietia chingii in Miao'ershan. To characterize regeneration sites, forest eco-units of different types in different phases of development were distinguished, following Oldeman's concept (1983, 1990).

Beech regeneration sites were more abundant in Daba than in Miao'ershan. In both sites a dense bamboo layer restricted beech regeneration. However, in Daba, patches of dead bamboos and forest patches without bamboos occurred regularly, probably because of the cold; the forest understorey was rather sparse. There, Fagus hayatae regenerated in growing (or canopy gaps) and decaying eco- units in all sites, and in some mature eco-units on ridges. Quercus aliena saplings were absent and Carpinus cordata mainly regenerated in moist sites. In Miao'ershan, a dense undergrowth of bamboos, evergreen trees and shrubs limited Fagus lucida regeneration. There, the beech regeneration was confined to large growing eco-units (or canopy gaps) located on ridges and upper slopes, where plants other than bamboos were sparse. Beech regeneration and growth performance of juvenile beeches were best in large abandoned sites next to beech stands, which were probably created by felling in combination with burning or clearing. This implies that large abandoned sites without prior established vegetation are optimal for beech regeneration.

Population structures suggested a sporadic regeneration pattern in both Fagus hayatae and Fagus lucida and a continuous regeneration pattern in the three evergreen species studied. Quercus aliena was represented by tall trees only. Fagus lucida's dominance over the evergreens in the overstorey in Miao'ershan was probably due to its tolerance of glaze or snow storms.

In Chapter 5, the growth and architectural strategies of three evergreen and three deciduous broadleaved shade-tolerant canopy or subcanopy tree species in the two beech forests (Daba & Miao'ershan) are described. The species studied are the same as in Chapter 4 except for Quercus aliena (saplings). The relative light intensity directly above the sampled saplings (1-5 m tall) was assessed using Ozalid paper following Friend (1961). The six species studied were grouped according to two types of branch morphology, i.e. orthotropy and plagiotropy (including plagiotropic mixed axes; see GLOSSARY), and their basic architectural characteristics were described following Hallé & Oldeman (1970). These species include both trees that typically form monopodial trunks and trees form sympodial trunks when conforming to their architectural models. However, sympodial trunks formed by axillary replacement, i.e. reiteration of the model, were common in shaded saplings of all six species.

Shoot growth rates over the last five years in the saplings, and radial stem growth rates over the last five years in some of the saplings studied and in some poles per species were measured. The relations between growth rates and percentage of daylight above the saplings were analysed using multiple linear regression, in which mean growth rates over the last five years were considered as dependent variables and the logarithm of percentage of daylight and the logarithm of sapling height were used as independent variables.

In low light intensities, juvenile trees of the species studied followed a common strategy by investing relatively more resources in height growth than in diameter growth, in order to grow up and reach brighter environments. The light affected extension and diameter growth more strongly in saplings with orthotropic branches than in those with plagiotropic branches. In the shaded understorey, the species with orthotropic branches tended to optimize their height growth so as to reach brighter environments, whereas the species with plagiotropic branches tended to persist in the understorey by efficiently spreading their interceptive and assimilative organs. Root collar sprouting in the saplings of Manglietia chingii and Castanopsis lamontii appears to be a reiteration strategy for survival in the shaded understorey. These diverse strategies should allow diverse tree species to co-exist in the forest.

In the southern forest, the young trees of all three evergreen species had faster top-shoot and diameter growth rates than the deciduous Fagus lucida in all light environments they encountered. Obviously, this favours the evergreen trees over the beech to grow up and reach the forest canopy. However, these evergreen trees were so susceptible to ice and snow storms that they failed to dominate in the forest canopy.

Chinese beeches are adapted to a wide range of habitats. The resistance to ice storms and snow storms favours their dominance in the canopy. Major "disturbances" and simultaneous bamboo mortality over a large area can allow beech regeneration at any, unpredictable moment. The plagiotropic mixed axes give beech flexibility in growth and architecture in response to change of light and growing space, and to physical injury.

Because of the beeches' wide ecological amplitude, growth and architectural flexibility and its resistance of ice and snow storms, it seems probable that beech dominance in the canopy will be maintained in the Chinese beech forests in the northern areas and also in most of the southern areas, despite the present scarcity of juvenile beeches in the southern beech forests, if the macro-climate remains similar.

Based on findings of this study, it is suggested that the aims of biodiversity conservation are best served by establishing nature reserves over a range of sites which experience different frequencies and intensities of climatic hazards, so as to conserve most of the associated species adapted to different degrees of forest dynamics and the related environments. It is also suggest that statistical modelling on tree species or forest response to global climatic change should not only account temperature and precipitation but also climatic hazards. This study confirms that beech is useful for afforestation in montane zones of subtropical China, and that it is feasible to plant beech in the area near the Bohai Bay in northeastern China.