Ecology of beech forests in the northern hemisphere

<p>Beech forests are dominated or codominated by at least one <em>Fagus</em> species. The beeches are a homogeneous group of 11 deciduous tree species growing in the Northern Hemisphere (Figure 1.1). They often dominate forest ecosystems throughout their ranges. The optimum for beech is on acidic and mesic loam soils. The ranges are limited by summer water deficits in continental and southern climates, and low winter temperatures and late spring frost in the north.<p>The purpose of my research was to find out why beeches are such widespread successful trees. I tried to answer several questions:<br/>1. Do these beech species have something in common as forest organisms? What makes them different from codominant trees? In Chapter 3, I selected radial growth and size parameters for such comparisons among trees. Size parameters were tree height, stem diameter and crown projection.<br/>2. What are common characteristics of beech forests over the whole range? In Chapter 4, I analyzed geographical trends in woody species composition and forest architecture. Within each site, I compared tree-height distribution and tree regeneration and related them to forest architecture.<br/>3. Are there common characteristics in the dynamics of beech forests? The relation between suppressed and released growth in trees, as well as release frequency, yields information about stability and change in the forest. In Chapter 5, I compared suppression and release in tree-ring chronologies of trees in different study sites. The per tree number of major releases was counted over one century. For each study site, I used this parameter to calculate the per year per tree probability of a major release.<p>In Chapter 2, I described the 16 study sites (Figure 2.1, Table 2.1) which were representative of beech forests in different geographic areas. They were selected in sites with minimal human influences. In each study site, I selected at least two plots. The plots included different phases of forest development and were between 300 and 2400m <sup>2</SUP>in size. Tree height and stem diameter were measured in trees taller than 5m, and in a subplot, trees between 0.5m and 5m were measured. Their stem position and crown projection were mapped. Among plants lower than 0.5m, I recorded abundance of beech seedlings and estimated percentage cover of important plant species. Increment cores were taken from trees growing in the forest canopy, in canopy gaps and in the understory.<p>Generally, beeches are more shade-tolerant than their deciduous broad-leaved codominants and less shade-tolerant than their evergreen broad-leaved codominants. During its lifecycle a beech tree can pass several periods of suppression (Table 3.7). Compared with beech, height/stem-diameter ratios were relatively higher in deciduous and lower in evergreen broad-leaved codominants. With the exception of <em>Acer saccharum</em> and <em>Cyclobalanopsis multinervis,</em> beeches had more variable height/ stem-diameter relationships than any codominant analyzed (Table 3.3). Beech had a more flexible growth strategy than codominant tree species.<p>The architecture of beech forests was most simple in some European beech forests and became increasingly complex in eastern Asiatic beech forests (Chapter 4). The eastern Asiatic beech forests were richest in woody species and had the highest ratio of evergreen vs deciduous broad-leaved tree species. In each study site, the forest canopy covered about 70% of the area and potential trees dominated in the canopy gaps. Exceptions were some Asiatic sites where medium-tall tree species or shrub and dwarf bamboo species dominated in the canopy gaps, and beech seedlings could not establish themselves, Beech juveniles were also absent from Chinese study sites where evergreen broad-leaved trees were abundant in the understory. The beech forests are very different in architecture and species composition.<p>There was a relationship between per year per tree probability of major release and the codominance of other tree species (Chapter 5). High probabilities of major release in the study sites where deciduous broad-leaved tree species could occur, favored these tree species over beech. Low probabilities of major release in the study sites where evergreen broad-leaved tree species could occur, favored these tree species over beech. In the study sites with evergreen broad-leaved trees, codominance of beech is probably maintained through infrequent and large-scale major releases. The interval between changes in the forest architecture, and consequently light levels in the forest, is important in determining the dominance of beech.<p>The <em>Fagus</em> species form an ecologically homogeneous group of tree species that are successfully dominating or codominating the forest ecosystems where they occur. Shade tolerance and flexible growth strategies form the basis for the success of beech. Beeches have a wide tolerance for macroclimate and soil as long as these are sufficiently humid, however the extent of their dominance is determined by the frequency of wind impacts