An Introduction to the Functional Diversity of Temperate Forest Trees

Abstract

Because of the ease of handling, the study of the functional significance of plant diversity became in large a domain of grassland or old field research. Research with simple, synchronized herbaceous/grass communities of various composition helped in establishing a conceptual framework, in essence explaining effects of plant species diversity on ecosystem functioning as either being related to some sort of complementarity of resource use, to species interactions (facilitation and other mutualisms), or to insure against system failure in cases of severe disturbance or stress that affect different species to different degree (e.g., Loreau 2000; Scherer-Lorenzen et al., Chap. 17, this Vol.; see also the Preface to this volume). While not yet fully understood in these simpler systems, these plausible concepts still await their test in longer-lived plant stands, natural ecosystems and forests in particular. Short-lived plants can replace each other in one to few years and may spread clonally with associated ecosystem processes following rapidly. It may take more than 100 years to change the composition of forests and even longer for the associated ecosystem (soil) adjustments. Given the longevity of trees, year by year compound-interest effects of small trait differences may accumulate and become effective within the life of an individual. This chapter explores some of these trait differences among temperate forest tree species and presents a few comparisons with species in other climatic zones. Does it matter which and how many tree species compose the forests that cover a third of Europe’s land area? This question is not as new as one might think.When wood was the sole source of heating energy, the overall productivity of forests was more important than the production of a certain type of straight timber. Early forest research of the 19th century had explored the yield of monospecific vs. two- or multi-species stands and produced clear evidence that the yield of mixed forests (e.g., evergreen conifers with deciduous species) can, on average, be 10–20 % (some extreme cases up to 50 %) higher than that of any monoculture in the same area (e.g., Gayer 1886; Table 2.1; see also Pretzsch, Chap. 3, this Vol.).However, the positive effects were commonly found on good soils,whereas on poor soils, mixtures may yield even less than certain monocultures (Kenk 1992). According to Wiedemann (1951), the range may go from +18 to –17% of volumetric yield for mixed stands of spruce/beech and oak/beech, depending on location (soil). Primarily mass-oriented considerations (productivity) became less significant when the dominant economic role of forests shifted from fuel to timber and energy was supplied from fossil sources. A massive rebuilding of the forested landscape followed, leading to forest types and species abundance strongly in contrast to the potential (Fig. 2.1). Today, 70 % of the European forests are conifer forests and 30 % broadleaved deciduous forests, the reverse of the pattern one would see in a natural landscape (Table 2.2).Most of these stands are close to monospecific. In this volume, several authors are synthesizing current knowledge about the effects of forest species diversity on forest functioning and integrity. This introductory chapter will explore the extent to which common temperate zone forest tree species differ in their functional traits.As will be shown, these differences are substantial, and thus the presence or absence of a certain species could potentially influence ecosystem processes significantly. However, some of these traits may become functional only when different species interact and be less significant in monospecific stands. Other traits may emerge more strongly in monospecific stands and become insignificant in mixed stands.With the current evidence it seems nearly impossible to differentiate the expression and function of traits by species neighborhood. This assessment will thus consider variations in traits irrespective of the assemblage type in which sample trees have grown. Although the temperate forest tree diversity in Europe is only about 10 % of that of temperate North America and less than 5 % of that found in temperate East Asia (largely because of past glaciation cycles and the east-west orienta- tion of the continental divide), the ca. 20 native key tree species composing European temperate forests (out of a total of ca. 60 European temperate tree species) cover a wide range of functional traits, possibly as wide as their American and Asian counterparts. Some of this variation will be reviewed here.