The Woody Planet: From Past Triumph to Manmade Decline

Abstract

Woodiness evolved in land plants approximately 400 Mya, and very soon after this evolutionary invention, enormous terrestrial surfaces on Earth were covered by dense and luxurious forests. Forests store close to 80% of the biosphere's biomass, and more than 60% of the global biomass is made of wood (trunks, branches and roots). Among the total number of ca. 374,000 plant species worldwide, approximately 45% (138,500) are woody species-e.g., trees, shrubs or lianas. Furthermore, among all 453 described vascular plant families, 191 are entirely woody (42%). However, recent estimations demonstrate that the woody domination of our planet was even greater before the development of human civilization: 1.4 trillion trees, comprising more than 45% of forest biomass, and 35% of forest cover disappeared during the last few thousands of years of human dominance on our planet. The decline in the woody cover of Planet Earth did not decelerate during the last few centuries or decades. Ongoing overexploitation, land use and climate change have pushed ten thousand woody species to the brink of extinction. Our review highlights the importance, origin and past triumph of woody species and summarizes the unprecedented recent decline in woody species on our planet.

Keywords: biodiversity loss; global change; lianas; shrubs; trees; woody plant families.

Figure 1

Woody plants form the principal components in forests and many other ecosystems on our planet, providing food and habitats for innumerable organisms. (a) Quercus bumelioides, tropical rainforest, Tapanti National Park, Costa Rica; (b) Salix polaris, tundra, Svalbard, Norway; (c) Rhizophora stylosa, mangrove, Iriomote, Japan; (d) Pinus cembra, alpine forest, Prealps of Fribourg, Switzerland; (e) Mopane woodlands, Etosha National Park, Namibia; (f) Semidesert vegetation, Perekushkul, Azerbaijan. Pictures: E. Kozlowski (a,c,e,f), G. Kozlowski (b,d).

Figure 2

Selected members of the Fagaceae, a large and entirely woody plant family. (a,b) Fagus sylvatica, Ueberstorf, Switzerland; (c,d) Quercus robur, Enney, Switzerland. Both species belong to the most common trees in European forests; (e) Castanopsis delavayi, endemic to Southwest China. Yunnan, China; (f,g) Quercus litseoides, Shenzhen, Wutong Mountain, China. The species is one of the rarest and most endangered oaks worldwide. Pictures: E. Kozlowski (a–e), Y.-G. Song (f,g).

Figure 3

Zelkova abelicea (Ulmaceae), an example of an endangered tree species endemic to the Mediterranean island of Crete (Greece). Only large trees can produce fruits (a,b). However, the majority of fruit is empty, which is probably due to unfavorable climatic conditions such as pronounced and recurrent droughts. The overwhelming majority of individuals, however, are dwarfed and nonfruiting due to overbrowsing by goats (c,d). The regeneration of populations via seedlings is nearly impossible due to the overgrazing, trampling and soil erosion caused by omnipresent large caprine and ovine flocks (e). Pictures: G. Kozlowski (a), H-R Siegel (b,d), L. Fazan (c,e).