The Academy Lecture

The Evolution of Wood Anatomical Diversity and its Significance

Pieter Baas

Pieter Baas
Leiden University and Naturalis Biodiversity Center, National Herbarium section
The Netherlands


Woody plant evolution has produced a wonderful diversity in xylem anatomical patterns which we increasingly understand in terms of adaptations to changing environments and climates. From the co-occurrence of many different wood anatomical types in a single biotope it is evident that adaptive evolution has harnessed the plant kingdom with a great number of alternative strategies that all apparently work well.  Yet high demands on hydraulic conductivity and safety,  on biomechanics,  and on biological defense against herbivores and pathogens all have to be met throughout the lifespan of a shrub or tree. Integration of functional, comparative wood anatomy with whole plant biology (stomatal conductance, leaf and petiole anatomy, root system depth, etc.) is needed to understand when and where xylem patterns are limiting in the survival or extinction of woody clades, but it is evident that these patterns are extremely important in evolutionary success.
In this talk I will review global patterns of wood anatomical diversity, mainly related to hydraulic architecture: vessel element dimensions, perforation plates,  intervessel pits, imperforate tracheary elements, and certain character syndromes such as dendritic vessel patterns associated with vascular tracheids and helical wall thickenings. These global patterns in natural woody floras are significantly associated with climatic variables such as moisture availability,  temperature, and seasonality. They can also be used to reconstruct past climates for fossil assemblages, and to predict the fate of modern forests under climate change. Although much research is currently focussed on the role of wood anatomy in tree biology, the potentials of ecological and evolutionary wood anatomy have yet to be fully developed.
Many of the adaptive innovations  in wood evolution have become highly conserved (e.g. vestured pits and type of vessel perforation), and contain strong phylogenetic signals of diagnostic value. There is therefore no conflict between adaptive and systematic significance of certain wood anatomical traits: they are two sides of the same coin. 

The Lecturer

Pieter Baas (1944) studied Biology at Leiden University, and completed his PhD thesis on the phylogenetic position of Ilex and putative relatives in 1975. From 1968—1969 he did research at the Royal Botanic Gardens Kew, under Dr. C.R. Metcalfe.  From 1969 onwards he is associated with National Herbarium of the Netherlands (NHN), currently part of the Naturalis Biodiversity Center. From 1991-2005 he was director of the NHN. Currently he is active as Professor Emeritus of Systematic Botany and honorary staff member of Naturalis
His main interest is in the evolution of wood anatomical diversity and its significance in tree biology and global change research. Related interests are systematic and phylogenetic plant anatomy, microscopic wood identification, biodiversity and conservation, biohistory, and wood culture, and the role of botanical gardens in research and education.
He was elected a Fellow of the Royal Academy of Arts and Sciences of the Netherlands (KNAW) and of the International Academy of Wood Science (IAWS). From 1976 onwards he is the Editor-in-Chief of the International Association of Wood Anatomists Journal. He published over 220 scientific papers, and 6 books.

The lecture was presented during the 2013 IAWS Plenary Meeting in Nanjing/China

The Evolution of Wood Anatomical Diversity and its Significance