Classification of plant functional types

Background/Question/Methods

The term 'plant functional type' (PFT), now a generic name for almost all concepts of basic ecological plant types, was coined originally for groups of species with similar responses to environmental conditions, especially climatic change. Unlike more integrative concepts, PFTs involved seasonality but no reference to plant form, as if this were irrelevant. The 2-by-2-plus-alpha "vegetation" units now used in most large-area modeling are also often called PFTs but are actually only highly generalized cover classes.

 Earlier ideas of basic ecological plant types include 'growth forms', form-function units called 'life forms' (sensu vero), more variable 'living forms', and the "ecophysiognomic forms" first used in large-area modeling. All of these concepts recognized that plant function is intimately related to plant form, including its seasonal variations. The first attempt to use plant types in geographic modeling (Box 1981) required 90 types to cover the main features of world terrestrial vegetation. That list now includes about 130 basic types, which express the form characters needed for physiological models of different plant types. 

Results/Conclusions

Classifications for geographic modeling have generally been top-down exercises involving what has been called 'geographic regression', i.e. postulating types, predicting their occurrence, and treating what remains uncovered. This presentation reviews classification approaches, especially the necessary form attributes of plant permanence, size, structure, and seasonality pattern. Structure involves not only physiognomic types such as trees but also morphological adaptations such as leaf "hardness". Seasonality must include some degree of plasticity. 

 Also presented, for high-latitude application, are: a hypothesized classification of relevant plant types; comparison of these types with species lists representing northern North America, Scandinavia and Russia; and an expanded classification based on the results. The fraction of species not classified adequately measures the validity of any classification.

 For better modeling, three things must be admitted: covering the important functional features of world terrestrial vegetation requires many more types than commonly used now; more plasticity is needed in defining their characteristics; and plant function involves integrated, interdependent manifestations that cannot be represented by independent 'traits'.