Box 2. Why measure plant traits and which traits to measure?

Plant functional traits give better insight into the constraints and opportunities faced by plants in different habitats than
does taxonomic identity alone (Southwood 1977; Grime 1979). They also provide understanding of how functional
diversity in the broad sense underpins ecosystem processes and the benefits that people derive from them (Chapin et
al. 2000; Díaz et al. 2007), and offer the possibility of comparing distant ecosystems with very little taxonomic
overlap (Reich et al. 1997; Díaz et al. 2004; Cornwell et al. 2008). The plant-trait approach often provides unique
mechanistic insights into several theoretical and practical questions, although it is not necessarily less laborious or less
expensive than other methods.

Which traits to measure to answer which questions?

No methods handbook can answer the question of what are the best traits to measure, because this strongly depends on
the questions at hand, the ecological characteristics and scale of the study area, and on practical circumstances. For
instance, there is not much point in comparing multiple species for succulence within wet environments or for
flammability within areas that burn only very rarely, whereas such data might be useful as a reference in larger-scale
studies. In addition, rather than setting limits to researchers’ curiosity, this trait handbook aims at inspiring others to
come up with and measure traits not covered here, including ‘new’ traits, to help answer exciting novel questions.
Some examples of additional interesting traits not covered here are in the introductory text of Cornelissen et al.
(2003). The first and foremost criterion in deciding what traits to aim for is the process of interest. Is the intended
study about fundamental plant or organ design in response to environmental variation in the present or about the
evolution that gave rise to today’s spectrum of designs? Is it about plant growth, reproduction or dispersal over the
landscape? Does it involve plant survival in response to resources or disturbance? Is the main question about response
to or effects on water, soil nutrient or fire regimes? Is it about vegetation feedbacks to atmosphere and climate? Does it
involve the juvenile stage, the persistence of adults? Does it involve pollinators, dispersers or herbivores? Does the
target process occur above or below ground? Is the focus on coarse differences across or among regions or continents
or on subtle differences among individuals of two slightly different local populations? Are specific ecosystem services
to people assessed or predicted? All these and further types of questions will have a direct impact on the selection of
traits.

Although there is no limit to the number of relevant traits in different research contexts, a small number of traits
have been considered relevant almost universally, because they are at the core of the plant life cycle (Grime et al.
1997; Westoby 1998). These are plant size (usually expressed as height), seed size (usually expressed as seed mass)
and the structure of leaf tissue (often expressed as specific leaf area or leaf dry-matter content). Beyond this, there are
some ‘core lists’ of plant traits that are considered important for plant resource use, regeneration, dispersal and
response to widespread disturbances (e.g. Hodgson et al. 1999; McIntyre et al. 1999; Weiher et al. 1999;
Lavorel and Garnier 2002; Knevel et al. 2003). A discussion of these is beyond the scope of the present manual,
and readers are referred to these papers for a first introduction. For a particular question, the brief ecological
background, and especially the list of references provided for each trait, should help identify the most appropriate traits
to measure. Logistic and financial considerations are equally relevant. For example, if resources are limited for
measuring relative growth rate on hundreds of species representing a large gradient of productivity, the specific leaf
areas and stem-specific densities of these species might serve as less precise but still useful proxies for broad patterns
of variation in growth and vegetation productivity. Similarly, the choice of traits would be slightly different if the
limiting factor is labour force or access to sophisticated analytical laboratories, or if the project involves an intensive
one-off measurement campaign carried out by highly trained specialists or recurrent measurements by third parties. The
recipes provided here, including the sections on Special cases or extras, should assist in making those decisions.