threedimensional
subsets of the six-dimensional data set are shown in
Figs 2b and c.Multi-dimensional analyses including from four to all
six of the traits similarly showed the large majority of variation
explained with a single axis (Table 2). With the six traits included,
74% of all variation lay along the first principal axis.
The extent to which each trait contributed to the principal axis of
variation is indicated by a loading (or weight) assigned to each trait.
The directionality of these loadings (Table 2) indicates that the axis
can be thought of as a leaf economics spectrum. This spectrum runs
from species with potential for quick returns on investments of
nutrients and dry mass in leaves to species with a slower potential
rate of return. At the quick-return end are species with high leaf
nutrient concentrations, high rates of photosynthesis and respiration,
short leaf lifetimes and low dry-mass investment per leaf area.
At the slow-return end are species with long leaf lifetimes, expensive
high-LMA leaf construction, low nutrient concentrations, and low
rates of photosynthesis and respiration.
Within growth forms or functional groups the principal axes of
variation had the same directionality of trait correlations as for the
total data set (Table 2). Similarly, species grouped by major biome
type (Fig. 1), or by MAT or MAR classes, yielded the same pattern
(data not shown). The concordance of these results is of special
significance, indicating a coordination of these key leaf traits that is
consistent across major plant functional types, growth forms and
biomes. The amount of variation captured by the principal axis in
the different species groupings was also similar to that across all
species in most cases.
The main exception was among deciduous trees and shrubs
where, as expected, there was substantially less variation in LL
(5-fold versus 100-fold), and where LL–LMA relationships were
partially uncoupled. Still, whereas different growth forms and
functional groups were differentiated along the leaf economics
spectrum when trait means were considered, the overlap between
species groups was large (data not shown). Evergreen trees and
shrubs had longer mean LL and higher LMA than deciduous
species, but evergreens had much wider ranges for both traits,
extending to LLs almost as short as for the shortest-LL deciduous
species, and to similarly low LMA. Similarly, on average, shrubs and
trees had higher LMA and longer LLs but lower Nmass, Amass and
Rdmass than herbs and grasses, yet trees and shrubs spanned almost
the entire range of any of these leaf traits. Another example: N2-
fixing species had higher mean Nmass than non N2-fixing plants, yet
the range of Nmass was larger and extended higher in non N2-fixing
species.