Interspecific correlations among ecologically important plant traits capture the attention of evolutionary ecologists because they may reflect two distinct phenomena. First, they may indicate physical, physiological or developmental ‘constraints’ that limit the independent variation and evolution of the focal traits. Second, the correlations may be the adaptive outcome of natural selection favouring particular combinations of traits over others, in which case the set of traits are often described as forming an ecological ‘strategy’ dimension (Westoby et al., 2002
). Distinguishing between these explanations and understanding the basis for trait-based strategy dimensions is important because it gives us insight into life-history trade-offs that operate within and between environments, and thus also into phenomena such as niche differentiation, species coexistence and the broad shifts in plant traits that occur along geographic gradients. Furthermore, emergent properties of ecosystems such as rates of net primary productivity and nutrient cycling are not only determined by site properties (e.g. rainfall, temperature, irradiance), but also by the traits and relative abundances of the species occurring therein (Reich et al., 1997
; Perez-Harguindeguy et al., 2000
; Garnier et al., 2004
The position of a species along a strategy dimension should relate to how the species makes a living or where it is most competitive (Grime et al., 1997
; Weiher et al., 1999
; Westoby et al., 2002
; Ackerly, 2004
; Diaz et al., 2004
). Rankings of species along a dimension should be consistent (at least approximately) in the face of within-species variation due to plasticity, acclimation or ecotypic variation. Four trait-based strategy dimensions identified to date (each described below) describe variation in (1) leaf structure and physiology, (2) seed size and seed output, (3) leaf size and twig size, and (4) typical maximum plant height (Westoby et al., 2002
). In this study we compiled data for focal traits from each of these dimensions, as well as for wood density, for more than 2100 woody species from seven Neotropical forests. Wood density (WD) was of particularly interest: while it has been suggested to be involved in several different strategy dimensions (see below), to date there have been few large-scale quantifications of its relationship to other key plant traits.
We had three aims. First, to quantify the pattern of ‘cross-species’ (interspecific) correlations among these ecologically important traits and, by implication, the correlation structure among the strategy dimensions that they represent. Convincingly demonstrating that two trait dimensions are orthogonal (not correlated) is at least as important for understanding plant ecological strategies as demonstrating that they are correlated: orthogonal dimensions convey independent information about plant strategies (Ackerly, 2004
). Second, using ‘phylogenetic’ analyses (Felsenstein, 1985
), we tested whether evolutionary divergences in trait-pairs showed similar correlation patterns as the cross-species analysis. Where results from the two types of analyses differ this may indicate that taxonomic biases in the dataset contributed substantially to the cross-species results. Third, we assessed how general the trait-relationships were by comparing cross-species trait correlations calculated for each of the seven forests separately. Below, we describe in more detail the trait-dimensions that were studied via their focal (representative) traits.
Plant species are arrayed along a ‘leaf economics spectrum’ running from high to low specific leaf area (leaf area per dry mass; SLA), leaf N and P concentration and gas exchange rates, but from short to long average leaf lifespan (Grime et al., 1997
; Reich et al., 1997
; Reich et al., 1998
; Wright et al., 2004
). Although many herbs and grasses and species native to fertile soils occur towards the high-SLA end of the spectrum, and many evergreen species from infertile habitats occur towards the low-SLA end, a range of leaf economic strategies can still be seen within growth forms, plant functional types and biomes (Reich et al., 1997
; Wright et al., 2004
; Wright et al., 2005
). Leaf economic traits also scale-up to canopy and whole-plant properties of shrubs and trees. For example, species with low SLA and long leaf lifespan (LL) accumulate greater total foliage mass per unit ground area than species with higher SLA and shorter LL (Reich et al., 1992
; Gower et al., 1993
; Read et al., 2006
), and have also been shown to have lower rates of height growth (Reich et al., 1992
). In this study, SLA was the focal trait representing the leaf economics spectrum.
Seed mass varies by 5–6 orders of magnitude among species (Leishman et al., 2000
). The probability of successful seedling establishment in the face of environmental hazards increases with seed mass (Westoby et al., 2002
). However, since species that produce larger seeds produce fewer seeds per unit of reproductive effort (Jakobsson and Eriksson, 2000
; Aarssen and Jordan, 2001
; Henery and Westoby, 2001
), the probability of dispersal to a safe site where establishment is possible decreases with increasing seed mass. This trade-off underpins the ‘seed size–seed output’ strategy dimension (Moles and Westoby, 2004
). Here, the trait chiefly representing this dimension was seed size (volume), but fruit size (volume) was also recorded.
Leaf size also varies by 5–6 orders of magnitude among species. Species with larger leaves tend also to have thicker twigs, forming a ‘leaf size–twig size’ trait-dimension among species (Westoby et al., 2002
). Here, the position of species along this dimension was represented by their average leaf size (area of individual leaves or leaflets). Species with larger leaves and twigs tend also to have less frequent branching and to bear larger fruits than species with smaller leaves/twigs; this set of relationships has become known as ‘Corner's Rules’ (Corner, 1949
; Ackerly and Donoghue, 1998
; Cornelissen, 1999
; Westoby and Wright, 2003
). Still, the adaptive significance of interspecific variation in leaf size is not well understood (Westoby et al., 2002
). In theory, larger leaves have thicker boundary layers and thus overheat more easily than smaller leaves, leading to higher respiration and transpiration costs (Givnish, 1978
). While this may help explain why community-mean leaf size tends to decrease with increasing site aridity (Givnish, 1978
), 1000-fold variation in leaf size is commonly seen among sets of co-occurring species (Fonseca et al., 2000
), suggesting that there must be additional costs and benefits associated with variation in leaf size. For example, in Australian evergreens both the degree of self-shading (Falster and Westoby, 2003
) and stem support costs per unit leaf mass (Pickup et al., 2005
) decrease with increasing leaf size, while herbivory levels have been shown to increase (Moles and Westoby, 2000
Typical maximum height of adult plants was the fifth trait that we recorded. Maximum height ranges from 1 cm to 100 m, four orders of magnitude, and can be considered as a strategy dimension in its own right (Weiher et al., 1999
; Westoby et al., 2002
). Taller species or individuals have an advantage over shorter plants in that they are able to intercept more light. At the same time, the accrued cost of investment in stems increases with increasing height, as does the continuing cost of maintaining stem tissues (respiration); further, taller individuals suffer from an increased risk of breakage (Niklas, 1992
; Givnish, 1995
; Ryan and Yoder, 1997
; Becker et al., 2000
). Trade-offs such as these mean that species with a wide range of maximum heights often co-occur. Maximum plant height also tends to be positively correlated with seed size among species, but for reasons that are as yet unclear (Moles et al., 2004
The final trait requiring introduction is wood density. Wood density (WD) is associated with several, somewhat inter-related aspects of ecological strategy. Firstly, sapling and adult mortality rates of Neotropical forest trees decrease with increasing WD (Zimmerman et al., 1994
; Muller-Landau, 2004
), presumably because higher WD confers greater resistance of stems to pathogen attack and to mechanical damage (Turner, 2001
). Secondly, species with higher WD tend to have slower stem-diameter and volumetric growth rates than lower WD species (Enquist et al., 1999
; Roderick, 2000
), this relationship largely underpinning a successional continuum in tropical forests from fast-growing, light-demanding species to slow-growing, shade-tolerant species (Lawton, 1984
; Poorter and Arets, 2003
; Muller-Landau, 2004
; King et al., 2005
). Finally, WD is also linked to several hydraulic properties of plants. Species with low WD tend to have highly conductive sapwood and store considerable water in their stems, while those with higher WD tend to be more resistant to xylem cavitation, and their leaves show larger daily fluctuations in leaf water potential (Stratton et al., 2000
; Meinzer, 2003
; Ackerly, 2004
; Bucci et al., 2004
; Santiago et al., 2004
; Hacke et al., 2005
In this study we compiled trait data for woody species from seven Neotropical forests, the traits being SLA, seed and fruit size, leaf size, plant maximum height and wood density. Our expectations were as follows.
- Due to the physical constraint that fruit size constrains the maximum possible size of seeds, seed and fruit size would show a ‘triangular’ relationship (small-fruited species would have small seeds only, whereas large-fruited species would have a wide range of seed sizes).
- Leaf size and fruit size would be positively correlated (Corner's Rules).
- SLA and wood density would be negatively correlated, reflecting the continuum from fast-growing pioneer species with low WD to slow-growing climax species with high WD.
- Taller species would have larger seeds (Moles et al., 2004, 2005).
- Otherwise, we expected the traits to be essentially unrelated, indicating orthogonality of the ecological strategy dimensions they represented.