Land-plant ecology on the basis of functional traits

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The tissue traits and architectures of plant species are important for land-plant ecology in two ways. First, they control ecosystem processes and define habitat and resources for other taxa; thus, they are a high priority for understanding the ecosystem at a site. Second, knowledge of trait costs and benefits offers the most promising path to understanding how vegetation properties change along physical geography gradients. There exists an informal shortlist of plant traits that are thought to be most informative. Here, we summarize recent research on correlations and tradeoffs surrounding some traits that are prospects for the shortlist. By extending the list and by developing better models for how traits influence species distributions and interactions, a strong foundation of basic ecology can be established, with many practical applications.

Section snippets

Schimper world and Hubbell world

Our world view follows Schimper [1]. In Schimper world, different plant species are more successful in different parts of the landscape, and this is because they have different quantitative traits, such as leaf nitrogen concentrations, rooting depths, wood densities, leaf sizes and potential canopy heights. Schimper world is set in real physical geography, along gradients of rainfall, temperature, and geomorphology.

Plant stems, canopy architecture, foliage and litter are so influential in

Data sets and models

Data sets about species traits are approaching global coverage, following decades of effort from many contributors. Seed mass data now cover >12 000 species [9], wood anatomy >5000 species (http://insidewood.lib.ncsu.edu/) and leaf economic and stoichiometric data >2000 species 10, 11, 12. The distributions of species are becoming better characterized as data sets of point locations rather than as presence or absence in grid squares (e.g. http://salvias.net/pages/, //nvs.landcareresearch.co.nz/

Ecologically significant plant traits and their variation across species

Because conceptual strategy dimensions such as competitiveness or shade tolerance are difficult to compare across habitats, recent plant strategy thinking (e.g. 16, 17, 18, 19, 20, 21, 22) has often emphasized measurable traits. These trait dimensions provide the means to compare species worldwide. Nevertheless, individual traits should not be considered in isolation, because pairs of traits are often coordinated (Box 2). An important part of trait research addresses interrelations among

Xylem hydraulics, wood density and leaf size

Baas et al. [31] discussed xylem evolution within the framework of a ‘tradeoff triangle’. Conductive efficiency was expected to trade off with resistance to embolism (the formation of gas bubbles in vessels, blocking the movement of water). Conductive efficiency was also expected to trade off with mechanical strength.

There are two mechanisms of embolism risk 31, 32. Under freeze–thaw, the risk that gas bubbles will form is proportional to vessel diameter. Under drought, gas bubbles are seeded

Roots in relation to shoots

First, one should consider the physiological or functional issues about root–shoot relations. Functional coordination is expected, because root-acquired resources are transferred to the shoot and vice versa. But does this mean that particular aboveground traits are always found in combination with particular belowground traits or, alternatively, is there a wide variety of root traits cooperating with a particular type of aboveground plant? Second, roots are important for ecosystem outcomes

N:P ratio in leaves, nutrient limitation and growth strategy

A leaf N:P ratio of ∼15 is thought to divide situations where growth responds more strongly to P addition (N:P>16) from situations where growth responds more strongly to N addition 61, 62. Recently, it has become evident that there is a strong gradient of leaf N:P increasing with mean annual temperature and towards the tropics 11, 12, 63, 64 (Figure 3a). Discussion of possible causes for this revolves around the influence of temperature on soil weathering and on growth rate, P being used more

Conclusion

In reviewing plant traits, we have emphasized questions that remain unclear. We are however optimistic and expect the next 15 years to set in place a coherent understanding of Schimper world. The elements that need to come together are first, a strong grasp of trait costs and benefits, in the context of a competitive environment and expressed as quantitative models; second, worldwide trait data sets that can position each species in the context of the full spread of ecologies that has evolved;

Acknowledgements

We thank Drew Kerkhoff and four anonymous reviewers for their comments. Kerkhoff, Joe Craine, Hafiz Maherali and Karl Niklas kindly provided data for figures. This work was funded by the Australian Research Council through the ARC-NZ Research Network for Vegetation Function.

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