Plant functional trait variation and ecological strategies

Peter Vesk

School of Botany

The University of Melbourne

Diverse solutions to challenges

• 250,000 to 400,000 species of seed plants – Dominated by Angiosperms – 700 - 900 extant Gymnosperms

• Honed by natural selection

Ecological study

• Autecology -- how a particular species makes its way in the world

• Communities -- how species interact with one another and with the abiotic environment

• Is every species a special case?

• Organizing principles

Theophrastus (d. 287 BC)

• Successor of Aristotle, director of Lyceum.

• Father of Botany• De historia

plantarum• De causis

plantarum

Historical origins

• Early origins of ecology• Plant geographers• Descriptive, broad scale

– von Humboldt (1807), – de Candolle (1874), – Warming (1895), – Schimper (1903),– Merriam (1890), – Du Rietz (1931), – Raunkiaer (1934)

A) phanerophyte: tree or shrub with perennating buds held >25cm above ground

B) and C) chamaephyte: semishrub, buds <25cm

D) hemicryptophyte: perennial herb with bud at ground level

E) geophyte: herb with bulb or other perennating organ below ground

F) therophyte: annual plant surviving only as seed (G)

Raunkiaer life-form categories

Raunkiaer leaf-size classes

• Leptophylls - < 25 mm2 (<5x5mm)

• Nanophylls – 25 – 225 mm2

(<15x15mm)

• Microphylls – 225 – 2025 mm2

(<45x45mm)

• Mesophylls – 2025 – 18225 mm2 (<135x135mm)

• Macrophylls – 18225 – 164025 mm2 (<405x405mm)

Ecological strategy schemes

• A strategy is how a species sustains a population. Or what they do and how they live.– Not conscious choice, but pattern of life history and

allocation that has been moulded by natural selection

• schemes that arrange species in categories or along spectra according to their ecological attributes.

Periodic table or personality

• Unlikely to be flexible enough as an analogy for plant ecology3 PFT1 + 2 PFT2 = ?? Veg type2

• Perhaps personality type testing schemes more useful

• Response to question or situation,• Several core types identified from many correlated

descriptors combined into few principal components of variation or factors (Factor analysis)

Theophrastus’ character types:

surliness

• The surly man when he is asked, 'Do you know where so-and-so is?' will say, 'Don't worry me ' or if addressed, will refuse to answer. If he wishes to sell something he never names to the intending purchaser the price he will take, but always asks him what he is to get. Those who send him presents for the festival as a mark of esteem are informed that he is sure it is not a present. He has no pardon for anyone who has accidentally bespattered him with mud or jostled him or trodden on his foot……..

Classification schemes

• Human psychology is very suited to classification: like and not like.

• Grouping of similar species, typologies: plant functional types

• An important question is whether to work with ‘types’ or with ‘spectrum’– Introvert to extrovert– Leaf size; small to large

Strategy schemes• Realised niche: distribution (or response) along

environmental gradients:– Ellenberg’s soil moisture and N; – Noble and Slatyer’s (1980) vital attributes (fire), grazing response

(Dyksterhuis, 1949). • Physiognomic:

– Raunkiaer (1934) bud position and leaf size, • Conceptual: relating to ecological opportunity (and/or

evolutionary pressures):– r-K and adversity selection (MacArthur & Wilson, 1967; Southwood,

1977; Greenslade, 1983), – early and late successional spp. (Bazzaz, 1979), – exploitative and competitive spp (Bormann and Likens, 1979) gap and

non-gap species in forests, density dependence, etc– Grime’s (1974, 1977) CSR scheme– types of time, Westoby (1980) theory for arid zone vegetation.

r-K spectrum• Species selected under

“r” vs “K” conditions• suggested correlates

– mortality density-independent vs density-dependent

– multiplication-dominated vs competition-dominated

– opportunistic vs equilibrium

– climate unpredictable vs predictable

– early vs late succession

r

K

Species traits supposedly favoured by r-selection relative to K-selection

• High maximal rate of increase rmax

• many small offspring

• Rapid development

• early reproduction

• small body size

• short lifespan

Traditional r-K spectrum mixes together two different allocation “decisions”

• 1. Reproductive effort vs growth and survival– partitioning of reproductive value

• 2. Within reproductive effort, many small vs few large offspring

• Limitations of r-K particularly obvious in plants (and other sessile ecologies) – plant populations rarely grow exponentially through a series

of generations– growth to large body size and survival to long lifespan

doesn’t necessarily go along with few large offspring

Introduction of a second dimension of variation, from

favourable to "stress" or "adversity"

Intensity of stress Frequency or intensity of disturbance

Low High

Low Competitors Stress-tolerators High Ruderals No viable strategy

Competitor

Stress-tolerator

Ruderal

C

SR

C-S

S-R

C-R

C-S-R

Grime’s CSR triangle

• Competitor, Stress-tolerator, Ruderal

• Ruderal (weed, pioneer) corner reflects response to recent disturbance

• S-->C axis expresses potential for rapid growth given favourable conditions

Features of CSR triangle

• High-stress-high-disturbance corner claimed not viable

• Not actually the habitat that is being classified, but the way the species deals with the habitat– in particular, in highly productive

environments one sees also S species– environment is resource-poor for them

because of the presence of C species

• Conceptual strategy dimensions (competitiveness, shade tolerance) difficult to compare across habitats

• Hence, measurable traits emphasized recently.

Recent approach: major measurable dimensions of ecological variation

• Seed mass - seed output (SM-SO)• Specific Leaf Area - leaf lifespan (SLA-LL)

– area/dry mass of leaf

• Height– of canopy of the species when full-grown

• wood density, twig size and leaf size– Proportional relationship

• degree of consensus emerging about first 3, at least

Merit of using readily-measurable dimensions

• Species from different continents or veg’n types can be “positioned” relative to each other without requiring info on species distributions relative to each other or to environmental factors– brings worldwide meta-analyses within reach

• Axis-traits need to express something important about ecology– SLA, seed mass, height represent basic tradeoffs– species spread widely along each axis

Emergence of “Macroecology”

• “The division of food and space among species on continents"

• Statistical patterns in datasets covering 100's to 1000’s of species

• Simple traits

• Environ. data

Specific Leaf Area SLA

• Plants invest dry mass in leaves, acquire further dry mass, re-invest in more leaves

• SLA (mm2 mg-1 dry mass) is light-capture area deployed per mass invested– analogous to a potential rate of return

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.

Low SLA = heavy fabric of leaf

• Thick lamina or high tissue density – Can feel with fingertips

– banksia to basil

– grasstree to lettuce

• SLA varies widely (~tenfold) between coexisting spp– why are low-SLA species not

competitively inferior?

Low-SLA species are competitive because their leaves have longer lifespans: revenue-

stream over longer duration

• LL varies 400-fold between species, up to 50-fold within a habitat

• SLA varies 100-fold between species, up to 40-fold within a habitat

Leaf lifespan (mo)

Global variation in Leaf economics

The worldwide leaf economics spectrum Ian J. Wright, et al Nature 428, 821-827(22 April 2004) doi:10.1038/nature02403

SL

A

Longer leaf lifespan in wetter areas, but leaf construction more important

seed mass* - seed output spectrum

*mass of an individual seedEg dust-like seed of orchids, through to coconuts

Benefit of low seed mass: high seed output

• within a m2 of canopy outline, seed output = (reprod effort)/(seed mass + accessory costs)

• seed mass varies > 100X as widely as reprod effort, hence is the dominant influence on seed output

• slope = -1.0Henery & Westoby (2001), 47 woody spp from sclerophyll veg’n.

Seed output / m2 canopy

Benefits of large seed during seedling establishment: survival of hazards

counts of published expts with >5 spp, >10-fold range of seed mass

• Why not a single best compromise between costs and benefits?

Westoby M, Falster DS, Moles AT, et al. (2002). Annu. Rev. Ecol. Syst. 33:125-59.

Wide spread of seed mass within habitat

• Seed mass correlated with dispersal morphology• 70% of var’n within dispersal mode• Very different vegetation types and continents

• Correlation pattern reasonably consistent

alloblack.jpg

Allocasuarina torulosa (Casuarinaceae)

Global Ecology and Biogeography Vol. 16, 1 Pages: 109-116Copyright © 2006 The Authors Journal compilation © 2006 Blackwell Publishing Ltd

Smaller seeds

towards the poles

• What else could explain seed mass

Ground ivy (Glechoma hederacea) reproduces clonallyYucca plant

Plant height: Medicago to Mountain Ash

Moles et al 2009 Jecol 97:923-932Copyright Journal compilation © 2009 British Ecological Society

Plant height

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are needed to see this picture.

Moles et al 2009 Jecol 97:923-932

Much variation at a site, primary productivity explains between sites

Stem economics spectrum?

• Stem density: balsa to teak

Cahve et al Ecology Letters Vol. 12: 351-366 Copyright Journal compilation © 2009 Blackwell Publishing Ltd/CNRS

Geography of wood density

• The geographical distribution of wood density in N and S America. the mean wood density value of all unique species occurrences in that cell. (b) Predicted mean wood density value on the basis of climatic variables. MAT, MAP

Plants with denser stems grow more slowly but with

lower mortality risk

Figure 5 Relationship between wood density and relative growth rate (log-transformed, a), and mortality rate (log-transformed, b), for two tropical forest sites (Barro Colorado Island, Panama, white circles, and Pasoh, Malaysia, black circles).

Chave, J., Coomes, D., Jansen, S., Lewis, S. L., Swenson, N. G. & Zanne, A. E. (2009) Towards a worldwide wood economics spectrum. Ecology Letters, 12, 351-366.

Plant ecological strategy schemes and functional traits

• Framework for organization of species and studies • Generalization of specific cases• Nomenclature/identity can’t help us with prediction about

unknown species • Understanding what are the major ways in which plants

differ• Leaf construction, plant height, stem construction, seed

size and number• Fundamental understanding of the role of environment and

gradients on structure and function• Meta-analysis of ecological and physiological experiments