Transcript Document

Research Questions on Plant Form and
Function
Question 1: What controls the evolution and distribution of diversity?
Question 2: What have been the important constraints /principles which
have shaped the evolution of plant form, physiology, and plant life histories?
Question 3: How do plant species/taxa differ from one another?
Question 4: What have been the central principles guiding the evolution of
plant form and function?
Question 5: What are the abiotic constraints on how plants operate in their
environments? How does this relate to diversity?
Question 6 What has been the role of biotic interactions in shaping the
evolution of plant diversity? and visa versa?
Question 7 How has the evolution of plants shaped the evolution of the
biotic and abiotic environment?
Lecture after Enquist (UA, pers comm)
To approach these questions - Visualize Evolutionary
Relationships
1. Clade - group of taxa that share a common ancestor
2. A cladogram (phylogeny) is a hypothesis showing
evolutionary relationships between taxa (NOTE: taxon
is singular)
3. Goal of cladistics - to group species or higher taxa
(clade) so that they share a common ancestor (W.
Hennig 1966)
4. Branching of cladogram is determined by the
distribution of characters among taxa used to construct
the cladogram (or ‘tree’)
Chl a
&b
Innovation in
cell division
Archegonium and
antheridium
Indeterminant
growth
Niklas 1992, 1997
Water and sap conducting
tissue
Plant Functional Forms
- A Different Perspective on Biodiversity
1. Functional forms are not distributed
randomly
across the earth
--> unique distributions throughout the globe
unique in functional diversity and evolutionary
history
2. Functional diversity reflects
adaptation (evolution via natural selection) to local
biotic and abiotic environments
Diversity of “functional forms”
A. Plant Size
Span ~20 orders of magnitude
B. Life Forms
1. Variety in the
habit (shapes) and
phenology of plants
epiphyte
-grows on host plant
at all life stages
Liana
-woody climber
2. Palms,
Succulents
(Cactus), Trees,
Annuals, grasses
etc. (many can be
seen in and around
one tropical tree!
hemiepiphyte
-starts in the tree,
sends roots to the
ground
-this part is a root
secondary
hemiepiphyte
-starts on the grounds
(terrestrial stage), grows
up trunk (liana stage),
loses its connection with
soil as base of stem
deteriorates (epiphytic
stage); finally sends roots
back to ground
(hemiepiphytic stage)
-e.g. Philodendron,
Monstera
-starts on the
ground, grows
upward
vine
= herbaceous
climber
tree
shrub
-a single trunk
many stems,
these normally
much smaller in
diameter
liana
-this part is a stem (cf.
hemiepiphyte).
-note the leaf-bearing
branch near base; a
hemiepiphyte root would
not have this.
Tropical rainforest (dominated by large angiosperms - contains
diversity of most clades)
Deserts (Most recent to be ‘colonized’)
Diverse in a cladistic sense but “convergent” in form,
physiology, and life history
Tundra - Alpine (unique physiology, small plants)
Grasslands - Unique physiologies and anatomy
Desert/Tundra
Grasslands
Continental forests
Wet swamp forests
History of diversification of the Land Plants
Diversity Packing and Filling of ‘Ecological Space’
Global Plant Distribution of Diversity
High diversity in some places – low diversity in others
What have been the important constraints
and or principles which have shaped the evolution
of plant form, physiology, and plant life histories?
All plants . . .
1. Have to compete for limiting resources in order to grow,
maintain homeostasis, and reproduce (fitness).
--> Ecological stage (biotic and abiotic environment)
2. Evolutionary ‘goal’ to maximize fitness
--> Result is the evolutionary play
which yields diversification
“Constraints” on evolutionary diversification =
any physical or biological process which
‘limits’ the phenotype possible for organic
evolution
A. Physical constraints
1. Limit what is physically possible
2. Organisms must obey the laws of physics and chemistry
-->Biomechanics
-->Laws of thermodynamics
-->Allocation of energy where Energy in = Energy out
Euler equation for the biomechanics
of stem height and diameter:
Plants must obey this equation if they are to become
larger
Young’s Elastic Modulus
Stem diameter
E 
2/3
Hmax  C  D
 
1/ 3
Tissue density
Maximum height to which a vertical cylindrical stem can
grow before it elastically buckles under its weight

Fick’s Law of Diffusion:
In order to exchange ‘resources’ with the environment
plants must follow diffusion laws
Diffusion coefficient
(varies with temp and concentration)
c j
J j  D j
x
Flow of certain resource, j,
per unit area per unit time

Concentration Gradient
(change in resource
concentration
of j with distance x)
“Constraints” continued…
B. Evolutionary constraints - Limitation to
diversity from evolutionary dynamic
1. Developmental: A bias on the production of variant
phenotypes caused by the structure, character,
composition, or dynamics of the developmental system.
2. Optimal Arguments: Adaptive explanations - limit
possibilities to only what maximizes fitness.
How do plant species/taxa differ from one another?
How are they similar?
1. Tremendous variety of Foliar Diversity
Evolutionary/Anatomical/
Physiological/Ecological/ Trends
How do plant species/taxa differ, continued…
1. Tremendous variety of foliar diversity
2. Evolutionary ‘divergence’ in physiology, anatomy and
life-history
--> Important physiological differences between clades
and environments
--> Important life-history differences (timing of growth
allocation, reproduction, mortality) between clades
and environments (e.g., Annuals vs. Giant Sequoias)
3. Evolutionary ‘convergence’ in form and function
--> Despite anatomical and physiological differences
Strong
selection to be
a tree but many
ways
to be a tree!
1. Convergence in
tree form but
dramatically
different anatomy
2. Common ancestor
for each clade
was small in
stature
Niklas 1992, 1997
What are the abiotic influences on how plants
operate in their environments? How does this relate
to functional diversity?
1. All plants ultimately need similar resources (water, light,
nutrients)
2. So why are they so diverse? Part of the answer is:
--> Distribution of limiting resources is spatially and
temporally heterogeneous
Global Average Monthly Rainfall
Monthly Mean Temperature (1961-1990), data from the Climate Research Unit, University of East Anglia
Principle challenges must be met by all plants
Niklas 1992, 1997
Whole-plant structure and function
All plants have four basic requirements
1. Interception of sunlight - energy source for
photosynthesis Exchange of gases between the plant
body and the atmosphere
2. Mechanical stability - sustain weight elevated
above the ground
3. Hydraulic transport - conduct water and nutrients
from one part of the plant body to another.
4. Reproduce - successfully complete the life cycle
Functional approach to plant evolution
1. The performance of the four functions can be evaluated
in terms of physical laws or principles
2. Evolution by natural selection leads to maximized plant
fitness within limits of constraints
3. Fitness is some combination of these functions
-->Local abiotic and abiotic environment will influence
relative importance of functions
4. Evolution strongly ‘guided/constrained’ by physical laws
or principles
Macroevolutionary perspective
Time
Seeds
Leaf diversity
Plumbing diversification
Architecture, Leaf/root anatomy
Xylem anatomy, Sporangia
Adaptive radiation of Tracheophytes - linked to key anatomical/
physiological innovations. Allowed plants to occupy new habitats and
access resources not being used
Niklas 1992, 1997
Finishing the Semester Special Topics in Ecological Anatomy
Apr 21
Lect ure: Special Topics in Ecological Anat omy – A
Leaves and t he ph ot osynt het ic environment
Apr 26
Lect ure: Special Topics in Ecological Anat omy –
Moving W at er
Apr 28
Lect ure: Special Topics in Ecological Anat omy –
St ru ct ura l I nt egr it y
May 3
Lect ure: Global C hange and S ignals i n P lant
St ru ct ure s
May 5
Prese ntati on of Pr oje cts f or En tire Cl ass Period