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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 C D 1/ 3 H max 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 E cological Anatom y – A Leaves and t he ph ot osynt he t ic enviro nment Apr 26 Lect ure: Special Topics in E cological Anatom y – Moving W at er Apr 28 Lect ure: Special Topics in E cological Anatom y – Stru ct ura l I nt egr ity May 3 Lect ure: Global C hange an d S igna ls i n P la nt Stru ct ure s May 5 Presentati on of Pr oje cts f or Ent ire Class Period