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Schedule
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ESC
200 - Spring 2002
Trees
in Our Environment
Prof: Linda Brubaker
LECTURE OVERVIEW
Basic statement:
Trees carry out the same life functions as smaller plants. However, their large size and long-life spans make these functions more challenging than for many other plants.
Key ideas
We will consider the special biology and stresses of trees. For example, 1) Trees have many parts that require communication (among buds, among branches, between roots and shoots). 2) Trees can move water from the soil to more than 100 feet above the ground without expending energy. Human devices that move water to these heights use pumps that must be fueled. Scientists still debate the forces that move water through trees. 3) Trees must endure large environmental fluctuations in different seasons and throughout their life span. They use a combination of phenotypic and genotypic solutions to this challenge. 4).... We will add to the list.
LECTURE OUTLINE (powerpoint version)
INTRODUCTIONTrees are distinguished by very large sizes and very long lifespans (largest and oldest living things, except fungi perhaps). Trees have been around for a long time, and many other organisms have evolved in them (i.e. trees have been the selective environment for the evolution of many species—e.g., primates, orchids, bromeliads, insects etc.)
Requires especially favorable conditions:
- long warm period to make enough food for large size
- sufficient water to support large leaf area
When this occurs, trees have advantage over other organisms by shading them out.
TODAY: What are some of the costs/challenges of
- large sizes
- long lifespans
How are these challenges met?
Challenges of Large Size
1. GROWTH
Approaches:
"Modular": addition of parts from buds, allows flexibility and resilience
"Expansion": radial growth by vascular cambium (a special feature of trees), allows for structural support and movement of water and nutrients
From roots, through stem, out of leaves- often moves 100-200 ft.
What "drives"
this movement?
(Addison
Wesley Longman Inc)
- water not pumped from below
- favorite current theory: evaporation from leaves "pulls" water through stem
- nutrients are in solution and move passively with water
- 3. COORDINATION
OF PARTS: Trees are made up of many different parts that are interdependent
- Trunk, branches, roots and leaves "communicate" via hormones (chemicals produced in one place, moved to another place where they cause a response)
- Because hormones move, they are a method of communication and coordination among parts
Inter-branch communication: results in coordinated branching pattern
- "auxin"
produced in apical bud, moves to lateral bud and suppresses growth
to varying amounts—results in short shoot vs long shoot developmen
- abscisic acid produced in roots when water is scarce, moved to branches, causes leaf drop to reduce water loss (e.g., black cottonwood drops leaves in late-summer droughts)
Challenges of Long Life Span
4. REPAIR
- modular growth: addition of parts by growth of new parts from buds
- totipotency:
when "shocked" meristematic tissue can produce different
specialized tissue (e.g. roots stimulated to produce branches, stumps
sprout new branches)
- compartmentalization:
- wounded
areas are walled-off or isolated and grown over (stem damage
is surrounded by resin to prevent infection and grown over by
new tissue)
- damaged parts are dropped (e.g.,leaf fall in response to disease)
- damaged tissue is not used (e.g., if water conducting tissue is damaged
- wounded
areas are walled-off or isolated and grown over (stem damage
is surrounded by resin to prevent infection and grown over by
new tissue)
5. ORDERLY SENESCENCE
Not all parts can live entire life span of tree (not possible to have infinite number of leaves)
There are mechanisms to:
- drop leaves after predetermined length of time (1 season, X yrs) depending on environment: high elevation species carry leaves longer droughty speices—WHY?
- recycle important elements before leaf drop and reuse in production of new leaves (older needles yellow before the drop from tree)
- some species drop lower branches quickly (escape fire)
6. PHENOTYPIC PLASTICITY
Environments fluctuate greatly during a tree's life time.
7. GENETIC BUFFERING
There are also genetic mechanisms that allow trees to respond to environmental fluctuations during their life time.
- Trees have highest level of heterozygosity of all organisms (i.e., diploid organisms with different forms of genes at given location on chromosomes)
- Heterozygosity may mean that there are different forms of key enzymes. Thus the tree has an "supply" of different "genetic software" that can be used under different climatic conditions
Contact Linda Brubaker at: lbru@u.washington.edu
[ University of Washington ] [ College of Forest Resources ]