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ESC 200 - Spring 2002
Trees in Our Environment

Prof: Linda Brubaker

LIFE-HISTORY STRATEGIES

LECTURE OVERVIEW

Basic statement:

The biological environment is an important agent of natural selection. Changes during forest succession are thought to select for different life-history features of tree species. These life-history features, in turn, affect strategies of forest management and uses of trees in urban settings.

Key ideas:

Tree species are found in recurring combinations called forest communities. Each community is broadly associated with a set of physical conditions (e.g., climate, soil type) and/or disturbance regimes (e.g., fire, flooding). Even in constant environments, forest communities change gradually over an often predictable sequence of stages. The life-history characteristics of species comprising this sequence also change in a predictable pattern. These characteristics include attributes such as the amount of energy allocated to growth vs. reproduction, the numbers and sizes of seeds, and the time spent in juvenile and adult life stages. Because the amount of energy derived from photosynthesis is finite and less than that necessary to meet all of the possible life-history demands, energy allocated to one process is not available to another. Thus, different strategies of allocating limited resources are selected in early vs. late stages of succession.

Important terms:

allocation

succession

intolerant

life-history traits

pioneer

tolerant

LECTURE OUTLINE

INTRODUCTION

Today we will think about timing of events in the life cycle of trees

Diagram of life cycle

STAGE	SEED	JUVENILE	ADULT

Question: Are there predictable patterns related to succession in how much time and energy is allocated to these stages?

It is widely recognized that there are differences in age of 1st reproduction, seed # and size, growth rates etc. between early and late successional species.

Background concepts and information:

1. Allocation-- Tree is faced with alternatives and must make "choices"

Source of energy for plant needs-- photosynthesis

Potential uses of energy

growth (leaves, roots, branches every year)
reproduction
transport of some materials (across membranes)
tissue repair
defensive chemicals

There is finite amount of energy produced each year but nearly infinite potential to use it, but energy used for one function/structure is not available for another

How is the allocation between different needs made??

NOTE: Often observed conflict between growth and reproduction

Seed production and growth

Large cone crops in Douglas-fir are often associated with less radial growth
Large seed crops in birch and maple correspond with lower leaf production

Age of first reproduction and growth

Dwarf apple trees-- produce fruit early, at small size-- but apple crop is small
Old-fashion large apple trees- produce fruit later, but larger tree size and larger apple crop

2. Environment in different stages of successtion (see figures)

light

temperature

3. Reproductive relationships (see figures)

reproductive output vs size

effect of age of 1^st reproduction

PREDICTIONS:

Make predictions about the following characterisitics in early versus late successional features, based on the likely consequences of the relationships described above.

seed size: SCROLL DOWN Early successional: smaller, for long dispersal into disturbed area; Late successional: larger, food needed for seedling to establish in shady environment

seed number: SCROLL DOWN Early successional: greater, food available to make many seeds, since individual seeds are small Late successional: fewer, each seed requires a large amount of food, so fewer seeds can be produced

seed defensive chemicals: SCROLL DOWN Early successional: lower, seeds are less likely to be eaten. Late successional: greater, due to predation by animals (large seeds are food supply for mammals, birds, and insects)

frequency of seed production: SCROLL DOWN Early successional: yearly, no selection for less frequent seed production. Late successional: irregular, but synchronized to prevent growth of predator populations. In mast years there are too many seeds for predators to eat and some escape. Small seed crops in following yearslead to starvation of predators (no seeds escape). All individuals of species growing together must be synchronized for this to work.

growth rates: SCROLL DOWN Early successional: fast, ample light for photosynthesis and multilayer leaf display. Late successional: slow, low light intensities in understory

age at 1st reproduction: SCROLL DOWN Early successional: Younger, fast growth allows trees to reach reproductive size early, also natural selection favors early age of reproduction (see figure of geometric growth). Late successional: Older, no selection for early reproduction since individuals would be too small to produce seeds at a young age.

leaf size #, leaf placement, crown architecture (concepts of adaptive geometry) SCROLL DOWN Early successional: smaller leaves, multilayer leaf placement and excurrent form to maximize leaf area that can carry out high rates of photosynthesis and to minimize heat stress and water loss. Late successional: larger leaves, monolayer leaf placement to capture light efficiently under low light intensities (no heat stress).

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Contact Linda Brubaker at: lbru@u.washington.edu

[ University of Washington ] [ College of Forest Resources ]