Conifers are a relatively small group of plants (673 Gymnosperm species and 520 conifer species)

Angiosperms are thought to be more widely adaptable group-- all life forms (very small herbs with short lifespan to large trees very long potential life span through vegetative propagation eg. Populus tremuloides clones may be 8000 yrs old)

Possible reasons

• pollination by animals might promote speciation
• seeds produced in ovaries that develop into fruit-- with a wide variety of dispersal characteristics
• efficient water conducting vessels (flow rate of water through vessels is ca. 250 times that of tracheids)
• ability to vegetatively reproduce, allow survival in environments where plants are frequently physically damaged (fire, herbivory, avalanches) and extreme climatic stress (don't have to go through seedling stage.

Nevertheless, conifers are able to dominate large portions of northern hemisphere

• high northern latitudes
• semi-arid parts of North America and Asia
• mild, maritime climates of the Pacific Northwest

What are the traits responsible for the widespread success of conifers in these regions?

Conifers tend to carry out high rates of photosynthesis at relatively low temperatures compared to deciduous species

Douglas-fir:

Douglas-fir caries out relatively high rates of photosynthesis in non-summer months-- spring and fall

• Cold, clear days in winter are too cold
• Warm summer days are too warm

Generally true for other conifers-- may help them be successful in areas of cold climates

Allows trees to photosynthesize as soon as conditions become favorable

• good for short warm season at northern latitudes, don't have to wait for the production of new leaves (don't waste valuable portion of short growing season on time it takes for leaves to flush)
• good in arid areas (eg New Mexico, Arizona, California), can take advantage of rains whenever they might occur (in non-growing season)

Conserves nutrients- don't have to produce entire set of leaves each year (good for soils with low nutrient content and environments with slow decomposition rates)

• long, narrow-diameter cells
• narrowness prevents bubbles formation in spring when stems thaw
• air bubbles form as water freezes and air molecules go out of solution to form gas (like ice cubes in freezer)
• small size of tracheids only allow small bubbles to form
• small bubbles are likely to redissolve
• in vessels of Angiospersm-- bubbles often do not dissolve
• air bubbles are break in water column and prevent water from being drawn up to leaves (like air holes in straw)

Thus, tracheids of conifers may not be as efficient at moving water as are vessels of Angiosperms, but they may be "safer" way to ensure reliable supply of water to crown of tree and necessary for living in cold winter climates

Sunken stomates with waxy deposits

Sunken stomates reduce evaporation because chamber around stomate is not exposed to dry air and turbulent airflow

Reduction in water loss is important for to contrasting situations

1. dry climates (as you might expect)-- generally reduces water loss
2. cold climates-- helps prevent winter desiccation, when air warms in spring but soil is still cold/frozen and water can't move into tree fast enough to replenish loss of water from leaves

We've discussed before (2 contrasting types of fires)

• low intensity, gound fires -- adaptations = thick bark, high crown, flamable litter, live through fire eg Pinus ponderosa, Sequoiadendron giganteum

• high intensity, crown fires -- adaptations = serotinous cones, highly flammable dead branches that remain on trees (fire ladders) e.g., Pinus contorta -- numerous others

Answer: unusual seasonality of PNW: mild, wet winters plus mild BUT DRY summers.

• Evergreen conifers can photosynthesize during mild winters, when deciduous species don’t have leaves.
• Summer droughts make warm season stressful, selecting against deciduous species (which can’t compensate for adverse summer conditions by photosynthesizing in fall, winter, spring.