 parameters. An Earth model has been included for comparative reference.
Model illustrations on the left, provide an equatorial view of the planet with latitudes shown on the extreme left and temperatures on the right.
The models on the right, are shown in a polar view. The numbers entered above the pole are latitude and number below are the average temperatures for the given latitude.
Environmental Limiting Factors & The Plant Community
The planetary models we looked immediately above, have no axial inclination, we were determining the average temperature for a latitude; the length of the day to night cycle made no contribution. The parent star will rise and set in the same locations daily. Each day at local noon, the sun will have risen to it’s highest point in the morning sky and will begin its move lower in the afternoon sky. The elevation the sun rises to above the horizon al local noon will depend on ones latitude on the planetary surface.
On a planet with no axial inclination and 0ºC APST, plants in the equatorial zone, below 25º latitude, would be exposed to nearly perpendicular sunlight at local noon.
On planets with0 to 30ºC APST, most of the plant communities exist below 60º latitude. In these locations, the noon sunlight comes from a position relatively high in the sky. This results in the tropical and cool forest developing ‘ vertical light gathering stratification’. Hence we would find fungi located in the lower strata, rising above the fungi would be the shade lovers, then the shade tolerant and finally a canopy of sun seekers.
On 0º axial inclination planets, with 45ºC to 60ºC APST, the majority of the plant community exists above 45º latitude. Under these conditions, the tropical and cool forest would tend to develop ‘horizontal light gathering stratification’. On the equatorial side of the forests reaching deeper and deeper into the hotter zones, plants would decrease in size or biomass, attempting to adapt to extreme temperatures. On the polar side of the forests, plant life would again decrease in size or biomass, but this time light would be the limiting factor.
The following, illustration, Relative Plant Size and Biomass Per Unit Area, uses APST, latitude and various biological limiting factors to assist in developing a feeling for plant height and /or biomass per unit area, for various latitudes across a planet’s surface.
Also, remembered that gravity affects height, so that the largest or tallest plants on a 3.0 Earth mass planet will be considerably shorter than the tallest plants on a 1.0 or 0.5 Earth mass planet—that is, if all parameters are equal except planetary mass.
The 0ºC APST’ model in the illustration shows short plants growing at around 30º latitude, this planets tundra zone. This biologically limiting, cold environment, rapidly gives way to a cool forest which extends from about 25º ‘north (and south’) latitude to the equator. With the sun passing nearly over the habitable equatorial zone, we would expect to find the plant communities of the cool temperature forest well developed into vertical light gathering strata.
Illustration: Relative Plant Size and Biomass Per Unit Area
The 60ºC APST (140ºF average planetary surface temperature) model has short, ‘shade loving’ plants in the biologically limiting, dimly lit, twilight like polar region. With a decrease in latitude and subsequent increase in elevation of the mid day sun, the cool temperature forests develop a fierce competition for light. Around 60º latitude, the damp. Warm and fairly well illuminated conditions produce a tropical type forest canopy which carries the tallest plants on the planet. Below 60º latitude, increasing temperatures tend to overcome moisture availability where upon plant size or biomass per unit area must be reduced to thwart starvation.
Approaching 30º latitude, the high pressure system creates arid conditions. This area is not only arid, but extremely hot, with air temperatures 10ºF above that required to destroy protein. From about 30º latitude and down to the equator, temperatures tend to become the bio