It means being able to a) compete for resources (like light, space, and nutrients); b) avoid predation (by both mammals, such as deer and porcupines, and by insects); and c) resist the impacts of disease and severe weather. Being taller gives a plant an obvious advantage in the competition for light. ... Indeed, at times the forest floor is coated with seedlings. ... But even during that era, as during our own, the limiting condition on forests was the same: moisture. ... Look at photos from around the world and you'll see that forests cover many types of landscapes--from...

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• Grade Level: High School

Atmospheric Structure The gaseous area surrounding the planet is divided into several concentric spherical strata (31k jpeg) separated by narrow transition zones. The upper boundary at which gases disperse into space lies at an altitude of approximately 1000 km above sea level. More than 99% of the total atmospheric mass is concentrated in the first 40 km from Earth's surface. Atmospheric layers are characterized by differences in chemical composition that produce variations in temperature. Troposphere The troposphere (14k jpeg) is the atmospheric layer closest to the planet and c...

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