How are we to know the answers without knowing about which groups the question pertains.
Competition, they try to grow taller than other trees to get more sunlight
Forests and other green plants uses carbon dioxide in photosynthesis thus lowering its amount in the atmosphere. In assumption, we can claim that in the ice ages, the low level temperatures may have caused the carbon dioxide levels to neutralize over these phenomenons, also to note that there were less oxygen-breathing organisms and higher carbon-dioxide dependent performing photosynthesis.
