As you discuss each need, have one student hold the prop representing that need over or around a tree for all to see. Look around at the trees and describe how each gets those needs fulfilled.
Water comes from the rain, in drier climates some crops are irrigated.
Alder likes wet feet, while oak and Christmas trees like a dry site. Space is provided by planting the trees in 4.5 by 6 feet spacing. Nutrients come naturally from the ground in the forest, and trees die, decompose and add those nutrients back to the soil. In a Christmas tree plantation, trees are harvested and removed from the site. They do not die and decompose there.