落葉 and 常緑: collected leaves, leaf fall and year-round greenery
Compare 落葉 as gathered leaves or the start of leaf fall with 常緑 in planted evergreens and a green hill. Separate an object, an onset and a continuing state.
落葉 as leaves gathered into a bag
落葉 (らくよう) means fallen leaves or leaf fall. 「庭に積もった落葉を集めて袋に入れた。」 means “I gathered the fallen leaves in the yard and put them in a bag.” The objects gathered and bagged are leaves already accumulated in the yard.
「気温が下がると、この木では落葉が始まる。」 means “The leaves on this tree begin to fall when the temperature drops.” This 落葉 is the phenomenon of leaf fall, and 始まる marks its beginning. It does not report that every leaf is already off the tree.
常緑 in planting and a hill's greenery
常緑 (じょうりょく) means evergreen. 「庭の周囲に常緑の木を植えた。」 means “We planted evergreens around the garden.” It reports the past act of planting trees around the garden.
「この丘は常緑樹に覆われて一年中緑が濃い。」 means “This hill is covered with evergreens and stays lush throughout the year.” This describes the hill's year-round lush state, not another past planting action.
An object, a phenomenon and a green state
袋に入れた落葉 names leaves that were collected, whereas 落葉が始まる names a change beginning on the tree. Distinguish these uses before comparing them with the greenery expressed through 常緑.
Evergreen and the described year-round greenery do not guarantee that no individual leaf ever falls. The examples supply neither a particular tree species nor permanent retention of every leaf.
What does 落葉 name in the bagged-leaf sentence and 落葉が始まる, and does 常緑 guarantee that no leaf falls?
Show the answer and why
The first names gathered fallen leaves; the second names the onset of leaf fall. The 常緑 examples describe planted evergreens and a hill remaining green throughout the year. That name does not guarantee permanent retention of every individual leaf.