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The increasing darkness in the Northern Hemisphere this time of year "indicates to the plant that is coming on. So it starts recouping materials from the leaves before they drop off." Evergreens protect their needle-like foliage from freezing with coatings and natural "antifreezes." But broadleaf plants, like sugar maples, birches, and sumacs, have no such protections. As a result, they their leaves. But before they do, the plants first try to important nutrients such as nitrogen and phosphorus.