Final answer:
The willow tree's additional mass largely comes from carbon dioxide in the atmosphere, which it transforms into biomass through photosynthesis, in accordance with the law of conservation of mass.
Step-by-step explanation:
The additional mass of a willow tree primarily comes from the carbon dioxide (CO₂) in the air. Through the process of photosynthesis, trees convert CO₂ and water into glucose and oxygen. The glucose molecules then serve as building blocks for the tree's biomass, contributing to its mass.
This means that while some mass may come from minerals absorbed by the roots from the soil, a significant portion of a tree's mass is derived from the carbon it captures from the CO₂ in the atmosphere.
This is aligned with the law of conservation of mass, which states that matter is neither created nor destroyed but rather transformed from one form to another.