Research Summary: A model proposed that lichens increase nitrogen available to young forest soil. Data: Plot treatment -> soil nitrate after 6 months: bare rock -> 0.4 mg/kg; moss only -> 0.7 mg/kg; lichen cover -> 2.8 mg/kg; lichen removed -> 0.6 mg/kg. Claim: Lichen cover contributes to higher soil nitrogen in early succession. Which result best supports the claim?