( ( ( Ozone ( generating reactions ) ) ( in ( the stratosphere ) ) ) ( ( is ( the ( main ( heat source ) ) ) ) . ) )
(ROOT (S (NP (NP (NN Ozone) (NN generating) (NNS reactions)) (PP (IN in) (NP (DT the) (NN stratosphere)))) (VP (VBZ is) (NP (DT the) (JJ main) (NN heat) (NN source))) (. .)))
Ozone generating reactions in the stratosphere is the main heat source.
(ROOT (S (NP (DT The) (NN stratosphere)) (VP (VBZ is) (NP (DT the) (NN heat) (NN source))) (. .)))