( ( ( ( ( ( Nonrenewable resources ) ( , ( however ( , ( exist ( in ( limited amounts ) ) ) ) ) ) ) , ) and ) ( ( once ( they ( are ( used up ) ) ) ) ( , ( they ( ( can not ) ( be replaced ) ) ) ) ) ) . )
(ROOT (S (S (NP (JJ Nonrenewable) (NNS resources)) (, ,) (ADVP (RB however)) (, ,) (VP (VBP exist) (PP (IN in) (NP (JJ limited) (NNS amounts))))) (, ,) (CC and) (S (SBAR (IN once) (S (NP (PRP they)) (VP (VBP are) (VP (VBN used) (PRT (RP up)))))) (, ,) (NP (PRP they)) (VP (MD can) (RB not) (VP (VB be) (VP (VBN replaced))))) (. .)))
Nonrenewable resources , however, exist in limited amounts, and once they are used up, they cannot be replaced.
(ROOT (S (NP (JJ Nonrenewable) (NNS resources)) (VP (MD can) (RB not) (VP (VB be) (VP (VBN replaced) (ADVP (ADVP (RB as) (RB easily)) (SBAR (IN as) (S (NP (PRP it)) (VP (VBZ is) (VP (VBN consumed))))))))) (. .)))