( ( During ( periods ( of drought ) ) ) ( , ( ( ( ( trees died ) and ) ( ( prairie plants ) ( ( took over ) ( ( previously forested ) regions ) ) ) ) . ) ) )
(ROOT (S (PP (IN During) (NP (NP (NNS periods)) (PP (IN of) (NP (NN drought))))) (, ,) (S (NP (NNS trees)) (VP (VBD died))) (CC and) (S (NP (JJ prairie) (NNS plants)) (VP (VBD took) (PRT (RP over)) (NP (ADJP (RB previously) (JJ forested)) (NNS regions)))) (. .)))
During periods of drought, trees died and prairie plants took over previously forested regions.
(ROOT (S (SBAR (IN Because) (S (NP (NNS trees)) (VP (VBP add) (NP (NN water) (NN vapor)) (PP (TO to) (NP (NN air)))))) (, ,) (S (VP (VBG cutting) (PRT (RP down)) (NP (NNS forests)))) (VP (VBZ leads) (PP (TO to) (NP (NP (JJR longer) (NNS periods)) (PP (IN of) (NP (NN drought)))))) (. .)))