( ( ( The relationship ) ( between ( ( trees and ) ( ( certain mushrooms ) ( that ( grow ( beneath them ) ) ) ) ) ) ) ( ( is ( beneficial ( to ( both organisms ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN relationship)) (PP (IN between) (NP (NP (NNS trees)) (CC and) (NP (NP (JJ certain) (NNS mushrooms)) (SBAR (WHNP (WDT that)) (S (VP (VBP grow) (PP (IN beneath) (NP (PRP them)))))))))) (VP (VBZ is) (ADJP (JJ beneficial) (PP (TO to) (NP (DT both) (NNS organisms))))) (. .)))
The relationship between trees and certain mushrooms that  grow beneath them is beneficial to both organisms.
(ROOT (S (NP (DT A) (-LRB- -LRB-) (NN n) (-RRB- -RRB-) (NN beneficial) (NN relationship)) (VP (VBZ exists) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT a) (JJ certain) (NN type)) (PP (IN of) (NP (NP (NN tree) (POS 's)) (NNS roots)))) (VP (VBP need) (S (NP (DT a) (NN fungus)) (VP (VB present) (SBAR (IN in) (NN order) (S (VP (TO to) (VP (VB grow) (ADVP (RB normally)))))))))))) (. .)))
A(n) beneficial relationship exists when a certain type of tree’s roots need a fungus present in order to grow normally.