Oh my. I do not really remember this part of the story.
A group of tree spirits are based in Wood Immortal Temple (木仙庵) at Bramble Ridge (荊棘嶺). The four most senior spirits are Jinjie Shiba Gong (勁節十八公), Guzhi Gong (孤直公), Lingkongzi (凌空子) and Fuyun Sou (拂雲叟), who appear in human form as four old men. Their true forms are a pine tree, a cedar tree, a juniper tree and a bamboo tree respectively. Tang Sanzang encounters them in the temple and discusses poetry with them. Other spirits include the Naked Demon (赤身鬼) and the Apricot Immortal (杏仙), whose true forms are a maple tree and an apricot tree respectively, while their servants are flower spirits. Sun Wukong sees through their disguise and tells Zhu Bajie when the latter asks him if he detects any demons in the vicinity. Zhu Bajie then destroys all the trees. Tang Sanzang is shocked and he scolds Zhu Bajie for killing innocents because the spirits never harmed him, but Sun Wukong explains that it is best to eliminate the spirits now in case they become evil in the future.