It happened Monday afternoon at Berryessa Creek Park.
Michael Aceves is shocked this large Eucalyptus tree came down.
He recalls someone from the city or water district studying the area 3 year ago.
"I asked them actually about this tree that fell and they said that's actually one of the healthiest trees you have here at the creek," said Aceves.
At SJ park where tree fell yesterday/killed a man. Neighbor says victim is always here picking lemongrass pic.twitter.com/inBGLyPGE5— Melanie Woodrow (@MelanieWoodrow) February 2, 2016
Late Tuesday afternoon the city's arborist arrived.
"I pulled a chunk off the bottom of this that's right here in the creek and it falls apart like confetti," said Russell Hansen.
Monday night, SJPD said the victim may have been riding his bicycle in the area.
Aceves says he worries about people walking through the park, especially children.
"It needs to be removed, but they also need to monitor what's going on with the rest of the trees," he said.
He says before someone else gets hurt or worse.