The Hawaiʻi ecoregion description for high mountains (alpine) defines it as the mountains above 9,000 feet and has a good short description and maps: http://www.hawaiiecoregionplan.info/alpinesystem.html.
A brief but informative summary of unique flora and fauna on Mauna Kea is here: https://bigislandguide.com/flora-fauna-mauna-kea.
The National Park Service Haleakala website has a short entry: https://www.nps.gov/hale/learn/nature/naturalfeaturesandecosystems.htm.
A Wikipedia entry is here: https://en.wikipedia.org/wiki/Hawaiian_tropical_high_shrublands.
A study on changes in the alpine systems for Pacific Islands (Hawaiʻi and New Zealand) by Frazier and Brewington (2019) and including maps and photos is here: https://www.eastwestcenter.org/sites/default/files/filemanager/Research_pdfs/Frazier__Brewington_2019_CurrentChangesAlpineEcosystemsPacificIslands.pdf.