I purchased trees for my rental property in 2018. How do I add them to the basis of the land?

They are not added to the basis of the land. They are depreciated over a 20 year life. From Pub 946: Although you cannot depreciate land, you can depreciate certain land preparation costs, such as landscaping costs, incurred in preparing land for business use. These costs must be so closely associated with other depreciable property that you can determine a life for them along with the life of the associated property.

Here are some more capital improvements:

• Remodels and room additions (including decks and porches)
• New or upgraded landscaping, irrigation, sprinkler system
• Hardscape such as pavement, block or retaining wall, patio
• Fencing
• Swimming pool, spa
• Storm windows, doors 
• New roof 
• Central vacuum or security system 
• Upgraded wiring, plumbing, duct work
• Central heating, AC, humidifier 
• New furnace, water heater 
• Filtration, soft-water, or septic system
• Built-in appliances 
• New flooring or wall-to-wall carpeting 
• Upgraded insulation
• Satellite dish