TL;DR

A group of trees is not a forest. Forests are complex ecosystems.
Planting trees isn’t always good for the climate or biodiversity. It’s more important to focus on conserving old forests than planting new ones. And when planting new trees, they must be the right trees for the right location.
Globally, forests are on the decline and about half of reforestation projects come in the form of intensively managed, largely monoculture, commercial plantations that trap 40 times less CO2 than natural forests and do not promote biodiversity.
It is possible to “lock up” carbon in wood products like houses, furniture, and even paper to an extent. These more permanent uses of wood are much better for the climate than burning wood for fuel, which emits more CO2 than burning coal or natural gas, but eventually the rate of decay will outpace any sequestration benefits (by about 2030).

A primer on forests

Tree coverage ≠ forest area. A forest is not just a group of trees. It is a complex ecosystem consisting of a variety of flora, fauna, and microorganisms that all function together. Our forests sequester an estimated two billion tonnes of CO2 every year (since 2000) and so forestry offers one of the most immediate and practical solutions for climate change mitigation.

There are three general types of forests (with additional subtypes based on the amount of precipitation received). Forests cover about 31% of our global land area (as of 2020), with the majority distributed across only ten countries. More than half of all forests are found in only five: Brazil, Canada, the US, China, and Russia. Russia has the largest forested area with 20% of all global forests located there.

Source: CarbonBrief

Tropical

Centered around the equator
Greatest species diversity
Nutrient-poor & acidic soil

Temperate

Found in the mid-latitudes
Distinct seasons w/ a well-defined winter
Rich soils

Boreal