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For example, if today I want to contribute to mass reforestation, what actions can I take? What about on the scale of a year, or a lifetime? This may make the question somewhat broad, but I envision an answer (maybe best as a community wiki) that lists specific things at different scales, overall answering the question: What are the most effective ways one can support mass reforestation?
I'll start with my own answer including some examples I'm aware of to help set context, and I'll highlight in bold some uncertainties. This will be a community wiki so we can build this list together and clarify uncertainties over time.
Contribute to non-profits and charities focused on forest conservation, reforestation, and sustainable land use (forest) policy. Many (re)forestation organisations offer the option to compensate your CO2 usage for e.g. flying.
Make consumer decisions which are forest-friendly, for example:
choosing paper and wood instead of fossil fuel based resources
preferring fiber products from certified sustainable forestry
Use Ecosia web browser and support other organizations that focus on
Actions This Year
Change your diet toward foods that conserve and create new forested landscapes
Are there any certifications of agroforestry or conservation agriculture based products?
Local diets can be easier to tailor to polycultural farms, which tend to support more biodiversity and trees than monocultures
Consuming tree-based foods and using Non Timber Forest Products (NTFP) (like maple syrup, forest-grown mushrooms or silvopasture grown animal products) when suitable can support forest enterprises
Make your charitable contributions a regular thing, 'set it and forget it'
Actions For Long-Term
Work in a career that is based on forested land-use, and maintain or restore forested land in property you manage (e.g. forestry, forest farming, agroforestry). This includes consulting, research, and extension work to help various industries and land-uses be more forest-friendly.
Volunteer with community forestry organizations
Work in a career supporting forested land-use, conservation, restoration, and sustainable use, either in policy or planning
Buy degraded/abused land in strategic locations (which should be cheap as a result). (Re-)Plant trees. Place a conservation covenant over the property so that future owners are legally required to preserve it.
In Victoria, Australia, organisations like the Trust For Nature can help with this process.
If you're not sure what a conservation covenant is, TFN has produced an easy-to-read brochure:
A conservation covenant (deed of covenant) is a voluntary, legal agreement made between a private landowner and Trust for Nature (“the Trust”). Its purpose is to permanently conserve and protect the natural, cultural or scientific values of the land. The Trust's conservation covenants are entered into under the Victorian Conservation Trust Act 1972, registered on Title and are legally binding forever.
Reforestation is a wishy washy subject. It's difficult, labor intensive and time consuming. But done well you can successfully reestablish forest cover. Jadav Payeng planted an entire forest by himself over 30 years so it's not impossible. If one man can do it in 30 years, 30 can do it in one...
Reforestation is achievable in a sequence steps.
Allocate the land you wish to plant. Private property is easier than public, If you own it, you're in charge of it or obtain permission from the owner to do so.
Berm the land so the soil does not wash away. Use geotextile bags, sand bags, rockwalls, or a silt fence.
Build a hill(s): Sounds unusual, but perfectly flat terrain is seldom what you want in an environment. scrap rocks, covered in dirt and built up, even to a moderate elevation.
Determine the trees necessary native to the environment formerly found.
Upon finding the necessary trees, find trees in the "Fabaceae" family. As they are nitrogen fixers they can grow in bad soils.
Microbe colonization: Mycorrhizal fungi and beneficial bacteria are as essential under the soil as the trees above it.
Fertilize: At this point, once the trees are growing, depleted soils don't regenerate quick enough to support them. It's time to fertilize, that can be done with compost, manure, artificial fertilizer
Water routinely until the tree's no longer require intervention and can sustain themselves on natural rainfall.
At the point the trees are self sustaining, begin planting forbs, herbs, ferns and understory vegetation.
Your forest prototype however is a mono-culture of one tree species. At this point it's necessary to replicate environment of forest succession, by planting other varieties.