Tree planting and natural regeneration

Forests support biodiversity by providing many micro-habitats, as do countless other habitat types. Habitat heterogeneity supports broad biodiversity.

In natural regeneration, succession starts when pioneer species seeds arrive on the wind or via birds, which can tolerate nutrient-poor soil. Succession is gradual, with pioneer and early-successional species contributing organic matter to the soil, supporting bacteria in the nutrient cycle and developing nutrient-rich soil. When soil is good enough, faster-growing plants and saplings can establish themselves. Succession through natural regeneration appears to achieve more stable, native & biodiverse populations (Prach & Walker, 2011) (Hodač​ová & Prach, 2003) (Broughton, 2021).

However, natural regeneration is slow, sometimes taking decades to develop good enough soil for an ecosystem to reach intermediate succession stage and grow large trees, delaying biodiversity and carbon sequestration.

The community is most vulnerable in its early stages of succession, threatened by fluctuitous climate conditions and invasive species. Non-native species can invade easily, dominating space, draining resources and bringing disease. Climate change makes succession trajectory unpredictable and challenges survival of native species, most of which cannot handle elevated heat and extended drought.

Species of tree and plant can be selected & planted based on desirable traits like fast growth, high carbon sequestration rates, survival in changing climate (higher temperatures and drought), tolerating waterlogged soil, supporting biodiversity and accelerating habitat succession. Planting trees accelerates forest establishment; their roots decrease soil erosion and water loss, protecting nutrients from leaching; their trunks & branches store massive amounts of carbon; their canopy leaves provide shade cover, protecting creatures from the Sun and reducing evapotranspiration.

C₄ Photosynthesis

C₄-photosynthetic plants sequester carbon most efficiently, can in low-nitrogen conditions and use little water, giving them the advantage in nutrient-poor soils, which are characteristic of early-successional sites. No trees are C₄​ but many fast-growing grasses & shrubs are, and could be planted in non-tree habitats. (Osborne & Beerling, 2006)

Planting trees into unprepared sites will usually result in most dying due to poor soil and lack of supportive habitat. Allowing and/or supporting natural regeneration of a site into succession will prepare the environment for trees to establish and flourish. Additionally, pre-existing tree-free habitats may already support biodiversity and sequester large amounts of carbon. Wetlands hold the most (EEA, 2022), and other plants like grasses sequester more into their roots, and expunge it throughout the soil, much deeper than trees do. Tree carbon input to SOC is primarily via litter and shallow roots (Smith & Johnson, 2003). Research shows no gain, and even net ecosystem carbon loss, decades after planting trees on heather plots, due to increased soil respiration and decreased SOC (Friggens et al., 2020). Not all sites need trees!