The problems with carbon offsets from tree-planting

If a forest burns, the carbon will be released back into the atmosphere.

One of the most common types of carbon offsets on the market comes from tree-planting projects. Many companies sell these offsets and claim they can be used to offset greenhouse gas emissions from other sources, such as air travel, automobiles, etc. While planting trees obviously has environmental benefits, selling carbon offsets from these projects is problematic in a number of ways:

Methodological challenges: Much scientific uncertainty surrounds the precise quantification of carbon sequestered in trees and soils. For example, studies suggest that more carbon is stored in the soil of many forest types than in the above-ground biomass (trees and other plants), and this soil carbon can be disturbed and released by harvesting and reforestation activities. Without an accurate understanding of the carbon inventory, it is difficult to quantify the net carbon benefit of planting a tree.
Permanence issues: Although a newly planted tree will remove carbon from the atmosphere while it is growing, a forest is never permanent and may one day succumb to disease or insect attacks (e.g., mountain pine beetle outbreaks), fire or logging, releasing the carbon into the atmosphere again. Planting trees is therefore at best a temporary solution. This makes issuing carbon offsets from forests problematic, as without some kind of insurance scheme (i.e., issuing temporary credits based on ongoing monitoring of the forest health) it is impossible to assure that the offsets sold won't become worthless at some point (e.g., due to forest fire 75 years after planting). On the other hand, avoided emissions (due to renewable-energy projects) or reduced emissions (due to energy-efficiency projects) are, in essence, permanent, because they prevent carbon from entering the carbon cycle in the first place. Keeping the carbon sequestered in the ground in the form of coal, oil or gas is a much better solution than burning it and then later trying to sequester it with trees.

Lack of space: It is simply not possible to plant sufficient numbers of trees to deal with the increased carbon dioxide emissions that are expected over the next half century. For example, scientists have estimated that to soak up just the United Kingdom's annual greenhouse gas emissions, we'd need to plant a new forest the size of Devon and Cornwall every year, and look after them in perpetuity.
Forests as a source of carbon emissions: Recent evidence suggests that global warming itself is stressing ecosystems and turning forests and forest soils into failing forests and, in the long run, into net sources of CO2. Thus, if we don't curb our use of fossil fuels, it won't matter how many trees we plant because these forests will be overcome and die as the climate continues to warm.
Lack of additionality (i.e., above and beyond business as usual): Many forestry offsets are from tree plantings that would, or should, have replaced logged forests anyway. It is therefore difficult to claim that such plantings can be used to offset emissions from elsewhere.
Monoculture plantations: Old-growth forests are often replaced by tree-farm plantations that are heavily managed (including with chemicals and fossil fuel-intensive machinery) and do not offer the same biodiversity benefits as natural forests.
No root-cause solution: Investment in forestry offsets does not contribute to reducing society's dependence on fossil fuels, something that is ultimately needed to address climate change. Responding to climate change means fundamentally changing the way we produce and use energy. Since funds to invest in emissions reductions projects are limited, the voluntary carbon market (i.e., purchasers of offsets) will have a larger impact by directing scarce capital to energy-related projects.

For these reasons, many environmentalists oppose the use of tree-planting to mitigate climate change.