# For a tree over its entire existence, does it actually have a net negative effect on atmospheric CO2?

A tree while alive converts CO2 + water -> carbohydrates + O2. However, once the tree dies, it decays, releasing CO2 back into the atmosphere. My question is, over an individual tree's overall existence, does a tree actually contribute to a reduction in atmospheric CO2?

I'm aware there's other pathways a tree could end up as a more long term carbon store (carbonaceous rocks), but mostly interested in if a tree were to die and fall in a forest, decay in 50-150 years, would it have contributed to a net reduction in CO2, or does a tree typically act as more of a temporary 100+ year store of CO2?

• Some amount of carbon (e.g. from leaf litter and tree roots) will be sequestered in the soil for longer than the life of the tree, but presumably this is a minor amount (I am searching the literature now to find a relevant number). Presumably carbon in the soil cycles on the order of a thousand years (citation needed :-). Trees harvested for lumber may extend the carbon cycle time by incorporation into buildings. On the other hand, carbon from trees may be cycled more quickly due to forest fires. Feb 10 at 23:42
• Do you mean a tree (which might just be replacing and then replaced by another in a forest), or planting trees where there currently are none? Also note that over a long timeframe, part of the dead trees (and discarded leave/needles) will get converted to a layer oif duff & humus, which can persist for long times. Feb 12 at 1:46
• @jamesqf either. My interest was more if a single tree (or forest of trees) were to die and follow its natural decay process, would it have a net absorption of atmospheric CO2. More or less is a tree just a means of carbon storage in its normal life cycle or is there a long(er) term CO2 reduction I wasn't aware of? Feb 12 at 2:06

A brief review of recent non-paywalled available literature indicates that such an effect likely exists but that it is difficult to quantify based on currently available data.

Some amount of carbon from trees can be sequestered in the soil for periods time significantly longer than the typical above-ground decomposition time of organic matter, potentially for millennia. This clearly lengthens the carbon cycle time, but it is not clear to me whether this represents carbon storage, as there does not seem to be a well established minimum cut-off time for this. The primary source for soil-sequestered carbon are tree roots, with leaf litter constituting a secondary source.

The following paper (preprint online) addresses the question in the specific context of agroforestry, i.e. cropland interspersed with trees. The paper notes multiple times that the processes involved in soil sequestration are not well understood and that quantitative measurements and estimates vary widely, as one would expect based on differences in climatic and soil condition. Note on units: A Mg corresponds to a metric ton.

Klaus Lorenz and Rattan Lala, "Soil organic carbon sequestration in agroforestry systems. A review." Agronomy for Sustainable Development, Vol. 34, No. 2, April 2014, pp. 443-454.

The soil organic carbon (SOC) pool, in particular, is the only terrestrial pool storing some carbon (C) for millennia which can be deliberately enhanced by agroforestry practices. [...] The SOC storage in agroforestry systems is also uncertain and may amount up to 300 Mg C ha−1 to 1 m depth. [...] Further, between 30 and 300 Mg C ha−1 may be stored in agroforestry soils up to 1-m depth [...] The SOC sequestration depends primarily on the soil C input and soil stabilization processes. Plant root and rhizosphere inputs, in particular, make a large contribution to SOC [...] Litter fall and in turn SOC sequestration may be affected by stand-density management as, for example, higher stocking levels of trees enhance the vegetation C pool.

Another paper that deals specifically with urban trees mentions in passing that the soil accounts for a significant part of the carbon sequestered in non-urban forests (presumably both naturally growing and human-managed):

David J. Nowak and Daniel E. Crane, "Carbon storage and sequestration by urban trees in the USA". Environmental Pollution 116 (2002) 381–389

Another area of the urban forest carbon cycle that this paper does not analyze is urban soils. Sixty-one percent of the total carbon in non-urban forest ecosystems in the USA is stored in the soil environment (Birdsey and Heath, 1995). The amount of carbon from trees that is retained in urban soils, its residence time, and the amount of carbon currently stored in these soils remains to be investigated. It is likely, however, that urban soils contain less carbon per hectare than forest soils due to lower carbon inputs and increased soil decomposition rates due to warmer air and soil temperatures (e.g. Pouyat et al., 1997).

• This paper finds an accumulation rate of 6.0 to 18.0 g C m$^{−2}$ year$^{−1}$ in boreal peat forests. (More from peat than trees, as I read it.) Feb 11 at 3:10