How will increased $\ce{CO2}$ affect forests and other vegetative areas?

Question

Some researchers contend that increases in atmospheric CO₂ (e.g., from anthropogenic emissions) will enhance plant growth in general.

1. What is the level of that enhancement?

2. Does the associated climate change simply make it easier for invasive species to propagate?

3. Or is there really a valid case to be made that at some point, if concentrations of CO₂ get high enough, forests and vegetative zones will somehow benefit?

4. In general, how will increases in CO₂ and related climate change affect forests and other vegetative areas?

Answer 1

If you increase CO₂ concentration and keep all other parameters at there current level, then biomass production should go up. The reason is simply that CO₂ is one of the building blocks biomass is made of in photosynthesis.

In the real world it's trickier than that of course. Here you have to take into account how natural cycles will be affected by increased CO₂ levels. Temperatures e.g. will go up which is expected to be beneficial for biomass production for moderate increases and becomes increasingly bad for higher increases. Precipitation patterns change, wind patterns and therefore soil erosion change etc. etc.

Chapter 7 of the IPCC report deals with the question how climate change affects biogeochemical cycles; biomass production is one of the parameters that go into the carbon cycle.

To my knowledge biomass production is one of the parameters that benefit from moderate temperature increase but are expected to turn to their opposite around the 2-degree benchmark.

Answer 2

One of the consequences that I find more fascinating with the increased CO₂ problem is the changes that affect specific components of the environment. One example that I like is the effect on poison ivy. Researchers at Duke University (Mohan et al., 2006), as part of their Free-Air CO₂ Enrichment (FACE) experiment, reported a large change in poison ivy activity when they added CO₂. They claim:

"...elevated atmospheric CO₂ in an intact forest ecosystem increases photosynthesis, water use efficiency, growth, and population biomass of poison ivy."

For more information on the experiment and the many results they have obtained from it, check here. They have more than 250 publications on the findings of the experiment.

So now, on top of all the other changes, we have to put up with poison ivy on steroids.

However, it is important to keep in mind that

... species that respond strongly to elevated Carbon in [nonresource-limited] experiments are unlikely to also be the most responsive in resource-limited field conditions... [so] we cannot directly extrapolate ... which species will be most responsive to elevated Carbon in the long term.

(Ali et al., 2013)

and

The rise in [CO2] will probably alter the prevalence of invasive species, but the nature of this change is difficult to predict. Whereas alien species may benefit from higher[CO2] in some regions, native species may benefit in others. Plants with certain CO2-responsive traits are likely to benefit from the rise in [CO2], especially if they are growing in ecosystems where those traits are rare. For instance, C3 species growing in C4-dominated ecosystems are likely to benefit from the rise in [CO2] (but under some circumstances may not). Invasiveness and CO2-responsiveness are not clearly linked.

(Dukes, 2000)

Answer 3

CO₂ can only enhance plant growth when other resources such as nutrients or water are not limiting growth. If N for instance is scarce, no matter how much you increase CO₂, plants will not take advantage of the increased CO₂ concentration.

Answer 4

It seems that tropical forests absorb more carbon dioxide than many scientists have believed before. NASA and NCAR scientists have shown that tropical forests absorb 1.4 billion metric tons out of a total global absorption of 2.5 billion metric tons. This estimate is much larger than previous estimates and the tropical contribution seems to be much larger than other areas like temperate or boreal forest.

Previously, most climate models showed mid-latitude forests in the Northern Hemisphere absorbing more carbon than tropical forests. Those results were data-limited and suggested that deforestation was causing tropical forests to release more carbon dioxide than they were absorbing.

The new results seem to indicate the opposite and are based in a more comprehensive analysis of available data.

... reconciles results at every scale from the pores of a single leaf, where photosynthesis takes place, to the whole Earth, as air moves carbon dioxide around the globe.

Answer 5

It seems that the CO2 fertilization has been over-estimated in past studies.

From Nature Climate Change article,Large divergence of satellite and Earth system model estimates of global terrestrial CO2 fertilization by W. Kolby Smith, Sasha C. Reed, Cory C. Cleveland, Ashley P. Ballantyne, William R. L. Anderegg, William R. Wieder, Yi Y. Liu & Steven W. Running

Atmospheric mass balance analyses suggest that terrestrial carbon (C) storage is increasing, partially abating the atmospheric [CO2] growth rate, although the continued strength of this important ecosystem service remains uncertain. Some evidence suggests that these increases will persist owing to positive responses of vegetation growth (net primary productivity; NPP) to rising atmospheric [CO2] (that is, ‘CO2 fertilization’). Here, we present a new satellite-derived global terrestrial NPP data set, which shows a significant increase in NPP from 1982 to 2011. However, comparison against Earth system model (ESM) NPP estimates reveals a significant divergence, with satellite-derived increases (2.8 ± 1.50%) less than half of ESM-derived increases (7.6  ±  1.67%) over the 30-year period. By isolating the CO2 fertilization effect in each NPP time series and comparing it against a synthesis of available free-air CO2 enrichment data we provide evidence that much of the discrepancy may be due to an over-sensitivity of ESMs to atmospheric [CO2], potentially reflecting an under-representation of climatic feedbacks and/or a lack of representation of nutrient constraints. Our understanding of CO2 fertilization effects on NPP needs rapid improvement to enable more accurate projections of future C cycle–climate feedbacks; we contend that better integration of modelling, satellite and experimental approaches offers a promising way forward.