This question has been around for decades and it requires truly interdisciplinary efforts to answer.
The major ecosystems of the planet are critically important for humans and for all creatures, and they are currently subject to very strong pressure from climate change and from human-induced ecological disturbances, such as agriculture and invasive species. Ecosystems change both slowly and abruptly in apparent response to extreme events that may be embedded in long-term change. Ecosystems have strong feedback effects on the atmospheric budgets of heat, moisture, greenhouse gases and aerosols, and, therefore, on climate.
To find out how climate change will affect forests and dry-land vegetation, and how those changes will affect atmospheric composition, we need more observations. Observations need to be on timescales of decades or longer to reflect processes in the atmosphere and of the major ecosystems. They also need to span local, regional and global spatial scales. Finally, we need ecosystem models integrated with atmospheric models that work like real systems.
In the United States, there are efforts to study parts of this problem and to make the requisite observations, supported by the National Science Foundation, the Department of Energy, NASA, the U.S. Forest Service and NOAA. What remains to be done is to coordinate and invigorate these efforts, and to commit to solving the problem on the relevant time and space scales. We have certainly seen a lot of progress in the last few years, but it has mostly served to point to the serious the gaps in our knowledge.
Ecosystem health goes hand-in-hand with human health and economic vitality. Atmosphere-biosphere interactions are powerful and have vast implications.
IMHO,
although CO2 concentration has increased by 40% it has had little effect on the air temperatures in tropical forests. This is because water vapour is the dominant greenhouse gas there. In the tropical forests the water vapour is saturated and and remains constant due because it is recycled daily by tropical storms. Moreover, evaporation of water vapour has a cooling effect as does the formation of clouds during the day. The land surface is also shaded by the tree canopy.
These factors do not apply to dry land. There is little cooling from evaporation and transpiration, and due to the lack of water, clouds do not form and shade the land during the day. It is often necessary to supply irrigation water to maintain crop yields. This makes these land vulnerable to droughts.
Thus the conversion of jungle by felling forests to create pasture land is likely to pose a greater threat to humanity in those areas than anthropogenic greenhouse gases.
I can anticipate the results of a personal research on forests.
The main forest associations were well characterized by Sandro Pignatti in his book on “Boschi d’Italia” (1998), giving the possibility to calculate the biotic indices of Ellenberg at the moment of their first relevés. These data can be compared with the Ellenberg indices of recent forest surveys (2005-2015). In this period of about 50-60 years, the raise of climatic temperature was strong: after 1985 the mean T°C increased in Italy of about 1-2 °C and the perennial snow limit of altitude on the Alps passed from 2980 to 3080 m.
The analysis of 72 forest samples indicate two different behaviours, (1) the boreal-Alpine conifer formations present a little decrease in thermophile species, while (2) the temperate and Mediterranean formations increased their thermophile species. So, even altitudinal belts follow a similar behaviour. Anyway, at this time the increase of thermophile species (case 2) results 7.47% in an altitude belt of 180 m (mean), while the decrease (case 1) results 3.0 % in an altitude belt of 1520 m (mean).
I cannot give you an exhaustive answer to your question, but I can share what I know from the top of my head.
Climate change implies changes in temperature, precipitation and of course many more abiotic factors that interacts with forest ecosystems. Even 1°C change in temperature can already shift plant distributions by kilometers, since many plants can only tolerate a range of temperatures. Temperature changes can also affect plant metabolism, including respiration and photosynthesis, which influences the rate at CO2 taken from the atmosphere and O2 released to the atmosphere. Normal rates of evapotranspiration will be affected such that warmer temperatures will increase the amount of water vapour that fills the atmosphere; while greater humidity caused by increased precipitation will limit evapotranspiration.
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