Not exactly my expertise, so take that into account.  Trees that live long lives, with deep strong roots and able to withstand disturbances well, are probably as good as fast growing trees.  If trees grow fast, it still is important that wood is used for long term uses.  The slow growing trees are often stonger and more suitable for long term structures as homes.  Tree species that tend to be disease resistent are also better for building for long term uses.  Trees along streams that fall into streams and are buried in sediment and stay under water can sequester carbon for centuries, so old activities such as snaking out buried or water logged trees from rivers are pulling out sequestered carbon, but most of that is used for long term furniture, ornamental pieces, so most is still sequestered.  Old growth trees that are eventually felled and used for long term products are probably going to sequester carbon longer than when the trees die and fall over.  

Fast growing trees that are not useful for any long term purpose to extend the sequestration may be storing and then recycling carbon back into system at a faster rate.

I also am not an expert on this subject, and have read a limited number of studies.

An interesting article is:

Kirby, Kathryn R., and Catherine Potvin. "Variation in carbon storage among tree species: implications for the management of a small-scale carbon sink project." Forest Ecology and Management 246, no. 2 (2007): 208-221.

It shows differences between carbon sequestration in forest and agro-forests (human created forests), and suggests agro-forests are less efficient a carbon sequestration.

There are studies that show that sequestration varies with the age of different species of trees as well

Kaul, M., Mohren, G. M. J., & Dadhwal, V. K. (2010). Carbon storage and sequestration potential of selected tree species in India. Mitigation and Adaptation Strategies for Global Change, 15(5), 489-510.

I haven't used this, but know that the US Department of Agriculture, Forest Service has a tree carbon calculator.  I;m not sure what kinds of trees are included in the data base.

http://www.fs.usda.gov/ccrc/tools/tree-carbon-calculator-ctcc

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Please have a look at these links for said query.

https://www.researchgate.net/publication/292567830_Carbon_sequestration_of_fast_growing_tree

https://www.researchgate.net/publication/233757094_Environmental_benefits_of_planting_fast-growing_trees_on_eroding_cropland_in_Java

http://www.foresteurope.org/news/importance-big-old-trees-carbon-sequestration-recognized-new-study

http://learn.eartheasy.com/2014/01/10-carbon-storing-trees-and-how-to-plant-them/

Article Carbon sequestration of fast growing tree

Article Environmental benefits of planting fast-growing trees on ero...

Thanks William, Micheal and Arvind for you inputs on the subject . I will study the matter linked by you.

 I think this is not true, however, fast growing trees can be a good option because they sequestrate CO2 faster than the others. But on a certain age or stage might be saturated so in long-term slow forest tree species may be better than fast-growing trees. This may also depend on forest management and land use such as coppicing and plantation on arable lands. I would be glad to see more opinions on this topic. Thanks. 

Your question is good but requires lots of space to be answered. Very shortly, carbon sequestration depends on many different factors where plant biology is only a part of the phenomenon. Just to remind some of the factors involved in carbon sequestration,  the approach differs from the individual plant level, population, and forest complexity. For example, the ecophysiological responses vary with these levels, with the interactions with other species (from  insects to mammals to plants, fungi, parasytes, etc.), climate variability, kind of management if there is any, soil properties, suitability of the site/environment for the plants species, and more. Thus, I would recommend that your approach to the estimation of the rate of carbon sequestration first of all sets what are the 'parity' conditions of the plant species growth that you may want to compare (softwood/hardwood), second you calibrate the relationships between rate of growth and density of wood, and finally you can start to verify what is the species with better carbon sequestration 'rate & amount', and where.

Adding to the on-going discussion, myriad of factors need to be taken into consideration. Some of which have been mentioned by previous contributors.  However, what I will like to add is that, it is quite important to look at the question and address it also from a lifecycle perspective whereby it is investigated on a comparable basis for both fast growing species and other species, from the lens of:

1. net carbon balance linked to working the land either tilling (partial tilling or no-tilling) of the land. Here he similar soil types and soil qualities, with equivalent carbon content should be used for both species. Also on the assumption that the same or similar machinery (here looking at embedded carbon) and energy efficiency of machinery and the fuel type used.

2. In the case that of fertilization is utilized to achieve high plantation productivity this needs to be looked at also. For example, nitrous oxide emissions embodied in synthetic fertilizers get released in the field. But let's remember that nitrous oxide (N2O) is a worse greenhouse gas than carbon dioxide (CO2). For example, one tonne of N2O gas has the equivalent global warming impact as 298 tonnes of CO2. So it is important to take that into consideration. Preferably, organic manure or compost is preferred over synthetic fertilizers.  

(3) Investigating the carbon capture or sequestration during the life span (indeed in a given time frame) for both fast growing species and the other compared specie(s)

(4) the fate of the wood after harvest. When utilized for example in products (e.g. buildings, furniture etc) the carbon is stored and not released. If at the end of life, these products are burnt or buried in landfills, the stored carbon get released. The same happens if the wood is immediately converted to energy or allowed to decay after harvest.  

You will find additional literature below:

Lippke, B., Oneil, E., Harrison, R., Skog, K., Gustavsson, L., & Sathre, R. (2011). Life cycle impacts of forest management and wood utilization on carbon mitigation: knowns and unknowns. Carbon Management, 2(3), 303-333.

Skog, K. E. (2008). Sequestration of carbon in harvested wood products for the United States. Forest Products Journal, 58(6), 56.

Smith, J. E., Heath, L. S., Skog, K. E., & Birdsey, R. A. (2006). Methods for calculating forest ecosystem and harvested carbon with standard estimates for forest types of the United States.

Skog, K. E., & Nicholson, G. A. (1998). Carbon cycling through wood products: the role of wood and paper products in carbon sequestration. Forest Products Journal, 48(7/8), 75

Micales, J. A., & Skog, K. E. (1997). The decomposition of forest products in landfills. International Biodeterioration & Biodegradation, 39(2), 145-158.

I hope that you might perhaps find this information useful.

Thanks Emmanneul for details of CS and added refrences. I will study them.

Not really.

Not always. Ocean is great sink for Carbon because of Phytoplankton. It may happen that fast growing species may have higher Photosynthetic capacity, may be for couple of years, then their P/R ratio declines and may go to infact zero. This is true in case of Social forestry exotic species tried in India. These species being softwood becomes susceptible to diseases, winds and cyclones, so do not provide environmental benefit for long time.

Thanks Dr. Upadhaya sir. I mean to say say here if in short rotation if industrial plantations we are raising fast growing trees and harvest in 4-8 years and again replant it. By this cycle are we sequestrating more carbon than if we planted a long rotation slow growing trees for 80-100 years.

Definitely because of high biomass, but lamina area also matters. Kindly consult this Govt. report