Trying........

[Tangled-Universe]

Be aware that Matt's estimation also is quite spot on These are difficult models to light and render. Looking forward to see the result!
I don't know about your workflow, but you can save a lot of time just by performing crop renders to check if you get proper shadowing and lighting, instead of rendering full frames only.

[Tangled-Universe]

Rendering an example now...see if I can get close to it...

Which models do you use by the way?

[yossam]

The adult Giant Sequoia on the xfrog DVD. Good luck

[Tangled-Universe]

Ah ok, I have those too, so may be I'll use those in a second attempt...

Anyway, here's a very quick doodle using NWDA HQ Pines.
It's not completely there yet, but if it works with these models then it should definitely work as well for the Sequioa's.

I used the settings I suggested before, although I also increase 'strength on surface' in the enviro light node as well as camera exposure to 2.
I'll post a tgc/tgd here when it's improved.

[Tangled-Universe]

How close does this come?

[yossam]

Pretty damn close...................mine will be finished in about 4 to 5 hours

I will post then and we can compare..........

[yossam]

Well. this one only took about 40 hours. I got rid of all the extra light sources and basically followed T-Us' recommendations. I changed the haze to 8 and checked receive shadows from surfaces to get rid of a hotspot. I want to thank everyone for their suggestions and I totally agree with Matt.....some of these models are a pain to light.

[Tangled-Universe]

40 hours?
What are your system specs and what were the rest of your rendersettings?

Mine took only 3 hours with GI 3/6/8 and AA16 (quite adaptive, though).

[yossam]

My render machine is an old quad core 2.4 Ghz with 8 G of ram. I had it set to use only 2 cores and 400 M of memory. The render settings were:

Quality-.85
AA-6
GI-2/2/8