Would there be any problem of using aluminum as the reservoir? I'm thinking of using a bunch of aluminum soda cans. Was just concerned about aluminum leaching into the soil and into the tree.

take a look at this

http://www.spectrumanalytic.com/support/library/ff/Soil_Aluminum_and_test_interpretation.htm

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3056430/

Therefore, Al can cause severe health problems in particular populations, including infants, elderly people, and patients with impaired renal functions, and unnecessary exposure to Al should be avoided for such patients

Thanks guys.. Guess I should have googled it first.  Right after posting, I found some other negative information too.  Was thinking they'd make a good reservoir.  But not I don't think so.

its funny but we use lots of al in cooking utensils.....
still the exposure is short , if with soil especiallly warm it might become a problem .