I heard back from PLD's IR department, who was kind enough to respond the same day of my request.  She said that while 90% of rental income (NOI) is required to be distributed in order for PLD to maintain its REIT status, the reason that PLD's payout ratio is only 50% is that only a portion of PLD's earnings stems from rental income.  She said that PLD already pays out 90% or more of its rental income. 

She said she expects the overall payout ratio not to exceed 65% at any given time.

I am personally concluding from this not to count on any increase in the payout ratio as part of my analysis.  However, I also see the fact that PLD is reinvesting the other 50% of its income into growth as a positive thing for its stock price, in comparison to a REIT that pays out 90%+ of its overall income.

In other words, I'm happy with a 3.5% dividend provided that their investments in growth bear fruit.  This has been the case in the past few years . . . the big question is whether this will continue . . . which I look forward to continuing discussion on!

Thanks for the follow up. I new the rule, but was not aware of the % of income from rentals. I see they have agreed to some China partnerships so it may be the growth will be worldwide.