To qualify as a real estate investment trust for federal income tax purposes, a company must (among other things) pay out 90% of its ordinary taxable income ("TI"). Some companies also choose to supplement their payouts by also distributing their capital gains income, but this is not required. The distribution of the 90% of TI must be made in the taxable year to which they relate or, if declared before the timely filing of the tax return for such year and paid not later than the first regular dividend payment after such declaration, in the following taxable year. Basically, companies may choose to have their taxable income "spillover" into the next calendar year to support future distributions. On the other hand, some companies prefer to pay out 100% of their taxable income in order to maximize their dividends paid deduction. Suffice to say, taxable income is the metric that drives the dividend for mortgage REITs.
It is important to note that the IRS definition of taxable income and the FASB definition of GAAP income are two completely different animals. There are a number of differences between book and tax income, but the two largest adjustments affecting mortgage REITs are the provision for loan losses and mark-to-market valuation adjustments. For tax purposes, these types of losses may only be deducted when realized. Therefore, the provision for loan losses and mark-to-market writedowns are added back to book income to derive taxable income. Given the current market environment, you can see how there could be very large book/tax differences due to the writedowns and that taxable income may greatly exceed book income.
JER Investors Trust (JRT) summarizes the risk created by the book/tax differential very well in its latest 10-Q:
Certain of our investments...may generate substantial mismatches between taxable income and available cash. In order to meet the requirement to distribute a substantial portion of our net taxable income, we may need to borrow, sell assets or raise additional equity capital...there can be no assurance that we will be able to do so on terms acceptable or available to us, if at all.
The front-end loading of taxable income generated by mREIT business functions accelerates dividend obligations ahead of the more straight-lined receipt of cash flow from their investments. Therefore, most mREITs must borrow, securitize, or consistently tap the equity markets to maintain sufficient liquidity. If access to the capital markets is suddenly closed, many mREITs don't have vital access to cash and must resort to asset sales at fire-sale prices. This phenomenon has affected many companies, including NovaStar Financial (NFI), Thornburg Mortgage (TMA), and Luminent Mortgage (LUM).