What's the difference between these Palo Verde trees?

As usual, this spring I have spent quite a bit of time hiking the desert near my home.  For the past week the Foothills Palo Verde trees (Parkinsonia microphylla) have been in bloom, dotting the hill and mountain sides with yellow.  The Palo Verde is typically the most abundant and important upland trees within the Sonoran Desert.  Every spring it is filled with flowers, which attracts swarms of insects (especially bees), and the insects in-turn attract lots of birds.  The blooming Palo Verde literally becomes a buzzing, chirping, swarm of activity each spring.  However, as I was walking I noticed a peculiarity, some of the Palo Verdes were blooming while others were not.  Why was this?  Here is some of what I observed.

Some of the Palo Verdes were blooming like this one.

Others had very few flowers or none at all like the one above.

So why the difference between the above two Palo Verde trees?  The trees both received the same amount of rain and both were in the same type of soil.  The answer to the difference in blooms lays in the pictures above.  So I hypothesized that the Saguaro was out competing the Palo Verde for water, resulting in the Palo Verde not having enough water to produce flowers (second photo).  The Palo Verde in the top photo was not associated with a Saguaro, and according to my hypothesis would have plenty of water.  To test this I decided to survey Palo Verde trees associated with and without saguaro cacti to determine how flowering was affected.  So, spending about a days worth of hiking around a mountain side I recorded the percent flowering of Palo Verde trees associated with Saguaros and not associated with saguaros. 

This is what I found.  Palo Verdes associated with Saguaros (Saguaros had to be at least three meters high) averaged 30% flowering within their canopy.  Palo Verdes not associated with Saguaros averaged 42% of their canopy flowering.  For all us statistical nerds, yes this was statistically significant for one-tailed, two-tailed, and paired t-tests!  So the test does support our hypothesis that Saguaros are our competing Palo Verdes for water.  Thus Palo Verde flowering is reduced.

This is all quite odd considering the Palo Verde is a well know nurse plant for the Saguaro Cacti.  A nurse plant aids the establishment and growth of another plant, "nursing" another plant to life.  The shade, nutrients, reduced temperature, and greater moisture under a Palo Verde are well known to "nurse" the establishment of young Saguaros.  But apparently the Saguaro "bites the had that feeds it" later in life, out competing the Palo Verde for water and eventually killing it (according to other researchers).