I am a bit late in commenting about this.

Trees are a big problem near roses if they are to thrive. The root system
on a mature tree is very extensive.

An example given by an arborist; Picture a champaign goblet placed on a
dinner plate. The goblet represents the tree and the plate the root system.
The root system spreads well beyond the foliage canopy. A large tree can
uptake more than 100 gallons of water per day!

Competition for moisture is fierce and the tree will dominate.

Sun exposure is usually limited by the tree canopy, the rule of thumb is
the rose needs at LEAST 6 hours of direct sun light per day.

These are the BASICS.

Other factors MAY be toxins from the tree.

As another poster suggested contact the county agriculture extension and/or
an arborist.

The American Rose Society www.ars.org has links to local societies that may
be able to offer assistance.

Depending on your location the soil conditions and available nutrients will
affect growth.

A soil test is ALWAYS recommended. DON'T add lime to the soil unless you
are certain that the pH is too low.
The western US typically has alkaline soil conditions and adding lime simply
adds to the problem.

Tim
American Rose Society
Consulting Rosarian