This is a tough question because there can be many factors.  Most of the time, the biggest factor is drought.  When a tree doesn’t get enough water, especially in the summers, it can decline rapidly.  Sometimes, it may take a year or 2 to show the symptoms or signs of decline, but the drought most definitely had an impact on the tree. Secondary issues that are common are borers, other insects like carpenter ants, and fungi.

Great question!  First, call a certified arborist to evaluate your trees if they look like they are not doing well.  Secondly, here are some good things to consider for tree care maintenance:

Fertilizing – You can fertilize mature trees once every 2-3 years.  Fall OR Spring, are the times for fertilizing trees.  Tree Care

Mulching a tree ring – this is one of the best things to do for a tree.  To keep lawn mowers away, to retain moisture, and don’t put to much mulch because tree roots need oxygen.

Aerating – even though this is most of the time for grass, it definitely can help oxygenate the soil and help de-compact the soil as well.

Vertical Mulching when the ground is hard – If you notice that the ground is very hard, you may want to consider this option.  The ground can get hard for many different reasons, rain runoff, to much foot traffic, heavy equipment, cars, etc.   Tree Care

Soil tests – Fairfax County offers free soil tests.  It is pretty easy to get a soil test and can be one of the most beneficial things you can do to help your tree!

Assess a tree for cavities, hypoxylon canker, borer activity, tip dieback, etc – a certified arborist can evaluate a tree in decline and provide good recommendations.    Certified Arborists