Q: My maple tree's leaves look awfully small this year. Last year it looked fine, but it's done this in the past, too. What's going on?
A: I'm going to guess that your maple over-produced seeds this year, which will cause the tree's leaves to emerge later, and be smaller, than usual. This gives the tree the appearance of being sparse.
It has close to the same number of leaves that it would have had it not over-produced seeds, it's just that the leaves are smaller, so the tree looks thinner.
Dry Springs seem to trigger the most over-production of maple seeds, although wet, cold or hot Springs seem capable of doing the same thing (thus we've covered all the bases!). Stress is the actual, suspected underlying cause, and all these conditions can stress a tree.
Prior conditions can also stress a tree, such as root damage due to construction, or a drought. The tree "senses" this stress, and the production of an abundant seed crop may be its way to ensure its species will continue, should it die itself.
If you've been fertilizing your tree regularly it should have ample energy reserves to survive this drain on its energy. If not, have your tree fertilized on an annual basis to help provide the elements and minerals it needs not only for normal healthy growth, but also for stressful periods it may have to endure.
In the meantime, be sure to water the tree during any extended dry spells, spring through late summer. A dry spell is more than 2 weeks without 1-2 inches of a ground-soaking rain. I prefer using an oscillating sprinkler that covers a wide area very evenly, and putting an empty coffee can next to it to collect the water. This way I can measure how much I've put down, just using a ruler.
The recommended amount of water is an average of 1-2 inches every 7-10 days. DON'T water every day! Instead, water infrequently (but regularly) and DEEPLY. This gets the water down to the roots that do most of the absorbing work - they're 6-12" below the surface.
Be sure to cover the entire area beneath the tree, too, particularly out to the ends of the branches (a.k.a., the dripline), as this is where the majority of the roots "reside" that absorb the water. Laying a hose near the trunk doesn't adequately reach these roots, especially those of a large tree, which could be dozens of feet away!
I usually tell people to picture that their tree trunk is in the center of a doughnut, where the hole is. The roots that do the absorbing are the doughnut itself, so start watering about 1/3 the distance from the trunk to the dripline and out. If there's ground space beyond the dripline, water out there, too, as roots are likely to be that far out.
One thing: DON'T rely on underground lawn sprinklers to water your trees adequately. Lawn sprinklers are designed just for THE LAWN! If you water your trees the way described above, your lawn will get watered adequately at the same time.