If you are referring to this year's foliage, it's too early for maple trees to be showing enough leaf bud break to judge canopy health. If you are referring to how the canopy leafed-out in prior years, though, then a thinning (becoming sparse) may indicate a problem with the tree's health. We recommend you have the tree evaluated by a certified arborist, which may need to wait until the canopy is leafed-out about a month from now (but see what an arborist says...they might want to evaluate it sooner if they are checking other parameters to fix, like any girdling roots).

Often, trees that decline tend to start doing so months or years before symptoms become obvious, including if an insect pest (like borers) or disease (like wood decay) is responsible. Sometimes, secondary pests or diseases that are more visible due to their damage are blamed for starting the decline when in fact they merely were attracted to the weakened tree that was first compromised by some other issue, typically due to something in its growing conditions.

Maples are vulnerable to developing girdling roots, especially if they were originally planted too deeply (or had extra soil added to their root zone at some point) or had mulch piled against the trunk base; girdling roots can "strangle" a trunk and cause canopy stunting and dieback as the problem root gets larger with age. Some can be fixed by an arborist if they're not too advanced.

A lack of soil nutrients is rarely the problem causing tree decline, especially if the foliage has not discolored and yellowed (yellowing leaves whose leaf veins remain greener is a typical symptom of certain nutrient deficiencies). Despite the fact that tree companies seem to routinely recommend this approach, fertilizer should not be needed or applied without verifying that soil nutrients are lacking by performing a laboratory soil test. There can be a range of reasons why foliage might look sickly that are not due to nutrients being deficient in the soil, in which case adding fertilizer won't have any benefit (and at worst it can pollute runoff as wasted material or stress roots further). Plants do not have immune systems in the way people do. While nutrients are needed for a plant to manufacture defensive chemicals used to ward-off some insects and pests, fertilizing when it's not needed can also contribute to certain pests actually doing better on a plant whose nutrient surplus they're feeding on, which is why it's important to diagnose the likely problem(s) and/or pests present before deciding to fertilize.

If needed, you can always seek an assessment of a second arborist (consulting arborists are certified but might not be employed by a tree care company, so may be less likely to recommending actions to take that could be costly or not reliably effective) before deciding what to do. Tree decline can't always be reversed or cured, though, especially if something outside of your control, like a drought that stressed the tree a year or two ago (we're still in a drought, as it happens), was the main factor.

Miri

Thank you for your prompt reply, Miri. I was referring to the foliage in the previous couple of years, as they have not started to leaf out yet. They do have a pretty heavy bloom right now if that’s any indication of health.

Unfortunately blooming well isn't a reliable sign of plant health, but trees can recover from some stresses. Silver maples are not always long-lived trees, as they can be prone to storm breakage due to their fast-growing nature, but we've seen specimens at least ~75 years old. While watering during drought can help alleviate tree stress, we acknowledge it's not always feasible, especially if someone is on well water.

If soils are not nutrient-deficient but a plant shows nutrient deficiency symptoms in the foliage (like chlorosis, where yellowing with green veining is the general pattern), that can occur from poor root health, potentially from one or more girdling roots near the base of the trunk, or from a soil acidity level (pH) that is outside of the plant's ideal range (not acidic enough, generally). Other than natural soil characteristics that vary from one site to another, one cause of a pH shift over time could be exposure to lime, such as if a nearby lawn is limed regularly to raise the pH. Maples prefer acidic soil, though have some flexibility for soils around neutral (pH 7.0), so soil pH alone might not be a contributing factor unless it's outside of a range of around 5.0 to 7.0, which a lab soil test can measure.

Miri

Thanks.  I would not plant silver maple due to these issues. These were mature when we bought house 1989.  I think they are at least 60 years old. Neighbor, slightly up slope, used to get lawn treated but stopped that some years ago. Maybe it’s natural senescence. I just don’t want to pay for useless treatment especially if it’s potentially harmful to wildlife. I’ll get soil tested and ask for thorough examination of the root area for girdling. 

You're welcome.