The heat this past week is a reminder of the summer of 2012 and the extended drought that year. Lawns took a real hit that summer, yet topdressing, watering, sodding and over seeding brought the lawns back the following year. The same thing cannot be said of our woody plants since 2012.
There a lots of factors as to why some landscape plants are flourishing and others not. Plants chosen for the landscape would be one of them. Plants with questionable cold hardiness are immediately put to the test when our winters are colder than expected and that stressed the plants at the least and often leads to bud and twig dieback and maybe cankers on the trunk and stems. Plant location is another. Some plants like full sun, others tolerate or prefer partial shade. Others do well in moist soils while other plants have their roots rot away. Whenever plants are not in their best location, there can and will be stresses on the plant and those get worse as plants grow and age.
Woody plants and evergreens had a reduction in root system size as the roots could not survive as they had without adequate soil moisture, so the Spring of 2013 had our plants budding out and growing without the same root system prior to 2012. That is a big stressor on any plant.
I have seen shrubs in just a few days' wilt, turn off color and then brown and die. Evergreens like Arborvitae take on a dull green needle color, have sections of the plant brown. Needle evergreens like spruce and pine quickly turn brown. In most cases this is due to the damage done in 2012 and pile on other factors like site, soil type and the recipe for decline and death are there.
Larger, older evergreen trees, especially Austrian Pine have been severely impacted and now already stressed have been finished off by needle diseases that are that "nail in the coffin". The major needle disease on Austrian pine is one called Diplodia Tip Blight. This needle disease attacks the brand new buds as they begin to expand and seems to kill that expanding candle at about 2-3 inches. With very little good needle growth from previous years, those pines continue to decline and will be gone yet this summer if they are not dead already.
If you have plants that outwardly at least look healthy, keep them that way by watering and do not wait for a drought to be declared. Evergreens use more water than a deciduous plant. A spring fertilization can help any plant rebuild root systems. Read all you can before replanting to be sure you have the best plant for the location when the time comes.