There is a defoliation problem with evergreen trees which started last year. Fertilization and added soil inoculants can help, but the effects of multiple years of environmental stress (drought) are taking its toll. The problem is region-wide and is serious. Please see the following report from my alma mater, UMass, that goes into great detail concerning this matter.
Based on my own experience, the UMass research, and several discussions with fellow arborist - I summarize the problem as follows:
1. Several years of drought have greatly affected trees with low energy reserves by weakening their defense systems to the point of exposing them to secondary pathogens that never cause the demise of healthy, well hydrated trees. Unfortunately, Darwin's theory of 'survival of the fittest' is at play here and you may expect the loss of some trees as they are just too weak to survive. There is no magic potion that would have prevented the death of these trees (rain would have helped) and to blindly blast them with poisonous fungicides would be irresponsible. [More information from the UMass Extension Plant Diagnostics Lab]
2. Excessive layering of mulch is also a contributing factor to the weakening of these trees. Please see the attached report from Rutgers University that explains in detail the problems associated with mulching. My biggest concern is the stripping away of needles (spring and fall clean up) that the trees produce to counterbalance the effects of drought. Trees know when they are stressed and the defoliation is the tree's way of increasing the amount of organic matter in the soil to sustain specific microbial vitality. 'Air spading' in the fall/early winter of to can help reverse the effects of excessive mulching. [More information from the Rutgers NJAES]
3. SALT - There was a time when sand/salt mix was an acceptable winter roadway application. About 10 years ago, this changed and now we are collectively addicted to salt in the greater route 128 belt. This has a negative impact on trees and in my opinion is a leading cause to the decline of trees that line our roadways. Please see the attached fact sheet from UMass regarding the negative effects of salt. Aside from the obvious problem with salt wicking moisture from living cells, my biggest concern is the binding up of the soil on a molecular level, otherwise known as 'cation exchange'. This will result in neutralizing the soil to extent that the trees are not able to absorb nutrients. [More information from UMass]
Hopefully this information based on science will bring greater awareness to the challenges that trees are now facing. If you are experiencing defoliation in your evergreens, or any other symptoms that indicate your trees are damaged, please take advantage of a free consultation with a Certified Arborist.