Your evergreens are constantly expelling moisture (transpiring) through their foliage into the air. Typically, this moisture is replaced through the roots. However, should water become unavailable for any reason (e.g. frozen soils or drought) the plant then begins using its reserve. If a lack of water continues, and reserves become depleted the tips of the leaves may start to shrivel, brown or die back. Once die back occurs, these areas will not recover or fill back in without new growth, which could take several years or more. Excessive moisture loss may cause branch die back or even death of the whole plant in extreme cases. Treating your plants with an anti-desiccant can greatly reduce or prevent this damage from occurring.