The causes range from destruction of their local habitats to global depletion of the ozone layer (scientificamerican.com)
This paper says «Our results suggest modern conifer forests may be considerably more vulnerable to anthropogenic ozone layer depletion than expected.» (ecoshock.org)
Under full compliance with the Montreal Protocol, the ozone layer is expected to recover to 1980 benchmark levels - the time before significant ozone layer depletion - before the middle of the century in mid-latitudes and the Arctic, and somewhat later in the Antarctic. (sciencedaily.com)