This paper deals with the results of long-term lichen monitoring in the Netherlands. We review the results related to ammonia (NH3) air pollution over the period 1989–2022. NIW (nitrophyte abundance) and AIW (acidophyte abundance) were used as parameters to unite lichens with positive and negative responses to ammonia, respectively. Spatial patterns and temporal changes at large numbers of sampling sites with Oaks (Quercus robur) are compared and discussed. A steady decrease of acidophyte abundance (AIW) since 1989 is apparent, and continues up to now. The way in which AIW behaves strongly suggests that de-acidification of tree bark is the main cause of decline. This process is likely to be cumulative, i.e., proceeds as long as ammonia pollution continues. Recovery of bark acidity and acidophytic epiphytes is unlikely under current conditions. Loss of acidophytes is considered to be an important warning signal for habitat degradation. Changes in nitrophyte abundance (NIW) are not just the opposite of the AIW. Until 1998, there was a marked increase in nitrophyte abundance. After reaching its maximum in 1999, the NIW decreased again by 25–30%, probably as a result of measures to control the ammonia emissions. Nitrophytes had initially (< 1999) increased fast due to the rapid de-acidification of tree bark. Many sites, at least in areas with high cattle densities, were fully de-acidified over the last 25 years. Measurements show that de-acidification eventually stops at a pH of about 6 or 6.5. Nitrophytes can be dominant at such sites, but can also disappear again, leaving a community with mainly crustose species typical for neutral bark. This is probably caused by a decrease in available nitrogen. Recent (since 2000) temporal changes of nitrophyte abundance (NIW) are likely reflecting changes in the load of ammonia.