I couldn’t find anything scientific specifically about cigarette ash, so I’m just reasoning from common sense.

Part of the problem of filter tips are heavy metals. I hypothesize that not all the metals are converted to gas and sucked into the filter while the cigarette is lit. Therefore there must be heavy metals remaining in the ash. These seep into the ground and ground water, whether the ash starts out on a parking lot or in a park.

If that’s dangerous to plant life depends on the plants living there. Tobacco, for instance, seems to handle them well (that’s how the metals get into the ash in the first place: tobacco sucks them out of the ground). “[S]ensitive plants growing in sites with heavy metals exposure show altered metabolism, growth reduction, and reduced biomass production and reduced yield.” [1]

https://link.springer.com/chapter/10.1007/978-3-030-41552-5_4