Experiment:
Measure wood regeneration time, flax harvest, vegetable yield, and the value of the 5 ecology skills at a location. Then drop clinker, wait for it to 'disappear', and remeasure above properties of the land at some specified later time.
Result:
To date, 1-10k clinker have been dropped in single piles. No change in any category has been observed as a result, both initially after the 'disappearance' of the clinker, as well as 12 hours afterwards.
However, tat2mikki has seen a decrease in flax harvest when dropping several clinker piles. I have tried, but not been able to reproduce this result using single piles of clinker, and will soon test the effect of dropping multiple piles. Our results are clearly not yet fully understood.
Odis has recently posted another example of flax yield on crossbred flax decreasing due to clinker pollution. His theories have yet to be confirmed, but seem plausible and consistent with our results and with the information he shows. see: http://www.atitd.org/wiki/tale3/Pollution