There's a few things you can look at.
I'm not familiar with Rowan trees. I hope you can send me more information on them.
But I am familiar with gardening and orchards. So I have a few ideas you can look at.
1) Look at the leaves on the tree. Do they look crinkled or like they are wilting? This would indicate if they aren't getting enough water.
2) If the leaves are different sizes instead of fairly uniform this also can indicate water issues. This can also apply for fruit. Like when you look at a peach tree if the peaches are mostly similar size then your watering cycle is good. But if the fruit are very different in size then this can mean its not getting enough water, nutrients.
3) Is there a competing tree or plant that's sucking up the water that was going to that tree? (Maybe something very large nearby?)
4) When was your first frost for the year? Then compare this to when that tree flowered.
5) Check for animal and bird stress (damage).
6) Has the ground where that tree is at been depleted of nutrients? (You can try to refresh this)
7) Some fruit trees will take a rest year if the previous year they had produced above normal yields. Some people think that pruning helps with avoiding this and that a fruit tree needs pruning every year.
8) Pruning; you want to prune but don't prune above 20 or 25% of the tree. If you prune too much you can actually cause the tree to not be able to fruit the next year. They often quote the limit of how much you can prune at 30%, so I try to stay below that.
9) Other; some species of trees or plants are naturally more stable than others. Some are less stable and will have off years.
An example of stability is PLUM TREES. Plum trees are one of the very most stable fruit trees. In fact, you could put them at the top of the ranking, more than any other fruit tree. Yet they aren't popular; they should get more attention. For my area plums rank highest on stability, then apricots, then apples, and pears, then peaches.... at the bottom of the list is cherries. This is the ranking for stability of southwest (dry climate) orchard tree stability. For other areas it won't show the same. (Also this was a published list, not my research. But I found it to be accurate).
Where I'm going with this is, we had a cherry tree, it was always sick. It would malfunction often, more than half the time it would malfunction and not fruit right. Or have issues. It was more prone to parasites and bug damage than other trees also.
But our plum tree, had zero maintenance. Never had any problems, it would always produce.
I don't know where rowan trees would fit into this. I don't have data on them. But its possible that some of them might not be as stable. But its also true that there can be many varieties of a certain type of plant species, where some of the varieties are better than others. It would be interesting to see how many rowan tree varieties there are and what you can do with that.
10) A lot of places now are also saying the sun and sunlight in their areas seems very screwy this year. Many places are acting extremely hot and like the wrong climate. Like 43 degrees Celcius in Japan, China, etc... which shouldn't even happen.
