British Columbia: Trees struggling on rooftop patio

I live in a three year old building in Vancouver. There are a number of trees on the roof, which I believe are dogwoods (but I don't know which variety). This summer they fared very poorly. Three of them outright lost all of their leaves, and all of them seemed to lose a few up at the top and suffered from brittle leaves in general. I'm wondering what could be the issue?

They're south facing and in the sun for nearly the entire day and so I wonder if they were simply baked by the hot sun? We double checked the drip irrigation system and it was indeed functioning as expected.

Interestingly after the hottest part of the summer, once it cooled down some, the one tree that shed all of its leaves regrew a bunch of them. A photo of this is available below showing a mixture of old dead leaves amongst new growth.

Hopefully all the trees bounce back next year and I'm wondering what I should do to prevent whatever caused this issue this summer.

 

Looks like Cornus kousa from the fruit, though the pic is a mite small to see for certain.

Yes, it'll be due to the hot dry summer that Vancouver has had. Dogwoods like moist, often somewhat shady, situations, and a rooftop isn't exactly ideal for that. They might recover, but if they don't, it gives you the opportunity to replace them with something else more tolerant of heat and drought. Something like Ceanothus would probably work better.

 

Poor leaf color probably indicates need for fertilization. This is especially likely if the are planted in soilless potting medium.

 

Kornus Kousa don't get anthracnose disease like our native B.C. Dogwood do, so that isn't a concern.

Do you have a landscape contractor? Or is the roof top tree maintenance not included in the budget?

As Ron suggested, have the soil checked. And then determine what fertilizing is required for plant health.

Most likely your building won't approve a change in the trees.
There will be weight restrictions, so adding soil is probably not allowed.
But, step up the amount of watering. Sprinkler systems usually need to be tweeked throughout the season.
We had a record long drought season this year.

 

a hot rooftop is hardly the place for any plants without some shade or wind, your plants will bake not only in the hot sun, but from the reflective material rooftops are made of, to reflect heat during the summer months. soaking the root zone will bring the temps down somewhat, but the entire plant would also need to be misted during heat wave, as well as the roof around it.
ron is right, if your plants are planted in soiless mix, not only will they need their food adjusted,but this hydroponic soil heats up fast, dries incredibly fast in sunlight/and heat from the roof, baking into a solid block, making water impossible to penetrate fully, has no nutritional value, as its usually used by control freaks like me who want absolute control over the feeding of their plants and comes guaranteed clean of bugs, moulds, fungus and weeds.
if they are in soiless mix, i recommend taking a chop stick and breaking up the soil a few times a year. one way to know for sure, is soiless mixes are very high in vermiculite,the other ingredients are parilite, and sphagnum peat moss. personally, i use sunshine mix, have had no problems with any of my plants and trees since. good luck.