How do bees help flowering trees survive?
One of the goals of every living organism (including plants), thus it seems, is to create offspring for the next generation. This is the way plants (angiosperms) have sex.

That said; I personally define pollination as:


  The act of transferring pollen grains from the male anther of a flower
  to the female stigma.


It should be noted that not only Bees play a very important role in pollination, but also other insects like wasps, beetles, mosquitoes etc.



When a bee sits on the flowers to collect nectar, its feet pick up pollen grains ( the powdery stuff) from the anthers and transfer it to the female stigma (which is sticky to hold the pollens).

Michigan State University:


  When a bee lands on a flower, the hairs all over the bees' body attract pollen grains through electrostatic forces. Stiff hairs on their legs enable them to groom the pollen into specialized brushes or pockets on their legs or body, and then carry it back to their nest. Individual bees tend to focus on one kind of flower at a time, which means it is more likely that pollen from one flower will be transferred to another flower of the same species by a particular bee. 


Then the pollen grain makes its way into the ovary of the flower through the long stalk (style) and fertilizes the egg. Then the flower matures to for a fruit and the ovules become it's seeds.
It's a symbiotic relationship where the plant provides food and the bees pollinate them in turn.
Also, certain bright colouration of flowers and their sweet smell attract more insects.

A declining population of pollinating insects signal the decline of the whole ecosystem including plants and trees.
Entomologists keep a check on various insect populations. They have found a decline in the population of bees world wide! This is an alarming situation. This will cause huge damage to fruit industries including kiwifruit, watermelon, squash and many others. It's time we take the environmental issues seriously.The reports aren't vague!

Penn State College of Agricultural Science:


  Historically, managed honey bee populations in the US and Europe have been monitored due to their vital role in providing pollination services in agricultural systems.  Several studies indicate that American and European beekeepers are suffering large annual losses.  In the US, beekeepers have lost ~30% of their colonies every year since 2006, with total annual losses sometimes reaching as high as 42% 




Edit

Citations:

https://en.m.wikipedia.org/wiki/Pollinator#Honey_bees

https://en.m.wikipedia.org/wiki/List_of_crop_plants_pollinated_by_bees

A brochure from FAO on importance of bees as pollinators :
http://www.fao.org/3/I9527EN/i9527en.PDF

Penn State College of Agricultural Science on declining population of pollinators :
https://ento.psu.edu/pollinators/resources-and-outreach/globally-pollinators-are-in-decline

Wikipedia on the same issue:
https://en.m.wikipedia.org/wiki/Pollinator_decline