It all starts with sugar.

Let me explain by first taking you on an imaginary trip to a winery. You’re standing in a tasting room with some friends, sipping a bit of sweet dessert wine, maybe a Reisling or a Muscat. Someone asks a question about how the wine was made and what makes it so sweet. The winemaker explains that in the fall, they allow certain grapes to ripen longer on the vine. Water evaporates from the grapes and they shrivel up like raisins, concentrating their sugars and becoming more acidic. Then the grapes are harvested and made into dessert wines. That’s one reason why dessert wines are sweet – and potent – with at least a 14 percent alcohol content.

Now consider what happens to certain leaves in the fall. Their moisture content drops, concentrating the sugars and increasing the acid content. When the sugar concentration is high enough, the sugar and certain plant proteins in the cell sap react in the presence of sunlight to produce anthocyanins.

Anthocyanins are plant pigments and can be either red or purple, depending on how much acid is present. They’re what make apples red or plums purple.

In the fall, we don’t see too many plants with red leaves in our local parks and open spaces. The one exception is poison oak, which, as you can see from the photo, turns a vivid red. I spotted plenty of poison oak in the creek bed while hiking recently in Wiley Canyon. That’s because poison oak grows best in a moist riparian habitat.

One more thing – I recommend you don’t pick the leaves to make a festive fall bouquet. Like dessert wine, poison oak is potent in its own way.

You can listen to stories like this every Friday morning at 7:10 a.m. on "The Hike Report", brought to you by your hometown radio station KHTS (AM1220) and by the Mountains Recreation and Conservation Authority.

For our complete hike and activity schedule and for trail maps, go to www.LAMountains.com.

To see what’s playing on radio station KHTS, go to https://www.hometownstation.com/or tune in to AM 1220.