The autumn nip usually associated with turning leaves is finally in the air, and Palo Alto's trees are putting on a show of vibrant color.

Trees are turning scarlet, flaming orange and shimmering gold along with more subtle shades of brown. The city has more than 39,000 trees in streets and parks alone, with whole streets that can be ablaze in autumnal glory.

Tree experts, such as arborists and Palo Alto urban-forest nonprofit Canopy, say now is the time to enjoy the painterly display.

"The 'best' trees change from year to year, depending on climate," Dave Dockter, city managing arborist, said during a Nov. 12 tree walk of the University South neighborhood. Canopy sponsors the monthly walks, which explore different neighborhoods.

Dockter has his favorites: blazing red liquidambar, multi-hued Chinese pistache, and the delicate gingko decked in gold.

These trees, called deciduous, go dormant in the winter. Cool weather triggers the process that brings on the color changes. Chlorophyll, the chemical giving a leaf its green color, is necessary for photosynthesis, when plants use the sun's energy to convert carbon dioxide into sugars. But a tree does not continue photosynthesis while dormant, so it stops producing chlorophyll, Dockter said. As the existing chlorophyll begins to break down, other present chemicals become dominant, creating a variety of showy leaf colors.

Carotenoids, which provide yellow, orange and red colors (think carotene, the chemical in carrots), and flavonoids, which cause yellows, are present in leaves all of the time, but they degrade more slowly than chlorophyll, he said.

Trees manufacture another set of chemicals, anthocyanins, as daylight hours shortens and sugars increase in the leaves. These chemicals create deep red, purple and magenta shades, according to Compound Interest, a website about chemical compounds people come across each day.

The time trees change color varies based on multiple factors, including sunlight and amount of water the tree receives, according to Dockter.

"Trees will turn color sooner in drought conditions," he said.

Walk around town and notice how the same kinds of trees might turn at different times. Some will remain green much longer because they have received more water; others turn color on one side while staying green on the other based on how much sun they receive on that side, he said.

Walking around University South, Dockter noted that some trees have yet to show their colors, but just wait. One of the rarest trees, the dawn redwood, will soon turn a brilliant russet, he said.

A magnificent example of the tree exists next to the Hamilton Avenue Post Office along Waverley Street. Once known only from fossils dating back 252-66 million years (there were about 20 species), a small grove of one redwood species was discovered in China in 1944.

Palo Alto's massive tree was planted from seed coming from that grove in 1949. The dawn redwood is a deciduous conifer, meaning it looks like an evergreen tree (such as a redwood), but it turns rusty red and drops its needles in fall and winter.

"In one month, this tree will be a blazing tree," he said.

One of the most brilliantly colored species found in the city also has a variety named "Palo Alto." An example exists outside of City Hall. Liquidambar, or American sweet gum, is native to the southeastern U.S. In the 1960s, former Parks and Recreation Department head George Hood discovered a liquidambar tree on Pitman Avenue that changed brilliantly. The cultivar he named after the city came from that tree, Dockter said.

Pitman Avenue's liquidambars, which line the street, have six different leaf colors, and they will continue to put on a good show dominated by red in the coming weeks, he said.

On nearby Greenwood Avenue off Newell Road, the color is predominantly gold. The small, fan-shaped leaves of the gingko shimmer in a mass of trees lining the street. A native of China, the gingko is also known from fossils dating back 270 million years. Also called the maidenhair tree, it is found throughout the city in gardens and as street trees.

The gingko has made Dockter's "top picks" list for this year. In addition to Greenwood, he recommends touring the Genencor headquarters at 975 Page Mill Road. For reds, he also suggests Page Mill Road between El Camino Real and Foothill Expressway.

His "top five" trees for this year, in addition to liquidambar and gingko, include Chinese pistache, producing orange, red and yellow (Cowper and Waverley streets at Embarcadero Road), Shumard oak (yellows and apricot) (Porter Drive at Page Mill) and Cimmaron ash (apricot) (Alma Street and Homer Avenue).

Other spots offer an artist's palette of colors. Old Palo Alto, for example, has many streets of mature trees bursting with a variety of colors, and the view down side streets such as Waverley near Oregon Expressway offer opportunities to marvel at nature's variety in the shifting light.

Mike Willemsen, who led "The Very Best Colors in Palo Alto" tree and photography walk on Thursday, said his favorite spots this year include Byron Street between Oregon Expressway and North California Avenue and along Bryant Street south of Forest Avenue, where there is a mix of colorful foliage worthy of photography. Canopy also offers information and more detail about the science of fall color on its blog at canopy.org.

Got a favorite street for autumn color this year? Please let us know. Contact Sue Dremann at sdremann@paweekly.com.