Sep. 12—From the cliffside aspens of the Pecos Wilderness to the vibrant maples of the Manzano Mountains, the symphony of fall colors is about to commence in New Mexico
But what’s conducting this complex biochemical orchestra in the forest? The process is called leaf senescence.
Over centuries humans have developed scientific methods and technologies to measure time, the tilt of the planet and to forecast temperatures. Trees have these mechanisms built in, though we’re yet to fully understand them, said Andrei Toca, a research scientist at New Mexico State University’s Forestry Research Center in Mora.
As idle as they may seem, trees are always counting.
Trees have evolved to measure the duration of the night, allowing them to time their transformation precisely, said University of New Mexico Biology Professor Will Pockman. That timing, however, can be influenced by weather like heat and dryness, or other environmental stressors.
As the northern hemisphere tilts away from the sun, and the night lengthens, trees anticipate the coming winter by extracting chlorophyll from their leaves, Pockman said. Chlorophyll allows plants to collect energy from the sun and gives leaves their signature green color.
In winter, trees store their chlorophyll internally, hunkering down for the arrival of spring when new leaves will need the green pigment.
“So the process itself is of recycling,” Toca said.
Once the chlorophyll is gone, what’s called an “accessory pigment” is left behind within the leaves, Pockman said. These pigments are the yellows, oranges and reds we associate with fall, which are not visible while the chlorophyll is around.
The duration of leaf senescence can differ depending on environmental conditions, Toca said, meaning some years vibrant colors can span six or seven weeks, while in other years it lasts just a few weeks.
The longer the process, the more nutrients trees are able to reabsorb, Toca said. So a long colorful fall season can be a marker of a forest’s good health.
Drought, frost and out-of-season inclement weather can all shorten this period. Those weather patterns are intensifying due to climate change, Toca said, making it more difficult for trees to time their transformation.
Research suggests that due to global warming the leaf senescence period is beginning six days later each decade.
Though the color change is mainly triggered by the duration of night, some trees have adjusted to the longer growing season caused by climate change and are holding out for a few more weeks of nutrients.
“It’s a very risky strategy,” Toca said.
If trees stay green later into October and even November, the likelihood for frost increases. Though it may be unusually warm during the day, nighttime temperatures can plunge causing an unprepared tree to lose all its green leaves, starving it of nutrients for next year.
This can, at times, kill trees, Toca said, or at least hamper their growth in future seasons.
Drought, however, has the opposite effect, causing trees to change their leaves sooner, Toca said.
Toca studies forest ecophysiology, learning how trees react to stressful conditions such as drought, or fire. During research trips, Toca has spent time in the Calf Canyon/Hermits Peak burn scar.
The forest is rebounding from the devastation with new growth of aspen, Gambel oak and fireweed, he said.
“It’s a nice place also to see not only the colors, but the revival of the forest after such an intense fire,” Toca said.
The Jemez, with its contrast of evergreens and aspen groves is another stunning spot to see the changing leaves, Pockman said.
“I love seeing the color scattered around in Jemez, or any of the mountain ranges in New Mexico,” Pockman said. “It’s really super spectacular. So I don’t have just one favorite, I love the whole landscape.”
Aspens are communicative plants. Many groves are made up of clones, hundreds or thousands of identical trees that share one root system and consequently change color at the same time, according to the National Forest Service.
These duplicating trees in New Mexico are found throughout the mountainous regions of the state, including in Bandelier National Monument, the Santa Fe National Forest, Carson National Forest and in the Sandias.
For leaf peeping close to home, drive up the winding crest road for oaks and aspens in the Sandias’ canyons or walk along the Rio Grande to view the golden yellow cottonwoods of the bosque.
For locations more than an hour’s drive, head to Fourth of July Canyon, outside of Torreon on the east side of the Manzanos for an uncommon concentration of bright red and orange maples or cruise Highway 4 in the Jemez to spot yellow aspens.
Or, drive to Santa Barbara Campground outside of Peñasco where a trail from the campground leads into the Pecos Wilderness. See swathes of yellow aspens along the creek and meadows and avoid the crowds closer to town.
Gillian Barkhurst is a reporter for the Albuquerque Journal. She can be reached at gbarkhurst@abqjournal.com or on Twitter @G_Barkhurst.