Forest service: Yellowing pine needles normal in fall

Brown needles on a ponderosa pine / Colorado State Forest Service
Brown needles on a ponderosa pine / Colorado State Forest Service

WOODLAND PARK, Colo. — Although thousands of evergreen trees along the southern Front Range are beginning to display dying yellow or brown needles, most are simply going through a natural shedding process – and are not infested by bark beetles or tree disease.

Michael Till, forester at the Colorado State Forest Service Woodland Park District, says that fall needle drop is usually brought on by seasonal changes and weather events. In his district, which includes El Paso, Teller and Park counties, conifer species that commonly shed needles in September and October include ponderosa pine, Douglas fir and Colorado blue spruce.

Every autumn, many Colorado evergreen tree species shed some of their older, interior needles as part of an annual growth cycle. Needles on the lower portion of the crowns or closest to the trunk are most commonly shed, but trees stressed due to drought or root damage may shed more needles to keep the tree in balance with its root system. Soon-to-be shed needles typically turn yellow first, then a reddish-brown color before dropping off; very small branches with few needles on them also may die.

Till says evergreen trees that shed fall needles have a different appearance than trees infested by bark beetles. The needles on a beetle-infested tree typically change color throughout the entire tree, initially starting with an off shade of green and turning to reddish-orange by the following summer. In addition to changing needle colors, bark beetle-infested trees also will show other signs of attack, such as fine sawdust at the base of the tree.

The seasonal discoloration and loss of pine needles frequently is called “needle cast,” but the term technically refers to a fungal disease of spruce and fir trees.

For more information about tree and forest health, contact the CSFS Woodland Park District at 719-687-2951 or visit http://csfs.colostate.edu/pages/woodlandparkdist.html.

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