The dark green, finely textured foliage of pine trees (Pinus spp.) provides shade, wind block and winter holiday decoration in a landscape. Seeing the lush needles turn yellow and brown is cause for concern, since it demonstrates the first signs of a potential environmental stress or threat. Healthy pines shed only their oldest needles when prospering in a moist, well-draining soil with lots of sunshine.
While most call pine trees "evergreens," their needles do not last forever. Depending on species, the needles endure between two to five years before they naturally die, shed and are replaced. Typically the natural annual shedding of old needles occurs in autumn, the same time when deciduous trees such as maples drop theirs. A close look at a pine tree in autumn reveals a hint of yellow-green as the lower background needles turn yellow and tan while the needles at the tips of branches remain deep green. The needles shed quickly once dry and do not persist on the pine tree, which is likely why reason few people know that the pine loses any foliage.
Lack of Water
Regardless of species, pines in general need a well-draining soil and appreciate moisture. While some pines tolerate seasonal droughts or drier climates, too little water results in tree stress. The first stages of water stress manifest as needle tips yellowing and then browning. This occurs either in the growing season or in winter. Though the tree is dormant in winter, moisture in the branches and foliage must be replenished because of the drying winds or warming sun. Watering pine trees heading into winter after a drought alleviates stress before winter and assists in survival.
Pests and Disease
Changing and moving populations of insect pests and diseases attack pines. Whether attacked by a tree-boring insect, spider mites or a fungus, a pine may sacrifice healthy foliage during the illness. Sucking insects or borers physically rob foliage of moisture, while a fungus disrupts the vascular tissue. Both result in dying needles, buds and eventually branches if severe.
Drift from herbicides that reach pine foliage or pine trees growing where air pollution occurs will prematurely turn the trees yellow and brown. In agricultural areas, wind may accidentally blow herbicides onto trees adjacent to treated crops. Or pines located near busy roadways or downwind from manufacturing plants with smokestacks may suffer from inhospitable fumes or subsequent acid rain.
Do not rule out that browning needles on a large pine tree heralds its natural degradation. Rule out other factors before assuming old age causes needle browning, since drought, pests or other factors may exacerbate the demise of a pine tree at any age.