Why do Norway inner needles turn yellow then brown and fall off?

Norway spruce (Picea abies) needles turn yellow and brown in the winter due to a process called seasonal leaf senescence. This process is triggered by a combination of factors, including changes in day length, temperature, and moisture levels.

Day length: As the days get shorter in the fall, Norway spruce trees produce less chlorophyll, the green pigment that plants use to photosynthesize. This causes the needles to start to turn yellow.

Temperature: As the temperatures drop in the fall, the water content of Norway spruce needles decreases. This causes the needles to become more susceptible to damage from frost and wind.

Moisture levels: Norway spruce trees need a lot of water to survive. If there is a drought in the fall, the trees will not be able to take up enough water to replace the water that is lost through transpiration. This can cause the needles to turn brown and fall off.

In addition to these factors, Norway spruce trees may also turn yellow and brown due to nutrient deficiencies, pests, or diseases. If you are concerned about the health of your Norway spruce tree, you should contact a certified arborist.