Conifers that we call “evergreens” naturally drop their oldest, innermost needles each autumn. The age at which needle drop occurs varies between conifer species.
Needles of ponderosa pine last for 3-4 years and Austrian pine for 4-5 years. It could be expected that about 20-33% of the needles of ponderosa and Austrian pine would yellow, brown and drop each autumn. In years that evergreens have had good growing conditions, a larger amount of new growth and needles will occur. When the “drop-dead date” for needles on that above average growth arrives, it may appear that a greater proportion of the total number of needles is yellowing, browning and falling.
Keep in mind that occasionally, home landscapes may include a “deciduous conifer” such as baldcypress or larch. It is a normal event for these conifers to lose all their needles in the autumn, but a healthy tree replaces them the following spring.