A major advantage of being a conifer is being able to photosynthesize in the late fall and early spring, well after the deciduous trees have all shed their leaves. Some conifers, like white pines, only keep their needles for 2 growing seasons, while others hold onto their needles for well over a decade. In the last post we looked at how to estimate the age of twig. In this post, we’ll use that information to estimate the age of a conifer’s needles (needle longevity)!
Method 1: Fractions
There are two methods for estimating how long a conifer holds onto its needles. The first relies on fall colors, so isn’t that helpful right now. When the needles have changed colors (and before they’ve fallen from the tree), you can easily estimate the fraction of needles on the twig that have changed colors. In white pines, roughly 1/2 of the needles will be yellow. The needles live 2 years (technically growing seasons, with a few stragglers extending into the 3rd growing season). In white cedar, it’s closer to 1/3 that change colors. meaning their needles live about 3 years. But this doesn’t always work as some conifers will lose a significant number of green needles during the winter (up to 20% in black spruce). This skews the ratio so that the needles appear to have shorter lifespans.
Method 2: Aging twigs
A second, and more accurate method for calculating needle longevity, is by counting annual rings (those terminal bud scale scars) along the twig. The vast majority of conifers can only sprout new leaves from their buds (pitch pine, which has latent buds under the bark, is a notable exception), so the oldest needles on the branch are the ones closest to the trunk. Count each ringlet of bud scale scars as you work your way back along the branch until you reach the last section with any needles on it. This number is the number of Mays ago the needles “hatched.”
So if it’s October of 2022 and you count 4 rings to the last needle, then the needles on that branch burst open in May of 2019. To carry the hatching analogy further, these needles were “conceived” in July of 2018, remained dormant through the winter, and were “born” in May of 2021 when the buds opened. The first few rings are rather easy to count this way (there should be a distinct color change across the annular ring line), but the older rings can be trickier to find and so takes practice and a little guess work.
Maximum needle longevity
Below are some of the maximum ages of needles that I’ve calculated for various conifers. Note that the lifespan of needles may vary within a tree: needles grown in the shade (where they’re more protected from wind, snow, ice, and harsh UV radiation) tend to live longer. Additionally, within a species, individuals growing in warmer climates tend to retain needles longer than in colder climates (source, source).
|Species||Est. max age of needles|
|White pine||2-3 growing seasons|
|White cedar||3-4 growing seasons|
|Scots pine||4 growing seasons|
|Canada yew||7 growing seasons|
|Jack pine||7 growing seasons|
|Red spruce||9 growing seasons|
|Eastern hemlock||11 growing seasons|
|Norway spruce||14 growing seasons|
|Balsam fir||16 growing seasons|
The limitation with this method is that the needles can live so long that the annual ring becomes nearly indiscernible. You can always cut a twig just behind the oldest needle and count the growth rings. Similar problem here for slender branches where the small rings can be exceptionally difficult to make out.