Plants of Southern California:
Pinus jeffreyi and P. ponderosa cone lengths
Pinus jeffreyi and P. ponderosa cone lengths
The cone length is by far the easiest way to distinguish these two species, and it works so well the majority of the time that a simple glance at the cones on the forest floor can give you the identification, once you get a feel for the size of the cones for each species. In fact, this method works even doing drive-by botany at speeds of 30 mph or less.
In southern California, at least in the areas where we have measured cones, only Jeffrey cones are longer than 12 cm, so if any of the cones on the ground around your feet are longer than 12 cm, you've nailed the determination. (Caveat: if we ever have a wet year again, it is possible that cones will be larger; see below.)
Normally, only ponderosa cones are 7 to 11 cm in length, so in most cases where you observe cones of that size, you've found ponderosas. The exception is that in severe drought years, like 2014, Jeffrey pine cones can be 7 to 12 cm in length, almost exactly the normal ponderosa size. However, in all cases where we have seen such small Jeffrey pine cones, older pine cones found on the ground intermixed with the 2014 cones are normal Jeffrey size. Similarly, in 2014, we have found fresh ponderosa cones only 6 to 9 cm long, mixed in with older cones 9-12 cm in length.
Figs. 1 and 2 show a histogram of cone sizes measured for trees at San Gabriel Mountains (Big Horn Mine Trail and Lone Pine Canyon); San Jacinto Mountains (7 Pines Trail; Stone Creek Trail; Sawmill Trail); and San Bernardino Mountains (survey along SR38 from Forest Falls to Big Bear City).
Fig. 1 shows all our data, which includes oversampling Jeffrey trees with the unusually-small 2014 cones. Fig. 2 excludes three Jeffrey trees that were targeted for measurement while specifically surveying for trees with unusually-small cones. Fig. 2 is probably more representative of conditions in most years.
Fig. 1. Histogram of cone lengths for all measurements, including targeted searches for Jeffrey trees with small 2014 cones.
Fig. 2. Histogram of cone lengths for all measurements except for three Jeffrey trees with small 2014 cones.
Although there is minimal overlap in the sizes of the cones in most years, in most cases measuring ten cones will give you a good idea of the range of sizes, and pin down the species. In normal years there is almost no overlap in cone size; Jeffrey cones are typically 11-18 cm long; ponderosa cones are 7 to 12 cm long.
However, in drought years, which have occurred three times now in the last 14 years, if you find cones less than 12 cm long, you need to take one extra step to verify the determination. The easiest step is to look for the older cones, and see what their size range is. The other test is to check the color of the scales on both sides away from the knob at the end. Jeffrey cones will be roughly the same color on both sides, but ponderosa cones will be noticeably darker on the stem side of the cone.
As mentioned above, cone lengths can vary with year, and can be significantly smaller in drought years, or from trees in drier areas. For example, a Jeffrey tree at the head of Lone Pine Canyon (SnGb) had an average cone length of 15.9 cm in 2013, and just 10.7 cm in the poor rainfall year of 2014. A ponderosa tree along the Big Horn Mine Trail (SnGb) had an average cone length of 10.7 cm in 2013, and 8.4 cm in 2014. If one compared the lengths of these 2013 ponderosa cones with the 2014 Jeffrey cones, one would conclude there was no difference in length. But in each separate year the ponderosa cones were significantly smaller.
Figs. 3 and 4 show the histogram of cone lengths for two years from the same trees.
Fig. 3. Histogram of cone lengths for three Jeffrey trees with small 2014 cones, for the 2014 cones and for the older cones, presumably from 2013.
Fig. 4. Histogram of cone lengths for two ponderosa trees with small 2014 cones, for the 2014 cones and for the older cones, presumably from 2013.
Fig. 5 shows the median cone length for 2014 cones plotted vs. the median cone length for older cones, presumably from 2013, for each of the five trees histogrammed in Figs. 3 and 4.
Fig. 5. Median cone length for 2014 cones plotted vs. the median cone length for older cones, presumably from 2013, for each of the five trees histogrammed in Figs. 3 and 4.