Alder (Alnus: Betulaceae) pollen grains are common in coal beds of the Miocene Beluga and the Pliocene Sterling formations exposed in the Kenai lowland, Alaska. All alder pollen grains of the Beluga Formation and the lower part of the overlying Sterling Formation are dominated by 4-pored grains (42–67% of total alder), with no exceptions. There is a striking transition to a 5-pored dominance (32–67%) with an accompanying increase in 6-, 7-, and even 8-pored grains in outcrops of the Sterling Formation along the Cook Inlet shore, north of Clam Gulch and at the head of Kachemak Bay, in upper Swift Creek Canyon, and in two canyon tributaries to Fox Creek. The general zone of this transition includes a previously correlated volcanic ash and gives credence to the rock strata being synchronous in these widely separated outcrops. It should thus be possible to correlate these strata across the Kenai lowland by the alder pore numbers combined with the volcanic ash. The cause of the transition to a ≥ 5-pored Alnus dominance in the Pliocene of the Kenai lowland can only be suggested. Using pore numbers to determine alder species is not an accurate method because pollen of different alder species may have similar percentages of pore numbers. Nevertheless, when taking into account percentages of 5-, 6-, and 7-pored grains combined with the presence or absence of polar arci, it can be suggested that a shift occurred from the 4-pore dominated subgenus Alnus, represented by Alnus incana, to the 5-pore dominated, cold—adapted subgenus Alnobetula through dispersion and hybridization of Asian species such as Alnus maximowiczii, Alnus firma, and possibly Alnus sieboldiana. The pollen of these three species contain between 2–40% circular polar arci, which are also present in high-pore-numbered grains above the transition in this study.