Jack Pines on Long Island: Most Cones Open Without Waiting for Fire

Most of the jack pine cones on Long Island, in Wisconsin's Apostle Islands, open and drop their seeds without waiting for a fire. A National Park Service study counted cones on beach and forest plots to find out how many pines hold their seeds closed until a disturbance such as fire releases them.

Closed (serotinous) cones. NPS photo.

What serotiny is

Serotiny is the ability of some trees to keep seeds in their crowns for years until a disturbance such as fire creates conditions that favor release. Non-serotinous cones release seeds more regularly in response to site conditions. Jack pine (Pinus banksiana) shows both types, a variable trait that lets the species hold its populations under changing site conditions and disturbances and that is linked to the history of natural disturbance. The study compared jack pine cone serotiny in Great Lakes coastal habitats at two stages of succession, open beach and forest.

Open (non-serotinous) cones. NPS photo.

How they counted

The researchers assessed temporary 10-m radius plots, 4 on the beach and 15 in forest. In each plot they recorded the number and species of trees, trunk diameter, and distance from each tree to the plot center, and took tree cores. On 4 branches in the upper third of each tree they counted cones and sorted them into open (non-serotinous), closed (serotinous) and new.

What they found

Most cones were the non-serotinous type in both zones, with a higher share on the beach. Open cones made up 61 to 83 percent of cones in the beach zone and 29 to 93 percent in the forested zone.

Jack pine cones by type and habitat. NPS graphic.

A lower level of serotiny on an island is consistent with other work. The study reads the high share of open cones as a sign that the trees reproduce in response to dry site conditions, not past fires. That matters because a single crown fire can favor serotinous cones and quickly reduce the genetic variation in a population. A mix of both strategies keeps jack pine going under lower and higher fire disturbance and, as one cited study puts it, is "crucial for their conservation." Climate change, which will likely influence historic fire regimes, adds to the importance.

Why Long Island is unusual

Long Island is the main place in the park with jack pine. The park's other 20 islands sit on Precambrian sandstone bedrock, but Long Island is estimated to have formed between 1,050 and 1,700 years ago and is the park's only sand barrier spit. It formed when the mouth of the Kakagon River shifted from the southern Chequamegon Bay side to the open lake on the north, which supplied enough sediment through longshore currents. The island has gained sediment on the north side and eroded on the south, and as the 1938 and 2015 aerial photos show, that continues: 36 acres of new habitat have accumulated within the past century.

Long Island, 1938. NPS photo.

An age map of the trees shows the oldest in the darkest blue and younger ones lighter. The youngest trees are all in habitat that formed after 1938, as expected from when the beach and dune area formed, and the ages further inland suggest a young forest at this site.

Estimated origin dates for jack pines on Long Island. NPS graphic.

Sources

• National Park Service, Apostle Islands National Lakeshore, Investigating Jack Pine Cone Serotiny on a Dynamic Barrier Spit; rewritten in hubnx's own words.