For Many Conifers, It’s All in the Seeds

By Maxwell McCormack

In last month’s issue, Lloyd Irland hit the nail squarely on the head. White pines had a banner year for production of that yellow powder, pollen from their male flowers. White pines dispersed most of the yellow stuff, but there were contributions from other pines, some spruces, and fir as well. This year, with its erratic spring conditions, tree pollen was still flying in mid-July. The hardwoods also participated, but they are never as dramatic as the conifers.

Pollen dispersal coincides with female flowering and is an indicator of forthcoming seed crops. These processes bear watching as they aid in projecting potential for recruitment of seedlings as natural regeneration. In Maine, the window for most of the action is mid-May through June.

Seedling development depends on an array of factors. Viable, mature seeds – often while still in the cones – can fall prey to a gauntlet of feeding or hoarding wildlife. Climate and soil conditions must provide suitable microsites with required sunlight or shade, desirable seed bed conditions, and available moisture where seedlings will have space to grow.

Large seed crops do not guarantee successful regeneration. Germinated seeds face formidable challenges that vary from region-to-region, site-to-site, and year-to-year, Seedlings and young saplings face continuing threats from insects, diseases, and browsing wildlife.

Frequencies of seed crops for five selected conifer species are summarized in the accompanying table. Let’s put hardwood species on the side shelf for now, since they involve a wider variety of complex processes. Time spans in the table (“General Seed Crop Characteristics”) outline a range of possibilities. These data, for example, identify one cause for the preponderance of fir regeneration in mixed softwood stands. Balsam fir produces seed regularly, often in bumper crops.

For planning purposes, some short-term projections are possible. Pine cones require two years to mature. We gain a one-year advance notice by observing quantities of one-year, immature cones on high crown branches. With adequate sunlight they can be observed using a scope or binoculars. For fir and some spruces, in the autumn. astute observers can spot the buds that will form female flowers the following spring. This can be done easily when freshly fallen crowns are accessible after windstorms or harvesting.

Mother Nature provides interesting relationships during pollen dispersal and pollination. The accompanying photograph shows a Korean fir (Abies koreana Wils.), a species used for illustration because it readily illustrates true fir’s typical distribution of female flowers (upper left inset) in the upper portion of the crown and the male flowers (lower right inset) restricted to the lower part of the crown.

These particular flowers are slightly past peak, but the female structure is erect with open, light green scales positioned to capture pollen moving through the air. After fertilizations, ideally for two seeds per scale, the cones will remain attached and erect. At maturity they will shatter, dispersing the winged seeds and bracts. The cone axes persist on the branches for an extended period.

The male structures in the photo have just completed dispersal of their pollen loads. The relative positioning in the crown requires upward transport of the pollen to reach the females. As the pollen moves upward it tends to drift laterally in the air currents, thereby fertilizing females of adjacent fir trees. This tendency minimizes possible self-pollination and fosters genetic variation in the progeny.

There’s plenty of subtle activity in woodlots when that yellow powder of spring is blowing around and coating our windshields.

Max McCormack is research professor emeritus at the University of Maine, and lives in Unity.