Oak trees (Quercus spp.) have non-ornamental male and female gendered flowers on their branches in springtime. While the male, pollen-shedding flowers open just prior to the tree's reddish female flowers, wind is the means by which pollen reaches female flowers, later forming acorns. Several environmental factors can affect pollination of oaks, but the exact factors vary by oak species.
Number of Flowers
Simply put, the more flowers, the greater the chance of getting acorns later. Trees that fail to produce large numbers of flowers cannot produce large numbers of acorns. Some oak species flower more heavily in alternating years, or can be affected by drought stress or other environmental factors that would cause fewer flowers to be produced.
The primary mechanism for oak tree pollination is wind. Although both male and female flowers occur on the same tree, typically the male flowers begin shedding their pollen before the female flowers open. Thus, female flowers are receptive to pollen after the same tree's male flowers are waning; this promotes cross-pollination. Wind is vital to ensure the pollen from a separate tree is carried to the receptive female flowers on another.
During the spring flowering, air temperatures can delay or hasten the development of flowers on an oak tree. Although cool temperatures slow flower development or opening, a fast weather change to warm air can cause flowering to overlap or occur simultaneously. Conversely, a warm air environment may lead to rapid male flowering, but a sudden change to cooler air can delay the naturally later-occurring opening of female flowers.
Research conducted by Cecich and Sullivan in Missouri on white (Quercus alba) and black oak (Quercus velutina) revealed a species difference between duration or survival of flowers on increased rainy days. While more flowers survived the bouts of rainy weather on black oak trees, fewer flowers endured on white oak.
Pollen transfer through the air is more efficient when the air is not cool and moist (humid), conditions that are often present in springtime rainfall events.
Fluctuations of air temperatures can slow or hasten growth of living plant tissues. The exposure to inhospitable conditions, such as freezing temperatures, can fully harm the tender growth of young male and female flower buds in oaks. Depending on the prevailing weather patterns, latitude or elevation, oak species may be brought out of winter dormancy only to succumb to a late-season frost, causing emerging flowers to be killed or greatly reduced in numbers.
- "Canadian Jrnl. of Forest Research" Influence of weather at time of pollination on acorn production of Quercus alba and Quercus velutina; Robert A. Cecich and Neal H. Sullivan; 1999
- Univ. of Tulsa: Pollen Study
- Danr.ucop.edu: Habitat Fragmentation Limits Pollen Availability and Acorn Production in Blue Oak
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