This area used to be a bog. Acidic water leached minerals for hundreds of thousands of years, forming a hardpan, making drainage very slow. The soil is unfavorable for plant growth in several ways – it is extremely acidic, aluminum levels are high enough to be called toxic, nutrients are low, and the poor drainage results in seasonally saturated soil. By comparison, the compacted urban soils most sidewalk trees grow in are paradise.I recently took a field trip to a pygmy forest in coastal Mendocino County through the City College of San Francisco Biology Dept. If you didn’t know what you were looking at, you’d think the trees are “sick.” These plants would normally grow much larger under more ideal growing conditions but are stunted here.
The dominant trees are Mendocino Cypress (Cupressus goveniana pygmaea) and Bolander Pine (Pinus contorta bolanderi), a subspecies of Shore Pine (Pinus contorta contorta). I was surprised to see they weren’t all shrub size – some tall trees had managed to break through the hardpan – but they were still small for their age and appeared unhealthy.
The understory shrubs are mostly members of the Ericaceae plant family. This is not surprising because many members of this family thrive in acidic soils (rhododendrons, blueberry, heathers). Dwarfed versions of Evergreen Huckleberry (Vaccinium ovatum), 2 different rhododendrons (R. columbianum and R. macrophylla), Salal (Gaultheria shallon) and shatterberry (Arctostaphylos nummularia) grow in the understory of the dwarf conifers. Very few herbaceous (non-woody) plants are present, and lichens grow directly on the ground.
No deciduous trees here – one must photosynthesize year round just to stay alive! Woody plants have the advantage so they can store energy for the bad times of year. Not too many animals live here – too exposed, not enough food.
Pygmy forests are found worldwide, but for different reasons. Near Morro Bay in southern California, the Elfin Forest contains Coast Live Oak (Quercus agrifolia), stunted by dry, nutrient-poor, sandy soils and salty ocean winds. The small trees grow on the leeward side of the dunes, sheltered from wind. They grow only as tall as the dunes that protect them.
On the island of Oland in Sweden, a pygmy forest is found on a very thin layer of soil on top of a limestone plain. In contrast to the highly acidic soils found in Mendocino, the Stora Alvet has very alkaline soils.
Trees and plants make amazing adaptations to survive less than ideal conditions. Appreciate your local urban tree today for its accomplishments.
* In Iceland, there are few tall trees; the Vikings cut them all down not long after arriving in the 9th century, not realizing that trees grow slowly in this cold environment. Native Icelanders say “If you get lost in an Icelandic forest, just stand up!” Which is good advice for getting lost in a pygmy forest as well.
Ellyn Shea is a consultant and garden educator in San Francisco.