The United States has a rich flora – we stand 11th in terms of number of plant species worldwide with around 15,500 – but those plants are not evenly distributed across our territory. Some areas are especially rich and hold many rare and unique species. The Klamath-Siskiyou ecoregion, which straddles the California-Oregon border, acted as a refuge during our most recent ice age allowing plant species to survive there and not elsewhere. As a result, this unique ecosystem is home to many plants not found in other parts of the U.S. The geology of the region differs greatly from that of the surrounding volcanic Cascades, leading to the development of soils dissimilar to those beyond the ecoregion, also contributing to the richness of plants. While several rock types can be found in the region, including granitic, sedimentary, and metamorphic, it is serpentine that is notable.

Photos 1-4: Ceanothus cuneatus, Darmera peltata on Illinois River Siskiyous, Lewisia leeana on Serpentine, Serpentine fern Aspidotis densa.

The Klamath Mountains ecoregion contains the largest expanse of serpentine in North America. Serpentine derives its name from the sometimes scaly (or snake-like) appearance of the rock, which is rich in iron and magnesium. Soils derived from serpentine can be challenging for plants to grow upon. They typically contain low levels of the essential nutrient calcium, plus high levels of magnesium, which inhibits calcium uptake. Heavy metals such as nickel and chromium are relatively abundant, sometimes to toxic levels. Lastly, nitrogen is also in short supply and is one of the three primary macronutrients all plants need to survive. Plants growing on serpentine soils must adapt to these challenging soil conditions, a process that has driven the evolution of new, unique species in this ecoregion.

Plants have adapted to this challenging environment in several different ways. California lilacs (Ceanothus) have nodules on their roots filled with Frankia bacteria, allowing them to fix atmospheric nitrogen. Carnivorous cobra lilies (Darlingtonia californica) get their nitrogen by dissolving and absorbing insect bodies. Alpine pennycress (Noccaea fendleri) can accumulate nickel from the soil and store it safely in its leaves. While natives of serpentine soils have adapted to this rare soil chemistry, these plants do not need to live on serpentine, and several grow happily here at Heronswood. Serpentine does have the effect of reducing competition, by removing otherwise vigorous plants from the ecosystem. When growing serpentine natives outside their own environment, gardeners must ensure overly vigorous plants do not swamp these species, which are less competitive.