Tree Adaptations at 7,000 feet
Quinn Rider, BS Ecology, Behavior, & Evolution
The Sierra Nevada is characterized by a number of different bioregions that are heavily influenced by altitude. Though a number of species persist in multiple bioregions, they are distinct in their makeup. Bear Valley resides in the Upper Montane. This line between the Upper and Lower Montane is defined by where 50% of precipitation occurs as snowfall, generally between 6,000 and 8,000 feet of elevation.
The Upper Montane region presents a number of challenges for the trees that inhabit it. The soils lack a clay rich subsoil and tend to be weakly developed, providing poor nutrient availability. The climate is dry and, often, very cold. The cold temperatures can freeze water bearing structures within trees, causing splits in the bark that open a tree to fungus and pests. It also brings ice and snow, abrasive loads that can snap branches and canopies. The warmer summer brings yet another challenge: short growing seasons and wildfire, though fire adaptations are a topic for another day. How then are the trees that live here adapted to survive our environment?
It is obvious to all who have visited Bear Valley that the dominant tree type is conifers. Conifers are generally more adapted to stressful environments with limited resources than are deciduous trees. Perhaps the most obvious difference is in their ever green cone-like shape. The spread of their limbs within the conical shape promotes maximum solar exposure (and absorption). It also sheds snow and ice well, as anyone who has been unloaded upon by a tree in the winter can attest. This prevents heavy build up that could snap a tree. They retain their leaves throughout the year, with an average needle life of around 3 years. Making leaves is expensive, both energetically and in resources like nitrogen. Conifers save resources through maintenance. Waste not want not, after all.
Their leaves differ from deciduous trees not only in their longevity, but also in their fleshy, needle-like (or scale-like in the case of cedars) shape. The form of these leaves are adapted to the harsh environment in a variety of ways. Their dark green color promotes heat absorption, which can help the tree maintain temperatures that allow for photosynthesis (45 - 85 degrees) even when the air temperature is cooler. Their fleshy, thick composition insulates the water bearing structures beneath the photosynthetic tissue, reducing the risk to the leaf from both freezing and evaporative water loss. The needle-like shape reduces surface area, another factor important in preventing water loss through evaporation. Conifer leaves are covered in a waxy coating called the cuticle, which serves a number of purposes. They prevent water loss through evaporation, protect the leaf from nutrient leaching during precipitation, are durable against abrasion from ice and snow, and they make the leaves less palatable in environments where food is often scarce.
A deciduous tree, with a much larger open leaf, has a much higher surface area. This allows for maximum solar absorption, but also opens a large area susceptible to damage and water loss. They lose their leaves in the fall when shifts in the available light spectrum indicate the coming winter, when photosynthesis becomes more of a struggle. Rather than photosynthesize year round, they capitalize on the ready availability of water, light, and nutrients to gather enough stores to last them through the relatively snow free winter.
How then do the seemingly fragile aspens survive the winter with their dropped leaves and lack ofneedles? Aspens are a generalist species; they are, in fact, the most widely distributed tree species in North America. Quaking aspens are somewhat spindly, lacking broad canopies that might break under snow loads. Rather than capitalize on the abundance experienced by lower land deciduous trees, they, like conifers, focus on maintenance. While they do drop their leaves, the structure of their thin bark allows sunlight through. This permits them to photosynthesize in their cortex, the green layer between the bark and their vascular tissue exposed if you scratch the bark. Heating of the trunk by the sun can keep them warm enough to photosynthesize even when temperatures drop below 45 degrees. Some other trees have this ability, but they lose it much more quickly as their bark thickens and their chlorophyllic layer sloughs off. This maintains the tree’s sapwood health through the winter, preventing costly repairs come spring. Aspens exist in colonies, reproducing from shoots sent out from their roots. Individuals resulting from these shoots are clones, genetically identical to the plant they originated from. Aspens do produce seeds, but the survival rate ofseedlings is low in areas with short growing seasons followed by harsh winters like our own. While individual trees rarely live long, the cloned colonies may be hundreds of years in age, with the oldest known clone being around 40,000 years old. This approach to reproduction allows aspens to quickly colonize disturbed or burned areas, capitalizing on those more sun rich environments. They contain a wealth of interesting adaptations, more than we have covered today, that allow them to persist amongst a sea of conifers.
Tree adaptations at high altitudes focus on maintenance rather than replacement, conserving water and nutrients while maximizing absorption of solar energy, both to warm the tree and to power photosynthesis. Individual species host their own unique adaptations, along with others beyond the scope of this discussion. Our forests are vital to the health of the ecosystem we enjoy and recreate in.
