Mountains, climate, and trees: the role of climate in shaping forested landscapesJeremy LittellClimate is not something we think of "seeing" - after all, climate goes by boring descriptions like "the statistics of weather". But climate is much more interesting than that! Think for a minute, even from your chair in the Puget Lowlands around Seattle, and see if you can imagine what a subalpine larch tree (Larix lyallii) in the high central Cascades is witnessing in its climate. Right now, it is 50 degrees (F) outside and there are azaleas blooming in Seattle, but some small larches I am familiar with are buried under 65 inches of snow, and the air temperature above that snow this afternoon at 2pm was 30.2 degrees (F). Why do I care, and more importantly, how do I know this?
Climate - and its fingerprints - are all around you. From a tall building in Seattle, you can sometimes see the snows of the Olympics and the Cascades, and sometimes all you can see is the inside of clouds. In the spring and summer, you can watch the changes in the snowpack over weeks - on Rainier or on Mt. Constance in the Olympics - from the relatively balmy Puget Sound. All this tells you something about the climate in those places. But how do trees see the climate? Anyone who has ever hiked up a tall mountain knows that climate changes as you go up in elevation - it generally gets cooler and wetter, and the tree species change as you move up, eventually giving way to the alpine - a place with no trees. These zones of different trees tell us a lot about climate. Where a tree species can germinate and how fast it will grow are partially determined by climate, and there are species of trees that are characteristic of certain environments and climates. Larch is one of them - it likes the cold, snowy, dry drainages of the high eastern Cascades. But I'm also an armchair scientist - there is a SNOTEL station at Hart's Pass that records snow, temperature, and other data and posts it to the internet in near-real-time, so I know what my little larches are witnessing without having to be there myself, shivering in a tent or bundled up to take data even in a warm year.
On the Olympic Peninsula, I've done work focusing on a single tree species (Douglas-fir) as well as the group of tree species that inhabit upper treeline - the place where forest gives way to alpine meadows. The Douglas-fir work I do focuses on understanding how climate controls the growth of trees of the same species in different climates. For example, the Douglas-fir growing just above the Hoh River live in a relatively warm, wet climate for Douglas-fir, but six hundred meters up in elevation, on the same mountain, it is much cooler and wetter. The same species at that elevation is subjected to a much snowier environment, and this is reflected in the growth rings of the trees. The trees at lower elevations are sensitive to summer drought - even in the Olympics! - but the trees up high are more sensitive to snowpack than to drought, and years with too much snow cause them to grow less, not more. Another example is the trees living at the edge of the forest, at treeline on Hurricane Ridge. Here, the trees are mostly species that are the most tolerant to long winters, deep snows, and low temperatures. Like the higher elevation Douglas-fir, their growth is limited by snow. But one of the potential impacts of climate change is to encourage establishment of more seedlings in places where trees couldn't grow well before, such as in the alpine zone just above treeline.
During the experiential spring break, we will go to the Hoh Rainforest and spend some time trying to understand how trees see climate, and try to think about what some of the oldest trees (some older than seven centuries) have witnessed in terms of climate. Depending on the weather and conditions, I plan to show you how a dendroclimatologist (tree ring climate) does basic field work. I am fortunate enough to have a job that takes me to the mountains every summer to try to better understand what climate does to trees and what trees can tell us about climate. I got started in that job because I loved being in the mountains, and the skills I learned there allowed me to study things most people can't, or won't. We will use a combination of tools to try to understand these things - micro temperature sensors, increment borers, special camera lenses - but also just our wits. Some things science will help you verify, but your observation skills and imagination are arguably more important than all the measurement tools because that's where the cool questions come from. Weather permitting, we will visit treeline, though we won't see many seedlings. But what we can see will tell us a lot about how treeline may change, and why it is the way it is currently.
We are going to go places where tires are not allowed - and for good reason. But this also means we need to be prepared - the sun may shine, but it may not. It may snow, or it may not. Certainly, it will be muddy in places, and Nature does not much care who gets wet, cold, and miserable. On a given day in the mountains, it is possible to freeze and fry, get drenched and go thirsty, be too full and then starving. If nothing else, the trees that have the most to tell about climate seem to like extremes. If we are lucky, we will have a pleasant stroll in the woods, but if we are luckier, we will know why the mountains capture our imaginations. What I hope you come away with from this is a sense of having had a good time in the forests and treelines of the Pacific Northwest - in many ways, there is no place like this on Earth. I also hope to share my own sense of awe, at the power of climate and at the perseverance of some ancient trees, but also to share the idea that science of this sort is exciting and full of opportunities.
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