Washington's Forests: Status and Prospects
This is the transcript of a talk I gave on 2022.11.03 to the Central Puget Sound chapter of the Washington Native Plant Society. It discusses the observed and foreseeable future effects of climate change, through the rest of the 21st century, on the forests of Washington state in the USA.
Good evening, and thank you for asking me here tonight.
Of late I have been thinking a lot about the forests of Washington and their dynamics over time, particularly over the period from about 1800 to 2100, the American era of history. That story cannot be told without speaking of climate change, which even now is causing profound changes in our forests, and which will bring even greater changes in the remainder of this century. And because climate change is a human thing, I must also talk about the options we still have to minimize the ecological and social impacts of climate change in our forests.
Climate controls terrestrial vegetation. Washington is fortunate to have a great diversity of climates, and most of the world’s great biomes can be found somewhere within our state, excepting only the tropical and subtropical biomes. Certainly there are many factors besides climate that help determine what plants will grow in a place, but climate imposes inflexible limits. This means that when climate changes, vegetation changes. Remember this, it explains more of the variation in both our past and future forests, than any other simple fact.
Washington is about half forested, with the western half of the state mostly forested and eastern Washington forests limited to the higher elevations. There are some big urbanized areas, but urbanization doesn’t exclude forests, although it heavily modifies them. About four-fifths of our forest are unreserved, meaning they are available for economic exploitation, mostly as timberland and grazing land.
Since climate is the biggest control on vegetation, it usually works out that a map of temperature and precipitation will show where forests occur and what kind of forests they are. In Washington, both temperature and precipitation are mostly controlled by elevation and distance from the ocean. Our densest forests, with the greatest biomass accumulation, are in the west, with temperate rainforests found along the coast and in the wettest Cascade valleys. Eastern Washington forests are drier, with forests in the driest areas limited mostly to riparian areas along streams.
I’ve just given you a snapshot of current conditions in Washington’s forests. Now we’re going to look at what the remainder of the 21st century may look like. That show is mostly driven by two things: how we change climate, and what we do about it. I’ll start with how we change climate, and for that, I will start by reviewing seven basic facts about climate change and forests. This is the most important thing we’re going to talk about tonight. Most of it is stuff that the sound-bite-driven media will not tell you, but if you want to understand what climate change will really mean for us in the coming years, you need to think about these facts and their implications. So we’re going to spend a few minutes thinking about this stuff.
First, climate change is effectively irreversible. Yes, it has always been with us, and always will be. However the changes that we have seen so far in the 21st century will be with us for a long time, longer than the lifespan of anything that lives on this Earth. That means we have no choice: we, and by that “we ” I mean every living thing, must adapt to the changed climate. There is no going back. This map shows one aspect of irreversibility: tipping points, where a functional aspect of the climate system shows irreversible change.
Second, the problem isn’t the fact of climate change, it’s the speed of the change. This is the famous “hockey stick” graph from 1998 showing that modern climate change is happening with unprecedented speed. The Earth has always had slow climate change and living things can adapt. They can migrate, they can evolve. But once climate change happens too fast for them to adapt, they die. That’s what’s happening now: a mass extinction event driven by a sudden shift to a climate that an organism can’t live in. This means that to the extent we cannot slow down climate change, we will need to find ways to help organisms adapt, and that mostly means helping them to migrate into areas with suitable climates.
Third, as far as plants are concerned, warmer temperatures cause drought, even if there is no change in precipitation. This is due to something called vapor pressure deficit, which is another way of saying that warm air can hold more water than cold air, so just warming it up causes things to evaporate more.
Fourth, do you remember what happened to native Americans when Europeans arrived? Diseases. Climate change allows old diseases to move into areas that have new victims. This is already causing widespread dieback of western redcedar and bigleaf maple, and throughout the West, climate change has been linked to the rapid expansion of bark beetles. These maps show projected expansion of bark beetle habitat under climate scenarios through the rest of the 21st century.
Fifth is an idea called stress complexes. This means that if you subject a plant, or a population, to more than one stress at the same time, it is more likely to decline than if those stresses came at different times. For a long time our forests have been stressed by fire suppression, by development, by pests and pathogens... now we’re adding in climate change. Species are lost, biomass declines, forests change to nonforests. This is what ecological collapse looks like. It’s already happening in parts of the west, and it may be happening in parts of eastern Washington.
Sixth, uncertainty. There are three types of climate change uncertainty:
Finally, threshold shift refers to the slow incremental changes that you do not see, but that drive long term ecosystem change. None of us saw the great herds of buffalo, and so we do not see the costs of their disappearance. More recently, almost every Eucalyptus tree in Seattle dates from after the great blizzard and freeze of December 1990. Some of you will remember that storm, when snow blanketed the city for weeks under icy clear skies. Such storms happened every decade or so through the 20th century, but never since 1990. Threshold shift is slow enough to be invisible to most people and is one of the main reasons climate change is denied, ignored, and not addressed.
This is a summary of projected climate change affecting Washington forests in the rest of the 21st century. The basic message here is drought, coming from several directions at once. Summers will be much hotter, causing greater vapor pressure deficits. Summers will have less precipitation, too. Winters will have less snowpack, so we will have less snowmelt to support spring and summer runoff and streamflow, and that will lead to hotter and drier riparian forests.
This is a model of Year 2010 Douglas-fir habitat suitability in Washington, in work done by Richard Waring’s lab at OSU. Red here is good, showing high habitat suitability. As we see, most Washington forests are great for Douglas-fir.
This is the model for 2030. Habitat suitability has dropped greatly in eastern Washington and pretty severely in western Washington. In these yellow areas, Doug fir will not grow well and may not grow at all, depending on details such as seed sources and year-to-year weather variation.
Here is the model for 2090. By that time suitable habitat will be restricted to mostly high-elevation sites in western Washington. Large parts of eastern Washington currently occupied by Doug fir forests will be incapable of supporting the species at all. Similar findings apply to all of our dominant tree species. Also, please consider that these are optimistic findings, based only upon climate, not considering effects of diseases, fires, unforeseen weather events, or human action. Generally those other stressors would further reduce the extent and health of forests.
So, what can we do about it?
I think of the traditional human relationship to the forest as one of exploitation or neglect. Exploitation means we interact with the forest, as a hunting-ground or source of timber for instance. Neglect just means we don’t change the forest, as in the case of wilderness areas, many of which have formal protection from exploitation. The problem is that climate change is global, which means there is no longer any forest, anywhere on Earth, that humans do not change. Since we are now everywhere, all the time, neglect can no longer preserve the forest. In the future, everything that comes to us from the forest will be, in some way, a product of human action. If those actions aren’t going to be purely destructive, we will need to manage the forest for desired results, and that needs a working forest ecosystem. To get anything out of the forest, whether it be timber or just oxygen, we will need to manage for ecosystem services.
Here are some examples. Top left is exploitation for timber, an example from Capitol Forest near Olympia. Bottom left is exploitation for development, an example from the Sammamish Plateau. Note how this heavily developed area still has about 30% forest cover. Bottom right is exploitation for agriculture, in the Skagit Delta, which results in almost complete loss of forest cover. The neglect example at top right is a forest above Lake Chelan in the Sawtooth Wilderness, where fire suppression and bark beetles have left a mostly dead forest.
The 21st century will bring a lot of people to Washington, especially since climate change will leave so much of the U.S. brutally hot and dry. By 2040 over 90% of our population growth will come from migration into Washington. This will place strong development pressures on the forest.
Current human impacts on the forest include climate change, which we’ve covered. In addition there are ongoing pressures that either remove forest cover outright, or change its distribution and structure through development, timber harvest, and fire suppression. Also, I previously talked about migration of diseases into Washington, but there are also likely to be further introductions of new and non-native pests and pathogens, as has happened several times in the past.
I just wanted to say a few words about wildfire. It’s a big topic and a big issue. This map shows recent fires in Washington, through 2018. In much of eastern Washington, forests have already burned in the 21st century, and by the end of the century it’s likely that almost everything east of the crest, and much of the forest west of the crest, will have burned. But, fire is also a time of change. It’s a time when the forest can come into equilibrium with the new climate, provided that suitable seed sources are available.
The current fire situation is mostly the product of two human actions, climate change and fire suppression. There’s a lot of argument about the relative importance of these two factors. The truth is, it varies a lot between different types of forest, but most Washington forests are showing both impacts to some degree. Climate change is producing hotter, drier conditions that lengthen the fire season and reduce fuel moisture loads. Fire suppression has caused extensive areas with elevated fuel loads, thus it is physically possible for fires to burn over very extensive areas. Also, both human actions and climate change have increased the frequency of ignitions. Thus far, and through the rest of the century, these impacts will alter the productivity of our forests, so that future forests will be thinner, more open, and with smaller trees. We will see higher tree mortality both through fire and through other causes - drought, pests, pathogens - and this will affect forest composition and the potential for the forest to provide ecosystem services.
With regard to pests and pathogens, this map shows the spread of white pine blister rust following its 1910 introduction via contaminated white pines imported from Asia. It has now infected every white pine in the West except the Great Basin bristlecone pine, and has caused extremely heavy mortality throughout the affected species. As I’ve mentioned, bark beetles are also widespread. Some of the threats that are just developing in Washington include Sudden Oak Death, emerald ash borer, and Swiss needle cast. The spread of these diseases has been linked to warmer weather, especially, to less severe winters.
If we accept that a viable forest must be managed to achieve desired ecological services, such as watershed function or timber production, then we should be doing some things to make that happen. One is to minimize the risks of catastrophe by managing impacts like fire and insects using tools such as thinning to achieve resilient forest structures. Another is to intervene rapidly when a catastrophe occurs, such as replanting burned areas with species expected to do well under forecast 21st century climates. In many cases, those species will not be found in the forest currently growing on the site. We need to designate and protect ecological refugia. Refugia tend to occur in mountainous areas, where a given climate change may be resolved by simply moving a species uphill, whereas in flat country a migration of tens or hundreds of miles might be needed. Finally, we need to set targets for ecological metrics such as diversity, resilience, connectivity, and economic return, and measure progress on those metrics, because we really still have a lot to learn about reducing uncertainty in this work. To manage forests where the climate is continuously changing, we need to monitor.
The most important climate adaptation strategy is simply assisted migration. When the climate is changing too rapidly for plants to migrate using natural mechanisms like wind and animal transportation, then we have to fill the gap, or the plants will die out. There has been a lot of theoretical work on this but so far assisted migration is not being widely used in our forests. Assuming there will be a learning curve before this works as well as we would like it to, assisted migration should be widely implemented as quickly as possible. A more difficult problem is, we can help trees to migrate, but what about whole ecosystems? Is it even possible to find and move the thousands of species that depend upon a keystone species like Doug fir? The question remains open.
This is a bit of a digression, but since I’ve been a WNPS member for 35 years, I figured I’m allowed to raise a thorny question. Do native plants still matter, given that a plant’s habitat is determined by climate and we have changed all of the climates? I think the real answer is that we ecologists stopped managing for native plants a long time ago. We now manage for desired ecosystem services. We still hate stinky Bob and reed canarygrass, but now for a better reason: they degrade ecosystems. That reason will stay valid no matter how the climate changes.
Well, I thank you for your patience. I know this was a lot of stuff, but I want to leave you with a few more points.
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