Why Disappearing Sierra Nevada Meadows Are Bad News for Water

September 25, 2017

Mountain meadows are starting to get some respect. For over a century, meadows were the first alpine environments targeted for development, grazing and farming, because they tend to be flat and packed with rich soil and nutritious plants. But we’re starting to understand that meadows have a much more important role to play for society at large.

Meadows, it turns out, are water banks. As winter snows melt, the runoff flows into meadows, where deep organic soil holds the moisture like a sponge and then releases it slowly. This helps minimize downstream flooding during spring. Meadows release that runoff over a longer period, helping stretch valuable water supplies through the long, dry summer months.

Efforts are underway to restore meadows, which have lost some of their water-holding ability as they’ve been compacted and eroded by grazing, logging and other activity. Unfortunately, a new threat has emerged: climate change.

A new study by researchers at University of California, Merced, found that Sierra Nevada meadows are shrinking due to encroachment by trees – primarily lodgepole pines. And not just some meadows, but virtually all meadows throughout the Sierra Nevada.

Warmer temperatures are likely to blame, according to the study, creating conditions more favorable to trees. As a result, lodgepole pines are creeping into areas that have been historically meadow environments, sinking deeper roots that create a new year-round water drain on meadow environments.

And the future doesn’t look good. As temperatures warm further due to climate change, more trees are expected to encroach on meadows. The authors reach a startling conclusion: By the end of this century, the average meadow will shift entirely to forest. Eventually, meadows may only be found in sparse locations at high elevation, where lodgepole pines can’t thrive.

What are the implications for biodiversity, for water supply and flood prevention? A lot of these answers aren’t yet known. But to find out more, Water Deeply recently talked to Lara Kueppers, a co-author of the study and a research scientist at the Sierra Nevada Research Institute, based at U.C. Merced.

Water Deeply: What’s unique about this study?

Lara Kueppers: I think the main thing is the extent of ground-based observations that we did. When you are in the field observing just a single meadow or a small number of meadows, you can sort of draw conclusions about those few meadows you looked at. But we were really after a large area and wanting to understand: Is this phenomenon something that’s widespread across a big section of the Sierra? So we surveyed meadows across a pretty broad area, from Sequoia-Kings Canyon National Park on up to the Lake Tahoe area.

The other way people try and cover a lot of ground is usually by using satellite images. But we were interested in the number of trees that were coming into these meadows that might be smaller and difficult to detect remotely. So being on the ground enabled us to do that.

I think in the end we surveyed over 340 meadows. Not all with the same level of intensity.

Lara Kueppers is a research scientist at the Sierra Nevada Research Institute, based at U.C. Merced, and co-author of a new study about disappearing meadows in the Sierra Nevada. (Photo courtesy Lawrence Berkeley National Laboratory)

Water Deeply: Are meadows really likely to disappear by the end of the century?

Kueppers: The conclusion isn’t that all meadows will disappear. The conclusion is that the average meadow will disappear. Basically, what we were finding is that many meadows are experiencing encroachment. There are some that aren’t, however, and those may continue to be resistant to encroachment in the future.

So, for example, in some meadows at higher elevations, where lodgepole pine isn’t abundant, you don’t see the same kind of encroachment as you do at lower elevations. But anyone who hikes in the backcountry of the Sierra has surely noticed trees creeping into meadows, if you return over time.

That’s why we say the “average” meadow, because a lot of those meadows that are out there are experiencing encroachment. It may not be full encroachment by the end of the century, but it will have experienced some encroachment.

Water Deeply: Your results show that climate change is a major reason for meadow shrinkage. How did you reach that conclusion?

Kueppers: If by climate change you mean human-caused climate change, we actually didn’t examine that question. What we were focused on is figuring out what are the causes of encroachment we see in terms of specific factors. So we looked at landscape factors like topographic position, elevation, what are the tree species around the meadow. Then we also looked at climate factors.

We found that climate factors are important drivers of the patterns we’re seeing, but we didn’t really try to quantify whether past human-caused climate change was complicit in that. We don’t really conclude the encroachment we’ve seen so far has been caused by human-caused climate change. Our conclusion is that climate is an important driver of encroachment. And then we said, let’s look to the future and projections of climate change.

Given what we know about factors that seem to lead to tree recruitment in these meadows, that’s what led us to the conclusion that future climate change is going to be a really strong contributor to encroachment. For example, temperature is one of the important factors that explains variability in the number of [tree] recruits in a meadow in any given year. So when we look at future temperatures increasing, we see that drives an increase in the number of trees in the meadows.

Water Deeply: How does snowpack affect meadow encroachment?

Kueppers: High snowpack actually promotes encroachment. We think that’s in part because, if there’s low snowpack, over winter these young trees can be exposed to very cold temperatures, and that can be detrimental to their ability to grow and survive. So when you have high snowpack, the really young juvenile trees are buried in snow, and protected over the winter. Then they can emerge and grow really strong in the summer.

The other thing snowpack is important for is a sustainable source of water in the drier meadows during spring and summer. These meadows are collecting meltwater, not just from the meadow itself but from the surrounding watershed, because the meadows are in topographically low locations. Again, when these trees are young and getting established, if the meadow dries out they’re going to have a hard time making it through the summer. But if there’s a high snowpack, there’s that sustained input of water and they’re less liable to dry out.

Meadows like this one, on the Toiyabe National Forest south of Lake Tahoe, are at risk of encroachment by lodgepole pine trees as temperatures warm over the decades ahead, according to a new study. (Photo courtesy UC Davis)

Water Deeply: How big of a role do meadows play in water storage?

Kueppers: They’re an integral part of the hydrology of the High Sierra. A big role that many meadows play is sort of as a regulator in the water system. They collect snowmelt from the surrounding slopes and they store it and sort of release it slowly over the spring and summer season. They can absorb a large amount of water and then release it. Some is coming from surface runoff, but other water is coming underground through cracks in the rock and then emerging in the meadows.

The soil, especially in the wettest part of meadows, is very organically rich. That’s because, over time as these grasses and other sedges and wildflowers that are well adapted to the meadows grow and die, the organic matter decomposes very slowly because there’s so much water around. So this organic matter just builds up, and more organic-rich soil can hold more water. And they release that water slowly over the summer.

Water Deeply: What are the water-supply implications of shrinking meadows?

Kueppers: We don’t fully know what the full-sum impact might be. There are a couple different ways that trees encroaching into meadows could alter hydrology. One explanation we have would be that trees typically use more water than the meadow grasses and wildlife. As a consequence, as trees become more abundant and larger, they would use more of the water coming into meadows, which means less water would flow downstream.

Another is that as the meadows dry, if the trees are using more of the water, there would be faster decomposition of organic matter, and that organic matter wouldn’t necessarily be replaced in kind by the trees.

If there are fewer roots with the trees than there were with the grasses and forbs, then the amount of organic matter flowing might not keep up with the losses from the drying conditions. So you would lose that sponginess in the soil, and it would weaken its ability to absorb and release that water over time.

Water Deeply: Will the Sierra look different in future?

Kueppers: I think there will still be some meadows. There will still be areas that remain wet. You just might have to look harder. You probably will have to go higher to get to them. Maybe not over our lifetimes, but in our children’s lifetimes. Luckily for us, changes happen very slowly. So while we can see signs of this process underway, at least all of us can still enjoy the meadows for what they are right now.

Water Deeply: What can we do to reverse this trend of meadow loss?

Kueppers: Our simple modeling exercise suggests that supporting solutions to climate change is a really important part of slowing meadow encroachment, because the critical factor driving encroachment overtime is the temperature increase. Another thing we can do is really limit the direct effects on meadows, such as trampling by grazing animals, because that just contributes to impacts on these meadows.