Big Things Come in Small Climates

Imagine a mountain overlooking a grassy plain. As rising temperatures and other impacts attributed to climate change take hold, the plants and animals covering its slopes are dying off. Birds and small mammals are migrating toward higher elevations in search of cooler climates — all except in a tiny patch of land at the base of the mountain, where these species are mysteriously continuing to thrive.

tree in field, Cambodia

Lone tree on a hilltop in Cambodia. (© Conservation International/photo by Sopha Sett)

Why? This is what we’re trying to figure out, to see if it may give us clues for how species — and humanity itself — can adapt to climate change.

Extinction risk from climate change may be determined by patches of habitat so small that scientists have overlooked them for decades. A few trees clinging to a cool hillslope or a patch of grass riding out a dry spell near a spring may make all the difference in how vegetation — and the animals that depend on it — survive climate change.

For almost 30 years, scientists studying historic climate change ignored small patches of vegetation, lumping them in with broader changes taking place as glaciers gave way to green plant life. Studies of future climate change missed them because climate models work on continental scales, not the scale of your living room. So these small patches of vegetation remained below our threshold of detection, yet they held a big secret.

It all changed with a very cold tree. A fossil of a tree was found in northern Europe, right near where the ice sheet was during the last ice age. How did it get there, and how did it persist so close to a massive ice sheet?  That first find was followed by others, until it became clear that many small patches of trees once grew here.

The existence of trees right next to a wall of ice seemed to contradict our whole understanding of ice ages. But it turned out that there were small pockets of warmth, or microclimates, near the ice sheet.

The last ice age was punctuated by periods of extreme, rapid climate change. Trees and other vegetation moved hundreds of kilometers in these climate flickers, sometimes in as little as a decade. Or did they? Instead of actually migrating southwards, some of these species may have remained in fragments of their previous habitat, undetected by researchers because their remaining pockets were so small.

Once we started noticing the ice age trees, they popped up everywhere. In Europe, North America, New Zealand and South America, rapid change in big ecosystems was made possible by relicts sheltered in microclimates — our hidden ecosystems.

Now that we understand this about the past, we’re looking for it in the future. Scientists began to look for microclimates in other settings, and found them — shady south-facing slopes and steep gorges, places where cold air collects in valley bottoms, places where fog is common, and other nooks and crannies in rough topography that may stay cool as global temperatures warm.

I and five fellow scientists recently co-authored a paper in the journal Trends in Ecology & Evolution (TREE), looking at progress in modeling these subliminal ecosystems.

We examined whether we could view microclimates and small vegetation patches using computer mapping software — and if so, what these patches might mean for the future of the natural world as climate continues to change. We called the vegetation patches “microholdouts”to reflect their role in helping plants and animals hold on in unusual places.

Looking for holdouts in models of the future requires adapting climate models to zoom in on areas the size of a vacation cottage, rather than their typical view which sees the world in chunks the size of Nebraska. It also requires acknowledging that small is important. To truly understand how plants will respond to microclimates, we need to observe them in different life stages, from seedling to fully mature.

To get this picture, scientists are wiring landscapes with miniature electronic thermometers to detect where microclimates currently exist. We then use computers to simulate future predicted conditions in those microclimates. This gives us an idea of the physical changes plants may experience.

Next we need to find out how plants react to these physical changes. Newly emerged seedlings are the most vulnerable to heat stress or lack of water. In four study sites in the mountains of California, we are planting seeds in all kinds of conditions — warmer and cooler, wetter and drier — to get a handle on how many seedlings will emerge and survive.

Finally, we combine models of fine-scale climate with models of plant response to simulate forest establishment and growth in the future. These complex computer models are giving us new insights into how microholdouts may influence response to the climate change impacts we’re experiencing and will continue to experience this century.

Microholdouts can serve as way stations through which genes adapted to warmer weather get passed from southern (warmer) to northern (cooler) parts of a species’ range. They can help rare plant species hang on for decades even when the surrounding climate has become inhospitable.

What microholdouts can’t do is provide permanent refuge from climate change. Even rare microclimates will eventually warm too much for the plant species that they currently hold to survive.

Therefore, we have to use the time provided by microholdouts wisely, to design conservation strategies that help plants and animals keep pace with climate change.

Management of fire and human intervention can help these holdouts last longer. For instance, if properly protected a microholdout on a mountain ridge can be a source of pollen and seed for surrounding valleys. That pollen or seed can be the source of new plant growth that may spring up in areas becoming newly suitable for the species, providing habitat for other plants and animals. Trees in the microholdout may provide shade that holds winter snowpack, maintaining water availability for downstream agriculture.

Will microholdouts be as big a deal for the future as they were for the past? We think so. In fact, I think we are on the verge of a small revolution in climate change biology.

Some of the world’s most unusual climates may be critical in helping ecosystems and the services people get from them to keep pace with climate change. Areas we once ignored may hold the key to the future — preventing extinctions and bringing water and other benefits to people.

Lee Hannah is senior scientist for climate change biology in CI’s Betty and Gordon Moore Center for Science and Oceans. Learn more about the paper published in TREE


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