Guest Post by Kathy Condon, 2024-2025 Sustainability Leadership Fellow and Ph.D. Candidate in the Department of Biology and Graduate Degree Program in Ecology at Colorado State University

The world is warming, and with that comes more extreme weather, more often. We are already seeing increases in extreme weather and climate conditions – from more frequent fluctuations between floods and droughts in the Midwest to more severe hurricanes affecting the coasts. It’s easy to see the greatest effects of extremes while they are happening: plants die during drought, fields are flattened by floods, heat waves damage our gardens.

We can choose to evacuate when warned about severe weather, can add water to our crops during drought, and can crank up the AC during a heatwave. Animals can hide in the shadows during the hottest part of a day. They can expand their ranges to seek out more resources during a dry period. But what happens to plants that can’t avoid these extremes? How quickly can an affected ecosystem bounce back? As extreme events are expected to continue to increase in frequency and severity, there is a growing interest in understanding not just how these extremes affect an ecosystem during the event, but also how long the effects will last in a system. For instance, imagine an area like a wetland, where soil is consistently somewhat damp. If an extended drought hits that area, the soil will dry not just on the surface, but deep into the earth. How much rain is needed to rewet that soil again? Is the “normal” rain enough to recharge the ground to the deepest dry areas? If the plants are still alive, how much water will they take up from the soil right away? Will that slow down the recharge deeper in the ground? These are the types of questions that we may ask when studying legacies of climate extremes.

The ecological consequences of extremes occur both during the extreme (think: plants drying up and dying), but also continue after the extreme has ended (think: all the plants that died were the same species and now that species is much less common in the community than it was before). These lasting changes are often referred to as legacy or memory effects. Legacy effects can happen in the non-living (i.e., soils, dead litter and waste) or living (i.e., growth rates, microbial processes) parts of an ecosystem. Most of the time, we think of legacy effects following extreme weather events in a negative sense. Drought legacies can lead to reduced soil moisture, which may cause slower plant growth due to water stress, slower soil nutrient cycling by microbes, and may leave the system vulnerable to shifts in community composition, if, for example, some invasive species was able to survive the drought more successfully than the native species.

However, legacy effects can also be positive, even for “bad” events like drought. One common example in grassland ecosystems is this: During a drought, plants are not growing as much because they are stressed by the lack of water. When water is limiting plant growth, they do not need as many nutrients from the soil. After the drought ends, there are “leftover” nutrients that remain in the soil from being unused during drought. Additionally, when the rain kick-starts microbial processing in the soil, the microbes also break down material in the soil that produces more nutrients. The leftover nutrients and burst of microbial activity can have a fertilization-like effect on the plants, increasing their growth post-drought…so plants may grow even more than they would usually under normal conditions when conditions before were bad.

In the bigger picture, legacy effects apply not just to scientists, but to anyone who has an interest in understanding their environments. Colorado’s native shortgrass steppe grasslands are often used for cattle grazing (among their many other uses), and understanding the year-to-year variability in how much grass will be available for cattle is extremely important for livestock managers. We can look for patterns from wet and dry previous-years to determine how current-year conditions affect the available forage. For example, consecutive wet years may lead to an increase in forage, and a wet year before a dry year may have a buffering effect. When livestock managers see the forecasts for the current growing season, knowing how the previous growing season conditions may interact with the forecasted conditions can allow for better-informed decision making. But legacies can also happen at smaller scales, such as within one growing season. Maybe for you, that means understanding how a wetter or drier spring may affect any perennials in your garden. For scientist, especially those that work in the field in natural and agricultural systems, we must consider legacy effects when making broader conclusions about some phenomenon in a system. In field experiments, it’s important to consider not just the effects of your treatments, but also if there are any effects from the history of a site that may change your interpretation.

Around the world, climate change is leading to increases in extreme events and extreme seasons and years. We need to understand how the severity, duration, and frequency of events will continue to effect ecosystems to best understand how recovery from extremes will occur. Wherever you are on the globe and in whatever ecosystem you live, work, or visit, legacy effects will play a role at some point – so considering what the plants and environment around us have been through before will help us to really understand what we’re seeing now and into the future.