Many scientists study how different plants and animals may react to
climate change. One of the things scientists in this study wanted to
know was how two species of pine trees, Jeffrey pines and ponderosa
pines, might react to a warming climate. To discover how the trees might be affected by warmer conditions, scientists would need to simulate the growing conditions of a possible future climate. How would they do this?

They might be able to grow plants in a greenhouse and control the surrounding temperature and water availability to simulate a warmer climate, but growing pine trees for many decades in a greenhouse would be very difficult!

Instead, the scientists in this study looked at the native range for Jeffrey and ponderosa pines. Jeffrey pines usually grow at higher elevations than ponderosa pines (figure 1). Growing at high elevations means that Jeffrey pines are adapted to cold temperatures and short growing seasons. At lower elevations, to which ponderosa pines are more adapted, temperatures are warmer and the growing season is longer.

These conditions may be like future climate warming conditions. Scientists could use lower—and warmer—elevation planting sites that are still within a plant’s native range as models for future climate change. They could then make predictions about how Jeffrey and ponderosa pines may be impacted by the changing environment by gathering data about how the trees grow in warmer conditions.

Imagine you found an unusual plant during a walk in the woods, and you
wanted to describe that plant to your teacher to see if they could help
you identify it. What might you tell them? You could describe its size, its color, how many leaves it has, what shape those leaves are, how its leaves or flowers smell, and what its seeds look like. All these traits make up the plant’s phenotype. A phenotype is the set of observable characteristics of an organism.

An organism’s phenotype can be influenced by both its genes and its environment. Genes are the information stored in DNA that tell an organism how to develop. The set of genes that an organism carries is called its genotype. Some traits, like eye color, are determined by an organism’s genotype. Other traits can be caused by the environment; for example, hydrangeas that are pink, purple, or blue get their color from chemicals in the soil rather than from their genes (figure 2).

In this study, scientists collected seeds with different genotypes of pine trees and planted them in new locations. They then collected information about the trees’ phenotypes, such as how tall and large they grew and when their buds appeared in the spring. By studying how different genotypes were affected by new environments, scientists could learn about how both genotype and environment can impact which trees survived and grew best in new locations.

Forest restoration is one of the many important jobs of the Forest Service. Forests can be damaged by different events, like wildfires, invasive insects, disease, or drought. Natural resource professionals then work together to replant trees in these damaged forests. Usually, forest restoration is done with trees that are native to the area to avoid changing the local ecosystem (figure 3).

However, one important factor that scientists and land managers must consider is climate change. Scientific evidence points to a warming climate, and scientists are making predictions about what the climate may be like in the future. Trees take a long time to grow, and they can live much longer than some other kinds of plants. Many live for several decades or even centuries!

Over a tree’s lifespan, the local climate may change a lot; temperatures may become warmer, growing seasons may be longer, and water availability may change. Trees can’t get up and move if their local environment becomes too challenging for them to survive; they must either adapt to new conditions or die.

In this study, scientists were able to use data from the work of previous scientists. The previous scientists collected seeds in 1973 from Jeffrey pines growing in three different locations and ponderosa pines growing in four different locations. All the locations were in the Sierra Nevada, a mountain range in California (figure 4).

Scientists made sure to collect seeds from high, middle, and low elevations. These seven collection sites are the seeds’ home sites (figure 5). Then scientists planted the seeds at three different planting sites—one at a high elevation, one at a middle elevation, and one at a low elevation.

During the next 40 years, those scientists and later scientists collected a variety of data. Scientists recorded which trees survived at each location. They also measured the surviving trees’ growth. To determine how much each tree grew, they recorded the diameter at breast height and the overall height of each tree. (Learn more about diameter at breast height below!) They collected these data in three different years: 1982 (after 9 years of growth), 1990 (after 17 years of growth), and 2014 (after 41 years of growth).

Scientists also recorded bud development observations from the trees planted at the low elevation site. They took pictures of the buds on the trees and classified the buds into one of six different stages of development (figure 6). They recorded these data weekly for both Jeffrey and ponderosa pines from mid-April to the end of May in 2014.

Finally, scientists collected climate data from each of the home sites and each of the new planting sites. Using computer programs, they modeled the mean annual temperature, mean annual precipitation, the days without frost, and how much moisture was available for growing plants in each location. They graphed their data to see the relationship, if any, among these variables and how trees survived and grew in each location.

The diameter of a tree is the length of a line drawn straight through the center of a tree trunk. But where do scientists take this measurement? Trees can have different diameters close to the ground or further up the tree. In this study, scientists measured the diameter at breast height (or DBH). DBH is the standard way that all scientists measure the diameter of big trees.

The diameter at breast height is the diameter of the tree 1.37 meters (or 4.5 feet) above the ground. When a tree is growing on a hill, the DBH is measured on the uphill side of the tree. Scientists can use a special measuring tape that calculates the diameter of the tree.

Survival Rates

High-elevation Jeffrey pines, adapted to colder temperatures, that were planted in lower elevations had the highest survival rates among Jeffrey pines. The difference in climate from home site to planting site did not seem to affect ponderosa survival rates. For both ponderosa and Jeffrey pines, the highest elevation planting site showed the greatest differences in survival rates.

Growth

Jeffrey pines from low elevation and high elevation had the same heights regardless of where they were planted, but ponderosa pines did have different heights. Ponderosa pines from low elevations grew taller than ponderosa pines from high elevations no matter where they were planted. In other words, this low-elevation genotype has adapted to longer growing seasons and that helps the trees spend more resources on growing.

In contrast, seeds taken from a high-elevation site have adapted to conserve their resources in cold temperatures, so they don’t spend those resources on growth. Those trees tended to grow less no matter where they were planted.

Bud Development

Bud development on both ponderosa and Jeffrey pines was only recorded at the low-elevation planting site. At this site, all the buds on the ponderosa pines opened earlier than the buds on the Jeffrey pines. The scientists found a difference among the Jeffrey pines from different elevations. The Jeffrey pines from the low-elevation site transitioned across the bud stages more slowly than the trees from middle and high elevations. The two species appear to have different requirements for bud development.

Ponderosa pines appear more adapted to their home sites and are slower to adapt to their new planting elevations. As the climate warms, scientists and land managers may plant new ponderosa pines at higher elevations within their species range to help the trees survive and thrive.

Jeffrey pines were less likely to survive in climates that were different from their home sites. Whether they survive in warmer future climates may depend on how they withstand temperature and water stress.

Land managers could possibly extend the growing season for Jeffrey pines by planting more Jeffrey pines from high elevations because those trees developed buds earlier than the low-elevation trees. However, when trees adapted to colder conditions are planted in warmer elevations, those trees may struggle to survive in the new climate.

The scientists found many differences in how these two species responded to being planted in new growing sites. More studies like this one of other species of trees will be important for forest restoration efforts in the future as the climate warms.