Seasons of Change

By Humphrey Lin | SQ Staff Writer | SQ Online (2013-14)

The arrival of spring each year brings, as usual, the pleasantries of flowering and leafing plants. Plants which had appeared lifeless during the winter seem to suddenly burst into life, bringing forth vibrant, colorful appendages. The first flower and leaf of spring mark important events in a plant’s life cycle. Exactly when these events occur over the plant’s lifetime can significantly alter its survival and its competitive interaction with other species. Such timing of these life events is called phenology. For years, scientists have documented changes in plant phenology as a result of habitat change, and conducted isolated experiments to create predictions of future behavior. But recent research shows that these experiments may not provide the most accurate predictions.

Phenological shifts have largely been attributed to temperature changes, leading to many controlled, experimental studies in which plants are subjected to various temperature conditions in order to track the impact on their phenology. How accurately these models reflect phenological changes in the real world, however, had not been carefully studied before. In their recent publication in Nature magazine, UC San Diego postdoctoral researcher Lizzie Wolkovich and principle investigator Dr. Elsa Cleland found that small scale warming experiments tend to underpredict plant phenological response to changing climate. This means that current warming experiments are yet unable to accurately predict the magnitude of plant response. This has significant global implications as scientists worldwide try to understand and predict the effects of climate change on the habitat

“[We found that] we cannot fully explain the observed responses based solely on the experiment data,” Dr. Cleland said. To assess the discrepancy between what scientists see in small scale experiments and what they observe in real life, Wolkovich and Cleland used a data synthesis technique. Collaborating with over 20 research groups worldwide, they gathered from them both small scale experimental data and observational field data. The studies concerned abundant plant species of the temperate zone whose timing of the first flowering and first leaf were documented according to number of days of shift per degree change (for example, flowering 7 days earlier per one degree temperature rise). Compiling and comparing the data, Wolkovich and Cleland found that in small scale warming experiments, these shifts were significantly smaller in magnitude.

Wolkovich and Cleland offer two hypotheses for these staggering discrepancies: First, it may be that additional global factors such as rainfall, nitrogen deposition, and invasive species, which are not accounted for in controlled experiments, impact phenology.

“Very little attention is paid to factors changing alongside temperature,” Dr. Cleland said.

Second, temperature changes in warming experiments may not accurately reflect temperature changes in nature. “In natural environments, plants experience changes in temperature extremes, maximum and minimum temperature, and temperature difference between night and day,” Dr. Cleland said. “All of these nuances may be influencing phenology.” Most warming experiments track only the mean temperature change over time, and not these subtleties. In light of the found discrepancies, Dr. Cleland proposes that researchers carefully measure and archive the climate data beyond just mean temperature change.

This finding has important implications for the field of plant ecology. “The ultimate goal is to understand how systems will respond, and predict into the future,” Dr. Cleland said. “Right now, we are accurately predicting the direction of phenological change, but not the magnitude.” This research finding reveals the challenge of accurately modeling nature and its many subtleties and complexities. “It really challenges us to continue to delve deeper into the mechanisms that underlie realistic responses,” Dr. Cleland said.