Global Environmental Change Affects the Timing of Plant Growth and Flowering
by Nancy Falxa-Raymond
In a study published in the September 12, 2006 issue of the Proceedings of the National Academy of Sciences, lead author Elsa Cleland and investigators from the Carnegie InstitutionÕs Department of Global Ecology and StanfordÕs Department of Biological Sciences found that global environmental change may alter the timing of plant growth and flowering. Their study was conducted in the Jasper Ridge Global Change Experiment (JRGCE) - a site designed to simulate environmental conditions predicted to exist one hundred years from now. Their results show that global environmental change could disrupt the progression of plant flowering, shaping the patterns in which limited resources are used in primary production - the production of organic compounds by organisms at the base of the food chain.
The Jasper Ridge Global Change Experiment
One of the longest running experiments of its kind, the JRGCE began in 1998 and analyzes four different variables of environmental change: doubling of atmospheric carbon dioxide, a temperature increase of 2 degrees Fahrenheit, a 50 % increase in precipitation, and increased nitrogen deposition. Each of these four experimental treatments are applied to grassland plots singly and in all possible combinations. The experiment includes control plots, and each treatment is replicated eight times.
An important feature of the JRGCE is its natural community setting. Dr. Christopher Field, director of the Department of Global Ecology, notes that Òit is difficult to do realistic experiments in artificial settings.Ó Many experiments that try to mimic global environmental change use communities created for the experiment, not ones that already exist. The natural community of Jasper Ridge is ideal for the Global Change Experiment because the plants are small enough to be measured and have annual life cycles that allow research to span many generations. To test plant growth timing changes, the scientists monitored the timing of both flowering and primary productivity, estimated by canopy greenness. Timing is Everything
The study of seasonal life cycle timing events in plants and animals, phenology, is one dimension of the influence of climate change on plant growth. Phenological complementarity (when plants of different species grow, flower, or mature at different times) promotes coexistence in multispecies plant communities. It is one of the mechanisms that determines critical ecosystem processes, such as nutrient capture and primary production. In Mediterranean-type ecosystems like those at Jasper Ridge, the majority of plant growth occurs in a short growing season, so even small shifts in phenology can disrupt the complementarity among species.
According to the studies, where global warming is strongest, the increase in temperature has a dominant effect on phenology, accelerating it and thus causing earlier flowering of grasses and wildflowers. However, where there is less warming, elevated carbon dioxide is the dominant factor affecting phenology. Increased carbon dioxide concentration and nitrogen deposition causes the flowering of grasses and wildflowers to converge significantly. For example, grasses normally flower earlier in the growing season than wildflowers. With increased carbon dioxide and nitrogen, flowering of grasses is delayed, and flowering in wildflowers is accelerated, reducing phenological complementarity.
Through the reduction in phenological complementarity, the overall productivity of the ecosystem could be reduced by altering the timing of flowering necessary to coincide with the needs of some species. Additionally, reduction in phenological complementarity could increase competition between the species, possibly reducing productivity and biodiversity because some species would outcompete others.
Dr. Nona Chiariello, research coordinator at Jasper Ridge, points out that the plants used in the study are not important globally and the results are not directly applicable to other areas. She does assert, however, that Òusing the Jasper Ridge results as a starting point, we can look for the same mechanisms elsewhere and determine whether they are functionally the same.Ó
The Future of Plant Phenology
Although the detailed measurements of each species are no longer being recorded, the ecologists at the JRGCE are still measuring primary productivity and how it changes over time, both throughout the growing season and over the years. Future research aims to determine whether the phenologically-altered ecosystems could become susceptible to changes such as invasive species replacing native populations that could not survive the changes.
Research at the JRGCE is providing scientists with valuable information about how the earth will change in the decades to come. Once the data has been collected, says Field, ÒPeople from a wide variety of perspectives can assess the information.Ó Then decisions can be made to preemptively address environmental concerns that humans may face in the near future.