Herbarium Specimens as a Novel Tool for Climate Change Research

Abstract

any changes in plant and animal Mbehavior point to the effects of-L v global climate change. In recent years, biologists have observed birds migrating earlier in the spring, tropical frog populations declining, and insects relocating to higher altitudes on mountain slopes. Yet the most convincing evidence that living organisms are responding to global warming comes from flowering plants, which are especially responsive to warm weather in the spring. The data on plant flowering times is particularly compelling because there is so much of it. Thanks to the professional and amateur botanists who have kept annual records for a variety of plant species and locations, there exists more long-term data on first flowering dates than on other biological phenomena. As described in a 2003 article in Arnoldia, analyses of these records show conclusively that both wild and cultivated species are flowering earlier than in the past because of warmer growing conditions. Although these data sets have proven valuable for understanding how certain plants in certain places have already been affected by global climate change, there are too few long-term data sets available to predict future impacts worldwide. To build a more geographically complete picture, scientists must seek new sources of data. Botanical gardens and museums might be those sources. In the spring of 2002, we decided to mvestigate whether the extensive collection of herbarium specimens at the Arnold Arboretum could be used in conjunction with the Arboretum's living collections to determine how plants in Bos-ton are responding to global warming. If so, our study would show that herbarium collections throughout the world could be valuable tools for studying plants' responses to climate change. Boston, like other large cities, is an especially good place to study climate change because its average annual temperatures have risen over the last hundred years by 1.5 degrees centigrade (2.7 degrees Fahrenheit)-more than in less urbanized parts of the world. Only half of Boston's increase corresponds to the increase in the average global temperature. The other half is related to urbanization-more paved surfaces to absorb sunlight and radiate it as heat, less plant cover to remove heat through transpiration, and more sources of greenhouse gas emissions, such as buildings and cars-a phenomenon collectively labeled as the "urban heat island effect." As a result, the increase in Boston's average temperatures has already reached the magnitude expected for the entire planet later in this century. Thus, Boston provides a preview of the warming that will occur elsewhere in the world. Prior to 2002, the Arnold Arboretum did not systematically collect phenological data-that is, records of the dates of seasonal biological events such as flowering or fruiting, which have been used in previous studies of climate change. However, the Arboretum does possess an alternative source of phenological data extending well over a hundred years: the Arnold Arbo-retum Herbarium, a collection of 80,000 dried plant specimens, most of which were taken from the woody plants of the living collections as part of the Arboretum's standard documentation process. The record that accompanies each dried and pressed specimen includes the name of the species, the identification number of the plant, the date of collection, and-importantly-the phenological state of the plant on that date, such as flowering, past flowering , or in fruit. Many of the plants from which