Information about forest stressors and the factors that influence them probably has expanded more in the past 25 years than in the entire previous century. Our knowledge of climatology, soils, pathogens, insects, plant physiology, anthropogenic influences, and other factors has increased at an almost geometric rate. Perhaps more importantly, scientists have come to appreciate the role that these stressors play in interaction with one another.

In other words, our knowledge of the cause-effect relationship is clearer than ever before. Earlier land managers and scientists often erred in assigning one condition as the cause (e.g., southern pine beetle) without recognizing the underlying influences that triggered the condition (e.g., not matching the planted tree species to the site). Today, investigative forest resource scientists take a much more holistic approach to analysis of conditions as reflected in stressed trees.

Coincident with this increased understanding of tree-environment dynamics has been a fortuitous and equally impressive ability to track, record, and analyze stressed conditions. This understanding has come none too soon. Without sophisticated detection and analysis equipment, the vast amounts of data accumulated over the past two or three decades could scarcely be tabulated, let alone analyzed and interpreted. To cope with the explosion of information, high-technology tools are necessities, not luxuries. Examples of such tools are geographic information systems (GIS), global positioning systems (GPS), satellite imagery, and highly refined aerial reconnaissance methods. Our present ability to collect and tabulate an enormous volume of data has led to an opportunity probably never envisioned by most scientists in the first half of the 20th century-the capacity to perform complicated, computerized, statistical, and spatial analysis on investigations into large-scale environmental change. As just one example of such high-technology investigative work, scientists have given birth to a new branch of dendrochronology (the study of tree age) in which analyses are made of tree rings to determine the chemical composition of the environment decades ago so it can be compared with the composition today.

Many of these new analysis methods have applications not only with contemporary information but also with data chronicled long ago. Retired scientists are no doubt heartened to see their work reanalyzed and depicted using contemporary methodology. One example is historical records on the southern pine beetle included in figure 22.

Using modern tools and land managers, environmental scientists are establishing baseline data that doubtless will prove valuable in determining whether conditions such as global warming and tropical deforestation are impacting geographic areas ranging in size from a stand of white pine trees in North Carolina to the health of the entire planet earth.

As dramatic as advances have been in recent years, technology continues to grow more and more sophisticated at an exponential rate. Software and hardware engineers, photogrammetrists, statisticians, and environmental scientists are coordinating their efforts to maximize the utility of their different disciplines. So rapid are these advances that the technology described in this publication likely will become obsolete in a few years.

Forest Health Protection is important to the public in many ways, from the forest products used in everyday life to the quality of the environment. Also, the same insects and diseases that damage forests can affect trees in residential and other nonforest areas.

To assist readers of this publication who are not familiar with forestry terms, a glossary has been placed at the end of the text. A list of the common and scientific names of the pests and the hosts that are discussed appears in appendix A.