As in the previous period, the mathematical subjects, namely, forest measurement and forest valuation, were more systematically developed than the natural history basis of forestry practice; the slower progress of the latter being caused by the greater difficulties of studying natural history and of utilizing direct observation.
In botanical direction, descriptive forest botany was first developed, and several good books were published by Walther, Borkhausen, Bechstein, Reum, the latter (1814), of high value, and also by Behlen, Gwinner and Hartig.
In the direction of plant physiology, Cotta, early and creditably, attempted (1806) to explain the movement and function of sap, but remained unnoticed. Mayer’s (1805-1808) essay on the influence of the natural forces on the growth and nutrition of trees, contains interesting physiological explanations for advanced silvicultural practice. But these sporadic attempts to secure a biological basis were soon forgotten. Not until Theodor Hartig (1848) published his Anatomy and Physiology of Woody Plants was the necessity for exact investigation of forest biology as a basis for silvicultural practice fully recognized. With the development of general biological botany or ecology, a new era for silviculture seems to have arrived. Perhaps in this connection there should be mentioned as one of the earlier important contributions of much moment, G. Heyer’s Verhalten der Bäume gegen Licht und Schatten (1856) in which the theory of influence of light and shade on forest development was elaborated.
Among those who placed the study of pathology of forest trees on a scientific basis should be mentioned first Willkomm (1876), followed by R. Hartig.
In zoölogy, the early writers began with a description of the biology of game animals. Next, interest in forest insects became natural, and, in 1818, Bechstein in his Encyclopædia devoted one volume (by Scharfenberg) to the natural history of obnoxious forest insects. Toward the middle of the century, with the planting of large areas with single species, insect pests increased, hence the interest in the life histories of the pests grew and gave rise to the celebrated work by Ratzeburg, “Die Waldbverderber und Ihre Feinde” (1841). A number of similar hand-books on insects and on other zoölogical subjects followed; the latest, a most complete work on insects, being still based on Ratzeburg’s work, is that of Judeich and Nitzsche, in two volumes (1895). Of course, the general works on forest protection always included chapters on forest entomology. The first of these text-books on forest protection was published by Laurop (1811), and others by Bechstein, Pfeil, Kauschinger and recently by Hess (1896), and Fürst (1889).
Knowledge of the soil was but poorly developed in the encyclopædic works of the earlier part of the period.
Not till Liebig’s epochmaking investigations was a scientific basis secured for the subject. Then became possible the improvements in the contents of such works as Grebe (1886), Senft (1888), and of Gustav Heyer, whose volume (Lehrbuch der Forstlichen Bodenkunde und Klimatologie, 1856), well records the state of knowledge at that time. But only since then has this field been worked with more scientific thoroughness by Ebermayer, Schrœder, Weber, Wollny, and by Ramann, whose volume on Bodenkunde (1893) may be still considered the standard of the present day (newest edition, 1910).
The question of the climatic significance of forests is one which first became recognized as capable of solution by scientific means when the movement for forest experiment stations began to take shape and the systematic collecting of observed data was attempted. Most of the problems are still unsolved.