Forest Tree Pollen in South Swedish Peat Bog Deposits - Lennart von Post (1967[1916])

Introduction by K. Faegri and J. Iversen

The Lennart von Post paper (1967) begins with an introduction stressing the vital importance of his 1916 Lecture to the 16^th convention of Scandinavian naturalists in the progression of pollen analysis. Primarily a Quaternary geologist, von Post attempted to use pollen analysis as a method for investigating climatic change during the Quaternary period. Unfortunately, since his presentation was never published, the paper was difficult to access. Furthermore von Post’s “aristocratic arrogance” in believing his work to stand on its own merit, made the paper know more for its implications than its wording.

The Lecture (Translated by M.B. Davis and K. Faegri)

In the 25 years prior to 1916, investigations followed two approaches for determining the age of peat bog strata. The paleofloristic method used Japetus Steenstrup as a model, focusing on isolating and identifying plant remains from peat. The second approach led by Rutger Sernander and C.A. Weber, was paleophysiognomic. This method carefully examined the stratigraphy in an attempt to interpret the ontogeny of each bog. While both approaches are limited in their ability to provide dates to peat-bog sequences, von Post considered the paleophysiognomic approach the most successful.

Using the paloephysignomic approach, Sernander was able to identify a distinct, regularly occurring stratigraphic marker. Termed the Grenzhorizon, this subboreal-subatlantic contact was determined to coincide with the postglacial climatic deterioration. According to von Post, this horizon can be used as a starting point when using stratigraphic methods for dating peat.

Because previous methods of dating peat bog strata proved unreliable, von Post suggested that fossil pollen within the different stratigraphic layers could be used to illuminate the history of the vegetation. von Post realized during his investigations that the character of preserved pollen flora in peat was constant and characteristic for each sample. Therefore, this composition must reflect the average configuration of the pollen rain at the time of deposit. As such, pollen rain itself must be a general reflection of the forest at that time. Realizing this, von Post believed that reconstruction of the changes in pollen flora composition through a sediment profile should make it possible to observe the changes of the general makeup of the forest through time.

To test the usefulness of using pollen to reconstruct the content of peat strata, von Post examined the pollen flora of South Swedish peat bogs collected in a systematic investigation throughout 1915. Peat bogs were selected using a set of five principles: (1) large bog size; (2) be above the maximum limit of the marine transgression; (3) general features of sediments and developmental history had to be known; (4) the subboreal-subatlantic contact was clearly developed; and (5) the profiles must be from peat that completely preserved the pollen flora.

Identification of pollen flora was undertaken using 200x magnification, with higher and lower magnification settings employed depending on the coarseness of the pollen. The composition of pollen flora in a particular peat strata was calculated as a percentage based on the total number of grains in a series of preparations.

In his presentation, von Post discussed his results using a “generalized diagram based on the averages of the individual curves” (387). Unfortunately, the translators could not locate the original diagrams of the pollen curves. According to the footnotes, pollen percentages from each bog were plotted along the x-axis, while the y-axis represented individual pollen types.

von Post separated the eight main curves plotted on the diagram into three groups:

1.      The first group, consisting of birch and pine pollen. This group exhibited high frequencies at the beginning of post-glacial time that subsequently declined until its rebound in subatlantic time.

2.      The curves from the second group, mixed oak-forest, alder, and hazel pollen, show a rapid increase over time as birch and pine pollen began to decrease. Their frequency diminishes following the onset of subatlantic time.

3.      The third group, made up of beech, hornbeam, and spruce, are represented sporadically at the beginning of post glacial time. These species begin to increase along with the pine and birch during the subatlantic.

At this point in the paper, von Post discusses the precautions of using pollen diagrams. He states that in the absence of pollen productivity and dispersal indices of various trees, the diagrams are not sufficient in expressing the composition of the forest communities. Rather, they should be studied for the trends they represent of changes in frequency between forest types (390).

Taking these considerations into account, von Post concludes that trees adapted for warmer climates made their way into southern Sweden after the post-glacial period began. von Post further tested this conclusion and the potential for the use of pollen sequences in chronological relation to sea-level change following deglaciation by analyzing pollen from peat and gyttja sequences of Ancylus lake (9500-8000 BP) and Litorina sea (7500-4000 BP). Although he could not prove it, he was certain that southern forest elements immediately replaced glacial period flora in the area. An important aspect of this conclusion concerns the beech-spruce pollen limit horizon.

During his analysis, von Post found that beech and spruce occurred in low percentage frequencies throughout the profile. The beech in southern Scandinavia first appeared during the Atlantic period. Spruce pollen, however, occurred in much lower frequencies. However, von Post was inclined to believe that its history was similar to that of beech pollen. While they were most likely subordinate species during the time of mixed oak-forest, they most likely took advantage of the drier climatic conditions during subboreal time and, eventually, became the dominant species: beech in the south and spruce in the north. von Post credited the differential dominance of spruce versus beech not only to climatic conditions but also human activity.

From the pollen analyses of Ancylus lake and Litorina sea, von Post concluded that:

1.      Postglacial time can be divided into a time of mixed-oak forest and a beech-spruce forest epoch.

2.      The Ancylus maximum epoch of southern Sweden are characterized by a pollen floral with dominating pine and birch, and low frequencies of alder, elm, lime or hazel (399). These flora were later to be replaced by beech-spruce forests, distinguished by higher frequencies of these pollen.

At the end of his lecture, von Post suggested that the use of pollen curves derived from analyzing sedimentary deposits could assist providing a better understanding the changes in climate during the late Quaternary period. This conclusion was made on the basis that during the presentation he showed that the variation of pollen flora in peat diagrams could be correlated with the stratigraphy of peat bogs. In all, he hoped to someday use this research to develop comparative forest maps of the post arctic period for the different regions of the world.

Questions for Discussion

1. What were the primary contributions of this study to ecology?

2. What are the strengths and weaknesses of this paper? (Consider in both a historical and modern context)

3. Is this paper suitable as a paper included in the "Methodological Advances" section of this volume?