The use of annual arboreal pollen deposition values for delimiting tree-lines in the landscape and exploring models of pollen dispersal

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Abstract

In reconstructing past vegetation by pollen analysis there is an increasing need to quantify plant abundance and to specify the distribution of vegetation units in the landscape. Fundamental to such reconstructions is an understanding of the pollen–vegetation relationship. Monitoring pollen deposition by a standardized method in different vegetation situations provides numerical data for this relationship. Here monitoring is by modified ‘Tauber’ traps following the standardization of the Pollen Monitoring Programme (PMP). Results are presented for an 18-year period (1982–1999) at ten localities within the latitudinal boreal forest zones of northern Fennoscandia. Particular focus is placed on the tree-lines as these are regarded as climate-sensitive boundaries in the landscape. At an annual temporal resolution, pollen deposition reflects climate, primarily the temperature of the growing season of the year before pollen emission. It is the long-term average pollen deposition which reflects the presence/absence and abundance of trees in the surroundings of the monitoring site. Pollen influx values are given for varying degrees of coverage of Betula, Pinus and Picea. The tree-lines of these three taxa are crossed when pollen influx rises above 500, 500 and 50 grains cm−2 year−1 respectively. The comparable pollen influx thresholds for the presence of forest are: 1000, 1500 and 100 grains cm−2 year−1 respectively. These numerical values apply to openings in the forest cover of c.1 ha. However, inside the forest, pollen deposition values can be some three times higher. These data contribute towards the validation of models of pollen–vegetation relationships and the results enable a more objective interpretation of fossil pollen assemblages from terrestrial deposits in terms of both tree-lines and climate.

Introduction

The primary aim of plant palaeoecologists has always been to reconstruct as precisely as possible the vegetation situations of the past. Increasingly these reconstructions are being quantified. Although some questions can still be answered by knowing what range of plant species was present at a site at a specific point in time, farther reaching questions require knowledge of the abundance of each species in terms of its areal coverage and exactly where in the landscape it was located. Increasingly, too, past vegetation situations are used as proxies of past climate and this approach, similarly, requires quantification. The underlying assumption is that, given suitable edaphic conditions and an absence of human interference, there are optimal climate conditions for every major vegetation unit and that the spatial limit of the vegetation unit represents the threshold of the climate necessary for its existence. Under a changing climate, plants growing at the outer limits of the unit are expected to react most sensitively. A long-term record of such limits, e.g. tree-lines 1, gives evidence of how climate has changed in the past, provides a measure of the scale and rapidity of change and brings out trends which can be used as a reference in predicting potential future change. The refinement of techniques which enable both former tree-lines to be defined with a high degree of accuracy and the composition of the forest to be quantified is, therefore, important.

The most commonly used investigation method in plant palaeoecology is pollen analysis. It has long been known, however, that pollen–vegetation relationships are dependent on a number of factors the most important of which are the pollen production of individual species; pollen dispersal (dependent upon the falling velocity of the pollen grains and windspeed); the spatial distribution of vegetation around the sampling site; and the basin size of the sampling site. The various aspects of pollen dispersal and how they affect the interpretation of past pollen assemblages have recently been reconsidered in detail by Jackson and Lyford (1999) and the relevance of pollen analysis to investigating timberlines and tree-lines, in particular, has been reviewed by Birks et al., 1996, Birks and Birks, 2000.

Three main approaches to quantification of the pollen–vegetation relationship have been used. These are: (1) the modern analogue approach with the collection of empirical data (Birks, 1980, Lamb, 1984, MacDonald and Ritchie, 1986, Caseldine, 1989, Caseldine and Pardoe, 1994, Jackson, 1991, Jackson and Dunwiddie, 1992, Jackson and Smith, 1994, Gaillard et al., 1994, Calcote, 1998, Davis et al., 1998); (2) the correction approach using R values or ERV (Davis, 1963, Davis et al., 1973, Andersen, 1973, Prentice and Webb, 1986, Jackson et al., 1995, Calcote, 1995, Jackson and Kearsley, 1998); and (3) the simulation approach through modelling (Prentice, 1985, Prentice, 1988, Sugita, 1993). All have made a valuable contribution towards a more precise and objective interpretation of past pollen assemblages, and it is becoming increasingly clear that the ‘pollen view’ picture of the past is greatly, if not primarily, determined by basin size and the related relevant source area (Davis and Sugita, 1997, Sugita et al., 1997). All the aforementioned approaches, however, involve expressing the presence of each taxon as a percentage of a specified pollen sum. This automatically places a restriction on making comparisons between sites and between different points in time (Prentice and Webb, 1986). Much of the empirical data and most of the simulations also focus specifically on lakes as the sedimentation basin.

The results presented here contribute both to the problem of pollen–vegetation relationships and to the use of pollen as a proxy for climate but from a different angle, namely by providing pollen deposition (influx) values (grains cm−2 year−1) from modern reference situations. They also concentrate on deposition in terrestrial environments. The advantage of calculating such ‘influx’ or, more correctly, pollen accumulation rates (see Bennett, 1994, also Hicks and Hyvärinen, 1999, for a discussion of the appropriate term) from lake sediments has been clearly demonstrated (Davis, 1969a, Davis, 1969b, Davis et al., 1973, Davis et al., 1980) and it is becoming apparent that, in terrestrial situations too, working with pollen influx values overcomes several of the percentage problems that pollen analysts encounter (see comments in Jackson and Smith, 1994). Results monitored under controlled conditions have a high temporal resolution (annual with exact dating) which allows quantification and calibration with both vegetation and meteorological records and also throws light on pollen dispersal and sedimentation mechanisms. The advantage over modern samples obtained from mosses and lake surface sediments is the freedom from having to work with percentages. On the other hand, however, the accumulating of a sufficiently large data set for multivariate analyses, for example, is much more arduous and time-consuming.

The aim of this paper is to demonstrate the contribution that annual pollen deposition values can make to quantifying the spatial limits of forests and trees and also to explore the extent to which these annual values reflect variations in climate, basin size and catchment area. The data come from a series of pollen traps which are located to form a network of points in northern Fennoscandia, and have been monitoring annual pollen deposition for some 18 years.

The ultimate object will be to use the monitored pollen deposition data to interpret fossil pollen assemblages. In order to do this it is necessary to obtain comparable data from sediments. This involves the calculation of pollen accumulation rates (PARs) which are susceptible to many imprecisions (Bennett, 1994). Moreover, the factors leading to these imprecisions differ considerably between peat and lake sediments. Whereas pollen deposition and incorporation into peats is very similar to pollen deposition to traps, in lakes the mechanisms of pollen deposition and sediment accumulation are much more complicated, as the vast body of literature on the subject testifies (Davis, 1968, Davis, 1973, Craig, 1972, Davis and Brubaker, 1973, Davis et al., 1973, Peck, 1973, Bonny, 1976, Bonny, 1978, Pennington, 1979, Davis and Ford, 1982; also the reviews in Jackson, 1994, Hicks and Hyvärinen, 1999).

In the following account emphasis is placed on the modern monitored data. Its application to interpreting fossil pollen assemblages, specifically from peats, will be addressed in a later publication.

Section snippets

Area of investigation

The pollen monitoring area (Fig. 1) comprises the major latitudinal vegetation zones of northern Finland, from the mixed spruce, pine and birch forests of the Arctic circle, northwards through the pine dominated forest and the mountain birch woodland zone to the areas of treeless Arctic/alpine vegetation on the Scandes mountains (Hämet-Ahti, 1989). The area covers the northern boreal and oroarctic zones (Ahti et al., 1968, Haapasaari, 1988) and the nature of the timberline forests has been

Materials and methods

A network of 30 pollen traps of a type modified from Tauber's original design (Tauber, 1974, Hicks and Hyvärinen, 1986) and conforming to the Pollen Monitoring Programme's standardization (http://www.ngdc.noaa.gov/paleo/pmp/pmp.html) are monitoring contemporary pollen deposition in northern Finland and the adjacent area of northeastern Norway. Although some monitoring sites date back to 1974 the majority have records from 1982 and some only from 1996. The results of ten traps, which together

Results and discussion

As has been recognized before (Andersen, 1974, Andersen, 1980, Hicks, 1994), the annual variation in pollen deposition at any one monitoring site is often as great or greater than the variation between sites (Fig. 2, Fig. 3). Major pollen years can be recognized in which pollen deposition of all the tree taxa (and frequently also the dwarf shrub taxa), at all sites, tends to be high, e.g. 1989 and 1998. In the same way there are low pollen years, such as 1982, 1988, 1996, in which very little

Pollen influx and climate

At an annual temporal resolution the overriding signal in the pollen influx is one of climate. On the small amount of calibration which has been done so far, it appears that, in northern boreal forests, where there is rarely a problem of moisture deficiency, this signal may be a temperature signal of the year before pollen emission: specifically the temperature of that period which is most relevant for the formation of pollen within the plant. If this is the case then there is considerable

Future applications

Monitoring pollen deposition under standardized conditions in the way described here contributes to an understanding of the factors which determine the amount of pollen reaching the surface of a terrestrial sediment. In addition to the original objective, that is a quantitative interpretation of fossil assemblages, the results also have a potential use in model development, and calibration with climate as part of a multiproxy data set. Since very little comparable data have been published it is

Acknowledgements

Such a monitoring programme could not have kept going over nearly 20 years without the help of a large number of people. During the period 1981–1999 Erkki Mäenpää, Seija Hytinkoski, Kasimir Tobolski, Heather Tinsley, and Kaisu Merenheimo have in turn accompanied me on the annual trip around Lapland to empty the pollen traps. Each in his/her own way has contributed with suggestions, observations and possible explanations, all of which I have consciously or unconsciously absorbed and incorporated

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