- Email: [email protected]
Palaeoecology and plant population dynamics
‘iBEE vol. 3, no. 12, December
1988
-.
Yhe pollen record of the past IO-20 thousand years is a source of data I.qoth on long-term climatic change ;nd on the dynamics of plant popuLotions in response to climatic change. Time sequences of pollen accumulation rates record invasions tf tree taxa over IO’-IO3 years. F’alaeoecologists have fitted such cata with simple population dynamic models that assume a constant climate. Population doubling times estimated from the pollen record are consistent with species’ life-history characteristics and with estimates based on the population structure cf modern forests. This palaeoecological approach complements studies of palaeoclimatological longer-term (10%705-year) population sL7ifts, in which population response i: assumed instantaneous. Both aoproaches depend on population responses being fast compared to t!le climatic changes that cause them. Pollen data also record the rlore complex interactions between c imate and vegetation that occur during periods of rapid climatic clange, and could be used to test nlore realistic models of vegetation dynamics in a changing environment.
McElroy’ observed that physical xientists ‘need a sense of history’. Tile mechanisms of environmental cilange can be studied with the help 0’ data describing changes that have actually occurred in the past. Ecolsimilarly, need to underogists, stand how environmental change and influenced organisms hils crlmmunities*. Quaternary palaeoecology aims to show how ecologic: I systems have responded to a changing environment. Like other sciences that deal with long-term or large-scale processes (e.g. meteorology, oceanography, cosmology), ~;llaeoecology is non-experimental; dataproceeds by careful gilthering, analysis of the data to stow patterns, and construction of cc nceptual or mathematical models to explain these patterns. Pollen analysis is the main techni que that palaeocologists use to reccnstruct changes in the composilion of plant communities. Pollen grains are abundantly produced and w dely dispersed. Pollen counts from la te muds, pe?+ - d mor humus give a lluantitative record of past changes
Co in Prentice is at the Institute of Ecological Bo any, Uppsala University, Box 559, S-751 22 Upxala, Sweden.
Palaeoecology andPlantPopulation Dynamics I. Cohn Prentice in the abundances of those taxa that are recognizable and well represented in the pollen record (e.g. most genera of temperate trees). The temporal resolution of the record varies, but it can be as fine as one sample every ten years. The highest resolution can be obtained from annually laminated lake sedimentss. The combination of long duration with high resolution implies that the pollen record contains information on different types of process that occur on distinct time scales4. PO&, many von Following palaeoecologists have focused on long-term climatic changes that can be inferred from the pollen record. Broad-scale spatial patterns in vegetation can be considered to be in dynamic equilibrium with climate, and their gross changes during the Late Quaternary (the past IO-20 thousand years) as a mirror of climatic change@. However, rapid changes observed with finer resolution during shorter intervals (e.g. the time course of increase in abundance of a taxon after it first appears in a region) may be rate limited by population processes and interpretable in terms of population dynamics7. This idea has been taken literally in several recent studies, in which palynologists have estimated intrinsic population characteristics of taxa by fitting simple population growth and competition model+16. These attempts rest on two assumptions: (I) that the pollen data provide an record of adequate quantitative population size; (2) that the changes in population size are biotically controlled, rather than directly tracking environmental changes. Pollen data and population size Sediment cores for pollen analysis are conventionally taken from lakes or bogs some tens or hundreds of metres from the forest edge. Simple dispersal physics shows that the potential atmospheric pollen source area at such locations is much larger than most ecologists would recognize as a stand of vegetation or population of plants (see Box 1)17. Nevertheless, a significant fraction of the atmospheric pollen input to lake or bog sites is, local in origin, and the local component may be augmented by waterborne input. The relative contribution of waterborne pollen varies according to lake and catch-
ment characteristicsl8; there is no basis for assumingssg that pollen from the catchment dominates. The average pollen output of a vegetated area reflects the composition of the vegetation, but with certain biases. The factor relating pollen production per unit time to abundance varies among taxa, as does the attenuation of pollen input with basin size (see Box 1). Pollen accumulation rates (PAR) in lake sediments can be estimated as the product of volumetric concentration and sediment accumulation rate, but PAR depends partly on sediment focusing and may vary over long periods as a result of variations in the sedimentation patternl8. Despite these complications, PAR changes in any one taxon, over short periods with homogeneous sedimentation, can be expected to approximate changes in population size. A proviso is that the ‘population’ thus measured is a weighted average of local and regional population sizes. Causesof changein plant abundance Persistent, long-term changes in plant populations are driven by climatic changes, but short-term variations may have other causes. Field studies of forest dynamics have shown the ubiquity of natural, patchy disturbance, which continually re-starts succession on individual patches’s, Natural disturbance and autogenic succession are often the main processes causing changes in tree populations observable over decades to centuries. Climatic change brings about longer-term compositional change, by affecting disturbance regimes and successional trajectories20. Conventional pollen diagrams do not record most successions because their source area includes many patches in different successional stages. (Regions where fire is the predominant type of disturbance are an exception, because fires affect large areas at a time; hence fineresolution pollen diagrams from boreal regions can show succession after individual fires3.1 Otherwise, the main opportunity to observe population dynamics in the pollen record arises during invasions, when a taxon’s threshold for growth is crossed so that for a while the is favourable for rapid climate population expansion in some or all 343
TREf vol. 3. no
12. December
1988
Box 1. Pokn and plant abitndame The amount of pollen deposited at the centre of a circular basin of radius r can be described by the general expression cc
W(z) -P
x(z) 7
dz
where P is the taxon’s pollen productivity, x(z) its abundance at distance z, and $&?I a function describing the proportion of its pollen remaining airborne at distance z from a source. This expression can also be written
P.x’.#r) where x’ is the population ‘seen’ by the pollen sample, i.e. a distance-weighted averaoe abundance: WI is the attenuation due to b&in size and’ &pen& on how well the pollen is dispersed. Potlen source area is therefore a function of pollen type and basin si2ez6. The atmospheric transport of pollen from continuous vegetation can be approximated by Sutton’s equationz7 for the dispersal and deposition of particles released at ground level (not the equation for tall sources such as chimneys, because the pollen does not have to reach the ground before it is deposited; the surrounding canopy acts as a deposition surface). Thus cb(z) = exp (-brv) where b =i 75v&, ve being the deposition velocity of the pollen grain (which increases with the square of diameter), u the effective wind speed, and y = 118.Taking r = 300 m,
and values of b = 2.2 for the large pollen of Picea abies (spruce) and 0.63 for the smaller, more buoyant pollen of Pinus sylvestris (pine), the distance needed to account for 20% of the pollen source area of spruce is only 440 m, but for 80% it is 3.4 km; the corresponding figures for pine are 1.1 km and 190 km. The attenuation factors
44300 ml are 0.011 for spruce and 0.277 for pine. Thus pine has a much wider source area and higher representation in moderatesized basins than spruce, but even pine can have a substantial atmospheric input of pollen from trees growing within 1 km of the basin.
of the habitats that make up the landscape. Invasions may be caused by climatic change, yet be rate limited by non-climatic factors. A taxon may be missing from the lower part of a 10 OOO-year pollen record but present above simply because the climate was unsuitable in the earlier part of the record. In the long term its abundance may be controlled by climate, acting both directly (on the taxon’s potential growth rate) and indirectly, via competition. Yet the time course of the invasion may be limited by how fast the taxon can reach a new equilibrium abundance. If so, then the rates of population increase estimated from pollen data
should be consistent with the modern population biology of the taxon; for example, slow-growing, shadetolerant trees might be expected to have slow population growth rates, and the estimated rates should agree with rates expected for population expansion during natural succession taking place in the absence of major environmental change. Fitting population models to pollen data Bennett*’ summarized evidence that supports these predictions: population doubling times of various tree taxa, estimated from pollen data during expansion phases lasting about 100-1000 years, were shown to be in the same order of magnitude as those estimated from transition matrix analysis of tree taxa (not the same taxa) in modern forests. At Lake Barrine, Queensland, doubling times of about 100 years for fastgrowing Trema contrast with over for slower-growing 250 years Agafhis and Podocarpusls. Doubling times of about 30-70 years for inof fast-growing vasions shadeintolerant trees (Corylus avellana, Betula spp., Pinus sylvesfris) at Hockham Mere, Englands, seem reasonable in terms of modern rates of succession and in comparison with about 100 years for the slowshade-tolerant T///a corgrowing, data. But estimates obtained at a nearby site9 are somewhat different from those at Hockham. More determinations are needed to establish consistent patterns of differences among taxa in any one region. These doubling times were estimated by fitting exponential or logistic curves to PAR data. The use of such population growth models should be regarded as descriptive rather than explanatory. The data often show systematic deviations from the fitted curves15. Also, a sigmoid population increase is not necessarily the result of the growth process implied in the derivation of the logistic equation. An alternative explanation would be a steady increase in the favourability of the climate, approaching the flat top of a bell-shaped ecological response curve. For example, the Holocene expansion of Crypfomeria japonica in Japan took several thousand yearslo and is therefore likely to have been under direct climatic control. The interest in the doubling times summarized by Bennett*’ lies in the fact that they are broadly consistent with expected doubling times based on the population biology of trees, thus indirectly supporting the hypothesis that these invasions were
rate limited by ecologlcal processes rather than climatic change. Similar curve-fitting procedures can be applied to more than one species at a time. Delcourt and Delcourt16 fitted the Lotka-Volterra two-species competition model to pollen data describing the replacement of Fraxinus nigra by Osfryai Carpinus in Missouri between 13 000 and 11 000 years BP. They obtained values for each species’ intrinsic rate of increase and carrying capacity and for the interspecific and intraspecific competition coefficients. However, this gradual shift of dominance might not reflect a competitive exclusion process in a constant environment (as the form of the model implies), but rather a gradual. climatically controlled shift in the amount of suitable habitat within the landscape; which considerably alters the interpretation of the ‘competition’ coefficients. The competitior! model inevitably fitted the data well because many parameters were estimated from few observations. Vegetation responsesto rapid climatic change The pollen record is evidently capable of adequate temporal resolution to give information on the transient response of plant communities to climatic change. The techniques described above are, however, limited in that they require climatic change to be slow relative to population dynamics, and they deal only with one or at most two taxa at a time. Similar limitations apply to gradient-analysis techniques that allow long-term population shifts (e.g. the major changes during the past 18 thousand years, mapped at intervals of three thousand years) to be interpreted in terms of direct climatic forcing?’ These techniques also rely on the relative rapidity of population re sponses, so that the long-terni changes are close enough to a ‘mov ing equilibrium’ with climate. Being equilibrium techniques, they too treat taxa independently and cannot take into account their dynamic interactions. More generally, pollen data retort: the interactions of complex climatic; changes and vegetation dynamics. Especially during times of relativelv rapid climatic change, e.g” around the time of most rapid deglaciation (about 13-9 thousand years BP) iri Europe and North America, the pollen data should yield information for an understanding of the transier?t response of vegetation to climatii change. Vegetation’s transient rt. sponse is important because it WI’! partly determine how ecosystems
;REE vol. 3, no. 12, December
1988
react to future climatic changes caused by human activities. The problem is how to model vegetation change in a way that takes into account climatic change and allows plant demographic characteristics to vary as a function of climate. Then, palaeoecological data can be used to tt?st the hypotheses underlying the nlodels. The key to this problem lies itI coupling what is known about the nature and mechanisms of climatic ciange to more realistic (i.e. less aDstract) models of vegetation proc ?sses. The strongest climatic signals durit-g the past 20 thousand years have b?en identified. They are associated ~.ith (a) the retreat of the continental ice sheets and contemporaneous changes in sea-surface temperatL res, and (b) the more direct effects 0. changes in insolation caused by o .bital variations. Together, these effects caused changes in atmosp:ieric circulation that are sufficient tc predict continental-scale patterns 0: change in climate and vegetation dllring the past 18 thousand yearsz2. Ritchie*3 used PAR data to show how plant communities in NW Canada responded to rapidly warming sL!mmers around 10000 years BP (v/hen the North American ice sheet was in rapid retreat and high-latitude sL;mmer insolation was near its and the subsequent maximum), gradual decrease in insolation and temperature. Ritchie suggested that the sequence of major population increases Populus-Juniperus-Picea bcatween about 11 000 and 9000 years BP was a biotically limited resl:onse to the warming, and that
equilibrium was not re-established until after Picea had established landscape dominance. This is the kind of hypothesis that can be tested indirectly by simulation. Nothing would be gained by fitting a population model that assumed a constant environment, but the shortterm changes in PAR could be compared with the output of a forest succession model forced by realistic climatic changes4,*4J5 derived from known boundary conditions via a physically based climatic mode122. Only such detailed, regionally specific modelling approaches offer the possibility of disentangling cause and effect on a time scale where climatic forcing and population response interact.
References 1 McElroy, M.B. (1986) in Sustainable Development of the Biosphere (Clark, W.C. and Munn, R.E., eds), pp. 199-211, Cambridge University Press 2 Davis, M.B. (1986) in Community Eco/ogy(Diamond, J. and Case, T.J., eds), pp. 269-284, Harper&Row 3 Saarnisto, M. (1986) in Handbook of Holocene Palaeoecology and Palaeohydrology(Berglund, B.E., ed.). pp. 161-180, Wiley 4 Prentice, I.C. (1986) \/egetario 67, 131-141 5 von Post, L. (1916) Geol. Foren. Forh. 38, 384-390 [English translation: (1967) Pollen Spores 9,375-4011 6 Webb, T., III (1987) Vegetatio 67,75-91 7 Watts, W.A. 11973) in Quaternary Plant Ecology(Birks, H.J.B. and West, R.G., eds), pp. 195-206, Blackwell Scientific Publications 8 Bennett, K.D. (1983) Nature 303, 164-167
9 Bennett, K.D. 603-620 10 Tsukada, M. 371-383 11 Tsukada, M. 159-187 12 Tsukada, M. 1091-1105 13 Tsukada, M. 113-118 14 Tsukada, M.
(1986) New Phytol. 103, (1981) Jpn. J. Ecol. 31, (1982) &n.
J. Ecol. 32,
(1982) Ecology63, (1982) Jpn. J. Ecol. 32,
and Sugita, S. (1982) Bat. Mag. Tokyo 95,401-418
15 Walker, D. and Chen, Y. (1987) Quat. Sci. Rev. 6,77-92 16 Delcourt, P.A. and Delcourt, H.R. (1987) Long-term Forest Dynamics of f,he TemperateZone, Springer-Verlag 17 Prentice, I.C. (1985) Quat. Res. 23, 76-86 18 Davis, M.B., Moeller, R.E. and Ford, J. (I 984) in Lake Sediments and Environmental History (Haworth, E.Y. and Lund, J.W.G.. eds), pp. 261-273, University of Leicester Press 19 Runkle, J.R. (1985) in The Ecology of Natural Disturbance and Patch Dynamics (Pickett, S.T.A. and White, P.S., eds), pp. 17-34, Academic Press 20 Prentice, I.C. (1986) in Forest Dynamics Research in Western and Central Europe (Fanta, J., ed.), pp. 32-41, PUDOC 21 Bennett, K.D. (1986) Philos. Trans. R. Sot. London Ser. 8 314,523-531 22 Webb, T., Ill, Bartlein, P.J. and Kutzbach, J.E. (1987) in North America and Adjacent Oceans During the Last Glaciation (Ruddiman, W.F. and Wright, H.E., Jr, eds), pp. 447-462, Geological Society of America 23 Ritchie, J.C. (1985) Ecolo!gyGE~, 612-621 24 Davis, M.B. and Botkln, D 6. 11985) Quat. Res. 23,327-340 25 Solomon, A.M. and Webb, T., III (1985) Annu. Rev. Ecol. Syst. 16,63-84 26 Bradshaw, R.H.W. and Webb, T.. Ill (1985) Ecology66,721-737 27 Sutton, O.G. (1947) Q J. R. Meteorol. Sot. 73.257-276
Letters to the Editor TREE
welcomes correspondence. Letters to the Editor may address issues raised in the pages of TREE, or other matters of general interest to ecologists and evolutionary biologists. Letters should be no more than 500 words long (preferably less!), and any references should be cited in the style of TREE articles. All correspondence should be sent to The Editor, Trends in Ecology and Evolution, Elsevier Publications Cambridge, 68 Hills Road, Cambridge CB2 ILA, UK. The decision to publish rests with the Editor, and the author(s) of any TREE article criticized in a Letter will normally be invited to reply.
345
1988
-.
Yhe pollen record of the past IO-20 thousand years is a source of data I.qoth on long-term climatic change ;nd on the dynamics of plant popuLotions in response to climatic change. Time sequences of pollen accumulation rates record invasions tf tree taxa over IO’-IO3 years. F’alaeoecologists have fitted such cata with simple population dynamic models that assume a constant climate. Population doubling times estimated from the pollen record are consistent with species’ life-history characteristics and with estimates based on the population structure cf modern forests. This palaeoecological approach complements studies of palaeoclimatological longer-term (10%705-year) population sL7ifts, in which population response i: assumed instantaneous. Both aoproaches depend on population responses being fast compared to t!le climatic changes that cause them. Pollen data also record the rlore complex interactions between c imate and vegetation that occur during periods of rapid climatic clange, and could be used to test nlore realistic models of vegetation dynamics in a changing environment.
McElroy’ observed that physical xientists ‘need a sense of history’. Tile mechanisms of environmental cilange can be studied with the help 0’ data describing changes that have actually occurred in the past. Ecolsimilarly, need to underogists, stand how environmental change and influenced organisms hils crlmmunities*. Quaternary palaeoecology aims to show how ecologic: I systems have responded to a changing environment. Like other sciences that deal with long-term or large-scale processes (e.g. meteorology, oceanography, cosmology), ~;llaeoecology is non-experimental; dataproceeds by careful gilthering, analysis of the data to stow patterns, and construction of cc nceptual or mathematical models to explain these patterns. Pollen analysis is the main techni que that palaeocologists use to reccnstruct changes in the composilion of plant communities. Pollen grains are abundantly produced and w dely dispersed. Pollen counts from la te muds, pe?+ - d mor humus give a lluantitative record of past changes
Co in Prentice is at the Institute of Ecological Bo any, Uppsala University, Box 559, S-751 22 Upxala, Sweden.
Palaeoecology andPlantPopulation Dynamics I. Cohn Prentice in the abundances of those taxa that are recognizable and well represented in the pollen record (e.g. most genera of temperate trees). The temporal resolution of the record varies, but it can be as fine as one sample every ten years. The highest resolution can be obtained from annually laminated lake sedimentss. The combination of long duration with high resolution implies that the pollen record contains information on different types of process that occur on distinct time scales4. PO&, many von Following palaeoecologists have focused on long-term climatic changes that can be inferred from the pollen record. Broad-scale spatial patterns in vegetation can be considered to be in dynamic equilibrium with climate, and their gross changes during the Late Quaternary (the past IO-20 thousand years) as a mirror of climatic change@. However, rapid changes observed with finer resolution during shorter intervals (e.g. the time course of increase in abundance of a taxon after it first appears in a region) may be rate limited by population processes and interpretable in terms of population dynamics7. This idea has been taken literally in several recent studies, in which palynologists have estimated intrinsic population characteristics of taxa by fitting simple population growth and competition model+16. These attempts rest on two assumptions: (I) that the pollen data provide an record of adequate quantitative population size; (2) that the changes in population size are biotically controlled, rather than directly tracking environmental changes. Pollen data and population size Sediment cores for pollen analysis are conventionally taken from lakes or bogs some tens or hundreds of metres from the forest edge. Simple dispersal physics shows that the potential atmospheric pollen source area at such locations is much larger than most ecologists would recognize as a stand of vegetation or population of plants (see Box 1)17. Nevertheless, a significant fraction of the atmospheric pollen input to lake or bog sites is, local in origin, and the local component may be augmented by waterborne input. The relative contribution of waterborne pollen varies according to lake and catch-
ment characteristicsl8; there is no basis for assumingssg that pollen from the catchment dominates. The average pollen output of a vegetated area reflects the composition of the vegetation, but with certain biases. The factor relating pollen production per unit time to abundance varies among taxa, as does the attenuation of pollen input with basin size (see Box 1). Pollen accumulation rates (PAR) in lake sediments can be estimated as the product of volumetric concentration and sediment accumulation rate, but PAR depends partly on sediment focusing and may vary over long periods as a result of variations in the sedimentation patternl8. Despite these complications, PAR changes in any one taxon, over short periods with homogeneous sedimentation, can be expected to approximate changes in population size. A proviso is that the ‘population’ thus measured is a weighted average of local and regional population sizes. Causesof changein plant abundance Persistent, long-term changes in plant populations are driven by climatic changes, but short-term variations may have other causes. Field studies of forest dynamics have shown the ubiquity of natural, patchy disturbance, which continually re-starts succession on individual patches’s, Natural disturbance and autogenic succession are often the main processes causing changes in tree populations observable over decades to centuries. Climatic change brings about longer-term compositional change, by affecting disturbance regimes and successional trajectories20. Conventional pollen diagrams do not record most successions because their source area includes many patches in different successional stages. (Regions where fire is the predominant type of disturbance are an exception, because fires affect large areas at a time; hence fineresolution pollen diagrams from boreal regions can show succession after individual fires3.1 Otherwise, the main opportunity to observe population dynamics in the pollen record arises during invasions, when a taxon’s threshold for growth is crossed so that for a while the is favourable for rapid climate population expansion in some or all 343
TREf vol. 3. no
12. December
1988
Box 1. Pokn and plant abitndame The amount of pollen deposited at the centre of a circular basin of radius r can be described by the general expression cc
W(z) -P
x(z) 7
dz
where P is the taxon’s pollen productivity, x(z) its abundance at distance z, and $&?I a function describing the proportion of its pollen remaining airborne at distance z from a source. This expression can also be written
P.x’.#r) where x’ is the population ‘seen’ by the pollen sample, i.e. a distance-weighted averaoe abundance: WI is the attenuation due to b&in size and’ &pen& on how well the pollen is dispersed. Potlen source area is therefore a function of pollen type and basin si2ez6. The atmospheric transport of pollen from continuous vegetation can be approximated by Sutton’s equationz7 for the dispersal and deposition of particles released at ground level (not the equation for tall sources such as chimneys, because the pollen does not have to reach the ground before it is deposited; the surrounding canopy acts as a deposition surface). Thus cb(z) = exp (-brv) where b =i 75v&, ve being the deposition velocity of the pollen grain (which increases with the square of diameter), u the effective wind speed, and y = 118.Taking r = 300 m,
and values of b = 2.2 for the large pollen of Picea abies (spruce) and 0.63 for the smaller, more buoyant pollen of Pinus sylvestris (pine), the distance needed to account for 20% of the pollen source area of spruce is only 440 m, but for 80% it is 3.4 km; the corresponding figures for pine are 1.1 km and 190 km. The attenuation factors
44300 ml are 0.011 for spruce and 0.277 for pine. Thus pine has a much wider source area and higher representation in moderatesized basins than spruce, but even pine can have a substantial atmospheric input of pollen from trees growing within 1 km of the basin.
of the habitats that make up the landscape. Invasions may be caused by climatic change, yet be rate limited by non-climatic factors. A taxon may be missing from the lower part of a 10 OOO-year pollen record but present above simply because the climate was unsuitable in the earlier part of the record. In the long term its abundance may be controlled by climate, acting both directly (on the taxon’s potential growth rate) and indirectly, via competition. Yet the time course of the invasion may be limited by how fast the taxon can reach a new equilibrium abundance. If so, then the rates of population increase estimated from pollen data
should be consistent with the modern population biology of the taxon; for example, slow-growing, shadetolerant trees might be expected to have slow population growth rates, and the estimated rates should agree with rates expected for population expansion during natural succession taking place in the absence of major environmental change. Fitting population models to pollen data Bennett*’ summarized evidence that supports these predictions: population doubling times of various tree taxa, estimated from pollen data during expansion phases lasting about 100-1000 years, were shown to be in the same order of magnitude as those estimated from transition matrix analysis of tree taxa (not the same taxa) in modern forests. At Lake Barrine, Queensland, doubling times of about 100 years for fastgrowing Trema contrast with over for slower-growing 250 years Agafhis and Podocarpusls. Doubling times of about 30-70 years for inof fast-growing vasions shadeintolerant trees (Corylus avellana, Betula spp., Pinus sylvesfris) at Hockham Mere, Englands, seem reasonable in terms of modern rates of succession and in comparison with about 100 years for the slowshade-tolerant T///a corgrowing, data. But estimates obtained at a nearby site9 are somewhat different from those at Hockham. More determinations are needed to establish consistent patterns of differences among taxa in any one region. These doubling times were estimated by fitting exponential or logistic curves to PAR data. The use of such population growth models should be regarded as descriptive rather than explanatory. The data often show systematic deviations from the fitted curves15. Also, a sigmoid population increase is not necessarily the result of the growth process implied in the derivation of the logistic equation. An alternative explanation would be a steady increase in the favourability of the climate, approaching the flat top of a bell-shaped ecological response curve. For example, the Holocene expansion of Crypfomeria japonica in Japan took several thousand yearslo and is therefore likely to have been under direct climatic control. The interest in the doubling times summarized by Bennett*’ lies in the fact that they are broadly consistent with expected doubling times based on the population biology of trees, thus indirectly supporting the hypothesis that these invasions were
rate limited by ecologlcal processes rather than climatic change. Similar curve-fitting procedures can be applied to more than one species at a time. Delcourt and Delcourt16 fitted the Lotka-Volterra two-species competition model to pollen data describing the replacement of Fraxinus nigra by Osfryai Carpinus in Missouri between 13 000 and 11 000 years BP. They obtained values for each species’ intrinsic rate of increase and carrying capacity and for the interspecific and intraspecific competition coefficients. However, this gradual shift of dominance might not reflect a competitive exclusion process in a constant environment (as the form of the model implies), but rather a gradual. climatically controlled shift in the amount of suitable habitat within the landscape; which considerably alters the interpretation of the ‘competition’ coefficients. The competitior! model inevitably fitted the data well because many parameters were estimated from few observations. Vegetation responsesto rapid climatic change The pollen record is evidently capable of adequate temporal resolution to give information on the transient response of plant communities to climatic change. The techniques described above are, however, limited in that they require climatic change to be slow relative to population dynamics, and they deal only with one or at most two taxa at a time. Similar limitations apply to gradient-analysis techniques that allow long-term population shifts (e.g. the major changes during the past 18 thousand years, mapped at intervals of three thousand years) to be interpreted in terms of direct climatic forcing?’ These techniques also rely on the relative rapidity of population re sponses, so that the long-terni changes are close enough to a ‘mov ing equilibrium’ with climate. Being equilibrium techniques, they too treat taxa independently and cannot take into account their dynamic interactions. More generally, pollen data retort: the interactions of complex climatic; changes and vegetation dynamics. Especially during times of relativelv rapid climatic change, e.g” around the time of most rapid deglaciation (about 13-9 thousand years BP) iri Europe and North America, the pollen data should yield information for an understanding of the transier?t response of vegetation to climatii change. Vegetation’s transient rt. sponse is important because it WI’! partly determine how ecosystems
;REE vol. 3, no. 12, December
1988
react to future climatic changes caused by human activities. The problem is how to model vegetation change in a way that takes into account climatic change and allows plant demographic characteristics to vary as a function of climate. Then, palaeoecological data can be used to tt?st the hypotheses underlying the nlodels. The key to this problem lies itI coupling what is known about the nature and mechanisms of climatic ciange to more realistic (i.e. less aDstract) models of vegetation proc ?sses. The strongest climatic signals durit-g the past 20 thousand years have b?en identified. They are associated ~.ith (a) the retreat of the continental ice sheets and contemporaneous changes in sea-surface temperatL res, and (b) the more direct effects 0. changes in insolation caused by o .bital variations. Together, these effects caused changes in atmosp:ieric circulation that are sufficient tc predict continental-scale patterns 0: change in climate and vegetation dllring the past 18 thousand yearsz2. Ritchie*3 used PAR data to show how plant communities in NW Canada responded to rapidly warming sL!mmers around 10000 years BP (v/hen the North American ice sheet was in rapid retreat and high-latitude sL;mmer insolation was near its and the subsequent maximum), gradual decrease in insolation and temperature. Ritchie suggested that the sequence of major population increases Populus-Juniperus-Picea bcatween about 11 000 and 9000 years BP was a biotically limited resl:onse to the warming, and that
equilibrium was not re-established until after Picea had established landscape dominance. This is the kind of hypothesis that can be tested indirectly by simulation. Nothing would be gained by fitting a population model that assumed a constant environment, but the shortterm changes in PAR could be compared with the output of a forest succession model forced by realistic climatic changes4,*4J5 derived from known boundary conditions via a physically based climatic mode122. Only such detailed, regionally specific modelling approaches offer the possibility of disentangling cause and effect on a time scale where climatic forcing and population response interact.
References 1 McElroy, M.B. (1986) in Sustainable Development of the Biosphere (Clark, W.C. and Munn, R.E., eds), pp. 199-211, Cambridge University Press 2 Davis, M.B. (1986) in Community Eco/ogy(Diamond, J. and Case, T.J., eds), pp. 269-284, Harper&Row 3 Saarnisto, M. (1986) in Handbook of Holocene Palaeoecology and Palaeohydrology(Berglund, B.E., ed.). pp. 161-180, Wiley 4 Prentice, I.C. (1986) \/egetario 67, 131-141 5 von Post, L. (1916) Geol. Foren. Forh. 38, 384-390 [English translation: (1967) Pollen Spores 9,375-4011 6 Webb, T., III (1987) Vegetatio 67,75-91 7 Watts, W.A. 11973) in Quaternary Plant Ecology(Birks, H.J.B. and West, R.G., eds), pp. 195-206, Blackwell Scientific Publications 8 Bennett, K.D. (1983) Nature 303, 164-167
9 Bennett, K.D. 603-620 10 Tsukada, M. 371-383 11 Tsukada, M. 159-187 12 Tsukada, M. 1091-1105 13 Tsukada, M. 113-118 14 Tsukada, M.
(1986) New Phytol. 103, (1981) Jpn. J. Ecol. 31, (1982) &n.
J. Ecol. 32,
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