Sensitivity of cool-temperate forests and their fossil pollen record to rapid temperature change

QUATERNARY

Sensitivity

RESEARCH

23, 327-340 (1985)

of Cool-Temperate Forests and Their Fossil Pollen Record to Rapid Temperature Change B. BOTKIN?

MARGARETBRYANDAVIS*ANDDANIEL *Department and

of Ecology ?Departtnent

and BehalGoral Biology, University of Minnesota, of Biology und Environmentul Studies Program, Santa Burbaru, Califowia 93106

Minneapolis, h4innesota University of Cuiifornia,

55455,

Received October 5. 1983 Simulations of cool-temperate forest growth in response to climatic change using the JABOWA computer model show that a decrease of 600 growing degree-days (equivalent to a 2OC decrease in mean annual temperature) causes red spruce (Piceu rubens) to replace sugar maple (Acer saccharwn) as the dominant tree. These changes are delayed 100-200 yr after the climatic cooling. producing gradual forest changes in response to abrupt temperature changes, and reducing the amplitude of response to brief climatic events. Soils and disturbances affect the speed and magnitude of forest response. The delayed responses are caused by the difference in sensitivity of adult trees and younger stages. The length of the delay depends on the life history characteristics of the dominant species. Delayed responses imply that fossil pollen deposits, even if they faithfully record the abundances of trees in forests, may not be able to resolve climatic changes within lOO200 yr, or to record very brief climatic events. This explains why pollen deposits do not as yet show responses to climatic changes during the past 100 yr. Only the Little ice Age, which lasted several centuries, caused sufficient forest change to be recorded in fossil pollen, and only at certain sites. m I985 University of Washington.

INTRODUCTION

On a broad scale, geographical ranges of plant species coincide with geographical patterns of climate, and the growth and fruiting of vegetation is affected by variations in weather. Such informal observations have led to the use of vegetation as an index of climate. While the actual magnitude and duration of the response of vegetation to climatic change is not well understood, variations in the earth’s climate are often deduced from changes in past vegetation recorded by fossil pollen. The sensitivity of pollen as an index of climatic change has been emphasized (Wright, 1982; Watts, 1982; Davis, 1978; Birks, 1981). We believe its usefulness might be enhanced, however, through a better understanding of the quantitative responses of plant communities to climatic change. The use of vegetation and fossil pollen to reconstruct climate has been based on cer-

tain crucial assumptions: (1) species respond instantaneously to climatic change (within the resolution limits of the fossil record); and therefore (2) species’ distributions and abundances are always in equilibrium with climate; (3) there are no other important factors that interfere with the way individual plant species respond to climate, such as interactions among species; (4) vegetation composition is accurately and consistently reflected by pollen assemblages. In this paper we consider the first three of these assumptions by inspecting the response of forests to rapid changes in temperature. We used a computer model (JABOWA) that simulates the growth of trees in forest plots. Climatic parameters in the model affect the growth rates of tree species differentially, and thus influence the outcome of competition. The model, therefore, can be used to predict changes in forest community composition resulting 321 0033-5894/85 $3.00 CopyrIght

0 I985 by the University

of Washmgton.

328

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AND

from changes in mean annual temperature and in precipitation. We have simulated the response of cool-temperate forests to temperature changes similar to those experienced in the Northern Hemisphere during the last few centuries. We have also inspected responses of forests in simulations that differ because of microclimate, soils, and disturbance regimes. Finally, we compare the theoretical results with the known responses of vegetation to recent changes in climate. The simulations are instrumental in determining the magnitude and duration of changes in temperature that we can expect to find recorded by fossil pollen, and thus in evaluating the sensitivity of pollen analysis as a proxy record of climate in the past. METHODS

The original version of JABOWA (Botkin et ul., 1973) simulated cool-temperate forests of northern New England; we have continued to simulate forests of this region. JABOWA and models that are derived from it or are similar in design, such as FORET, have been applied by other investigators to several major types of forest vegetation in different parts of the world (Aber et ul., 1978; West et ul., 1981). The forest model simulates the annual growth of individual trees on small plots (100 m2). For each year, the computer determines the number of seedlings of each species added to the population, the growth in diameter and height of each individual, and whether or not each individual survives. Reproduction and mortality are stochastic processes in the model. In our experiments, 20 replicate plots were aggregated to obtain the mean and variance for all measures. The growth of individual trees is defined by the following species-specific key characteristics (Table I): maximum age, maximum diameter, and maximum height; relationship between height and diameter; relationship between local leaf weight and diameter; relationship between rate of pho-

BOTKIN

tosynthesis and available light to an individual tree; relationship between relative growth and growing degree-days for the year; relationship between growth rate and soil moisture conditions: relationship between growth rate and soil nitrogen. The probability of mortality of an individual tree is a function of the maximum known age for individuals of that species and a minimum annual growth rate, below which the tree is assumed to lack sufficient vigor to resist additional sources of mortality. Reproduction, the maximum number of saplings that can enter a forest plot in any year, is influenced for each species by tolerance of shade, temperature, soil moisture, and soil nitrogen. The abiotic environment is defined by soil depth, soil moisture holding capacity (constrained by soil texture, percentage rock, depth of water table, and depth to bedrock), soil nitrogen, and monthly temperature and precipitation records. Temperature is represented by growing degreedays (gdd), i.e., the integral of degrees above 4P F (4.44“ C) times the number of days, integrated over the year. We have adapted temperature and precipitation from records at Woodstock, New Hampshire, a weather station near Experimental Hubbard Brook Forest (Bormann and Likens, 1979) with which our simulations can be compared, modifying rainfall to simulate mesic and dry conditions. Soil moisture is calculated monthly from precipitation and evapotranspiration and is summed for the growing season. Evapotranspiration is calculated from a modified Thornthwaite water balance calculation (Sellers, 1965: Thornthwaite and Mather, 1957). We have modified JABOWA to incorporate a more precise calculation of hydrological factors (Botkin and Levitan, 1977), to include the dependence of tree growth on soil fertility (Aber et al., 1978, 1979), and to allow several patterns of climatic change. The number of species was expanded to include 40 taxa which occur abundantly in northeastern United States.

POLLEN

RECORD

In some experiments “disturbances” were simulated. In these cases, there was a fixed yearly probability (0.01) that all trees on a plot would be eliminated at the same time. This stochastic process operated independently on each plot to simulate local disturbances. Since the experiments involved 20 replicate plots, each of which might be cleared with a probability of 0.01, on the average 1 plot out of the 20 would have been cleared every 5 yr. This “disturbance” factor operated independently of the normal stochastic mortality processes in the model. Three assumptions crucial for use of the model to simulate response to climate concern the form of the function relating growing degree-days to tree growth. This relationship is taken to be a parabola, and the points where the parabola crosses zero are derived from the present geographic range of a species and the average monthly temperatures at these limits. Thus it is assumed that (1) the current vegetation distribution is in a steady state with the 30-yr climatic average or a close approximation to it; (2) the climate is optimal at the midpoint between these limits and least favorable at the margins; and (3) the relationship between a species and growing degree-days has not changed during the Holocene. These assumptions could easily be changed if other data were available to provide a basis for the tree growth-growing degreeday relationship. Another important assumption is that the availability of seeds is a global rather than local characteristic. Our simulations permitted all 40 species to be available in the pool of seedlings to the extent that climate permitted growth. The measure of tree abundance reported here is basal area, the cross-sectional area of tree stems. Most palynologists comparing tree abundances to pollen use basal area as the measure of vegetation (e.g., Webb ef al., 1981). RESULTS

Two climatic

regimes, differing

by 2T

OF

329

CLIMATE

mean annual temperature

(corresponding

to

600 gdd), were used for most of our exper-

iments. A 2°C difference in temperature approximates the largest change of mean annual temperatures observed at temperate latitudes in the North Atlantic region during the past 500 yr. To approximate the length of the coldest part of the Little Ice Age (17th and 18th centuries) (Mitchell, 1977; Gribben and Lamb, 1978) a 200-yr period for change was chosen. Growing degreeday values were chosen to represent temperature differences across the deciduousconiferous forest ecotone in northern New Hampshire (Bormann et al., 1970). Simulations run at 2854 gdd result in plots dominated by sugar maple (Acer saccharuwz), while 2255 gdd result in dominance by red spruce (Picea r&errs). Both the period and the amplitude of the cooling episode were varied to establish the limits of forest community sensitivity to small and short-term climatic changes. Responses were simulated on fertile soils with abundant water, on poor soils, and on both sites with periodic random disturbances such as fires and windstorms (see Methods). In one experiment, climatic inputs simulated conditions near a hypothetical treeline. Forest responses on good soils to 200-yr cooling. In this experiment, a forest on a

site with adequate soil moisture and nutrients was subject to a 200-yr cooling of 600 gdd (Fig. 1). In Figures 1A and B, the initial climatic regime of 2854 gdd decreased in Year 800 to 2255 gdd and increased again in Year 1000 to the original value. Figure IA shows the effects in a forest without disturbances; Figure 1B shows the effects in a forest with occasional, random local disturbances; Figures IC and D are controls. Sugar maple dominated the warmer control forest (2854 gdd) (Fig. 1C). Red spruce dominated the cooler control forest (2255 gdd) (Fig. ID). The forest, subjected to a 200-yr-long cooling period (Fig. 1A), underwent a change in the dominant species: sugar maple declined and red spruce increased. The increase in spruce was de-

330

DAVIS

AND

BOTKIN TABLE

Sugar maple Beech Yellow birch White ash Mountain maple Striped maple Pin cherry Choke cherry Balsam fir Red spruce White birch Mountain ash Red maple Scarlet oak Hornbeam Green alder Spreckled alder Chestnut Black ash Butternut White spruce Black spruce Jack pine Red pine White pine Trembling aspen White oak Red oak White cedar Hemlock Silver maple Tamarack Pitch pine Gray birch American elm Basswood Bigtooth aspen Balsam poplar Black cherry Red cedar

118.7 87.? 143.6 147.5 72.6 109.8 227.2 233.3 102.7 50.7 190.1 155.6 213.8 128.7 144.4 143.3 196.9 195.2 96.2 192.2 91.8 32.0 142.0 156.4 141.2 173.7 72.0 107.7 3s.7 86.0 164.8 86.3 86.5 119.5 180.0 169.8 176.7 232.5 166.7 88.7

1.57 2.20 0.486 1.75 I.13 1.75 2.45 2.45 2.5 2.5 0.486 1.75 I.57 I.75 0.486 2.0 2.0 1.75 1.75 I.75 2.5 2,s 2.0 2.0 2.0 0.486 I.75 I.75 2.5 2.0 1.57 2.0 2.0 0.486 1.6 1.6 0.486 0.486 2.45 2.0

400 366 300 150 25 30 30 20

200 400 140 30 150

200 I50 30 30 200 70 90 200 250 185 275 450

100 600 400 400 600 125 200 200 50 300 140 70 150 258 250

170 160 100 150 13.5 22.5 28 10 34”/86 24”/60 30”/76 IO 150 1r/30 I?‘/30 5 8 4’/122 2’160 3’191 21”/53 18”/45.7 2w/50 3V/9l 4V/lOl 100 4’1122 IO0 100 150 4’1122 85 3’/9l 15”/38 5’1152 4.5’/137 24”/60 100 3,191 2’160

I.

PARAMETERS

1 IO’/3350 120’/3660 100’/3050 SO’/2440 500 1000 1126 500 75’12290 75’12290 1001/3050 500 l20’/3660 loo’/3050 50’/1520 300 400 90’/2740 70’12 130 loo’/3050 1IO’/3350 90’12740 100’/3050 100’/3050 150’/4570 100’/3050 lOO’/3050 lOO’/3050 80’/2440 120’/3660 120’/3960 100’/3050 lOO’/3050 30’/9lO l26’/3840 140’/4270 70’/2l30 SO’/2440 100’/3050 SO’/1520

USED

37.8 44.0 58.3 30.7 53.8 76.7 70.6 72.6 50.1 71.8 76.6 72.6 47.0 194.2 92.2 65.2 65.8 42.7 66.4 64.0 121.2 113.9 116.5 64.0 87.8 S8.3 47.8 58.3 46.0 47.0 67.7 6X.5 64.0 40.7 48.7 60.3 66.4 46.0 64.0 46.1

FOR 40 TREE

0.1 l I 0. I37 0.291 0.102 2.0 I.70 I.26 3.63 0.291 0.59n 0,504 3.63 0.156 3.23 I.53 6.52 4. I 0. I75 0.554 0.352 I.14 1.24 I.16 0,352 0.435 0.291 0.198 0.291 0.230 0. I56 0.257 0.403 0.352 0.535 0.160 0.220 0.554 0.230 0,352 0.384

Note. G = growth constant; C = leaf area constant: AgemaX = maximum age; D,,,aX = maximum diameter; Hmdy = maximum oew seedlings per plot per year: Degdm,” = degree-days at northern limit: DegdmaX = degree-days at southern limit; Phthr = water table: Wl,,,ay = maximum wilt tolerated (fraction of potential evapotranspiration); I-type = shade tolerance type: N-type

layed, however, until 100 yr after the cold period began, and spruce abundance peaked 150 yr later, 50 yr after the climatic change had been reversed. The abundances of red spruce and sugar maple changed gradually, responding with a lag to the abrupt changes in temperature. Similar but much smaller changes occurred in the less abundant beech (Fugues grundifolia) and balsam fir (Abies balsamea), while other minor species showed no significant change. Soil effects. Poor soils -xeric, coarse textured, and nutrient poor, with available

rainfall reduced to 75% of that in Experiment I (Fig. 2)-obscured the effects of the climatic change (Experiment 2). The poor soil forest was dominated by paper birch (Be&la papyrifera). White pine (Pinus wobus) was of secondary importance; red pine (P. resinosa) and trembling aspen (Populus tremuloides) occurred consistently in low abundance (Fig. 2C). The response of the forest to the sudden temperature change (Fig. 2A) was much less marked than on the mesic, nutrient-rich soil of Experiment 1 (Fig. IA). Paper birch remained dominant throughout, and its changes were

POLLEN

SPECIES IN THE SIMULATION

3 3 is IO 2 2 60 60 2 2 IO 2 3 3 3 IO IO 3 3 3 2 2 2 3 4 IO ItI IO 2 3 2 IO 2 10 3 3 3 3 10 3

2100 2000 2414 1a00 II00 1700 700 I300 700 1800 2006 3900 2750 540 2174 1700 3200 600 600 I150 2100 600 2966 2400 I500 2416 2700 600 1900 2300 2100 1000 3899

height; bz = constant relating light threshhold for seedlings; : nitrogen tolerance type.

MODEL

6,300 6,000 s,300 10,947 6,300 6,300 6,OOil IO*000 3,700 3,800 4,000 4,ow 12,400 8.000 IO.300 3,000 5,299 8,499 5.3OQ 6.500 3,750 3,800 4,000 4.100 6,000 5,600 10.200 9.600 9,706 6,559 9,OQo 33800 5,800 4,800 12.000 6,000 6,000 4,300 10,945 IO.204

RECORD

OF

331

CLIMATE

JABOWA

0 0 0.9 0.9 0 0 0 0 0 0 0.9 0.9 0.9 0 0 0.9 0.9 0.9 0.9 0 0 0.9 0 0 0.9 0 0.9 0.9 0 0 0.9 0.9 0 0 0.9 0 0 0 0.9 0

20,000 2o.OQo 400 400 20,000 20,000 50 SO 20,000 20,000 400 400 400 50 SO 400 400 400 400 50 400 400 50 SO 400 50 400 400 400 20.000 400 SO 50 SO 400 20.000 50 SO 400 50

0.56667 0.48889 0.6000 0.4 0.48889 0.56667 0.56667 0.56667 0.21111 0.48889 0.54444 0.54444 0.3222 0.93333 0.93333 0.3222 0.21111 0.93333 0.3222 0.93333 0.5444 0.1555 I.25 I.25 1.0 0.7 0.93333 0.93333 0.1 0.4889 0.4 0.155556 1.25 I.00 0.4 0.56667 0.4 0.4 OS6667 0.7

0.35 0.35 0.245 0.245 0.274 0.274 0.378 0.378 0.245 0.245 0.378 0.290 0.450 0.450 0.450 0.130 0.050 0.450 0.130 0.450 0.245 0.130 0.530 0.500 0.450 0.450 0.450 0.450 0.050 0.24s 0.187 0.050 0.530 0.450 0.245 0.290 0.187 0.378 0.378 0.450

3 3 2 2 3 3 I l 3 3 2 2 2 I 1 2 2 2 2 l 2 2 1 1 2 I 2 2 2 3 2 l 1 I 2 3 I I 2 1

2 2 2 1 2 2 I 2 3 3 3 2 3 3 2 3 3 3 I 2 I 3 3 3 3 2 2 2 I 3 1 2 3 3 1 1 2 I 2 3

height to diameter; b3 = constant relating height to diameter; sapnu= maximumnu&erS or Wltmx = maximum leaf weight per plot permitting seedling entry: l&I” = minimum depthto

not signiticantly different from those of the control (Fig. 2C). Of the more important species, only yellow birch (II. uliegheniea.rk) showed a statistically significant decrease. Disturbance further obscured the response to temperature change (Fig. 2B), in part because the basal area of all species was reduced. White birch changed little; yellow birch and white and red pine decreased slightly. Tree-line response. Experiment 3 simulated conditions at a hypothetical tree line where temperature was the factor limiting tree growth. Growing degree-days were

maintained at 747 gdd until Year 450, then warmed abruptly to 1197 gdd. Even though the absolute change in growing degree-days was smaller than in Experiments 1 and 2 (450 vs 600), the vegetation response in the simulation was more dramatic, with trees beginning to replace treeless vegetation within 50 yr (Fig. 3). Limits of sensitivity: length of climatic events. How brief a climatic event might affect forest composition was investigated in Experiment 4. The conditions were the same as Experiment lA, but the onset of climatic change was set at Year 400 for rea-

332

DAVIS

AND

sons of computational efficiency. In Experiment 4A the duration was the same as in Experiment I-200 yr (Fig. 4A). The duration was halved to 100 (Fig. 4B), 50 (Fig. 4C), and 2.5 yr (Fig. 4D). The first three produced significant changes in the basal area of the dominant species. When the event lasted only 25 yr (Fig. 4D), however, no species showed a significant change in abundance, despite the large amplitude of the change in temperature. Interestingly, when the climate was abruptly warmed at Year 500 in Figure 4B, sugar maple began to increase, but red spruce also continued to increase until Year 550, and then it began to decline. This resulted from growth of already established trees. Again, the response was delayed; in the lOO-yr event (Fig. 4B), red spruce did not reach its maximum until 150 yr after the cooler period began and did not return to its original abundance for another 100 yr (150 yr after the climatic change was reversed). With the 50-yr cooling (Fig. 4C), maximum spruce abundance occurred 125 yr after the cool period began (75 yr after the end of the cool period) and the magnitude of the spruce response was small. Limits of sensitivity: amplitude of climatic everzts. How small a temperature change might change forest vegetation under the conditions of Experiment 1 was investigated in Experiment 5. The amplitude of the change in temperature was halved, from 600 gdd to approximately 300 gdd (Fig. 5A), then to approximately 150 gdd (Fig. 5B). (The latter is equivalent to a 0.5’ C mean annual temperature decrease.) Sugar maple and red spruce showed statistically significant responses to the first case (Fig. 5A), but no species showed a significant response to the second, even though the duration was extended to 400 yr.

BOTKIN

DISCUSSION

AND CONCLUSIONS

Summary of simulation results. These results imply a sensitivity limit for the forest we have modeled. Even when climatic change persisted over a long time interval, changes of less than 300 gdd were too small to have an observable effect on forest composition. This suggests a limit for the potential sensitivity of fossil pollen for reconstruction of climatic history. In the model forest, a 200-yr, 600-gdd drop in temperature can cause a large vegetation change on mesic, nutrient-rich soils in the absence of disturbance (Experiment I). On xeric, nutrient-poor soils (Experiment 2) the change was smaller; the poor soil was a major controlling factor, preventing a strong response of the vegetation to temperature. Disturbance decreased the amplitude of the response of vegetation to climatic change. On both nutrient-rich and nutrientpoor soils, disturbance reduced forest basal area and encouraged the growth of early successional species, obscuring the response of the old-age forest dominants (Experiments 1B and 2B). Disturbance also increased the heterogeneity of the forest, with shade-tolerant species dominating some plots, and early- and midsuccessional species dominating other, more recently cleared plots. The heterogeneity made the response to climate less obvious than when all plots had a more uniform history and were mainly dominated by a single species. This held for both good and poor soils (Experiments 1 and 2). Because it hastens the removal of canopy trees, disturbance has the general effect of speeding up response to climate. Responses to climatic change were greater in transitional habitats, for ex-

FIG. 1, Basal area average for 20 plots for dominant species of trees plotted against time in a 1400yr simulation of forest growth on good soils. (A) Climate was changed from 2854 to 2255 gdd in Year 800, then back to 2854 gdd in Year 1000. (B) As in A, but with probability of clearing of plots 0.01 per year. (Cl Control with climate maintained at 2854 gdd throughout. (D) Control with climate maintained at 2255 gdd throughout.

POLLEN

RECORD

OF

333

CLIMATE

STANDARD SITE NO DISTURBANCE

0 4000

gdd=2854

lr;TANDARD SITE DISTURBANCE

3006 c-7 E 0 W

m al 2

a

1 gdd=2255.4

2000

1000

0

STANDARD SIT NO DISTURBAN

4000

3000

In tu m

2ooc

lOO(

0

4001

3001

zooc

1ooc

, 0

zoo

400

600

600

Year

1000

,200

14uo

334

DAVIS

AND BOTKIN

A

gdd=2854

4oot IwAT

rR

3ooc I-

N;;~;?Y~;:

COOLER gddzZ255.4 4

200~

iooc

al E 0 w

ta

o gdd=2854

4000

STANDARD DRY hE POOR SITE

-L-r gdd=2255.4

3000

a

:

DISTURBANCE 2000

-

1000

co w a

m

o 4000 CONSTANT CLIMATE gdd=2654 3000

STANDARD DRY & POOR SITE No

DISTURBANCE

2000

1000

0 0

200

400

600

800

1000

1200

1400

Year FIG. 2. Basal area for dominant species of trees plotted against time in a 1200-yr simulation of forest growth on poor soils. (A) Climate changed from 2854 to 2255 gdd in Year 800, then back to 2854 gdd in Year MOO. (B) As in A, but with probability of clearing of plots 0.01 per year. (C) Control with constant climate (2854 gdd).

ample, tree line, where a warming produced a rapid change from no trees to a white spruce-birch-aspen forest (Experiment 3). In this experiment our intention was to simulate a theoretical tree line de-

termined tree-line important scure the ture. For

by temperature alone. In a real situation, other effects could be and might either enhance or obresponse to changes in temperaexample, the precise elevation of

POLLEN 04

4000 gddzIl973 WARMER

E -

”

3000

lu F 2000 I a ;

COOL& gdd-74?

,~WHlTE BIRCH “WHITE SPRUCE ~TRi34Bu~~ ASPEN

335

RECORD OF CLIMATE

~ I

ucE ABEECH APLE . TREMBLING ASPEN

A

0 STANDARD SITE NO DISTURBANCE

1000

0

zoo

400

600

800

Year FIG. 3. Basal area for dominant species of trees plotted against time in a lOOO-yr simulation of forest growth in a hypothetical tree-line situation. Climate changed from 747 to I197 gdd in Year 450.

tree line in New England may be controlled by wind velocity and frost damage (Spear, 1985). Soils are peaty and acidic or stony with little organic content. Krummholz (i.e., stunted trees) occur at tree line, with balsam fir dominant, black spruce (P. HZLWiu~) rare, and paper birch a minor component (Spear, 1985). In all cases where growing degree-days were decreased, the response of the forest lagged 100 to 150 yr behind the climatic cooling. Delayed responses result from the longevity of trees and their tolerance to a wide range of climatic conditions. Mature trees have a high average survival rate even under climatic stress. Furthermore, a closed, mature tree canopy strongly influences light conditions near the forest floor, and therefore strongly influences regeneration. For example, without disturbance (Experiment lA), a closed canopy dominated by mature sugar maple produced a dense shade in which relatively little regeneration took place. As the climate was made colder, spruce and fir could enter the forest only gradually. Their seedlings grew

4000

TTT

STANDARD SITE 1000

@ m

0

C

gdd-2054

Ll

gdd-2255

T

- 3000 a In m 2000 a 1000

N0 DISTURBANCE

0

L

I-

DA

4000

3000

2000

,000

FIG. 4. Basal area for dominant species of trees plotted against time in a 1200-yr simulation of forest growth on good soils. Climate changed from 2g54 to 2255 gdd starting in Year 400 for CA) 200-yr interval; (B) IOO-yr interval; tC) SO-yr interval; and (D) 25.yr interval.

0 0

200

400

600

Year

800

1000

1200

336

DAVIS

AND BOTKIN

FIG. 5. Basal area for dominant species of trees plotted against time in a simulation of forest growth on good soils. (A) Climate changed from 2854 to 2549 gdd for a 200-yr interval, starting in Year 400. (B) Climate changed from 2854 to 2700 gdd for a 400-yr interval.

under low iight conditions, where growth was slow even though the climate favored them relative to other species. Meanwhile, the cooler climate had a much smaller effect on the canopy trees, slowing their growth considerably, but decreasing their probability of survival only slightly. Under these simulated conditions, mature sugar maple trees continued to dominate the plots for a period much longer than one would predict from a consideration of the adaptations of individual species to climate. In such a case, clear-cutting a plot would hasten the change in composition from warm-adapted to cold-adapted species. Our results agree with Smith’s (1965)

idea of community “inertia” in response to climatic change. He suggested a similar mechanism: When climatic change occurs, adult perennial plants continue to survive, but do not reproduce successfully. Young plants of other species, better adapted to the new climatic regime, enter the community gradually, and existing plants are replaced as they die by invading species. Forest responses to climatic change are often considered in terms of adaptations of individual species to climate; delayed responses demonstrate that biotic interactions alter the way species abundances within a forest community change in response to a climatic change. Delays within a community can be asymmetrical in response to warming and cooling, because they depend upon the life-history characteristics of the individual species that are increasing or decreasing (Shugart er ul., 1981). Climatic events of large amplitude and short duration caused changes in the simulations that were very similar to those caused by climatic events of longer duration but lesser amplitude (Experiments 4 and 5; compare Figs. 4B and 5A). These experiments also showed that the smallest durations and amplitudes of temperature change that produced forest response were 50 yr for a 2’ C mean annual temperature change, and I0 C mean annual temperature change for a climatic episode 200 yr in duration. The model forests did not respond to a change of 0.5’ C, even when it lasted 400 yr. Comparison of simulation results with forest responses to climatic changes in historical times. The sensitivity of vegetation to climate can be evaluated from responses to known changes in climate during the last 500 yr. Many long-term temperature records are from distant localities. However, it is not necessary to assume that climatic changes in New England were exactly similar. Our model has been useful in showing how a particular type of forest vegetation would respond to rapid temperature

POLLEN

RECORD

changes; we are interested, however, in the general case of forest response to climatic change. Three recent climatic events of different durations are known in the Northern Hemisphere: (1) a recent cooling trend lasted two to three decades (ca. 1940-1970 A.D.), with a 0.3’-0.P C decline in mean annual temperature (Mitchell, 1977): (2) a warming trend (with minor reversals) extended from 1880 to the late 194Os, during which the mean annual temperature rose 0.V C in the Northern Hemisphere (Mitchell, 1977); (3) a cold period known as the Little Ice Age extended for several centuries, ending about 1850 A.D. Mean annual temperatures fell about 2’ C, with still larger changes lasting a decade or two (Mitchell, 1977; Gribben and Lamb, 1978). The Little Ice Age is known from historical accounts, from geological evidence of glacier fluctuations, from studies of tree-ring widths, and from oxygen-isotope changes in cores of glacier ice (Fig. 6). All three climatic events are recorded in many regions throughout the Northern Hemisphere, including northeastern United States (Wahl, 1968; Davis ef al., 1980) where our model has validity, All are known to have affected the biota, especially migratory birds and motile insects (Lamb, 1977; Hustich, 1952; Ahlmann, 1953). Climatic changes in the last three decades (event I) have strikingly affected the productivity of crop plants (Schneider and Temkin, 1978), but they do not seem to IEngland

I

1 1000

1

”

w, YEARS

FIG. 6.

changes region.

, 1500

,

1 2000

A.D.

Climatic records of mean annual temperature during the last 1000 yr in the North Atlantic (Redrawn from Gribben and Lamb, 1978.)

OF

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have affected forests. A decline in spruce in New England (1964- 1979) is attributed to factors other than climate (Siccama ef al., 1982). However, climatic warming since 1850 (event 2) has affected forest vegetation in several regions. Alpine tree line has moved upward in Lapland and broad-leaved trees are expanding in southern Finland (Erkamo, 1952). A birch dieback in Nova Scotia in the 1940s was attributed to indirect climatic effects such as damage to mycorrhizal fungi (Nash and Duda, 195 1; Hauboldt q 1952). In the White Mountains of California large numbers of bristlecone pine seedlings have become established since 1850 above the existing tree line: wider annual rings show increased growth rates of trees (LaMarche, 1982). One might infer from such information that climatic warming between 1850 and 1950 (event 2) was of sufficient amplitude and duration to affect abundances of tree species; in addition, the warming exceeds the sensitivity limits in our simulations of cool-temperate forest. The delayed response predicted by our simulations suggest, however, that changes in forest communities would not be apparent so soon after the temperature change. An exception would be vegetation capable of exceptionally rapid response, such as a forest subject to disturbance, or tree-line situations. In fact, all examples of response are either from heavily disturbed forests, such as those in southern Finland, or tree-line situations. Changes of tree line in the last 100 yr have not been documented in New England where factors other than temperature appear to limit tree growth (Spear, 1985). Forest changes correlated with events I and 2 do not appear in the pollen record in sediments. In lakes where sediment mixing has been minimal, fossil pollen deposits can record rapid changes such as the abrupt decline of chestnut (Castanea dentata) caused by blight early in the 20th century (Anderson, 1974; Brugam, 1978; Allison et al., 1984). If climatically induced changes

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had occurred in the abundances of canopy trees, we could expect to find a record. Vegetation responses to the Little Ice Age (event 3), in contrast, are recorded at several localities and for several kinds of vegetation. In Finland historical records document the cultivation of oats and barley until the 13th century. These crops are grown there today, but were not grown during the Little Ice Age (Lamb, 1977). Similarly, the establishment of seedlings of bristlecone pines ceased at modern tree line during the 18th and 19th centuries, and growth rates of trees surviving at tree line declined (LaMarche, 1973). Forest changes recorded by fossil pollen include the expansion of beech during the last millennium (Fugl4s gra&f&~) several 1OSof kilometers beyond its former western limit in Upper Michigan (Woods and Davis, 1982); the increased abundance of hemlock (Tsugu cunudensis) within the last few centuries in northwestern Lower Michigan (Bernabo, 1981) and the development of maple-basswood (Acer-Tilia) Bigwoods forest during the last 400 yr along the prairie margin in Minnesota (Grimm, 1983). In the hemlock case, the response to climate was strong on sites located on rich soils, while only minor changes are recorded in lakes in regions where soils are strongly limiting (Bernabo, 1981). All three climatic events affected vegetation. However, the shortest and smallest (event 1) apparently did not affect forest vegetation; the intermediate event (event 2) has affected only tree-line or secondgrowth forests. Only the longest event (event 3) has produced changes in the abundances of canopy trees in forests that have affected the pollen deposited in sediment, and this response occurred only on better soils. These observations are consistent with the results from our simulation experiments. Our experiments lead us to suggest that adjustments of tree abundances to recent climatic events, such as climatic warming since 1850, will continue for at least another

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century. The abundances of canopy trees in old-growth forests may have been largely determined by conditions 100-200 yr ago, while the abundances of seedlings and saplings are probably changing now in response to the temperature changes during the last 50 yr. Thus the experiments provide insight into the assumptions that have been accepted in the reconstruction of paleoclimate from vegetation history. First, species abundances are not always in equilibrium with climate. Second, our experiments show that biotic interactions can have a strong effect on climatic response and can obscure and delay the response. Third, our experiments show that other environmental factors, especially soils and disturbance, must be taken into account in reconstructing climate from vegetation. Fourth, our experiments suggest that the pollen record in sediments, even if it records the vegetation perfectly, cannot resolve climatic changes more closely than within a century or two. This record may give the impression that short climate events occurred 100 or more yr later than the actual event. The pollen record cannot distinguish step changes from gradual changes. It cannot distinguish between larger, shortlived changes and smaller changes of longer duration. Very brief or very small changes in temperature may leave no record in pollen deposits. Vegetation responds so slowly that pollen deposits, no matter how closely they are sampled, are capable of recording only long-term climatic changes, providing a kind of running mean of climatic variation, rather than precise records of climatic changes from one decade to the next. ACKNOWLEDGMENTS This work has been supported by the National Science Foundation, Research Grant DEB 80-12159. We thank Tad E. Reynales for assistance in programming and for comments and suggestions about the manuscript. Ray W. Spear and Sara L. Webb also provided helpful critical comments.

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