Disturbance and climate warming influences on New Zealand Nothofagus tree-line population dynamics

Journal of Ecology

2001

89

, 1061–1071

© 2001 British Ecological Society

Blackwell Science Ltd

Disturbance and climate warming influences on New Zealand

Nothofagus

tree-line population dynamics

LOUISE E. CULLEN, GLENN H. STEWART, RICHARD P. DUNCAN and JONATHAN G. PALMER*

Ecology and Entomology Group, Soil, Plant and Ecological Sciences Division, PO Box 84, Lincoln University, Canterbury, New Zealand

Summary

1

We used forest stand history reconstruction to infer the relative roles of disturbanceand climate warming on the population dynamics of

Nothofagus menziesii

(silver beech)dominated tree lines in north Westland, South Island, New Zealand.

2

Stem recruitment in tree line forests over the last 300 years has been episodic, hastended to occur in small, scattered patches, and has been dominated by the productionof new stems from existing trees. Pulses of stem recruitment also coincide with episodesof abrupt decline in the radial growth of established trees. These patterns suggest thatinfrequent natural disturbances form localized canopy openings, damage trees thatsurvive the event, and initiate the establishment of new trees and the production of newstems by surviving trees that fill these openings.

3

Climate warming in New Zealand since 1950 has had little effect on the recruitmentof

Nothofagus

close to the tree line. There is a large seedling pool within the tree lineforests, but recruitment will probably require a disturbance-related canopy opening.

4

Natural disturbances drive the population dynamics of

Nothofagus

tree lines and maymodify their response to climate warming. Unlike many Northern Hemisphere treelines, there has been no recent upward movement of the tree line or increase in seedlingestablishment. This difference could reflect the greater importance of natural disturb-ance for recruitment in the abrupt closed-canopy tree lines formed by light-demanding

Nothofagus

species in the Southern Hemisphere. However, given the ubiquity of distur-bance effects in low-altitude forests in both hemispheres, future studies need to considertheir role when investigating any tree line response to climate warming.

Key-words

:

beech forest, forest disturbance, climate change, growth suppressions,temporal and spatial patterns

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, 1061–1071

Introduction

Global temperatures have increased by up to 0.6

°

Cover the past century and temperatures are predicted torise a further 1.5–4.5

°

C by 2100

(Houghton

et al

.1996). Faced with increasing temperatures, ecologistsand conservation managers need to understand theeffects of global warming on species, communitiesand ecosystems. Tree lines occur at the point wherecarbon gained during the summer is equal to, or onlyjust greater than, that lost due to respiration or tissue

mortality through lack of hardening to withstand wintertemperatures (Wardle 1965). Because summer temper-atures and the length of the growing season stronglyinfluence carbon production and shoot hardening,higher temperatures should increase tree productivityand survival at the tree line (Brubaker 1986).

Tree line responses to warming over the last centuryhave been detected in many parts of the NorthernHemisphere (see Kullman 1990; Rochefort

et al

. 1994for reviews) with seedling invasion of subalpine meadowsand recent establishment above the current tree line.Other studies have found upsurges in establishmentwithin tree line forests (e.g. Szeicz & MacDonald 1995;Taylor 1995). The relatively rapid response of NorthernHemisphere tree lines to climate warming has reinforcedthe belief that temperature is the dominant control ontree line dynamics.

*Present address: School of Archaeology and Palaeoecology,The Queen’s University of Belfast, Belfast BT7 1NN, NorthernIreland, UK.Correspondence: Richard P. Duncan (fax +64 3 325 3844;e-mail [email protected]).

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This may however, overlook the potential importanceof natural disturbance events. Natural disturbancesdrive the regeneration dynamics of most closed-canopyforests by creating opportunities for the establishmentof new individuals through canopy opening (Pickett &White 1985) and have been shown to play a role in theregeneration dynamics of tree line forests in some areas(e.g. Veblen

et al

. 1981). Increases in tree establishmentand changes in tree line position could, therefore, prim-arily be a response to disturbance, rather than climatewarming (Slayter & Noble 1992). Fire, for example, cancontrol the position of North American tree lines (Vale1981; Baker 1992), and regeneration above the currenttree line is often a recovery to the tree line positionbefore the fire (Shankman 1984). At or below the presentforest tree line, disturbances, ranging from small-scaletree-fall gaps (Taylor

et al

. 1996) to catastrophic forestdestruction by earthquakes (Veblen 1985), lead to periodsof increased tree establishment. Events following anappropriately timed disturbance could mimic the expectedresponses to climate warming. Disturbances could alsomodify the response of tree line populations to climatechange. Light-demanding tree species that rely on dis-turbances for recruitment may be unable to respond toclimate warming unless it coincides with canopy open-ing (Brubaker 1986), whereas the response of tree lineforests to warming may be enhanced during periods ofdisturbance (Overpeck

et al

. 1990).There are few studies examining the response of

Southern Hemisphere tree lines to climate warming.Warming in New Zealand has occurred mostly in thelast 50 years, with an increase of 1.0

°

C in average

summer temperatures since 1950 (Salinger

et al

. 1993).Since the limit of New Zealand beech tree lines reflectsboth summer temperatures and the length of the growingseason (Wardle 1965) we would expect that changes inposition and density would be similar to those in theNorthern Hemisphere. In fact, Wardle & Coleman (1992)suggest that

Nothofagus

tree lines in New Zealand haveresponded to the recent warming with a minor tree lineadvance (

c

. 10 m), however, any interpretation of treeline dynamics in response to climate warming shouldat least consider the role of natural disturbance (e.g.Veblen 1985; Baker 1992). Our aim was to examine therelative roles of natural disturbance and climate warmingin influencing the population dynamics of

Nothofagus

tree lines in South Island, New Zealand.Warming is likely to have affected New Zealand forests

only within the last 50 years, but we aimed to recon-struct the population dynamics of

Nothofagus

tree lineforests further back in time by ageing trees and datingperiods of past tree establishment. An understandingof tree establishment patterns over the past 200–300 years provides a context within which to interpretrecent changes as inferred from the present patterns oftree regeneration. To achieve this we examined: (i) thetemporal and spatial patterns of tree establishment incurrent tree line forests; (ii) evidence for recent establish-ment above the tree line; and (iii) tree growth patterns.

Nothofagus

species commonly form the tree line in theSouthern Hemisphere, including New Zealand, where

Fig. 1 Silver-beech dominated tree line at Mt Haast, South Island, New Zealand.

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Nothofagus menziesii

(Hook. f.) Oerst (silver beech)is dominant on the wetter western mountains and

N. solandri

var.

cliffortioides

(Hook. f.) Poole (mountainbeech) is dominant in drier eastern areas (Ogden

et al

.1996).

Nothofagus

tree lines generally vary little in theirlocal altitude, and are abrupt and floristically simple(Fig. 1), providing a relatively simple system in whichto investigate climate and disturbance influences. Ourstudy was carried out in the Rahu Saddle area (42

°

19

′

S,172

°

07

′

E), north Westland, South Island, New Zealand.Here tree lines dominated by silver beech are typical of

Nothofagus

tree lines in the Southern Hemisphere ingeneral, where closed-canopy forest extends right up toan abrupt tree limit (Wardle 1965).

The climate of New Zealand, and the South Island inparticular, is dominated by a prevailing west to south-west airflow (Salinger 1988). These winds are moisture-laden due to New Zealand’s oceanic location. The mainaxial mountain range in South Island runs from thesouth-west to the north-east, creating a significant barrierto airflow and generating a steep rainfall gradient fromthe west to the east coast (Salinger 1988). The nearestlong-term temperature station, Hokitika (

c

. 120 kmsouth-west of the study site), shows slight seasonalvariation in temperature, with February the warmestmonth (15.6

°

C) and July the coldest (7.1

°

C). The totalannual precipitation at Reefton (the nearest rainfallstation, 40 km west of the study area and at 700 m altitude)is 1930 mm. Precipitation at the tree line is likely to behigher because of the greater elevation (

c

. 1200 m),although much of this will fall as snow during winter.

Methods

We selected three sites near Rahu Saddle and estab-lished seven 0.05 ha plots at these sites to investigate

tree line responses to climate warming and distur-bance. Three north-east facing plots were established atMt Haast (Ht1–3, 42

°

19

′

S, 172

°

05

′

E), three south-facing plots at Rahu Valley (Rv1–3, 42

°

17

′

30

′′

S, 172

°

08

′

E)and one south-facing plot at Rahu Spur (Rs1, 42

°

18

′

S,172

°

07

′

E, Table 1). We subjectively located each plotin areas that avoided obvious avalanche tracks or areaswith large canopy openings that indicated recent majordisturbance. Each plot was 10

×

50 m, with the longerside extending downslope at 90

°

from the tree line. Treeline altitude, aspect and slope were recorded for eachplot (Table 1).

Interpretation of tree population dynamics requiresinformation on the temporal patterns of tree establish-ment, often inferred from age and size distributions(Henry & Swan 1974; Stewart & Rose 1990). A distinctpulse of tree establishment evident in an age-classdistribution is often an indication of regeneration inresponse to canopy-opening disturbance (Oliver &Stephens 1977; Stewart 1986). In contrast, we mightexpect a sustained upsurge in tree establishment,synchronous with warmer conditions, in response torecent climate warming.

In the tree line forests that we studied, multistemmedindividuals that branched just above the forest floor werecommon. We defined a tree as any individual with oneor more stems

≥

10 cm diameter at 50 cm above the forestfloor. We recorded the species, number and diameter ofall stems

≥

10 cm diameter at 50 cm for each tree rootedwithin the plot. We defined a sapling as any individualwith all stems < 10 cm diameter at 50 cm, but with oneor more stems

≥

1 cm diameter at the forest-floor level.Sapling diameters were measured at forest-floor level,rather than at 50 cm, because sapling stems oftenbranched extensively just above the forest floor. We

Table 1 Number of silver and mountain beech seedlings (< 1 cm diameter), saplings (1–10 cm diameter) and stems (≥ 10 cmdiameter), and total basal area (b.a.) of each species in each 0.05 ha tree line plot

Mt Haast Rahu Valley Rahu Spur

Ht1 Ht2 Ht3 Rv1 Rv2 Rv3 Rs1

Altitude (m a.s.l.) 1210 1170 1180 1150 1140 1140 1255Slope (°) 20 30 4–40* 29 29 31 27Aspect NE NE NE S S S SSWAvg. canopy height (m) 2 4 5 4 4 5 3No. silver beech stems

Seedlings 30 62 32 29 19 9 36Saplings 54 111 43 27 22 30 36Stems 126 142 83 102 93 80 81

Total silver beech b.a. (m2) 3.1 3.4 2.4 3.4 3.9 3.7 3.3No. mountain beech stems

Seedlings 1 32 0Saplings 33 34Stems 25 72

Total mountain beech b.a. (m2) 2.7 0.6

*Plot started with a 4° slope, and after c. 25 m downslope changed to a 40° slope.


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defined a seedling as any individual < 1 cm diameter atforest-floor level. All seedlings in each plot were counted.We constructed size-class frequency distributions fromthe above data. Because silver beech was much moreabundant in the tree line plots than mountain beech(Table 1), we only present the results for silver beech.

To construct age-class frequency distributions andto develop tree-ring growth chronologies, we cored allstems

≥

10 cm diameter, at a height of 50 cm above theforest floor. We usually extracted one core per stem, butsome stems were re-cored if the first core missed thepith. Cores were mounted and sanded using progres-sively finer grades of sandpaper until the growth ringswere clearly visible. Only cores which either passedthrough the stem pith or close by, so that the arcs of theinnermost rings were visible (‘arc’ cores), were retained.Growth rings were counted under a binocular micro-scope and ring widths were measured using a Hensonbench, binocular microscope and the

measuringprogram (Madera Software 1988). For the ‘arc’ cores,the number of missing rings was estimated using thegeometric model of Duncan (1989) which estimates thedistance from the largest visible arc to the pith, and amean cumulative radial growth curve to estimate thegrowth rate of the missing rings, using only those coresthat passed through the pith (see Villalba & Veblen 1997).

The time to reach coring height (50 cm) was estimatedfrom linear regressions of forest-floor level age (whichranged from 10 to 80 years) against total height (whichranged from 0.25 m to 2.30 m) for silver beech saplingscollected adjacent to, but outside, each plot. Due tosmall sample sizes the collections from around eachplot were combined by site so that the sample sizeswere: Mt Haast,

n

= 31, Rahu Valley,

n

= 26 and RahuSpur,

n

= 26. Saplings in large canopy gaps or under adense canopy were avoided to obtain an estimate of thetime to reach coring height for saplings growing in smallopenings. Twenty-two, 15 and 19 years were added tothe ages of silver beech stems at Mt Haast (sapling age–height regression,

r

2

= 0.55), Rahu Valley (

r

2

= 0.68)and Rahu Spur (

r

2

= 0.63), respectively. Because of thevariation in tree growth rates to coring height, varia-tion in the height at which trees were cored (due to low-growing branches), and error in estimating the numberof rings missed in cores that missed the pith, we groupedtree and stem ages into 20-year age classes. The age ofmultistemmed trees was taken to be the age of the oldeststem of that tree.

Stems < 10 cm diameter were not cored and their agescould not be reliably estimated from age-diameter regres-sions for stems

≥

10 cm diameter because the spread ofages around a particular diameter was large (e.g. 10 cmdiameter stems ranged in age from 75 to 150 years).Instead, we inferred the temporal patterns in establishmentof stems < 10 cm diameter from size-class distributions.

We used the New Zealand national temperatureseries, compiled from seven climate stations aroundNew Zealand (Salinger 1980), to relate tree-line estab-lishment patterns to temperature trends. Because tree

lines are likely to respond only to changes in summertemperature, we used the average of the mean monthlytemperatures from December to February to producean average summer series. In using the New Zealandtemperature series we assume that the recent increase inthe national average summer temperature (1.0

°

C since1950) has also occurred at the tree line. This is likely, aswarm and cool periods have been synchronous through-out New Zealand (Salinger 1980).

Positive spatial autocorrelation at small distances intree or stem ages would suggest that trees or stems wereestablished in even-aged patches, a pattern that couldreflect establishment in localized canopy openings inresponse to disturbance (Duncan & Stewart 1991; Frelich& Lorimer 1991). Alternatively, tree ages may not bepatchy but show a directional trend, such as increasingage below the tree line, which might indicate a progressiveincrease in tree line elevation in response to climatewarming.

To examine the spatial patterns of tree establish-ment, we recorded the position (as

x

,

y

co-ordinates tothe nearest 0.1 m) of all stems

≥

10 cm diameter at50 cm above the forest floor, and of all seedlings andsaplings at forest-floor level in each plot. The ages ofsilver beech trees and stems

≥

10 cm diameter in eachplot were analysed for spatial autocorrelation follow-ing the methods in Duncan & Stewart (1991), usingMoran’s

I

coefficient (Moran 1950). Correlograms wereconstructed to examine how the degree of autocorrelationin tree and stem ages varied with increasing distance,and the type of spatial structure in tree or stem ages wasinferred from the correlogram shape (Legendre & Fortin1989). Each correlogram was tested for overall sig-nificance using a global test at the

α

= 0.05 level. Whencorrelograms indicated significant spatial structure, weused agglomerative hierarchical clustering (

inthe package

, Belbin 1989) to identify even-agedpatches of trees or stems in each plot by grouping treesor stems on the basis of their similarity in age and spatiallocation (see Duncan & Stewart 1991). Groups identifiedfrom the cluster analysis were only accepted as even-agedpatches if they were spatially discrete, had a relativelyeven-aged structure, and had a different age structurefrom neighbouring patches (Duncan & Stewart 1991).

In each plot, the spatial distribution (either clumped,random or regular) of silver beech seedlings and sap-lings was assessed using the

K

(

t

) function (Ripley1977; with the edge correction given in Diggle 1983), at0.5 m intervals up to half the length of the shortest sideof each plot. Using Monte Carlo simulation, approx-imate 99% confidence envelopes were constructed fromthe high and low values obtained from 1000 simula-tions of a random point process. Positive or negativevalues of

K

(

t

) that fall outside these confidence enve-lopes indicate a significantly clumped or regular stemdistribution, respectively.


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We tested for a directional trend in recent recruit-ment by regressing the size of seedlings and saplingsagainst distance below the tree line. A trend towardssmaller (and presumably younger) stems at the treeline could indicate a progressive advance of the treeline and an active zone of regeneration at the currentboundary, a possible response to recent climatewarming.

Temporal patchiness in tree establishment is often aresponse to past disturbance (Stewart 1986; Taylor

et al

.1996) but, at the tree line, periods of increased estab-lishment could also be a response to climate warming.To help interpret the patterns of tree establishment, wesearched for abrupt, short-term (< 5 years) changes inthe radial growth of trees. Rapid increases in radialgrowth are often a response to canopy opening, wherethe death of a canopy tree leads to the ‘release’ of adjacentcanopy or understorey trees (Lorimer 1980). Alternat-ively, an abrupt decline in tree growth (suppression)can be a response to a severe event in which treeswere damaged and growth restricted (Jacoby

et al

.1997).

To detect abrupt growth changes, we applied growthsuppression and release enhancement filters (seeKitzberger

et al

. 1995) to chronologies developed foreach plot using the ring-width measurements of indi-vidual silver beech stems. Ring-width series from indi-vidual stems were cross-dated using a combinationof visual cross-dating and

(Holmes 1983).Chronologies were standardized (using

; Cook1985) to remove the biological growth trend using anegative exponential curve, or a negative linear regres-sion, or if both of these failed, a horizontal line was fittedwhich does not remove any growth trend. Autocor-relation, where growth in previous years affects sub-sequent growth, was not removed because this wouldlimit our ability to detect disturbance-related suppres-sions or releases. Chronologies were truncated at theyear where the number of ring-width series fell belowfive. The suppression/release filters are designed toremove most annual growth variability, leaving onlyperiods of strong suppression or release (Kitzberger

et al

. 1995). For each year, the indices were calculatedbased on the growth of the preceding and following2 years, with strong suppressions or releases produc-ing larger spikes.

To determine if the tree line altitude is increasing, weextended each plot directly above the current tree linein a 10-m wide transect and searched upslope for treesuntil no further trees were found. For any trees present,we recorded the species, diameter (cm) at the baseif > 1 cm (otherwise recorded as seedling; stems were

measured at their bases as most were prostrate and didnot grow taller than 50 cm), and distance from the treeline (m). A core was extracted from any stem > 5 cmdiameter near ground level.

Results

Current tree line in the plots ranged from 1140 m a.s.l.(metres above sea level) to 1255 m a.s.l. (Table 1). Silverbeech formed the tree line and was the dominant can-opy species in all seven plots. Mountain beech waspresent in high numbers in plot Ht3, low numbers inplot Rs1, and as one seedling in plot Ht1.

The combined age data for all seven tree line plots sug-gest that silver beech stems

≥

10 cm diameter have beenrecruited episodically in the last 300 years. There was ageneral increase in stem numbers starting around 1740

, but there was also a distinct pulse of stem recruit-ment over the period 1880–1920 (Fig. 2a). In Fig. 2(a),

Fig. 2 (a) Combined age-class distribution of silver beechstems ≥ 10 cm diameter from the seven study plots. Stems thatestablished from seed are indicated by dark grey and stemsthat established by vegetative reproduction by light grey. TheNew Zealand average summer temperature series (smoothedwith an 10-year running mean) is plotted on the right-handaxis. (b) Combined size-class distribution of all silver beechstems from the seven study plots; sd, seedlings.


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the lack of stems that were recruited after 1920 is partlyan artifact of not coring young stems < 10 cm diameter.Nevertheless, the size distribution of all stems alsoimplies a distinct recruitment pulse, with fewer saplings1–10 cm diameter than stems 10–20 cm diameter(Fig. 2b). Hence it appears that there was an upsurge instem recruitment in the period 1880–1920 that has notbeen sustained in recent decades. Significantly, themain peak in this recruitment pulse occurs during aperiod when the average summer temperature in NewZealand was low (1890–1915; Fig. 2a).

,

A period of high stem recruitment in the period 1880–1920 is evident in the age structure of all individualplots (Fig. 3). In two plots (Ht2 and Rs1) this is evidentas a discrete pulse of stem recruitment, but in otherplots it appears to be part of a longer period of stemrecruitment, sometimes with a distinct pulse initiatedearlier than 1880 (e.g. 1860 in Ht3 and Rv3).

At least three widespread episodes of radial growthsuppression occurred between 1880 and 1920 (indicated

by arrows on Fig. 3 and Fig. 4). The most consistent,and usually the largest of these suppressions, started in1898 (Rv1–3) or 1899 (Ht1, Ht3 and Rs1). The tworemaining episodes are evident as one or two suppres-sions in the late 1870s to early 1880s, the most con-sistent of which started in 1881 (Rv1–3, Rs1 and lessnoticeably in Ht3), and as a group of several suppres-sions in the late 1910s, the most consistent of whichstarted in 1915 in all chronologies except Rs1 (Fig. 4).Although periods of growth release are evident in mostplots (Fig. 5), few are synchronous between plots,suggesting that there were few widespread episodes ofgrowth release. The most consistent growth releasestarted in 1950 (Rv2 and Rv3) or 1951 (Ht1–3, Rv1 andRs1). The only other consistent growth release occurredin 1990 (Ht2–3, Rv1–3).

Although there was a major pulse of stem recruit-ment after 1880, earlier pulses in recruitment are alsoevident in some plots (Fig. 3). In Ht1, Ht3 and Rv2, forexample, there was a smaller pulse of stem recruitmentbetween 1820 and 1860, and in plots Rv3 and Rs1 therewas a small recruitment pulse between 1800 and 1840.In other plots older pulses could have occurred, but theymay have been destroyed by more recent disturbances,

Fig. 3 Age-class distributions of silver beech stems ≥ 10 cm diameter in each study plot. Stems which established from seed areindicated by dark grey and stems that established by vegetative reproduction by light grey. The New Zealand average summertemperature series (smoothed with an 10-year running mean) is plotted on the right-hand axis. Arrows indicate dates of majorgrowth suppression events evident in Fig. 4.


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Fig. 4 Suppression indices for the silver beech chronologies developed from each tree line plot using Kitzberger’s suppressionfilter. Arrows indicate dates of major growth suppression events.

Fig. 5 Release indices for the silver beech chronologies developed from each tree line plot using Kitzberger’s release filter. Arrowsindicates dates of major growth release events.


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such as those which initiated the recruitment pulsestarting around 1880. Although most of the chrono-logies did not extend far back enough to determinewhether the older recruitment pulses are also associ-ated with suppression events, Rv2 shows an 1820recruitment pulse following major growth suppressionaround 1815 (Fig. 4e).

Sapling diameter distributions suggest that recentrecruitment of silver beech has continued to be episodic(Fig. 6). Plots Rv1 and Rv2 show a peak at 6–10 cmdiameter, in addition to a larger peak 3–5 cm seen in allplots, which contrasts with the few saplings 1–2 cmdiameter and the large number of seedlings in all plots.This implies little recent recruitment from what mayrepresent a suppressed ‘seedling pool’ (Wardle 1984).

Tree ages (taken as the age of the oldest stem) showedlittle evidence of spatial structure. Only the correlo-

grams for plots Ht2 and Rv1 were globally significantat the 0.05 level (not shown), but their shape did notsuggest a spatial structure consistent with establishmentin even-aged patches.

In contrast, the ages of individual stems were signif-icantly spatially structured. The correlograms for allplots, except Ht2 (where the stem ages were all verysimilar), were globally significant and all showed sig-nificant positive autocorrelation in the smallest dis-tance classes (up to 6 m), with a switch to significantnegative autocorrelation in larger distance classes (9 mup to 24 m). These patterns are consistent with stemestablishment in even-aged patches, with significantpositive autocorrelation in small distance classes reflect-ing the clumping of stems of similar age within patches,and significant negative autocorrelation in larger dis-tance classes reflecting the separation between patchesof different age.

The cluster analysis of stem ages and locations failedto distinguish obvious, large even-aged patches. Ratherit appeared that stems were grouped in small (2–4 mdiameter) patches of similar age. Occasionally theseconsisted entirely of stems from a single tree, suggest-ing that a tree had established and rapidly producedmany stems to fill an available opening. More often,however, stems came from several trees, suggesting thatmany trees produced stems simultaneously to fill anadjacent canopy opening. Because of the small size ofeven-aged patches, coupled with the ability of trees toproduce many stems, individual trees often had stemsin several different age groups, probably reflectingresponses to multiple canopy openings. Seedlings andsaplings were significantly clumped at distances up to5 m in all plots (not shown), consistent with recruit-ment into small canopy openings.

There is little indication of an active zone of regen-eration at the tree line boundary. The only significantrelationships between sapling diameter and distancebelow tree line were found for plots Rv2 and Rv3(

r

2

= 0.50,

P

= 0.024 and

r

2

= 0.60,

P

= 0.001, respect-ively), but these relationships were negative, meaningthat larger stems were found closer to the tree line.

Silver beech had established above the tree line in onlythree plots: Ht1, Ht2 and Rv3 (Table 2), with mostestablishment in Ht2. Most individuals had established

Fig. 6 Size-class distributions of silver beech stems < 10 cmdiameter in the study plots; sd, seedlings.

Table 2 Establishment of silver beech stems in 10 m wide transects above the tree line plots

Plot No. stemsAverage diameter in cm (range)

Maximum distance above tree line (m) (average)

Highest alt. above tree line (m a.s.l.)

Age (diameter in cm)

Ht1 3 5.1 (3.7–6.1) 6 (3.7) 1215 34 (6.5)Ht2 40 3.9 (< 1–9.8) 8.1 (3.1) 1185 29 (9.8)Rv3 6 5.8 (3.5–8.5) 20 (6.0) n.r. 30 (8.5)

n.r., not recorded.


1069Disturbance and climate warming at tree line

© 2001 British Ecological Society, Journal of Ecology, 89, 1061–1071

within a few metres of the current tree line, althoughone individual was found 20 m above the tree line atRv3. Only one stem in each plot was large enough to becored and their ages suggest that establishment abovethe tree line has occurred within the last 30–40 years(Table 2; the ages are not adjusted for years to reachcoring height).

Discussion

-

Two lines of evidence suggest that natural disturbance hasa major influence on Nothofagus population dynamics atthe tree line in our study area. Recruitment has beenepisodic, which is a feature of disturbance-affected forests(Oliver & Stephens 1977; Stewart 1986), and episodesof growth suppression often coincided with upsurges instem recruitment. In particular, the largest growth sup-pression, evident in all plots and starting in 1898–1900,coincided with the peak in stem recruitment across allplots in 1880–1920. We suggest this is most likelycaused by natural disturbances that formed small can-opy openings through stem breakage, thus damagingthe surviving trees while allowing the recruitment ofnew stems into the openings. Because the openings weregenerally small and many trees survived the disturbance,the major response was the production of new stems bysurviving trees to fill the canopy openings, leading topatches of even-aged stems. The exception was plotHt2 where it appears that a catastrophic disturbancearound 1860 initiated widespread synchronous establish-ment of trees throughout the plot.

Although the major growth suppression starting in1898 occurred during a period of low summer temper-atures in New Zealand, it is unlikely that this is atemperature response because the suppression was nolonger evident when temperatures reached their lowest(1903–04). Rather, the suppression is most likely due toa severe south-easterly windstorm recorded from theadjacent Maruia Valley in 1898 (Foster 1931; Conway1952). This gale coincided precisely with the start of thesuppression at our tree line sites and with an episodeof small-scale gap formation and growth release inNothofagus fusca (red beech) forests (Stewart et al.1991). Because a delay of 1–2 years before the onset ofgrowth suppression following a disturbance event iscommon ( Jacoby et al. 1997), the growth suppressionsstarting 1898 and 1899 in all plots are probably syn-chronous and almost certainly reflect this windstorm.Other pulses of stem recruitment and growth suppres-sion in the tree line plots (Figs 3 and 4) most likelyreflect the impact of similar disturbances.

Overall, our results suggest that natural distur-bances, most probably from windstorms or possiblysnow damage, have been a major factor in initiatingstem and tree recruitment in Nothofagus tree lines inour study area over the last 100–150 years, and probably

back as far as 300 years. Previous research has alsosuggested that disturbance is important for forestregeneration at high altitudes in New Zealand (Allen &Wardle 1984), North America (Shankman 1984) andChina (Taylor et al. 1996). In particular, Nothofagus-dominated tree line forests in parts of South Americashow a very similar regeneration pattern to our sites,with forests dominated by a mosaic of small even-agedpatches (Veblen et al. 1981) and episodic regeneration(Cuevas in press). Recruitment of Nothofagus treelines may also be limited by poor seed germination andseedling survival (see Cuevas 2000).

Tree line position is not undergoing a substantial advance,with only limited localized recruitment above thepresent forest limit (mostly within 10 m), despite the1.0 °C increase in New Zealand average summer tem-perature since 1950 (Salinger et al. 1993). In contrast,recent establishment above current tree limits in responseto climate warming has been found in many parts of theNorthern Hemisphere (e.g. Daly & Shankman 1985;Kullman 1991; Luckman & Kavanagh 1998). There is alarge number of seedlings above the current tree line inplot Ht2. However, this plot is not typical since moststems and trees below were recruited as a distinct pulseafter 1860, suggesting re-colonization following astand-destroying disturbance, most likely an avalanche,and the tree line may be returning to a pre-avalanchestate, as seen with fire-affected tree lines (e.g. Shankman1984).

Wardle & Coleman (1992) also found only a limitedadvance (c. 10 m) of silver beech tree lines at other sitesin New Zealand, which they attributed to warming.Given the presence of trees > 100 years old at the treeline boundary, the low numbers of seedlings foundabove the tree line and their scattered distribution inour study, we favour their alternative interpretationthat limited recruitment above the tree line represents apermanent low-density population with continuousturnover (see also Szeicz & MacDonald 1995). In con-trast, within 10 m of Chilean tree lines there can be rel-atively high numbers of Nothofagus pumilio seedlings,but the restriction of seed dispersal mostly to within20 m of the tree line and poor seed viability above thetree line (Cuevas 2000) suggests that establishmentbeyond 10 m sufficient to create a new tree line isunlikely.

There has been no recent, continuous upsurge inNothofagus seedling establishment in the tree lineforests we studied; rather, recruitment continues to beepisodic. This is in contrast to the many NorthernHemisphere studies which document increased seedlingestablishment below the tree line in response to climatewarming (e.g. Scott et al. 1987; Magee & Antos 1992;Szeicz & MacDonald 1995; Taylor 1995; Rochefort& Peterson 1996). A recent study in South America(Daniels 2000) also found no increase in regeneration


1070L. E. Cullen et al.


within Nothofagus pumilio tree line forests despitehigher temperatures since 1970, an outcome that maybe a consequence of the overriding importance ofmoisture availability for successful N. pumilio establish-ment. We suggest that a lack of response to climatewarming below the tree line in the Nothofagus foreststhat we studied is a consequence of the overridingimportance of natural disturbance for stem recruitment.Regeneration of Nothofagus in these tree line forestsappears to depend on the creation of canopy openingsand is therefore unlikely in the absence of disturbance.

Our results may reflect a general difference betweenthe Hemispheres in tree line structure and physiog-nomy, and potentially the role of disturbance in driv-ing regeneration processes at the tree line. SouthernHemisphere tree lines are often dominated by light-demanding species such as Nothofagus, which form atall, closed-canopy forest right up to an abrupt limit(Wardle 1965). At least in our study area, disturbanceis required for substantial recruitment within theseclosed-canopy forests and a response to climate warm-ing is unlikely unless warming coincides with canopydisturbance. In contrast, tree lines in the NorthernHemisphere are typically dominated by coniferousspecies and often grade from closed-canopy forest tosmall clumps of trees to scattered individuals until thetree limit is finally reached (Kullman 1990). This moreopen structure may mean that coniferous tree lines areless dependent on canopy opening by disturbance forsuccessful establishment, so that recruitment respondsmore rapidly to changes in temperature.

Nevertheless, recent work from Switzerland hassuggested that the subalpine limit of Pinus sylvestrishas shown little response to climate warming becauseseedling establishment relies on disturbance, mostlyfrom fire (Hättenschwiler & Körner 1995). This suggeststhat there are tree lines in the Northern Hemispherewhere disturbance could affect responses to climatewarming in a similar way to that found in our study.Furthermore, the open coniferous tree lines of theNorthern Hemisphere are not typical of forests belowthe tree line in both hemispheres, in which disturbanceis usually important for forest regeneration. We suggestthat understanding the likely impact of climate warm-ing on the dynamics of forests in general will requirecareful consideration of the interactions between climatewarming and disturbance impacts.

Acknowledgements

This research was funded by Lincoln University, theLotteries Commission, the Robert C. Bruce Trust, andthe Canterbury Botanical Society. Financial supportfor the first author was provided by a Lincoln Uni-versity Doctoral Scholarship. We would like to thankAmanda Ridley, Bert Borger, Maartje de Deugd,Nancy Willems, Gillian Vaughan, Pavla Honzíckováand Gillian Dennis for their tireless help in the field andtwo anonymous referees for their helpful comments.

We also thank the Department of Conservation forpermission to core trees in the study plots.

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Received 11 January 2001 revision accepted 5 July 2001


Disturbance and climate warming influences on New Zealand Nothofagus tree-line population dynamics

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