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Climate Change Treeline
 

Climate Change Treeline

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    Climate Change Treeline Climate Change Treeline Document Transcript

    • PACE AND PATTERN OF RECENT TREELINE DYNAMICS: RESPONSE OF ECOTONES TO CLIMATIC VARIABILITY IN THE SPANISH PYRENEES J. JULIO CAMARERO and EMILIA GUTIÉRREZ Dept. Ecology, Fac. Biology, University of Barcelona, Avda. Diagonal, 645, 08028 Barcelona, Spain E-mail: chechu@bio.ub.es Abstract. Treeline ecotones are regarded as sensitive monitors of the recent climatic warming. However, it has been suggested that their sensitivity depends more on changes in tree density than on treeline position. We study these processes and the effect of climate, mainly air temperature, on tree recruitment and recent treeline dynamics. We selected three relatively undisturbed sites in the Spanish Pyrenees, dominated by Pinus uncinata, and analyzed their recent dynamics at local spatial (0.3–0.5 ha) and short temporal scales (100–300 years). We wanted to establish whether higher temperature was the only climatic factor causing an upward shift of the studied alpine treelines. The data we report show that treelines were ascending until a period of high interannual variability in mean tem- perature started (1950–95). During the late twentieth century, treeline fluctuation was less sensitive to climate than was the change in tree density within the ecotone. Tree recruitment and treeline position responded to contrasting climatic signals; tree recruitment was favored by high March temperatures whereas treeline position ascended in response to warm springs. We found a negative relationship between mean treeline-advance rate and March temperature variability. According to our findings, if the interannual variability of March temperature increases, the probability of successful treeline ascent will decrease. 1. Introduction Tree populations at their distribution margins are theoretically very sensitive to climate variability (Brubaker, 1986). This is the case for latitudinal and altitu- dinal treeline ecotones, where low temperature limits tree growth (Tranquillini, 1979). Their value and reliability as monitors of the recent climate warming is based mainly on studies of tree growth and recruitment within these ecological boundaries (Tranquillini, 1979; Payette and Filion, 1985; Slatyer and Noble, 1992; Lescop-Sinclair and Payette, 1995; Paulsen et al., 2000). In this study, we define the alpine forest-tundra ecotone as the area bounded by the treeline (maximum eleva- tion of live individuals with stems at least 2 m high) and the timberline (maximum elevation of a closed forest). Several studies on alpine treeline have documented al- titudinal shift during the first half of the twentieth century followed by tree-density increase within the ecotone during the last decades (Kullman, 1979; Rochefort et al., 1994; Szeicz and MacDonald, 1995; MacDonald et al., 1998). There have been also reports on treeline recession during recent cold episodes (Kullman, 1996). However, few detailed studies have so far described how climate, and mainly Climatic Change 63: 181–200, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.
    • 182 J. JULIO CAMARERO AND EMILIA GUTIÉRREZ temperature, influences recruitment and treeline-shift at altitudinal forest-tundra ecotones. Our main objective was to quantify the treeline-climate relationship at a small spatiotemporal scale (0.3–0.5 ha plot, 100–300 yrs). In order to do this, we focused on changes in treeline elevation and tree recruitment within relatively undisturbed treeline ecotones in the Spanish Pyrenees. A similar response at all the studied sites would indicate a common regional effect, most likely a climate factor. We were more interested in tree recruitment because of reports that it was more sensitive to climate variability than tree mortality was (Payette and Filion, 1985; Lloyd, 1997). We suggest that a greater variability in air temperature may be the ultimate climate factor favoring an upward shift of alpine mesic treelines. Like most Eurasian mountains, the Pyrenees provide a unique opportunity to study the effect of two global-change components, i.e., climate warming and land-use modification, on treeline ecotone. Between 1882 and 1970, mean an- nual temperature increased by 0.83 ◦ C at the Pic du Midi meteorological station (43◦ 04 N, 00◦ 09 E, 2862 m a.s.l.) in the Central Pyrenees (Bücher and Dessens, 1991). In addition, grazing pressure in this area has been falling since the 1950s, which has led to drastic modification in land use (García-Ruiz and Lasanta- Martínez, 1990). For instance, Bas (1993) estimated a 80% reduction of grazing pressure in a village in the Catalan Pyrenees paralleled by a shift from nomadic to stabling flocks of sheep. These combined processes have caused pronounced changes in the structure of the Pyrenean treeline ecotone (Soutadé et al., 1982). 2. Materials and Methods 2.1. STUDY SITES We reconstructed recent treeline dynamics at the following sites: Ordesa – O, 42◦ 37 N, 00◦ 02 W, 2110–2100 m a.s.l.; Tessó – T, 42◦ 36 N, 01◦ 03 E, 2360– 2330 m; Estanys de la Pera – EP, 42◦ 28 N, 01◦ 38 E, 2430–2360 m (Figure 1). The mean slope ranged 20◦ (O)–25◦ (T, EP). The three sites represent most of the geographical variability in the Pyrenean treeline ecotone marked by the W-E cli- mate gradient across the range (W – Atlantic influence; Central area – continental influence; E – Mediterranean influence). The studied ecotones are dominated by Pinus uncinata Ram., a shade-intolerant and long-lived (700 yrs.) conifer, which forms most treelines in the Pyrenees (Ceballos and Ruiz de la Torre, 1979; Bosch et al., 1992). The sites O and T have not been affected by local anthropogenic dis- turbances (logging, grazing) during the last 50 years. At these sites recent livestock estimates ranged 2–24 sheep month ha−1 according to Aldezábal et al. (1992) and Bas et al. (1994). According to historical documentation, site T has not been in- tensively disturbed since the eighteenth century (Bringue, 1995). Site EP could be regarded as slightly disturbed by grazing (estimated present livestock is ca. 25–50 sheep month ha−1 ).
    • TREELINE DYNAMICS IN THE PYRENEES 183 Figure 1. Distribution of P. uncinata (black areas) in the Iberian Peninsula and detailed maps of the studied treelines (O, T, EP). The black rectangles at each site correspond to the approximate location of the plot. The climate at the study sites is continental with Atlantic (T site) or Mediter- ranean (O and EP sites) influence. Mean annual temperature and total annual precipitation range from 2 to 5 ◦ C and from 1200 (EP) to 1600 mm (T), re- spectively. Estimated lowest and highest mean monthly temperatures are –4 ◦ C (January–February) and 11 ◦ C (July). Maximum snow-cover thickness is reached in winter-spring and ranges from 0.5 (O, EP) to 3 m (T). Snow cover differences are reflected in the understory vegetation. At site T, we found a community typical of longer and deeper snow-cover sites dominated by Rhododendron ferrugineum and Vaccinium myrtillus. At sites O and EP, more xeric species were found such as Juniperus communis subsp. alpina, Festuca rubra and Calluna vulgaris. 2.2. FIELD SAMPLING A rectangular plot (30·100 m, the EP site; 30·140 m, the O and T sites) was marked out at each site in topographically uniform parts of the treeline ecotone. The plot had its longer side parallel to maximum slope and included current treeline and timberline. The following variables were recorded in the field for every P. uncinata
    • 184 J. JULIO CAMARERO AND EMILIA GUTIÉRREZ individual within the plot: location in the plot (x and y coordinates); size (diameter at breast height, height); and growth form (arborescent, flagged-krummholz, and krummholz). Individuals whose height was equal or less than 0.5 m were regarded as seedlings. All individuals were tagged for their future monitoring. 2.3. DENDROCHRONOLOGICAL METHODS Germination date was estimated by taking out a core from each live individual’s main stem as close to the ground as possible. This was done for all living individ- uals located within the plot whose height was greater than 0.5 m. All cores were mounted, sanded, and crossdated using standard dendrochronological methodol- ogy (Stokes and Smiley, 1968). Inner-ring dates were corrected for (i) years to core height and (ii) years to center (missed pith). The correction was made using (i) age-height regression and (ii) age-diameter regression combined with the fitting of a circle template to the ring curvature so as to estimate the distance of the core to the center. At sites T and EP, we estimated nondestructively the ages of those individuals with height ≤0.5 m by counting the number of branch whorls and bud scars on the main stem. This age estimation was validated comparing it with the age obtained counting the tree-rings in basal disks taken from a subsample of trees located outside but near the plot. This procedure was not carried out at site O because of the multistemmed character of most of the individuals (Camarero and Gutiérrez, 1999). A static age structure of live trees is the expression of change in the rate of tree recruitment and mortality over time (Harcombe, 1987). In order to interpret changes in tree density as a result of tree recruitment dynamics, we assumed that mortality was nearly constant for all cohorts. To analyze tree recruitment dynamics as related to radial growth, seven residual chronologies from adult P. uncinata growing in high-elevation (2075–2360 m a.s.l.) subalpine forests were used (see also Tardif et al., 2003). These stands are near site T, and they are a reliable sample of the variability in P. uncinata radial growth in the Spanish Pyrenees (Gutiérrez et al., 1998). Trees were sampled following standard dendrochronological techniques, taking at least 2 cores per tree at 1.3 m. These cores were processed in a similar way as the basal ones, but ring width was measured using a semiautomatic system (Aniol-CATRAS) with a resolution up to 0.01 mm. Each ring-width series was standardized using a spline function with a 50% frequency response of 32 years (Cook and Peters, 1981). Autoregres- sive modeling was then performed to remove temporal autocorrelation, which was mostly of first order. Standardization involved transforming the ring-width value into a dimensionless index by dividing the observed values by the expected values given by the spline function (Fritts, 1976). Then the indexed residual series were averaged. Following this procedure, long-term (low frequency) trend was removed from the tree-ring series. To standardize the tree-ring series we used the program ARSTAN (Cook, 1985). Calculation details and chronology statistics can be found elsewhere (Gutiérrez et al., 1998).
    • TREELINE DYNAMICS IN THE PYRENEES 185 2.4. CLIMATIC INFLUENCE ON TREELINE DYNAMICS In order to calculate the rate of treeline shift, maximum elevation (y coordinate) of live individuals with stems at least 2 m high (treeline) was determined for 25- yr intervals (maximum treeline elevation). Each plot was divided into 3 altitudinal subtransects (10-m wide), so as to take the treeline spatial heterogeneity into ac- count. We calculated mean maximum elevation of the treeline and its standard deviation for 25-yr intervals, using data from the three subtransects per site. Since results did not differ greatly from those obtained using the maximum treeline ele- vation of the entire plot, we used the latter and simpler variable – which in addition was more related to our interest in the effect of temperature on tree recruitment within the treeline ecotone. The treeline-shift rate (m · yr−1 ) was calculated by dividing the change in treeline elevation, between successive intervals, by the time elapsed. On average, a 1-year old seedling took ∼40 years to become a 2-m tree (Camarero, 1999), and we assumed that this time span was constant. The treeline advanced (rate >0) or remained stable (rate = 0) during all the considered periods at the studied sites. In order to study the effect of climate on tree recruitment, the number of pine recruits within every ten year interval for the 1880–1980 period was calculated. These data were related to monthly mean minimum temperatures (from the Pic du Midi station) averaged for the same 10-yr intervals. Temperature was the most re- liable climatic variable for this station (Bücher and Dessens, 1991). No significant temporal autocorrelation was detected for recruitment data. In order to quantify the climatic influence of temperature on treeline dynamics, we used a reconstruction of monthly mean surface-air temperatures (1781–1997) for lake ‘Estany Redó’ (42◦ 39 N, 00◦ 46 E, 2240 m a.s.l.). It was based on lowland instrumental climate records (Agustí-Panareda et al., 2000). The temperatures were averaged for 25- yr intervals and related to maximum treeline-advance rate estimated for the same time intervals during the period of 1775–1995 (1775–99, 1800–24, . . . , 1975–95). In this case we attempted to avoid circular inference by using climate data instead of dendroclimatic reconstruction. In all the cases, Spearman’s rank correlation co- efficient (rs ) was used. If in any paired comparison one or two variables showed a significant (P ≤ 0.05) temporal trend their correlation value was not taken into account. 3. Results and Discussion The main changes suggested by repeated historical photographs taken along the twentieth century (Figure 2) were: (i) increase in tree size and density within the treeline ecotone; (ii) reduced or even null altitudinal ascent of the treeline. A similar trend was observed in all the studied treeline ecotones. It could suggest a common regional cause such as climate. The data in Figure 3 show this tendency
    • 186 J. JULIO CAMARERO AND EMILIA GUTIÉRREZ Figure 2. Structural changes at a treeline ecotone in the Spanish Pyrenees during the twentieth century. The most prominent change is the increase in tree size and density across the treeline ecotone. Maximum treeline elevation has not increased (white line). These two photographs were taken in 1909 (Lucien Briet, Mus´ e Pyr´ n´ en, Lourdes) and 1997 (J. L. Ac´n Fanlo) at Punta Diazas e e e ı (42◦ 38 N, 00◦ 03 W, Ordesa y Monte Perdido National Park), near site O. at site T. Firstly, there has been a recent increase in tree establishment and density within the ecotone, specially during the period of 1900–49. Secondly, the treeline ascended greatly during the last half of the nineteenth century, reached maxi- mum elevation at the beginning of the twentieth century and then remained stable throughout the rest of that century. Mean treeline elevation followed a parallel trend and remained approximately stable during the past century. Maximum spatial
    • TREELINE DYNAMICS IN THE PYRENEES 187 Figure 3. Spatiotemporal variability in tree density and treeline position (maximum elevation of live individuals with stems at least 2 m high) within the alpine forest-tundra ecotone at site T. The figure shows the same plot (30 m · 140 m; the y axis follows the altitudinal gradient upslope – the arrow points upslope) during different time periods (1750–1849, . . . , 1950–97). Various limits are shown: uppermost treeline (MAX – gray thick line) of all individuals present during each period, mean treeline (AVG – black thick line – average of maximum elevation of the treeline formed only by individuals established during each 25-yr interval) and its standard deviation (±SD – black thin line). Each filled black symbol represents an individual that established and became a tree (live indi- viduals with stems at least 2 m high) during the period indicated above. Unfilled symbols represent trees established during periods previous to the one indicated above. Different symbols represent tree-establishment periods (e.g., circles = 1750–1849). Current timberline is at y = 40 m.
    • 188 J. JULIO CAMARERO AND EMILIA GUTIÉRREZ heterogeneity (standard deviation) of mean treeline elevation increased during the period of 1850–99 and decreased during that of 1950–97. That pattern might be the result of the high tree-recruitment rate during that period as well as during the previous decades. Overall, these changes agree with those observed in repeated historical photographs, i.e., increase of tree density within the ecotone but minor or null treeline shifts (Figure 2). During the first half of the twentieth century, the spatial clustering of pine recruits was greater at short distance (less than 9 m) than during the second half (Camarero et al., 2000a). These patterns developed at a time characterised by very warm springs and summers and low temperature variability during the first half of the twentieth century, but warm fall-winter seasons and high interannual temperature variability during the second half of the last century (Bücher and Dessens, 1991; Manrique and Fernández-Cancio, 2000). These contrasting climatic trends were also reflected in the temporal variability in radial growth (Tardif et al., 2003). High-elevation subalpine P. uncinata stands in the Central Pyrenees showed two contrasting patterns of radial growth during the twentieth century (Figures 4A–C). The first half of that century was characterised by: (i) low interannual variability (low frequency of wide and narrow rings, Fig- ure 4A, low moving standard deviation of mean radial-growth index, Figure 4C), (ii) reduced similarity in growth among sites (low mean inter-site correlation, Figure 4B). A reverse pattern was found during the last half of the twentieth and during the nineteenth century. Pine recruitment at treeline was very high during the periods of 1925–49 (site T) and 1950–74 (sites O and EP; Figure 4D). Conversely, the treeline-advance rate was null during the last half of the twentieth century and it reached maximum values at the end of the nineteenth century (1850–74, sites O and T) or at the beginning of the twentieth century (1900–24, site EP; Fig- ure 4E and Table I). The mean ‘regional’ maximum treeline-advance rate reached its highest values during the periods of 1850–74 and 1900–24. If these data were analyzed subdividing each plot into three 10-m wide transects running along the slope (n = 9 transects per site), the results varied slightly. During the period of 1900–49, mean treeline-advance rate reached its highest value at sites T and EP. Furthermore, during the period of 1925–49 most of the subtransects showed alti- tudinal shift above maximum treeline elevation reached during previous periods (Table I). However, and also during the last 50 years, most of the subtransects did not show any altitudinal ascent. Climate affected tree recruitment and treeline-advance rate in different ways (Figure 5). Tree recruitment was favored by high monthly mean minimum temper- ature during March–April, July, and October (Figure 5A). The positive effect of warm spring and fall could be related to the increase in seedling mortality due to frost during those seasons. However, such relationships may be more complex. For instance, high temperature in spring could speed up snowmelt, and lead to more drying of soils in summer. This would intensify the negative effect of summer drought on seedling survival (Puig, 1982; Lloyd, 1997). Other climatic factors, such as winter-spring snow cover, could improve reproductive success at treeline
    • TREELINE DYNAMICS IN THE PYRENEES 189 Figure 4. Relationship between radial-growth (A–C), tree recruitment (D) within three treeline eco- tones (sites O, T, EP), and maximum treeline-advance rate (E) within these sites in the Central Pyrenees. (A) Relative frequency (percentage) of wide (> +1.5 SD; values above 0) and narrow (–1.5 SD<; values below 0) annual ring-width indices in seven subalpine P. uncinata stands. Max- imum and constant sample size reached around 1850 (n = 177 cores). (B) Mean (±SD) fixed (big dots) and moving (small dots) correlations (Pearson’s r; n = 21) between the seven residual chronologies every 25-yr period. (C) Mean (10-yr moving average) and standard deviation (SD, 25-yr moving average) of annual ring-width indices in seven stands. (D) Tree-recruitment at treeline ecotones (O, T, EP sites). (E) Maximum treeline-advance rate at these sites (bars) and estimate of the variability of mean annual temperature (line; CV, coefficient of variation for each period based on temperature reconstructed for the nearby ‘Estany Red´ ’ lake, 1781–1995). Note that tree recruitment, o treeline-advance rate, and the CV of temperature are given for 25-yr periods except for the first and last ones. Treeline-advance rate at site T during the period of 1850–74 was 4 m · yr−1 . The colors of bars correspond to the different treeline sites in (D) and (E).
    • 190 Table I Mean, standard deviation (±SD) and maximum treeline-advance rate (m · yr−1 , in parentheses) at three altitudinal treeline ecotones in the Central Pyrenees (O, T, and EP sites) during the last 150 years. The fifth column shows mean maximum rate (±SD) of treeline advance averaging the values for the three sites (‘regional’ mean). The last column shows relative frequency (%) of substransects (n = 9) showing altitudinal ascent during each period. Mean value and standard deviation were calculated for the three altitudinal subtransects at each plot. The highest mean and maximum values for each site are in bold Period O T EP Mean Subtransects (%) 1850–74 0.64 ± 0.84 (0.82) 0.87 ± 1.50 (3.99) 0.03 ± 0.05 (0) 1.60 ± 2.11 50.00 1875–99 0.22 ± 0.39 (0) 0.39 ± 0.39 (0) 0.84 ± 1.35 (0.05) 0.02 ± 0.03 66.67 1900–24 0.17 ± 0.29 (0.50) 0.90 ± 1.25 (0.36) 0.63 ± 0.99 (0.80) 0.55 ± 0.22 55.55 1925–49 0.24 ± 0.24 (0) 0.47 ± 0.40 (0.13) 0.97 ± 1.23 (0.62) 0.25 ± 0.33 77.78 1950–74 0 (0) 0.27 ± 0.47 (0) 0.39 ± 0.67 (0) 0±0 22.22 J. JULIO CAMARERO AND EMILIA GUTIÉRREZ 1975–95 0 (0) 0 (0) 0 (0) 0±0 0
    • TREELINE DYNAMICS IN THE PYRENEES 191 Figure 5. Climatic influence on tree recruitment and treeline advance at P. uncinata treeline eco- tones. (A) Relationship between tree recruitment (absolute recruit number per hectare and year) and monthly mean minimum temperature (data from the Pic du Midi station, 1880–1980) at three treeline ecotones (O, T, EP sites) in the Spanish Pyrenees. The correlation was obtained for 10-yr periods (n = 10). (B) Relationship between monthly mean temperature reconstructed for a nearby alpine lake (Estany Red´ , 1781–1995) and maximum treeline-advance rate for 25-yr periods (sites o T and EP, n = 9; site O, n = 6). In both cases, the correlation (Spearman index, rs ) calculated for all months and seasons (WIN, January–March; SPR, April–June; SUM, July–September; FAL, October–December). Significance level is indicated by the symbol over the bar (∗ = P ≤ 0.05; ¶ = P ≤ 0.01). (Kullman, 1979; Frey, 1983). In any case, most of them are affected directly or indirectly by air and soil temperatures. The positive relationship between spring-summer temperature and tree recruit- ment at treeline has been observed both at altitudinal and latitudinal treelines (Kullman, 1983; Szeicz and MacDonald, 1995; Gutiérrez et al., 1998; Camarero and Gutiérrez, 1999). This relationship is related to temperature requirements, over a period of several summers, for successful seed production, germination and seedling establishment (Zasada et al., 1992; Scott et al., 1997). Appropriate temperatures are rarely attained at treeline environments, which causes the absence of sexual regeneration (Arseneault and Payette, 1997). In addition, extremely cold episodes have caused rapid treeline recession in the Swedish Scandes (Kullman, 1996). The recent reduction in tree recruitment at the studied treeline ecotones, and
    • 192 J. JULIO CAMARERO AND EMILIA GUTIÉRREZ that observed at the turn of the twentieth century, could also be explained within such context. If temperature interannual variability increased, the probability of successive favorable springs or summers would decrease. Therefore, tree recruit- ment would also decrease. This is shown in our data where ring-width is used as a proxy for climatic variability (Figure 4A–E). The hypothesis should be tested by comparing forests with treeline ecotones, so as to quantify possible temporal lags between tree recruitment peaks and warm episodes in such contrasting areas. Tem- poral lags up to 20–30 years have been found in boreal and subalpine forests (Scott et al., 1987; Gutiérrez et al., 1998; Suárez et al., 1999; Gervais and MacDonald, 2000). These lags could be related to the greater importance for tree recruitment of temperature at treelines vs. disturbances at forests (Zackrisson et al., 1995). The advance of treeline was consistent and negatively related to high mean March and November temperatures (Figure 5B). Such negative correlation between mean March temperature and maximum treeline-advance rate for 25-yr was found again when comparing temperature data with the regional mean value (rs = −0.64; P = 0.06). However, this relationship was positive and weaker in the case of spring temperature (April–June). The positive correlation between the regional treeline-advance rate and spring temperature was also significant (rs = 0.68; P ≤ 0.05). Paradoxically, November temperature of the year prior to growth is amongst the main climatic variables controlling positively the radial growth of P. uncinata during the year of growth (Camarero, 1999; Tardif et al., 2003). Thus, warm Novembers are related to high rates of radial growth in adults. However, higher November temperatures were associated with lower rates of treeline advance at sites O and T (Figure 5B). In the Pyrenees, warm Novembers are associated with cyclonic conditions and high snow precipitation (Del Barrio et al., 1990), which may induce higher rates of mortality among established treeline individuals because of direct physical damage or snow avalanches (Furdada, 1996). Overall, the responses of tree recruitment and treeline advance were rather sim- ilar at sites O and T as compared with site EP. It could be explained by four facts: (i) site EP is under the strongest Mediterranean influence, (ii) it is the farthest site from the meteorological observation points used in both analyses, (iii) it is slightly disturbed, and (iv) this site’s spatial pattern is clumped, dominated by ‘tree islands’. Such pattern can strongly buffer the responses of tree growth and tree recruitment to climate variability (Scott et al., 1993). The great increase in re- cruitment at site EP in 1950–74 could be related with the general decline in grazing since the 1950s in the Spanish Pyrenees (Bas, 1993). Indeed, if recruitment at the most undisturbed sites (O, T) for this period is subtracted from recruitment at site EP, a value of recruitment controlled by non-climatic factors might be established. At the Ordesa site we found strong inverse correlations between the rate of tree- line invasion and summer temperature (Figure 5B). This finding might be explained because of the Mediterranean influence at this site, which implies the presence of frequent summer droughts. Warmer summers at site O could induce a greater evaporative demand and increase the mortality rates of established P. uncinata
    • TREELINE DYNAMICS IN THE PYRENEES 193 seedlings, which are very sensitive to soil water availability in July (Camarero and Gutiérrez, 1999). A similar but less pronounced relationship is also observed at the more mesic site T. Analogously, Jacoby and D’Arrigo (1995) found at Alaska treelines evidences of moisture stress induced by recent climate warming, which is limiting the radial-growth response of trees at those sites. The treeline-advance rates for site EP showed a different response to summer temperature, which could be due to its greater level of local disturbance, and its clumped spatial pattern described before. Interestingly, several authors have noted that drought has become a main stress factor at boreal forests because of recent global warming in the late twentieth century (D’Arrigo and Jacoby, 1993; Jacoby and D’Arrigo, 1995; Barber et al., 2000). This has caused radial-growth declines in response to warmer temper- atures, specially at the more xeric treelines (Lloyd and Fastie, 2002). This might be the case of the Ordesa site, where there is a strong Mediterranean influence in spite of its western location in the Pyrenees (Camarero and Gutiérrez, 1999). Therefore, drought might be the prevalent factor explaining the negative influence of summer and September temperatures on treeline advance at this site. Lloyd and Graumlich (1997) showed how severe multi-decadal droughts increased the mortality of the uppermost individuals causing treeline descent in the Sierra Nevada, U.S.A. We found a negative relationship between mean treeline-advance rate and radial-growth variability for high-elevation chronologies (Figure 6A). In fact, the three longest periods with very few trees (0–15%) showing wide or narrow tree-rings, i.e., ‘stable’ periods for radial growth, were: 1744–64, 1861–77, and 1905–52 (Figure 4A). These periods preceded or coincided with treeline-advance episodes, which were also characterised by low interannual variability in mean temperature (Figure 4E). Since ring-width is not an independent variable and may be related with recruitment (circular argument), we analyzed the treeline-shift rate and standard deviation of mean monthly and seasonal temperatures reconstructed for a nearby alpine lake. The only significant correlation was found between the treeline-advance rate and standard deviation of mean March temperature (rs = −0.66; P ≤ 0.05; Figure 6B). The synchrony between periods of high March-temperature variability and peri- ods of lower treeline-advance rate may be viewed within the recent climatic context in the Central Pyrenees characterised by: (i) the rise of mean temperature (Bücher and Dessens, 1991), and (ii) the increase of interannual variability in mean tem- perature (Figure 4E; see also Tardif et al., 2003). A treeline ascent implies the occurrence of several consecutive processes: production of viable seeds, disper- sal, availability of adequate regeneration sites, germination, successful seedling establishment, vertical growth up to ca. 2 m (treeline individual), and survival and persistence until the individual is sampled. Climate variability affects all these sequential stages, but its influence is probably different for each process (Earle, 1993). For instance, the same climatic variable can enhance one of these processes while inhibiting another one.
    • 194 J. JULIO CAMARERO AND EMILIA GUTIÉRREZ Figure 6. Negative relationship between treeline-advance rate and radial-growth variability (A) or climatic variability (B). The scatter diagrams represent the relationship between: (A) mean ‘regional’ rate of treeline shift and standard deviation (SD) of mean residual chronology (average of seven high-elevation P. uncinata chronologies); (B) this rate and SD of mean March temperature (recon- structed for the nearby alpine lake Estany Red´ , 1781–1995). Values are means for 25-yr periods, o except for those of 1781–99 and 1975–95. In both cases, the 1850–74 period value is an outlier (mean treeline-advance rate = 1.6 m · yr−1 ; see Table I). The exponential function was fitted in (A) excluding this outlier (r = −0.81, P = 0.015), and only to highlight the negative correlation.
    • TREELINE DYNAMICS IN THE PYRENEES 195 Overall, warmer springs (April–June) are positively related with seedling estab- lishment, and treeline advance (Figure 5). This mode of treeline dynamics would correspond to a decrease in mortality rate of the uppermost trees, and an increase in recruitment rate. Higher temperatures in March favor tree recruitment, but they are negatively related with the rate of treeline-advance (Figure 5). In addition, a higher dispersion of March temperatures are also negatively related with the occurrence of treeline ascents (Figure 6). This mode of treeline decline would correspond to an in- crease in the mortality rate of marginal upright treeline individuals, while seedling establishment remains high. The transition from winter to the spring (March) seems to be critical for successful treeline ascent. Maximum snow thickness at the studied sites may be reached in late winter-early spring but its interannual variability is very high depending on temperature fluctuations (Furdada, 1996). Warmer March temperatures could speed up the snowmelt, reduce the thickness of the protec- tive snow cover, enhance the negative effects of wind abrasion on upright treeline individuals, and increase the mortality of these marginal trees (Tranquillini, 1979; Frey, 1983). This could explain the negative influence of March temperature and its variability on the rate of treeline advance. This is also supported by the negative re- lationship between winter temperature and the rate of treeline advance (Figure 5B). We suggest that the described relationships between recruitment, treeline shift and temperatures should be applicable to similar treelines where spring temperatures and the associated snow melt limit tree recruitment through temporal variability. Little attention has been paid in the literature to the relationship between treeline dynamics and climatic variability. For instance, long-term reconstructions of tree- line dynamics in the Sierra Nevada (U.S.A.) during the Holocene concluded that increase in treeline elevation was favored by higher temperature and wet conditions (LaMarche, 1973; Lloyd and Graumlich, 1997). A similar long-term study in the Polar Ural treelines stated that the forming of an adult generation of Larix sibirica at treeline required favorable conditions during at least 50 years (Shiyatov, 1993). This is one of the few studies emphasizing the importance of climatic stability in understanding treeline dynamics. Some authors have reported an increase in tree density within treeline ecotones but minor treeline fluctuation in response to the recent climate warming (Kullman, 1979; Payette and Filion, 1985; Scott et al., 1987; Szeicz and MacDonald, 1995; MacDonald et al., 1998). It would suggest that tree abundance might be a more sen- sitive monitor of climate change than treeline position (Slatyer and Noble, 1992). In fact, we found a more significant correlation between tree recruitment and temper- ature than between treeline-shift rate and temperature (Figure 5). Long periods of treeline stasis punctuated by brief periods of change seem to be a common feature of treeline dynamics (Kullman, 1979, 1990; Slatyer and Noble, 1992; Shiyatov, 1993; Lloyd and Graumlich, 1997). The described scenario would agree with a con- ceptual model based on non-linear treeline response to climatic thresholds (Slatyer and Noble, 1992; Arseneault and Payette, 1997). The treeline would remain static or ascend gradually during long periods, but it could descend suddenly in response
    • 196 J. JULIO CAMARERO AND EMILIA GUTIÉRREZ to intense disturbance, including extreme climatic events (Kullman, 1990). The data we report show that the studied treelines were ascending until a period of high climatic variability started (1950–95). Minor changes in treeline position during the last fifty years coincided with recent episodes of tree recruitment. Both of the variables responded contrastingly to apparently similar climatic signals. Pinus uncinata recruitment was favored by high March temperature whereas treeline ascended in response to warm spring (Figure 5). Similar results were found for P. sylvestris regeneration at Swedish treelines (Kullman, 1990). Treeline response usually lags behind climatic fluctuation and tree recruitment peaks (treeline inertia) because of the great longevity and phenotypic plasticity in tree species dominant at treeline ecotones (Kullman, 1979, 1990, 1996). Such plasticity can explain why the response of treelines to climatic fluctuation is asym- metric. Some treeline individuals (e.g., krummholz) can persist for decades to centuries during harsh climatic periods and respond with an accelerated vertical growth in reaction to improved climatic conditions. However, if the climatic thresh- old is surpassed due to an extreme climatic event (severe frost or drought, period of scarce snow cover, intense warming) the treeline response can be fast (Scott et al., 1987; Kullman, 1990). A greater interannual variability in mean temperature would increase the probability of surpassing these climatic thresholds. This could produce unexpected treeline-shifts in response to climate fluctuations. Local factors also modulate the relationship between climate and tree recruit- ment (Hobbie and Chapin, 1998). It has been pointed out in several studies at different treeline ecotones (Payette and Filion, 1985; Szeicz and MacDonald, 1995; Lloyd, 1997; Camarero et al., 2000a), but never proved conclusively, that the modification of microenvironmental conditions by trees or krummholz within the ecotone could enhance further tree establishment (nurse effect). It is clear that krummholz patches modify snow conditions in a different way than isolated upright trees do (Scott et al., 1993). If a positive-feedback switch, as the one proposed above, is operating at a treeline under unfavorable climate conditions, abrupt limit should appear (Wilson and Agnew, 1992). In fact, sharp boundaries have been described at site O as a result of past tree-establishment episodes (Camarero et al., 2000b). The appearance of such abrupt limits could be regarded as a spatial indication of the existence of a positive feedback. Our results suggest that the turn of the twentieth century marked a climatic transition, in the Spanish Pyrenees, between the end of the Little Ice Age and the current period. The period of 1881–1895 in Spain is regarded as a very cold episode marking the start of a warming trend at the beginning of the twentieth century (Font Tullot, 1988). The past century can be divided in the Spanish Pyrenees into two well defined climatic periods. During the first one (∼1900–49), the interannual temperature variability was low and spring-summer temperatures were responsible for the observed warming. The following period (∼1950–99) was characterised by a greater interannual temperature variability and higher fall-winter temperatures (Bücher and Dessens, 1991; Agustí-Panareda et al., 2000). These factors allowed
    • TREELINE DYNAMICS IN THE PYRENEES 197 the treeline to remain static while tree density increased within the ecotone. There- fore, higher temperature was not the only climatic factor stimulating an upward shift in the studied altitudinal treelines. We predict that the recent global warming is unlikely to cause an altitudinal ascent of the studied treelines, if it is accompanied by an increase in temperature variability. Both the climatic and dendroecological data we report confirm such recent increase for subalpine forests in the Spanish Pyrenees (Tardif et al., 2003). Other findings suggest that it is a trend valid for most of the Iberian Peninsula (Manrique and Fernández-Cancio, 2000). Acknowledgements We sincerely thank many people for their help in the field (O. Bosch, X. Lluch, M. Manzanera, E. Muntán, M. Ribas, M. A. Rodríguez, J. A. Romero, R. Ro- mano, P. R. Sheppard, J. Tardif, and L. Viñolas). ‘Aigüestortes i Estany de Sant Maurici’ and ‘Ordesa y Monte Perdido’ National Parks provided logistic help. Drs. A. Bücher, J. Dessens, J. Catalán, and R. Thompson provided climatic data. We thank Mrs. G. Marsan (Musée Pyrénéen, Lourdes, France) and J. L. Acín Fanlo for providing permission to reproduce the 1909 and 1997 photographs at Punta Di- azas. This research was funded by the Spanish CICyT (AMB95–0160) and the EU project FORMAT (ENV4-CT97–0641). We thank Daria Generowicz-Wasowicz and two anonymous reviewers for their helpful comments. References Agustí-Panareda, A., Thompson, R., and Livingstone, D. M.: 2000, ‘Reconstructing Temperature Variations at High Elevation Lake Sites in Europe during the Instrumental Period’, Verh. Int. Ver. Limn. 27, 479–483. Aldezábal, A., Bas, J., Fillat, F., García-González, R., Garín, L., Gómez, D., and Sanz, J. L.: 1992, Utilización ganadera de los pastos supraforestales en el Parque Nacional de Ordesa y Monte Perdido, Report, Instituto Pirenaico de Ecología (C.S.I.C.) - I.C.O.N.A., Jaca, Spain. Arseneault, D. and Payette, S.: 1997, ‘Reconstruction of Millenial Forest Dynamics from Tree Remains in a Subarctic Tree Line Peatland’, Ecology 78, 1873–1883. Barber, V., Juday, G. P., and Finney, B.: 2000, ‘Reduced Growth of Alaskan White Spruce in the Twentieth Century from Temperature-Induced Drought Stress’, Nature 405, 668–673. Bas, J.: 1993, Les pastures supraforestals a la Vall Ferrera i la Vall de cardós (Pallars Sobirà). Valoració de la capacitat ramadera de les pastures de Lladorre, M.Sc. Thesis, Univ. Lleida, Lleida, Spain. Bas, J., Moreno, A., Luna, A., Martínez, J., Sanuy, D., and Fanlo, R.: 1994, ‘L’explotació ra- madera a les pastures del Parc Nacional: Dades preliminars’, in La investigació al Parc Nacional d’Aigüestortes i Estany de Sant Maurici, III Jornades sobre Recerca, Generalitat de Catalunya, Espot, pp. 237–247. Bosch, O., Giné, L., Ramadori, E. D., Bernat, A., and Gutiérrez, E.: 1992, ‘Disturbance, Age and Size Structure in Stands of Pinus uncinata Ram.’, Pirineos 140, 5–14. Bringue, G. M.: 1995, Comunitats i bens comunals al Pallars Sobirà, segles XV–XVIII, Ph.D. Thesis, Univ. Pompeu Fabra, Barcelona.
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