Radial growth of Pinus sylvestris growing on alluvial terraces is sensitive to water-level fluctuations


Author for correspondence: Walter Oberhuber Tel: +43 0512 507 5948 Fax: +43 0512 507 2715 Email: Walter.Oberhuber@uibk.ac.at


  • • Along the Alpine river Lech (Tyrol, Austria), poorly grown Scots pine (Pinus sylvestris) stands dominate on dry alluvial terraces, which are made up of coarse calcareous gravel. Here we evaluated the impact of environmental factors, such as precipitation, temperature and water table, on annual variability of radial growth.
  • • Tree-ring chronologies from six stands comprising different age classes were developed by extracting two core samples from more than 15 trees per plot. Pearson correlations were applied to determine the influence of environmental factors.
  • • Close inverse correlations (r2 > 0.35) between maximum water table during the growing period and annual increments of adult stands indicated that water surplus in the rooting zone was the primary growth-limiting factor. Drought stress reduced growth only in some extremely dry years.
  • • Results provide evidence that dominance of P. sylvestris on gravelly alluvial terraces is caused not only by tolerance of soil dryness and nutrient deficiency, as previously assumed, but also by a dynamic multilayered root system which allows adaptation to highly variable soil-moisture conditions.


Annual rings of trees are most valuable and long-term climate proxy data at an annual or even subannual resolution (for reviews see Fritts, 1976; Dean et al., 1996; Schweingruber, 1996). At the limits of tree growth, as at the Alpine treeline or within inner Alpine dry valleys, the year-to-year variability of ring widths primarily reflects variation in temperature and precipitation, respectively (LaMarche & Fritts, 1971; Eckstein & Aniol, 1981; Oberhuber et al., 1998; Rigling et al., 2001). Likewise, trees growing along riversides may be influenced by water-level fluctuations. The benefits of short-term flooding (the presence of water in soil in excess of field capacity) for growth of flood-tolerant species can be attributed to increased soil moisture and nutrient deposition, whereas tree growth of flood-intolerant species is hampered by inundation due to development of anaerobic conditions in the rooting zone. Water and nutrient supply through roots is reduced, and the hormonal balance governing root and shoot development can be disturbed (Kozlowski, 1982). In general, woody angiosperms tolerate flooding better than most gymnosperms, and inundation primarily impairs growth and development of flood-intolerant species if it occurs during the growing season (Gill, 1970; Kozlowski, 1984, 1997).

Within inner Alpine dry valleys, Scots pine (Pinus sylvestris) dominates not only on steep, south-facing slopes on stony, slow-weathering dolomite and lime-rich substrate, but also along riversides on very free-draining calcareous gravel beds (Ellenberg, 1988). These gravelly alluvial terraces, which are found in some river valleys of the limestone Alps and their northern foothills (Lech, Isar and Rhine valleys) are only occasionally subject to flooding, and are characterized by nutrient-deficient thin soils with low water-holding capacity. The dominance of poorly grown P. sylvestris stands at these sites is thought to be caused by tolerance of periodic drought stress and nutrient deficiency of the substrate (Seibert, 1958; Ellenberg, 1988; Müller & Bürger, 1990). Fluctuating water tables, however, might exert additional stress on growth of P. sylvestris, a species regarded to be flood-intolerant, as deep-reaching tap roots are subjected to frequent inundation during the growing season alternating with drained conditions and periodic drought.

As limiting factors of P. sylvestris growth have not yet been elucidated at alluvial limestone gravel terraces, a study was initiated to determine the influence of year-to-year variability in climate factors (precipitation and temperature) and water table on annual increments of this coniferous tree species. Dendroecological methods applied in this study allow a retrospective analysis of long-term data sets of radial tree growth.

Materials and Methods

Study area

The study area (approx. 900 m asl) is situated along the river Lech (Tyrol, Austria), which in upper reaches belongs to one of the last near-natural riverine ecosystems of the Alps. There Pinus sylvestris L. dominates on coarse, calcareous gravel beds (Fig. 1a). The plant community of selected stands is a Dorycnio–Pinetum sylvestris (Wallnöfer, 1993), whereby on most xeric sites with thin soils (soil depth < 5 cm), pioneer vegetation prevails in the ground flora and the canopy coverage reaches approx. 30%. According to the FAO soil unit classification scheme, soil types within the study area belong to Leptosols: very shallow soils in unconsolidated, very gravelly material.

Figure 1.

(a) Pinus sylvestris stands on a limestone gravel terrace at the bank of the River Lech (Tyrol, Austria). (b) Close-up view of alluvial terrace, approx. 2 m high, covered by variously aged P. sylvestris stands.

Field collection, sample preparation and chronology development

Plots were selected at alluvial terraces with varying distance from the river bank to obtain stands of diverse age (Fig. 1b). Sampled trees (n > 15 trees per plot) were free of major stem or crown anomalies such as lightning, wind or snow breakage, in order to reduce noisy signals in the ring-width series. Two core samples were extracted with an increment borer at approx. 30 cm above ground when trees were < 2 m height or at breast height (1.3 m) from opposite sides of each tree. Cores were mounted on grooved boards and the surface prepared with a sharp razor blade to produce flat surfaces on which ring boundaries were clearly defined under magnification (Pilcher, 1990). Ring widths were measured to the nearest 0.01 mm using an incremental measuring table. The correct dating of tree-ring series was checked using cofecha program (Holmes, 1994; Grissino-Mayer, 2001) which identified misdated segments within each ring series. Because of individual growth characteristics or missing rings, approx. 10% of total sample size (one to two trees per stand) could not be cross-dated accurately and were therefore dropped from the analysis. Missing rings occurred in < 1% of rings measured and did not occur on both tree radii of a given year.

Standard and residual chronologies were calculated using the arstan program (Cook & Holmes, 1984; Cook, 1985). A two-stage detrending procedure was chosen to remove most of the low-frequency variability in each ring series that is assumed to be unrelated to climate, such as tree ageing and forest stand development (Cook, 1987). In a first step, a negative exponential curve or a linear regression line was fitted to the ring series. In a second step, a cubic smoothing spline with a frequency–response cutoff set at two-thirds of the length of each series was applied. Dimensionless indices were formed by dividing the observed ring-width value by the predicted ring-width value.

Residual chronologies were produced accordingly with the residuals derived from the auto-regressive moving-average (ARMA) modelling with a robust mean value function applied to discount the effect of statistical outliers (Cook, 1985; Holmes, 1994). Residual chronologies are commonly used in dendroclimatic studies because removal of serial autocorrelation, which is generally considered to be biological in origin (e.g. through stored photosynthates; Fritts, 1976), is required for some statistical analyses.

Chronologies of stands showing comparable age structure were pooled provided a high cross-correlation among cores was determined. Two different cross-dating methods were applied to quantify the agreement among site chronologies (Wigley et al., 1987). The year-to-year ring-width changes can be simplified to a binary variable (an increase or decrease in ring width from one year to the next), and agreement quantified by counting the number of agreements and disagreements. Expressed as a percentage, this is referred to as the W statistic or ‘Gleichläufigkeit’ (Eckstein & Bauch, 1969). Agreement was also quantified parametrically using the product-moment correlation coefficient which, in turn, was adjusted for the amount of overlap between chronologies using the standard t statistic (Baillie & Pilcher, 1973).

Cambial age at sampling height and statistical indices of the standardized chronologies were calculated (Table 1). The standard deviation measures the variability of the measurements at all wavelengths. Mean sensitivity is a measure of the mean relative change between adjacent ring widths, and is calculated as the absolute differences between adjacent indices divided by the mean of the two indices. Higher values of mean sensitivity and higher standard deviations are indicative of more climatically responsive chronologies (Fritts, 1976). The first-order autocorrelation assesses relationships with prior growth. One of the most useful parameters for evaluating the quality of a chronology is the common variance among trees included in a chronology. Higher common variance accounted for by the first principal component indicates a greater climatic influence on tree growth (Fritts, 1976; Briffa & Jones, 1990).

Table 1.  Site description, characteristics of selected stands, and chronology statistics
Chronology codeAge class (approx. (yr)Mean distance to river (m)Above water level (m)Stand height (m)CC (%)n trees*Age (yr)RW (1/100 mm)MS (%)SDAutocorrVariance first EV (%)
  • CC, canopy coverage; RW, ring width, mean ± SD; MS, mean sensitivity; SD, standard deviation; Autocorr, First-order autocorrelation; EV, Eigenvector.

  • *

    Each tree with two radii from opposite sides.

  • Cambial age at c. 30 cm and 1.3 m above ground for stands B, G, and all other stands, respectively. Mean values (range: min/max).

  • Calculated on the basis of detrended ‘standard’ chronologies.

B 15 252.0 2.53016 11 (8/14)2.02 ± 0.31 90.071−0.2652
G 15 252.0 43013 14 (10/18)2.83 ± 0.34 80.080−0.2470
A 50 252.5 88016 49 (15/67)1.41 ± 0.64130.123 0.2439
F 50 503.5106016 62 (55/75)1.64 ± 0.32100.150 0.6249
D1502504.25166015144 (82/174)0.76 ± 0.16120.167 0.5742
C1501003.75208015147 (71/298)0.90 ± 0.37150.238 0.6345

Climate data sets and water-level records

Records of monthly maximum, minimum and mean water levels of the River Lech were available since 1976 from a hydrological station (Griesau), approx. 15 km from the study area. Water levels reached their peak during the summer months (Fig. 2), fluctuating between maximum and minimum values and prevailing at maximum level for a few days to a week. Hydrological conditions within the study area are in contrast to those in temperate oceanic zones, where precipitation and hence water levels of rivers and lakes are highest during winter (Walter & Lieth, 1960, 67).

Figure 2.

Long-term (1976–2002) mean monthly water level (maximum, mean and minimum) above zero point of water gauge (1014 m asl) and precipitation sum (dotted line) within the study area during the course of the year.

Since 1951, precipitation and temperature have been collected at meteorological stations in Forchach and Reutte (approx. 1.5 and 13 km from the study area), respectively. The rain gauge (ombrometer) was installed 1 m above ground. Temperature data were recorded 2 m above ground in free-standing double-louvred white screens (Stevenson screen). Long-term (1951–2002) mean annual precipitation was 1308 mm with a maximum during summer (June–August, 493 mm). Mean annual temperature reaches 6°C. Maximum water levels of the River Lech are reached in late spring and summer (May–August) because of the synergistic effect of snowmelt and increased precipitation in summer (Fig. 2).

Environmental influences on P. sylvestris growth

To identify the environmental factors most closely associated with variations in tree growth, Pearson product-moment correlation analysis (r) was calculated for relationships between climatic and water-level variables and residual ring-width chronologies. Response-function analysis, which is a form of multiple regression in which the predictor variables are principal components of monthly mean temperature and total precipitation values (Fritts, 1976), were not applied, as Blasing et al. (1984) argue that correlation functions are more easily replicated and also more stable than response functions. Environmental variables used in the correlation analysis included monthly mean temperature and total monthly precipitation for individual months May–December of the year before growth and January–August of the year of growth. Correlation coefficients must also be computed between ring indices and climate variables for several months before the growing season, because the width of an annual ring is an integration of climatically influenced processes occurring over a longer period (Fritts, 1976). The statistical relationship between ring width and each monthly climatic variable was examined over the period common to the chronology and the instrumental climatic record at the 95% confidence level.


Six site tree-ring chronologies (Fig. 3) were developed with mean tree ages at coring height ranging between approx. 15 and 150 yr (Table 1). Based on determined age structure, stands were grouped into three age classes whereby age classes 1–3 comprise stands showing mean tree ages of approx. 15, 50 and 150 yr, respectively. Mean sensitivity and autocorrelation coefficients within these age classes ranged from 8 to 15% and from negative values to 0.63, respectively. Principal components analyses on the individual samples from each chronology showed that the variance accounted for by the first component ranged between 39 and 70%. Chronologies of stands belonging to age classes 2 and 3 were pooled, which was justified by highly significant agreements (P < 0.001) in year-to-year ring width changes and t values considerably exceeding the minimum value for acceptable cross-dating match of 3.5 (Table 2).

Figure 3.

Ring-width chronologies of selected stands depicted for periods with sample depth (number of trees included in the chronologies)  10 trees. Capital letters denote chronology codes, as in Table 1.

Table 2.  Synchronization between developed Scots pine (Pinus sylvestris) chronologies
Site codeBGAFCD
  1. Agreement of the year-to-year ring width changes between standardized chronologies is quantified by the percentage of sign agreement W (significance level: ***, P < 0.001; **, P < 0.01; *, P < 0.05) and the product-moment correlation coefficient adjusted for the amount of overlap between chronologies using the standard t statistic (for details see Materials and Methods). Statistics are shown in the sequence W/t-value/overlapping years).

  2. ns, W not significant at P < 0.05 and t < 3.5.

G (100/100/18)nsnsns78*/3.1/18
A  (100/100/67)72***/5.4/6765**/6.1/6767**/5.6/67
F   (100/100/75)64**/7.3/7563*/5.8/75
C    (100/100/298)74***/9.9/174
D     (100/100/174)

Relationships between both climate variables (mean monthly and seasonal temperature; total monthly and seasonal precipitation) and fluctuations of the water level and radial growth, which were analysed by Pearson product-moment correlations, are shown in Tables 3 and 4. A highly significant (P < 0.001) inverse relationship exists between radial growth of P. sylvestris and maximum water level during spring and summer, whereby an age-dependent effect can be deduced. Whereas radial growth of very young stands (mean age approx. 15 yr) showed no response to year-to-year changing water levels, incremental growth of adult stands (age classes approx. 50 and 150 yr) was significantly limited by maximum water levels during spring–summer (P < 0.01). The closest relationship existed between May–June maximum water level and radial growth of stands with mean tree age approx. 50 yr, whereby r2 = 0.45 (45% of the year-to-year variability in ring width is explained by variation in water level). Precipitation, however, had no direct effect on radial growth, but was even severalfold inversely related to radial growth during summer (P < 0.05). Mean monthly temperature, however, sporadically exerted a direct influence on growth. The inverse relationship between precipitation during spring–summer and tree growth is mainly caused by the influence of precipitation on the water level.

Table 3.  Summary of the significant Pearson correlation functions (P < 0.05) based on residual chronologies showing effects of climate variables and maximum water level on ring-width indices of Pinus sylvestris for the period 1951–2002 (n = 52) and 1976–2002 (n = 27), respectively
Site code/ age classPrecipitation*Mean temperatureMaximum water level
  • Positive and negative correlations indicate that above-average tree growth is associated with above- and below-average values of the climate variable, respectively. Spurious correlations were detected and eliminated by evaluation of scatter plots.

  • *

    The correlation coefficient between July precipitation and July temperature was −0.54 (P < 0.001; n = 52).

B                  +                            
F   +                                         
D        +                                   
C                             +              
c. 50 yr                                           
c. 150 yr        +                    +          
Table 4.  Pearson correlation coefficients (r) between pooled age class residual chronologies and seasonal climatic variables (precipitation, mean temperature, water level) for the periods 1951–2002 (climate variables) and 1976–2002 (water level)
Climatic variableAge class (approx. yr)
  • *

    , P < 0.05;

  • **

    , P < 0.01;

  • ***

    , P < 0.001.

Spring temperature (Mar–May)−0.021−0.003
Summer temperature (Jun–Aug) 0.068 0.077
Spring–summer temperature (Mar–Aug) 0.012 0.030
May–Jun temperature−0.214−0.165
Spring precipitation−0.191−0.216
Summer precipitation−0.139−0.261
Spring-summer precipitation−0.221−0.327*
May–Jun precipitation−0.224−0.212
Spring maximum water level−0.416*−0.531**
Summer maximum water level−0.431*−0.600***
Spring–summer maximum water level−0.469**−0.626***
May–Jun maximum water level−0.667***−0.507**
May–Jun minimum water level−0.461*−0.394*
May–Jun mean water level−0.569**−0.382*

As shown in Fig. 4, periods of declining and rising water levels favour and limit radial growth of P. sylvestris, respectively. Maximum water levels exerted their influence on tree growth by a mean fluctuation of ±40 cm during spring–summer. In May 1999, extreme flooding caused radial growth to decrease strikingly, whereas in 2003, which was characterized by an extreme summer heatwave (Schär & Jendritzky, 2004), radial growth of adult stands (age classes 50 and 150 yr) increased.

Figure 4.

Time-series plot comparing variations in Pinus sylvestris growth with anomaly of spring–summer (March–August) maximum water level for the period 1976–2003. RW, detrended ring-width chronologies of age classes approx. 50 and 150 yr; WL, maximum water-level anomaly.

Although growth–climate relationship did not show a direct influence of precipitation on radial growth, a long-term comparison of ring series with those from drought-exposed inner Alpine sites revealed synchronous growth patterns. As depicted in Table 5, the oldest stand (age class 150 yr) showed significant agreement in year-to-year variability of radial growth with several stands from dry inner Alpine sites. Furthermore, distinct growth reductions, which were detected in all ring series in, for example, 1952, 1973 and 1976, can be related to drought periods and/or heatwaves documented in spring and/or summer (Fig. 5).

Table 5.  Synchronization between pooled age-class chronologies from the Lech Valley (this study) and inner Alpine drought-exposed sites within the Inn Valley (Oberhuber & Kofler, 2000, 2002)
Age class (approx. yr)Chronology codes (Inn Valley)*
  • Statistics of synchronization are given as in Table 2. Sample depth (number of trees included in the chronologies)  10 trees.

  • *

    Inner Alpine dry sites are approx. 55 (K101) and 30 km (K134, K135, K137) east-south-east and south-east, respectively, in linear distance off the study area.

Figure 5.

Comparison of high-frequency variations between detrended Pinus sylvestris ring-width chronologies from the study area (age classes approx. 50 and 150 yr) and an inner Alpine drought-exposed site (K130CHRI). Years when drought periods and/or heatwaves were recorded during spring and/or summer (Fliri, 1998) are indicated. Sample depth (number of trees included in the chronologies)  10 trees.


Growth limitation caused by soil dryness

Pinus sylvestris stands dominate in dry valleys of the inner Alps and in nutrient-deficient habitats such as raised bogs. They are also a natural element of floodplains on very free-draining calcareous gravel beds (Ellenberg, 1988). The water table in the gravelly subsoil is generally > 1–2 m below the surface and remains out of reach of the drought-tolerant ground flora. Dominance of P. sylvestris at these sites is therefore related to dry and poor soil conditions (Seibert, 1958; Müller & Bürger, 1990). It has been reported that drought stress in trees affects growth directly by reducing cell turgor and interfering with metabolism and cell enlargement, and indirectly by decreasing the synthesis of auxin and carbohydrates and slowing down their translocation to the cambium (Kramer, 1964; Zahner, 1968).

However, the inverse relationship found between precipitation and ring width during the growing season indicates that drought does not appear to play a major role in radial growth of trees within the study area. As temperature in July is directly related to growth at some sites, and considering that there is an inverse relationship between temperature and precipitation in July (P < 0.001), it can be deduced that warm and dry summers even favour growth. On the other hand, identical growth patterns (P < 0.001) between P. sylvestris ring series from this study and inner Alpine dry sites of the Inn Valley (Oberhuber et al., 1998; Oberhuber & Kofler, 2002), where annual precipitation is almost 50% lower, reveal that at alluvial terraces drought stress also exerts an influence on tree growth. Likewise, synchronous growth reductions in years showing spring and/or summer drought, as in 1952 and 1976, indicate an impact of a strained water balance on annual increments.

These contradictory results can be explained by the fact that one of the limitations of using mean monthly temperature and precipitation data is that short-term, extreme climatic events are not well represented. Stressful events such as drought often occur over a period of only several days or weeks, yet might substantially affect current year tree growth. Schweingruber (1996) critically discussed the significance of long-term climate-growth relationships (response functions; Fritts, 1976) without consideration of extreme climatic events.

Highest sensitivity of old trees (150-yr age class) to drought stress is possibly caused by decreasing ability to adapt adequately to fluctuations in the water table, or increasing water demand with tree age. However, differences in site conditions (e.g. soil texture and structure) that influence root growth through effects on root penetration and available water content and aeration cannot be excluded unequivocally.

Radial growth reductions that coincide with extreme drought periods might be related to growth inhibition of the fine root system in the upper soil layer (Kaufmann, 1968). Kramer (1983); Körner (1989); Irvine et al. (1998) suggest that extreme water deficits might cause damage to fine roots even though P. sylvestris is considered well adapted to drought-prone sites. Furthermore, a common response to soil drying is that root respiration gradually decreases (Bryla et al., 1997; Burton et al., 1998), which is part of the general decline in carbon assimilation and overall metabolism associated with slow growth. Root shrinkage under drying conditions can also reduce the uptake of water and nutrients (Faiz & Weatherley, 1982; Veen et al., 1992). Roots showing enhanced geotropism on exposure to drought (Sharp & Davies, 1979) will increase water uptake, but the root system developed at greater soil depths will suffer from low oxygen availability in years when the water table rises.

Growth limitation caused by high water table

It is well known that, on nitrogen-deficient soils with low water-holding capacity, P. sylvestris develops a ‘dual’ root system: deep-reaching tap roots and mycorrhiza-associated fine roots spreading just beneath the soil surface (Laitakari, 1929; Kalela, 1950; Köstler et al., 1968). Soil inundation, however, causes reduced root growth of most coniferous plants by inhibiting root formation and branching, growth of existing roots and mycorrhizal colonization, and inducing root decay (Theodorou, 1978; DeBell et al., 1984; Kozlowski, 1984; Stenstrom, 1991). Shallow root systems are characteristic of sites with high water tables, such as raised bogs (Kokkonen, 1923; Köstler et al., 1968; Lieffers & Rothwell, 1986), where tree growth is also favoured by a more continental climate. In this respect Ellenberg (1988) argues that longer-lasting and more pronounced dry periods bring about better aeration of the upper peat layers, which benefits the growth of roots and their mycorrhiza. An excess of soil water displaces air from the noncapillary pore space, producing oxygen deficiency and accumulation of CO2 which induces anaerobic decomposition of organic matter. Rouvinen et al. (2002) reported that flooding was the most conspicuous disturbance causing mortality of coniferous trees on flat areas near streams, lakes and mires. Although it has been reported that flowing water is less harmful to plant growth than stagnant water because of the higher O2 content of the former (Kozlowski, 1984), it can be assumed that, within the study area, stream velocity rapidly declines in gravel beds which develop parallel to the flow direction.

Our results suggest that the effects of water-level fluctuations on P. sylvestris radial growth depend to a large degree on the maximum water table. That high groundwater levels strikingly reduce aboveground stem growth is therefore related to the limited tolerance of coniferous roots of greater anoxia under an elevated water table (Yamamoto et al., 1987; Kozlowski et al., 1991) which, at alluvial terraces, can repeatedly develop and prevail for a few days to a week during the growing season. With temporary saturation of the soil by excess water, depletion of dissolved oxygen by respiration of roots and soil microorganisms may require only a few hours to days, thus detrimental effects to root growth and function occur before anaerobic conditions in the soil develop (Drew & Stolzy, 1991). Extensive growth inhibition by flooding during the warm season is associated with the high O2 requirements of growing roots with high respiration rates. Reduced cambial activity caused by flooding during the growing season has been reported for many flood-intolerant species (Kozlowski, 1997). Drainage of raised bogs also caused an instant increase in annual increments of coniferous species (Grünig, 1955; Dang & Lieffers, 1989; Linderholm, 1999) and forest expansion (Freléchoux et al., 2000). Inhibition of shoot growth and suppressed needle formation by flooding has also been demonstrated for other pine species (Hunt, 1951; Tang & Kozlowski, 1983).

Furthermore, wet soils are apt to be cold, which inhibits root growth and lowers the mineral salt and water uptake (Kramer & Kozlowski, 1960; Bowen, 1991). Loss of root biomass caused by flooding can also diminish drought tolerance afterwards, because root growth usually is reduced more than stem growth, which causes the root/shoot ratio to decrease and results in an inadequate water supply to the shoot by the partly dead or injured root system (Lyr & Hoffmann, 1967; Kozlowski et al., 1991). In addition, Kozlowski (1984) and Smit & Stachowiak (1988) report that flooding can result in increased resistance to water movement across the root cortex, and decreased water absorption. Flooding also reduces the number of mycorrhizal fungi, which are strongly aerobic, and suppresses formation of new mycorrhizal populations (Filer, 1975; Theodorou, 1978), which leads to reduced uptake of macronutrients (especially N, P and K) and higher sensitivity to drought (Davies et al., 1996; Kozlowski, 1997).

Hence the reductions in P. sylvestris radial growth that we observed may have been the result of impairment and frequent restructuring of the root system in response to flooding followed by a reduced tolerance of drought stress. Keeland & Sharitz (1997) also suggested that reduced growth of wetland species at periodically flooded sites may have resulted from damage to the root system in response to alternately flooded and drained conditions. Although flooding of soil during the growing season has repeatedly been shown to cause rapid and drastic reduction in the rate of photosynthesis of many tree species because of stomatal closure (Kozlowski & Pallardy, 1997), Zaerr (1983) found no significant response in net photosynthesis of P. sylvestris seedlings after 4 d flooding. The missing growth response of young trees (15-yr age class) is related to their less extensive and deep-reaching root system (Kalinin, 1983 cited by Polomski & Kuhn, 1998), their more pronounced regeneration capacity, and/or their minor water demand because of lower transpiration and hence generally lower sensitivity to drought stress.

We suggest that the dynamic dual root system (deep tap roots which respond to year-to-year fluctuations of the water table and mycorrhiza-associated fine roots which exploit nutrient-deficient xeric soils just beneath the surface) enables P. sylvestris to dominate at alluvial terraces, which are made up of coarse calcareous gravel characterized by poor and highly variable soil moisture conditions.


Water-table fluctuations alternating with soil drought during the growing period render alluvial terraces made up of calcareous gravel extreme environments for tree growth. Dendroecological analysis of the radial growth of P. sylvestris revealed that fluctuations in the water table exert a more persistent influence on growth than previously assumed (Ellenberg, 1988), indicating the importance of environmental constraints on belowground organs and their effect on the success and survival of the tree (Körner & Paulsen, 2004).

Finally, this study has shown that ring series of P. sylvestris growing on alluvial terraces are suitable for developing a dendrohydrological model to reconstruct water-level fluctuations of this inner Alpine river for the past 200 yr (Jones & Briffa, 1984; Woodhouse, 2001).


We thank two anonymous reviewers for valuable suggestions and comments on improving the manuscript. This study was supported in part by Tiroler Landesregierung, Abt. Umweltschutz, which is greatly acknowledged. Climate and hydrological data were provided by Zentralanstalt für Meteorologie and Hydrographischer Dienst, Innsbruck, respectively.