Fine root dynamics of mature European beech (Fagus sylvatica L.) as influenced by elevated ozone concentrations

Environmental Pollution 157 (2009) 2638–2644

Contents lists available at ScienceDirect

Environmental Pollution journal homepage: www.elsevier.com/locate/envpol

Fine root dynamics of mature European beech (Fagus sylvatica L.) as influenced by elevated ozone concentrations Raphael Mainiero a, *, Marian Kazda a, Karl-Heinz Ha¨berle b, Petia Simeonova Nikolova b, Rainer Matyssek b a b

Department for Systematic Botany and Ecology, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany ¨t Mu ¨ nchen, Ecophysiology of Plants, Department of Ecology, Am Hochanger 13, 85354 Freising, Germany Technische Universita

Doubling of ozone concentrations in mature European beech affected the seasonal timing of fine root turnover rather than the turnover rate.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 December 2008 Received in revised form 20 April 2009 Accepted 3 May 2009

Fine root dynamics (diameter < 1 mm) in mature Fagus sylvatica, with the canopies exposed to ambient or twice-ambient ozone concentrations, were investigated throughout 2004. The focus was on the seasonal timing and extent of fine root dynamics (growth, mortality) in relation to the soil environment (water content, temperature). Under ambient ozone concentrations, a significant relationship was found between fine root turnover and soil environmental changes indicating accelerated fine root turnover under favourable soil conditions. In contrast, under elevated ozone, this relationship vanished as the result of an altered temporal pattern of fine root growth. Fine root survival and turnover rate did not differ significantly between the different ozone regimes, although a delay in current-year fine root shedding was found under the elevated ozone concentrations. The data indicate that increasing tropospheric ozone levels can alter the timing of fine root turnover in mature F. sylvatica but do not affect the turnover rate. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Fine root growth Mortality Longevity Soil environment Fagus sylvatica

1. Introduction Fine roots are the most important plant organs for the acquisition of water and nutrients. Because uptake capacity is highest in young root segments and depletion zones have not yet developed (Ha¨ussling et al., 1988; Van Praag et al., 1993; Volder et al., 2005), the formation of new fine roots plays an important role in influencing a plant’s overall water and nutrient uptake capacity (Caldwell and Richards, 1989; Eissenstat and Caldwell, 1988; Hodge, 2004). Perennial plants therefore continuously display fine root turnover, i.e. form new fine roots while replacing older, inefficient ones. In temperate forests, fine root dynamics (i.e. growth and mortality) are adapted to the seasonally changing soil conditions, as fine root growth roughly follows the seasonal course of soil temperature (Tierney et al., 2003).

* Corresponding author. Permanent address: Institute for Applied Plant Biology, Sandgrubenstrasse 25, 4124 Scho¨nenbuch, Switzerland. Tel.: þ41 61 481 32 24; fax: þ41 61 481 34 36. E-mail addresses: [email protected] (R. Mainiero), [email protected] (M. Kazda), [email protected] (K.-H. Ha¨berle), [email protected] (P.S. Nikolova), [email protected] (R. Matyssek). 0269-7491/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2009.05.006

Fine root turnover, in terms of biomass, is determined by the standing biomass and the turnover rate, the latter being a function of the fine root longevity. Many tree species are characterized by low fine root longevity that therefore relieves the considerable resource costs of fine root turnover (about 1/3 of the total carbon (C) is invested belowground; (Hendricks et al., 2006; Lauenroth and Gill, 2003; Ostonen et al., 2005)). As the timing of fine root dynamics changes in relation to soil environmental conditions and so does fine root longevity, turnover is a crucial factor for the root system’s efficiency in resource acquisition and, therefore, plant functioning (Eissenstat and Yanai, 1997; Yanai et al., 1995). Fine roots as heterotrophic organs depend on the C supply from the shoot. One factor that alters C supply is tropospheric ozone (O3). O3 has become a widespread air pollutant of increasing global importance (Ashmore, 2005; Fowler et al., 1999; Sitch et al., 2007; Vingarzan, 2004) primarily affecting leaf physiology and, as a consequence, tending to limit belowground carbon allocation (Andersen, 2003; Ashmore, 2005; Matyssek and Sandermann, 2002). The current knowledge on the effects of this on fine roots, however, mostly relies on static parameters such as standing biomass, neglecting the potential relevance of changes in fine root longevity and dynamics (Andersen, 2003).

R. Mainiero et al. / Environmental Pollution 157 (2009) 2638–2644

European beech (Fagus sylvatica L.) is an ecologically and economically important broadleaved tree species in Central Europe. On the basis of several root parameters studied in beech saplings, fine roots turned out to be amongst the most sensitive tree organs under elevated O3 impact (Braun and Flu¨ckiger, 1995; Landolt et al., 2000; Paludan-Mu¨ller et al., 1999). Also in mature beech, C investment into the root system was lowered under increased O3 concentrations (Braun et al., 1999; Thomas et al., 2002). Alterations in fine root longevity of F. sylvatica were recently shown to contribute to changes in fine root turnover (Meier and Leuschner, 2008). However, knowledge is scarce about the influence of elevated ground-level O3 on the temporal dynamics of fine root turnover in mature F. sylvatica trees. Using minirhizotrons, the present study focuses on the dynamic performance of the fine root system of mature F. sylvatica. Fine root dynamics were analysed under stand conditions in plots with ambient or twice-ambient O3 concentrations (Nunn et al., 2002). It was hypothesized that: (i) elevated O3 concentrations increase the longevity and decrease the turnover rate of fine roots, so that the C need of the fine root system is reduced. (ii) Given the temporal variation in C supply to the root system (Wardlaw, 1990), the increased O3 load alters the seasonal pattern of fine root dynamics, and, as a consequence, the relationship changes with the soil environment (soil temperature and moisture). 2. Materials and methods 2.1. Site description The study site is located in a managed mixed forest ‘‘Kranzberger Forst’’ close to Freising, Germany (48 250 N, 11 390 E). Climatic and edaphic conditions are summarized in Table 1. Soils in this area developed from quaternary loess (60–80 cm) covering a layer of gravel and clay. Soil layering is homogenous. The study was carried out in a patch of mature beech (ca. 50 trees, 400 m2). At the time of the study, trees were ca. 70 years old and reached more than 25 m in height. European beech is assumed to constitute the autochthonal vegetation at this site (Galio-odorati-Fagetum). Understorey vegetation was scarce with some individuals of Rubus fruticosus L. and Carex brizoides L. indicating high N availability and slightly impeded drainage. 2.2. Ozone fumigation Experimental free-air O3 fumigation was performed at the site (‘‘Kranzberg Ozone Fumigation Experiment’’, KROFEX; (Karnosky et al., 2007; Nunn et al., 2002) as the ambient O3 regime was measured above and within the adjacent canopy of five untreated trees that served as controls (1  O3). Within the canopy of another five neighbouring trees, the ambient O3 regime was experimentally doubled (2  O3) on a half-hourly basis. A detailed description of the experimental set-up and distribution of ozone levels is provided by Nunn et al. (2002). O3 concentrations in the 2  O3 regime were restricted to below 150 nl l1 to prevent acute injury. The fumigation started in April 2000 and continued throughout the growing seasons (April/May to October) of the following years until 2007. O3 regimes were expressed as SUM0 (i.e. the sum of all O3 concentrations) and AOT40 (i.e. the sum of O3 concentrations above 40 nl l1). In 2004, the study year, the fumigation resulted in an increase of 63% and 264% above control levels, respectively for SUM0 and AOT40 (Table 2). 2.3. Root observation Fine root dynamics was studied using minirhizotrons as has been described previously (Mainiero and Kazda, 2006). Eight minirhizotrons, grouped in four pairs, Table 1 Site characteristics at the study site ‘‘Kranzberger Forst’’. Altitude Mean annual precipitation (1970–2000) Mean annual temperature Exposition Soil Soil type Humus form Soil water pH

490 m a.s.l. 786 mm 7.8  C 1.8 N Haplic luvisol Loamy/silty clay Moder 3.8–4.5

2639

Table 2 Cumulative ozone dose at the experimental site ‘‘Kranzberger Forst’’ in the study year 2004. Cumulative ozone dose is provided as SUM0, i.e. the sum of all ozone concentrations, and AOT40, i.e. the accumulated exposure above a threshold value of 40 nl l1. Data are taken from Lo¨w et al. (2006).

SUM0 (ppm h) AOT40 (ppm h)

1  O3

2  O3

142.7 17.3

232.6 63.0

were installed in April 2002 in the 1  O3 and 2  O3 regimes. Tubes were made of glass with a total length of 50 cm and an inner diameter of 50 mm. Minirhizotrons in the present study were installed vertically (Smit et al., 2000). Images were obtained using a rigid borescope with a cold light source (Olympus, Hamburg, Germany) and a fixed digital camera (Olympus Camedia C-5050). Imaging was done at six equal depths ranging from 3.4 cm to 41.6 cm belowground and in four directions each (i.e. 48 images per minirhizotron pair). Each image covered 13.14 cm2 resulting in a total area observed of 631 cm2 per minirhizotron pair. Distances between the minirhizotrons of the 1  O3 regime and those under the 2  O3 plot ranged between 15 and 20 m. Root observation was done from January 2003 to February 2005 with targeted time intervals of 14 days during the growing season and 28 days during winter. Root length measurements were performed for the time period November 2003 to December 2004 (i.e. 371 days). 2.4. Root data As wide-angle imaging of a curvilinear inner tube surface results in strongly distorted images, root length in the present study was determined using the line intersect method (Tennant, 1976; Upchurch, 1987). A 1  1 mm grid, identically distorted, was superimposed on the minirhizotron images (Adobe Photoshop 7.01) counting the number of quadrants intersected by fine roots. The system was calibrated by applying the same procedure on lines of known length that were photographed with the same minirhizotron/borescope combination. Orientation and curvature of the lines were random, as the lengths of the lines were similar to those of the fine roots depicted (4–20 mm). Linear regression revealed a nearly 1:1 relationship between counted quadrants (c) and length (L) (L ¼ 1.04  c, p < 0.001, R2 ¼ 0.96, n ¼ 246). This equation was used to convert quadrant counts to root length. Similarly, a 0.5  0.5 mm grid was used to assess fine root diameter (<1 mm). Raw data for length-related parameters were assessed cumulatively for each image. Standing root length density (RLD) was determined by calculating the root length and relating it to the image area. Fine roots were assigned depending on the time of emergence (t) or disappearance (t þ 1). Fine roots that disappeared from the minirhizotron surface were defined as dead. If disappearance was a gradual process, fine roots were labelled as dead as they started shrivelling, indicated by changes in the shape or the contrast to the background. Accordingly, fine root mortality rate (rm) was derived by relating the respective sum of root length to the image area and the time interval between sampling dates (mm m2 day1). Area-related fine root growth (i.e. length increment by formation of new roots plus root elongation) was derived from the following relationship: RLDtþ1 ¼ RLDt þ RLDg  RLDm ð1Þ or RLDg ¼ RLDtþ1  RLDt þ RLDm ð2Þ where RLDg and RLDm are the root length growth and root mortality, respectively, between the sampling dates at t and t þ 1 (mm day m2). Root growth rate (rg) was then calculated by relating RLDg to the time interval between sampling dates (mm m2 day1). Because of non-steady state conditions, the fine root length turnover rate was calculated according to Meier and Leuschner (2008) as the ratio between total root growth and mortality within the 371-day study period (m m1). In addition, a survival analysis was performed on the basis of root number (total of 188 roots). Only fine roots that appeared during the growing season of 2004 (May 26, 2004–August 19, 2004) were considered. To this end, non-mycorrhizal fine roots and ectomycorrhizal fine roots were analysed separately. The latter were distinguished according to the presence of a hyphae mantle and/or typical branching as ‘‘Christmas tree like structures’’. Mycorrhizal roots consisted entirely of short laterals. If losses of the hyphae mantle occurred, mycorrhizal roots were defined as dead. Censoring of fine roots was progressively (Lee, 1992). 2.5. Soil water and soil temperature Fine root dynamics was studied in parallel to the seasonal courses of soil temperature and soil water content (% vol.), as the respective sensors were placed between each pair of minirhizotrons. Soil temperature probes (Delta T, Burwell, UK) were installed at 5 cm and 25 cm depth. Soil water content was measured using time

2640

R. Mainiero et al. / Environmental Pollution 157 (2009) 2638–2644

domain reflectometry (TDR probes type ML2x, Delta T Devices, Burwell, UK). In order to facilitate installation and therefore to minimize disturbance, TDR probes were installed in an angled position (45 ). In this way they integrated soil water content between 13 cm and 17 cm depth. Soil temperature and water content data were logged continuously at 30 min intervals by a data logger system (data logger DL2, Delta T, Burwell, UK). Between 28th December 2003, and 17th March 2004, low temperatures caused breakdowns in the energy supply for the 2  O3 data logger, so that soil data were not sampled continuously. Accordingly, data for regression analysis were derived by interpolation between the respective sampling dates.

a

2.6. Statistical analysis Each pair of minirhizotrons was treated as one replication and data were pooled for each pair (n ¼ 4). Root growth rate and mortality rate were calculated for all six depth levels and differences were tested using the Kolmogorov–Smirnoff test. Differences in the fine root length turnover rate were examined by t-test. Relationships between fine root dynamics and soil parameters were tested using multiple linear regression. In order to account for vertical gradients in soil water content (Torreano and Morris, 1998), only root data from the soil layer next to the TDR probe (i.e. 10–15.2 cm) were considered. Therefore, soil temperature was set as the mean of the two probes (5 cm and 25 cm). Survivorship functions, i.e. the proportion of fine roots surviving until time (t), were compared using the Gehan–Breslow test. This test was used in order to account for prolonged imaging intervals during late seasons. Cohorts were built for time periods early (26th May/15th June 2004), central (30th June/21st July 2004) and late in the growing season (4th August/19th August 2004). There was no significant difference for fine root survival between cohorts in either of the two O3 regimes. Therefore, survivorship functions are presented cumulatively for the three cohorts. Statistical tests were performed using the software packages ‘‘Statistica’’, Rel. 6.1 (StatSoft Inc., Tulsa, Okla.) or ‘‘SigmaStat’’, Rel. 3.1 (SysStat Sofware Inc.), respectively. For all tests, a level of significance for p < 0.05 was used.

b

3. Results 3.1. Soil water and soil temperature During the study period, soil water content and soil temperature showed typical seasonal variations (Fig. 1). Soil temperature ranged between 1.3 and 16.1  C under 1  O3 and 2.1–15.7  C under 2  O3 (means each between 5 cm and 25 cm soil depth). Soil water status was fairly constant until June/July 2004. Then, a pronounced summer drought occurred with soil water content reaching the limit for plant availability between August and October. Soil water recovered to moderate values in late autumn. 3.2. Fine root growth Under 1  O3 and 2  O3, fine root growth and fine root mortality appeared simultaneously (Fig. 2a). Under 1  O3, the fine root growth rate showed a considerable intra-annual variation ranging between 1.0  0.4 and 17.1  4.9 mm m2 day1 (mean  1SE). Here, root growth showed a pronounced maximum in early summer (June/July), i.e. a time period that was characterized by high soil water availability and rising soil temperature. The root growth rate rapidly decreased during August/September, although increasing again in late autumn to a secondary maximum. Root growth rate under 2  O3 showed a similar variation (1.9  1.6–17.6  5.5 mm m2 day1, mean  1SE) and did not differ significantly from 1  O3. The temporal patterns, however, differed considerably between the two O3 regimes as the root growth rate under 2  O3 peaked during August/September, whereas a minimum was found in June/July. As a result, root growth rate in the two plots showed different relationships to the measured edaphic parameters. Under 1  O3, a significant correlation between fine root growth rate and soil temperature was found, whereas soil water content did not appear as a significant parameter in the model (Table 3). Conversely, a statistically significant relationship between root growth rate and the soil parameters was absent under 2  O3.

Fig. 1. Soil temperature (a) and soil water content (b) in the 1  O3 and the 2  O3 plot during the study period from December 2003 to December 2004. Temperature values are given for 1  O3 (solid line) and 2  O3 (dashed line). Points in (b) give mean soil water content at the root observation dates (1 SE). The horizontal line in (b) indicates the level at which soil water is no longer available to the plant (14% vol.).

3.3. Fine root mortality Fine root mortality rate also showed a large intra-annual variation (1  O3: 2.0  0.6–28.6  10.5 mm m2 day1; 2  O3: 1.4  1.0–24.9  20.5 mm m2 day1; means  1SE) which did not differ significantly between the two O3 regimes (Fig. 2b). Moreover, the temporal course of fine root mortality was similar under both O3 regimes with mortality rates reaching maxima in early summer (June) and during autumn (September/October). Temporal patterns of fine root growth and mortality under 1  O3 were very similar as indicated by significant correlation (p < 0.01, r ¼ 0.44). In contrast, under 2  O3, no significant relationship was found between fine root growth and mortality. Irrespective of the O3 regime, fine root mortality significantly correlated with soil parameters (Table 3). Accordingly, fine root mortality scaled positively to soil temperature and soil water content. The correlation between root mortality rate and soil parameters was stronger under 2  O3. 3.4. Fine root longevity and fine root turnover rate The turnover rate across the 371-day period amounted to 0.57 m m1 under 1  O3 and did not differ significantly from that under 2  O3 (0.62 m m1). Cohort analysis across all root types for each O3 regime revealed a median longevity of 77 days under 1  O3 and 91 days under 2  O3 in the absence of a statistically

R. Mainiero et al. / Environmental Pollution 157 (2009) 2638–2644

a

a

b

b

Fig. 2. Fine root growth rate and fine root mortality rate for F. sylvatica during the study period. Data refer to all six depth levels (3.4 cm to 41.6 cm) and are given as means  1 SE.

significant effect. Mycorrhizal fine roots of both O3 regimes were short-lived with median longevity of 70 days and 77 days under 1  O3 and 2  O3, respectively (Fig. 3a). For non-mycorrhizal fine roots, the survivorship function was higher under elevated O3 for most of the study period (Fig. 3b; median longevity: 1  O3 ¼ 112 days, 2  O3 ¼ 170 days). However, the survivorship Table 3 Significance and parameter estimates for multiple linear regression in the form y ¼ a  T þ b  W þ c, where y, T, W and c are the fine root mortality or growth rate, soil temperature, soil water content and intercept, respectively. Root growth rate and mortality rate implied in the model refer to the soil depth next to the measurements of soil water content (i.e. 10–15.2 cm). Results for the models and parameters are presented only for statistically significant data with a ¼ 0.05. Regression coefficient B Root growth rate 1  O3 Model a (soil temperature) b (soil water content) c (intercept) 2  O3 Model Root mortality rate 1  O3 Model a (soil temperature) b (soil water content) c (intercept) 2  O3 Model a (soil temperature) b (soil water content) c (intercept)

1.23 – 5.64

2641

Significance

R2

<0.001 <0.01 n.s. >0.10

0.20

n.s.

<0.01 <0.05 <0.001 <0.05

0.18

4.07 3.15 81.04

<0.001 <0.01 <0.001 <0.001

0.32

2.50 2.27 60.3

Fig. 3. Survival probability for (a) ectomycorrhizal fine roots (n ¼ 86) and (b) non-mycorrhizal fine roots (n ¼ 102). Survival analyses include all fine roots that appeared in the time period from May 26, 2004 to August 19, 2004. Data were censored progressively.

function under elevated O3 declined rapidly for ageing roots. Accordingly, survival of non-mycorrhizal fine roots did not differ significantly between the O3 regimes although shedding of newly formed fine roots was delayed under elevated O3. 4. Discussion 4.1. Fine root longevity and dynamics under ambient O3 concentrations Fine root turnover is a common characteristic of perennial plants, with large variation in fine root longevity across species and environments (Eissenstat and Yanai, 1997; Lauenroth and Gill, 2003). Minirhizotron studies performed in mono-specific stands of mature trees revealed a range of median fine root longevity from 30 days in Populus sp. (Block et al., 2006) to ca. 400 days in Picea abies (Majdi and Andersson, 2005). The value of 77 days found in this study for mature F. sylvatica therefore falls into the lower part of the species range. In the present study, only the growing season was considered for longevity estimates. According to Andersson and Majdi (2005), the values found will differ from a whole year approach but seem to mirror true values more reliably. Indeed, as about one half of the variation in fine root longevity can be attributed to inherent differences in species (Peek, 2007), low fine root longevity values, i.e. much lower than 1 year, seem to be characteristic for mature F. sylvatica. In beech forests, the intra-annual variation of soil moisture and temperature results in largely co-varying availability of water and

2642

R. Mainiero et al. / Environmental Pollution 157 (2009) 2638–2644

nutrients (Cecchini et al., 2002; Chang and Matzner, 2000; Ritter, 2005). In addition, a fine root’s water and nutrient uptake capacity varies throughout the year as a result of varying soil water and temperature status (Coners and Leuschner, 2005; Marschner et al., 1991). Thus, given the low fine root longevity in F. sylvatica, total resource uptake by fine roots is maximized only if their lifespan coincides with favourable soil environmental conditions, i.e. high temporal precision of fine root growth (Hodge, 2004). Indeed, in accordance with previous studies dealing with mature F. sylvatica (Ladefoged, 1939; Mainiero and Kazda, 2006), the present work showed that fine root growth took place at very different rates. As was shown for other temperate forest ecosystems (Tierney et al. (2003) and citations therein), fine root growth roughly tracked the seasonal course of soil temperature, whereas soil water content did not appear as a significant determinant. As a result, new fine roots were produced preferentially in early summer (June/July), i.e. a time period with rising soil temperature and prior to summer drought. Thus, under 1  O3, a remarkable coincidence existed between the appearance of new fine roots and favourable soil conditions. In addition, there was a coincidence with the seasonal course of canopy transpiration, with the latter driving the mass flow of nutrients to the root surface (Coners and Leuschner, 2005; Marschner et al., 1991; Schipka et al., 2005). The temporal pattern of fine root growth found under the 1  O3 regime thus should exert a positive influence on the potential water and nutrient uptake of the whole plant. However, fine root mortality scaled positively with soil temperature and soil water content. Determining the factors that control the timing of fine root shedding is still a matter of active research (Eissenstat and Yanai, 2002). In their study on fine root dynamics in Q. ilex, however, Lo´pez et al. (2001) suggested recent fine root growth as a trigger for fine root shedding. Accordingly, older and therefore less efficient fine roots were not maintained as new roots were produced, resulting in a simultaneous increase. Likewise, the present study found that fine root mortality and growth were positively correlated. The relationship between recent fine root growth and mortality, as suggested by Lo´pez et al. (2001), therefore appears to exist in F. sylvatica too, and may explain the counterintuitive finding of increased fine root mortality under favourable soil conditions. Regarding survival analysis, only roots produced during the current year were considered, providing insight into age-specific mortality. Accordingly, the rapid decline in survivorship function for young roots and the low fine root longevity regardless of mycorrhization indicate rapid replacement also of current-year roots. In conclusion, F. sylvatica accelerated fine root turnover under 1  O3 during favourable conditions by increasing fine root growth in concert with fine root mortality. 4.2. Fine root dynamics under elevated O3 concentrations For F. sylvatica, recent studies showed that fine root longevity varies with environmental conditions (Meier and Leuschner, 2008; Nikolova, 2007; Peek, 2007). In contrast to the first hypothesis, however, neither fine root survival nor turnover rate responded significantly to elevated O3 concentrations in the canopy. However, in accordance with the second hypothesis, distinct differences emerged if the temporal patterns were considered. Indeed, fine root mortality and growth occurred concurrently and in the same range in both plots. Contrasting with 1  O3, however, the relationship between fine root growth and mortality was obscured under 2  O3. The findings are consistent with those of Kelting et al. (1995), who studied the effect of twice-ambient O3 concentrations on fine root dynamics in Quercus rubra. They found elevated O3 concentrations affected the annual fine root biomass production

and mortality although the corresponding ratio between both root parameters stayed unchanged. Hence, mean fine root longevity apparently did not change. However, the O3 treatment caused an asynchrony between root production and mortality (Kelting et al., 1995). More recently, in their long-term study on young Populus tremuloides, Pregitzer et al. (2008) also found no significant influence of elevated O3 on fine root survival. In the present study, the temporal pattern of fine root mortality was similar for the two O3 regimes. However, assessing the time of root death by means of minirhizotrons is problematic as morphological rather than physiological changes are used as criteria for root death. Changes in morphology due to root death necessarily involve parts of the decomposition process (Comas et al., 2000). Therefore, O3-induced differences in the temporal pattern of fine root mortality might still have existed but the corresponding soil environmental conditions (e.g. drought, frost) delayed root decomposition until morphological changes were detected. Such bias is not possible for fine root appearance indicating that the differences between the two O3 regimes were most pronounced for fine root growth. Change in root growth may have also caused the differences in median fine root longevity for non-mycorrhizal fine roots. Accordingly, the delay in shedding of current-year fine roots appears to compensate for the lowered fine root growth rate at concurrently unaltered mortality in early summer. Subsequently and consistent with the root dynamics under 1  O3, the large emergence of new fine roots in late summer allowed replacement of ageing fine roots thus causing the rapid decrease in the survivorship function of ageing fine roots. As a result of an altered fine root growth pattern under 2  O3, the apparently fundamental relationship between fine root growth rate and soil temperature (Tierney et al., 2003) vanished. Root growth under elevated O3 thus took place under different soil environmental conditions and only roughly matched the seasonal course of nutrient availability or canopy transpiration. Rather, the growth of new root segments peaked during summer drought. Conversely, the regression model revealed increased fine root mortality during favourable conditions. Such altered temporal pattern of fine root turnover likely decreases the potential benefits (water and nutrients). Changes in root development were suggested as one reason for decreased nutrient supply in plants exposed to elevated O3 concentrations (Le Thiec et al., 1995). No apparent difference in fine root longevity was found for ectomycorrhizal fine roots. In the same trees and during the same study period, however, significant differences were found concerning the ectomycorrhizal community structure, as the fungal species diversity was enhanced under 2  O3 (Grebenc and Kraigher, 2007). Therefore, the survivorship functions presumably averaged over different fungal communities and the large number of fungal species (Grebenc and Kraigher, 2007) might have obscured O3-induced differences in fine root lifespan. However, the studies of Gorissen et al. (1991) and Andersen and Rygiewicz (1995) indirectly support the present finding. The authors studied C-partitioning in tree saplings infected with only two or one fungal species, respectively. Elevated O3 concentrations decreased the overall C allocation to the mycorrhizal fungus but when normalized for biomass, 14CO2 release did not differ between O3 regimes. Specific respiration of mycorrhizal fine roots hence did not change (Andersen and Rygiewicz, 1995; Gorissen et al., 1991). As the latter correlates with fine root longevity (Eissenstat et al., 2000; Tjoelker et al., 2005), O3 fumigation presumably did not influence the lifespan of mycorrhizal fine roots. The present study covered a 1-year period with significant ozone-induced effects on leaf parameters (Lo¨w et al., 2006). However, as the influence of O3 can vary with climatic conditions and duration of fumigation (Ashmore, 2005; Nunn et al., 2005;

R. Mainiero et al. / Environmental Pollution 157 (2009) 2638–2644

Pregitzer et al., 2008), O3-induced differences in fine root dynamics might also vary between years. Moreover, the present study focused on fine root turnover rate. Annual fine root construction costs, however, are determined by the respective biomass turnover, which is the product of turnover rate and standing biomass (Hendricks et al., 2006). Altered fine root biomass turnover was suggested by other studies on the same site which found evidence for enhanced turnover within the ectomycorrhizal network (Grebenc and Kraigher, 2007) and stimulated fine root production under 2  O3 (Nikolova, 2007). In accordance with the results of Kelting et al. (1995) and (Pregitzer et al., 2008) however, the present study precludes alterations in fine root turnover rate as an agent for altered fine root biomass turnover in mature F. sylvatica. 5. Conclusions The present study indicates that elevated O3 concentrations can influence the temporal pattern of fine root dynamics in F. sylvatica. As a result, the relationship between fine root dynamics and seasonal changes in the soil environment are altered accordingly. As there was no indication of a compensatory change in root morphology or respiration rate (Nikolova, 2007), the alterations might lower the fine root system’s efficiency (Eissenstat and Yanai, 1997, 2002). Acknowledgements This study was part of the SFB 607 research centre ‘‘Growth and parasite defence’’ and was financed by the German Science Foundation (DFG) (grant no. Ka941/61,2). We gratefully acknowledge this support. References Andersen, C.P., 2003. Source-sink balance and carbon allocation below ground in plants exposed to ozone. New Phytologist 157, 213–228. Andersen, C.P., Rygiewicz, R.T., 1995. Allocation of carbon in mycorrhizal Pinus ponderosa seedlings exposed to ozone. New Phytologist 131, 471–480. Andersson, P., Majdi, H., 2005. Estimating root longevity at sites with long periods of low root mortality. Plant and Soil 276, 9–14. Ashmore, M.R., 2005. Assessing the future global impacts of ozone on vegetation. Plant, Cell and Environment 28, 949–964. Block, R.M.A., Van Rees, K.C.J., Knight, J.D., 2006. A review of fine root dynamics in Populus plantations. Agroforestry Systems 67, 73–84. Braun, S., Rihm, B., Schindler, C., Flu¨ckiger, W., 1999. Growth of mature beech in relation to ozone and nitrogen deposition: an epidemiological approach. Water, Air and Soil Pollution 116, 357–364. Braun, S., Flu¨ckiger, W., 1995. Effects of ambient ozone on seedlings of Fagus sylvatica L. and Picea abies (L.) Karst. New Phytologist 129, 33–44. Caldwell, M.M., Richards, J.H., 1989. Competing root systems: morphology and models of absorption. In: Givnish, T.J. (Ed.), On the Economy of Plant Form and Function. Cambridge Univ. Press, Cambridge, pp. 251–273. Cecchini, G., Carnicelli, S., Mirabella, A., Mantelli, F., Sanesi, G., 2002. Soil conditions under Fagus sylvatica CONECOFOR stand in Central Italy: an integrated assessment through combined solid phase and solution studies. Journal of Limnology 61 (Suppl. 1), 36–45. Chang, S.-C., Matzner, E., 2000. Soil nitrogen turnover in proximal and distal stem areas of European beech trees. Plant and Soil 278, 117–125. Comas, L.H., Eissenstat, D.M., Lakso, A.N., 2000. Assessing root death and root system dynamics in a study of grape canopy pruning. New Phytologist 147, 171–178. Coners, H., Leuschner, C., 2005. In situ measurements of fine root water absorption in three temperate tree species – temporal variability and control by soil and atmospheric factors. Basic and Applied Ecology 6, 395–405. Eissenstat, D.M., Caldwell, M.M., 1988. Seasonal timing of root growth in favourable microsites. Ecology 69, 870–873. Eissenstat, D.M., Wells, C.E., Yanai, R.D., Whitbeck, J.L., 2000. Building roots in a changing environment: implications for root longevity. New Phytologist 147, 33–42. Eissenstat, D.M., Yanai, R.D., 1997. The ecology of root lifespan. Advances in Ecological Research 27, 1–60. Eissenstat, D.M., Yanai, R.D., 2002. Root life span, efficiency, and turnover. In: Waisel, Y., Eshel, A., Kafkafi, U. (Eds.), Plant Roots the Hidden Half. Marcel Dekker Inc., New York, pp. 221–238.

2643

Fowler, D., Cape, J.N., Coyle, M., Flechard, C., Kuylenstierna, J., Hicks, K., Derwent, D., Johnson, C., Stevenson, D., 1999. The global exposure of forests to air pollutants. Water, Air and Soil Pollution 116, 5–32. Gorissen, A., Joosten, N.N., Jansen, A.E., 1991. Effects of ozone and ammonium sulphate on carbon partitioning to mycorrhizal roots of juvenile Douglas fir. New Phytologist 119, 243–250. Grebenc, T., Kraigher, H., 2007. Changes in the community of ectomycorrhizal fungi and increased fine root number under adult beech trees chronically fumigated with double ambient ozone concentration. Plant Biology 9, 279–287. Ha¨ussling, M., Jorns, C.A., Lehmbecker, G., Hecht-Buchholz, C., Marschner, H., 1988. Ion and water uptake in relation to root developments in Norway Spruce (Picea abies (L.) Karst.). Journal of Plant Physiology 133, 486–491. Hendricks, J.J., Hendrick, R.L., Wilson, C.A., Mitchell, R.J., Pecot, S.D., Guo, D., 2006. Assessing the patterns and control of fine root dynamics: an empirical test and methodological review. Journal of Ecology 94, 40–57. Hodge, A., 2004. The plastic plant: root responses to heterogenous supplies of nutrients. New Phytologist 162, 9–24. Karnosky, D.F., Werner, H., Holopainen, T., Percy, K., Oksanen, T., Oksanen, E., Heerdt, C., Fabian, P., Nagy, J., Heilman, W., Cox, R., Nelson, N., Matyssek, R., 2007. Free-air exposure systems to scale up ozone research to mature trees. Plant Biology 9, 181–190. Kelting, D.L., Burger, J.A., Edwards, G.S., 1995. The effects of ozone on root dynamics of seedlings and mature oak (Quercus rubra L.). Forest Ecology and Management 79, 197–206. Ladefoged, K., 1939. Untersuchungen u¨ber die Periodizita¨t im Ausbruch und La¨ngenwachstum der Wurzeln bei einigen unserer gewo¨hnlichsten Waldba¨ume. Det forstige Forsørgsvœsen i, Denmark, Copenhagen. Landolt, W., Bu¨hlmann, U., Bleuler, P., Bucher, J.B., 2000. Ozone exposure-response relationship for biomass and root/shoot ration of beech (Fagus sylvatica), ash (Fraxinus excelsior), Norway spruce (Picea abies) and Scots pine (Pinus sylvestris. Environmental Pollution 109, 473–478. Lauenroth, W.K., Gill, R., 2003. Turnover of root systems. In: de Kroon, H., Visser, E.J.W. (Eds.), Root Ecology. Springer-Verlag, Berlin, pp. 61–83. Le Thiec, D., Dixon, M., Garrec, J.P., 1995. Distribution and variations of potassium and calcium in different cross sections of Picea abies (L) Karst and Fagus sylvatica (L) leaves exposed to ozone and mild water stress. Annals of Forest Science 52, 411–422. Lee, E.T., 1992. Statistical Methods for Survival Analysis. John Wiley & Sons Inc., New York. Lo´pez, B., Sabate´, S., Gracia, C., 2001. Fine-root longevity of Quercus ilex. New Phytologist 151, 437–441. Lo¨w, M., Herbinger, K., Nunn, A.J., Ha¨berle, K.-H., Leuchner, M., Heerdt, C., Werner, H., Wipfler, P., Pretzsch, H., Tausz, M., Matyssek, R., 2006. Extraordinary drought of 2003 overrules ozone impacts on adult beech trees. Trees 20, 539–548. Mainiero, R., Kazda, M., 2006. Depth-related fine root dynamics of Fagus sylvatica during exceptional drought. Forest Ecology and Management 237, 135–142. Majdi, H., Andersson, P., 2005. Fine root production and turnover in a Norway Spruce stand in northern Sweden: effects of nitrogen and water manipulation. Ecosystems 8, 191–199. Marschner, H., Ha¨ussling, M., George, E., 1991. Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [Picea abies (L.) Karst.]. Trees 5, 14–21. Matyssek, R., Sandermann, H., 2002. Impact of ozone on trees: an ecophysiological perspective. Progress in Botany 64, 349–403. Meier, I., Leuschner, C., 2008. Genotypic variation and phenotypic plasticity in the drought response of fine roots of European beech. Tree Physiology 28, 297–309. Nikolova, P.S., 2007. Below-ground Competitiveness of Adult Beech and Spruce Trees: Resource Investments versus Returns. Technische Universita¨t Mu¨nchen. Nunn, A.J., Reiter, I.M., Ha¨berle, K.-H., Werner, H., Langebartels, C., Sandermann, H., Heerdt, C., Fabian, P., Matyssek, R., 2002. ‘‘Free-air’’ ozone canopy fumigation in an old-growth mixed forest: concept and observation in beech. Phyton (Horn, Austria) 42, 105–119. Nunn, A.J., Reiter, I.M., Ha¨berle, K.-H., Langebartels, C., Bahnweg, G., Pretzsch, H., Sandermann, H., Matyssek, R., 2005. Response patterns in adult forest trees to chronic ozone stress: identification of variations and consistencies. Environmental Pollution 136, 365–369. Ostonen, I., Lo¨hmus, K., Pajuste, K., 2005. Fine root biomass, production and its proportion of NPP in a fertile middle-aged Norway spruce forest: comparison of soil core and ingrowth core methods. Forest Ecology and Management 212, 264–277. Paludan-Mu¨ller, G., Saxe, H., Leverenz, J.W., 1999. Responses to ozone in 12 proverances of European beech (Fagus sylvatica): genotypic variation and chamber effects on photosynthesis and dry-matter partitioning. New Phytologist 144, 261–273. Peek, M.S., 2007. Explaining variation in fine root longevity. Progress in Botany 68, 382–398. Pregitzer, K.S., Burton, A.J., King, J.S., Zak, D.R., 2008. Soil respiration, root biomass, and root turnover following long-term exposure of northern forests to elevated atmospheric CO2 and tropospheric O3. New Phytologist 180, 153–161. Ritter, E., 2005. Litter decomposition and nitrogen mineralization in newly formed gaps in a Danish beech (Fagus sylvatica) forest. Soil Biology and Biochemistry 37, 1237–1247. Schipka, F., Heimann, J., Leuschner, C., 2005. Regional variation in canopy transpiration of Central European beech forests. Oecologia 143, 260–270. Sitch, S., Cox, P.M., Collins, W.J., Huntingford, C., 2007. Indirect radiative forcing of climate change through ozone effects on the land-carbon sink. Nature 448, 791–794.

2644

R. Mainiero et al. / Environmental Pollution 157 (2009) 2638–2644

Smit, A.L., George, E., Groenwold, J., 2000. Root observations and measurements at (transparent) interfaces with soil. In: Smit, A.L., Bengough, A.G., Engels, C., Van Noordwijk, S., Pellerin, S., Van de Geijn, S.C. (Eds.), Root Methods. SpringerVerlag, Berlin, pp. 235–256. Tennant, D., 1976. A test of a modified line intersect method of estimating root length. Journal of Ecology 63, 995–1001. Thomas, V.F.D., Hiltbrunner, E., Braun, S., Flu¨ckiger, W., 2002. Changes in root starch contents of mature beech (Fagus sylvatica L.) along an ozone and nitrogen gradient in Switzerland. Phyton (Horn, Austria) 42, 223–228. Tierney, L.T., Fahey, T.J., Groffman, P.M., Hardy, J.P., Fitzhugh, R.D., Driscoll, C.T., Yavitt, J.B., 2003. Environmental control of fine root dynamics in a northern hardwood forest. Global Change Biology 9, 670–679. Tjoelker, M.G., Craine, J.M., Wedin, D., Reich, P.B., Tilman, D., 2005. Linking leaf and root trait syndromes among 39 grassland and savannah species. New Phytologist 167, 493–508. Torreano, S.J., Morris, L.A., 1998. Loblolly pine root growth and distribution under water stress. Soil Science Society of America Journal 62, 818–827.

Upchurch, D.R., 1987. Conversion of Minirhizotron-root Intersections to Root Length Density. ASA, Madison, WI. Van Praag, H.J., Sougnez-Remy, S., Waterkeyn, L., 1993. L’absorption-translocation spe´cifique d’e´le´ments nutritifs en relation avec la structure anatomique des radicelles de heˆtre (Fagus sylvatica) et d’e´pice´a (Picea abies) I. Mesures de l’absorption-translocation spe´cifique. Belgian Journal of Botany 126, 175–183. Vingarzan, R., 2004. A review of surface O3 background levels and trends. Atmospheric Environment 38, 3431–3442. Volder, A., Smart, D.R., Bloom, A.J., Eissenstat, D.M., 2005. Rapid decline in nitrate uptake and respiration with age in fine lateral root of grape: implications for root efficiency and competitive effectiveness. New Phytologist 165, 493–502. Wardlaw, I.F., 1990. The control of carbon partitioning in plants. New Phytologist 116, 341–381. Yanai, R.D., Fahey, T.J., Miller, S.L., 1995. The efficiency of nutrient acquisition by fine roots and mycorrhizae. In: Smith, W.K., Hinckley, T.M. (Eds.), Resource Physiology of Conifers: Acquisition, Allocation and Utilization. Academic Press, San Diego, pp. 75–103.