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Drought responses of Arrhenatherum elatius grown in plant assemblages of varying species richness
Acta Oecologica 39 (2012) 11e17
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Original article
Drought responses of Arrhenatherum elatius grown in plant assemblages of varying species richness Dennis Otieno a, *, Juergen Kreyling b, Andrew Purcell c, Nadine Herold d, Kerstin Grant b, John Tenhunen a, Carl Beierkuhnlein b, Anke Jentsch b a
Department of Plant Ecology, University of Bayreuth, D-95440 Bayreuth, Germany Department of Biogeography, University of Bayreuth, D-95440 Bayreuth, Germany Biological Sciences, University of York, Heslington, York YO10 5DD, UK d Max Planck Institute for Biogeochemistry, Hans-Knöll-Str. 10, 07745 Jena, Germany b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2011 Accepted 31 October 2011 Available online 22 November 2011
Evidence exists that plant community diversity influences productivity of individual members and their resistance and resilience during and after perturbations. We simulated drought within the long-term EVENT experimental site in the Ecological-Botanical Garden, University of Bayreuth to understand how Arrhenatherum elatius (L.) responds to water stress when grown in three different plant assemblages. The set up consisted of five replications for each factorial combination of drought and plant assemblages differing in functional diversity. Leaf water potential (JL), leaf gas exchange, natural d13C, plant biomass and cover were measured. Imposed drought had different effects on A. elatius, depending on plant assemblage composition. Severe water stress was however, avoided by slowing down the rate of decline in JL, and this response was modified by community composition. High JL was associated with high stomatal conductance and leaf photosynthesis. Biomass production of A. elatius increased due to drought stress only in the least diverse assemblage, likely due to increased tillering and competitive advantage against neighbors in the drought-treated plants. Our results indicate that beneficial traits among plant species in a community may be responsible for the enhanced capacity to survive drought stress. Resistance to drought may, therefore, not be linked to species richness, but rather to the nature of interaction that exists between the community members. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Arrhenatherum elatius Drought stress Productivity Plant assemblages Species interactions
1. Introduction The relationship between plant species richness and productivity has long been recognized (Naeem et al., 1994; Tilman et al., 1997; Hector et al., 1999) and is thought to be a central component of community ecology (Pfisterer and Schmid, 2002; Bruno et al., 2003). Controversies however, surround the nature of the interactions between community complexity and stability, or resistance to stress and disturbance on the one hand and productivity on the other hand (Loreou and Hector, 2001; Hooper et al., 2005; Roscher et al., 2008). Currently, the influence of changing environmental conditions, such as altered precipitation patterns and climate warming, on the stability of plant performance and community interactions provides a novel research frontier. Initial evidence suggests that drought events may have substantial
* Corresponding author. Tel.: þ49 921 552325; fax: þ49 921 552564. E-mail address: [email protected] (D. Otieno). 1146-609X/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.actao.2011.10.002
ecological impact on productivity, belowground biotic processes, phenology and community invasibilty (Jentsch and Beierkuhnlein, 2008; Kreyling et al., 2008; Mirzae et al., 2008). Yet, the ecological importance of extreme weather events, such as drought, is expected to increase in the near future in many parts of the world (IPCC, 2007). There is however, a substantial lack of knowledge regarding how such events, particularly in terms of their altered frequency and magnitude, will affect physiological performance, species interactions and ecosystem functioning (Jentsch et al., 2007; Jentsch and Beierkuhnlein, 2008). Ecological interactions are bound to vary with changes in the abiotic environment. A number of studies indicate that positive effects become stronger as abiotic stress increases (Bartness and Yeh, 1994; Greenlee and Callaway, 1996). It has been demonstrated that the stability of community functions, in the face of abiotic stress or extreme events, becomes stronger as community diversity increases (Hector et al., 1999; Jentsch et al., 2009; van Ruijven and Berendse, 2010). Interactions among plants in a community have generally been shown to shift from competition
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to facilitation, depending on the prevailing abiotic conditions, and gains from biodiversity could be the result of complementarity in resource use (Caldiera et al., 2001; Michalet et al., 2006) and facilitation (Hooper and Vitousek, 1998; Hector et al., 1999). Better understanding of the ecophysiological mechanisms of resource use underlying these associations of competition versus facilitation in plant interactions could help define the nature of relationships among plant community attributes. Since water stress is one of the main factors constraining productivity and diversity in most plant communities (Boyer, 1982; Pugnaire and Luque, 2001; Fridley, 2002), an understanding of water utilization by members of a community may help elucidate some of the underlying mechanisms associated with the positive relationships between species richness and productivity, as well as survival. Most ecophysiological studies on plant responses to drought focus on leaf level measurements, individual performance and possible links to entire plant structure changes in growth strategy. On the other hand, in community ecology, various concepts, such as the insurance hypothesis (Yachi and Loreau, 1999), address the role of biodiversity in buffering the effects of disturbance on community performance. Yet, these concepts fail to take account of the role of community composition on individual plant-level response to disturbance. Consequently, more studies are still needed in order to understand the coupling between individual species responses and community composition and structure. Here, we report on stress response in a common European grassland species, Arrhenatherum elatius (L.), as influenced by the composition of its nearest neighbors (‘community’). We assume that the above-mentioned mechanisms, which have primarily been discussed at the plant community level so far, also account for stability of performance at the species level, i.e. of a single widespread species within mixtures of different species composition. We hypothesized that; A. elatius individuals in species-rich mixtures are more resistant to drought stress, maintaining higher tissue water potentials, leaf gas exchange and productivity.
2. Materials and methods 2.1. Experimental site This research was conducted in the EVENT-I experiment (Jentsch et al., 2007) in the Ecological Botanical Gardens of the University of Bayreuth, Germany (49 5501900 N, 11340 5500 E, 365 m a.s.l.). The site is characterized by a mean annual temperature of 8.2 C, and a mean annual precipitation of 724 mm (1971e2000). Precipitation is distributed bi-modally with a major peak in June/ July and second peak in December/January (data: German Weather Service). Here, we focused on the drought tolerance of A. elatius, a widespread and agriculturally important European grass species (Michalski et al., 2010), in three different species assemblages. The experiment was carried out with two fully crossed factors: (1) precipitation (drought, control) and (2) species assemblage (A. elatius in combination with one other grass species (G2), with one other grass and two herb species (G4), and with one other grass and two herb species of which one was a legume (G4þ), see Table 1). The total set up consisted of 5 replicates of each factorial combination, 30 plots in total of 2 m 2 m in size. The factors were applied in a randomized block design with the species assemblages blocked and randomly assigned within each rainfall manipulation (Jentsch et al., 2007). The texture of the previously homogenized and constantly drained soil body consisted of loamy sand (82% sand, 13% silt, 5% clay), with pH ¼ 4.5 in the upper (0e20 cm depth) and pH ¼ 6.2 in the lower (20e80 cm depth) soil. Data acquisition
Table 1 Plant assemblage compositions for A. elatius in the EVENT I experiment. The G4 mixture lacks Lotus, which is a legume. Abbreviation
Species richness
Functional diversity
Species
G2
2
G4
4
1 functional group (grass) 2 functional groups (grass, herb)
G4þ
4
Arrhenatherum elatius, Holcus lanatus Arrhenatherum elatius, Holcus lanatus, Plantago lanceolata, Geranium pratense Arrhenatherum elatius, Holcus lanatus, Plantago lanceolata, Lotus corniculatus
3 functional groups (grass, herb, legume herb)
was carried out in the central square meter of each plot only, in order to circumvent edge effects. The drought manipulation simulated a 100-year extreme drought event in accordance with local climate data series, i.e. 32 days without rainfall induced by one rainout shelter per droughttreated block. The rainout shelters were constructed with a steel frame (6 m by 8 m, center height: 3 m; Hochtunnel, E & R Stolte GmbH, Germany), and covered with a transparent plastic sheet (material: 0.2 mm polyethylene, SPR 5, Hermann Meyer KG, Germany), with 95% PAR transmission and set up only for the manipulation period. Any lateral surface flow was avoided by plastic sheet pilings around treatment blocks and around each single plot reaching down to a depth of 10 cm. Strong greenhouse effects resulting from the shelters were avoided by starting the roof at 80 cm height and allowing for near-surface air exchange. The plots were also oriented in the common wind direction to facilitate air circulation under the roof. Control plots were watered regularly using portable irrigation systems. These were applied in equal parts during the manipulation period to ensure high water supply so that the plants did not suffer any water stress during the experimental period. The drought treatment lasted from May 21st (DOY 141) to June 21st (DOY 172) in the year 2007. Five widespread grassland species were chosen from the regional flora. Species were selected with respect to their affiliation to defined functional groups (grasses, herbs, legumes), life-span (perennials), their overall importance in nearby and Central European grassland systems, and based on the fact that they do naturally grow on substrate similar to the one used in this experiment. 100 plant individuals per plot in defined quantitative composition were planted in a systematic hexagonal grid, with 20 cm distance between individuals, in early April 2005. Density between individuals was, therefore, constant. Grass and herb individuals used in the experiment were grown from seeds in a greenhouse in the preceding fall. The communities were established at two levels of species diversity (2 and 4 species) and three levels of functional diversity (1, 2, 3 functional groups), thus resulting in three species combinations, or plant assemblages, in total (Table 1). Once installed, these species compositions were maintained by periodic weeding. 3. Measurements 3.1. Microclimate Weather conditions at the experimental site were continuously monitored from a climate station constructed 50 m away in an open location, so as to avoid any interference from the constructions. Parameters measured included air humidity and air temperature (Fischer 431402 sensor, K. Fischer GmbH, Drebach, Germany) and photosynthetic active radiation (LI-190 Quantum sensor, LI-COR,
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i h Rsample =Rstandard 1 1000;
USA) at 2 m above the ground surface. Data was recorded every 30 s, averaged and stored every 30 min using a data logger (DL2e, Delta-T Devices Ltd., Cambridge, UK). Daily precipitation was measured with a tipping bucket rain gauge (precipitation sensor 7041, Theodor Friedrichs & CO), while soil water content (SWC) within the G4 plots was monitored continuously using FD sensors (Echo EC5; Decagon, USA) installed at 5e10 cm soil depth. Installation depth for the sensors was based on a previous study, which showed that about 70% of the roots are restricted to the upper 5 cm of the soil and 96% to the upper 15 cm (Kreyling et al., 2008). Additional information on SWC was obtained from a TDR tube access probe (Diviner 2000, Sentek technologies, USA), that was used to read soil moisture in previously installed tubes at 12, 20 and 27 cm depth in all the plots at the end of the drought manipulation.
expressed in units of per thousand (&). 13C/12C ratios were calculated against the P.D. Belemite Standard (precision of 0.2&). The results were compared with other measurements to determine changes associated with shifts in 13C. Every measurement was replicated two times and the accuracy in d-values was better than 0.1&. Biomass estimation: On July 2nd (DOY 183) all the aboveground plant material (biomass) from four A. elatius individuals per plot was harvested for biomass determination. The aboveground biomass was oven-dried at 80 C for a period of 48 h before weighing.
3.2. Plant water status
3.5. Determination of plant cover
Midday leaf water potential (JL) was measured on three replicate leaves of A. elatius per plot using a portable pressure chamber (PMS Instruments Co. Corvallis, OR, USA). During measurements, the leaves were cut while enclosed in a plastic bag to reduce further moisture loss during transfer and fixing into the chamber. Moist tissue paper was introduced into the chamber to reduce water loss during the measurements. Measurements were confined to the period between 12:00 and 14:00 when lowest water potentials were expected. We focused on midday leaf water potential rather than pre-dawn leaf water potential as a better indicator of productivity since higher leaf water potentials promote stomatal conductance and photosynthesis.
A pin-point method was used to determine species-specific plant cover by counting the number of hits on plant organs by one hundred pins that were inserted vertically, 10 cm apart, into a 1 m2 quadrat. The results were treated as percentage (%) cover. Measurements were carried out on May 10th (DOY 130), June 26th (DOY 177) and September 2nd (DOY 245).
3.3. Leaf gas exchange Leaf gas exchange parameters, namely leaf transpiration (E), leaf stomatal conductance (gs) and leaf photosynthesis (A) were monitored between DOY 130 and DOY 170. A series of weekly measurements were carried out using a portable gas exchange system (LI-6400, LI-Cor, Lincoln, NE). A set of 3 grass tufts on each plot were identified and marked for measurements. On any single measurement day, 2-3 suitable leaf blades were selected from each of the tufts per plot and were set parallel in the cuvette, with their upper surfaces well exposed so that they were fully illuminated during measurements. Each set of measurements lasted one to 2 min; once a steady state was attained, 10 readings were logged per measurement at 10 s intervals. The selected leaves were marked and similar leaves were monitored either during midday (12:00 to 14:00) or throughout the day (from sunrise to sunset), when diurnal course measurements were conducted. The measured leaves were then excised at the end of measurement period and the leaf area (LA) of the section of leaf enclosed in the cuvette was then determined using a leaf area meter (CI-202 CID, Camas, WA). LA information was used to standardize the leaf gas exchange data. 3.4. d13C determination On DOY 183, a set of three fully matured leaves of A. elatius from every plot was selected. In each plot (factorial combination of watering regime and plant mixture), two sun exposed leaves of five individual plants were sampled and combined. The samples were oven-dried for 48 h at 80 C. The dry leaves were ball-milled and sub-samples of 1 mg were analyzed for d13C with an elemental analyzer attached to an isotope ratio mass spectrometer using ConFlo III interface (Laboratory of Stable Isotopes, UFZ, Leipzig, Germany). The carbon isotope composition (d13C) of a sample was calculated as follows:
d13 C ¼
4. Statistical analyses Linear mixed effects models, combined with analysis of variance (ANOVA), were applied to test for significant differences between groups at single points of time. The split-plot design was also taken into account by using block identity as a random factor. Homogeneous groups of factor combinations (drought manipulation, plant mixture) were identified by Tukey HSD post hoc comparisons. The level of significance was set to p < 0.05. For time series, linear mixed-effects models were employed to test for effects of drought manipulation and the respective interactions of plant mixtures, while taking the split-plot design and the repeated measurements into account (time was used as additional random factor). Prior to statistical analysis, data was log- or square-root-transformed, if conditions of normality were not met. This was also done to improve homogeneity of variances. Both characteristics were tested by examining the residuals versus fitted plots and the normal qqplots of the linear models. All statistical analyses were performed using R (R Development Core Team, 2010). 5. Results Mean daily PAR recorded between May and June, during the time when the treatments were carried out, was 40 mol m2 d1 (Fig. 1a). These values are about 3e5% higher than the PAR values recorded within the rainout shelters, based on parallel PAR measurements with the gas exchange measurement equipment, LI6400. Near-surface air temperature in the drought-treated plots was slightly (wþ1.3 C) increased by the roofs during the relatively short rainfall manipulation period of 32 days. We assumed that this didn’t influence the plant physiology significantly. Rainout shelters effectively prevented rainwater from reaching the drought-treated plots (Fig. 1c and d) and significantly lowered SWC (Fig. 1b). Due to equipment failure at the onset of experimentation, soil moisture data did not cover the whole rainfall manipulation period. Data from subsequent years however, indicate that drought stress builds up slowly and that the period with soil water content dropping below the permanent wilting point was almost completely covered in the study year (Jentsch et al., 2011). Compared to the wellwatered (control) plots, soil moisture at the end of the drought treatment was significantly lower at all soil depth levels (12 cm:
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Fig. 2. Midday leaf water potentials (JL) of A. elatius grown in drought-treated and in well-watered plots with varying plant mixture diversity levels. Results are shown for measurements conducted 10 days, DOY 152 (a), and 22 days, DOY 170 (b), after the commencement of drought treatments. Bars are SE. Statistics are shown on the right.
Fig. 1. Daily photosynthetic active radiation (PAR) (a), Soil water status in wellwatered and drought-treated plots (b), and precipitation amounts on well-watered (c), and drought-treated plots (d). Precipitation in well-watered plots was compensated through irrigation in order to replace the amount of water that was lost through evapotranspiration. A solid bar in Fig. 1d indicates the period when drought was imposed.
F ¼ 310.5; p < 0.001; 20 cm: F ¼ 397.4; p < 0.001; 27 cm: F ¼ 107.9; p < 0.001) and no differences were observed among the plant assemblages (12 cm: F ¼ 0.1; p ¼ 0.917; 20 cm: F ¼ 0.5; p ¼ 0.590; 27 cm: F ¼ 0.4; p ¼ 0.673) and the interaction of both factors (12 cm: F ¼ 0.0; p ¼ 0.995; 20 cm: F ¼ 0.0; p ¼ 0.981; 27 cm: F ¼ 0.1; p ¼ 0.889). Imposed drought stress resulted in a decline in leaf water potential (JL) in A. elatius (Fig. 2), except in the G4 plant assemblages. These minimum JL values however, remained stable 22 days after the imposition of drought stress (Fig. 2b). The fourspecies mixture (G4) experienced the highest JL (midday JL ¼ 1.6 MPa), followed by the four-species mixture containing legumes (G4þ) (1.8 MPa). The lowest JL (1.9 MPa) occurred in the two-species mixture (G2) (Fig. 2b). Midday leaf water potential of the control treatments showed mixed responses among the members of an assemblage at different times. On clear sky days (PAR > 1000 mmol m2 s1), control plants exhibited higher leaf gas exchange compared to their counterparts in the drought-treated plots (Fig. 3, DOY 165). Measurements conducted on DOY 170, a cloudy day when mean PAR values of 10 mol m2 s1 were recorded, showed no significant difference in gas exchange parameters among the A. elatius individuals in the
control and drought-treated plots. Instead, leaf assimilation (A), stomatal conductance (gs) and leaf transpiration (E) declined in most of the treatments, likely due to low radiation. Significant (p < 0.001) differences in shoot d13C signals resulting from rainfall manipulation and also plant mixture were observed (Fig. 4). Generally, more negative d13C values occurred in the wellwatered plants compared to the drought-treated plants. The lowest d13C signal in the well-watered plants was 29& and occurred in the G4 mixture, while highest was 28.3& and occurred in the G2 mixture. The lowest and highest d13C values in the droughttreated plants were 28.6& and 27.6& in the G4 and G4þ mixtures respectively. d13C signals were significantly lower in the A. elatius individuals grown in the G4 mixture than in the other 2 communities. On average, biomass per individual in the well-watered treatments was lower than in the drought-treated plants (Fig. 5). Individuals in the G2 mixture accumulated significantly (p < 0.001) more biomass than the other two plant assemblages. The highest biomass per individual was found in the G2 mixture subjected to water stress, while no significant difference was found between all other factorial combinations. Results from ground cover measurements showed higher cover in the drought-treated than in the nonstressed plants (Fig. 6). At the end of the growing period, individuals of A. elatius subjected to drought had more tillering than the non-stressed ones, thus resulting in higher ground cover, irrespective of the assemblage composition. In summary, interaction effects between rainfall manipulation and community composition in affecting the stress response of A. elatius were significant for JL and aboveground biomass. However, interaction effects were not significant for leaf gas exchange.
6. Discussion We tested the hypothesis that species diversity increases the resistance to drought among A. elatius individuals growing in plant assemblages, and that this may improve their survival chances and
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Fig. 3. Midday leaf CO2 assimilation, stomatal conductance and transpiration of A. elatius grown in well-watered and drought-treated plots with varying plant mixture diversity levels. Values are mean of measurements conducted between 12:00 and 14:00 h. Bars are SD.
productivity during drought. As expected, imposed drought resulted in a significant reduction in leaf water potential (JL) of A. elatius by about 0.6 MPa. This was in response to a 30% decline in soil moisture content (down to 5%-vol), which demonstrates the ability of A. elatius to resist rapid decline in JL during drought. This resistance may be achieved through intrinsic characteristics that lead to improved water uptake during drought (Knapp, 1984;
Barker et al., 1993; Asbjornsen et al., 2008; Saha et al., 2009), or increased resistance to water loss through stomatal regulation (Jones, 1992; Fay et al., 2002; Chen et al., 2009). Since we observed a general response in all of the treatments, irrespective of plant mixture composition, retention of favorable tissue water status during drought, as observed in this species, points to an inherent adaptation in A. elatius. Such an inherent characteristic may, however, be modified by species interactions within assemblages. Soil moisture status was monitored within the 5e27 cm soil depth in this study, yet grasses are known to extend their roots to depths greater than 30 cm, which remain moist for longer during drought (Le Roux and Bariac, 1998; Asbjornsen et al., 2008). Although we did not examine plant root distribution in our study, a separate study conducted on the same plots reported no roots of A. elatius below 15 cm depth (Kreyling et al., 2008), thus eliminating the chances of deep rooting and water uptake from the deeper soil
Fig. 4. d13C determined on leaf samples obtained from well-watered and droughttreated plots with different plant community composition. Samples were obtained on DOY 184 after the end of drought-treatment. Bars are SE.
Fig. 5. Biomass of A. elatius grown in well-watered and drought-treated plots with varying plant species composition and species richness. The results are means of 4 harvested individuals per plot. Biomass harvest was conducted on DOY 184.
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Fig. 6. Plant cover estimates of A. elatius grown in well-watered and drought-treated plots. Cover was estimated at three times during the growth of the plants. The last estimate is performed on re-growth after the first harvest on DOY 184. Bars are SE. Note that the rainfall manipulation started two years before the year displayed, explaining the differences in cover before the manipulation period.
layers as possible reasons for the slow decline in JL of A. elatius during drought. In most plant species, favourable tissue water status is translated into bioproductivity through increased stomatal conductance and CO2 assimilation (Saha et al., 2009). Plants that exhibit higher leaf water potentials are, therefore, likely to exhibit higher CO2 assimilation rates than their counterparts with low tissue water status. Considering leaf gas exchange measurements conducted on DOY 165, in the middle of drought and when the vegetation was mature and actively photosynthesizing, well-watered individuals of A. elatius, with higher JL, exhibited higher stomatal conductance and photosynthesis compared to the stressed plants. Equally, A. elatius plants grown in the G4 mixtures had higher stomatal conductance and assimilation rates compared to their counterparts in the other two plant mixtures. These results are supported by the d13C measurements, which showed the well-watered plants, as well as stressed A. elatius in the G4 mixture, to have less enriched tissues. This suggested that these plants carried out gas exchange (or had wide-open stomata) over extended periods of time during experimentation (Ehleringer and Cooper, 1988; Farquhar et al., 1989). These results discount stomatal closure as one reason for the observed favourable JL in A. elatius grown in the G4 mixture, but support the fact that maintenance of high water potential leads to increased leaf stomatal conductance and photosynthesis during drought. Soil moisture declined uniformly in all the plots during drought. Consequently, the differences in JL arising during drought, as observed between A. elatius grown in the G4 and the other plant assemblages, could only be attributed to the interactions among members of this assemblage. Competitive balance between the members of the different assemblages appears to be the key for this response, with an inferior competitor in the G4 assemblage (Geranium pratense produced 4 g m2 in the drought and 1 g m2 in the control treatment) being replaced by a more dominant competitor in the G4þ assemblage (Lotus corniculatus: 204 g m2 in drought and 154 g m2 in the control treatment). Species identity and interactions rather than raw numbers determine species performance. Specifically, A. elatius in the G4 mixture with less competitive pressure is better placed to avoid water stress, continue photosynthesizing and survive longer during drought
(Sperry et al., 2002). The ability to survive longer is of great significance and could make real differences as far as generation continuity is concerned. We recommend more detailed investigations to identify mechanisms that could contribute to the observed response of A. elatius in the G4 assemblage. High photosynthetic rate over an extended period of time is likely to lead to higher biomass accumulation. Interestingly, biomass production did not strongly shift due to the watering regime, but was more associated with species compositions. Leaf level results did not reflect the total biomass accumulated in the different treatment plots. Similar results were reported by Kreyling et al. (2008). Integrated results showed that well-watered A. elatius, and also those grown in the G4 mixture, exhibited higher stomatal conductance and photosynthesis, and hence were expected to be more productive. During drought however, A. elatius increased its tillering (high ground cover), thus accounting for the unusually high biomass in the drought-treated plants. Such responses were not captured by leaf-level measurements, which were limited to the individual leaves. Although A. elatius grown in the G4 mixture showed higher resistance to drought, its biomass production per individual was highest in the species poor G2 mixture. There was also no clear link between species richness and biomass production, both in the perturbed and non-perturbed systems. Resistance to drought however, will ensure survival of the species during perturbation (Naeem, 2002). According to Yachi and Loreau (1999), resistance to stress should increase with species richness. Pfisterer and Schmid (2002) however, found no definite link between resistance and community diversity, but observed that productivity was linked to diversity in the absence of perturbation. In our study, the lowest biomass of A. elatius was recorded in plant assemblages with highest diversity and vice versa. Our results are similar to those reported by Fridley (2002). There was also no clear relationship between resistance to drought and species richness. Instead, our results showed that drought resistance in A. elatius may depend on the nature of association among its nearest neighbors. These results, therefore, point to the significant role played by the nature of plant interactions, rather than simple species numbers, in conferring stability during drought. The scope of measurements however, did not allow us to identify reasons for high JL in A. elatius, and how it is modified by community association. We could also not link leaf level responses to total aboveground production. Further investigations are recommended in order to identify the specific traits that are involved, and to validate some of the results. Author contributions Dennis Otieno: Designed and carried out the measurements, carried out data analysis and wrote the manuscript. Andrew Purcell, Nadine Herold, Kerstin Grant and Juergen Kreyling carried out measurements. Juergen Kreyling also helped with statistical analysis and revised the manuscript. Anke Jentsch wrote and revised the manuscript and also sought project funding. John Tenhunen and Carl Beierkuhnlein read and revised the manuscript. References Asbjornsen, H., Shepherd, G., Helmers, M., Mora, G., 2008. Seasonal patterns in depth of water uptake under contrasting annual and perennial systems in the corn belt region of the Midwestern U.S. Plant and Soil 308, 69e92. Bartness, M.D., Yeh, S.M., 1994. Cooperative and competitive interactions in the recruitment of the marsh elders. Ecol 75, 2416e2429.
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Beierkuhnlein, C., 2011. Climate extremes initiate ecosystem-regulating functions while maintaining productivity. J. Ecol.. doi:10.1111/j.1365-2745. 2011.01817.x. Jones, H.G., 1992. Plants and Microclimate: A Quantitative Approach to Environmental Plant Physiology, second ed. Cambridge Univ. Press, Cambridge. Knapp, A.K., 1984. Water relations and growth of three grasses during wet and drought years in a tallgrass prairie. Oecol 65, 35e43. Kreyling, C., Wenigmann, M., Beierkuhnlein, C., Jentsch, A., 2008. Effects of extreme weather events on plant productivity and tissue die-back are modified by community composition. Ecosyst 11, 752e763. Le Roux, X., Bariac, T., 1998. Seasonal variations in soil, grass and shrub water status in a West African humid savanna. Oecol 113, 456e466. Loreou, M., Hector, A., 2001. Partitioning selection and complementarity in biodiversity experiments. Nature 412, 72e76. Michalet, R., Brooker, R.W., Cavieres, L.A., Kikvidze, Z., Lortie, C.J., Pugnaire, F.I., Valiente-Banuet, A., Callaway, R.M., 2006. Do biotic interactions shape both sides of the humped-back model of species richness in plant communities? Ecol. Lett. 9, 767e773. Michalski, S.G., Durka, W., Jentsch, A., Kreyling, J., Pompe, S., Schweiger, O., Willner, E., Beierkuhnlein, C., 2010. Evidence for genetic differentiation and divergent selection in an auto tetraploid forage grass (Arrhenatherum elatius). Theor. Appl. Gen. 120, 1151e1162. Mirzae, H., Kreyling, J., Hussain, Z., Li, Y., Tenhunen, J., Beierkuhnlein, C., Jentsch, A., 2008. One extreme drought event enhances subsequent carbon uptake in experimental grassland communities. J. Plant Nutr. Soil Sci. 171, 681e689. Naeem, S., Thompson, L.J., Lawler, S.P., Lawton, J.H., Woodfin, R.M., 1994. Declining biodiversity can alter the performance of ecosystems. Nature 368, 734e737. Naeem, S., 2002. Biodiversity equals stability. Nature 416, 23e24. Pfisterer, A.B., Schmid, B., 2002. Diversity dependent production can decrease the stability of ecosystem functioning. Nature 416, 84e86. Pugnaire, F.I., Luque, M.T., 2001. Changes in plant interactions along a gradient of environmental stress. Oikos 93, 42e49. R Development Core Team, 2010. R: a Language and Environment for Statistical Computing R version 2.11.1. R Foundation for Statistical Computing. ISBN 3900051-07-0, URL: http://www.R-project.org (Vienna, Austria). Roscher, C., Temperton, V.M., Buchmann, N., Schulze, E.-D., 2008. Community assembly and biomass production in regularly and never weeded experimental grasslands. Acta Oecol. 35, 206e217. Saha, S., Strazisar, T.M., Menges, E.S., Ellsworth, P., Sternberg, L., 2009. Linking the patterns in soil moisture to leaf water potential, stomatal conductance, growth, and mortality of dominant shrubs in the Florida scrub ecosystem. Plant and Soil 313, 113e127. Sperry, J.S., Hacke, U.G., Oren, R., Comstock, J.P., 2002. Water deficits and hydraulic limits to leaf water supply. Plant Cell Environ. 25, 251e263. Tilman, D., Lehman, C.L., Thomson, K.T., 1997. Plant diversity and ecosystem productivity: theoretical considerations. Proc. Natl. Acad. Sci. 94, 1857e1861. van Ruijven, J., Berendse, F., 2010. Diversity enhances community recovery, but not resistance, after drought. J. Ecol. 98, 81e86. Yachi, S., Loreau, M., 1999. Biodiversity and ecosystem productivity. The insurance hypothesis. Proc. Natl. Acad. Sci. USA 96, 1463e1468.
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Acta Oecologica journal homepage: www.elsevier.com/locate/actoec
Original article
Drought responses of Arrhenatherum elatius grown in plant assemblages of varying species richness Dennis Otieno a, *, Juergen Kreyling b, Andrew Purcell c, Nadine Herold d, Kerstin Grant b, John Tenhunen a, Carl Beierkuhnlein b, Anke Jentsch b a
Department of Plant Ecology, University of Bayreuth, D-95440 Bayreuth, Germany Department of Biogeography, University of Bayreuth, D-95440 Bayreuth, Germany Biological Sciences, University of York, Heslington, York YO10 5DD, UK d Max Planck Institute for Biogeochemistry, Hans-Knöll-Str. 10, 07745 Jena, Germany b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2011 Accepted 31 October 2011 Available online 22 November 2011
Evidence exists that plant community diversity influences productivity of individual members and their resistance and resilience during and after perturbations. We simulated drought within the long-term EVENT experimental site in the Ecological-Botanical Garden, University of Bayreuth to understand how Arrhenatherum elatius (L.) responds to water stress when grown in three different plant assemblages. The set up consisted of five replications for each factorial combination of drought and plant assemblages differing in functional diversity. Leaf water potential (JL), leaf gas exchange, natural d13C, plant biomass and cover were measured. Imposed drought had different effects on A. elatius, depending on plant assemblage composition. Severe water stress was however, avoided by slowing down the rate of decline in JL, and this response was modified by community composition. High JL was associated with high stomatal conductance and leaf photosynthesis. Biomass production of A. elatius increased due to drought stress only in the least diverse assemblage, likely due to increased tillering and competitive advantage against neighbors in the drought-treated plants. Our results indicate that beneficial traits among plant species in a community may be responsible for the enhanced capacity to survive drought stress. Resistance to drought may, therefore, not be linked to species richness, but rather to the nature of interaction that exists between the community members. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Arrhenatherum elatius Drought stress Productivity Plant assemblages Species interactions
1. Introduction The relationship between plant species richness and productivity has long been recognized (Naeem et al., 1994; Tilman et al., 1997; Hector et al., 1999) and is thought to be a central component of community ecology (Pfisterer and Schmid, 2002; Bruno et al., 2003). Controversies however, surround the nature of the interactions between community complexity and stability, or resistance to stress and disturbance on the one hand and productivity on the other hand (Loreou and Hector, 2001; Hooper et al., 2005; Roscher et al., 2008). Currently, the influence of changing environmental conditions, such as altered precipitation patterns and climate warming, on the stability of plant performance and community interactions provides a novel research frontier. Initial evidence suggests that drought events may have substantial
* Corresponding author. Tel.: þ49 921 552325; fax: þ49 921 552564. E-mail address: [email protected] (D. Otieno). 1146-609X/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.actao.2011.10.002
ecological impact on productivity, belowground biotic processes, phenology and community invasibilty (Jentsch and Beierkuhnlein, 2008; Kreyling et al., 2008; Mirzae et al., 2008). Yet, the ecological importance of extreme weather events, such as drought, is expected to increase in the near future in many parts of the world (IPCC, 2007). There is however, a substantial lack of knowledge regarding how such events, particularly in terms of their altered frequency and magnitude, will affect physiological performance, species interactions and ecosystem functioning (Jentsch et al., 2007; Jentsch and Beierkuhnlein, 2008). Ecological interactions are bound to vary with changes in the abiotic environment. A number of studies indicate that positive effects become stronger as abiotic stress increases (Bartness and Yeh, 1994; Greenlee and Callaway, 1996). It has been demonstrated that the stability of community functions, in the face of abiotic stress or extreme events, becomes stronger as community diversity increases (Hector et al., 1999; Jentsch et al., 2009; van Ruijven and Berendse, 2010). Interactions among plants in a community have generally been shown to shift from competition
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to facilitation, depending on the prevailing abiotic conditions, and gains from biodiversity could be the result of complementarity in resource use (Caldiera et al., 2001; Michalet et al., 2006) and facilitation (Hooper and Vitousek, 1998; Hector et al., 1999). Better understanding of the ecophysiological mechanisms of resource use underlying these associations of competition versus facilitation in plant interactions could help define the nature of relationships among plant community attributes. Since water stress is one of the main factors constraining productivity and diversity in most plant communities (Boyer, 1982; Pugnaire and Luque, 2001; Fridley, 2002), an understanding of water utilization by members of a community may help elucidate some of the underlying mechanisms associated with the positive relationships between species richness and productivity, as well as survival. Most ecophysiological studies on plant responses to drought focus on leaf level measurements, individual performance and possible links to entire plant structure changes in growth strategy. On the other hand, in community ecology, various concepts, such as the insurance hypothesis (Yachi and Loreau, 1999), address the role of biodiversity in buffering the effects of disturbance on community performance. Yet, these concepts fail to take account of the role of community composition on individual plant-level response to disturbance. Consequently, more studies are still needed in order to understand the coupling between individual species responses and community composition and structure. Here, we report on stress response in a common European grassland species, Arrhenatherum elatius (L.), as influenced by the composition of its nearest neighbors (‘community’). We assume that the above-mentioned mechanisms, which have primarily been discussed at the plant community level so far, also account for stability of performance at the species level, i.e. of a single widespread species within mixtures of different species composition. We hypothesized that; A. elatius individuals in species-rich mixtures are more resistant to drought stress, maintaining higher tissue water potentials, leaf gas exchange and productivity.
2. Materials and methods 2.1. Experimental site This research was conducted in the EVENT-I experiment (Jentsch et al., 2007) in the Ecological Botanical Gardens of the University of Bayreuth, Germany (49 5501900 N, 11340 5500 E, 365 m a.s.l.). The site is characterized by a mean annual temperature of 8.2 C, and a mean annual precipitation of 724 mm (1971e2000). Precipitation is distributed bi-modally with a major peak in June/ July and second peak in December/January (data: German Weather Service). Here, we focused on the drought tolerance of A. elatius, a widespread and agriculturally important European grass species (Michalski et al., 2010), in three different species assemblages. The experiment was carried out with two fully crossed factors: (1) precipitation (drought, control) and (2) species assemblage (A. elatius in combination with one other grass species (G2), with one other grass and two herb species (G4), and with one other grass and two herb species of which one was a legume (G4þ), see Table 1). The total set up consisted of 5 replicates of each factorial combination, 30 plots in total of 2 m 2 m in size. The factors were applied in a randomized block design with the species assemblages blocked and randomly assigned within each rainfall manipulation (Jentsch et al., 2007). The texture of the previously homogenized and constantly drained soil body consisted of loamy sand (82% sand, 13% silt, 5% clay), with pH ¼ 4.5 in the upper (0e20 cm depth) and pH ¼ 6.2 in the lower (20e80 cm depth) soil. Data acquisition
Table 1 Plant assemblage compositions for A. elatius in the EVENT I experiment. The G4 mixture lacks Lotus, which is a legume. Abbreviation
Species richness
Functional diversity
Species
G2
2
G4
4
1 functional group (grass) 2 functional groups (grass, herb)
G4þ
4
Arrhenatherum elatius, Holcus lanatus Arrhenatherum elatius, Holcus lanatus, Plantago lanceolata, Geranium pratense Arrhenatherum elatius, Holcus lanatus, Plantago lanceolata, Lotus corniculatus
3 functional groups (grass, herb, legume herb)
was carried out in the central square meter of each plot only, in order to circumvent edge effects. The drought manipulation simulated a 100-year extreme drought event in accordance with local climate data series, i.e. 32 days without rainfall induced by one rainout shelter per droughttreated block. The rainout shelters were constructed with a steel frame (6 m by 8 m, center height: 3 m; Hochtunnel, E & R Stolte GmbH, Germany), and covered with a transparent plastic sheet (material: 0.2 mm polyethylene, SPR 5, Hermann Meyer KG, Germany), with 95% PAR transmission and set up only for the manipulation period. Any lateral surface flow was avoided by plastic sheet pilings around treatment blocks and around each single plot reaching down to a depth of 10 cm. Strong greenhouse effects resulting from the shelters were avoided by starting the roof at 80 cm height and allowing for near-surface air exchange. The plots were also oriented in the common wind direction to facilitate air circulation under the roof. Control plots were watered regularly using portable irrigation systems. These were applied in equal parts during the manipulation period to ensure high water supply so that the plants did not suffer any water stress during the experimental period. The drought treatment lasted from May 21st (DOY 141) to June 21st (DOY 172) in the year 2007. Five widespread grassland species were chosen from the regional flora. Species were selected with respect to their affiliation to defined functional groups (grasses, herbs, legumes), life-span (perennials), their overall importance in nearby and Central European grassland systems, and based on the fact that they do naturally grow on substrate similar to the one used in this experiment. 100 plant individuals per plot in defined quantitative composition were planted in a systematic hexagonal grid, with 20 cm distance between individuals, in early April 2005. Density between individuals was, therefore, constant. Grass and herb individuals used in the experiment were grown from seeds in a greenhouse in the preceding fall. The communities were established at two levels of species diversity (2 and 4 species) and three levels of functional diversity (1, 2, 3 functional groups), thus resulting in three species combinations, or plant assemblages, in total (Table 1). Once installed, these species compositions were maintained by periodic weeding. 3. Measurements 3.1. Microclimate Weather conditions at the experimental site were continuously monitored from a climate station constructed 50 m away in an open location, so as to avoid any interference from the constructions. Parameters measured included air humidity and air temperature (Fischer 431402 sensor, K. Fischer GmbH, Drebach, Germany) and photosynthetic active radiation (LI-190 Quantum sensor, LI-COR,
D. Otieno et al. / Acta Oecologica 39 (2012) 11e17
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i h Rsample =Rstandard 1 1000;
USA) at 2 m above the ground surface. Data was recorded every 30 s, averaged and stored every 30 min using a data logger (DL2e, Delta-T Devices Ltd., Cambridge, UK). Daily precipitation was measured with a tipping bucket rain gauge (precipitation sensor 7041, Theodor Friedrichs & CO), while soil water content (SWC) within the G4 plots was monitored continuously using FD sensors (Echo EC5; Decagon, USA) installed at 5e10 cm soil depth. Installation depth for the sensors was based on a previous study, which showed that about 70% of the roots are restricted to the upper 5 cm of the soil and 96% to the upper 15 cm (Kreyling et al., 2008). Additional information on SWC was obtained from a TDR tube access probe (Diviner 2000, Sentek technologies, USA), that was used to read soil moisture in previously installed tubes at 12, 20 and 27 cm depth in all the plots at the end of the drought manipulation.
expressed in units of per thousand (&). 13C/12C ratios were calculated against the P.D. Belemite Standard (precision of 0.2&). The results were compared with other measurements to determine changes associated with shifts in 13C. Every measurement was replicated two times and the accuracy in d-values was better than 0.1&. Biomass estimation: On July 2nd (DOY 183) all the aboveground plant material (biomass) from four A. elatius individuals per plot was harvested for biomass determination. The aboveground biomass was oven-dried at 80 C for a period of 48 h before weighing.
3.2. Plant water status
3.5. Determination of plant cover
Midday leaf water potential (JL) was measured on three replicate leaves of A. elatius per plot using a portable pressure chamber (PMS Instruments Co. Corvallis, OR, USA). During measurements, the leaves were cut while enclosed in a plastic bag to reduce further moisture loss during transfer and fixing into the chamber. Moist tissue paper was introduced into the chamber to reduce water loss during the measurements. Measurements were confined to the period between 12:00 and 14:00 when lowest water potentials were expected. We focused on midday leaf water potential rather than pre-dawn leaf water potential as a better indicator of productivity since higher leaf water potentials promote stomatal conductance and photosynthesis.
A pin-point method was used to determine species-specific plant cover by counting the number of hits on plant organs by one hundred pins that were inserted vertically, 10 cm apart, into a 1 m2 quadrat. The results were treated as percentage (%) cover. Measurements were carried out on May 10th (DOY 130), June 26th (DOY 177) and September 2nd (DOY 245).
3.3. Leaf gas exchange Leaf gas exchange parameters, namely leaf transpiration (E), leaf stomatal conductance (gs) and leaf photosynthesis (A) were monitored between DOY 130 and DOY 170. A series of weekly measurements were carried out using a portable gas exchange system (LI-6400, LI-Cor, Lincoln, NE). A set of 3 grass tufts on each plot were identified and marked for measurements. On any single measurement day, 2-3 suitable leaf blades were selected from each of the tufts per plot and were set parallel in the cuvette, with their upper surfaces well exposed so that they were fully illuminated during measurements. Each set of measurements lasted one to 2 min; once a steady state was attained, 10 readings were logged per measurement at 10 s intervals. The selected leaves were marked and similar leaves were monitored either during midday (12:00 to 14:00) or throughout the day (from sunrise to sunset), when diurnal course measurements were conducted. The measured leaves were then excised at the end of measurement period and the leaf area (LA) of the section of leaf enclosed in the cuvette was then determined using a leaf area meter (CI-202 CID, Camas, WA). LA information was used to standardize the leaf gas exchange data. 3.4. d13C determination On DOY 183, a set of three fully matured leaves of A. elatius from every plot was selected. In each plot (factorial combination of watering regime and plant mixture), two sun exposed leaves of five individual plants were sampled and combined. The samples were oven-dried for 48 h at 80 C. The dry leaves were ball-milled and sub-samples of 1 mg were analyzed for d13C with an elemental analyzer attached to an isotope ratio mass spectrometer using ConFlo III interface (Laboratory of Stable Isotopes, UFZ, Leipzig, Germany). The carbon isotope composition (d13C) of a sample was calculated as follows:
d13 C ¼
4. Statistical analyses Linear mixed effects models, combined with analysis of variance (ANOVA), were applied to test for significant differences between groups at single points of time. The split-plot design was also taken into account by using block identity as a random factor. Homogeneous groups of factor combinations (drought manipulation, plant mixture) were identified by Tukey HSD post hoc comparisons. The level of significance was set to p < 0.05. For time series, linear mixed-effects models were employed to test for effects of drought manipulation and the respective interactions of plant mixtures, while taking the split-plot design and the repeated measurements into account (time was used as additional random factor). Prior to statistical analysis, data was log- or square-root-transformed, if conditions of normality were not met. This was also done to improve homogeneity of variances. Both characteristics were tested by examining the residuals versus fitted plots and the normal qqplots of the linear models. All statistical analyses were performed using R (R Development Core Team, 2010). 5. Results Mean daily PAR recorded between May and June, during the time when the treatments were carried out, was 40 mol m2 d1 (Fig. 1a). These values are about 3e5% higher than the PAR values recorded within the rainout shelters, based on parallel PAR measurements with the gas exchange measurement equipment, LI6400. Near-surface air temperature in the drought-treated plots was slightly (wþ1.3 C) increased by the roofs during the relatively short rainfall manipulation period of 32 days. We assumed that this didn’t influence the plant physiology significantly. Rainout shelters effectively prevented rainwater from reaching the drought-treated plots (Fig. 1c and d) and significantly lowered SWC (Fig. 1b). Due to equipment failure at the onset of experimentation, soil moisture data did not cover the whole rainfall manipulation period. Data from subsequent years however, indicate that drought stress builds up slowly and that the period with soil water content dropping below the permanent wilting point was almost completely covered in the study year (Jentsch et al., 2011). Compared to the wellwatered (control) plots, soil moisture at the end of the drought treatment was significantly lower at all soil depth levels (12 cm:
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Fig. 2. Midday leaf water potentials (JL) of A. elatius grown in drought-treated and in well-watered plots with varying plant mixture diversity levels. Results are shown for measurements conducted 10 days, DOY 152 (a), and 22 days, DOY 170 (b), after the commencement of drought treatments. Bars are SE. Statistics are shown on the right.
Fig. 1. Daily photosynthetic active radiation (PAR) (a), Soil water status in wellwatered and drought-treated plots (b), and precipitation amounts on well-watered (c), and drought-treated plots (d). Precipitation in well-watered plots was compensated through irrigation in order to replace the amount of water that was lost through evapotranspiration. A solid bar in Fig. 1d indicates the period when drought was imposed.
F ¼ 310.5; p < 0.001; 20 cm: F ¼ 397.4; p < 0.001; 27 cm: F ¼ 107.9; p < 0.001) and no differences were observed among the plant assemblages (12 cm: F ¼ 0.1; p ¼ 0.917; 20 cm: F ¼ 0.5; p ¼ 0.590; 27 cm: F ¼ 0.4; p ¼ 0.673) and the interaction of both factors (12 cm: F ¼ 0.0; p ¼ 0.995; 20 cm: F ¼ 0.0; p ¼ 0.981; 27 cm: F ¼ 0.1; p ¼ 0.889). Imposed drought stress resulted in a decline in leaf water potential (JL) in A. elatius (Fig. 2), except in the G4 plant assemblages. These minimum JL values however, remained stable 22 days after the imposition of drought stress (Fig. 2b). The fourspecies mixture (G4) experienced the highest JL (midday JL ¼ 1.6 MPa), followed by the four-species mixture containing legumes (G4þ) (1.8 MPa). The lowest JL (1.9 MPa) occurred in the two-species mixture (G2) (Fig. 2b). Midday leaf water potential of the control treatments showed mixed responses among the members of an assemblage at different times. On clear sky days (PAR > 1000 mmol m2 s1), control plants exhibited higher leaf gas exchange compared to their counterparts in the drought-treated plots (Fig. 3, DOY 165). Measurements conducted on DOY 170, a cloudy day when mean PAR values of 10 mol m2 s1 were recorded, showed no significant difference in gas exchange parameters among the A. elatius individuals in the
control and drought-treated plots. Instead, leaf assimilation (A), stomatal conductance (gs) and leaf transpiration (E) declined in most of the treatments, likely due to low radiation. Significant (p < 0.001) differences in shoot d13C signals resulting from rainfall manipulation and also plant mixture were observed (Fig. 4). Generally, more negative d13C values occurred in the wellwatered plants compared to the drought-treated plants. The lowest d13C signal in the well-watered plants was 29& and occurred in the G4 mixture, while highest was 28.3& and occurred in the G2 mixture. The lowest and highest d13C values in the droughttreated plants were 28.6& and 27.6& in the G4 and G4þ mixtures respectively. d13C signals were significantly lower in the A. elatius individuals grown in the G4 mixture than in the other 2 communities. On average, biomass per individual in the well-watered treatments was lower than in the drought-treated plants (Fig. 5). Individuals in the G2 mixture accumulated significantly (p < 0.001) more biomass than the other two plant assemblages. The highest biomass per individual was found in the G2 mixture subjected to water stress, while no significant difference was found between all other factorial combinations. Results from ground cover measurements showed higher cover in the drought-treated than in the nonstressed plants (Fig. 6). At the end of the growing period, individuals of A. elatius subjected to drought had more tillering than the non-stressed ones, thus resulting in higher ground cover, irrespective of the assemblage composition. In summary, interaction effects between rainfall manipulation and community composition in affecting the stress response of A. elatius were significant for JL and aboveground biomass. However, interaction effects were not significant for leaf gas exchange.
6. Discussion We tested the hypothesis that species diversity increases the resistance to drought among A. elatius individuals growing in plant assemblages, and that this may improve their survival chances and
D. Otieno et al. / Acta Oecologica 39 (2012) 11e17
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Fig. 3. Midday leaf CO2 assimilation, stomatal conductance and transpiration of A. elatius grown in well-watered and drought-treated plots with varying plant mixture diversity levels. Values are mean of measurements conducted between 12:00 and 14:00 h. Bars are SD.
productivity during drought. As expected, imposed drought resulted in a significant reduction in leaf water potential (JL) of A. elatius by about 0.6 MPa. This was in response to a 30% decline in soil moisture content (down to 5%-vol), which demonstrates the ability of A. elatius to resist rapid decline in JL during drought. This resistance may be achieved through intrinsic characteristics that lead to improved water uptake during drought (Knapp, 1984;
Barker et al., 1993; Asbjornsen et al., 2008; Saha et al., 2009), or increased resistance to water loss through stomatal regulation (Jones, 1992; Fay et al., 2002; Chen et al., 2009). Since we observed a general response in all of the treatments, irrespective of plant mixture composition, retention of favorable tissue water status during drought, as observed in this species, points to an inherent adaptation in A. elatius. Such an inherent characteristic may, however, be modified by species interactions within assemblages. Soil moisture status was monitored within the 5e27 cm soil depth in this study, yet grasses are known to extend their roots to depths greater than 30 cm, which remain moist for longer during drought (Le Roux and Bariac, 1998; Asbjornsen et al., 2008). Although we did not examine plant root distribution in our study, a separate study conducted on the same plots reported no roots of A. elatius below 15 cm depth (Kreyling et al., 2008), thus eliminating the chances of deep rooting and water uptake from the deeper soil
Fig. 4. d13C determined on leaf samples obtained from well-watered and droughttreated plots with different plant community composition. Samples were obtained on DOY 184 after the end of drought-treatment. Bars are SE.
Fig. 5. Biomass of A. elatius grown in well-watered and drought-treated plots with varying plant species composition and species richness. The results are means of 4 harvested individuals per plot. Biomass harvest was conducted on DOY 184.
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D. Otieno et al. / Acta Oecologica 39 (2012) 11e17
Fig. 6. Plant cover estimates of A. elatius grown in well-watered and drought-treated plots. Cover was estimated at three times during the growth of the plants. The last estimate is performed on re-growth after the first harvest on DOY 184. Bars are SE. Note that the rainfall manipulation started two years before the year displayed, explaining the differences in cover before the manipulation period.
layers as possible reasons for the slow decline in JL of A. elatius during drought. In most plant species, favourable tissue water status is translated into bioproductivity through increased stomatal conductance and CO2 assimilation (Saha et al., 2009). Plants that exhibit higher leaf water potentials are, therefore, likely to exhibit higher CO2 assimilation rates than their counterparts with low tissue water status. Considering leaf gas exchange measurements conducted on DOY 165, in the middle of drought and when the vegetation was mature and actively photosynthesizing, well-watered individuals of A. elatius, with higher JL, exhibited higher stomatal conductance and photosynthesis compared to the stressed plants. Equally, A. elatius plants grown in the G4 mixtures had higher stomatal conductance and assimilation rates compared to their counterparts in the other two plant mixtures. These results are supported by the d13C measurements, which showed the well-watered plants, as well as stressed A. elatius in the G4 mixture, to have less enriched tissues. This suggested that these plants carried out gas exchange (or had wide-open stomata) over extended periods of time during experimentation (Ehleringer and Cooper, 1988; Farquhar et al., 1989). These results discount stomatal closure as one reason for the observed favourable JL in A. elatius grown in the G4 mixture, but support the fact that maintenance of high water potential leads to increased leaf stomatal conductance and photosynthesis during drought. Soil moisture declined uniformly in all the plots during drought. Consequently, the differences in JL arising during drought, as observed between A. elatius grown in the G4 and the other plant assemblages, could only be attributed to the interactions among members of this assemblage. Competitive balance between the members of the different assemblages appears to be the key for this response, with an inferior competitor in the G4 assemblage (Geranium pratense produced 4 g m2 in the drought and 1 g m2 in the control treatment) being replaced by a more dominant competitor in the G4þ assemblage (Lotus corniculatus: 204 g m2 in drought and 154 g m2 in the control treatment). Species identity and interactions rather than raw numbers determine species performance. Specifically, A. elatius in the G4 mixture with less competitive pressure is better placed to avoid water stress, continue photosynthesizing and survive longer during drought
(Sperry et al., 2002). The ability to survive longer is of great significance and could make real differences as far as generation continuity is concerned. We recommend more detailed investigations to identify mechanisms that could contribute to the observed response of A. elatius in the G4 assemblage. High photosynthetic rate over an extended period of time is likely to lead to higher biomass accumulation. Interestingly, biomass production did not strongly shift due to the watering regime, but was more associated with species compositions. Leaf level results did not reflect the total biomass accumulated in the different treatment plots. Similar results were reported by Kreyling et al. (2008). Integrated results showed that well-watered A. elatius, and also those grown in the G4 mixture, exhibited higher stomatal conductance and photosynthesis, and hence were expected to be more productive. During drought however, A. elatius increased its tillering (high ground cover), thus accounting for the unusually high biomass in the drought-treated plants. Such responses were not captured by leaf-level measurements, which were limited to the individual leaves. Although A. elatius grown in the G4 mixture showed higher resistance to drought, its biomass production per individual was highest in the species poor G2 mixture. There was also no clear link between species richness and biomass production, both in the perturbed and non-perturbed systems. Resistance to drought however, will ensure survival of the species during perturbation (Naeem, 2002). According to Yachi and Loreau (1999), resistance to stress should increase with species richness. Pfisterer and Schmid (2002) however, found no definite link between resistance and community diversity, but observed that productivity was linked to diversity in the absence of perturbation. In our study, the lowest biomass of A. elatius was recorded in plant assemblages with highest diversity and vice versa. Our results are similar to those reported by Fridley (2002). There was also no clear relationship between resistance to drought and species richness. Instead, our results showed that drought resistance in A. elatius may depend on the nature of association among its nearest neighbors. These results, therefore, point to the significant role played by the nature of plant interactions, rather than simple species numbers, in conferring stability during drought. The scope of measurements however, did not allow us to identify reasons for high JL in A. elatius, and how it is modified by community association. We could also not link leaf level responses to total aboveground production. Further investigations are recommended in order to identify the specific traits that are involved, and to validate some of the results. Author contributions Dennis Otieno: Designed and carried out the measurements, carried out data analysis and wrote the manuscript. Andrew Purcell, Nadine Herold, Kerstin Grant and Juergen Kreyling carried out measurements. Juergen Kreyling also helped with statistical analysis and revised the manuscript. Anke Jentsch wrote and revised the manuscript and also sought project funding. John Tenhunen and Carl Beierkuhnlein read and revised the manuscript. References Asbjornsen, H., Shepherd, G., Helmers, M., Mora, G., 2008. Seasonal patterns in depth of water uptake under contrasting annual and perennial systems in the corn belt region of the Midwestern U.S. Plant and Soil 308, 69e92. Bartness, M.D., Yeh, S.M., 1994. Cooperative and competitive interactions in the recruitment of the marsh elders. Ecol 75, 2416e2429.
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