Effects of extreme rainfall events are independent of plant species richness in an experimental grassland community

Global climate models predict more frequent periods of drought stress alternated by heavier, but fewer rainfall events in the future. Biodiversity studies have shown that such changed drought stress may be mitigated by plant species richness. Here, we investigate if grassland communities, differing in species richness, respond differently to climatic extremes within the growing season. In a 3-year outdoor mesocosm experiment, four grassland species in both monoculture and mixture were subjected to a rainfall distribution regime with two levels: periods of severe drought in the summer intermitted by extreme rainfall events versus regular rainfall over time. Both treatments received the same amount of water over the season. Extreme rainfall combined with drought periods resulted in a 15% decrease in aboveground biomass in the second and third year, compared to the regular rainfall regime. Root biomass was also reduced in the extreme rainfall treatment, particularly in the top soil layer (− 40%). All species developed higher water use efficiencies (less negative leaf δ13C) in extreme rainfall than in regular rainfall. These responses to the rainfall/drought treatment were independent of species richness, although the mixtures were on an average more productive in terms of biomass than the monocultures. Our experimental results suggest that mixtures are similarly able to buffer these within-season rainfall extremes than monocultures, which contrasts with findings in the studies on natural droughts. Our work demonstrates the importance of investigating the interactions between rainfall distribution and drought periods for understanding effects of climate change on plant community performance.</p

Femme, sport, traumatologie : mieux comprendre 'l'avant' pour éviter 'l'après'

Advances in root ecology have revealed that root standing biomass is higher in species-rich plant communities than in species-poor communities. Currently, we do not know whether this below-ground diversity effect is the result of enhanced root production or reduced root mortality or both, which is essential information to understand ecosystem functioning, as it determines C sequestration and N dynamics in soil. Minirhizotron observations were combined with root coring in five different plant communities (four monocultures and the respective mixture). Molecular markers were used to quantitatively determine species abundance in mixed root biomass samples in order to track shifts in below-ground species composition. In addition, a litterbag experiment was performed to study root decomposition independent of root mortality. Root length production was greater and root length loss was lower in the mixture than expected from monocultures in all years. Simulations suggest that at least two species must have had reduced losses in mixture compared to monoculture. However, the diversity effects on root mortality may partially be explained by selection effects as the species with the longest root life span became dominant in the mixtures. Root length loss from minirhizotrons was very low; the combination of minirhizotron length measurements with root biomass estimates from coring suggested underestimation of root loss in minirhizotrons over time. Root decomposition was not affected by diversity. Diversity enhanced root length production and decreased root loss, resulting in below-ground overyielding. With decomposition unaffected, our results suggest that root mortality is reduced with increasing diversity. Future studies have to reveal the generality of our observations in larger scale biodiversity experiments by using species having a wider variety of root traits

Effects of extreme rainfall events are independent of plant species richness in an experimental grassland community

Global climate models predict more frequent periods of drought stress alternated by heavier, but fewer rainfall events in the future. Biodiversity studies have shown that such changed drought stress may be mitigated by plant species richness. Here, we investigate if grassland communities, differing in species richness, respond differently to climatic extremes within the growing season. In a 3-year outdoor mesocosm experiment, four grassland species in both monoculture and mixture were subjected to a rainfall distribution regime with two levels: periods of severe drought in the summer intermitted by extreme rainfall events versus regular rainfall over time. Both treatments received the same amount of water over the season. Extreme rainfall combined with drought periods resulted in a 15% decrease in aboveground biomass in the second and third year, compared to the regular rainfall regime. Root biomass was also reduced in the extreme rainfall treatment, particularly in the top soil layer (− 40%). All species developed higher water use efficiencies (less negative leaf δ13C) in extreme rainfall than in regular rainfall. These responses to the rainfall/drought treatment were independent of species richness, although the mixtures were on an average more productive in terms of biomass than the monocultures. Our experimental results suggest that mixtures are similarly able to buffer these within-season rainfall extremes than monocultures, which contrasts with findings in the studies on natural droughts. Our work demonstrates the importance of investigating the interactions between rainfall distribution and drought periods for understanding effects of climate change on plant community performance.</p

Mommer et al 2015 FE - Minirhizotron data from biodiversity experiment

Data on root standing biomass and total root length of four grassland species growing in monocultures and 4 species mixtures in the Nijmegen Phytotron. From minirhizotron images over three years we calculated root length production and root length loss for these plant communities

ANCOVA results for available nutrients in soil solution.

<p>Diversity of species (<i>F. rubra</i> monoculture, <i>P. lanceolata</i> monoculture and mixture of the two species) and soil depth were fixed factors, and time after plantation was a covariate.</p>*<p>P<0.05;</p>**<p>P<0.01;</p>***<p>P<0.001;</p>ns<p>P>0.09. Bold shows significant effects.</p

Experimental setup and biomass data.

<p>Planting scheme (<b>a</b>); shoot (<b>b</b>) and root mass in the poor top (<b>c</b>) and rich bottom layer (<b>d</b>); percentage belowground biomass at harvest (<b>e</b>), in <i>Festuca rubra</i> (<i>Fr</i>) and <i>Plantago lanceolata</i> (<i>Pl</i>) monocultures and mixtures. Horizontal lines in <b>b–d</b> show expected values for mixtures in case of competitive-equivalence (i.e., 50% of monocultures, or a relative yield of 0.5), and arrows depict the percentage deviation. Asterisks show significant differences between observed and expected values after t-tests. Data are means ± SE, N = 3–4. (*) P<0.06; * P<0.05; ** P<0.01; *** P<0.001.</p

Root growth observed through minirhizotron tubes.

<p>(<b>a</b>) Root length production over time (m m⁻² image) of <i>Festuca rubra</i> (<i>Fr</i>) and <i>Plantago lanceolata</i> (<i>Pl</i>) in mixtures, obtained from minirhizotron observations at 10 cm depth. Solid lines are for observed values, dashed lines for expected values from monocultures (½ of monocultures). On each date, t-test were run separately to detect significant differences between observed and expected values in each species. <i>P</i>-values were then adjusted using the Sidak correction for multiple comparisons. After correction, * P<0.009; ** P<0.002; *** P<0.001. (<b>b</b>) Linear regression of expected <i>versus</i> observed root length of <i>Plantago</i> and <i>Festuca</i> in mixtures over the whole experiment, and null expectation expected = observed (1∶1). Significance of deviation of slopes from unity is shown by p-values. Data are means ± SE, N = 3–4.</p

Nutrients dynamics in soil solution over time.

<p>Nitrate (<b>a, b</b>), ammonium (<b>c, d</b>) and phosphate (<b>e, f</b>) concentration in <i>Festuca rubra</i> and <i>Plantago lanceolata</i> monocultures and in mixtures of the two species, at 7 and 35 cm depth. In nitrate, different letters in legends show significant differences between species over time, after ANCOVA_{layer x species}. Data are means ± SE, N = 3–4. No significant second and third order interactions involving species and time were detected (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055805#pone-0055805-t001" target="_blank">Table 1</a>), meaning that similarities/differences between species were consistent all over the experimental period. Soil nutrient concentrations were derived from regular sampling of soil water over the course of the experiment through porous suction cups that had been placed in the soil layers.</p

Effects of cell number and cell size on petiole length variation in a stoloniferous herb

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