Suchergebnisse

15 páginas, 8 figuras, 3 tablas. ; We studied the effects of water stress, fertilization and time course on foliar volatile terpene emission rates by Quercus ilex and Pinus halepensis in a garden experiment. The terpenes mostly emitted by both species were α-pinene, β-pinene, β-myrcene and Δ3-carene. P. halepensis emission rates (average 31.45 μg g−1 DM h−1) were similar to those of Q. ilex (average 31.71 μg g−1 DM h−1). The effects of drought (reduction to one-third of full watering) and fertilization (250 kg N ha−1, 250 kg P ha−1, or both) were different depending on the species: the drought treatment significantly increased the terpene emissions from Q. ilex by 33%, and the fertilization treatments reduced the terpene emissions from P. halepensis by 38%. Terpene emission rates increased with time course in parallel to raising summer temperatures in P. halepensis and Q. ilex, whose emission rates were temperature related (r = 0.42 and r = 0.68, respectively) and light related (r = 0.32 and r = 0.57, respectively). There was a positive relationship for P. halepensis, and a negative relationship for Q. ilex, between emission rates and relative water contents. No relationship was found between emission rates and N or P foliar concentrations. The results of this study show complex species-specific responses with stronger and faster short-term responses in terpene-non-storing than in storing species and indicate that terpene emissions may significantly change in the warmer, drier and more fertilized conditions predicted for the next decades in the Mediterranean region. ; Our research was partly supported by the European Union's ecological and physiological functions of biogenic isoprenoids and their impact on the environment (ISONET) (Marie Curie network contract MC-RTN-CT-2003 504720), by grants CGL2004-01402/BOS and CGL2006-04025/BOS from the Spanish Government, by a 2004 grant from the Fundación Banco Bilbao Vizcaya Argentaria (BBVA), by the Catalan Government project (SGR2005/00312) and by the European Union project Assessing Large Scale Risks for Biodiversity with tested Methods (ALARM) (Contract 506675). ; Peer reviewed

10 páginas, 4 figuras. ; Several volatile organic compounds (VOCs) have been reported as having a communication role between plants and also between plants and animals. We aimed to test whether methanol, a short-chain oxygenated VOC, could also have a signalling role between plants. We monitored photosynthetic performance and VOC exchange rates of Quercus ilex L. saplings before and after two different treatments: (a) clipping of some leaves to simulate an attack by herbivores and (b) fumigation with gaseous methanol for 5 h to simulate the amount of methanol a plant could receive from surrounding plants if those had been already attacked by herbivores. The clipping treatment enhanced the photosynthetic rates, the chlorophyll a to b ratio and the carotenoid to chlorophyll ratio of nonclipped leaves, suggesting an activation of plant protective metabolism. Also, a small but interesting systemic (in nonclipped leaves) increase in methanol emission rates was observed, which agrees with the possibility that methanol may act as a signalling cue. The methanol fumigation treatment induced an increase in the actual photochemical efficiency of PSII and also in the carotenoid to chlorophyll ratio. Methanol fumigation also promoted a 14% increase in the monoterpene emission rate, 1 day after the treatment, a similar response to the ones induced by other signalling VOCs. The enhanced monoterpene emissions could add to the blend of VOCs emitted after stress and be part of further signalling pathways, thus forwarding the message started by methanol. This study suggests that clipping and methanol fumigation at natural concentrations elicit significant neighbour plant physiological responses and further BVOC emissions. ; This study was supported by the Spanish Government grants CGL2006-04025/BOS, CGL2010-17172, and Consolider-Ingenio Montes CSD2008-00040, the CSIC grant PIF08-006-3 and the Catalan Government grant SGR 2009-458. Roger Seco gratefully acknowledges a FPI fellowship (BES-2005-6989) from MEC (Spanish Government). ; Peer reviewed

5 páginas, 3 figuras, 1 tabla. ; A rain exclusion experiment simulating drought conditions expected in Mediterranean areas for the following decades (15% decrease in soil moisture) is being conducted since 1999 in a Mediterranean holm oak forest to study its response to the forecasted climatic changes for the coming decades. The maximum PSII quantum yield of primary photochemistry (Fv/Fm) was measured in Quercus ilex, and Phillyrea latifolia, the co-dominant species of the studied forest, from 1999 to 2009 in four plots: two of them were control plots and the other two plots received the rain exclusion treatment. In both species, the Fv/Fm values were highly dependent on air temperatures, and in a second term, in water availability. P. latifolia was the species with the larger decrease in Fv/Fm values induced by low air temperatures, while in hot seasons, the Fv/Fm values in P. latifolia were even higher than in Q. ilex. Rainfall exclusion decrease Fv/Fm values significantly only in few monitoring dates. The most drought resistant species P. latifolia was more affected by the experimental rainfall exclusion than Q. ilex that instead lost number of leaves per tree. There was a synergic effect of drought stress and winter cold in P. latifolia not observed in Q. ilex, but a more conservative strategy in P. latifolia maintaining leaves with a down-regulation of the linear photosynthetic electron transport. These results indicate that, although other physiological and reproductive strategies at whole plant level must be also taken into account, the warmer and drier environment expected for the following decades could favour the species more sensitive to cold and more resistant to drought, the shrub P. latifolia, in detriment of the tree Q. ilex as already observed in the field after severe heat-drought episodes. ; This research was financially supported by the Spanish Government CGL2006-4025/BOS, CGL2010-171722/BOS, and Consolider-Ingenio MONTES CSD2008-00040 projects, by the CSIC Frontera project PIFO08-006-3, and by the Catalan government SGR2009-458 project ; Peer reviewed

9 páginas, 4 tablas, 2 figuras. ; We aimed to test the effect of soil drought conditions projected by general circulation models and ecophysiological models for the next few decades on emission rates of isoprenoids by Mediterranean shrublands. We conducted a field experiment in which we generated soil drought (ca. 12%–20% decreased soil moisture) using automatically sliding curtains, and we measured foliar isoprenoid emissions in the three dominant species of the studied shrubland (Erica multiflora L., Globularia alypum L., and Pinus halepensis L.) in two different annual periods. Monoterpene emissions were detected in the three studied species, but isoprene emissions were significantly detected only in E. multiflora. Main volatile terpenes emitted by the three species were α‐pinene, β‐myrcene, limonene, and Δ3‐carene. In general, maximum isoprenoid emission rates were found in the hottest periods, and minimum emission rates were found in winter. Isoprene emissions in E. multiflora ranged between practically 0 μg g−1 (dry matter) h−1 in winter 2005 and 57 μg g−1 (dry matter) h−1 in summer 2003. Isoprene emissions were 75% lower during the dry second annual period, 2004–2005, than during the first year, 2003–2004. In E. multiflora, there was an overall decrease of 19% in isoprene emissions in response to soil drought. Conversely, monoterpene emissions increased by 26.4% in drought treatment. In G. alypum, there was an overall increase of 75% in terpene emissions in response to soil drought. In P. halepensis, drought treatment increased terpene emission rates by 156%. Drought treatment affected the emissions mainly in the hottest seasons, spring and summer. There were strong compound‐specific, species‐specific, interannual, and seasonal changes in the emission rates and in their response to the treatments. These data might help to improve prediction algorithms, inventories, and modeling of isoprenoid emissions and of their response to climate change (decreased isoprene emissions and increased monoterpene emissions under moderate or short‐term drought and decreased emissions under severe or long‐term drought), but the great variability highlights the difficulty of the task. ; Our research was partly supported by ISONET (Marie Curie network contract MC‐RTN‐CT‐2003 504720) from the European Union, grants CGL2004‐01402/BOS and CGL2006‐04025/BOS from the Spanish government, a 2004 grant from the Fundación BBVA, a European Union grant (ALARM contract 506675), and a SGR2005‐00312 grant from the Catalan government. ; Peer reviewed

13 páginas, 6 figuras, 1 tabla. ; This study examined the effects of the combination of UV radiation and water limitation on the leaf photosynthesis, stomatal conductance, and terpene emissions of four Mediterranean species. 1-year-old seedlings of these Mediterranean species [Daphne gnidium L., Pistacia lentiscus L., Ilex aquifolium L. and Laurus nobilis L.] were grown under one of three UV treatments (without UV, with UVA, or with UVA ? UVB) and two watering regimes (high and low water supply). In general, UV treatments did not affect significantly leaf photosynthesis or stomatal conductance, although UVA and UVB radiation in September led to a reduction in leaf stomatal conductance in D. gnidium. Leaf photosynthesis rates did not differ significantly between the two watering treatments, even though, in three of the species, leaf stomatal conductance was significantly higher among the well-watered plants. The effects of UV on terpene emissions were species-specific; D. gnidium had the highest terpene emission rates when grown under UVA ? UVB radiation, which was also true for L. nobilis in September. Overall, UV treatments did not have a significant effect on total terpene emission rates in I. aquifolium, but UVB and UVA in July and September, respectively, reduced emission rates in P. lentiscus. A limited water supply reduced the terpene emission rates in D. gnidium, increased emissions in L. nobilis, and did not affect the emission rates in the other two species. ; Grants from the Spanish Government (CGL2006-04025/BOS, CGL2007-64583/BOS, and CGC2010- 17172), Consolider-Ingenio Montes (CSD2008-00040), and the Catalan government (SGR2009-458) supported the research. ; Peer reviewed

11 páginas, 6 figuras , 1 tabla. ; Host recognition is a key process in oligolectic bees but the mechanisms through which they find and recognize appropriate pollen host plant are not entirely clear. Hoplitis adunca is a monolectic bee collecting pollen only from Echium spp. (Boraginaceae). We aimed to test whether Echium vulgare floral scent plays a major role in the attraction of H. adunca females, and to identify components of E. vulgare scent that may be involved in this specific attraction. We used a combination of behavioral and chemical (GC/GC–MS, PTR-MS) analyses. In order to identify the chemical cues likely to be involved in the specific attraction of H. adunca, we compared the scent of fresh flowers, nectar, pollen, and whole plants of E. vulgare and Anchusa officinalis, another Boraginaceae, which does not attract H. adunca. H. adunca females were attracted to the scent of E. vulgare flowers when offered against a blank or against the scent of A. officinalis flowers. However, H. adunca females were not attracted to the scent of A. officinalis flowers when offered against a blank. The emission spectra of the two plant species differed markedly, as did the emission spectra of various flower components (pollen, nectar and whole flowers) within a species. Pollen presented a low volatile release, but emitted significantly higher amounts of mass 55 (butanal, 1,3-butadiene, or other volatiles of molecular mass 54), and mass 83 (hexanal, hexenols, hexenyl acetate, or other volatiles of molecular mass 82) in E. vulgare than in A. officinalis. Nectar produced a particular emission spectrum with high emission rates of masses 109 and 123. Mass 109 may likely correspond to 1,4-benzoquinone, a volatile specifically measured in E. vulgare in parallel studies to this one. The flower emission spectrum was mainly a combination of the pollen and the nectar scents, although it also contained additional volatile compounds such as those of mass 63 or mass 81. As for terpenes, E. vulgare emitted limonene, longicyclene, junipene, trans-caryophyllene and a-humulene, that were not detected in A. officinalis, and the most emitted monoterpenes were a-pinene, junipene and limonene whereas the most emitted terpenoid by A. officinalis was a-pinene. After identifying these chemical cues, olfactory/behavioural assays with specific volatiles and combinations of volatiles are necessary to understand the chemical interactions of the H. adunca-E. vulgare system. ; This research was supported by Spanish MEC grants CGL-2006-04025/BOS, CGL2010-17172, and Consolider-Ingenio Montes CSD2008-00040, the European Science Foundation "VOCBAS" program, and a Catalan government SGR2009-458 grant. ; Peer reviewed

13 páginas, 7 figuras. ; We studied the effects of experimental warming and drought on the plant biomass of a Mediterranean shrubland. We monitored growth at plant level and biomass accumulation at stand level. The experimentation period stretched over 7 years (1999–2005) and we focused on the two dominant shrub species, Erica multiflora L. and Globularia alypum L. and the tree species Pinus halepensis L. The warming treatment increased shoot elongation in E. multiflora, and the drought treatment reduced shoot elongation in G. alypum. The elongation of P. halepensis remained unaffected under both treatments. The balance between the patterns observed in biomass accumulation for the three studied species in the drought plots (reduction in E. multiflora and P. halepensis and increase in G. alypum) resulted in a trend to reduce 33% the biomass of the drought treatment plots with respect to the untreated plots, which almost doubled their biomass from 1998 to 2005. The results also suggest that under drier conditions larger accumulation of dead biomass may occur at stand level, which combined with higher temperatures, may thus increase fire risk in the Mediterranean area. ; This research was funded by the EU under the projects CLIMOOR (Contract ENV4-CT97-0694) and VULCAN(Contract EVK2-CT-2000-00094) and we also received additional financial help from the Spanish Government (grants CGL2006-04025/BOS and Consolider Montes CSD2008- 00040), the Catalan Government (grant SGR2005-00312) and FP6 NEU NITROEUROPE (Contract GOCE017841). ; Peer reviewed

11 páginas, 5 figuras. ; We conducted a night-time warming and drought field experiment for 7 years (1999–2005) in a Mediterranean shrubland. We focused on the two dominant shrub species, Erica multiflora L. and Globularia alypum L. and the tree Pinus halepensis L. and the final years to study the effects of the experimental night-time warming and drought on Fv/Fm, photosynthesis, and stomatal conductance. Warming treatment increased mean air temperature and mean soil temperature through the years by an average of 0.7 and 0.9 C respectively, and drought treatment reduced soil moisture through the years by an average of 19%. Warming tended to increase photosynthetic rates in E. multiflora, G. alypum and P. halepensis mostly in the cold seasons, when plants were more limited by temperature, as shown by the lowest values of Fv/Fm being detected in winter in the three studied species. A negative effect of warming was only detected for E. multiflora in summer 2003. Drought treatment generated different responses of net photosynthetic rates depending on the species, season and year. Stomatal conductance showed the same pattern as photosynthesis for the three studied species, displaying seasonal and inter-annual variability, although with an overall negative effect of drought for P. halepensis. Photosynthetic rates decreased significantly in the dry winter 2005 and spring 2005 in comparison to the same seasons of 2003 and 2004. There were positive correlations between the photosynthetic rates in different seasons for E. multiflora, G. alypum and P. halepensis and the soil moisture of the week prior to measurements. The great variation in the photosynthetic rates was thus explained in a significant part by soil moisture levels. The lowest Fv/Fm values usually corresponded with lowest stomatal conductances suggesting that drought stress could be associated to stress by low temperatures in winter. ; This research was funded by the EU under the projects CLIMOOR (Contract ENV4-CT97-0694) and VULCAN (Contract EVK2-CT- 2000-00094) and we also received additional financial help from the Spanish Government (grants CGL2006-04025/BOS and Consolider Montes CSD2008-00040), the Catalan Government (SGR2005- 00312), and the European project FP6 NEU NITROEUROPE (Contract GOCE017841). ; Peer reviewed

8 páginas, 3 tablas, 3 figuras. ; Isoprene and monoterpenes emission fluxes emitted by Quercus robur seedlings were measured at a leaf scale in the field. Isoprene emissions were always predominant over the monoterpene ones, a-pinene, b-pinene and camphene being the most abundant emitted monoterpenes. Emission fluxes were normalised at standard conditions of temperature and photosynthetically active radiation, PAR, (30 C and 1000 mmol m 2 s 1 respectively) using temperature and light dependent algorithms. The standardised emission fluxes ranged from 43 to 73 mg gDM 1 h 1 for isoprene and from 0.04 to 2.95 mg gDM 1 h 1 for the monoterpenes. The values reported in the literature are within our experimental intervals. These standardised fluxes were used to estimate the Quercus robur emissions in Galicia (NW Spain). This region is characterised by its abundant forest extensions where Quercus robur occupies the second place in species abundance with a total of 195029 ha of Quercus robur pure stands. To estimate the region emission fluxes, both the extension and distribution of the forest areas and the regional climatic conditions over five years (2002–2006) were taken into account. The averaged annual fluxes regarding the forest extension were 0.04 t ha 1 yr 1 for isoprene and 0.52 kg ha 1 yr 1 for monoterpenes. This means averaged annual fluxes of 9730 t yr 1 and 114 t yr 1 for isoprene and monoterpenes, respectively. These values place Galicia as an important isoprene emitter power in Spain as a consequence of the extension of its forests more than of the climatic condition ; JP acknowledges funding from grants from the Catalan Government (SGR2005-00312), and the Spanish Ministry of Education and Science (CGL2006-04025/BOS and Consolider Montes CSD2008-00040). ; Peer reviewed

6 páginas,4 figuras. ; We compared the role of instantaneous temperature and temperature history in the determination of α-pinene emissions in Mediterranean conifer Pinus halepensis that stores monoterpenes in resin ducts, and in Mediterranean broad-leaved evergreen Quercus ilex that lacks such specialized storage structures. In both species, α-pinene emission rates (E) exhibited a significant exponential correlation with leaf temperature and the rates of photosynthetic electron transport (JCO 2 +O 2 ) started to decrease after an optimum at approximately 35 o C. However, there was a higher dependence of E on mean temperature of previous days than on mean temperature of current day for P. halepensis but not for Q. ilex. JCO 2 +O 2 showed a maximum relationship to mean temperature of previous 3 and 5 days for P. halepensis and Q. ilex respectively. We conclude that although the best correlation of emission rates were found for instantaneous foliar temperatures, the effect of accumulated previous temperature con ; Our research on volatile compound emissions has been supported by the funding from the European Science Foundation (Programme VOCBAS), the European Commission Marie Curie research training network ISONET (MC-RTN-CT-2003 504720), the Spanish Government (CGL2006-04025/BOS and ConsoliderIngenio Montes CSD 2008-00040), the Catalan Government (SGR 2009-458), the Spanish National Research council (CSIC-PIF08- 006-3), the Estonian Science Foundation (grant 7645) and the Estonian Ministry of Education and Science (SF1090065s07) ; Peer reviewed

19 páginas, 6 figuras, 2 tablas. ; Leaf elemental and secondary metabolite contents and morphological traits are important measures of time-dependent ecosystem changes. We aimed to test whether plants from older tropical forests have lower nutrient contents and different elemental stoichiometry than plants from younger ecosystems (''soil age'' hypothesis) and whether they had different contents of carbon based secondary compounds (CBSC) and morphological traits as a result of a longer evolution under tropical conditions. We conducted a phylogeny-independent study of the foliar chemical and structural traits in two sets of 86 species each measured in two different-aged tropical forests, a young soil forest in Hawaii and an old soil forest in Borneo. The leaf contents of nutrients and micronutrients tended to be higher in Hawaii than in Borneo but leaf N:P content ratio was not different. The ''soil age'' hypothesis was thus only partially supported by the results indicating that several other factors influence plant elemental content. Total phenolic content was twice larger in Hawaiian than in Bornean plant species. Terpene contents were not different in terpene-containing species but the percentage of species containing terpenes was much higher in Borneo (97%) than in Hawaii (34%) suggesting that the longer time of evolution in Borneo has allowed a more widespread development of very diverse defensive, allelopatic and information relationships of plants with specialist herbivores and other plants. Principal component analyses separated Hawaii and Borneo species on the basis of leaf elemental composition, total phenolics and terpene contents and leaf dry mass per area (LMA). The results collectively support the ''leaf economic spectrum'' and ''carbon excess'' paradigms because in both sets of species and also in the combined set of Borneo and Hawaiian species, there are negative relationships of N content with LMA and total phenolics. The results suggest thus that changes throughout time in N and P availability can be important but do not explain all the variability underlying the evolutionary changes in leaf chemistry and structure in these tropical forests. Other factors determining species biogeochemical niche such as K, Mg or S elemental stoichiometry, leaf economic traits and changes in plant defence and communication strategy are also likely to be involved. ; This research was supported by the Spanish Government projects CGL2006-04025/BOS, CGL2010- 17172, Consolider Ingenio Montes (CSD2008-00040), by the Catalan Government project SGR 2009-458 and by the Estonian Ministry of Education and Research (grant SF1090065s07) ; Peer reviewed

12 páginas, 4 tablas, 2 figuras. ; Aim Alien plant invasion is prominent in the Hawaiian Islands. There are many factors involved in invader success. To date, there is a general lack of information about one of them, which we aim to study here: the terpene emission capacity of both Hawaiian native and alien plants. Location Oahu (Hawaii). Methods We screened 35 alien and 35 native dominant plant species on Oahu Island for monoterpene emissions. The emission rates were measured from field-grown plants under standardized conditions of temperature and quantum flux density in the laboratory. Results The emission rates of total terpenes ranged from 0 µg g−1 h−1 to 55 µg g−1 h−1, and altogether 15 different terpenes were emitted in detectable amounts by the overall set of species. A phylogenetic signal was observed for total terpene emissions. Total terpene emission rates were higher in aliens than in native species (12.8 ± 2.0 vs. 7.6 ± 1.9 µg g−1 h−1, respectively). Main conclusions The greater terpene emission capacity may confer protection against multiple stresses and may partly account for the success of the invasive species, and may make invasive species more competitive in response to new global change-driven combined stresses. These results are consistent with aliens coming from very diverse ecosystems with generally higher biotic and abiotic stress pressures, and having higher nutrient concentrations. On the contrary, these results are not consistent with the 'excess carbon' hypotheses. These results indicate changes in vegetation terpene emissions brought about by alien plant invasions. ; This study was supported by the University of Hawai'i (G. P. Wilder research funds) and the grants from the Spanish Government (CGL2006-04025/BOS and Consolider-Ingenio Montes CSD 2008-00040), the Catalan Government (SGR 2009- 458), the Spanish National Research Council (CSIC-PIF08-006- 3), the Estonian Science Foundation (grant 7645), and the Estonian Ministry of Education and Science (SF1090065s07). ; Peer reviewed

17 páginas, 3 figuras, 4 tablas, 1 apéndice. ; Capacity for terpene production may confer advantage in protection against abiotic stresses such as heat and drought, and also against herbivore and pathogen attack. Plant invasive success has been intense in the Hawaiian islands, but little is known about terpene content in native and alien plant species on these islands. We conducted a screening of leaf terpene concentrations in 35 native and 38 alien dominant plant species on Oahu island. Ten (29%) of the 35 native species and 15 (39%) of the 38 alien species contained terpenes in the leaves. This is the first report of terpene content for the ten native species, and for 10 of the 15 alien species. A total of 156 different terpenes (54 monoterpenes and 102 sesquiterpenes) were detected. Terpene content had no phylogenetic significance among the studied species. Alien species contained significantly more terpenes in leaves (average ± SE=1965±367 μg g −1 ) than native species (830±227 μg g −1 ). Alien species showed significantly higher photosynthetic capacity, N content, and lower Leaf Mass Area (LMA) than native species, and showed higher total terpene leaf content per N and P leaf content. Alien species, thus, did not follow the expected pattern of "excess carbon" in comparison with native species. Instead, patterns were consistent with the "nutrient driven synthesis" hypothesis. Comparing alien and native species, the results also support the modified Evolution of Increased Competitive Ability (EICA) hypothesis that suggests that alien success may be favored by a defense system based on an increase in concentrations of less costly defenses (terpenes) against generalist herbivores. ; This research was supported by grants from the Spanish Government (CGL2006-04025/BOS and Consolider-Ingenio Montes CSD 2008-00040), the Catalan Government (SGR 2009-458), Estonian Science Foundation (grant 7645), and the Estonian Ministry of Education and Science (SF1090065s07) ; Peer reviewed

15 páginas, 3 tablas, 8 figuras. ; Polar forests populated high-latitude landmasses during the Cretaceous and early Paleogene when the climate was warm and the atmospheric CO2 concentration ([CO2 ]) was at least double that of the present-day. We investigated emission rates and composition of monoterpenes in relation to photoinhibition and photoprotection of two evergreen and three deciduous ''living fossil'' tree species representative of these ancient forests, after growth in a simulated Cretaceous polar (75 N) environment at either ambient (400 ppm) or elevated (800 ppm) [CO2 ] for four years. Total monoterpene emission rates peaked during the polar summer, when prolonged periods of 24 h light would have been conducive to enhancing the formation and chemical lifetime of ozone, and hence its radiative forcing. During most of the growing season, a-pinene was the main compound emitted, although the magnitude and chemical speciation of emissions was highly species specific. Growth at elevated [CO2 ] changed the composition of monoterpene emissions in evergreen and deciduous species, but only altered total leafscale emission rates of the deciduous species. The continuous daylight (photon flux: 300– 400 mmol PAR m 2 s 1 ) of the simulated polar summer (14.5 weeks) did not result in leaf photoinhibition or increased photoprotection via the xanthophyll cycle. Nevertheless, species with high a-pinene emission rates showed also high values of photochemical efficiency at ambient [CO2], suggesting a possible function of this monoterpene in protecting the photosynthetic apparatus. We suggest that high monoterpene emissions from forests during the long polar summers may constitute a regional high-latitude climatic feedback through alteration of tropospheric composition, and secondary organic aerosol formation. These biotic effects may be important to represent in Earth system models of ancient ''greenhouse'' climates. ; This study was supported by a European Union Marie Curie postdoctoral fellowship to L.L. (contract EVK2 –CT – 2002 – 50022) and by the Spanish Government REN2006 – 4025/BOS and the Catalan Government SGR2005 – 00312 projects. ; Peer reviewed

14 páginas, 6 figuras, 3 tablas. ; Information about soil VOC inventories and exchange rates in different soils is very scarce. Seasonality of soil VOC exchange rates is also largely unknown, despite the increasing interest in some soil volatile compounds, such as monoterpenes, because of their important role in soil ecology. We aimed to explore and quantify soil VOC exchange rates in a Mediterranean shrubland and their seasonality. Measurements of soil VOC exchange were taken using GC-MS and PTR-MS techniques, together with soil temperature, soil moisture and soil CO2 efflux measurements, during two annual campaigns with contrasting precipitation. Methanol, acetic acid, ethyl acetate, acetaldehyde, acetone, C3 and C4 carbonyls (such as methyl ethyl ketone), a-pinene and limonene, showed the highest emission rates. Maximum soil monoterpene emission rates were very low (0.003 nmol m 2 s 1 ) compared with foliar monoterpene emission rates. The emission rates of the other VOCs were also low (maximum 0.8 nmol m 2 s 1 ) except for methanol (1.2 nmol m 2 s 1 ). Maximum soil uptake rates for some VOCs, such as methanol and acetonitrile (ranging from 0.1 to 0.5 nmol m 2 s 1 ) were, however, comparable with foliar uptake rates. Further studies are needed to corroborate these results and the possible importance of the soil VOC sink in regional chemistry-climate models. Long-term severe drought increased soil monoterpene emission rates in this Mediterranean shrubland. The increases seem to be linked to changes in the soil's physical properties induced by low soil moisture. Unlike monoterpenes, other soil VOC emission rates decreased when soil moisture was low. The results suggest a seasonal control of soil temperature on the emission rates of monoterpenes and other VOCs. The emission rates increase with soil temperature. Positive correlations between the VOC exchange rates and the soil CO2 fluxes suggest that phenology of roots and microorganisms also controls seasonal changes in soil VOCs in this Mediterranean shrubland. ; This research was supported by Spanish MEC grant CGL2006- 04025/BOS. We also gratefully acknowledge partial funding by the ISONET European Commission contract MC-RTN-CT- 2003-504720, the European Commission contract ALARM 506675, the European project NITROEUROPE GOCE017841 and the Catalan Government SGR2005-003/12 grant. ; Peer reviewed