Romà Ogaya

Estimation of plant water concentration by the reflectance Water Index WI (R900/R970)

Abstract Water Index WI (R900/R970) was used for the estimation of plant water concentration (PWC) by ground-based, reflectance measurements. Reflectance and PWC were measured for adult plants growing in the field throughout an annual cycle and in potted seedlings submitted to progressive desiccation. The species studied were characteristicly Mediterranean: Pinus halepensis, Quercus ilex, Quercus coccifera, Arbutus unedo, Cistus albidus, Cistus monspeliensis, Phillyrea angustifolia, Pistacia lentiscus and Brachypodium retusum . WI was significantly correlated with PWC when all the species were considered together, and with almost all the species considered individually, especially when a wider range of PWC was obtained by extreme dessication of experimental plants. The correlations increased when normalizing WI by NDVI. The wavelength of the trough corresponding to water absorption band tended to shift from 970-980 nm to lower wavelengths 930-950 nm with decreasing PWCs. Infrared measurement of plant temp...

Comparative field study of Quercus ilex and Phillyrea latifolia: photosynthetic response to experimental drought conditions

Quercus ilex and Phillyrea latifolia growing in a holm oak forest in Prades mountains (NE Spain) were subjected to experimental drought conditions. Soil water availability was reduced about 15% by plastic strips and funnels that partially excluded rain throughfall and by ditch exclusion of water runoff. Diurnal courses of maximum photochemical efficiency of PSII (Fv/Fm), apparent photosynthetic electron transport rate (ETR), net photosynthetic rate (A), transpiration rate (E) and water use efficiency (WUE) were measured in sunlit and shade leaves of both species during 2 years. Moreover, the responses of photosynthetic rates to PPFD and CO2 concentrations were also measured. Q. ilex experienced lower E rates and higher A rates and WUE than P. latifolia throughout the experimental period, but during summer drought these differences disappeared. Q. ilex exhibited a less cold sensitive behavior whereas P. latifolia showed a more heat–drought resistant behavior. Under severe summer drought conditions none of the two species was able to reach a positive carbon gain. Drought treatment produced a slight decrease in Fv/Fm values of Q. ilex plants and a strong decrease in Fv/Fm values of P. latifolia only in winter 2000, when drought stress coincided with cold stress. Drought treatment produced also a slight decrease in ETR values of both species. During midday, A and E rates decreased in drought plots in both species associated to lower photochemical efficiencies. In those drought plots, only P. latifolia was able to increase WUE by reducing transpiration losses during midday. Both species tended to present higher A rates for a given soil humidity in drought than in control plots. However, whereas Q. ilex A rates increased with soil humidity, P. latifolia A rates did not increase above 17% soil humidity, showing no water availability response above such threshold. It is very likely that mesic species such as Q. ilex lose competitive advantage in the drier environment forecasted for next decades than the more xeric P. latifolia.

Contrasting foliar responses to drought in Quercus ilex and Phillyrea latifolia

Leaf morphology, longevity, and demography were examined in Quercus ilex and Phillyrea latifolia growing in a holm oak forest in Prades mountains (northeast Spain). Four plots (10 × 15 m) of this forest were submitted to an experimental drought during three years (soil moisture was reduced about 15 %). Leaf area, thickness and leaf mass per area ratio (LMA) were measured in sun and shade leaves of both species. Leaf longevity, the mean number of current-year shoots produced per previous-year shoot (Sn/Sn-1), the mean number of current-year leaves per previous-year shoot (Ln/Sn-1), and the percentage of previous-year shoots that developed new ones were measured once a year, just after leaf flushing. LMA and leaf thickness increased since leaf unfolding except in summer periods, when stomatal closure imposed low photosynthetic rates and leaves consumed their reserves. LMA, leaf area, and leaf thickness were higher in Q. ilex than in P. latifolia, but leaf density was higher in the latter species. Drought reduced the leaf thickness and the LMA of both species ca. 2.5 %. Drought also increased leaf shedding up to ca. 20 % in Phillyrea latifolia and decreased it up to ca. 20 % in Q. ilex. In the later species, Sn/Sn-1 decreased by 32 %, Ln/Sn-1 by 41 %, percentage of shoots developed new ones by 26 %, and leaf area by 17 %. Thus the decrease of leaf number and area was stronger in the less drought-resistant Q. ilex, which, under increasingly drier conditions, might lose its current competitive advantage in these Mediterranean holm oak forests.

Comparative seasonal gas exchange and chlorophyll fluorescence of two dominant woody species in a Holm Oak Forest

Summary Diurnal courses of net CO 2 uptake rate, stomatal conductance (g S ), maximum photochemical efficiency of PSII (Fv/Fm) and apparent photosynthetic electron transport rate (ETR) were measured in Quercus ilex and Phillyrea latifolia in a holm oak forest throughout the seasons of the year. These measurements were complemented with response curves of photosynthetic rates to PPFD and CO 2 concentrations. P. latifolia was better adapted to drought and warm conditions and showed higher net CO 2 uptake, g S and Fv/Fm values than Q. ilex in summer. But in autumn and specially in winter P. latifolia was more sensitive to low temperatures and experienced lower net CO 2 uptake, g S , ETR and Fv/Fm values than Q. ilex . The maximum net CO 2 uptake values for P. latifolia occurred under summer high temperatures whereas maximum net CO 2 uptake values for Q. ilex occurred under winter low temperatures. However, in summer during midday, both species presented null or slightly negative net CO 2 uptake rates. Since in the summer season both species experienced similar ETR values, the lower net CO 2 uptake values of Q. ilex suggest that Q. ilex presented greater photorespiration rates. During winter, very low Fv/Fm values were found especially for P. latifolia , indicating that maximal photochemical efficiency of PSII is very sensitive to low temperatures. However, they were not accompanied by low net CO 2 uptake rates showing that cold photoinhibition determined a potential but not an actual decrease in photosynthetic performance. Under well watered conditions and with high CO 2 concentration and saturated PPFD, Q. ilex was able to increase its photosynthetic rates whereas P. latifolia had lower plasticity to make a profit of optimal environmental conditions. These results show different strategies between these two dominant co-occurring species. They also indicate that the warmer and drier conditions expected for the Mediterranean region in the near decades as a result of climate change will favour drought resistant species with lower photosynthetic rates such as P. latifolia in detriment of more mesic species such as Q. ilex .

Dampening effects of long‐term experimental drought on growth and mortality rates of a Holm oak forest

Forests respond to increasing intensities and frequencies of drought by reducing growth and with higher tree mortality rates. Little is known, however, about the long-term consequences of generally drier conditions and more frequent extreme droughts. A Holm oak forest was exposed to experimental rainfall manipulation for 13 years to study the effect of increasing drought on growth and mortality of the dominant species Quercus ilex, Phillyrea latifolia, and Arbutus unedo. The drought treatment reduced stem growth of A. unedo (-66.5%) and Q. ilex (-17.5%), whereas P. latifolia remained unaffected. Higher stem mortality rates were noticeable in Q. ilex (+42.3%), but not in the other two species. Stem growth was a function of the drought index of early spring in the three species. Stem mortality rates depended on the drought index of winter and spring for Q. ilex and in spring and summer for P. latifolia, but showed no relation to climate in A. unedo. Following a long and intense drought (2005-2006), stem growth of Q. ilex and P. latifolia increased, whereas it decreased in A. unedo. Q. ilex also enhanced its survival after this period. Furthermore, the effect of drought treatment on stem growth in Q. ilex and A. unedo was attenuated as the study progressed. These results highlight the different vulnerabilities of Mediterranean species to more frequent and intense droughts, which may lead to partial species substitution and changes in forest structure and thus in carbon uptake. The response to drought, however, changed over time. Decreased intra- and interspecific competition after extreme events with high mortality, together with probable morphological and physiological acclimation to drought during the study period, may, at least in the short term, buffer forests against drier conditions. The long-term effects of drought consequently deserve more attention, because the ecosystemic responses are unlikely to be stable over time.Nontechnical summaryIn this study, we evaluate the effect of long-term (13 years) experimental drought on growth and mortality rates of three forest Mediterranean species, and their response to the different intensities and durations of natural drought. We provide evidence for species-specific responses to drought, what may eventually lead to a partial community shift favoring the more drought-resistant species. However, we also report a dampening of the treatment effect on the two drought-sensitive species, which may indicate a potential adaptation to drier conditions at the ecosystem or population level. These results are thus relevant to account for the stabilizing processes that would alter the initial response of ecosystem to drought through changes in plant physiology, morphology, and demography compensation.

Establishment of co-existing Mediterranean tree species under a varying soil moisture regime

We investigated the responses of two co-existing Mediterranean trees with different regeneration strategies (Phillyrea latifolia seedlings and Quercus ilex sprouts) to experimental drought below the forest canopy. We considered different recruitment stages and used leaf isotopic discrimination to estimate water use efficiency (WUE) and nitrogen availability and use. Drought decreased the emergence and survival of seedlings and sprouts. Survival and growth of older saplings were not influenced by drought. Seedling emergence of P. latifolia was higher than Q. ilex sprout production, but Q. ilex sprouts had higher survival and growth rates. These differences disappeared in the sapling stage. Carbon isotopic discrimination suggested that Q. ilex sprouts had higher WUE than P. latifolia seedlings. Drought increased WUE of recruits, particularly in Q. ilex. Water use regulation increased with ageing, particularly in P. latifolia. Q. ilex had higher d15N values than P. latifolia; these were also higher under drier soil conditions. Current year seedlings had higher d15N than saplings, particularly in P. latifolia, suggesting they exploit superficial soil layers. These results suggest that sprouts obtain benefit from resources stored in parent plants. At earlier stages, they perform better than seedlings. This response is not coupled to adult vulnerability to drought for these species, revealing the difficulty of predicting species dynamics during climate change.

Phenological patterns of Quercus ilex, Phillyrea latifolia, and Arbutus unedo growing under a field experimental drought

Abstract: A holm oak forest located in the Prades Mountains (northeast of Spain) was subjected to an experimental drought to determine its phenological responses. Soil water availability was reduced by 15% by plastic strips and funnels that partially excluded rain throughfall and by ditch exclusion of water runoff. We monitored eight phenological events: leaf shedding, leaf flushing, flower bud formation, flowering, flower senescence, fruit growth, fruit maturation, and fruit abscission in the three dominant species, Quercus ilex, Phillyrea latifolia, and Arbutus unedo once a week from winter 1999 to winter 2001. We also collected litterfall in circular baskets randomly distributed on the ground every 15 d from winter 1999 to winter 2001 and every 2 months from winter 2001 to winter 2003. Arbutus unedo showed a higher proportion of individuals flowering and fruiting than Q. ilex and P. latifolia. Arbutus unedo was also the most sensitive species to water availability since drought treatment delayed its phenophases, whereas this treatment did not significantly affect the timing of the other two species’ phenophases. The flower and fruit production was also greater in A. unedo than in the two other species, but inter-annual variability was high, and no significant drought effect was found in any of the three species. Stem litterfall was greater in drought plots than in control plots during the overall studied period. In a drier environment, as predicted for Mediterranean areas in the near future by global circulation models, drought-resistant species such as P. latifolia could present greater ability to produce reproductive structures than less resistant species such as Q. ilex or A. unedo. This different response among species could produce changes in seedling recruitment and resprouting ability and, in the longer term, in species distribution.

Influence of water and terpenes on flammability in some dominant Mediterranean species

In the Mediterranean basin, fires are a major concern for forest and shrubland ecosystems. We studied flammability, its seasonality and its relationship with leaf moisture and volatile terpene content and emission in the dominant species of a Mediterranean shrubland and forest in Catalonia (NE Iberian Peninsula). We measured temperatures and time elapsed between the three flammability phases: smoke, pyrolysis and flame, for four seasons. We sampled twice in spring because of an occasional drought period during this season. Flammability had a significant relationship with leaf hydration, in the shrubland and in the forest. Few and only weak correlations were found between terpene content and flammability. In the future, arid conditions projected by climatic and ecophysiological models will increase fire risk through decreased hydration and subsequent increased flammability of the species.

Urgent need for a common metric to make precipitation manipulation experiments comparable

In recent decades, researchers have increasingly focused attention on how climatic change will affect ecosystem functioning. Much of this research has centred on the effects of rising atmospheric CO2 concentrations and increasing temperatures (e.g. Ainsworth & Long, 2005; Wu et al., 2011). The consequences of altered precipitation patterns, by contrast, have received far less attention. The results of precipitation manipulation experiments were only recently synthesized for the first time (Wu et al., 2011), and the conclusions that can be drawn regarding global patterns remain preliminary. In general, reduced water inputs slow ecosystem processes, while increased rainfall enhances plant productivity (Wu et al., 2011), but it remains unclear how this response differs among ecosystems. When synthesizing water manipulation experiments, firm conclusions are not only precluded by the insufficient number of data points (Wu et al., 2011), but also by the difficulty of defining the magnitude of the perturbation experienced by the biota (further coined the ‘actual’ treatment). Unlike in the case of, for example, elevated CO2, the magnitude of the imposed treatment (e.g. )20% precipitation vs +300 ppm CO2) does not clearly indicate the actual treatment. Many factors influence the way an ecosystem experiences a change in rainfall; of greatest importance to the ecosystem is not the amount of incoming precipitation, but rather the amount of water that plants have access to. This ‘plant available water’ strongly depends on factors such as soil texture and rooting depth (Tolk, 2003), and the latter can show substantial seasonal and interannual variation (Knapp et al., 2008) and may differ between treatments. Additionally, runoff water and stem flow can complicate estimations of the magnitude of an imposed manipulation (Cotrufo et al., 2011), and plants that access groundwater supplies can complicate interpretations of treatment effects. If we want to understand why plant responses to altered precipitation differ among ecosystems, comparisons of effects of precipitation manipulations must use a more ecologically meaningful metric to describe the actual treatment than merely the change in precipitation. Without such a relevant ‘common denominator’, observed differences in ecosystem responses to altered precipitation may reflect differences not only in ecosystem properties, but also in the actual treatment. Potential metrics for quantification of the actual treatment

Increased sensitivity to climate change in disturbed ecosystems

Human domination of the biosphere includes changes to disturbance regimes, which push many ecosystems towards early-successional states. Ecological theory predicts that early-successional ecosystems are more sensitive to perturbations than mature systems, but little evidence supports this relationship for the perturbation of climate change. Here we show that vegetation (abundance, species richness and species composition) across seven European shrublands is quite resistant to moderate experimental warming and drought, and responsiveness is associated with the dynamic state of the ecosystem, with recently disturbed sites responding to treatments. Furthermore, most of these responses are not rapid (2-5 years) but emerge over a longer term (7-14 years). These results suggest that successional state influences the sensitivity of ecosystems to climate change, and that ecosystems recovering from disturbances may be sensitive to even modest climatic changes. A research bias towards undisturbed ecosystems might thus lead to an underestimation of the impacts of climate change.