Due to climate change, drier summers have been observed over the last ten years in Mediterranean areas. Increasing drought levels may have a different weight in influencing the stomatal versus photosynthetic activity of forests, altering the water-use efficiency (
In a Mediterranean-type climate, forest ecosystems are typically subjected to high temperature and scarce soil water availability during most of the summer. Moreover, due to the impact of climate change, more extreme drought periods are expected for the coming decades as a consequence of reduced precipitation during summers (
In turn, climate change may result in a negative impact on plant carbon assimilation through the increase of stomatal limitation to photosynthesis (
Previous models were based on the simplifying assumption of a constant linear relationship between gs and Amax, with a simple dependence upon vapour pressure deficit (VPD -
The use of stable carbon (δ13C) isotope as a powerful tool for investigating the balance between Amax and gs (see isotope theory) has grown steadily during the past two decades. The positive relationship between WUE and δ13C arises through their independent linkages to the ratio of internal to ambient CO2 concentrations (
The broader purpose of this study was to test in the Mediterranean environment a dual isotope conceptual model (
The stable isotope technique has been revealed to be an important tool in identifying medium and long-term effects of environmental factors on CO2 and H2O gas exchanges in plants. The carbon isotope composition (δ13C) of leaf organic matter reflects the fractionation processes occurring during the diffusion of 12CO2 and 13CO2 through stomatal pores and photosynthetic assimilation. In C3 plants, discrimination against 13C is linked to photosynthesis
where
where
The oxygen isotope composition (δ18O) of leaf organic material can be used for distinguishing between possible causes of variation in δ13C, thanks to the link of δ18O with the isotopic fractionation of water during transpiration in leaves. During transpiration, molecules of water containing lighter isotopes (H2 16O) tend to diffuse faster from the site of evaporation to the atmosphere. In this way, water becomes enriched in the heavier isotopes of 18O, compared to water coming from the soil. The oxygen isotopic composition of leaf water at the sites of evaporation (ΔE) is expressed as follows (
where ΔV is the oxygen isotopic composition of water vapour in the air, ε* is the temperature-dependent fractionation associated with the lower vapour pressure of H218O compared to that of H216O, εk is the kinetic fractionation during evaporative water diffusion through the stomata and boundary layer, and ea/ei are the vapour pressures in the atmosphere and intercellular air spaces, respectively.
Thus, according to this equation, the degree of leaf water enrichment depends on the rH. The latter represents the evaporative driving force, and a reduced rH causes an increase of δ18O in the leaf water. This enrichment is then expected to be reflected in the organic matter (
The dual isotope conceptual model proposed by
The research was carried out in a drought-prone 50 ha of Mediterranean macchia. The site was dominated by the coppice
Two different levels of volumetric soil water content (SWC, volume of water per volume of soil, multiplied by 100) have been imposed by an alteration of the amount of precipitation reaching the soil. This was done during the summers of 2004 and 2005 (June to August) on three replicated plots (~100 m2). A mean value of 7% in SWC was obtained by partial rain exclusion (-20%), using a system of pipes suspended about 1.8 m above the forest floor (water-depleted plots, D). A mean value of 14% in SWC was obtained by adding water through a sprinkler net to simulate rain events (watered plots, W). A 10% threshold of SWC, established in a pre-treatment experiment, represented the dry (below) and well-watered (above) conditions (
SWC was measured within the D and W treatments by probes (Campbell Scientific, INC, Logan, Utah, USA). These probes consisted of two 30 cm long stainless steel rods, fully inserted into the soil at six different locations per replicate. The time domain reflectometry (TDR) method was used to translate the readings in SWC (
Eight intensive field campaigns were carried out during 2004 and 2005 to assess plant water status. Predawn leaf water potential (
Maximum CO2 assimilation at saturating light (Amax) and stomatal conductance (gs) were measured using a portable infrared gas analyzer (LI-6400 Li-cor, Lincoln, NE, USA). Measurements were performed on 10 sunlit leaves of six trees growing in the central portion of each replicated plot to avoid the “edge effect”. Hours between 11:30 am and 15:30 pm on cloudless days were chosen for all measurements. This is when environmental conditions were most stable and when photosynthetic photon flux density (PPFD) was above 1200 μmol m-2 s-1 (above saturating light conditions for
Six samples of non-fully expanded leaves were collected from each replicate plot in five field campaigns (June, July 2004 and June, July, September 2005) to measure carbon (δ13C) and oxygen (δ18O) isotopes. Leaf samples were collected from the same shoots where gas exchange measurements were performed.
Bulk leaf samples were dried, ground to a fine powder and then weighed in tin capsules, using a value of 0.6-0.8 mg for δ13C and 1.1-1.3 for δ18O. Bulk leaf material was combusted to CO2 for carbon isotope analysis in an elemental analyzer (EA-1108, Carlo Erba, Italy), which was connected to a mass spectrometer (Delta-S Finningan MAT, Germany)
δ13C and δ18O were calculated as (
where
Values of treatments (W, D) are presented as the mean ± standard error and compared using the Student-Newman-Keuls test. Statistical significance was defined as
As a consequence of treatments, significant differences in soil water content (SWC) emerged between water depleted (D) and watered (W) plots, although such differences were higher in 2005 than 2004 (
In general, predawn water potentials (Ψpd) reflected the SWC conditions during the two years of the experiment (
Leaf vapour pressure deficit (VPD) showed a bimodal pattern with a minimum in winter-spring and a maximum in summer, although July 2004 (4.8 kPa) was drier than July 2005 (3.5 kPa -
Drier conditions during the summer had a strong effect on gas exchange activities. Significant reductions of stomatal conductance (gs) and maximum CO2 assimilation (Amax) were observed over the seasons, in response to changes in water availability (
Significant differences in Amax between the D and W treatments were observed during the experiment, in particular during the summer drought. However, Amax values in D plots did not fall below 60% of those observed in the W treatment (
Because of parallel declines in gs and Amax, computed intercellular CO2 (ci) concentration and instantaneous water-use efficiency (WUEinst) did not differ between treatments (
The carbon isotope composition (δ13C) was not affected by changes in SWC and VPD during the experiment (
In contrast, δ18O was found to be more sensitive to variations in soil water availability than δ13C. In fact, δ18O differed significantly between D and W plots in July 2004 and July/September 2005 (
δ18O significantly increased in response to reduced stomatal aperture (
As a test of the dual isotope conceptual model, δ13C was found not significantly correlated with δ18O (r = 0.42, P = 0.23 -
Although the study period was slightly wetter than the area’s long-term average (
As a result, the enhanced drought affected water and carbon relations, leading to reductions in maximum CO2 assimilation (Amax) and stomatal conductance (gs) for
Thus, the reliable correlation observed between δ18O and gs suggests that bulk leaf material was a suitable medium to infer the physiological performance of trees. This was previously observed by
The combination of δ13C and δ18O in a semi-quantitative model revealed qualitative variations of Amax and gs across two different soil water regimes. Thus, in this investigation, the dual isotope model proposed by
The down regulation of
Although the dual isotope approach has been shown to be a reliable tool to infer the relationship between gs and Amax, it is not able to explain in details the underlying mechanism involved. In fact, under Mediterranean conditions, it is often necessary to assess the contribution of stomatal and non-stomatal limitations in driving changes in Amax and gs and their weight in this ratio that, in turn, affects the WUE.
As a result of the parallel decrease in gs and Amax, we observed little or no differences in either WUEinst and integrated WUE as assessed by δ13C. This result contrasts with most findings in Mediterranean areas, where a significant increase in WUE has been found (
The large fluctuations in WUEinst over the two seasons appear to be mainly related to changes in VPD. Thus, VPD is considered an important parameter in driving gas exchanges in Mediterranean environment (
The combination of δ13C-δ18O, in a semi-quantitative conceptual model, proved to be a valid tool for investigating the time integrated gs-Amax relationship under Mediterranean conditions. In fact, a constant δ13C and an increase of δ18O isotopes in response to reduced air and soil water availability were consistent with a parallel decline of either gs and Amax, as assessed by gas exchange. The good correlations found between δ13C and ci or between δ18O and gs confirm this result. Furthermore, either instantaneous (from gas exchanges) and integrated WUE (from δ13C isotopes) data are in agreement in showing that, as a consequence of a parallel decrease of either gs and Amax, soil water restriction had no or slight influence on WUE. VPD was shown to have a larger impact on WUE than SWC. Such a response should result in a negative feature under climate change scenarios that may further reduce the carbon sequestration and the productivity of Mediterranean forests.
This research was supported by the EU Project n. EKV2-CT-2002-00158 MIND “Mediterranean terrestrial ecosystem and Increasing Drought” and the MIUR-PRIN Project prot. 003073315_003 “Drought and Mediterranean forests: stomatal mechanisms in the regulation of plant gas exchanges”. The authors would like to thank the coordinator of the MIND project, Dr. Franco Miglietta, and the Principal Investigator of the research group, Prof. Marco Borghetti. The authors also express their gratitude to Raddi S, Nolè A, Lapolla A, Anichini M, Cantoni L and Vicinelli E for helpful support in field measurements.
The conceptual isotope model from
Correlation of instantaneous water-use efficiency (WUEinst) and vapour pressure deficits (VPD) in water depleted (D) and watered (W) plots during the experiment.
Correlation between leaf carbon isotope composition (δ13C) and intercellular to ambient CO2 concentrations (ci/ca) assessed by gas exchange in water depleted (D) and watered (W) plots during the summer in 2004 and 2005. Symbols represent the mean value ± standard error for each date of measurement.
Correlation between leaf oxygen isotope composition (δ18O) and stomatal conductance (gs) assessed by gas exchange in water depleted (D) and watered (W) plots during the summer in 2004 and 2005. Symbols represent the mean value ± standard error for each date of measurement.
The δ13C-δ18O relationship (above left panel) in water depleted (D) and watered (W) plots during the summer in 2004 and 2005. The top right panel provides information derived from the δ13C-δ18O relationship: this result is consistent with case c) in the Scheidegger model with the arrow pointing to right side; the model output of gs-Amax indicates a decrease of either gs or Amax. The bottom panel shows the correlation between Amax and gs from gas exchange measurements. Symbols represent the mean value ± standard error for each date of measurement.
Site, environmental conditions and stand characteristics. (1): Soil Taxonomy; USDA Soil Survey Staff 1999.
Location | Allumiere, Lazio, Italy |
---|---|
Latitude | 42° 11’ |
Longitude | 11° 56’ |
Altitude (m a.s.l.) | 180 |
Slope | Uniform |
Soil type | Andsols(1) |
Soil depth (cm) | 31 |
pH | 4 |
Annual rainfall (mm)1951 - 2005 | 919 |
Summer rainfall (mm) 1951 - 2005 | 120 |
Annual temperature (°C) 1951 - 2005 | 13.6 |
Age of trees (years) | 25 |
Tree density (trees ha-1) | 4070 |
Stem diameter at 1.3 m aboveground (cm) | 5 |
Height (m) | 5 |
Leaf area index (m m-2) | 5.5 |
Total basal area (m2 ha-1) | 19.6 |
Variations in vapour pressure deficit (VPD), soil water content (SWC), predawn water potential (Ψpd), stomatal conductance (gs), maximum CO2 assimilation (Amax), intercellular CO2 concentration (ci), instantaneous water-use efficiency (WUEinst), bulk leaf carbon (δ13C) and oxygen (δ18O) isotope composition in water depleted (D) and watered (W) plots during the experiment. Values marked with asterisks are significant for P < 0.05.
Date | Plot | VPD (kPa) | SWC (%) | Predawn Ψ (MPa) | gs (mol m-2s-1) | Amax (μmol m-2s-1) | ci (ppm) | WUEinst (μmol mol-1) | δ13C (‰) | δ18O (‰) |
---|---|---|---|---|---|---|---|---|---|---|
Jun-04 | D | 3.2 | 13* | -0.54 | 0.107 | 5.63 | 254 | 1.82 | -26.7 | 29.11 |
W | 3.3 | 17* | -0.46 | 0.113 | 5.21 | 257 | 1.79 | -26.9 | 29.30 | |
Jul-04 | D | 4.6 | 12* | -0.97* | 0.044* | 3.20* | 235 | 2.21* | -26.6 | 30.37* |
W | 4.4 | 15* | -0.86* | 0.078* | 5.24* | 230 | 1.81* | -26.8 | 31.22* | |
Jan-05 | D | 0.9 | 16 | -0.07 | 0.108 | 6.56* | 253 | 7.20 | - | - |
W | 0.9 | 18 | -0.05 | 0.125 | 7.86* | 246 | 7.30 | - | - | |
Feb-05 | D | 1.1 | 17 | -0.47 | 0.084* | 6.06 | 239 | 6.34* | - | - |
W | 1.1 | 20 | -0.45 | 0.110* | 7.05 | 252 | 5.52* | - | - | |
Apr-05 | D | 4.4 | 23 | -0.57 | 0.130* | 7.92* | 267 | 1.80 | - | - |
W | 4.7 | 23 | -0.52 | 0.161* | 9.02* | 273 | 1.68 | - | - | |
Jun-05 | D | 2.9 | 8* | -0.65 | 0.121* | 7.80 | 227 | 2.77* | -25.8* | 28.60 |
W | 3.1 | 14* | -0.61 | 0.148* | 8.93 | 235 | 2.15* | -26.3* | 28.70 | |
Jul-05 | D | 3.3 | 5* | -0.89* | 0.078* | 5.84* | 216 | 2.55 | -26.1 | 28.80* |
W | 3.1 | 15* | -0.44* | 0.162* | 10.11* | 228 | 2.37 | -25.9 | 29.26* | |
Sep-05 | D | 2.2 | 10* | -0.34 | 0.122* | 7.88* | 239 | 3.50 | -26.3 | 27.20* |
W | 2.3 | 21* | -0.41 | 0.151* | 8.95* | 238 | 3.39 | -26.5 | 27.90* |