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sur le développement des zones arides et semi-arides









Water is a resource and a limiting factor for plants. Water transport from roots to leaves occurs mainly through the xylem. The capacity of this tissue to transport water (i.e., its hydraulic conductivity) it is not constant but varies as a function of the conditions at which the transport takes place. In particular, the probability that the water column inside the plant breaks increases when water potentials become very negative (drought) or when temperatures are very low (frost). This dysfunction, called xylem embolism, causes a decrease in the hydraulic conductivity of the tissue. The vulnerability to xylem embolism depends on the species and tissue considered, and has been recognized as one of the key factors in the response of plants to several environmental stresses. In this thesis we have studied the implications of xylem embolism for drought and frost resistance in several woody plants, with the following objectives : (1) to study the mechanism of freezing-induced xylem embolism in Larrea tridentata, (2) to compare xylem structure and function between roots and stems, specially with regard to their possible role in limiting water transport, (3) to test the existence of a trade-off between conducting efficiency and security in the xylem in species from a given community, and explore its possible causes and consequences, (4) to study the safety margins at which the different species of a given community operate, (5) to characterize the strategies to cope with drought stress of several Mediterranean species from different communities, (6) to predict the likely effects of climate change on the species studied. The vulnerability to freezing-induced xylem cavitation of Larrea tridentata in the Chihuahuan desert The vulnerability of Larrea tridentata to freezing-induced xylem embolism was studied in a Chihuahuan desert population (central New Mexico, USA). In the field we measured wood, air and soil temperatures, soil and leaf water potentials, and native embolism in roots and branches. These measurements were combined with anatomical studies and laboratory experiments in which an insulated chamber attached to a temperature bath was used to freeze stem and root samples under controlled conditions. We compared our results with the measurements by Pockman & Sperry (1997) in a Sonoran desert population of L. tridentata. The comparison showed that although the Chihuahuan population was slightly more resistant to freezing-induced xylem embolism, the impact of freezing was also greater in this population because of the much lower temperatures. This result agrees with the observation of ca. 50% stem mortality in the population studied, and suggests that low temperatures contribute to limit the present distribution of L. tridentata in central New Mexico. Within plants, stems were more limited by freezing than roots, as opposed to what happens with drought-induced embolism. Our laboratory experiments corroborated the findings by Pockman & Sperry (1997) regarding the existence of a relationship between minimum freezing temperature and % embolism. The correlation observed between the area of the low temperature exotherms produced during the freezing treatments and the resulting % embolism strongly suggested that the freezing of water inside parenchyma cells was associated with the occurrence of xylem embolism. This finding adds to the existing evidence supporting an active role of living cells in long-distance water transport. Hydraulic properties of roots and stems of nine woody species from a Holm oak forest in NE Spain The hydraulic architecture of roots and stems of nine woody species was studied in a Holm oak forest in NE Spain. The species studied differed in phylogenetic relationships, biogeographical origin, wood type and leaf-habit. Xylem anatomy, hydraulic conductivity, vulnerability to drought-induced xylem embolism, and seasonal water relations during a period of acute drought were measured. Between-tissues differences in hydraulic properties were consistent with previous findings reported in the literature. The diameter of xylem conduits, hydraulic conductivity and vulnerability to xylem embolism were always higher in roots than in stems. Surface roots were shown to live closer to their hydraulic limit than stems of similar diameter. Our results also showed large differences in hydraulic properties among species. The pressure at which a 50% of conductivity was lost due to embolism ranged between 1.3 and >8 MPa for roots and between 3.1 and >8 MPa for stems. Vulnerability to xylem embolism followed a pattern of Quercus ilex = Acer monspessulanum = Arbutus unedo = Sorbus torminalis = Cistus laurifolius > Cistus albidus = Ilex aquifolium > Phillyrea latifolia > Juniperus oxycedrus. All species showed large reductions in stomatal conductance associated with the progression of drought. Safety margins (defined as the difference between the pressure causing 95% embolism and minimum leaf water potentials under field conditions) differed markedly among species, with some of them (J. oxycedrus, I. aquifolium, P. latifolia) showing a xylem overly resistant to cavitation. A non-linear, negative relationship between hydraulic conductivity and resistance to cavitation was found when species and tissues were combined. The shape of this relationship, a power function with exponent ca. -2, is explained using a simple model that assumes a linear relationship between vessel diameter and the size of its larger pore. The implications of the trade-off implied by the above relationship are discussed in relation to drought tolerance of the species studied. Hydraulic architecture of Pinus halepensis, P. pinea and Tetraclinis articulata in a dune ecosystem of Eastern Spain The hydraulic architecture of Pinus pinea, P. halepensis and Tetraclinis articulata was studied in a coastal dune area from Eastern Spain. The measured variables include vulnerability to xylem embolism, hydraulic conductivity and carbon isotopic discrimination in leaves. Leaf water potentials were also monitored in the three studied populations during an extremely dry period. Our results showed that roots had always wider tracheids and higher hydraulic conductivity than branches. Roots were also more vulnerable to xylem embolism and operated closer to their hydraulic limit (i.e., with narrower safety margins). Although it was not quantified, extensive root mortality was observed in the two pines. T. articulata was much more resistant to embolism than P. pinea and P. halepensis. At the same time, T. articulata experienced lower water potentials. As a result, the safety margins were reasonably similar for the three species. The study outlined very different water-use strategies for T. articulata and the pines. Whereas T. articulata had a conservative strategy that relied on the low vulnerability of its conducting system, the two pines showed regulatory mechanisms at different levels (i.e., embolism, root demography) that constrained the absorption of water when it became scarce. Drought-induced mortality and hydraulic architecture in pine populations of the NE Iberian Peninsula The summers of 1994 and, to a lesser extent, 1998 were particularly dry in eastern Spain. As a result, several plant species were severely affected. We estimated drought-induced mortality in populations of three pine species that co-exist in the study area (Pinus nigra, P. pinaster and P. sylvestris). Hydraulic conductivity, vulnerability to xylem embolism, and tree-ring width were also measured for each population. Results showed that mortality only affected P. sylvestris, and that there were significant differences between two populations of this species. Although maximum hydraulic conductivity and vulnerability to embolism were almost identical among species and populations, they differed in other aspects of their hydraulic architecture. In particular : (1) hydraulic conductivity per unit of leaf area was lower in the most acutely affected P. sylvestris population. Lower leaf-specific conductivity causes higher water potential gradients and, hence, higher levels of embolism (if vulnerabilities are alike). We suggest that this difference was the main cause of the observed mortality pattern. (2) P. pinaster showed higher water-use efficiency (inferred from _13C data) than the other two species. Regarding the response to drought at the population level, the most affected P. sylvestris population slightly increased growth after the 1994 drought, which we relate to a relaxation of competition among surviving individuals. The important drought-induced mortality observed in the study area suggests that a drier climate (as predicted by climate change simulations) may endanger several P. sylvestris populations in the Mediterranean basin. Sap flow of three co-occurring Mediterranean species under varying atmospheric and soil water conditions We studied the seasonal patterns of water use of three woody species co-occurring in a Holm oak forest in NE Spain. The three species studied (Quercus ilex L., Phillyrea latifolia L. and Arbutus unedo L.) constitute *99% of the total basal area of the forest. The study period included the dry seasons of years 1999 and 2000. Water use was estimated by means of Granier-type sap flux sensors. Standard meteorological variables, soil water content, and leaf water potentials were also monitored. Daily sap flux (JS) was strongly influenced by environmental variables. In general, JS was mainly correlated with atmospheric water pressure deficit during the spring (wet period), while during the summer (dry period) soil moisture deficit explained most of the variance in JS. The average reduction in JS between early summer and the peak of the drought was 60% for A. unedo, 48% for P. latifolia, and 80% for Q. ilex. This study was also part of a drought simulation experiment. A. unedo responded to the experimental decrease in water availability reducing sap flux during the summer. This species also modified its water use between years according to their different seasonal patterns of precipitation. Estimated stand transpiration during the "relatively wet" 1999 year was 98% of net rainfall, further supporting that the community was limited by water availability. Our results, combined with previous studies in the same area, outlined very different strategies to cope with water shortage for the three species. A. unedo and Q. ilex needed a strong stomatal control over water loss to avoid low water potentials that could cause dangerous levels of xylem embolism. On the other hand, low vulnerability to xylem embolism allowed P. latifolia to have a less strict stomatal regulation and maintain transpiration at lower water potentials than the other two species. Q. ilex showed a long-term reduction of sap flux which we relate to the high levels of embolism predicted for this species. Our results suggest that Q. ilex and, to a lesser extent, A. unedo, may be at their limit to cope with water stress in many Mediterranean areas, in agreement with the acute impact that the 1994 drought had on Q. ilex populations in NE Spain. These results are discussed in relation to the possible impacts that climate change will have on Q. ilex-dominated forests. A hydraulic model to predict drought-induced mortality in woody plants : an application to climate change in the Mediterranean The potential effects of climate change on vegetation are of increasing concern. In the Mediterranean region, the dominant effect of climate change is expected to be the modification of water balance. In this paper we present a model developed to predict drought-induced mortality of woody plants under different climatic scenarios. The model is physiologically-based and simulates water transport within individual woody plants, which can be isolated or competing for a common water pool. The key assumption of the model is that, within a drought event, plant mortality is caused by the inability of plants to transport water to the leaves. In the particular application that we report in this study, we compare two evergreen species, Quercus ilex and Phillyrea latifolia, which were very differently affected by the acute drought that occurred in E Spain in summer 1994. While in some Q. ilex populations the amount of individuals that dried completely was up to 80%, P. latifolia showed no apparent damage. This two species grow together in a Holm-oak forest in NE Spain that was monitored during the years 1999 and 2000. A Generalised Likelihood Uncertainty Estimation (GLUE) approach was used to calibrate the model against sap-flow measurements. The only difference between species that was introduced a priori was that Q. ilex was more vulnerable to xylem embolism than P. latifolia (based on our own measurements in the study area). During the calibration the information provided by the measured sap flows was used to retain the more likely parameter sets for each species. These parameter sets were used in all the following simulations. The model was able to accurately simulate both transpiration dynamics and measured mortalities for the two species in the study area. In the simulations under climate change two factors were explored : the increase in mean temperature (+1.5, +3 and +4.5ºC) through its effect on ET, and the duration of summer drought. Under any of the scenarios, mortalities were much higher for Q. ilex : while this species was predicted to survive with less than 5% mortality droughts of up to 84-94 days, the mortality of P. latifolia reached 5% between the day 133 and the 150. For droughts longer than three months, which is approximately the current drought duration in the study area for dry years, the mortality of Q. ilex increased sharply. These results are discussed in relation to the possible long-term impacts of climate change on Q. ilex-dominated forests


Présentation et version intégrale (Tesis Doctorals en Xarxa)

Page publiée le 20 mars 2006, mise à jour le 31 janvier 2019