Adaptive strategies of Cistus species to Mediterranean stress factors

Plant functional diversity is achieved by trade-offs between physiological and morphological traits which are now widely used to define plants’ adaptive strategies to cope with environmental changes. Under Mediterranean conditions, species have to face two main stress factors, namely winter chilling and summer drought. Such factors show a great spatial variability through the Mediterranean Basin and contributed to diversify plant species adaptive strategies. Furthermore, local plant populations within the Mediterranean Basin can be exposed to different selective pressures possibly affecting their adaptive strategies in response to the same environmental cues. Common garden experiments have been proved valuable approaches to disentangle differences in leaf traits among populations of Mediterranean species, particularly in widespread ones. Among them, the genus Cistus (Cistaceae) comprises 21 species which developed with the advent of the Mediterranean climate and contributed to shape the current composition and diversity of the Mediterranean area. They are dominant elements in the Mediterranean shrub communities of European-African-Mediterranean ecosystems. In this context, the aim of the present thesis was to analyze some aspects of the adaptive strategies of the three widespread Cistus spp., namely C. monspeliensis L., C. salvifolius L. and C. creticus subsp. eriocephalus (Viv.) Greuter & Burdet) in response to the main limiting factors of the Mediterranean climate. Plants were grown from seeds of different provenances under common garden conditions. In particular, common pattern of response across species growing under the same environmental cues as well as the intraspecific variability in such responses were analyzed. From an ecological point of view, three main features make these species important to investigate: (i) they have pioneer character, making early growth a key point of their adaptive strategies; (ii) being seasonal dimorphic species they display two leaf types (winter and summer leaves) characterized by a relatively short leaf longevity and relatively low leaf construction costs during a growing season, which seems to be an advantage in habitat under intense stress; (iii) they possess a marked short-term physiological plasticity in response to environmental changes. However, within these three features, some aspects of Cistus spp. adaptive strategies have not yet been clarified. In detail: (i) the early growth of this species in relation to environmental changes has been overlooked so far; (ii) it is not yet clear if these species can actually rely on the formation of different functional leaf ‘populations’ within a single leaf type in order to face environmental changes; (iii) there is no previous study on the profits that can be achieved by adopting a marked short-term physiological plasticity in response to environmental changes. To address the broad objective and fill the highlighted gaps of knowledge, different experimental approaches were used making up a total of 3 Chapters (plus General Introduction, General Discussion and Conclusions). In Chapter 2, by fitting non-linear models on seedling height data, it was possible to analyze the relationship between early growth parameters and daily air temperature changes. A parameter that could summarize the temperature dependency of the maximum relative growth rate (RGRmax) during the early growth stages for the selected provenances was quantified. Nevertheless, a greater temperature sensitivity of RGR lead to a reduced maximum height reflecting a negative trade-off between the extent of RGR temperature responsiveness and the length of the developmental phases. The findings quantitatively defined provenance dependent strategies by which the selected species cope with daily air temperature variations during early growth. In Chapter 3 leaves produced under different climatic conditions through the winter season (i.e. chilling stress) to spring were analyzed in order to highlight the leaf traits involved in determining potential resilience of three Cistus spp. to changing environmental conditions and to what extent intraspecific differences could affect such response. The results highlighted a complex mechanism that involves diachronic leaf trait adjustments that are parameter-dependent. The findings also demonstrated the existence of different functional leaf populations in Cistus spp. which further extend the current understanding of winter leaves structuring and functioning. Such leaf functional differentiation allows the species to minimize leaf construction costs while maximizing resource acquisition and provide a less costly resilience mechanism in the transition from winter to spring season. In Chapter 4, the short-term physiological response of the considered Cistus spp. during an imposed experimental drought and subsequent recovery was analyzed. A positive trade-off between stress and recovery responses during a short-term drought experiment was found, and it was also consistent across species and provenances. The study highlighted a short-term functional strategy which involves a rapid recovery of gas exchange parameters to avoid the progression of photoinhibition. Such response was associated with the lack of leaf shedding. Such short-term strategy is advantageous by allowing the species to hold the more costly summer leaves (compared to winter leaves) in order to maintain relatively high productivity to face prolonged drought events. Overall, the results demonstrated that Cistus spp. are affected by stress imposition but they can recover as soon as favorable conditions are restored. This is achieved through their ability to respond to climate unpredictability by adjusting their photosynthetic organs, morphologically and/or physiologically, in order to enhance their performance during favorable climatic conditions. Nevertheless, such ability can constrain their response to stochastic changes in thermal amplitudes during a growing season due to climate change thus threatening their competitive ability. The characterization of plant species responses to the major targets of climate change is important in order to highlight mechanisms that can be further investigated both experimentally and in the field. The reported findings provide a conceptual framework, in terms of experimental designs, traits to be included and mechanisms, that deserve to be applied by including other Cistus spp. in the field in order to extend the results at the genus level.