Abstract
The current ongoing increase in the atmospheric CO2 concentration is an unquestionable fact. Thus, plants are bound to live in a more enriched CO2 world in a not far-off future. In this new framework, regeneration of forest tree species may be modified as a consequence of the change in the current patterns of seedling response to other environmental resources, such as water or light. We studied the impact of an elevated CO2 concentration on the interaction of drought and light upon the water relations of cork oak (Quercus suber L.) seedlings. In a complete factorial design of contrasting light (HL vs LL), water (WW vs S) and CO2 levels (800 ppm vs 370 ppm), we analysed the influence of each factor and its interaction in the modification of different leaf water parameters in potted seedlings after a 6 months experimental period. These parameters were derived from the construction, with leaf materials, of the P-V curves: osmotic potential at full turgor (Ψπ100), osmotic potential at zero turgor (Ψπ0), modulus of elasticity at full turgor (εmax), and the ratio dry/turgor weight (DW/TW). Doubling of the CO2 levels over the current concentration (380 ppm) did not change any of the studied leaf water parameters, while light and water availabilities had a significant influence. This result does not exclude changes in other basic physiological parameters that could modify the pattern of cork oak regeneration responding to a CO2 enriched atmosphere in the future, and under climatic conditions different to the current ones.
Highlights
In Mediterranean forests, water stress and light may be considered as the main climatic limiting factors that control regeneration success of forest tree species
Ψπ0 and Ψπ100 decreased in response to water stress in high light (HL), as well as in low light (LL) seedlings (Table 2)
The decrease was steeper in HL seedlings, since the slopes and intercepts from the relationships between Ψpd and Ψπ0 or Ψpd and Ψπ100 were higher for HL (Fig. 2)
Summary
In Mediterranean forests, water stress and light may be considered as the main climatic limiting factors that control regeneration success of forest tree species. A new environmental factor must currently be taken into account, the progressive increase in the levels of the atmospheric carbon dioxide concentration (IPCC, 2007) Such increase, in conjunction with other greenhouse gases, is considered to be responsible for climatic change. One of the effects predicted in the future is the likely increase in the occurrence and intensity of drought in the Mediterranean basin (Pastor and Post, 1988; Peñuelas et al, 1998). Whether these changes may induce displacements in the actual recruitment of seedlings in the understory of forest ecosystems and the patterns of segregation in the regeneration niches is an important issue that needs to be unravelled (Bazzaz and Miao, 1993; Hättenschwiler and Körner, 2000). How the interaction of both factors, co-occurring in nature, will be modified in the future enriched CO2 world is uncertain, as are the changes promoted in forest composition
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