Effects of ozone on forests. 1) The left of the figure shows a healthy forest being under low ambient ozone levels while 2) the right parts indicated a forest being under high ozone levels. 3) Photosynthesis is the major process in ecosystems; It take place during daylight hours. It comprises CO2 uptake, assimilation and storage as biomass, and O2 production, as well as water vapor release. 4) Along with CO2 uptake, O3 is also untaken. The exchange gases occurs through stomata (tissue openings in the leaf surface). 5) O3 moves through intercellular spaces, penetrates cell membranes and triggers a cascade of intracellular reactions, causing chlorosis and finally cell collapse and development of necrotic spots, senescence and prematurity. 6) Thus, at community level, ozone disturbs forest stands by altering the inter- and intra-specific competition between individuals depressing the O3-sensitive individuals and species to the benefit of the O3-tolerant ones; it also makes the O3-sensitive individuals more vulnerable to other diseases. Thus, O3 , at long-term, may alter plant species diversity. 7) Besides, O3 reduces root size (biomass and length) and affects root symbiotic endo- and ecto-phytic microorganisms -including mycorrhiza- of O3-sensitive individuals, through the reduction and composition(?) of feeding secretions.
The Gordian Knot of the Forest-Ozone-Carbon interactions. In the pre-industrial epoch, carbon is stored via photosynthesis (1) and leads to long-term carbon sequestration into above- and below-ground (roots and soil) wood biomass. However, in the climate change epoch with higher temperatures and increased greenhouse gases, including ambient ozone (O3), the situation is complex. The higher CO2 levels, alone, in the atmosphere are expected to “feed” forest growth and have beneficial effects. The increased O3 levels, alone, depress forest trees, contributing to “forest decline syndrome”, i.e., visible injury, lower photosynthesis, lower carbon sequestration, lower carbon storage (7) and biomass decay, which also releases CO2 in the atmosphere (8). In a positive feedback, the depressed forest vegetation emits more BVOCs (4), further increasing O3 levels. Concurrent elevated concentrations of CO2 end O3 may outcome to a sustained increase in Net Primary Productivity (NPP), while the adverse long-term effect of increased O3 on NPP may be lesser than projected. Elevated CO2 levels negate or even overcompensate the negative O3 effect on ecosystem functions and the cycles of carbon and nitrogen. Anthropogenic emissions of CO2, NOx, and volatile organic compounds (VOCs) (3) as well as biogenic VOCs (BVOCs) emitted by forests (4) contribute to increased O3 levels in the atmosphere. Soil microbial processes contribute to soil-emitted BVOCs and NOx (O3 precursors) (Gray et al., 2010) as well as CO2, N2O and CH4 (5). Under advanced climate change, forest fires are expected to be more frequent and larger than in the pre-industrial epoch, due to reduced availability of moisture that dries out vegetation, providing fires’ fuel. These fires release carbon monoxide (CO), organic carbon (OC), NOx (all of which contribute to O3 formation), and black carbon (BC) (which influences photosynthesis by increasing diffuse radiation) as well as CO2 (which further intensifies global warming)(9). Besides, rising temperatures are expected to increase insect outbreaks. Moreover, in a warmer future, it is expected the currently dominant forest types to be completely displaced by other forest types of adapted tree species (Thomas et al., 2009). Thus, the overall enigma remains unsolved……
