ELEVATED CO2 AND TREE ROOT-GROWTH - CONTRASTING RESPONSES IN FRAXINUS-EXCELSIOR, QUERCUS-PETRAEA AND PINUS-SYLVESTRIS

Root growth and respiration in elevated CO2 (700 mu mol mol(-1)) was s tudied in three tree species, Fraxinus excelsior L., Quercus petraea. L. and Pinus sylvestris L. grown in open-top chambers (OTCs) during a long-term exposure (20 months), during which root systems were allowed to develop without restriction imposed by pots. Root growth, measured as root length using root in-growth bags was increased significantly in trees exposed to elevated CO2, although the magnitude of the respon se differed considerably between species and with time of sampling, th e greatest effect observed after 6 months in ash (ratio of elevated:am bient, e:a; 3.40) and the smallest effect observed in oak (e:a; 1.95). This was accompanied by changes in specific root length, with a signi ficant decrease in all species after 6 months, suggesting that root di ameter or root density were increased in elevated CO2. Increases in ro ot length might have resulted from an acceleration in root cell expans ion, since epidermal cell size was significantly increased in the zone of elongation in ash root tips (P < 0.05). Contrasting effects of ele vated CO2 were observed for root carbohydrates, with significant incre ases in soluble sugars for all species (P < 0.05), but both increases and decreases in starch content were observed, depending on species, a nd producing a significant interaction between species and CO2 (P < 0. 001). Exposure to elevated CO2 increased the total root d. wt for whol e trees of all three species after 8 months of exposure, although the magnitude of this effect, in contrast to the root in-growth study, was greatest in Scots pine and smallest in ash. No significant effect of elevated CO2 was observed on the root:shoot ratio. Further detailed an alysis of whole root systems after 20 months confirmed that species di fferences in root responses to elevated CO2 were apparent, with increa sed coarse and fine root production in elevated CO2 for Scots pine and ash respectively. Lateral root number was increased in elevated CO2 f or all species, as was mean root diameter. Root respiration rates were significantly reduced in elevated CO2 for all three species. These re sults provide firm evidence that exposure of trees to future CO2 conce ntrations will have large effects on root system development, growth, carbohydrate status and respiration. The magnitude and direction of su ch effects will differ, depending on species. The consequences of such responses for the three species studied are discussed.