PARENT SESSION
Oral Session #24: Forests: Carbon allocation, carbon budgets. Presiding: C. Reid.
Tuesday, August 7, 2001. 8:00 AM to 11:15 AM. Hall of Ideas F.
Fine root biomass and carbon turnover in a jarrah (Eucalyptus marginata Donn ex Sm.) forest in south-western Australia.
Grierson, Pauline1, Adams, Mark1,2, 1 2
ABSTRACT- We studied seasonal patterns of fine root biomass and soil carbon in Jarrah (Eucalyptus marginata Donn ex Sm.) forest. Sampling was stratified by understorey composition - either a dense Banksia grandis Willd. understorey (Banksia), or no B. grandis and only a diffuse understorey of other small shrubs (Jarrah). Root samples were sorted into >2mm and <2 mm diameter classes by depth. The <2 mm diameter class was further separated into "root mat" (cluster and/or mycorrhizal mat of laterals) and other roots. Fine root biomass increased rapidly in response to greater water availability in winter and spring. The root mat fraction senesced in the following dry summer and decomposed in autumn. Fluctuations in root biomass were more pronounced in Banksia plots owing to intensive proliferation of cluster roots, where annual changes in root biomass (0-20 cm) were about 15 t ha-1 yr-1 compared to 10 t ha-1 yr-1 in Jarrah plots. Soil and root 
13C signatures were significantly (P<.0001) and positively correlated in Jarrah plots but not in Banksia plots. Possible explanations for de-coupling of root and soil C under Banksias include rapid loss of C from cluster roots through respiration and/or greater fractionation of carbon isotopes with decomposition. These data demonstrate considerable seasonal and spatial variation in fine root growth and the importance of increasing representation of roots in models that describe carbon and nutrient cycling in mediterranean-type ecosystems.
KEY WORDS: Banksia grandis, cluster roots, mycorrhizas, 13C