Seasonal Changes of Soil and Ecosystem Respiration in a Young Forest J Hunt, T McSeveny and F Kelliher 4-6 th Feb 2008, Weintal Resort, Tanunda, Australia.

Carbon storage New Zealand is a signatory to the Kyoto Protocol –Required to create inventory of its GHG sources and sinks –Reduce GHG emissions to a fixed amount NZ GHG emissions are increasing –Increased fuel use –Increased reliance on coal power stations –Increase in methane emitters –Reduction in pine plantations Effort going into carbon storage in marginal lands, esp. re-afforestation with kanuka)

The forest Kanuka (Kunzia ericoides, Myrtaceae) Short lived ( yrs) Fast growth habit, light demanding Secondary invader, reclaims old pasture Exotic grass/herb understory 7m tall trees, ha -1 First forest on site, 35 yrs old 49 tC ha -1,equivalent to a net gain of 1.4 tC ha -1 y -1

Site Location Island in the middle of the braided Rakaia River Forest occupies 800 m x 2000 m 50 km SW of Christchurch (43 ºS) 60 masl Soils 100 yrs old NW foehn + katabatic winds X

Instrumentation Solar powered, closed-path, eddy covariance Profiling system Background met. Soil moisture and temperature Monthly soil surface respiration and biomass measurements

Seasonal understory biomass Tree leaf biomass was ~ 200 g m -2 Unimodal change in total understory biomass Change in live:dead ratio between spring and autumn

Seasonal ecosystem respiration Summer drought reduced ecosystem respiration by up to 7-fold Spring and autumn respiration have similar response to temperature

Ecosystem respiration under well watered conditions At same temperature, autumn respiration 13% lower then in spring Suggests change in carbon substrate supply or quality

Response of ecosystem respiration to changes in soil VWC Fit R 10 to all data to remove temperature effect If VWC < 12% then linear response of R 10 to increasing soil moisture

Comparison between soil surface and ecosystem respiration Soil and ecosystem respiration bimodal Ratio of Rs/Re remained stable until rewetting in April After rewetting 3-fold increase in R s Sustained increase in R s for 2 months

Conclusions Response of ecosystem respiration to temperature greater in spring than autumn Due to increased substrate availability with active growth and greater biomass When VWC > 12%, temperature and substrate availability were main drivers of ecosystem respiration Rs/Re varied from 50-90%, not caused by differences in soil-canopy temperatures Dynamic forest understorey can influence magnitude and seasonal maxima of ecosystem respiration