Canopy Conductance in a Two-Storey Siberian Boreal Larchforest, Russia

Abstract:

A larch forest in eastern Siberia was characterized by the presence of two distinct stories, the overstory with a small leaf area index (LAI) and a dense understory with a relatively large LAI. To understand the roles of the over- and understory in forest-atmosphere water exchange, canopy conductance (G_c), a critical parameter used in determining the energy and mass exchange, was calculated based on latent heat flux above the overstory and understory, measured separately. Results showed that G_c for the overstory (G_co) and understory (G_cu) experienced different seasonal fluctuations. G_co was smaller than G_cu during periods of leaf expansion and leaf fall and showed an increasing trend until one month after the onset of leaf expansion. In contrast, a sharp decrease in G_co was observed immediately before onset of leaf fall. Furthermore, G_co was slightly larger than G_cu during the fully foliated period. A simple model using solar radiation and vapor pressure deficit (D) as inputs successfully reproduced the G_c in fully foliated periods with acceptable accuracy. Furthermore, both the understory and overstory in this study have a large reference G_c (G_c at D = 1KPa) than their counterparts of other boreal forests and would not be able to sustain a constant leaf-soil water potential difference as D increases. We speculated that this confers the forest with an advantage allowing it to be able to sustain carbon assimilation during large D days and thus provides for the survival of the ecosystem during the short growing season at this site.