Factors promoting larch dominance in central Siberia: fire versus growth performance and implications for carbon dynamics at the boundary of evergreen and deciduous conifers

Abstract

The relative role of fire and of climate in determining canopy species composition and aboveground carbon stocks were investigated. Measurements were made along a transect extending from the dark taiga zone of central Siberia, where <i>Picea</i> and <i>Abies</i> dominate the canopy, into the <i>Larix</i> zone of eastern Siberia. We test the hypotheses that the change in canopy species composition is based (1) on climate-driven performance only, (2) on fire only, or (3) on fire-performance interactions. We show that the evergreen conifers <i>Picea obovata</i> and <i>Abies sibirica</i> are the natural late-successional species both in central and eastern Siberia, provided there has been no fire for an extended period of time. There are no changes in performance of the observed species along the transect. Fire appears to be the main factor explaining the dominance of <i>Larix</i> and of soil carbon. Of lesser influence were longitude as a proxy for climate, local hydrology and active-layer thickness. We can only partially explain fire return frequency, which is not only related to climate and land cover, but also to human behavior. <br><br> Stand-replacing fires decreased from 300 to 50 yrs between the Yenisei Ridge and the upper Tunguska. Repeated non-stand-replacing surface fires eliminated the regeneration of <i>Abies</i> and <i>Picea</i>. With every 100 yrs since the last fire, the percentage of <i>Larix</i> decreased by 20%. <br><br> Biomass of stems of single trees did not show signs of age-related decline. Relative diameter increment was 0.41 &plusmn; 0.20% at breast height and stem volume increased linearly over time with a rate of about 0.36 t C ha⁻¹ yr⁻¹ independent of age class and species. Stand biomass reached about 130 t C ha⁻¹(equivalent to about 520 m³ ha^&minus;1). Individual trees of <i>Larix</i> were older than 600 yrs. The maximum age and biomass seemed to be limited by fungal rot of heart wood. 60% of old <i>Larix</i> and <i>Picea</i> and 30% of <i>Pinus sibirica</i> trees were affected by stem rot. Implications for the future role of fire and of plant diseases are discussed.