ALBERT

Description: Understanding processes driving mortality in forests is important for comprehension of natural stand dynamics and for informing natural disturbance-based ecosystem management. There has been considerable study of mortality in forests during the self-thinning phase but we know much less about processes driving mortality in stands at later successional stages. We addressed this through study of five 1ha spatially explicit permanent plots in mature (111 to 186 years old in 2012) Pinus contorta stands in the Canadian Rocky Mountains using data from repeated measurements over a 45 year period, dendrochronological information, and point pattern analysis. We tested the hypothesis that these stands had completed the self-thinning/density-dependent mortality stage of succession. Contrary to our expectations, the self-thinning phase can persist for more than 140 years following stand establishment. Our findings suggest this was attributable to prolonged post-fire establishment periods due to surface fires in three of the plots while in the other two plots moist conditions and slow growth most likely delayed the onset of competition. Several pieces of evidence indicated the importance of density-dependent mortality in these stands over the study period: 1) The diameter distribution of individuals changed from initially right-skewed towards normality as a result of mortality of smaller diameter stems; 2) Individuals of lower canopy positions were proportionally more affected by mortality; 3) When compared to the pre-mortality pattern, surviving stems in all stands had an increasingly uniform spatial distribution. In two of the plots, recent windthrow and/or ingrowth initially hindered our ability to detect density-dependent mortality but our dendrochronological sampling and permanent plot data allowed us to untangle the different processes at play; in doing so we demonstrate for the first time how density-independent processes can mask underlying density-dependent mortality processes in older stands. Mortality of larger dominant canopy trees increased over the study period and mortality of dominant stems was a random process in all stands suggesting these stands were approaching the end of the self-thinning stage and that density-independent processes might soon become more important. Our results provide an improved understanding of mortality processes that can be applied to natural-disturbance based ecosystem management. This article is protected by copyright. All rights reserved.

Description: Question What are the processes driving regeneration and successional pathways in Pinus contorta ssp. latifolia Douglas ex Loudon forests? Location P. contorta forests of the Canadian Rockies in Alberta, Canada. Methods We used five 1 ha long-term permanent plots, dendrochronological stand reconstruction, and detailed spatial analysis of stem mapped data. Results Establishment of shade intolerant species mainly occurred within 20 years post-fire but important later recruitment of these species was observed in the drier plots, primarily in gaps in stands that had more open canopies. Shade-tolerant species established quickly following fires on moister sites but, despite apparent seed source availability after the stand-initiating fire, their establishment was delayed in the drier sites, most likely due to moisture limitations. Shade-tolerant regeneration was clustered around shade-tolerant trees that had established immediately post-disturbance; this suggests the latter served as seed sources and perhaps created favorable microsites for establishment. Conclusion Competition for light, dispersal limitation, and moisture limitations were all processes influencing regeneration and successional pathways in these P. contorta stands. Although initial studies in these stands had suggested some of the stands could potentially be self-perpetuating P. contorta forests, our results suggest that all plots studied will likely transition towards dominance by shade-tolerant species. However the rate of this transition will likely be slower in the drier sites compared to the moister sites since the density of shade-tolerant regeneration was much lower in these sites and took longer to establish (ca. 70-90 years) post-disturbance. This article is protected by copyright. All rights reserved.