Tree growth and forest dynamics at transitional zones between temperate and boreal forests of northeastern China and eastern Canada

To understand how tree growth responds to climate change at tree distribution limits and what are the potential changes in tree distribution ranges under the future climate, we used dendrochronological and field survey methods to investigate tree responses to climate var iability at the transitional zones in northeastern China and western Quebec. The selected study areas are dominated by mixed forests with many tree species growing at their distribution limits. In northeastern China, we focused on three dominant conifers of an elevational gradient (750-1800 rn) in Changbai Mountain. The trees studied included Korean pine (Pinus koraiensis Siebold et Zuccarini), Jezo spruce (Piceajezoensis Carr. var. komarovii (V.Vassil.) Cheng et L.K.Fu), and Manchurian fir (Abies nephrolepis (Trautv.) Maxim.) . We observed climatic controls of growth of the dominant species and potential shifts in altitudinal position of vegetational ecotones as a result of climate change. In eastern Canada, we analyzed climate-growth relationships of red maple (Acer rubrum L.) along a latitudinal gradient ( 47-49° N) at its northern distribution limit, and discussed direct and indirect climatic effects on its future distribution. Since windthrows are main disturbances affecting tree growth and forest dynamics in Changbai Mountain, we first reconstructed disturbance history in the study area over the period 1770-2000 (Chapter I), based on the analyses of growth release and recruitment patterns. Percentages of growth releases in subcanopy trees were below 6% in most decades, suggesting that disturbances initiating these releases were of low severity. Strong winds possible cause of modera te disturbance events. Two episodes with increased disturbance rates (19% and 13%) were dated to the 1920s and 1980s. Shade-intolerant Olga bay larch (Larix olgensis Henry) recruited mainly before the 1860s. Recruitment of mid-tolerant Korean pine occurred as several regeneration waves (1820s, 1850s, 1870-1880s, 1930s, and 1990-2000s) of moderate intensity. Shade-tolerant Jezo spruce and Manchurian fir regenerated continuously over the last 220 and 130 years, respectively. An enhanced recruitment of Korean pine, Jezo spruce, and Manchurian fir was observed during the 1930s and 1990s, which were coincided with an increased growth release frequency in the 1920s and 1980s, suggesting disturbance events of moderate intensity. We studied regeneration density, biomass accumulation, and growth sensitivity of Korean pine, Jezo spruce, and Manchurian fir at their respective distribution limits in Changbai Mountain (Chapter II). Regeneration densities did not differ significantly among the elevations except for the Jezo spruce, which showed a significantly lower regeneration at its lower limit than at upper limit. All three species showed a significantly higher basal area increment (BAI) at the middle part of their distribution ranges than at their limits. They also showed higher growth sensitivity to temperature than to precipitation at their upper limits and the inverse pattern was observed at their lower limits. At respective upper limits, summer temperatures of the current year (for Korean pine and Jezo spruce) and spring temperature of the current year (for Manchurian fir) positively affected tree growth. At respective lower limits, precipitation in autumn of the previous year had a positive effect on growth. The study suggested that elevational limits of forest vegetation were constrained by climate factors affecting growth of dominant species rather than those controlling regeneration density. We concluded that climate change would likely shift the elevational positions of the climate optima for the growth of canopy dominants, leading to changes in the species and ecotone ranges. In Chapter III, we examined growth rate and growth sensitivity to climate of red maple by analyzing dendrochronological data from nine sites located along a 300 km transe ct ( 47-49 °N) covering three bioclimatic domains in western Quebec. All three growth variables, i.e., growth rates during the first 30 years of maple lifespan, cumulative basal area increment over the most recent decade (2000 -2009) , and annual growth rate over the whole tree lifespan, were positively related to latitude. Annual variability of maple growth was positively affected by the previous year September temperature across the whole transect. July temperature of the current year and December precipitation of the previous year had a positive influence on the growth in the northern part of transect ( 48-49 °N). Mid-summer (July) drought limited the growth in the southern part of transect ( 47-48 °N). We interpreted an apparent discrepancy between the results of absolute growth analyses and response function analyses by the overriding effect of stand history on the growth pattern of red maple. Specifically, maples recruited after large disturbance events such as stand replacing fire in the north could take advantages of primarily higher light levels for the growth, compared to those in the south which likely occurred in canopy gaps. Expected climate change would likely favor red maple growth in the northem balsam fir - paper birch domain, resulting in an extension of maple distribution to the northem boreal mixedwoods. However, the natural and human disturbance regimes affecting the growth conditions at the site lev el appear to be dominant controls of the actual biomass productivity of red maple at its present distribution range. Our study suggested that distribution limits of studied species were controlled through a combination of direct climatic and disturbance-mediated effects on their recruitment and growth. The direct effects of future climate change may positively affect tree growth and recruitment at higher elevational (Changbai Mountain) or latitudinal limits (western Quebec). While the effects of changes in disturbance regimes on trees maintenance in the forest were differed in two study area, responses to these changes were mainly dependent on species life strategy and ecological traits. In Changbai Mountain, increases in temperatures and wind disturbances may facilitate trees migration towards higher elevation, leading to an upward shift of altitudinal position of broadleaf-Korean pine/spruce-fir ecotone. While in western Quebec, negative influences of increases in fire activity and severity may override positive effects of direct climate on maple growth. We believe that better projections in precipitation dynamics, which will directly control levels of fire activity in the context of climate warming, may enhance our ability to project future changes in northem distribution limit of red maple.