Temperate and Boreal tree growth declines in spite of increasing atmospheric CO2

text:Background/Question/Methods Despite the general assumption that recent changes in atmospheric CO2 are indiscriminately increasing plant growth, empirical evidence has shown contrasting results. Changes in growth can be species and site dependent and it remains questionable as to whether plants will have the ability to overcome new ecophysiological obstacles faced due to modified atmospheric condition. Here we used standard dendrochronological methods to evaluate changes in growth since 1850 in two coniferous (black spruce - Picea mariana (Mill.) B. S. P.; red pine - Pinus resinosa Ait.) and two deciduous (red oak - Quercus rubra L.; red maple - Acer rubrum L.) tree species along a latitudinal gradient representing the gradual transition between Temperate and Boreal forests in Ontario, Canada. Natural forest stands were chosen near the areas of: Turkey Point (42°42'N; 80°21'W); Algonquin Park (45°35N; 78°21'W); Sudbury (46°51'N; 80°37'W); Cochrane (49°10'N; 80°59'W) and Moosonee (51°17'N; 80°38'W). Species were sampled following their actual range of distribution; both deciduous species and red pine at the southernmost site (Turkey Point); all four species in the Algonquin Park and Sudbury sites and only black spruces in the two northern sites (Cochrane and Moosonee). Two wood cores taken from 20 – 35 trees per species at each site were analyzed for changes in ring width and annual basal area increment (BAI). Results/Conclusions With the exception of red pines in the Sudbury area, which present stable but low growth rates since 1900, a release in BAI coincided with increases in atmospheric CO2. This positive relation, however, was only highly significant until the 1980s and for all species annual BAI either stabilized or decreased after that. This general pattern was observed for both the youngest and the oldest individuals and, therefore, represents a stem growth decline that cannot be explained by natural senescence processes. Moreover, this trend appears to be weakly associated with or unrelated to historical disturbance or latitudinal variation for the different sites. Only black spruce was significantly influenced by latitude, as it had the most severe BAI decline at the southern sites (after 1980 in Turkey Point and 1998 in Algonquin Park), while in the two highest latitudes BAI either stabilized (after 2000 in Cochrane) or showed a continuous increment in stem growth (Moosonee). Further investigations of an intrinsic decline in CO2 sensitivity with tree size or in relation with recent local scale changes in climate are key to elucidate these trends and increase accuracy in estimating terrestrial carbon stocks.