| The global climate change, featured by rapidly raising temperature, has drawn intensive attention worldwide due to its huge influence on ecosystem and human society. The response of forest ecosystem to climate change, including topics on biomass accumulation, forest productivity, carbon sequestration and circulation, forest composition and tree species dynamic, etc, has been among the research hotspots. Tree-ring series honestly reflect the historical climate conditions during tree growth, therefore providing reliable records of the climate change along with its ecological results. In this paper we studied the radial growth and species dynamic of dominant tree species in the forest along an altitudinal gradient of 780-2000m a.s.l. on the northern slope of Changbai Mountain, China, using tree-ring materials, and explored the effects of tree age, elevation and site condition on growth-climate relationships, aiming to understand the responses of tree stem growth, forest composition and species dynamic to the changing climate.In order to understand the tree growth and distributional dynamic in response to the changing climate at spatial (eg, elevation and site) and temporal (eg, age) scale, tree cores were collected from Manchurian fir (Abies nephrolepis), Jezo spruce (Picea jezoensis), Korean pine (Pinus koraiensis) and Erman’s birch (Betula ermanii) on in total 10 elevations. Chronologies were built up for each species at each of their respective distributional elevations, and the relationships between tree growth and climate (temperature and precipitation) factors were analyzed. Additionally, the regeneration density, age structure and basal area increment (BAI) were investigated. The main results included:(1) The BAI and regeneration densities of each coniferous species were different across elevations. The BAI of Manchurian fir and Jezo spruce were less at their distributional margins compared with what at their interior distributional zones; the BAI of Korean pine at its lower distributional range was less than the higher elevations (p<0.05). The respective regeneration densities of Manchurian fir and Korean pine across elevations were not significantly different (p>0.05), whereas the regeneration density of Jezo spruce at its lower distributional range was lower than at the higher elevations (p<0.05).The growth-climate relationships were species specific across elevations. The correlations between growth of Korean pine and temperature were weakening with the ascending elevations, while the correlations between growth and precipitation became stronger. Contrastingly, the growths of Manchurian fir and Jezo spruce were more closely related to temperature rather than precipitation, and the growth-temperature relationships became stronger with ascending elevation. The growth of each species was not more closely related to climate at their respective upper or lower distributional margin than at it optimal distributional area; and at each of the studied elevations, the growth-climate relationships were species specific. Due to the complexity of the growth-climate relationships, the future climate warming was likely to affect the species growth and distribution in different ways. It was speculated that the distribution of Manchurian fir was likely to develop toward the lower elevations; Jezo spruce forest declined, and the distributional area of Korean pine forest was likely to be narrowed.(2) The responses of Korean pine chronologies with different lengths to climate were different due to the age effect. Overall, the radial growth of trees older than 130 years (large average cambial age samples, LCA) seemed to be more closely related to climate, compared with the growth of younger trees within 50-90 years (young average cambial age samples, YCA). Response analysis revealed that radial growth of YCA negatively responded to mean monthly temperature of current January and May, while LCA positively responded to mean monthly maximum temperature during current May. Correlation analysis showed that the mean monthly temperatures of previous October, as well as current January and May were significantly correlated with the tree-ring width of YCA. Whereas the radial growth of LCA was positively correlated with mean monthly maximum temperature of current January, February, April and September, and negatively correlated with mean monthly minimum temperature of current April. Besides, mean monthly precipitation of current May also prompted the growth of LCA. These results together showed that the assumption of age-independent climate-growth relationship was invalid for Korean pine in Changbai Mountain. Physiological processes and hydraulic constraints related to tree age, associated with the variation in environmental stress, could possibly be the main causes of these age-dependent diversities. The growth-climate regressions indicated that under the scenario of temperature increased by 4℃, and precipitation increased by 20% in northeastern China, the tree-ring growth of YCA was predicted to increase by 35.6%, whereas the tree-ring growth of LCA reduced by 25.2%.(3) The growth-climate relationships of Betula ermanii exhibited similarities over the altitudinal gradient. It was confirmed by principle component analyses (PCA) and redundancy analyses (RDA) that the tree-ring growths of Betula ermanii were governed by the regional climate rather than the micro-environment specified by elevation. Revealed by correlation analyses, in general, the low temperatures during previous June, July, December and current May, and the high temperatures during current June, July and August would benefit the radial growth of Betula ermanii. Abundant precipitations during growing seasons (previous June, July, September and current August), previous November and current May would also facilitate the tree-ring growth. The reason for this uniform growth behavior in Betula ermanii remained unclear. Betula ermanii in Changbai Mountain formed relatively pure stands with an open canopy, which might be partly contributing to the common climatic signals in tree-ring growth across the altitudinal gradient, but further work was required to test this assumption. The complexity existing in the growth-climate relationships of Betula ermanii suggested the birch forest was able to resist the future climate changes to some extent. Revealed by abundant regeneration of Betula ermanii on its lower distributional range, the birch forest had the potential of spreading downward along the altitudinal gradient.(4) At the same elevation, the chronologies of Erman’s birth from different sites responded differently to climate conditions, though the chronologies were highly correlated to each other (correlation coefficient>0.5). In general, compared with precipitation, the growth responses to temperature were more similar between sites, featured by the positive correlations with temperatures of previous September, current June and July, and by the negative correlations with temperatures of previous June to August as well as Demcember (p<0.05). The growth responses to precipitation were largely different between sites. On the elevation of 1750m a.s.l., two chronologies were positively correlated with the precipitation of previous November and September respectively; on the elevation of 1950m a.s.l., the chronologies of both sites were positively correlated with precipitation of previous June, current May and August, but only the chronology of the old site was positively correlated with precipitation of previous July and November. On the elevations of 1800m a.s.l., the chronology of new site presented little correlation with precipitation, while the chronology of old site was closely related with precipitation; oppositely, on the elevation of 1900m a.s.l., the chronology of new site showed correlations with precipitations of multiple months, whereas the chronology of old site had little correlation with precipitation. In our case, the reason for the site difference in the growth-climate (precipitation in particular) relationships however was less likely due to the site-specific soil textures and slope aspects. |
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