Tree rings have long been used to calibrate the net primary production (NPP) time-series predicted by process-based models, based on an implicit assumption that ring-width indices (RWI) can well reflect temporal NPP change. However, this assumption has seldom been tested systematically. In this study, 36 plots were set in three forest types from four sites along a latitudinal gradient in northeast China. For each plot, we constructed chronologies and stand NPP of the past 20 years to examine: is RWI a good proxy of inter-annual variation of forest NPP for different forest types under different climate? If it is, why? Our results indicate that RWI was closely related to stand NPP in most cases, and could be used as a good proxy of NPP in temperate forests. Standard and arstan chronologies were better related to NPP series than residual chronology. Stand NPP time-series were mainly determined by large trees, and the correlation between RWI and NPP was also higher for larger trees. We suggest that large trees and dominant species of canopy layer should be sampled for chronology construction. Large trees are major contributors of forest biomass and productivity, and should have priority in forest conservation in a rapid-warming world.

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Tree radial growth is widely found to respond differently to climate change across altitudinal gradients, but the relative roles of biotic factors (e.g. forest type, height and density) vs. climate gradient remain unclear. We sampled tree rings from 15 plots along a large altitudinal gradient in northeast China, and examined how climate gradient, forest type, height, tree size and density affect: (1) temporal growth variability [mean sensitivity (MS) and standard deviation (SD) of the chronologies], and (2) the relationship of ring width indices (RWI) with historical climate. We used BIC based model selection and variable importance to explore the major drivers of their altitudinal patterns. The results showed that: both growth variability and RWI-climate relationships changed significantly with altitude. Forest height was the most important predictor for altitudinal changes of MS and SD. For RWI-climate relationships, forest type was more important than climate gradient, while height and stem density were weak but necessary predictors. We showed that the altitudinal difference in growth response to climate change cannot be explained by climate gradient alone, and highlight the necessity to examine the influence of biotic factors (which covary with climate across geographic gradient) to better understand forest response to climate change.

Aims This research examined the different response of tree growth to climate change at the early, middle, late and matured successional stages of Korean pine (Pinus koraiensis) and broadleaf mixed forest. Methods This research used dendroecological methods to analyze radial growth at different successional stages of Korean pine forests in response to climatic change in the Shengshan Nature Reserve of Heilongjiang. Important findings Chronology statistics indicated that the sensitivity of radial growth to inter-annual climate variability decreased from early to later successional stage. Meanwhile, the influence of some climate indices to radial growth also changed during the successional process. Radial growth of matured forest was positively related to mean monthly temperature in June and December of previous year, revealing an obvious "lag effect" of temperature. The ring-width of matured forest had a negative correlation with mean monthly temperature but a positive correlation with monthly precipitation in June of current year, reflecting the limiting role of growing season water availability. However, these limiting effects gradually disappeared towards the earlier stages of forest succession. On the contrary, the growth of early successional forest was negatively correlated to precipitation in June of previous year, and this effect disappeared towards the later successional stages. These differences revealed increased demand of water by dominant species from early to later successional stages. A moving correlation analysis showed that the increased warming and drying climate in the research area had both positive and negative influences on radial growth of each forest type. Earlier successional forests did not show clear long-term growth