Influence of non-climatic factors on the relationships between tree growth and climate over the Chinese Loess Plateau

• We introduce the BRT method to analyze of influences of the non-climatic factors.
• The old trees and the cores taken from the south are more sensitive to drought.
• The slow-growing trees rely more on the nutrients of the previous year.

Modulations of non-climatic factors on climate–growth relationships can confound the interpretation of dendroclimatic reconstructions. Accordingly, we evaluated the influences of core direction, tree age, tree size, growth rate and species on the climate–growth relationships of 210 tree-ring cores from Kongtong Mountain and Xiaolong Mountain on the semi-arid Chinese Loess Plateau. We employed both the linear methods of correlation and response function analyses, and the nonlinear method of the Boosted Regression Trees (BRT). Most trees are sensitive to the moisture availability. Old trees are more sensitive to drought than young trees, which may be related to the intensified hydraulic resistances and reduced photosynthesis in response to drought. Tree cores taken from the southern direction are often more sensitive to drought than cores taken from other directions. The enhanced drought sensitivity for the southern cores may be possibly related to more branches and leaves for photosynthesis. Slow-growing trees show higher correlations with precipitation of the September prior to growth. This indicates that the extremely stressed, slow-growing trees tend to rely largely on nutrients produced prior to the growing season. For the reconstructions of annual drought in this region, we suggest collecting tree cores from the southern side of old trees growing at stressed sites.