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We aimed to reveal the latitudinal trends in foliar δ 13 C and δ 15 N of Quercus variabilis, a widely distributed species in East Asia, associated with two ontogenetic stages (juvenile and mature trees) along a North-South transect (26°-40° N). The results showed that mature trees had higher foliar δ 13 C and δ 15 N values than juveniles. Foliar δ 13 C and δ 15 N values of trees with both ontogenetic stages were nonlinearly increased and decreased with latitude, respectively. No interaction between ontogenetic stage and latitude for the changes of foliar δ 15 N and δ 13 C indicated that both ontogenetic stages across the transect consistently responded to latitudinal environmental variations. Results from the random forest models indicated that foliar δ 15 N of Q. variabilis was mainly affected by soil nutrient contents, e.g., soil organic matter, phosphorus, nitrogen, whereas dominated factors for foliar δ 13 C were related to moisture, such as relative humidity, precipitation of growing season.

To explore potential effects of soil nutrient on leaf thermal tolerance across geographical origins, five provenance, i.e., Chengbu of Hunan (CB), Zigui of Hubei (ZG), Neixiang of Henan (NX), Lincheng of Heibei (LC) and Pinggu of Beijing (PG), seedlings of Quercus variabilis were cultivated under nitrogen and phosphorus addition. Leaf thermal tolerance parameters (cold, heat and span), nutrient concentrations (nitrogen and phosphorus), nitrogen use efficiency (NUE) and phosphorus use efficiency (PUE), as well as non-structural carbohydrate (soluble sugar and starch) concentrations were analyzed. The results were shown as follows: Nutrient absorption and utilization efficiency varied between oak origins with no obvious geographical trends. PG had higher nutrient use efficiency (NUE and PUE), whereas NX had lower PUE under all treatments. CB had the highest PUE under phosphorus addition. Compared with the control, nutrient addition increased the cold tolerance of PG and LC, and to some extent, increased the heat tolerance of the three middle provenances (ZG, NX, LC). However, the thermal span was opposite to the cold tolerance, as nutrient addition decreased the thermal span of PG and LC but increased that of NX. The leaf cold tolerance had significantly positive correlation with soluble sugar concentration, while the heat tolerance negatively and positively correlated with leaf phosphorus and ratio of nitrogen to phosphorus, respectively. No significant correlation was found between leaf thermal span and leaf chemical substances. To sum up, nutrient use efficiency varying in provenances might be contributed by the original habitats and consequently presented with some local adaptation characteristics, which complicated the response of thermal tolerance to nutrient addition.

Age structures and spatial distribution patterns of Quercus variabilis populations were analyzed across geographical gradients (latitude, longitude and altitude) by using the size-grading method and the ratio of variance to mean. The results showed as following: Over the horizontal gradient, the northern, middle, southern and western populations of Q. variabilis exhibited an inverse-J shape, but the eastern populations declined. The spatial patterns of adult individuals were all clumped except the northern populations which were randomly distributed. Juveniles were clumped in the northern, middle and western populations, but were randomly distributed in the southern and eastern populations. The aggregation intensity of juveniles across latitude decreased with the order as the central, northern and southern populations, but as the central, southern and northern populations for adult individuals. The aggregation intensity of both juveniles and adults across longitude followed a decreased order as the central, western and eastern populations. Along the altitudinal gra-dient, the inverse-J type occurred only in the low- and middle-altitude populations, but populations in the high altitude declined. The juveniles in populations among altitude gradient all were clumped, but the adults were all clumped except the low-altitude populations which were randomly distributed. The aggregation intensities of both juveniles and adults were higher in the middle than the other altitudinal populations. Compared with adults, juveniles generally had higher aggregation intensities across various geographical gradients. Our results revealed that the age structure and spatial distribution pattern of Q. variabilis were mainly determined by environment variation across geographical gradients and the species' biological property, which supported the central-marginal hypothesis.

Knowledge of latitudinal patterns in plant defense and herbivory is crucial for understanding the mechanisms that govern ecosystem functioning and for predicting their responses to climate change. Using a widely distributed species in East Asia, Quercus variabilis, we aim to reveal defense patterns of trees with respect to ontogeny along latitudinal gradients. Six leaf chemical (total phenolics and total condensed tannin concentrations) and physical (cellulose, hemicellulose, lignin and dry mass concentration) defensive traits as well as leaf herbivory (% leaf area loss) were investigated in natural Chinese cork oak (Q. variabilis) forests across two ontogenetic stages (juvenile and mature trees) along a ~14°-latitudinal gradient. Our results showed that juveniles had higher herbivory values and a higher concentration of leaf chemical defense substances compared with mature trees across the latitudinal gradient. In addition, chemical defense and herbivory in both ontogenetic stages decreased with increasing latitude, which supports the latitudinal herbivory-defense hypothesis and optimal defense theory. The identified trade-offs between chemical and physical defense were primarily determined by environmental variation associated with the latitudinal gradient, with the climatic factors (annual precipitation, minimum temperature of the coldest month) largely contributing to the latitudinal defense pattern in both juvenile and mature oak trees.