Carbon isotopes of C3 herbs correlate with temperature on removing the influence of precipitation across a temperature transect in the agro-pastoral ecotone of Northern China

Plant δ13C–temperature (δ-T) relation has been established in many systems and is often used as paleotemperature transfer function. However, it is still unclear about the exact contributions of temperature variation to plant 13C discrimination because of covariation between temperature and precipitation (aridity), which reduces confidence in reconstruction of paleoclimate. In this study, we measured carbon isotope composition (δ13C) of 173 samples of C3 perennial herbs from 22 sites across a temperature gradient along the 400 mm isohyet in the farming-pastoral zone of North China. The results showed that precipitation obviously affected the correlations of temperatures and foliar δ13C. After removing the influence of precipitation by analysis of covariance (ANCOVA), a more strongly positive relationship was obtained between site-mean foliar δ13C and annual mean temperature (AMT), with a regression coefficient of 0.1636‰/°C (p = .0024). For widespread species, Artemisia lavandulaefolia and Artemisia capillaries, the slopes (or coefficients) of foliar δ13C and AMT were significantly steeper (larger) than those of foliar δ13C and AMT where the precipitation influence was not excluded, whereas the δ-T coefficients of Polygonum persicaria and Leymus chinensis showed little change across the transect after deducting the precipitation effect. Moreover, the positive relationship between temperature and δ13C over the transect could be explained by soil moisture availability related to temperature. Our results may afford new opportunities for investigating the nature of past climate variability.

植物碳同位素δ¹³C与温度（δ-T）的关联已在诸多生态系统中得到证实，常被用作古温度转换函数。然而，由于温度与降水（干旱度）存在共变关系，温度变化对植物¹³C分馏的确切贡献仍不明确，这降低了古气候重建结果的可信度。本研究沿中国北方农牧交错带内的400毫米等雨量线温度梯度，在22个样点采集了173份C3型多年生草本植物样品，测定了其碳同位素组成（δ¹³C）。结果显示，降水显著影响了温度与叶片δ¹³C之间的相关性。通过协方差分析（ANCOVA）消除降水影响后，样点平均叶片δ¹³C与年平均气温（AMT）之间呈现出更强的正相关关系，回归系数为0.1636‰/℃（p=0.0024）。对于广布种茵陈蒿（Artemisia lavandulaefolia）和Artemisia capillaries，其叶片δ¹³C与年平均气温的斜率（或系数）显著高于未排除降水影响时的对应值；而在扣除降水效应后，Polygonum persicaria和Leymus chinensis的δ-T系数沿样带未出现明显变化。此外，样带上温度与δ¹³C之间的正相关关系可通过与温度相关的土壤有效含水量得到解释。本研究结果为探究过去气候变率的本质提供了新的途径。