The inter-annual variability (IAV) of the terrestrial carbon cycle is tightly linked to the variability of semi-arid ecosystems. Thus, it is of utmost importance to understand what the main meteorological drivers for the IAV of such ecosystems are, and how they respond to extreme events such as droughts and heatwaves. To shed light onto these questions, we analyse the IAV of carbon fluxes, its relation with meteorological variables, and the impact of compound drought and heatwave on the carbon cycle of two similar ecosystems, along a precipitation gradient. A four-year long dataset from 2016 to 2019 was used for the FLUXNET sites ES-LMa and ES-Abr, located in central (39°56'25″ N 5°46'28″ W) and southeastern (38°42'6″ N 6°47'9″ W) Spain. We analyse the physiological impact of compound drought and heatwave on the dominant tree species, Quercus ilex. Our results show that the gross primary productivity of the wetter ecosystem was less sensitive to changes in soil water content, compared to the dryer site. Still, the wetter ecosystem was a source of CO₂ each year, owing to large ecosystem respiration during summer; while the dry site turned into a CO₂ sink during wet years. Overall, the impact of the summertime compound event on annual CO₂ fluxes was marginal at both sites, compared to drought events during spring or autumn. This highlights that drought timing is crucial to determine the annual carbon fluxes in these semi-arid ecosystems.

This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.

陆地碳循环的年际变率（IAV）与半干旱生态系统的变率密切相关。因此，了解此类生态系统 IAV 的主要气象驱动因素是什么，以及它们如何应对干旱和热浪等极端事件至关重要。为了阐明这些问题，我们分析了碳通量的 IAV、它与气象变量的关系，以及复合干旱和热浪对两个相似生态系统碳循环的影响，以及降水梯度。从 2016 年到 2019 年的四年长数据集用于 FLUXNET 站点 ES-LMa 和 ES-Abr，位于中部（39°56'25″ N 5°46'28″ W）和东南部（38°42' 6″ N 6°47'9″ W) 西班牙。冬青栎。我们的研究结果表明，与干燥场地相比，湿润生态系统的总初级生产力对土壤含水量的变化不太敏感。尽管如此，由于夏季生态系统呼吸作用大，潮湿的生态系统每年都是 CO _{2的来源。}而在潮湿年份，干燥场地变成了 CO _2汇。总体而言，与春季或秋季的干旱事件相比，夏季复合事件对两个地点的年 CO _{2通量的影响都是微不足道的。}这凸显了干旱时间对于确定这些半干旱生态系统中的年度碳通量至关重要。

本文是主题问题“2018 年欧洲严重干旱和热浪的影响：从地点到大陆规模”的一部分。