Data from: The role of nutritional impairment in carbon-water balance of silver fir drought-induced dieback

Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in the southwestern of Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (δ18O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical – chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate – growth relationships and lower δ18O than ND trees. The uncoupling between iWUE and δ18O, together with the positive correlations between P and K leaf concentrations and δ18O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies.

树种干旱分布极限处的后缘种群尤其易受干旱引发的森林衰退影响。欧洲西南部的银冷杉（Abies alba）林分便是典型代表。尽管此前已有针对银冷杉干旱诱导衰退模式的研究，但关于营养受损所起作用的信息仍有缺失，尽管其与树木碳-水分平衡存在潜在相互作用。本研究针对西班牙比利牛斯山脉4个林分中的衰退个体（declining, DD）与健康个体（non-declining, ND）银冷杉，对其径向生长、内在水分利用效率（intrinsic water-use efficiency, iWUE）、氧同位素（δ¹⁸O）及叶片养分浓度开展了对比分析。此外，本研究还评估了森林衰退易感性、种内性状变异（木材密度与叶片性状）与根际土壤理化性质之间的关联。衰退个体的生长衰退始于二十多年前，此后其应对干旱的生长恢复力较低。衰退个体的叶片中磷（P）、钾（K）、铜（Cu）和镍（Ni）等养分浓度始终低于健康个体。叶片养分状况对生长恢复力指数的显著影响，证实了矿质营养在干旱发生前、发生期间及发生后对树木生理功能与生长的关键作用。相较而言，木材密度与叶片形态性状的变异，以及土壤性质，与树木营养状况及干旱响应表现的相关性较弱。在低海拔、温度更高的样地中，衰退个体的气候-生长相关性更强，且其δ¹⁸O值低于健康个体。内在水分利用效率与δ¹⁸O之间的解偶联，以及叶片P、K浓度与δ¹⁸O的正相关关系，表明衰退个体利用更深层的土壤/基岩水源，且其养分吸收与水分吸收存在垂直解偶联现象。本研究为解析银冷杉衰退的驱动机制提供了新视角，并强调了将树木营养状况纳入森林衰退研究的必要性。