Tree growth response to drought partially explains regional-scale growth and mortality patterns in Iberian forests

Tree-ring data has been widely used to inform about tree growth responses to drought at the individual scale, but less is known about how tree growth sensitivity to drought scales up driving changes in forest dynamics. Here, we related tree-ring growth chronologies and stand-level forest changes in basal area from two independent data sets to test if tree-ring responses to drought match stand forest dynamics (stand basal area growth, ingrowth, and mortality). We assessed if tree growth and changes in forest basal area covary as a function of spatial scale and tree taxa (gymnosperm or angiosperm). To this end, we compared a tree-ring network with stand data from the Spanish National Forest Inventory. We focused on the cumulative impact of drought on tree growth and demography in the period 1981–2005. Drought years were identified by the Standardized Precipitation Evapotranspiration Index, and their impacts on tree growth by quantifying tree-ring width reductions. We hypothesized that forests with greater drought impacts on tree growth will also show reduced stand basal area growth and ingrowth and enhanced mortality. This is expected to occur in forests dominated by gymnosperms on drought-prone regions. Cumulative growth reductions during dry years were higher in forests dominated by gymnosperms and presented a greater magnitude and spatial autocorrelation than for angiosperms. Cumulative drought-induced tree growth reductions and changes in forest basal area were related, but initial stand density and basal area were the main factors driving changes in basal area. In drought-prone gymnosperm forests, we observed that sites with greater growth reductions had lower stand basal area growth and greater mortality. Consequently, stand basal area, forest growth, and ingrowth in regions with large drought impacts was significantly lower than in regions less impacted by drought. Tree growth sensitivity to drought can be used as a predictor of gymnosperm demographic rates in terms of stand basal area growth and ingrowth at regional scales, but further studies may try to disentangle how initial stand density modulates such relationships. Drought-induced growth reductions and their cumulative impacts have strong potential to be used as early-warning indicators of regional forest vulnerability.

This study was financially supported by Xunta de Galicia, Grant/Award Number PGIDIT06PXIB502262PR, GRC GI-1809; INIA, Grant/Award Number RTA2006-00117; CANOPEE, 2014-2020-FEDER funds, Spanish Science Ministry RTI2018-096884-B-C31, RTI2018-096884-B-C33, AGL2017-83828-C2-2R, RTI2018-096884-B-C3,1 and RTI2018-096884-B-C32 projects. Gabriel Sangüesa-Barreda was supported by a “Juan de la Cierva-Formación” grant from MINECO (FJCI 2016-30121). Antonio Gazol and Paloma Ruiz-Benito were supported by a project “2018 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation.” Ana-Maria Hereş was supported by the project PN-III-P1-1.1-TE-2019-1099 financed by the Romanian Ministry of Education and Research through UEFISCDI. Raúl Sánchez-Salguero was supported by VULBOS project (UPO-1263216, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014-2020). Paloma Ruiz-Benito was supported by the Community of Madrid Region under the framework of the multi-year Agreement with the University of Alcalá (Stimulus to Excellence for Permanent University Professors, EPU-INV/2020/010) and the University of Alcalá “Ayudas para la realización de Proyectos para potenciar la Creación y Consolidación de Grupos de Investigación.” Andrea Hevia was supported by PinCaR project (UHU-1266324, FEDER Funds, Andalusia Regional Government, Consejería de Economía, Conocimiento, Empresas y Universidad 2014-2020).

Peer reviewed