Abstract Rear‐edge tree populations are experiencing a combination of higher temperatures and more intense droughts that might push individuals beyond their tolerance limits. This trend towards rising atmospheric [CO 2 ] is concurrent with an increase in intrinsic water use efficiency (iWUE), which theoretically enhances photosynthesis and decrease evapotranspiration rates, consequently improving tree resistance to drought. However, it remains unclear whether iWUE is favouring tree growth under current climate conditions, particularly when climate and iWUE legacy effects are simultaneously considered. We evaluated this question with an extensive sampling along Iberian rear‐edge (dry) populations comprising four mountain ranges and two distinct altitudes. We simultaneously examined the effects of climate and iWUE on secondary growth using annually resolved basal area increments (BAIs) for the period 1901–2017. We used linear mixed models including second‐order autocorrelation and 1‐year legacy effects of iWUE and summer drought. BAI and iWUE increased across the studied period. iWUE increase was driven by changes in atmospheric CO 2 concentration and water availability during the growing season. Climate and iWUE exerted direct and lagged effects on beech growth. Water availability during growing season was the main driver of tree growth, combining direct and indirect effects through its impact on iWUE. Legacy effects of water availability and iWUE were more important than growing season conditions. The net effect of iWUE shifted when lagged effects were considered, resulting in a net negative impact on tree growth. Synthesis : Our results reveal that climate and iWUE legacy effects must be considered to assess the net iWUE effect on secondary growth. Considering lagged effects, the current increase in iWUE is constraining tree growth. Modelling efforts of tree growth response to climate warming should include climate and iWUE legacy effects to adequately assess terrestrial ecosystem carbon balance.
Wood samples in digitized cores with CaptuRING (CR_ in files) and scanned with Epson 750VPRO (scan_) for sample digitization comparison. Species: Fagus sylvatica (FS) Quercus faginea (QF) Quercus ilex (QI) Quercus pyrenaica (QY) Juniperus cedrus (JC) Juniperus thurifera (JT) Pinus nigra (PN) Pinus pinaster (PP) Graphical scale was fitted with empirical values for CR samples, and theoretical resolution value in scanning software for scanned ones. SupplementaryMaterials2.csv file includes summary table from image comparison data.
The response of European beech (Fagus sylvatica L.) to climate warming will depend on the ability of their populations to adjust tree performance to water shortage. By exploring inter- and intra-annual variations in secondary growth and mean vessel area (MVA), we assessed the effects of precipitation on cambial activity and hydraulic control during the vessel expansion phase along tree lifes. We sampled beech populations at low and high altitude from four mountain ranges across its southwestern distribution edge. We measured a total of 45,897 rings from 126 trees and 5.5 million vessels from 76 trees. We built chronologies for ring width and MVA between 1950 and 2017, calculated their climate responses and evaluated the effects of region, altitude and chronology type (ring-width vs. MVA) by means of ordinations (PCA) and constrained ordinations (pRDA). Precipitation controlled ring width and MVA along beech's southwestern distribution range, but at different time domains. Ring width responded primarily to summer precipitation during the previous growing season, whereas MVA responded to water availability during the vessel expansion phase, with timing shifting along the ring, according to the moment of vessel expansion. Regional differences were significant, but low, compared with the effect of chronology type. A large part of the variance explained by region was due to the strong difference between Western Pyrenees forests –growing under hyperhumid conditions– and the rest of forests under drier and warmer climate. Only minor differences between altitudes were found for the climate control of ring width and vessel size at annual scale, and no intra-annual effect on climate control of MVA. The stronger effect of chronology type on climatic response compared to the role of geographical location or altitude suggests common climate constraints on secondary growth and xylem anatomy along beech dry edge.