Long tree-ring chronologies reveal 20th century increases in water-use efficiency but no enhancement of tree growth at five Iberian pine forests

Abstract

We investigated the tree growth and physiological response of five pine forest stands in relation to changes in atmospheric CO 2 concentration (c a ) and climate in the Iberian Peninsula using annually resolved width and d 13 C treering chronologies since AD 1600. 13 C discrimination (D % c i /c a ), leaf intercellular CO 2 concentration (c i ) and intrinsic water-use efficiency (iWUE) were inferred from d 13 C values. The most pronounced changes were observed during the second half of the 20th century, and differed between stands. Three sites kept a constant c i /c a ratio, leading to significant c i and iWUE increases (active response to c a ); whereas a significant increase in c i /c a resulted in the lowest iWUE increase of all stands at a relict Pinus uncinata forest site (passive response to c a ). A significant decrease in c i /c a led to the greatest iWUE improvement at the northwestern site. We tested the climatic signal strength registered in the d 13 C series after removing the low-frequency trends due to the physiological responses to increasing c a . We found stronger correlations with temperature during the growing season, demonstrating that the physiological response to c a changes modulated d 13 C and masked the climate signal. Since 1970 higher d 13 C values revealed iWUE improvements at all the sites exceeding values expected by an active response to the c a increase alone. These patterns were related to upward trends in temperatures, indicating that other factors are reinforcing stomatal closure in these forests. Narrower rings during the second half of the 20th century than in previous centuries were observed at four sites and after 1970 at all sites, providing no evidence for a possible CO 2 'fertilization' effect on growth. The iWUE improvements found for all the forests, reflecting both a c a rise and warmer conditions, seem to be insufficient to compensate for the negative effects of the increasing water limitation on growth.

Plant-soil interactions in Mediterranean forest and shrublands: impacts of climatic change

“…The positive effect on growth generally decreased with time (Körner and Miglietta 1994;Jones et al 1995;Peñuelas et al 2008a;Andreu-Hayles et al 2011), suggesting acclimation to increase of atmospheric CO 2 concentrations (Navas et al 1995;Roumet et al 2000;Saurer et al 2003). The increased growth in the short-term is linked to increases in WUE associated with an atmosphere richer in CO 2 (Nijs et al 2000).…”

“…Some studies (Peñuelas and Azcón-Bieto 1992;Scarascia-Mugnozza et al 1996;Tognetti et al 1998;Nijs et al 2000;Morgan et al 2004;Peñuelas et al 2008a;Andreu-Hayles et al 2011;Maseyk et al 2011), and mathematical models (Osborne et al 2000) suggest that increases in concentrations of atmospheric CO 2 , by increasing WUE, can counteract the effects of the projected increase in drought in the coming decades (IPCC 2007). Long-term experiments have not tested whether Mediterranean plants could acclimate to elevated CO 2 to buffer these higher WUEs.…”

Contrasting growth and water use efficiency after thinning in mixed Abies pinsapo-Pinus pinaster-Pinus sylvestris forests

“…The high correlations between the variation in growth and iWUE for these species (negative for P. pinaster and positive for A. pinsapo) indicate the influence of climatic constraints on inter-annual variation in iWUE under silvicultural practices. In addition to the potential effects of thinning intensity on tree growth and physiological performance, Mediterranean species showed enhanced BAI during unfavourable years as iWUE increased (Andreu-Hayles et al 2011). …”

Process models and model-data fusion in dendroecology

“…The apparent fertilization effect of the models, related to water use efficiency (WUE) enhancement, itself due to a decrease in stomatal area, has not been observed in experimental chambers or in tree-ring series (Gedalof and Berg, 2010;Silva and Horwath, 2013). There is therefore much controversy as to whether the increase in CO 2 since the beginning of the industrial period has enhanced growth, with much recent evidence reporting changes in plant physiological processes but not in growth (Huang et al, 2007;Andreu-Hayles et al, 2011;Peñuelas et al, 2011;Girardin et al, 2012;Lévesque et al, 2014). Experiments are often done on young trees while the CO 2 effect is a long term process which cannot be easily investigated by short-term experiments (Medlyn et al, 1999).…”

“…Even when these experiments are done on old trees (30 years old for Alpine conifers, Handa et al, 2005), the duration of the experiments is necessarily limited in time (3 years in the FACE experiment (Handa et al, 2005). Even when an increased WUE is observed, it is not necessarily related to a tree-growth enhancement (Andreu-Hayles et al, 2011;Peñuelas et al, 2011). Nevertheless some studies have acknowledged that WUE enables a better resistance to drought (Linares and Camarero, 2012;Girardin et al, 2012) and thus a strong fertilization effect (Keenan et al, 2013).…”