Increased water‐use efficiency translates into contrasting growth patterns of Scots pine and sessile oak at their southern distribution limits

Abstract

In forests, the increase in atmospheric CO concentrations (C ) has been related to enhanced tree growth and intrinsic water-use efficiency (iWUE). However, in drought-prone areas such as the Mediterranean Basin, it is not yet clear to what extent this "fertilizing" effect may compensate for drought-induced growth reduction. We investigated tree growth and physiological responses at five Scots pine (Pinus sylvestris L.) and five sessile oak (Quercus petraea (Matt.) Liebl.) sites located at their southernmost distribution limits in Europe for the period 1960-2012 using annually resolved tree-ring width and δ C data to track ecophysiological processes. Results indicated that all 10 natural stands significantly increased their leaf intercellular CO concentration (C ), and consequently iWUE. Different trends in the theoretical gas-exchange scenarios as a response to increasing C were found: generally, C tended to increase proportionally to C , except for trees at the driest sites in which C remained constant. C from the oak sites displaying higher water availability tended to increase at a comparable rate to C . Multiple linear models fitted at site level to predict basal area increment (BAI) using iWUE and climatic variables better explained tree growth in pines (31.9%-71.4%) than in oak stands (15.8%-46.8%). iWUE was negatively linked to pine growth, whereas its effect on growth of oak differed across sites. Tree growth in the western and central oak stands was negatively related to iWUE, whereas BAI from the easternmost stand was positively associated with iWUE. Thus, some Q. petraea stands might have partially benefited from the "fertilizing" effect of rising C , whereas P. sylvestris stands due to their strict closure of stomata did not profit from increased iWUE and consequently showed in general growth reductions across sites. Additionally, the inter-annual variability of BAI and iWUE displayed a geographical polarity in the Mediterranean.

Limited evidence for CO2‐related growth enhancement in northern Rocky Mountain lodgepole pine populations across climate gradients

“…Our results are consistent with previous studies demonstrating that changes in isotopic discrimination and increases in iWUE are highly site‐specific and do not necessarily result in increased productivity (i.e., BAI) at the tree level (Andreu‐Hayles et al., ; Gómez‐Guerrero et al., ; Knapp & Soulé, ; Lévesque et al., ; Linares et al., ; Martínez‐Sancho et al., ; Peñuelas et al., ; Wu et al., ). Growth declines despite increasing iWUE have also been observed for the arid and semiarid systems of the Mediterranean (Andreu‐Hayles et al., ; Lévesque et al., ), central Mexico (Gómez‐Guerrero et al., ), lower elevation forests in northwestern China (Wu et al., ), as well as in the more mesic Tropics (Nock et al., ; van der Sleen et al., ).…”

Long‐term physiological and growth responses of Himalayan fir to environmental change are mediated by mean climate

“…Thus, C i /C a patterns at low elevation under dry conditions are driven by stomatal responses to water availability and VPD, in addition to CO 2 rise. Our results confirm findings of earlier studies showing that iWUE trends vary between regions and are modified by mean climate (Frank et al, ; Lévesque et al, ; Martínez‐Sancho et al, ; Peters et al, ; Saurer et al, ). We found substantial differences in iWUE trends across tree sizes (cf.…”

Warming Effects on Pinus sylvestris in the Cold–Dry Siberian Forest–Steppe: Positive or Negative Balance of Trade?

“…The increase in WUE i did not translate into enhanced growth, but rather the opposite; in particular, BAI decreased more strongly at the driest site of each region (−57% and −35% for Siberia and Spain, respectively) than at its wetter counterparts (−10% and −9%, respectively). In dry areas, high WUE i rates are associated with the cost of reduced CO 2 assimilation in isohydric conifers such as Scots pine [83], which is beneficial for protecting against xylem cavitation [84]. Our findings highlight the usefulness of a combined growth-isotope analysis and suggest a predictable pattern of radial growth relationships with WUE i that would be dependent on the difference in water deficit experienced among stands for the prevailing thermic regime of a region.…”