Summary The extent to which water availability can be used to predict the enlargement and final dimensions of xylem conduits remains an open issue. We reconstructed the time course of tracheid enlargement in Pinus sylvestris trees in central Spain by repeated measurements of tracheid diameter on microcores sampled weekly during a 2 yr period. We analyzed the role of water availability in these dynamics empirically through time‐series correlation analysis and mechanistically by building a model that simulates daily tracheid enlargement rate and duration based on Lockhart's equation and water potential as the sole input. Tracheid enlargement followed a sigmoid‐like time course, which varied intra‐ and interannually. Our empirical analysis showed that final tracheid diameter was strongly related to water availability during tracheid enlargement. The mechanistic model was calibrated and successfully validated (R2 = 0.92) against the observed tracheid enlargement time course. The model was also able to reproduce the seasonal variations of tracheid enlargement rate, duration and final diameter (R2 = 0.84–0.99). Our results support the hypothesis that tracheid enlargement and final dimensions can be modeled based on the direct effect of water potential on turgor‐driven cell expansion. We argue that such a mechanism is consistent with other reported patterns of tracheid dimension variation.

1. Climate change scenarios forecast rising temperatures for the Mediterranean region, which could enhance the vulnerability to drought stress in forest ecosystems. The long-term effects of climate forcing on tree performance can be, however, modulated by other environmental factors, such as competition and rising atmospheric CO 2 concentrations. 2. We assessed the concomitant effect of competition, climate and CO 2 concentrations on the treering d 13 C-derived intrinsic water-use efficiency (iWUE) and basal area increments (BAI) of species with different drought tolerance: two Mediterranean deciduous species (Quercus faginea Lam. and Quercus pyrenaica Willd.) and one conifer (Pinus sylvestris L.). Additionally, given that competition may be managed to mitigate the effect of increasing drought stress, we further examined the influence of this variable on iWUE and growth using data compiled from the literature, providing the first review on the response of iWUE to competition. 3. Competition had no significant effect on iWUE in any of the three species studied, whereas, as expected, growth rates were significantly higher under low-competition levels. This was consistent with the literature review, which showed that shifts in iWUE with competition changes are rare; supporting the hypothesis that leaf-level gas exchange tends to be a homeostatic trait. In the long term, the three species exhibited a significant increasing trend in iWUE due to the combined effect of increased CO 2 concentration, climate and age. Growth, however, was mostly affected by competition and climate and in most cases was not enhanced as a result of the increase in iWUE. 4. Synthesis. Regardless of their functional response to drought, trees respond to reduced competition through the structural shifts such as increased radial growth rather than leaf-level gas exchange adjustments. CO 2 and climate are, therefore, the main drivers of iWUE variability, rather than competition. Thus, if temperature-induced drought becomes limiting, reducing competition for resources may not offset the detrimental effect of increasing drought stress on tree physiology and growth decline may occur without a CO 2 fertilization effect.

Climatic scenarios for the Mediterranean region forecast increasing frequency and intensity of drought events. Consequently, a reduction in Pinus sylvestris L. distribution range is projected within the region, with this species being outcompeted at lower elevations by more drought-tolerant taxa such as Quercus pyrenaica Willd. The functional response of these species to the projected shifts in water availability will partially determine their performance and, thus, their competitive success under these changing climatic conditions. We studied how the cambial and leaf phenology and xylem anatomy of these two species responded to a 3-year rainfall exclusion experiment set at their elevational boundary in Central Spain. Additionally, P. sylvestris leaf gas exchange, water potential and carbon isotope content response to the treatment were measured. Likewise, we assessed inter-annual variability in the studied functional traits under control and rainfall exclusion conditions. Prolonged exposure to drier conditions did not affect the onset of xylogenesis in either of the studied species, whereas xylem formation ceased 1–3 weeks earlier in P. sylvestris. The rainfall exclusion had, however, no effect on leaf phenology on either species, which suggests that cambial phenology is more sensitive to drought than leaf phenology. P. sylvestris formed fewer, but larger tracheids under dry conditions and reduced the proportion of latewood in the tree ring. On the other hand, Q. pyrenaica did not suffer earlywood hydraulic diameter changes under rainfall exclusion, but experienced a cumulative reduction in latewood width, which could ultimately challenge its hydraulic performance. The phenological and anatomical response of the studied species to drought is consistent with a shift in resource allocation under drought stress from xylem to other sinks. Additionally, the tighter stomatal control and higher intrinsic water use efficiency observed in drought-stressed P. sylvestris may eventually limit carbon uptake in this species. Our results suggest that both species are potentially vulnerable to the forecasted increase in drought stress, although P. sylvestris might experience a higher risk of drought-induced decline at its low elevational limit.