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. 2017 Jul 11;8:1200.
doi: 10.3389/fpls.2017.01200. eCollection 2017.

Xylem and Leaf Functional Adjustments to Drought in Pinus sylvestris and Quercus pyrenaica at Their Elevational Boundary

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Free PMC article

Xylem and Leaf Functional Adjustments to Drought in Pinus sylvestris and Quercus pyrenaica at Their Elevational Boundary

Laura Fernández-de-Uña et al. Front Plant Sci. .
Free PMC article

Abstract

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.

Keywords: climate change; drought stress; experimental drought; phenology; sub-Mediterranean forest ecosystems; water deficit; wood anatomy; xylogenesis.

Figures

FIGURE 1
FIGURE 1
Cumulative precipitation, monthly standardized precipitation-evapotranspiration index (SPEI), daily vapor pressure deficit (VPD) and daily relative extractable water (REW) during the study period. REW is given for each of the rainfall exclusion plots and the control area. Vertical dashed lines mark the days in which gas exchange measurements were taken.
FIGURE 2
FIGURE 2
Pinus sylvestris (A,C,E) and Quercus pyrenaica (B,D,F) phenological phases in control and trees subjected to rainfall exclusion. Gray lines mark standard deviations. Q. pyrenaica cambial phenology is separated by earlywood and latewood.
FIGURE 3
FIGURE 3
Growth rates and anatomical properties for P. sylvestris in control trees and trees subjected to rainfall exclusion. Anatomical traits were also analyzed for the year prior to the establishment of the experiment. Lower case letters indicate differences among years in control trees and capital letters indicate differences among years in treated trees. Bold letters indicate differences between treatments within each year. r, growth rate; N tracheids, number of tracheids; LW/RW, proportion of latewood as a function of ring width; CWT, cell-wall thickness; DH, hydraulic diameter.
FIGURE 4
FIGURE 4
Quercus pyrenaica growth rates and anatomical properties per treatment. Anatomical traits include the year prior to the establishment of the experiment. Lower case letters indicate differences among years in control trees and capital letters indicate differences among years in treated trees. Bold letters indicate differences between treatments within each year. rEW, earlywood growth rate; rLW, latewood growth rate; LW/RW, proportion of latewood as a function of ring width; DH, hydraulic diameter.
FIGURE 5
FIGURE 5
Pinus sylvestris needle chemical properties, morning gas exchange and midday leaf water potentials per treatment. Lower case letters indicate differences among years in control trees and capital letters indicate differences among years in treated trees. Bold letters indicate differences between treatments. C/N, C/N concentration ratio; PNUE, photosynthetic nitrogen use efficiency; Δ, carbon discrimination; A, morning photosynthetic rate; gs, morning stomatal conductance; Midday Ψ, midday leaf water potential.

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