Summer-drought constrains the phenology and growth of two coexisting Mediterranean oaks with contrasting leaf habit: implications for their persistence and reproduction

Macaulay Institute, Craigiebuckler, Aberdeen, AB15 8QH UK
Trees (Impact Factor: 1.87). 08/2009; 23(4):787-799. DOI: 10.1007/s00468-009-0320-5

ABSTRACT This study analyses how coexisting evergreen and deciduous oaks adjust their phenology to cope with the stressful Mediterranean
summer conditions. We test the hypothesis that the vegetative and reproductive growth of the winter deciduous (Quercus faginea Lam.) is more affected by summer drought than that of the evergreen [Quercus ilex L. subsp. ballota (Desf.) Samp.]. First, we assessed the complete aboveground phenology of both species during two consecutive years. Shoot
and litter production and bud, acorn and secondary growth were monitored monthly. Second, we identified several parameters
affected by summer conditions: apical bud size, individual leaf area (LA), leaf mass per area (LMA) and acorn yield in both
species, and leaf-fall in Q. faginea; and analysed their variation over 10years. Q. ilex performed up to 25% of shoot growth and most leaf development during summer, whereas Q. faginea completed most of both phenophases during spring. Secondary growth was arrested in summer under drought conditions. Approximately,
30–40% of bud and 40–50% of acorn growth was undertaken during summer in both species. Summer drought related to differences
in LA, LMA and leaf senescence, but not to acorn yield. Both species had similar year-to-year patterns of acorn production,
though yields were always lower in Q. faginea. Bud size decreased severely in both species during extremely dry years. In Q. ilex, bud size tended to alternate between years of large and small buds, and these patterns were followed by opposite trends in
stem length. In Q. faginea, bud size was more stable through time. Q. ilex was more phenologically active during summer than Q. faginea, indicating a higher tolerance to drought. Furthermore, bud and fruit growth (the only two phenophases that both species
performed during summer) were more severely affected by summer drought in Q. faginea than in the evergreen. The differential effects of summer drought on key phenophases for the persistence (bud growth) and
colonization ability (fruit production) of both species may have consequences for their coexistence.

  • Source
    Forests and Global Change, Edited by D. A. Coomes, D.F.R.P. Burslem, W.D. Simonson, 10/2013: chapter Global change and Mediterranean forests: current impacts and potential responses: pages 47-75; Cambridge University Press.
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    ABSTRACT: Disentangling how variation in reproduction and growth are linked in plants across different ecological scales, and how allocation rules change in response to stress are fundamental aspects of life history theory. Although it is known that reproductive allocation is an allometric process and that environmental conditions can influence demographic traits, patterns of variation in vegetative and reproductive functions across and within individuals of tree species suffering drought-induced decline have rarely been documented. In this study we use Scots pine (Pinus sylvestris L.) as a model species to explore patterns of variation in cone production and growth in two declining populations at the southern edge of its distribution. A Bayesian approach was used to assess how these demographic traits vary as a function of drought effects and competition and covary across different ecological scales. The allometric trajectories relating tree size with cone production and growth differed along gradients of drought impacts and biotic interactions. Although reproduction and growth increased with tree size, cone production reached a maximum at intermediate sized trees and stabilized or decreased at larger sizes. Drought stress effects (defoliation at the tree level and overall decline at the plot level) and competition for resources reduced cone production and growth. Our results also showed differential effects of defoliation on cone production depending on tree size, with stronger effects on larger individuals. After accounting for these effects, much of the variation of demographic traits and correlations among them occurred at small ecological scales across individuals (i.e. within plots) and within individuals across years. This resulted in covariations between demographic traits among nearby individuals and within individuals through time, suggesting a consistent advantage in resource acquisition of some individuals within plots, and trade-offs between growth and cone production within trees across years. In conclusion, this study reports that drought-induced forest decline is associated with lower growth and cone production in Scots pine, which could contribute to explain the long-term impacts of drought in southern populations of this species and, in particular, its low regeneration capacity after severe drought.
    Perspectives in Plant Ecology Evolution and Systematics 06/2014; · 3.32 Impact Factor

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