Evolutionary ecology of fire-adaptive traits in a Mediterranean pine species

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Ruth C. Martín-Sanz (2018) - Evolutionary ecology of fire-adaptive traits in a Mediterranean pine species

Forests have high ecological, economic and social value, besides playing a key role in the maintenance of biodiversity and as carbon sinks, but the current global change can cause adaptation problems of forest species as well as modify forests distribution and functioning. Mediterranean environments are especially sensitive to climate change, where predictions suggest that temperature increase and rainfall decrease will be especially drastic, together with a higher frequency and intensity of disturbances (forest fires and epidemic outbreaks of pests and diseases). The scarce availability of water is the most evident resource limitation in these Mediterranean environments and may increase the evolutionary trade-offs among vital functions predicted by the life-history theory. This theory is based on the idea that forest trees, like other living beings, must optimize the amount of energy and resources they dedicate to each of their vital functions since the available resources are limited.
Pine trees are long-life large organisms with short age at first reproduction and several advantages for the study of adaptive traits in trees from an ecological-evolutionary approach. Mediterranean pine forests constitute reservoirs of adaptive genetic diversity of great value in the face of environmental change. The ability of those populations to persist in the medium term will depend to a large extent on the existence of sufficient genetic variation in relevant traits, on the exchange of genetic information among populations (genetic flow) and on their adaptive phenotypic plasticity. Local adaptation is expected to arise from genetic variability within and between populations, but phenotypic plasticity also plays a major role in the ability of species to cope with environmental changes and may allow the appearance of adapted phenotypes without the existence of an underlying genetic change. However, knowledge about the adaptive role and the plasticity of key life-history traits is still very limited in forest species, especially in Mediterranean environments. Among the life-history traits stand out the reproductive ones, such as the threshold size of reproduction or fecundity, but serotiny degree or bark thickness are other fundamental traits related to adaptation to fire that have received so far, less attention.
This thesis, framed in the field of evolutionary ecology, includes four works that correspond to scientific articles already published or manuscripts in preparation. It also includes various annexes with additional information. The main objective of this work was to compare how different environments, more or less favorable, condition the compromises among adaptive traits (life-history traits) in a typical Mediterranean pine (Pinus halepensis Mill., Aleppo pine), which can be considered a model of maximum resilience in Mediterranean ecosystems. Specifically, we focused on two key resilience traits: serotiny of female cones to build an aerial seedbank that ensures regeneration after intense crown fires, and bark thickness that allows survival of adult trees in front of less severe fires until reaching a sufficient aerial seedbank, without forgetting its interrelation with other traits such as reproduction (female and male) or growth. Both traits are complementary but not mutually exclusive. Understanding their genetic and environmental variation patterns, unraveling the complex interaction between genotype, phenotype and environment, together with the allometric effects (ontogenetic or developmental), constitutes a fundamental challenge to be able to foresee the response of these forests under the new environmental scenarios, and for the management and conservation of forest resources under the current global change. The use of provenance trials in contrasted common environments allowed us to separate the genetic effects from environmental effects and interacting developmental differences. In addition, climatic and fire information from the populations’ origin areas was used to identify ecotypic patterns of variation.
Throughout the different studies involved in this thesis, we found clear evidence of intraspecific genetic variation and high phenotypic plasticity, as well as genotype-by-environment interaction and signs of local adaptation in the different studied traits. This suggests the existence of potential evolutionary change to face new selective pressures, variable within the species distribution range. The quantitative genetic differentiation between populations was higher than the differentiation found with molecular markers for fire-adaptive traits under contrasting environments. The growth-limiting environments for P. halepensis, mainly continental conditions with high annual and/or daily thermal oscillation that reduce the vegetative period, and the shortage of precipitations in spring and summer, accelerated the early release of seeds and decreased the allocation to bark.
The detailed study of serotiny degree in P. halepensis pursued on the one hand to determine the phenotypic plasticity of this trait taking into account the allometric and genetic effects, and on the other hand, to examine whether or not there are endogenous factors that can affect the opening of serotinous cones. For this, measurements in the provenance trials were completed with field and laboratory manipulative experiments. We found that unfavorable environments for growth caused the early release of seeds and that the duration of serotiny in P. halepensis implies the supply of water to the cones through its peduncles by the bearing plant, which suggests the existence of maintenance costs of serotiny.
We verified the existence of phenotypic plasticity and allometric plasticity in bark thickness, a fire-adaptive trait poorly studied in conifer species. Confirming our hypothesis environments with lower resource availability limited both the relative allocation to the bark and absolute bark thickness. Importantly, this can increase immaturity risk in P. halepensis populations (death by moderately intense fires before reaching an aerial bank of seeds that ensures regeneration) under the dryer environments caused by climate change.
We also studied the relationship between ecogeographic variables and different adaptive phenotypes in P. halepensis -including growth, female and male reproduction and fire-adaptive traits-, as well as the correlations among traits. In general, we found evidence of local intraspecific adaptation. We also confirmed that trade-offs in terms of allocation of resources to adaptive traits related to growth, reproduction and defense against fire matched the predictions of life-history theory and differential allocation. Finally, further confirming the hypothesis of local adaptation, we verified that the quantitative genetic differentiation among populations was greater than the neutral genetic differentiation for serotiny and bark thickness.
All the results obtained confirm that adaptive traits must be taken into consideration within the new paradigm of adaptive forest management under global change. This means considering not only traits related to growth or reproduction, but also those directly related to fire resilience, in order to implement better conservation and management practices maintaining the adaptive potential of forest populations facing the multiple challenges expected in the near future.

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Dpto. de Ecología y Genética Forestal
Dr. José M. Climent Maldonado
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