Institute for Research in Sustainable Forest Management, University of Valladolid, 47002 Valladolid, SPAIN.
The study of life-history traits is central to evolutionary biology and ecology, as life- history traits are closely linked to the environment where organisms thrive. Among them, traits related to reproduction such as the threshold size for reproduction, fecundity and the schedule of reproductive investment along life are particularly relevant. Their study helps to understand past adaptive processes as well as to infer future ones. For forest trees, this knowledge is particularly valuable and urgent, given their founding role in ecosystems all over the world.
Life-history traits can be addressed at the species level but usually, intraspecific within- and among-population variation also exists. This is the case for numerous plant species, as flowering time is commonly very variable and found to be correlated with natural selective pressures. This correlation may be due to plastic or genetic causes. A requirement for genetic responses to take place is the existence of additive genetic variation, which can be revealed by quantitative genetic studies or artificial selection experiments. Moreover, according to life-history theory, the expression of particular fitness-related traits is limited by costs or trade-offs with other traits also related to fitness.
Pinus pinaster Ait. and P. halepensis Mill. are two Mediterranean pine species showing numerous advantages for the study of tree reproductive traits from an evolutionary- ecological standpoint. Using these species as a model, it is possible to integrate aspects such as population differentiation, past and future local adaptation, plasticity, genetic architecture and multi-trait adaptive relationships.
The aims pursued in the present study were, first, to document population differentiation in reproductive life-history traits. Second, to test the hypothesis that life-history traits are related to the environment by means of phenotypic plasticity and genetic change processes. Third, to estimate reproductive quantitative genetic parameters of key life-history traits. Fourth, to test the hypothesis that investment in reproduction is bound to costs or trade-offs with other adaptive traits, both at phenotypic and genetic levels.
-material and methods-
P. pinaster and P. halepensis are naturally distributed along extensive areas of the Mediterranean basin. P. pinaster has a particularly broad ecological niche, and there is wide genetic differentiation among populations, meanwhile P. halepensis, very tolerant to drought, shows smaller population differentiation, at least in neutral molecular markers. Data used in the present study were obtained from common garden experiments of three kinds: provenance, provenance-progeny and progeny common gardens, containing genetic material originated in natural populations or derived from breeding programs. Common garden data allowed to estimate to which extent phenotypic expression is genetically based and, when experiments were replicated under contrasting environmental conditions, to know whether this expression was also plastic. Furthermore, climatic data describing the areas where natural populations originated were obtained. Climatic data were then used to check their relationship with phenotypic traits. For P. pinaster, this relationship was checked after correcting for strong neutral genetic structure with nuclear microsatellite data.
Traits measured in the common gardens were related either to vegetative growth (total height, diameter at breast height, stem straightness) or with reproduction (presence/absence and quantification of reproduction, both male and female). For P. halepensis, reproduction was also manipulated experimentally seeking for the expression of somatic costs of reproduction. Data analysis was accomplished by linear (Gaussian data) and generalised (non-Gaussian data) mixed model analysis.
-results and discussion-
In both species, large population differentiation for reproductive life-history traits was a general finding. Moreover, mean population values –most notably regarding female function- covaried according to the environment where populations had evolved. A precocious and intense investment in reproduction was related to unfavorable environments for vegetative growth (low site index). This correlation is in agreement with the adaptive value of reproductive traits in forest trees; just as well as it has been described in other clinal traits such as timing of grow cessation. However, clinal trends were not found for the threshold size for male reproduction in P. pinaster. This could be related to sex allocation patterns strongly linked to tree height, as height is related to pollen dispersal conditions.
In P. halepensis, environmental effects (plasticity) on the expression of reproductive traits were also observed. In pararell to patterns of population genetic differentiation, a more growth-limiting environment was related to advanced ontogenetic development. In both species, it was noteworthy the absence of genotype by environment interaction for reproductive traits, contrasting with results for vegetative growth.
Plastic adaptive responses triggered in response to environmental change can reduce the need for genetic changes. On the other hand, high genetic variance and moderate to high heritabilities of reproductive life-history traits enable steady local adaptation processes. This possibility was backed by a neat response to artificial selection in P. pinaster.
At the somatic level, female costs of reproduction were confirmed in P. halepensis by means of a manipulation experiment, but not by correlational studies. In this species, investment in reproduction had a greater effect on future reproduction than on vegetative growth. At the genetic level, costs of reproduction in terms of growth showed a variable pattern depending on the species and the common garden environment. Nonetheless, several reproductive traits displayed a strong correlated response to selection for growth in P. pinaster i.e. evidence for the existence of a genetic trade-off between reproduction and growth. From the point of view of forest management, it is advised against the use of size as a single surrogate of fitness. Even though it is true that large trees tend to produce a greater number of offspring, this relationship is not tight and might be shadowed by trade-offs. On the contrary, under a new paradigm of adaptive forest management, the inclusion of reproduction to better define fitness is recommended.