Climatic adaptation of trees: Rediscovering provenance tests

Abstract

Common garden testing of populations of different origin started withforest trees more than two hundred years ago. Since then, so-calledprovenance tests have been established with most commercially importantspecies. Beyond the strictly silvicultural goals, the tests offer excellentopportunities to study intraspecific genetic variation patterns and representprobably the most powerful available tool for testing hypotheses of climaticadaptation in trees. Analysis of adaptive traits (mostly juvenile heightgrowth) in provenance experiments indicate the existence of very effectiveconstraints on adaptedness. The performance of populations plotted against anecological-climatic factor exhibits a characteristic pattern and can bedescribed by response functions. The population average of a fitness-relatedtrait for a locally adapted population is often significantly lower than thatof populations from other environments; usually the ones from milder climateperform better. The phenomenon is interpreted as adaptation lag. Suboptimaladaptation is compensated by a high level of genetic diversity. Moleculargenetic studies confirm the high level of allelic and individual geneticdiversity in forest trees. A consequence of individual homeostasis,phenotypic stability of populations is usually also high; the sensitivity toenvironmental changes is generally moderate. Phenotypically stablepopulations are valuable not only because of a wider range of potentialcultivation but specifically because of a greater ability to adjust tounexpected changes. This trait should receive more attention in the futurefor obvious reasons. The maintenance of a high within-population geneticvariance is favored by the genetic system of the investigated species(effective gene flow, outbreeding, high genetic load, etc.). Random eventsand long-lasting biotic interactions are further effects impairing theefficiency of natural selection. In view of expected climate instability,genetic adaptability of forest trees causes serious concern due to their longlifespan compared to the rapidity of expected changes in environmentalconditions. The potential of provenance tests to interpret long-termadaptational processes should be utilized to analyze, model and predictresponse of trees to climate change. Although seldomly appreciated,provenance research might be among the most important contributions offorestry to biological sciences.