There is increasing interest in understanding the costs and benefits of increased size and prolonged lifespan for plants. Some species of trees can grow more than 100 m in height and can live for several millennia, however whether these achievements are obtained at the cost of some other physiological functions is currently unclear. As increases in size are usually associated with ageing, it is also unclear whether observed reductions in growth rates and increased mortality rates are a function of size or of age per se. One theory proposes that reduced growth after the start of the reproductive phase is caused by cellular senescence. A second set of theories has focussed instead on plant size and the increased respiratory burdens or excessive height. We report on experimental manipulations to separate the effects of extrinsic factors such as size from those of intrinsic factors such as age for four tree species of contrasting phylogeny and life history. For each species, we measured growth, gas exchange and leaf biochemical properties for trees of different ages and sizes in the field and on propagated material obtained from the same genetic individuals but now all of small similar size in our common gardens. For all species, evidence indicated that size, not cellular senescence, accounted for the observed age-related declines in relative growth rates and net assimilation rates. Two species exhibited evidence of genetic control on leaf characters such as specific leaf area, although size also exerted an independent, and stronger, effect. We found partial support for the theory of hydraulic limitations to tree growth. The lack of a marked separation of soma and germline, an unlimited proliferation potential of meristem cells and the exponential increase in reproductive effort with size all help explain the lack of a senescence-induced decline in trees. It is possible that trees much older than the ones we sampled exhibit senescence symptoms.

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Hybridization and genetic swamping by planted exotic Populus taxa are putative threats for native Populus resources. We investigated the potential for hybridization between the exotic hybrid Populus x canadensis Moench and its wild relative, the European black poplar (Populus nigra L.), by a series of pollination experiments in the greenhouse. We also tested the effect of mixtures of pollen from the taxon’s own pollen and pollen of the foreign taxon on mating success in reciprocal crosses. We observed reduced pollen and seed viability of the hybrid clones of P. x canadensis compared to the clones of the parental species P. nigra. Surprisingly, when in combination with pollen of its wild relative P. nigra, pollen of exotic P. x canadensis sired significantly better on its own flowers. In poplar breeding, the pollen mentor technique which uses pollen mixes of artificially inactivated conspecific pollen and untreated incompatible foreign pollen, has been successfully used to overcome incompatibility barriers. The results of this study suggest that in the wild, where mixed pollen loads are common, similar effects as the pollen mentor effects may enhance the invasiveness of exotic P. x canadensis. This study helps to explain former field observations and contributes to the understanding of potential environmental impacts of commercial exotic and transgene poplar plantations

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