Evidence for age- and size-mediated controls of tree growth from grafting studies.

Vegetative propagation techniques such as grafting can be used, in conjunction with field studies, to decouple the relative effects of age and size on tree metabolism and growth. Despite interest in this approach, little attention has been paid to the best metrics for assessing the growth performance of grafted plants over time. Based on an analysis of the grafting literature and our own data, we show that the choice of metrics to assess tree growth can entirely change the conclusions reached about the relative importance of age versus size. We recommend that absolute as well as relative rates of growth are calculated and that scion size be standardized as much as possible at the start of the experiment. Once proper metrics are chosen, all of the available published evidence is largely concordant with two concepts: (1) age-mediated controls of tree growth are likely to be important during the first few years of a tree's life (before phase change); and (2) after the first few years of a tree's life, size-mediated factors largely prevail over age-mediated factors in determining tree growth rates. We found no support for theories invoking age-mediated sink limitations in old trees.

Plant size, not age, regulates growth and gas exchange in grafted Scots pine trees.

We studied the effect of scion donor-tree age on the physiology and growth of 6- to 7-year-old grafted Scots pine (Pinus sylvestris L.) trees (4 and 5 years after grafting). Physiological measurements included photosynthethetic rate, stomatal conductance, transpiration, whole plant hydraulic conductance, needle nitrogen concentration and carbon isotope composition. Growth measurements included total and component biomasses, relative growth rates and growth efficiency. Scion donor trees ranged in age from 36 to 269 years at the time of grafting. Hydraulic conductance was measured gravimetrically, applying the Ohm's law analogy, and directly, with a high-pressure flow meter. We found no effect of scion donor-tree age on any of the variables measured. There was, however, great variation within scion donor-tree age groups, which was related to the size of the grafted trees. Differences in size may have been caused by variable initial grafting success, but there was no indication that grafting success and age were related. At the stem level, hydraulic conductance scaled with total leaf area so that total conductance per unit leaf area did not vary with crown size. However, leaf specific hydraulic conductance (gravimetric), transpiration, photosynthesis and stomatal conductance declined with increasing total tree leaf area and needle width. We hypothesize that needle width is inversely related to mesophyll conductance. We conclude that canopy and needle size and not scion donor-tree age determined gas exchange in our grafted trees.

Size-mediated ageing reduces vigour in trees.

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.