Hi! I have been doing some research on the effects of density on the growth of containerised tree seedlings, using a Nelder design in some of the experiments. A similar effect to that observed in the field, as previously discussed in this list, was observerd with contenerised seedlings. It is difficult to know what was/were the variable/s that made some intermediate density optimal for growth. So, I will give a list possibilities: 1) Light quality (Red:Far-red ratio) changed with density, and this change occured even in sparse canopies (LAI < 1). At least pines and birch can respond to this changes by elongating the stem. 2) Soil temperature. Shaded soil remained cooler. Usually warmer root temperature leads to increase in the root:shoot ratio. 3) Shelter from wind. Wind can affect also morphology through mechanical effects (through hormones, e.g. ethylene). 4) Shelter from wind and transpiration from neighbours can decrease the leaf to air-at-the-leaf-surface vapour pressure difference, decreasing transpiration per unit leaf area. Water use efficiency was lower at low densities. 5) Of course in relatively dense canopies, shading (decrease in PAR) is an important factor, and it affects both growth rate and morphology... but by itself does not explain slow growth rates in low density conditions. The importance of each of these effects of density probably depends on the species (and provenance) and on the site, so I do not think that there is a simple way of modelling tree supression except to "magically" make the simulated seedlings "know" that they are under a canopy. A mechanistically complete (at the eco-physiological level) model of tree supression based on how trees really sense neighbours within a tree canopy would require a lot of eco-physiological experimentation both in the field and under controlled conditions, something worth-while doing, but which, I think, would take a few years! Any other hypotheses to add to the list? ;-) Pedro. ======================================================================= Pedro J. Aphalo FINNISH FOREST RESEARCH INSTITUTE FIN-77600 Suonenjoki, FINLAND Internet: pedro.aphalo@metla.fi fax: +358 79 513 068, tel: +358 79 513 811 ,,,^..^,,, =======================================================================
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