Hi Anya!

As far as I am concerned, this subject, tree health and it's defence against pest or other environmental stress, directly related to environmental or ecological factors including both abiotic and biotic factors affecting tree, forest and also whole plant community in terms of health and resistivity to any natural diseases.

May be there is indirectly a relation between tree growth and its defence but growing fast or not in trees  directly related to physiological and anatomical features under the same ecological condition.

So clearly, main answer is: Under the same ecological condition, we can not only say that fast growing trees healthier than the others. Its depend on ecological conditions.

SCENARIO 1: At general, Poplar is fast growing tree and it comsumes and needs more water than Oak species. So, If they grow in dry and semi dry condition, Poplar probably will have less resistivity than Oak. And this condition will be affect Poplar' physiological properties and finally it will be less resistivity to pest and the other env. stress. than oak. We can make an inference that.

SCENARIO 2: But, in humid contidion, we can not say that Poplar will be healtier than Oak. Can we?

Its one of the my little opinion:)

Good luck with your study.

Thank you Ugur, you bring up an excellent point that the relationship between growth and health, or lack there of, depends on environmental conditions.

Dear Anya,

What does " healthy" mean? If it is about tree's defense ability against pest or decay; then I think younger and vigor trees are stronger. Alex Shigo developed a concept about reaction of trees to wounding and decay named "compartmentalization of decay". According to that, younger trees have more live xylem cells in their trunk and grow faster; therefore, the process of healing the scar and isolating the decay  is faster.

http://www.nrs.fs.fed.us/pubs/gtr/gtr_nrs97.pdf

Regards,

Again an excellent point. Forest health has many definitions. Here, I was referring to defence against damaging insects and diseases.

Reza also bring up the fact that tree age would influence the probability of a tree being attacked by damaging insects or pests. Therefore, the variable of tree age would have to be removed from an experimental design that tests the relationship between tree growth and defence. Similar to environmental variables needing to be removed from an experimental design as Ugur pointed out previously.

Testing relationships between tree growth and defence would therefore need to be conducted under the same environmental conditions between trees of the same age. Does anyone know of a study like this? 

Thank you for the link, interesting information with beautiful images. Related to decay isolation is the role of calcium in wound recovery (ex. Huggett et al. 2006). 

Huggett, B.A., Schaberg, P.G., Hawley, G.J. and Eager, C. 2006. Long-term calcium addition increases growth release, wound closure, and health of sugar maple (Acer saccharum) trees at the Hubbard Brook Experimental Forest. Canadian Journal of Forest Research 37: 1692-1700

We can make  a guess but often plant defence reactions are evolutive and adaptative in a specific guest host game. Lignin pathway arms drive a lot of polyphenolic derivatives that are active in longevity, and wood density is a broad defence factor that increases with lignin. Pionieer plants are succeded in accordance and the sequence in forest clearances can be a defence failure of fast growth.  

Dear Reza has been commented that "younger trees are more stronger" . Partly I agree with him. We have to consider both abiotic and biotic stress compenent. Young trees may have more live xylem cells and grow faster so it may has more resistivity to biotic disturbances such as pest etc. BUT young trees have less tolerance in terms of abiotic stresses like water stress, root desiccation  and frozen tolerance etc. in open ecosystems. There are lots of study about that.

Dear Anya,

Many different reports appeared on plant responses that were varying among plants of different ages, among different organs of the same plants as well as among leaves of different developmental stage of the same plant . A trade-off between growth and defense has been presumed (Herms and Mattson 1992; Boege and Marquis 2005) and a range of theories exists that try to explain spatial and temporal variation in plant defense (for review see: Stamp 2003). For example, the optimal defense theory (ODT) postulates that defense is costly and the plant parts with the highest fitness value should be most defended. The second most prominent theory, growth-differentiation-balance hypothesis (GDBH) predicts that slower growing plant parts have more resources available for defense (consequently have higher defense level) than fast growing tissues. For both hypotheses there are several supporting as well as contradictory reports. Boege and Marquis (2005) and Fluch et al. (2008) stressed the role of development at the whole-plant level as an important factor in syntheses of plant defense theories and proposed a non-linear fashion of changes in defense and tolerance during plant ontogeny. Efficiency of defense is, however, not only about defense equipment but also about its dynamic - faster is better. The issue is really very complex and at current state of art I can hardly imagine you could perfom an experiment that can consider all possible factors. We all should proceed stepwise; nevertheless I wish you a  much success :-).

Behind the scene there is evolution and population diversity. The occurrence of a fast growing feature can be the reason for a specific extinction under generic stress, as well any bottleneck to get to reproductive age or for viruses and bacteria a fast mutation rate. Depending on molecular recognition and reactivity a set of outcomes is possible, without any wacthmaker. I would redirect to former J Gould issues, but the agriculture of clonal populations can be just a start for questioning about man made selections.