- Individuals have limitations, not communities - 581FORUM

Journal of Vegetation Science 15: 581-582, 2004© IAVS; Opulus Press Uppsala.

AbstractA reply is presented on the comments by Marrs, Weiher,

and particularly Lortie et al. on an earlier Forum paper. Themain point is that adapted alpine plants are not stressed, whichfollows, i.a., from their productivity which is equal to that intropical systems when the length of the growing season istaken into account. Another point is that individual-based andcommunity-based considerations should not be confused.

Keywords: Competition; Plant ecology; Resource; Stress.

When I was invited by JVS to write a perspectivesnote (Körner 2003a), I had selected the widespreadblack-box use of stress and limitation, because to me itis a classical case where a well-established and usefulscientific concept becomes misleading, when it finds itswell intended, though inappropriate application in fieldsto which it does not apply (thanks to R. Marrs 2004, forsympathizing with that view). I did not at all question thesignificance of interactions among organisms – the wholeperspective was written to underpin it. Since I really donot think this is a definitions issue – but I agree withWeiher (2004) that too narrow definitions are not helpful– I will largely focus on the reply by Lortie et al. (2004)and I used one of their statements for my reply’s header.

I could have left it with this headline, because thisstatement by Lortie et al. (2004) nicely encapsulates mymain message: ‘stress’ in this context is only a specialcase of limitation. If we agree on the headline statement,the rest remains trivial. Much of their reply really isbeating a strawman and a repetition of ecological basics,nobody would disagree with. In essence, my conclusionculminated in what these authors nicely had documentedin their neighbour removal experiments at high eleva-tions (Callaway et al. 2002), although they got theirinterpretation wrong (the final kick I needed to acceptthe JVS invitation). It is not because alpine plants arestressed that facilitation becomes so important. Theybecome stressed, when experimenters remove their

Individuals have limitations, not communities– A response to Marrs, Weiher and Lortie et al.

Körner, Christian

Institute of Botany, University of Basel, Schönbeinstrasse 6, CH-4056 Basel, Switzerland; E-mail ch.koerner@unibas.ch

FORUM

neighbours and thus create a new, much harshermicroclimate, for which the species assembled in thatplace after hundreds of years of community processeshad not been ‘qualified’ for. An alpine mat traps a lot ofwarmth and depends on it. If one destroys that aerody-namic boundary, a succession process is induced, withnew species coming and old species going. What elsedid I say? It is totally misleading to remove vital lifeconditions only to conclude from the disastrous conse-quences that stress is important in ‘normal’ life.

One could conclude, however, that microclimateplays a more important role for plant life at higherelevations and that dense canopies can facilitate somedecoupling of vegetation from atmospheric conditions,but there is nothing new with this (for references seeKörner 2003b).

If thermal conditions in the alpine environment wouldlimit growth in native species one would expect produc-tivity to be reduced. It may thus, come as a surprise tomany readers that in the humid parts of the globe, so-called ‘stressed’ alpine plants compose into vegetationwhich is as productive as humid tropical forests, pro-vided one makes a fair comparison and relates biomassaccumulation to periods only during which plants grow,for instance, 45 compared to 365 days (Körner 2003b).Adaptation has done a masterly job – it is time whichmatters! The misleading year basis for such compari-sons gives rise to such unfounded, anthropocentric viewsabout stress. I still like the broad comparative experi-mental approach by Callaway et al., but that particularpart of their conclusions is unfounded and oversellingan otherwise very useful study.

My central theme was a separation of individual-based and community-based considerations rather thanconfusing the two (Lortie et al. 2004). I would accept thecritique that somewhat simplistic jargon (in hopes tobreak the ice) carried me away from the grey shades ofreality to a black and white notion, as Weiher (2004)noted). This however, should not have disguised theimplicit intent.

582 Körner, Ch. FORUM

Lortie et al. (2004) state “He (Körner) rejects thepossibility that limitation occurs for plants growing intheir ‘natural’ habitats...” which is exactly the oppositeof what I state, namely that plant growth will nearlyalways be limited in nature. Needless to say that I cannotrightfully be accused for making a case for ‘perfect’evolution (“he fails to acknowledge that fitness is neverperfect”), with assuming all plants to live under optimalitywhere they are found (see the next paragraph). The keyissue is the word plant. What holds for the individualplant or species, does not hold for the community (see thetitle). Any change in resource supply (or climate) willcreate a new community at the loss of the old one. This isall but an issue of definition, this relates to the very basicsof how we view ecosystems. Comparing the change ofcommunity composition when the environment changes(e.g. when a dry place becomes moist) with communityconsequences of invasions (for what ever reason) byaliens into environments similar to their home conditions– is an apples vs pears comparison and does not help inclarifying the stress issue as discussed here.

Students learn about bell shaped species abundancecurves that should never misguide one on assumptions ofwhat is optimal for a species (in terms of growth andreproduction). Pinus sylvestris reaches peak abundanceon acid peat or sand drifts or on calcareous scree. It ismissing in late successional forests on ‘good’ soils.When grown alone (without competition) P. sylvestrisdoes best at that very end of the curve’s tail on the lattersoils (Sitte et al. 2002). There is no need to recall thecompetition classics. Simply because natural speciesassemblages reflect the environmental conditions andbiotic interactions, there is no basis for discussing ‘limi-tation’ at the community scale (see the title) but plentyof interesting work in this context at the species level. Icould not agree more.

Lortie et al. (2004) first find “... no logical reason tobelieve that groups of species may be any more or lesslimited or stressed than individuals...”, a statement theycontradicted a few lines later by what I made the headlineof this response. Yes, stress and limitation are individual-based concepts. Unfortunately, at the end of their textthey return to insisting that they prefer stress and limita-tion to be applied at all possible scales. Stress andlimitation, applied to communities of plants in an eco-logical, biodiversity oriented context mirrors a mislead-ing picture of the world, whereas the same conceptapplies well to the yield of a field of corn or massproduction per unit land area in general, and it alwaysapplies to individual species. Alpine plants may alwaysbe limited by low temperatures (in growth and otheraspects of metabolism), but a given alpine plant commu-nity is not.

References

Callaway, R.M., Brooker, R.W., Choler, P., Kikvidze, Z.,Lortie, C.J., Michalet, R., Paolini, L., Pugnaire, F.L.,Newingham, B., Aschehoug, E.T., Armas, C., Kikodze,D. & Cook, B.J. 2002. Positive interactions among alpineplants increase with stress. Nature 417: 844-848.

Körner, Ch. 2003a. Limitation and stress – always or never? J.Veg. Sci. 14: 141-143.

Körner, Ch. 2003b. Alpine plant life. 2nd. ed. Springer, Berlin,DE.

Lortie, C.J., Brooker, R.W., Kikvidze, Z. & Callaway, R.M.2004. Reply to Körner: Stressed over stress. J. Veg. Sci.15: 577-580 (this issue).

Marrs, R.H. 2004. Some limitations on limitations? J. Veg.Sci. 15: 573-575 (this issue).

Sitte, P., Weiler, E.W., Kadereit, J.W., Bresinsky, A. & Körner,Ch. 2002. Strasburger – Lehrbuch der Botanik. 35th ed.Spectrum, Heidelberg, DE.

Weiher, E. 2004. Why should we constrain stress and limita-tion? Why conceptual terms deserve broad definition. J.Veg. Sci. 15: 569-571 (this issue).

Received 10 March 2004;Accepted 14 April 2004.

Co-ordinating Editor S. Díaz.