Hello,

 Most likely the reasons you cite for undergrowth depletion are correct. The obvios lack of light due to a close canopy environment plus the thick layer of litter,  does not allow teh emergence of a previous seed bank, and those which manage will not obtain enoughh light. Teh microcliamte in teh surface will control weed emegance as well. In terms of allelopathy , it´d be interesting to find out. I will try and find you some literature.

Two factors that affect vegetation are (a) resource competition and (b) allelopathic effects.

Resource requirement of Casurina is  very parsimonious, thatswhy it is used in sandy coastal areas as wind break and for other purposes. Moreover in plantations, resource competition is not very intense.

There are some studies that describes the allelopathic effect of Casurina. They attribute diminished understory layer development in Casurina stands to release of phenolic and tannins from decomposing leaf and branch litter.

You can get more in formation on this through these papers.

1. Role of Allelopathy in Regulating the Understorey Vegetation of Casuarina Equisetifolia by Daizy R. Batish, Harminder Pal Singh. Environmental Forest Science

Forestry Sciences Volume 54, 1998, pp 317-323

2. Pinyopusarerk, Khongsak, and Alan Pennock Newton House. Casuarina: an annotated bibliography of C. equisetifolia, C. junghuhniana and C. oligodon. International Centre for Research in Agroforestry (ICRAF), 1993.

Dear Elemer and Mukesh,

Thanks for your valuable advice and suggestions. It is interesting to see your answer. Thank you again. 

i agree with the answers above and know little about casuarina.  but to check on the allelopathy you could do a little thinning in a few plantings and open up the ground layer to see if other plants emerge, maybe rake the litter back in some areas to create a better seed bed, maybe even introduce seed of local weeds.  i have seen plantations of other species so dense there is very little growing underneath.

I just briefly went through the answers above. My observation is that plants can very well establish under C. equitisifolia. I have seen this at the coast of Tamil Nadu as well as at the coast of Tansania (Baobab Farm). Casuarian is a pioneering species and supports growth of vegetation communities that come later in the succession. The reason why this is not often observed is high grazing pressure. Just look for Casuarina plantations behind walls or fences which are truly closed and you will see it.

In continuation of Carl Mize's comment: If the plantation is very dense and does not allow light to penetrate to the ground, plants cannot grow. However, I have never seen such dense C. plantations.

Joachim, glad you have first hand experience.  thinking over my answer, i have seen forests so dense that there is almost nothing in the understory, but all my examples are at least relstively shade-tolerant conifers.

Carl, sure such forests exist. But I have not seen it with Casuarina. So the absence of regeneration under C. is not due to lack of light, but through other factors.

Agreed and allelopathy is about the only other factor one can imagine.  black walnut (juglans nigra) uses allelopathy, although i have read it is difficult to prove that it happens. 

It is a very interesting question and the answers received are also quite apt.  In my experience, generally in casuarina plantations,undergrowth is very poor.  It may be due to many factors, very often, combination of factors that is leading to poor undergrowth. May be  the reason  could vary from location to location.  The predominant  reasons  one may have to find out. Lack of available light, litter deposit or allelopathic effect  all can play important role. In some sites, sometimes , biotic factors  such as grazing may also   contribute  to this. Detailed studies  are useful for throwing more light on this aspect. 

Dear All

I am really happy to see a professional discussion on my question. It is really interesting to see them. It also opened space to do carryout research further in this subject. Thank you.

Thanks Julio.

Allelopathy acts in different ways. Some allelopthic compounds inhibit germination, but not the development of plants that are already growing. Other allelopathic compounds may do the opposite. Some are more subtle. The pathway that Casuarina uses to inhibit other plants, and which plants are susceptible, is important to know, because it determines how Casuarina can be used in alley cropping or other forms of polyculture.

In addition, the allelopathic compounds may be present only in certain parts of the plants anatomy, so raking away litter may have no effect at all on the allelopathic processes occurring under a tree. A positive response by weeds to raking may simply be the benefit to plants whose germination is initiated by light reaching the seeds. Controlled experiment is the only sure way to distinguish the possible reasons why an intervention has an observed effect. Allelopathic compounds usually affect different species to very differing degrees. Allelopathy is complex and probably most inter-species effects are not reliably inferred beyond the rank of genus. This suggests the magnitude of inter-genus testing to be explored lies in the order of a quadratic function on the number of genera. Constraining this interaction number to only agriculturally important species still leaves a huge void of unknown interactions. Furthermore, some allelopathy seems to result from indirect effects involving other organisms such as disease agents or species of symbiotic fungal species.

Consequently much more study is needed in order to base polyculture on sound science rather than opinion. Until the monocultural mind-set of mainstream agriculture has been realigned to more ecologically sound approaches, there will be little funding for such research. One possible motivation for the monoculture agro-industry to explore allelopathy is the discovery of selective natural herbicides.

I have been looking, unsuccessfully for information on Casuarina ellelopathy in order to determine whether I can sparsely intersperse Casuarina as a support species for increased fertility, by planting them amongst berry bushes (Rubus and Ribes), while at the same time gaining the advantage of C. suppressing the germination of local annual weeds. I  have a client for whom this would be an excellent design solution if the allelopathic  pathways of Casuarina are appropriate.