The high nitrogen-use efficiency of rhododendrons and their ericoid mycorrhizal partners, together with the high recalcitrance to decomposition of evergreen rhododendron litter, is surely a factor in rhododendrons' ability to usurp space from other forest dominants after disturbance and inhibit their return. Allelopathy may play a role as well. Research on rhododendron or other ericaceous plants as keystone species in North American forests may offer some clues. For instance:

Mallik, A. U. 1995. Conversion of temperate forests into heaths: role of ecosystem disturbance and ericaceous plants. Environmental Management 19: 675-684.

Nilsen, E. T., J. F. Walker, O. K. Miller, S. W. Semones, T. T. Lei and B. D. Clinton. 1999. Inhibition of seedling survival under Rhododendron maximum (Ericaceae): could allelopathy be a cause? American Journal of Botany 86: 1597-1605.

Petraitis, P. S. and R. E. Latham. 1999. The importance of scale in testing the origins of alternative community states in ecosystems. Ecology 80: 429-442.

Certainly the distribution and adaptability of rhododendron in the Himalaya is unique. In fact the regeneration of rhododendron is also specific and it is pivotal for rural agro-pastoral activities and rural economy.

I think if some species is a keystone species depents on research question. For example: The keystone species of the (evolution of) cultural landscape of the Lunenburg Heath has been the moorland sheep.

In terms of ecosystem thinking - What kind of functions has this tree species among other tree species? Is it a pioneer species, ore do they occur in climax stadium of a forest. Do they have deep/flat roots and protect the system from soil errosion?

The high nitrogen-use efficiency of rhododendrons and their ericoid mycorrhizal partners, together with the high recalcitrance to decomposition of evergreen rhododendron litter, is surely a factor in rhododendrons' ability to usurp space from other forest dominants after disturbance and inhibit their return. Allelopathy may play a role as well. Research on rhododendron or other ericaceous plants as keystone species in North American forests may offer some clues. For instance:

Mallik, A. U. 1995. Conversion of temperate forests into heaths: role of ecosystem disturbance and ericaceous plants. Environmental Management 19: 675-684.

Nilsen, E. T., J. F. Walker, O. K. Miller, S. W. Semones, T. T. Lei and B. D. Clinton. 1999. Inhibition of seedling survival under Rhododendron maximum (Ericaceae): could allelopathy be a cause? American Journal of Botany 86: 1597-1605.

Petraitis, P. S. and R. E. Latham. 1999. The importance of scale in testing the origins of alternative community states in ecosystems. Ecology 80: 429-442.

Dear Sailesh, I also agree that it probably depends how you define a key stone species and probably there are various definitions for it. To make it even more complex you also have a term 'ecosystem engineer' species (which I prefer actually) - and it probably party overlaps. I would say that the 'ecosystem engineer' is a species that affects and modifies its environment in a way that it benefits its own growth / fitness. So that would be not only a species that eg dominates in terms of biomass in a given ecosystem, but also transforms its surroundings so gives it an advantage above other species (via some bio-chemical or biotic mechanisms - such as acidification of soil, or allelopathy). Some species can even effectively build its own ecosystems - a good example are Sphagnum mosses that build up a peat-bog where they create a habitat with a specific hydrology and small-scale hydrological gradients, or the mangroves that facilitate the coastal accretion and development of (wooded-shrubby) marshes growth so create its own habitat.

I dont think its a good approach to assign the status of a keystone species from an anthropological point of view (= a species that is somewhat important from human perspective and affects a system, but it does not mean that a system needs it).

Although I dont know much about Rhododendron forests - I think it is a natural ecosystems, that develop for a long time. I would call Rhododendron an ecosystem engineer = If it affect the the system (physically, by reducing erosion, and allowing for soil formation), modify the soil chemistry , probably also modify / effect soil microorganisms, affects the hydrological ballance (and microclimate), and provide habitat (produces large biomass, dominating ?) for specific fauna and flora communities (which do not exist elsewhere) - in a way that it modifies its environment and creates condition for favorable for itself . Its important to know what processes or mechanisms are specific for Rhododendron? Possibly a quality of litter is here an important characteristic (I know about few examples of it from other systems). Eg because its flowering massively it sustains a community of insects (birds) that are the scarce pollinators on higher elevations? - just hypothetical.

I work with such system - analysis (often interdisciplinary) which takes an ecosystem approach, mainly in the perspective of nature conservation, ecosystem restoration and understanding the ecosystem functioning (applied research). Ecosystem engineers are important for restoring functions of a system - that's why I am interested in them.

For even greater confusion I think a term keystone species is used for a species that are low in a abundance but somehow affect other species a lot - e.g. top predators in savanas. They regulate the system, and have a proportionally much larger effect on its working than any other species. Although its just a one species, and in terms of biomass its nothing, taking it "out of the system" changes a systems drastically.

This should not be confused with a (ecosystem-state) indicator species = eg a species that is depending on a various elements of its environment, both specific and sensitive, which can provide an insight about 'ecosystem health' or ecosystem state (although these are somewhat vague terms ). This species reacts to a changes in a system (quality, size, type of habitat) but is not necessary for functioning of a system and does not really significantly change the conditions around itself.

I think a best is to search in literature for ecosystem engineer, ecosystem builder, keystone species etc - and see what sort of species and why were called like that. You can also contact me directly (I have some pdfs of papers on ecosystem engineers).

There are a number of definitions of "keystone species" and "ecosystem engineers" on the web, but I recommend finding definitive and accepted definitions in key papers by leaders in the topic, and working from these; then define your criteria based on the primary literature (not general web pages), and work through and see if Rhododendron fits the accepted scientific definitions and criteria.

I am not sure that I find "ecosystem engineer" to be a terribly useful concept overall, let alone for ericads - it is obviously useful at one end of the scale (beavers building dams) but gets into shades of gray at the subtle end of the scale (nutritional effects of detritus, humus formation, etc.) - so it is only with tongue in cheek that I suggest that we need to divide ecosystem engineers into civil, mechanical, structural and chemical engineers if we wish the concept to be useful. (Ericads would probably be "ecosystem chemical engineers"?) In reality, I find it easier not to anthropomorphise ecological processes, which takes us down the road towards "strategies", and other terms which can inadvertently begin to lock us into human constructs in our thinking - useful for analogies, not useful if we start anthropomorphizing. ("Ecosystem engineer" was floated for a while by AU Mallik for Kalmia angustifolia; check out his papers and Powerpoints, web pages, etc.)

Allelopathy, N mineralization and anthropogenic disturbance (grazing sheep) have been mentioned in this thread as key processes in ecosystems where ericads play an important role; to these, you could add other anthropogenic (forest harvesting) and natural (fire, windthrow) disturbances (Kalmia angustifolia, Gaultheria shallon), and water relations. (We chased N mineralization hypotheses for many years re: Gaultheria shallon in coastal BC, but turns out that water use may have been more important - check out SCHIRP web pages at UBC; I believe one of the last papers has "water" in the title.) So in reviewing Rhododendron in the Himalayas, bear water relations in mind.

(Good to see you are still in this area, Roger! I regret that it has now been some time since I have worked with this fascinating group of plants!)

I like Roger's opinion. The Agata's explanation is interesting but if some species modify its environment in a way that it benefits its own growth / fitness, how that could be ecologically important species to particular ecosystem? Isn't that a kind of invasion, which compete and reduce space for native species? I agree with Titus's arguments and like suggestion regarding water relations which explaining the ecological importance of Rhododendron in Himalayas. Thank you all!

DEar B. Titus,

I agree about finding the criteria and definition (based on literature) and adapting it to a new research model.

Its an interesting opinion about the ecosystem engineers. Good to hear it!

I have encountered much broader understanding (see e.g. publication below). I am personally more flexible with this term. I think its useful when we talk about functional relationships in ecosystem (and in my opinion this term stresses this functional approach) and ecosystem functions, recently translated into ecosystem services. I know many researcher are reactant to this concept, but I work with it as it is necessary to win a support of decision-makers or of broad society for nature conservation or nature restoration.

For Rhododendrons in Himalayas - Next to water relations (and chemical effects via litter) possibly also heat and microclimate can be important (easy to check this out). These will be of course related to water as well.

"Ecologic engineers" is a term used often for a native species, so they take space from other native species due to competition, or succession or natural shifts in ecosystems.

Review in Ecological Engineering 37 (2011) 113–122: How ecological engineering can serve in coastal protection

Bas W. Borsjea,b,∗, Bregje K. van Wesenbeeckb, Frank Dekkerc, Peter Paalvastd,

Tjeerd J. Boumab,e, Marieke M. van Katwijkf, Mindert B. de Vries

Thanks you as well for inspiring discussion!

Rhododendron in Himalayas is not uniform their shape and forms changes with attitudinal gradient eg R. arboreum in 1500mts is a large tree but at 3500 mts ( alpine zone) its from creeping nature. So its adaptability in himalayn ecosystem makes it a good companion of most the wide spectrum of species in different altitudes. Further its flowers are so attractive that also harbour variety of pollinators and help other species too. Roots, litter and soil already discussed. certainly human / social dimension also important and recognised even as state tree of Uttarakhand and state flower of Sikkim in India due its mass invovment in socioeconomic and cultural value.

Dear Manmohan - you put an example of R. arboreum, it is not true that the shape and forms of this tree change with altitude gradient. You can encounter same sized tree in lower to higher elevation. What makes difference in size and shape is eco-types (subspecies and varieties) but not altitude. There is no second opinion about the attractiveness of this tree species, which is also reflected by their designation as the national flower, state flower or tree in their native area. I do not encounter or known any such pollinators which totally depend on R. arboreum; definitely they attract a wide range of pollinator but not single such pollinator whose life cycle is possible only because of R. arboreum. If you know any such species which are totally depend on R. arboreum you are welcome to share.

Cultural value or socioeconomic value does not make the species ecologically important. Ecosystem function such as maintaining in water conservation in the Himalayas is one of key point to discuss in this regard.

As Agata argued, beside water relations heat relations might be important, which could play important role in maintaining a micro - climate. Seems allelopathic effect of this species could be a useful point to argue. Agata's argue on ecological engineering still sound like an invasion to me, "take space from other native species due to competition or natural shift in the ecosystem (due to changing climatic comfort zone?)". Succession sounds good but is it OK to describe any species as keystone based on climax vegetation formed as a result of succession?

Sailesh, I may be straying into semantics here, but in my lexicon, by definition a species that is indigenous to an area (that is, not transported there by humans in recent times) cannot be invasive. Some native species can be aggressive and become dominant under certain conditions, but that doesn't make them invasive. My understanding is that species diversity within the genus Rhododendron is very high in the Himalayas and nearby mountain ranges extending into Yunnan and Szechuan. It seems unlikely that a single species of Rhododendron is "the" keystone species there but it's possible that several species form a functional group that is keystone to a particular successional trajectory and persistent end state. I view keystone species as those that may engineer a transition to a different successional pathway, often following severe disturbance, from the pathway that predominated before the disturbance or before local colonization by the species. I highly recommend that you peruse this book, in order to gain insight into how the keystone species concept relates to alternative successional trajectories and alternative persistent ecosystem states (the time commitment isn't onerous -- the text is only 160 pages long):

Petraitis, Peter. 2013. Multiple Stable States in Natural Ecosystems. Oxford University Press, Oxford, U.K. 188 pp.

Roger, thank you for clarifying. I like the points you mentioned, it is better to mention a group of Rhododendron rather than a single species.

Rhododendrons are among important, dominant and primitive group of flowering plants thrives in temperate and subalpine regions of Himalaya. These species have considerable ecological and economic importance. In western Himalaya, the wood of the species is used for fuel, flowers used to made into herbal drink. The drink is considered medicinal for heart diseases, bark used for digestive disorders. Due to anthropogenic interference associated with habitat destruction, climate change and over-exploitation have collectively put pressure on its natural population. Over-exploitation of flowers make the situation worse, as it has negative impact on the regeneration potential of the genus. Due to its direct impact on soil and moisture regime and its traditional uses by local inhabitant, Rhododendron could be considered as a keystone species for himalaya.

Keystone species play a crucial role in

maintaining the organization and diversity

of their ecological communities. Rhododendrons act as keystone species

in the high-elevation portions of the Eastern Himalayas.The species must be conserved soon before they become endangered or extinct. Certainly it is considered as the keystone species in Himalayan ranges.

Dear Sailesh- May be trues of subtype but is not adaptable nature of species made room for others survival. Certainly pollinators are there but specific insects yet to search. third water regime in Himalayas can be true as its close associate of Oaks in hiss but Oaks are considered main water recharging species.--- Is human being are not part of ecosystem??? My not be traditionally by ecologist but can it be redefined or another area of concern ---

Dear Vinod - Thank you for sharing experiences from the Western Himalayas (WH); talking about Western Himalayas it again comes back to R. arboreum. If there is any report or publication on vulnerability assessment of Rhododendron in WH please share it.

Dear Kadavul - Rhododendrons are very important in the Eastern Himalayas-Hengduan Mountains with 30 to more than 450 species of rhododendron occurring in different areas of this landscape. Definitely this landscape needs a conservation measures to be applied but that is not related to the issue I put here for discussion. Seems you are working in Southern part of India, could you provide some facts/literatures about R. arboreum subsp nilagiricum regarding the role and importance of this species in the Western Ghats and adjoining areas where it occurs? That information might be useful to describe it as very important species (not because of its occurrence in a remote area from their homeland in the Himalayas but for contributing some ecological functions).

Dear Manmohan - Sounds like you are asking several questions, I thinks you can get some in B. Titus answer (see above). Regarding association of Rhododendrons, they occur with many species ranging from sub-tropical elements to alpine elements, or formed a pure stand of their own. Mostly R. arboreum is associated with oak, but arboreum occurs in a wide range of habitat and species.

I am not much familiar with the ecology of Oak but you might find following paper interesting,

Katul, Gabriel; Todd, Philip; Pataki, Diane; Kabala, Zbigniew J.; and Oren, Ram. 1997. Soil water depletion by oak trees and the influence of root water uptake on the moisture content spatial statistics. Water Resources Research, Volume 33, Issue 4, pp. 611-623. DOI: 10.1029/96WR03978.

As being a wildlife researcher, I would term a Keystone species as the species that can provide food to animals even in adverse conditions or say year round even. In the pinch conditions, these keystone species attract an array of animals and most parts of the species are edible even. Like fruit, leaf, flower, pith, bark, sap etc. Rhododendrons to me are not the species that are to be kept under Keystone word. Even though the species of this genus attract a whole array of Birds to feed on their nectar and might be few mammal species on their flowers. But all this is seasonal mostly. So I donn think of adding this Genus under the section "Keystone".

Dear Dr Sailesh In Western Ghats , The Mukurthi National Park (MNP) is a 78.46 km² protected area located in the western corner of the Nilgiris Plateau west of Ootacamund hill station in the northwest corner of Tamil Nadu state in the Western Ghats mountain range of South India. The park was created to protect its Keystone species.Rhododendron arboreum (the national flower of Nepal or Rhododendron nilagiricum,) are seen throughout the grasslands and very large specimens are conspicuous around many sholas. Other common shola trees and shrubs among the 58 species found here include: Syzygium calophyllifolium, Daphiphyllum neilgherrense, Cinnamomum wightii, Vaccinium leschenaulti, Mahonia leschenaulti, Litsea sp., Lasianthes sp., Psychotria sp. and Michelia nilagirica.

Wild Yellow Raspberries grow on the edge of sholas and in disturbed soil along trails and roads .The Edges of most sholas are lined with the shrubs: Gaultheria fragrantissima, Rhodomyrtus tomentosa, Rubus sp., Bergeris tinctoria, Eurya nitida, Strobilanthes sp., and HelichrysumMukurthi National Park is managed by the Tamil Nadu Department of Forestry with the main objective to conserve the endangered shola-grassland ecosystem and its endemic flora and fauna.The effective effort is to protect Rhododendron sp.hence because of tourist disturbances may decline the population of Rhododendrons in these Biosphere reserve.

By definition, refers to a keystone species in ecosystems or vegetation systems play a key role in species, especially in maintaining the structure and function of the system or to ensure running. so whether Rhododendrons are keystone species or not , depending on its status and role in community or ecosystem mainly.

First of all, I would like to thank you for this wonderful question which had not been gotten much attention in past. Almost 10 years ago I was doing my master thesis in Tawang (Zemithang circle), Eastern Himalayas about about traditional forest management by Monpa people in oak forests. I saw many Rhododendron species (at least I alone identify 10 species with the help of "Flowers of the Himalaya") which are associated to temperate moist broadleaved forests. I saw incredible moss and lichens growing on the bark of Rhodos, however, could not identify them. I saw Eurasian jays feeding on oak acorns and nesting in the bushes of Rhodos. Rhodos are incredible species that you may find from early successional stage to late successional stage with high variation in shade tolerance. They are one of the most important component of the temperate broadleaved forest ecosystem in the Himalayas. However, I do not know how many species are directly depended on them? which is very important to know for answering your question. Therefore, I would say more research is needed in this field and this could be a nice starting hypothesis. By the way, I drank a fresh traditional juice from Rhodo flowers made by my Monpa friends in Tawang which was really tasty! All the best with your research on this amazing genus.

Dear Dr Kadavul - Thank you for your your very informative answer from the Western Ghats. Is it possible to access some publication or reports related to R arboreum nilagiricum (except Jain et al 2000; Giriraj et al 2008), please refer.

Dear Murali - I partially agree with your opinion regarding the concept of keystone species. Ecology function is important to define that may be biological (as you mentioned) or contribute to the ecosystem as Xang-hai mentioned.

Dear Somidh - Thank you for sharing your experience and knowledge on Rhododendrons from Tawang area. I read a couple of papers from this region and get familiar with Rhododendron diversity. You mentioned you spent a long time in this region as a researcher, could you share some information about R arboreum and specific pollinators of this species? May be we can search some point for collaborative work for promoting this group of plant species for conservation!?

As per my personal observation working in alpine regions of Sikkim. Rhododendron is a keystone species of sub-alpine and alpine regions of the Himalaya. Where we hardly see Abies sps as a upperstorey in patches. They are good source of food for many birds, alpine butterflies, moths etc and provide shelter to many himalayan species of mammals like musk deer, serow red fox , mountain weasel and small mammals like Pika and rodents.

I found many birds and insects visiting the rhododendron arboreum flower like fire tailed sunbird, green tailed sunbird, babblers, tits, rosefinces, and laughing thrushes and in insects like bees, bumblee bees and many flies. But it is very difficult to make a conclusion based on observation . They could be the pollinators or could just be the visitors.

Interesting topic to work on. Good luck for your research.

Dear Sunita - Thank you for sharing information about visitors to R. arboreum tree. It is interesting to know Rhodos provide food source for a wide range of animals, is there any publication (from Sikkim or adjoining region) regarding Rhodos as a source of aforementioned animals? If yes please provide a link or title. Once again thanking you for sharing your observation.

Conservation of Rhododendrons in Sikkim Himalaya: An Overview by Singh et al. 2009

I think Singh et al 2009 did not mention about animals visiting Rhodos.

Yes..but there are few studies on it. Various organization are working on hopefully by coming year we will have a data on this too.

Good luck for the work.

Sunita donn you think that all these are seasonal observations.?

Yes i partly agree with your opinion Murali Krishna the food resources available form rhododendron is seasonal. But if we consider their importance in structuring various avian and mammals community in that landscape (especially in sub-alpine and alpine region) is vital.

Yes more research in this topic is necessary to justify it.

Definitely, rhododendrons are key stone species for the montane temperate forests of the Himalaya. In the western Himalaya there are 5 species of rhododendrons (R. arboreum, R. campanulatum, R. barbatum, R. anthopogon, R. lapidotum) distributed from 1800- alpine region. R. arboreum has widest distribution in the western Himalaya from 1800 to tree line. It is one of the major associated species of oak forests (having highest biodiversity in temperate region) and some area pine forest etc. All the above mentioned birds and wild animals are common to these forests. These species also forms shelter for several rare, endangered and threatened species in the region. R. arboreum is highly used for making juice by the local communities. R. arboreum and R. campanulatum are prime source of fuelwood in temperate and sub-alpine regions.

I am not in a position to address the question of whether Rhododendron are a keystone species in the Himalayas but our flow cytometry research on Rhododendron species from this region indicates that closely related species often vary in ploidy level (diploid to decaploid) especially among those classified as lepidotes. Elepidote species from this region do not appear to show this variation in ploidy level.