(Arboricultural-styled) 'Fact of the Day'

[Kveldssanger]

Thinking about subscribing, but noticed they don't even do a print journal!

[Kveldssanger]

22/03/16. Fact #177.

 

Tree defoliators, namely insects, can cause serious damage when outbreaks are severe. In the urban environment, such outbreaks are considered to be more common, because of the urban heat island effect – this is because insects are ectotherms (they require an external heat source). Insects will, in theory, therefore fare much better in such urban environments, compared to nearby rural locations. Additionally, as the vigour of trees will change as a result of warmer urban climates, such alterations may have a positive (or negative) impact upon insect populations. For example, trees that fare less optimally may produce fewer secondary metabolites that dissuade defoliation, and emit fewer herbivore-induced plant volatiles that disrupt herbivory and attract predators and parasites of the insect defoliator. Of course, this has consequences for the host trees, as defoliation outbreaks may thus be more severe, more prolonged, more frequent, and thus more damaging.

 

In order to add weighting to this statement, the authors of this study investigated how the oak scale insect Parthenolecanium quercifex fared in urban environments across Raleigh, North Carolina, USA, with a specific focus upon how urban temperatures influenced their abundance both directly (attraction to warmer areas, increased fecundity in females) and indirectly (rate of parasitism). Because the scale insect only has one generation per year, where the first instar stage of the insect feeds upon leaf phloem tissue before over-wintering upon the bark and developing through the second instar stage prior to pupating and becoming adults the following spring, it was not expected for increased temperature to improve generational turnover rates, but simply enable females to lay more eggs / lay eggs that have a lower mortality rate. Furthermore, as the scale insect is very similar to an array of other insect species and genera, understanding how urban temperatures influenced its biology could give an indicator as to how other insect species would fare in similar conditions.

 

What the authors found, following a survey period, was that Parthenolecanium quercifex over-wintering in the second instar stage were 13x more abundant in warm locations than in cool ones (see the below figure). In addition to this, the ovisacs (where a single egg is layed) were 5.5x more abundant when deposited by the very same adult scale insects that were 13x more abundant, and these ovisacs gave way to first instar stage scale insects 7x greater than on trees in cold sites. Generations of scale insect that spent their entire life cycles in hot microclimates were also observed to, when placed in colder microclimates (in a greenhouse), still be found at higher abundances than scale insects that had spent their entire life cycle in cold microclimates. In this sense, scale insects may locally adapt (in a beneficial sense) to hotter locations, and it is suspected that the scale insect has this ability because populations are highly segmented and therefore site-specific adaptations can occur with relative ease (gene flow is ‘locked’ – meta-populations almost don’t exist beyond the level of but a single, or few, trees).

 

Second instar stage scale insect abundance, in different urban locations, per 30.5cm of tree stem.

 

Conversely, no correlation was identified between urban temperatures and the rate of parasitism upon the scale insect. A total of six parasites were studied, of which none were found to have a significantly increased rate of parasitism when temperatures were higher. In fact, rates were near identical in hot and cold sites, as shown in the graph below. It is suggested that this is because the scale insect’s natural enemies are simply found in less abundance in urban locations, because of the poorer habitat quality. By a similar token, females were not found to lag more eggs on trees in hotter microclimates, and nor was host tree ‘quality’ deemed to impact upon the abundance of scale insects.

 

The rate of parasitism upon the scale insect Parthenolecanium quercifex. Parasites studied included Coccophagus lycimnia, Encyrtus fuscus, and Eunotus lividus.

 

It was also mentioned that it is unlikely that stressed trees would be host to more scale insects, because scale insects would probably be found in lesser abundances where water and nutrients are lacking within the tree. Given urban trees typically struggle because of drought and a lack of nutrient availability, it is thus impropbable that tree quality is an influencing factor upon population levels of the scale insect. If it were then, because the scale insect is a sap-sucker that relishes nutrient-rich sap, a tree lacking this (because of drought and poor nutrient availability in the soil) would not be a able to support large numbers. Therefore, the increase in scale insects in hotter microclimates is likely to be independent of tree quality (condition).

 

To conclude, the authors remark that scale insects are more abundant in urban locations where the microclimate is warmer. As a consequence, if temperatures continue to warm in the urban setting, or become more homogenous (at a higher temperature) across a large urbanised spatial scale, scale insect populations may markedly increase and therefore be potentially very damaging for urban trees. Because urban trees are exposed to so many adverse conditions, an increase in pest activity is certainly not something that will help their case for survival. Such a weakened nature may also leave them exposed to other pests and diseases, which do rely upon weakened hosts to establish in great abundance.

 

Beyond the urban setting, if temperatures increase in rural locations, scale insects may also become more of an issue there. Granted, such rural locations are home to greater numbers of parasites (natural enemies), and thus an increase in numbers there may perhaps support an increase in parasitoid abundance as well. This is, however, just speculation.

 

Source: Meineke, E., Dunn, R., Sexton, J., & Frank, S. (2013) Urban warming drives insect pest abundance on street trees. PLoS One. 8 (3). p1-7.

[Kveldssanger]

23/03/16. Fact #178.

 

The pathogen Xylella fastidiosa has been hitting the headlines in the UK recently, because there is growing concern over the risk of it reaching the UK shores and not being caught quickly. This bacteria ultimately causes a leaf scorch that, over time, progresses throughout the crown of its host, and will eventually cause tree mortality. Because it’s not native to the UK, its introduction would potentially be very significant, given no native tree species have any form of resistance to it. Furthermore, as it may attack a huge variety of trees and plants, understanding a little more about its biology is certainly something that should be pursued. With that in mind, I thought I’d look at a study from the District of Colombia, Washington, USA that sought to investigate the population structure of the bacteria in an urban streetscene, with specific focus upon how the bacteria associated with the constituent tree species.

 

Within the District’s street scape, infected trees of the species Morus alba, Platanus occidentalis, Quercus coccinea, Quercus palustris, Quercus phellos, Quercus rubra, and Ulmus americana had foliar samples taken from their structure, which were then taken into the laboratory and the constituent Xylella fastidiosa sequenced (DNA sequencing). Most samples were taken from trees displaying visible symptoms of infection, though some were also taken from trees, or locations upon an infected tree, where there were no symptoms (asymptomatic). The table below gives a breakdown of exactly what samples were taken from which trees.

 

The number of samples taken from each tree species.

 

Following DNA sequencing of all the samples, it was identified that there were five different ‘sequence types’ of the bacteria Xylella fastidiosa. These sequence types were almost exclusively unique to a specific tree genus, though one sequence type (ST-9), whilst found predominantly in oak, was also found in an elm sample (this specific sequence type has been found in elm before, and also sycamore). This near exclusivity of different sequence types occurred almost always, in spite of the fact that many study locations were comprised of infected trees of more than one of the species studied (for example, a site may have contained both mulberry and oak, or elm and sycamore, and so on). Curiously, the elm infected by a type found otherwise only on oak was not near to any oak tree.

 

The host specificity of Xylella fastidiosa.

 

This host specificity is important, because it means that a sequence type infected mulberry is highly unlikely to ever be pathogenic towards oak, for example. However, other studies have shown that ST-8, found here only on sycamore, could also be found on oak and elm. Therefore, certain sequence types may have the ability to infect more than one tree genus, though this is certainly not true for all sequence types.

 

In this sense, a few comments can be made: (1) Monocultures are certainly to be avoided, for those tree species that are susceptible to Xylella fastidiosa. They can swiftly become very extensive inoculum bases for the bacteria as a group, and if they succumb to the infection then an entire swathe of trees can readily be wiped-out. By planting and maintaining healthy populations of an array of tree species of different genera, the impact of Xylella fastidiosa can be reduced (though only in urban areas – outbreaks in woodland settings would be far more impactful, because of a reduced diversity of tree species).

 

(2) Management of the bacteria can be worked down to the sequence type level, and this may prove to be a double-edged sword in terms of management. In a positive light, because cross-contamination of a sequence type is likely to be quite infrequent, targeted management approaches can be created that are specific to the particular strain of the bacteria and specific to the area of infection. Of course, this is also concerning, because one control method for one sequence type may not necessarily work for another, and for those sequence types that do have more than one potential host genus, adaptations on the genetic level may enable it to branch-out to infect other tree genera as well, or become more adept as infecting peripheral species and genera in its current host range. This latter consideration may be one reason why limiting monocultures is important, as the build-up of the pathogen in a suitable inoculum base may potentially increase its pathogenicity.

 

Source: Harris, J. & Balci, Y. (2015) Population Structure of the Bacterial Pathogen Xylella fastidiosa among Street Trees in Washington DC. PLoS One. 10 (3). p1-11.

[Kveldssanger]

24/03/16. Fact #179.

 

Many studies undertaken by scientific researchers are – by a sort of default – quite limited, in terms of the study’s spatial scale. Whilst this is not necessarily a bad thing, when the point of the research is to use a specific area (such as a city) as a case study, it can limit the effectiveness of problems that occur across a very large geographical area. One such example would be Cameraria ohridella (horse-chestnut leaf miner) presence within the UK, where it is an invasive pest (having arrived in 2002) progressing across the UK that may aggressively defoliate Aesculus hippocastanum (horse chestnut) and thus cause serious tree health problems.

 

An example of horse chestnut foliage being attacked by the leaf miner. Source: The Wild Diary.

 

Because the pest is found in many areas of the UK, understanding its characteristics as a meta-population is important, but tricky (in terms of cost and time) if relying upon the standard method of research. To remedy this issue, a form of science known as ‘citizen science’ can be used, which directly involves members of the public, across very large areas, gathering data and relaying it back to scientific researchers, where the data can then be processed. And would you know it, the study looked at here employs such a form of science, with the aim of understanding: (1) more about how serious leaf damage is from Cameraria ohridella, when comparing leaf damage extent to how long the miner has existed within the local geographical area, and (2) whether parasitism of the leaf miner (by parasitic wasps) is higher in areas where the leaf miner has existed for longer periods of time. The research project was dubbed ‘Conker Tree Science‘ (and is still ongoing, in a slightly different format).

 

In order to bring in accurate information from members of the public, which could be processed and transformed into conclusions, those aiding with the survey (around 875 people, for the first part) were asked to complete a few basic tasks. The first was to determine an average level of leaf mining damage upon foliage of the horse chestnut, with the aid of a very basic diagram (shown below). Such recording was undertaken during June – September of 2010, and alongside the numerical scoring the surveyors also submitted photos of the foliage inspected. This enabled an experienced individual to determine whether recording was accurate, and if not, bias would be accounted for in the data analyses. No bias was found, so the data gathered by the survey participants was considered to be accurate.

 

Ranging from 0-4, with 0 being no damage and 4 being major damage, survey participants reported to what extent the horse chestnut they were inspecting was mined.

 

Where a member of the public had stated the leaf was at least partly damaged, the researchers compared the location of the tree to how long Cameraria ohridella had been known to exist in the area. Of course, the time of year was factored into account, as leaf mining will progressively worsen as the larvae progress through their instar stages; as will it generally worsen as the summer progresses (there may be more than one life cycle of the pest in one summer). Furthermore, because the presence of the leaf miner may have been under-recorded prior to this study taking place (as people became used to its presence,a nd no longer recorded it), the researchers constructed a model that would predict where the miner should have progressed to in the years following 2002. Therefore, if reports came in from areas where there were no records of the leaf miner, the model would allow for comparisons to be made between mining severity and how long the miner had likely been present for in that locality.

 

With regards to ascertaining whether parasitism increased with how long the leaf miner had been present within the local environment for, survey participants were also asked to take small cuttings of foliage during early July 2010 and seal those cuttings in a plastic bag for two weeks. After this time, a count for Cameraria ohridella, parasitic wasps, and any other insects (in case of contamination) was undertaken. In order to identify the different organisms, the researchers provided an identification key to all survey participants. Much of this part of the data collection was done by schoolchildren, with the aid of trained researchers. Following counting of all insects, the data was compared to that gathered by experts, in order to check for bias. No significant issues were found with regards to identifying the leaf miner, so the data gathered by the school children was used in the study without being corrected in any manner. For the parasitic wasps however, it was found that the school children failed to fully identify them in some instances (under-estimation).

 

After collating the results and analysing them, the researchers found that the damage caused by Cameraria ohridella rose for the first three years; at which point, it begun to quickly ‘flatten’ (and thus, damage then remained rather constant, albeit significant). This is shown in the below graph. In this sense, in the fourth year, the leaf miner will most likely be causing maximum damage, and from here-on-in, such maximum damage will routinely occur (of course, it will still vary from year-to-year).

 

Foliar damage associated with Cameraria ohridella, in relation to how long it had been present in the local area of the UK.

 

As for whether parasitism of the leaf miner increased over time, the 2,208 cases of reared insects (1,810 of the cases were from school children) showed that the rate of parasitism increased aongside how long the leaf miner had been present in an area for. Consulting previous literature of parasitism on the leaf miner, the researchers suggest that initially pre-pupal stage generalist parasites will use the leaf miner as a host, though after a few years more specialised pupal stage parasites will succeed into the trophic system. However, once specialised parasites do arrive, it is not expected for there to be a continual rise in their population abundance, as research in mainland Europe has shown that parasitic wasp populations plateau after a some years. Therefore, parasitic wasps may not be able to greatly limit the damage caused by Cameraria ohridella.

 

Such results are certainly interesting, though the manner in which they were obtained is equally so. What we can draw from this is that citizen science certainly has the ability to work, and across a large geographical area; all whilst costing little money and ‘outsourcing’ the time spent on data collection to willing volunteers. This can be good as it engages people with the scientific process, thereby removing it from its pedestal and giving science a form of accessibility. The act of engaging with school children was particularly pleasing to read about, as one never knows whether such research could evoke a greater interest in scientific research for some of those children. Granted, citizen science isn’t a ‘cure-all’ approach, as there are many limiations. The researchers remark that one main limitation in this study was the inability to directly sample wasp parasites, and thus specific species couldn’t be identified. Furthermore, the data is only as good as those collecting it, and because volunteers are unlikely to be amateur gall enthusiasts, data collection must be simple, swift, and succinct. There’s also the need to verify the data after it has been collected, unlike with scientific researchers who will know how to gather data prior to gathering it (and thus eliminating bias, ideally).

 

Nonetheless, a good study, and hopefully citizen science can be used in the future for other projects!

 

Source: Pocock, M. & Evans, D. (2014) The success of the horse-chestnut leaf-miner, Cameraria ohridella, in the UK revealed with hypothesis-led citizen science. PLoS One. 9 (1). e86226.

[David Humphries]

24/03/16. Fact #179.......hopefully citizen science can be used in the future for other projects!

 

I'd concur that Citizen science isn't a cure all, it certainly has its place (and probably an important one in terms of education) but perhaps not so much when the FC have to chase down red herrings from CS in the form of Chalara being reported from elms and other non target hosts.

 

As an example I received a reply from the FR Tree Health Diagnostic & Advisory Service today on an enquiry I highlighted whilst engaged in a London wide Ceratocystis survey.....................from last August !

 

They are stretched and underfunded beyond belief.

 

 

.

[Kveldssanger]

I'd concur that Citizen science isn't a cure all, it certainly has its place (and probably an important one in terms of education) but perhaps not so much when the FC have to chase down red herrings from CS in the form of Chalara being reported from elms and other non target hosts.

 

As an example I received a reply from the FR Tree Health Diagnostic & Advisory Service today on an enquiry I highlighted whilst engaged in a London wide Ceratocystis survey.....................from last August !

 

They are stretched and underfunded beyond belief.

 

 

.

 

The lack of funding really is rather scary, and wasn't there that leaked FC staff survey recently that showed hardly anyone was ready for the new Stewardship programme? It's impossibly difficult to successfully operate with declining budgets and more demands.

 

Citizen science should be kept to simple things, I suspect. People don't report stuff out of malice (I hope!), though certainly they may be over-zealous. On an isolated levels that's OK, but if it becomes not so isolated then that's an issue.

[Kveldssanger]

26/03/16. Fact #180.

 

Associations (symbiosis) with mycorrhizal fungi are necessary for a vast number of tree species, and for those where associations are not necessary for (mainly pioneer species such as willow and birch) then such a lack of association will usually lead to the tree having significantly stunted growth. In this sense, a symbiosis between tree roots and mycorrhizal fungi is very much crucial. Therefore, where associations are lacking of abundance, which may particularly be the case in urban environments (because of soil compaction, construction damage, changing pH levels due to pollution, and so on), trees may markedly suffer. In fact, I have looked at a paper quite recently on such a lack of mycorrhizal symbiosis with fine root hairs in urban locations.

 

Granted, certain tree species do fare rather well in the urban environment, and therefore it may be the case that these species are able to associate more successfully with mycorrhizal fungi (amongst other reasons). By a similar token, the properties of the fine root mass of trees in urban areas may differ from rural locations, which of course would have implications for mycorrhizal associations. As a means of testing this, the well-known and well-used tree species horse chestnut (Aesculus hippocastanum) was studied across four urban and two rural sites in late autumn in Poznan, Poland, with specific focus upon the extent of arbuscular mycorrhizal colonisation in the trees’ root masses, the properties of the fine rooting masses, and soil chemical properties. The authors note that the horse chestnut is an ideal tree species to use for such a study, because is is well-adapted to the urban environment, and has been cultivated for ornamental planting for many centuries.

 

Briefly touching upon the actual study sites, the first two urban sites (URB1 & URB2) features horse chestnuts planted in an avenue between 1909-1911. this avenue is 10m wide, and on either side sits some form of public highway. The avenue has always had poor soil conditions, and in 1998 the two URB sites were mulched in an attempt to improve soil conditions. Site URB1 has, since then, received routinely mulch applications, whilst site URB2 hasn’t. The third urban site (URB3) is in the city centre and near a main public highway, within the Henryk Wieniawski Park. This site was planted with horse chestnuts (amongst other species) in 1907 and 1910. URB4 (a closed cemetery in General Jan Henryk Dabrowski Park) consists of many tree species, including horse chestnut, which were planted somewhere between 1830-1930. The two rural locations were both where horse chestnuts were situated in Wielkopolska National Park, which is 20km to the south of Poznan. The two locations were within horse chestnut avenues, bordering agricultural fields, established during 1800-1850, making them some of the oldest horse chestnuts in the region.

 

The avenue where sites URB1 & URB2 were located. Source: Wikimedia Commons.

 

With regards to the results, soil analyses identified that lead and copper levels were significantly higher in urban environments, and that elements such as calcium, carbon, magnesium, phosphorus, potassium, and sodium, also varied between all six sites to rather marked extents. No significant difference was found between sites for elements such as nitrogen and sulphur and, overall, the toxicity of urban soils (associated with higher levels of lead, and sodium, in particular) was considered to be only of a low level, and rural sites had a level toxicity as well because of chlorine (associated with fertilisers).

 

Despite this, the rural sites were shown to be significantly different in soil characteristics to the urban park sites (URB3 & URB4), which were in turn significantly different to the street-side urban sites (URB1 & URB2). Similarly, fine root characteristics of the horse chestnuts studied varied significantly between the rural and urban sites. In sites URB1 & URB2, for example, the fine root biomass was 1.6-2.2x greater, as was fine root length, surface area, and volume. This was very likely because of the mulching undertaken on these sites, which would have, particularly in the case of URB1, consistently improved soil conditions in the upper layer, thereby enabling better fine root growth.

 

However, fine root tip density was 1.3-1.4x lower in urban sites. Conversely, in rural sites, fine roots were thinner, and specific fine root lengths (the ratio of fine root length and the dry mass of such fine roots), and also specific fine root areas, were greater. This suggests that those horse chestnuts in rural areas, because of the better soil conditions (reduced bulk density and associated soil compaction), had the ability to produce finer rooting masses that could disseminate out into the soil with greater ease. In turn, nutrient and moisture uptake is, theoretically, easier for the tree.

 

Interestingly, this is where the differences end. In relation to the rate of colonisation by arbuscular mycorrhizal fungi (specifically, vesicles, hyphae, and coils), and even fungal endophytes, there was no significant difference between those horse chestnuts in rural environments and those in urban environments (though urban settings did yield greater abundances of arbuscular mycorrhizae and fungal endophytes, on average – around 1.2x higher for arbuscular mycorrhizae). The horse chestnuts at URB1 & URB2 had the highest abundance of vesicles, for example, and those at URB3 had the highest abundance of hyphae and coils (as shown in the table below). The abundance of vesicles is considered to be because they are most routinely found during late autumn, which coincided with the timing of this study.

 

Colonisation rates for each site.

 

Such results are also interesting because they are vastly different to the study I looked at in this blog post, which presented arbuscular mycorrhizal fungi colonisation rates down in the 20% range for urban sites and the 40% range for rural sites for the horse chestnut. Perhaps, the stressors that could impact upon such colonisation rates are not so evident in Poznan, when compared to Ontario, Canada. Additionally, as the horse chestnuts studied here are all very much mature, perhaps their likely longer existence in situ has enabled for a greater rate of mycorrhizal symbiosis.

 

Furthermore, the authors did note that the horse chestnuts in the rural locations were more extensively defoliated by horse chestnut leaf miner (Cameraria ohridella) and leaf blotch (Guignardia aesculi). Compiled with the fact that agricultural fertilisers would have been applied in the vicinity of the rural horse chestnuts (as known by the high chlorine content of the soils), such factors may have lead to lower colonisation rates by arbuscular mycorrhizal fungi – for the latter factor, there is a vast pool of research outlining how fertilisers adversely impact upon such associations.

 

In essence, therefore, one can conclude that, at least in this case, urban soils are not poor enough to impact upon mycorrhizal associations with horse chestnut roots. From this, it can be posited that, if urban soils are looked after so that they are not hugely adverse in properties, then they can support similar, or higher) levels of mycorrhizal fungi associations than rural locations. Because such associations are critical for the health of trees, it may enable for urban trees to live healthy lives that are not prematurely terminated by damaging environmental factors. Therefore, focus should be paid to the soil environment, as such an environment may be a key (of perhaps many) to safeguarding our urban tree populations.

 

Source: Karliński, L., Jagodziński, A., Leski, T., Butkiewicz, P., Brosz, M., & Rudawska, M. (2014). Fine root parameters and mycorrhizal colonization of horse chestnut trees (Aesculus hippocastanum L.) in urban and rural environments. Landscape and Urban Planning. 127 (1). p154-163.

[Gary Prentice]

23/03/16. Fact #178.

 

The pathogen Xylella fastidiosa has been hitting the headlines in the UK recently, because there is growing concern over the risk of it reaching the UK shores and not being caught quickly. This bacteria ultimately causes a leaf scorch that, over time, progresses throughout the crown of its host, and will eventually cause tree mortality. Because it’s not native to the UK, its introduction would potentially be very significant, given no native tree species have any form of resistance to it.

 

At the risk of sounding pedantic, how does this differ from Chalara? With that disease being introduced into an ecosystem that hasn't evolved with it.

 

The mortality rate of Chalara seem to be in the region of 95-98%, so some individual trees will survive it and in time, would reproduce to create a tolerant population. (As I understand occurs in Chalara's natural geographical environment.)

 

Are you saying that with Xylella fastidiosa, for some reason there will be no natural resistance all all? 100% mortality after colonization.

 

In the eighties DED appeared to be a disaster, changing the landscape forever. The landscape changed but did it really matter? Nature is always changing and adapting. Is globalization simply accelerating the process to the extent that we're becoming overly concerned about it?

 

I heartily agree with your comments about monocultures, whether in hectares of commercial woodland or urban street planting - the more diversity there is the lower the risk of a single pathogen seriously affecting the landscape.

[Kveldssanger]

At the risk of sounding pedantic, how does this differ from Chalara? With that disease being introduced into an ecosystem that hasn't evolved with it.

 

The mortality rate of Chalara seem to be in the region of 95-98%, so some individual trees will survive it and in time, would reproduce to create a tolerant population. (As I understand occurs in Chalara's natural geographical environment.)

 

Are you saying that with Xylella fastidiosa, for some reason there will be no natural resistance all all? 100% mortality after colonization.

 

In the eighties DED appeared to be a disaster, changing the landscape forever. The landscape changed but did it really matter? Nature is always changing and adapting. Is globalization simply accelerating the process to the extent that we're becoming overly concerned about it?

 

I heartily agree with your comments about monocultures, whether in hectares of commercial woodland or urban street planting - the more diversity there is the lower the risk of a single pathogen seriously affecting the landscape.

 

I wouldn't say it does differ. Mike Raupp did a talk at Barcham last year where studies showed that mortality of "naive" tree species (those that had not grown up in competition with the pests and pathogens) was near 100%, compared to the much lower mortality rate of tree species that had, over the course of I don't know how many years, evolved alongside such pests and pathogens. As ash dieback is considered to originate from Asia, it is of little wonder that the ash species from the US and Europe are getting battered by it. The same with EAB (emerald ash borer), and Manchurian ash (from Asia) is a good ash species to watch in that regard (as it has evolved alongside the EAB). But even then, bringing in non-native tree species to deal with non-native p&ds is equally as absurd on paper, as there are little obligate associations current native species would have with such trees. It compounds (though can admittedly reduce the problem - by patching an arterial bleed with the end of an earbud) a problem that really should never have existed in the first place.

 

From a slightly 'romantic' perspective, in a sense a pure ecological one, we'd have not seen the emergence of so many different exotic p&ds if we didn't facilitate their movement, and therefore if ash dieback, Xylella, or what have you, naturally succeeded, it'd be far more staggered and there wouldn't be the myriad of other p&ds to go with it (therefore taking away from the magnitude of the problem). However, when we have destroyed the elm, the poplar, the ash, and perhaps many other 'native' tree species as well, it just gets a little absurd. We talk of issues in isolation, as if ash dieback acts alone from everything else. It doesn't. It's a case of p&ds basically undertaking a pincer movement upon the defending tree forces, whilst they are simultaneously getting ruined on other sides already by existing (including exotic) p&ds. It's a case of ecosystem function, and frankly we 'aint seeing that to the level it needs to be at. Trees are primary producers, and form the basis of many ecosystems (alongside lower-tier plants - Xylella can also batter those, for example). Oak may be having acute and chronic decline, but so are our ecosystems (and let us be honest, the small 'victories' where there are small-scale successes, add to the delusion that current conservation efforts are 'okay' - on the micro-scale yes, but on the macro-level? heck no).

 

Saying the landscape is changing and that it'll fix itself is, in all fairness, and I mean this in the nicest of ways, a cop-out. It's a way of offloading a sense of moral responsibility for actually fixing a mess we have created. Okay, if it all naturally suceeded through no human intervention then cool, but if we had any part to play then we have a part to play in fixing that very mess. If it compounds into something of leviathan proportions then oh well, we must deal with it in the best way we can, and we have a moral responsibility for that.

 

Globailsation is a scourge (by-and-large), and whilst it has its benefits it decimates pretty much everything. Ironically, even the economy, to a degree. If globalisation did not occur, the current ecological climate would not be so dire - I can say that with near certainty. But we have what we have and we must work with it, to the best of our abilities.

Edited March 27, 2016 by Kveldssanger

[Kveldssanger]

27/03/16. Fact #181.

 

We may usually think of deadwood as something that benefits terrestrial species (bats, insects, fungi, mammals), though this is not the limit of what organisms may benefit from its presence. Many rivers and streams will, at some point in their existence, run through woodland and adjacent to hegderows, and beyond the beneficial cooling effects of a dappled shade courtesy of a canopy cover, the provisioning of deadwood (coarse woody debris) into the aquatic ecosystem may be of marked benefit for fish. Without question, the exact requirements of a woodland stream or river will vary depending upon constituent species, though by-and-large the presence of deadwood can be considered to be of benefit. The slowing-down of flow to create pools can aid with feeding, can certainly help salmon travelling upstream (in terms of enabling them to expend less energy on reaching egg-laying grounds), and can also reduce the build-up of silt on the stream bed by reducing bank erosion (‘clean’ gravel beds are critical for successful egg-laying). Benefits beyond this absolutely exist, and in this case we’ll be looking at the relationship between Chinook salmon redds (spawn sites) and large woody debris within the stream / river environment in a Californian river.

 

The study site chosen by the researchers was a 7.7km stretch of the lower Mokelumne River, where around 90% of all the river’s Chinook salmon redds are created. At its source, some 3,000m above sea level, its watershed is dressed with mountainous forests, before flowing westwards through oak woodlands and agricultural fields clad with tree belts and hedgerow. Riparian zones adjacent to the river are around 20m in width, and the constituent trees (alder, cottonwood, oak, walnut, willow, etc) reach heights of up to 25m. The river ends at its confluence with the San Joaquin River. The Mokelumne River is considered to be of medium size, with a channel width ranging from 15-83m (averaging at 31m). The below map gives more of a representation of the river’s location.

 

The location of the study site in the wider landscape.

 

Because of the historic management of the river (gravel bed extraction, salmon fishing, and so on), the river had actually been rather degraded up until more recently (management began in the 1960s to repair the degradation). Improvements to the river involved the addition of significant (over 100,000 tonnes) amounts of gravel and cobble (for improving spawning beds), and this has served to markedly increase Chinook salmon populations to the point that over 2,000 redds are created each year (up from only a couple of hundred). A great deal of the gravel augmentation took place in the 500m stretch of river west of the Camanche Dam (this dam sits just to the east of the study site), and was added in a way that made the river bed heterogenous in nature (providing riffles and pools, viable egg-laying sites, and even locations for adult salmon). Boulders were also added, as were large pieces of woody debris. This woody debris was buried in parts by gravel, so not to have the woody debris drift away.

 

The lower Mokelumne River. Source: Wikimedia Commons.

 

In this study, a total of 340 pieces (plus nearly 200 more where there we no redds) of large woody debris in the study area were surveyed and mapped (as redds were located nearby), where the average length was 6.9m (give or take 4m), and the average diameter was 23cm (give or take 12cm). The large majority (70%) of the large woody material was oriented in the direction of flow, and only 20% was oriented laterally (and never beyond 6m into the river). A total of 59% of the 340 deadwood pieces were located within the river but no more than 2.5m away from the river margin. The rest resided (at least partially) upon the bank. 65% of the deadwood was at least significantly decayed, and much of the identifiable deadwood was from alder.

 

In the first 3km of the study site, Chinook salmon utilised an area, 10m in radius, surrounding large woody material located in riffles (to a total of 68% of all redds). In the western 4.7km, this dropped to 44% of all redds. In many instances, these redds were situated downstream from large woody material, and were located in areas where there had been gravel augmentation. Furthermore, most of the redds were found within a 10m margin from the river bank, which is where 90% of all woody debris was also located. In this sense, one can identify how large woody material is certainly important for Chinook salmon redds, and the addition of such woody material to the river ecosystem is likely to have been a marked driver behind why Chinook salmon populations have increased. Reasons are because of, for example, altered flow velocities, increased levels of dissolved oxygen, cooler temperatures, and the shelter (against predators and other female salmon) provided by woody debris for egg-laying females (a larger list is given at the end of the journal article).

 

By a similar token, the addition of gravel would have provided improved conditions for redd creation by female Chinook salmon, though it is perhaps almost certain that the two, when combined, have the most beneficial impact. In fact, the researchers suggest that the much-increased habitat complexity associated with their presence is of great benefit and therefore, from a river restoration perspective, large woody material should be incorporated into management projects associated with ecosystem rejuvenation.

 

Source: Senter, A. & Pasternack, G. (2010) Large wood aids spawning Chinook salmon (Oncorhynchus tshawytscha) in marginal habitat on a regulated river in California. River Research and Applications. 27 (5). p550-565.