Bat Music - Harmonics and horseshoe bats

The really cool thing about horseshoe bats is their use of harmonics. Using their nose leaf they can filter down a note to focus in on a high frequency (around about 80 - 110khz) pure tone (well, nearly pure tone). To understand that you need to understand the fundamental frequency and harmonics.

Most notes you hear emitted by anything are what are termed "compound notes". This means the note is made up of loads of frequencies.

The part of the compound note we "hear" most prominently is the deepest and that is called the "fundamental". Over this are a series of overtones. Where these fall in a nice mathematical pattern of doubling themselves in integers they confer a pleasant sound that makes sense to our ears and we call them harmonics. This is what instruments do.

Where those overtones are more random and fall in an uneven sequence, it doesn't make a pleasant sound. Think of your car. You can definitely perceive a "note" the engine runs at.... you can hear the fundamental.... but its a messy NOISE rather than an organised NOTE.

What's really cool is that every emitter of sound highlights and cuts different overtones and this is what creates "timbre". This is why your voice sounds different to mine.... why an oboe sounds different to a guitar.... a car sounds different to a chainsaw.

Some things can cut out all those overtones and just produce the fundamental. Or focus in on a harmonic. Tuning forks and singing bowls are good examples. When you play harmonics on guitar you are highlighting one overtone and shaving away all the ones underneath it.

Bats in general are masters at using harmonics. It helps them to increase bandwidth and get a more detailed picture of the world. Sometimes you'll see an entire harmonic series filling up your sonogram when the pipistrelles are really going for it.

Most bats seem to be altering their delivery to increase the overtones they can produce. I'm not sure if any bats attempt to isolate harmonics like horseshoe bats do. It's not a perfect system... they put all their energy into the first or second harmonic but there is still a bit of the fundamental there although it's weak. They haven't quite mastered tuning fork!

The reason for all this sophistication is because they specialise in fluttering prey in highly cluttered environments. Most bats have to compromise between range and detail (lower frequencies punch further through the air) but range is pretty pointless for the clutter specialist. That allows these guys to really focus in on using the high detail, high frequency approach.

There are a lot of other interesting functions to their very unique form of echolocation, like picking up the doppler effect, but that's enough info for one day.

Have a listen to their cool, warbley alien call!

"Fields Yellow with Charlock"

On yesterday's session of government sanctioned exercise, this field in the eWoodhall estate near Tewin in Hertfordshire stuck out to me.

At first I thought it was rape coming into flower... I then noticed it seemed to be a volunteer crop... and the leaves were wrong. Then out of the blue a quote "...Fields yellow with charlock..." popped into my head. I took a closer look, got out my faithful friend Francis Rose's Wildflower Key and I checked it out.

It was indeed charlock (Sinapsis arvensis)

For me this sight had brought to life a quote from Arthur Young's General View on the Agriculture of Hertfordshire (1804). The work of this eminent agricultural improver captured a moment of transition between pre and post improvement agricultural practices.

As a conservationist it was a valuable read as it gave me a new perspective into the reality of those pre fertiliser, pre pesticide days of abundant plant life. It made me realise just how much farmers struggled with those crop weeds we now try desperately to conserve as they spiral towards extinction. Many of us are guilty of looking down on the agricultural improvers as callous destroyers of biodiversity, but they believed they were making things better for everyone, however tragic the story of loss that subsequently unfolded.

It bolstered the idea that conservation should never be all or nothing. To be truly sustainable, it needs to balance the needs of both nature AND humans.

Anyway, I digress.

In an excellent chapter about turnips (Young really loved a turnip), he commented with scorn on the state of the turnips "over-run with charlock in full bloom" all about Hertford and Hatfield (Young, 1804 p,103-4). On enquiry he found a decline rather than advancement in Hertfordshire husbandry.

Advanced practice held that one should hoe for weeds at least twice, but many mid Herts farmers were taking the lacksadaisical approach of hoeing but once, which he disparaged as "..a most reprehensible fault in their husbandry; every motive should incline them to the necessary exertion of giving a second hoeing", going on to emphasise that GOOD farmers in other counties might even give them a third going over.

He concludes his assessment of the situation with the the damning indictment that "..here they are contented with fields yellow with charlock", hoping that by drawing attention to it, he might shame farmers into upping their game and getting with the 20th century program!

Charlock was considered by other authors of the time (e.g. Long, 1938) as the most problematic arable weed in the UK. In another of Young's writings (sadly I cannot recall which), he dreams of a day when the scourge of Charlock is eradicated from the countryside. He would be ecstatic to see that today his dreams had largely come true.

All apart from this field, which takes us back to that very scene he described so venomously back in 1804.

Frequnt in most soil types, other than acid; charlock abundance increases with the calcium carbonate loams (Bond, Davies & Turner, 2007). In central Hertfordshire's geological mosaic, this is one such place; a hilltop patch where the chalk rises through the gravels and clays to be exposed at the surface. The green patches on the below map show where this is happening.

One soil charlock isn't so keen on is an acid soil (Bond, Davies & Turner, 2007). As many soils in this area are disposed to acidity, it's likely the old practice of chalking (to bring the acidity down to the neutral range) allowed it to gain a more widespread hold over the county.

Now the big question is why would this farmer have allowed this situation to arise? Charlock doesn't just compete for space and resources, it is a host of pest and disease, their seeds reduce the quality of oilseed rape oil and poisonous to stock. However the seeds are important to farmland birds (Bond, Davies & Turner, 2007)

Well from what I can gather, this and the adjacent Bramfield Park woods are probably an outlying part of the Woodhall estate, centred a few miles north at Watton at Stone. This is an innovative estate that works closely with the University of Hertfordshire on a wide range of agro-ecology research projects in the aim of marrying the twin goals of quality local food production with rewilding.

It could be they're testing control mechanisms on our local genetic variant whilst working out how it can be tolerated as part of the agricultural ecosystem, Whatever it is they're up to. I'd love to know more!

You can find out more at their website (https://www.woodhallestate.co.uk/).

Bond, W., Davies, G., Turner, R. (2007). The biology and non-chemical control of Charlock (Sinapsis. arvensis. L). The Organic Association. Available at: https://www.gardenorganic.org.uk/sites/www.gardenorganic.org.uk/files/organic-weeds/sinapis%20arvensis.pdf

Young, A (1804). General view of the Agriculture of Hertfordshire, drawn up for the consideration of the board of agriculture and international improvement. London: G & W Nicol.

Field Woodrush, an indicator of antiquity?

I took myself out for a goodly 5m hike for todays allocated exercise and to brush up on my botanising.

Here's one member of a patch of Field woodrush (Luzula campestris) scattered across the green in Ayot Green, Hertfordshire..

I am fond of this little guy. I like his yellow and russet head and the long messy hairs on his leaves. I like his diminutive size. He is small and neat, where rushes are usually large and bulky.

I am not fond of how he is referred to on many blogs as a "nasty lawn weed that's impossible to get rid of". It's a great little plant that can invade my lawn any day!

Field woodrush can be an indicator of nutrient poor, moderately acidic soils (Biological records centre, n.d) and can be an additional indicator of historic acid grassland or heath

But before you exitedly start wondering if your front garden was part of some ancient park, this is a common species that can pop up in any undernourished lawn or playing field. It can also tolerate mildly alkaline soil (Biological records centre, n.d) and may pop up in nutrient poor chalk grassland.

A quick scan through google seems to reveal a disagreement between sources as to whether it is typical of highly acidic soils or prefers soils closer to neutral.

Mabbet (2009) and The Royal Horticultural Society assert that acidification of topsoil through poor management and environmental means is a more important factor than native soil pH, particularly in calcarious grassland.

(If anyone can contribute with reputable knowledge and citations, I'd be appreciative).

All that considered, please don't consider it an "absolute" indicator of acidity or history.

You need to use your knowledge of the local area to ascertain whether or not it's likely to indicate either acidity or anything of historical interest. In my area, whenever I see it I suspect acidity and antiquity and I am usually right. Not because I'm dead clever, it's just I'm a massive local/ landscape history and soil science nerd who loves figuring out what plants can tell me about former land use!

Ayot Green is a prime example of antiquity. Dotted with veteran oaks, it is a hamlet set about a triangular green; a characteristic feature in mid Hertfordshire. These tend to be of Saxon origin, probably born from piecemeal enclosure of the woods (Branch, 1970; Munby, 1977)

However Ayot Green lies immediately adjacent to the historic Brocket estate; its houses once workers cottages. The estate, which dates from at least the 1200's may once have incorporated the green. The below historic overlay map from the National Library of Scotland (2020) allows you to visualise the place as it is now and as it was in the late 1800's.

I personally think the green, with its traditional "triangle green" setup may well pre-date the estate, however the green was probably swallowed into it as meadow or wood pasture as evidenced by this photograph, collected by V. Richards on Herts Memories (2010). This shows that in the early 1900's the village was managed as meadow. Tree guards are erected around young trees, indicating livestock have access.

Similarly to many places in Hampshire's new forest, there is a distinct entrance, marked by two (now defunct) gate posts. This provides more evidence that it was enclosed for livestock. From the ornatenes of the posts I suspect it was a part of Brocket park and represented a northern entrance

If any local history buffs have any sexy sources confirming or disproving my hypothesis, do enlighten me!

The soil is underlain by the gravels, clays and sands of the Lowestoft formation For any geology fans in the audience, I've included a map and key (Edina Digimap/British Geological Society, 2020).

The Lowestoft formation produces highly acidic soils, described as "sharp and burning" by Wooldridge & Smetham (1931). On top of this they are so free draining they struggle to hold on to water or nutrients. At the same time, springs formed in the underlying clays make them prone to waterlogging.

These traits made them incredibly difficult to farm and agricultural improver Arthur Young was wonderfully vocal about it. He had the misfortune to farm these soils for nine years and his disdain for them ran through his work as an ongoing theme (Young, 1804). This provided me with much entertainment whilst I was reading his work for my MSc dissertation! I shall write a future blog about this, as my exploration of their rubbishness was fascinating stuff!

These soils were clearly something else and their presence probably explains why Brocket Park and nearby Sherrardspark wood were never lost to arable farmland.

But field woodrush doesn't like highly acidic soil does it?

Well in this part of Hertfordshire, the gravels are influenced by the underlying chalk of the Chiltern dipslope, which comes to the surface in many places, creating an interesting geological mosaic of chalk, clay and gravel. The chalk gets into the soils, creating a a buffering effect on pH that gave rise to heaths and grasslands that were......you guessed it! MODERATELY acidic!

However, arable fields were still acidic enough to require a regular dose of chalk if the pH was to be maintained at the level optimal for crop growth (around 6). Today, the control of field wood rush in lawns is best achieved by addressing pH (RHS, n.d). We can therefore predict that any soils on which it occurs has only a minimal influence from the chalk and is pretty acidic.

Where I see field wood rush in and around Welwyn Garden City, I look for encapsulated old acid grassland. Thanks to the garden city ethos of marrying town and country, the retention of field trees and open space was central to its design, which left behind an interesting relic community.

Here, I think its presence is a pretty reliable sign of both antiquity and acidity..

References

Branch, J.W. (1970). Hertfordshire. London, Batsford.

Biological records centre (n.d.) Field Woodrush (Luzula campestris). Available at: https://www.brc.ac.uk/plantatlas/plant/luzula-campestris

Edina Digimap/British Geological Society, (2020) Geology Digimap. Available at: http://digimap.edina.ac.uk

Herts Memories (2010). Ayot St Peter - Ayot Green. Images of Ayot Green. Available at: https://www.hertsmemories.org.uk/content/herts-history/towns-and-villages/ayot-st-peter/ayot_st_peter_-_ayot_green

National Library of Scotland (2020) Overlay maps. https://maps.nls.uk/geo/explore/#zoom=4&lat=55.78537&lon=-3.16449&layers=1&b=1

Mabbett, T (2009). Field woodrush The grass lookalike weed. Available at: https://archive.lib.msu.edu/tic/bigga/gki/article/2009nov25.pdf

Munby, L. M. (1977). The Hertfordshire Landscape. Hodder & Stoughton Ltd.

Royal Horticultural Society (n.d). Woodrush in Lawns. Available at: https://www.rhs.org.uk/advice/profile?pid=481t

Wooldridge, S. W., & Smetham, D. J. (1931). The Glacial Drifts of Essex and Hertfordshire, and Their Bearing Upon the Agricultural and Historical Geography of the Region. The Geographical Journal, 78 (3), 243-265.

Young, A (1804). General view of the Agriculture of Hertfordshire, drawn up for the consideration of the board of agriculture and international improvement. London: G & W Nicol.