The Physical Space
Understanding the land's physical characteristics helps us learn about its biological potential.
The Climate
A mild, maritime climate, often cloudy and wet, and capable of supporting temperate rainforest. Climate change will result in a warmer climate and an increased water deficit.
The Ground
Heavy upland soil that is slowly water permeable. Quite acidic with low fertility. Beneath the soil, carbon-bearing mudstone, siltstone, and sandstone which is characteristic of the South Wales Coal Measures.
Topography
Relatively flat across most of the woodland. To the south, it falls gently towards Garnant beck. To the north, there is a steep incline down into the railway cut.
The Water
The land is wet, even in summer. Its six streams flow all year around, and in winter, water lies on the surface in many places. Because the soil is acidic, some streams have a rusty sludge from the gleying of the soils. Generally, the land is best understood as wet.
The Climate
Mild, often wet, weather
It is a maritime climate, with weather that is often cloudy and wet but mild.
The mean annual temperature is about 10 °C (4.4 – 16.3 °C), with a soil temperature of 10.6 °C (5 – 16.8 °C). Temperature varies by season and time of the day. In winter, temperatures are influenced by those of the surface of the surrounding sea, which are lowest in late February or early March, but there is little difference between January and February. Minimum temperatures are usually around sunrise, with the coldest nights being those when there is little wind, skies are clear, and there is a covering of snow. July is normally the warmest month, with mean daily maximum temperatures about 20 °C. Daily maximum temperatures usually occur 2 or 3 hours after midday.
The hilly terrain and its closeness to the Atlantic encourage cloudy weather, and a haar often hangs over the wood in the morning that paints the many cobwebs with droplets of water. The valley is quite dull with an average annual sunshine total of just over 1,400 hours, in line with the average for Wales but somewhat lower than the 1,750 hours on the south coast of Pembrokeshire. Mean monthly sunshine totals reach a maximum in May and are lowest in December. The key factor is the variation in the length of the day through the year, but cloud cover also plays a part.
It is quite wet, with an average annual rainfall of nearly 1,700 mm, higher than the average of around 1,300 mm for South Wales but this is much less than the 3,000 mm that Snowdonia gets. October – January are much wetter months than February – September. This seasonality is caused by the high frequency of winter Atlantic depressions and the relatively low number of summer thunderstorms.
It is relatively windy but much less so than along the coast. The annual average windspeed is around 9 mph (7.7 – 11.1 mph). The strongest winds are linked to the passage of deep areas of low pressure close to or across the UK. The frequency and strength of these depressions is greatest in the winter, especially November – February, which is when mean wind speeds and gusts are strongest.
Overall, the climate is oceanic and capable of supporting a temperate rainforest. Average rainfall is about 1,675 mm and so above 1,400 mm per annum, summer rainfall of c. 295 mm and so is 17% of the total, which over the 10% threshold for temperate rainforest, while the average temperature in July is 16.3 °C which is slightly above the 16 °C screen. Maps show the wood is located in the temperate rainforest climatic zone, with hygrothermy in the range 125 – 150, which is classified as ‘oceanic’, and lies very close to land to the east that is ‘hyperoceanic’, i.e., the Brecon Beacons.
The Geology
Typical of the South Wales Coal Measures
The geology is the South Wales Coal Measures. These are sedimentary rocks laid down 318 – 309.5 million years ago in the Carboniferous Period. The rocks are predominantly carbon-bearing mudstone, siltstone, and sandstone, with 3 thin bands of carbon-bearing sandstone across the land. The rock is impervious but may be fissured in some places. There are no younger, superficial deposits from glacial till overlaying this, although these occur in the valley at Garnant and Gwaun-Cae-Gurwen. There are at least 5 coal seams and there has been significant mining underground, with coal visible near the surface in many places.
The Soil
Heavy, clayey soils
The soil is heavy upland soil: slowly permeable, fine loamy over clayey soil with a peaty zone at the surface; this is known as Wilcocks 721c.
The composition varies across the land between clay loam and sandy silt loam, depending on the proportion of clay in the soil, but which is generally relatively low across the site (range: 12.5 – 24.4%). The soil is 50 – 100 cm deep across most of the land but less than 50 cm over the sandstone bands and where the peaty surface zone has been cut away to improve the soil through the farming practice of bietin, which pared the peaty zone, burnt it and returned the ashes to the soil. This has depleted the topsoil and reduced the fertility of the ground. It is very thin, less than 20 cm, in parts of several fields where the topsoil has been depleted over time through these poor farming practices, particularly in Cae Mawr, Cae Carkit, Cae Biettin, Cae Newydd and Cae Newydd Uchaf.
Chemical characteristics
The soil is very acidic (pH 4.2 – 5.2; average pH 4.7[1]) and has low fertility, with negligible amounts of phosphates (phosphorus: 1 – 15 mg/l; average 5.2 mg/l) and potassium (45 – 99 mg/l; mean 70 mg/l). These contrast with the levels suitable for farming: pH 6.5 – 7.0; phosphates 16 – 58 mg/l; potassium 120 – 180 mg/l. So, the soil has low fertility, is not productive farmland and does not appear to have been improved, with negligible levels of fertilisers having been added to the soil in the past.
Because of the soil’s acidity, there are fewer deep-burrowing earthworms to mix the surface organic layer with the mineral soil below. So, litter has built up at the soil surface, fungi play a bigger role in breaking down the material, and the upper organic ‘peaty’ layer is a distinct zone separated from the mineral region below, which is known as ‘mor’ humus.
Iron compounds have leached from the upper mineral layers because of the action of organic acids formed from the breakdown of the litter, leaving a greyish-whitish clay loam layer, and some of this iron has redeposited lower down. These mottled ‘gleyed’ soils are found across the woodland.
Variations to general characteristics
Differences include:
- The raised section of the tramway has been constructed from a coaly soil and has a relatively solid surface with very little topsoil.
- Coed Cae and the northern section of the tramway (where the tramway’s base has been removed) have higher levels of phosphates, so have higher levels of fertility compared to the rest of the land, so, for example, nettles grow well in this part of the South Tramway. This greater fertility may have been caused by manure heaps or, in the Tramway, it could be where sheep were kept in the early 2000s (there are a rusty shearing pen and several ‘water’ baths dumped here).
- The soils in Cae Biettin, Cae Mawr, Cae Newydd Uchaf and Cae Newydd have shallower depths and lower levels of organic matter. These fields may have been ‘pared and burned’, where the peaty surface would have been removed, dried, and burned, then the ashes added back to the soil. Indeed, one field is called Cae Biettin, which translates as ‘pared and burned field’.
Soil contamination testing[2] did not indicate any general industrial contamination of the farmland, but suggested some contamination of the soil of, and near, the tramway. This found polycyclic aromatic hydrocarbons that are likely from the coal which was used to construct the raised platform for the tramway.
[1] These relate to test results from Hill Court Farm Research (2021 – 2022). Tests by Oakshire Environmental were acidic but in the range pH 5.4 – 6.1 (2022 – 2023).
[2] Oakshire Environmental analysed soil samples collected by Axel and Sophie Steenberg in 2022 and 2023 for heavy metals (agrochemicals and industrial), pH, phenols (coal and petroleum), total organic carbon, polycyclic aromatic hydrocarbons (coal and petrol), benzene-toluene-xylene (petrol), volatile petroleum hydrocarbons, extractable petroleum hydrocarbons, and pesticides. These screens check for agricultural and industrial contamination of the land.
The Topography
Flat, sloping gently to the south and sharply in the north
The land is quite flat. It slopes gently to the northeast with some definite undulations. This flatter topography changes when the land falls away rapidly to the northeast into Garnant valley and the railway cut, and less sharply to the south into Garnant stream.
The ‘plateau’ begins about 750 m to the south, where the Betws hills fold into a flatter region before it curves down into Garnant valley. The land descends rapidly from 320 m above sea level at the top of Banc Cwmhelen to 240 m at the top of Bryncethen; it is 180 m at the southern edge of the wood and 170 m to the north, declining 10 m over about 150 m in distance (so, roughly 1 in 15, or a 6.7% gradient), before it falls quite sharply by 10 – 20 m to the railway cut at 160 – 150 m (about 1 in 4, 25% gradient).
There are 3 streams that cut across the land, flowing north eastwards and eastwards. A fourth stream begins where the land drops into Garnant valley and flows north eastwards. All streams enter Garnant river. The southern beck is a source of the Garnant because it starts on Blaen-y-garnant Farm, which translates as ‘Source of the Garnant’ Farm. There are two smaller streams in Coed Cae and Cae Lloi that flow southeast down the slope into the southern beck.
There is an extensive, surface drainage system that interacts with the natural drainage of the land into the streams.
The Water
A wet landscape with several becks
Water shapes the landscape, and water is the staff of life.
The land is wet, especially in the winter, but is not wetland. The six streams flow all year around, although the middle streams are more of a trickle during summer. There is a line of tea-coloured standing water during the winter in Waunygarnant, where the ground folds and the water table is above the surface. In the winter, there are many places where the water table is above the ground and water lies on the surface.
These streams and the standing water are fed by groundwater that moves through the topsoil above the clayey zone in the soil and the impervious sedimentary rock below. This groundwater comes from rain that falls on the Betws hills and the surrounding farmland, flows downhill and crosses the land, either naturally through the soil or through the drainage system. There does not seem to be any water springing from fissures in the rock, although the mined areas within the rock will be full of water. There is, however, a spring marked near to Bryncethin Road in Waunfain, which supplies potable water to Cwm Garnant farmhouse.
Open drainage system
The natural flow of water across the landscape has been altered by an extensive open drainage system, and some of the streams have themselves been straightened and in places constrained by drainage pipes, preventing them from shifting around. The drains move some of the groundwater across the landscape into the streams more quickly than it would naturally move through the soil. However, the topsoils remain saturated, because the heavy soils are not naturally freely draining, the drainage system is inadequate and would need mole drains to be effective, so the water table has not been shifted down from the surface.
Because of the drains, the water movements have been altered. For example, the groundwater now feeding the third stream via the drainage network would historically have flowed into the second stream; as a result, the second stream has a lower waterflow than in the past and could dry out in the summer. The drains alongside the Tramway no longer connect, so an intermittent ‘stream’ flows here and across the surface of the Triangle, resulting in a patch that can flood during rainy periods, is boggy in the winter and soft underfoot the rest of the year.
Water chemistry
The water is clean, although it can become ochred (a rusty sludge) in places because of gleying of the soils followed by oxidation of the water-soluble Fe2+ ions to insoluble Fe3+ within the drains and streams, as they mix with the oxygen in the water. Average data for the streamwater are: pH 7 (pH 5.6 – 8.6), salinity of 0.02 PSU (0.01 – 0.03 PSU), dissolved oxygen 71% (11 – 100%), with a temperature equal to the atmospheric temperature. In contrast, the groundwater’s temperature is more stable, being below air temperature in warmer weather and above it in colder weather, i.e., it is buffered in relation to the atmospheric conditions, but with much lower pH and dissolved oxygen. Average data for the groundwater are: temperature 10 °C, pH 2, salinity of 0.01 PSU, dissolved oxygen 16%.
Climate Change
A warmer future and an increased moisture deficit
Estimates for climate change in Wales and the woodland, on medium-high scenarios in comparison to the 1961 – 1990 baseline, are:
- A 2.3 – 2.6 °C increase in mean annual temperature by the 2050s and 3.3 – 4.1 °C for the 2080s. There are similar levels of increase in the winter and summer temperatures except that they are relatively higher in the summer.
- The wood’s temperature may be 12 – 13 °C by the 2050s and 13 – 14 °C by the 2080s.
- The accumulated annual temperature for the wood may be about 50% higher in the 2050s and about 66% higher in the 2080s becoming ‘very warm’ by the 2050s, compared to the current ‘warm’ rating for accumulated temperature.
- Whereas the overall levels of rain will be unchanged, there will be greater seasonality, with the winters wetter and the summers drier. The mean change may be 13 – 14% in winter and -17% for the summers in the 2050s, and a 19 – 26% increase in the winters and a -20 to -26% decrease for the summers in the 2080s.
- The moisture deficit of the woodland may worsen by 20% by the 2050s and by nearly 50% for the 2080s, but the ground may remain ‘moist’ although it could become only just ‘slightly dry’ by 2080.
- There is no predicted change in average or extreme wind speeds, and there is no change in the windiness rating from ‘moderately exposed’.
Overall, the wood’s climate in 2080 may be like mid-Glamorgan’s in 2022, so for somewhere between Port Talbot and Pontypridd.
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