OR/17/014 Discussion

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Archer, N A L, Everest, J. 2017. Final Report: Emergent Forest dynamics and Natural Flood Management. British Geological Survey Internal Report, OR/17/014.

Investigating hypothesis 1) as forests develop, the root system creates macropores and increases organic matter concentrations within the soil/root layer, optimising on the one hand to store water in the more organic soil and the other hand to allow water to flow via old root channels[edit]

To understand the dominant flows using the rationale in section 1.4 (for the Old Forest and Plantation sites), I it can be assumed that points having the highest Kfs and φm will have the greater macorporosity and may include greater preferential flow pathways, due to decomposition of root systems (Bengough, 2013[1]). Overall, the median Kfs value is highest in the Plantation site, allowing a median of 20.25 mm/15 min of rainfall to enter the soil, whereas the median Kfs value allows only 8.55 mm/15 min of rainfall to enter the soil. During rainfall measurement the highest rainfall intensity recorded was 17.2 mm in 15 min, during the winter in the Old Forest site, which means that the Old Forest is likely to experience some infiltration excess overland flow, unlike the Plantation site (Figure 4). However, the overall results are more complex than the dominant flow pathways suggested in the hydrological conceptual model in Figure 4.

The Old Forest has more significantly developed organic matter layers, where the organic soil and vegetation understory of bilberry, bearberry, heather and sphagnum are significantly deeper than the Plantation site (Figure 22). As shown in Figure 27, Kfs values for the Old Forest site is more heterogeneous than the Plantation site, because the range of Kfs values is larger in the Old Forest site (Table 3). The significantly greater Kfs values measured within the vegetated mounds in comparison to measurements taken in depressions in the Old Forest (Figure 29), suggests a 'dual' system of infiltration rates, where the organic understory allows rainfall to enter the soil system rapidly, but then the more peaty depressions have finer pores, which allows water to infiltrate more slowly. Taking into account the range of Kfs enables an understanding of the full range of possible infiltration rates between the two sites.

The organic understory in the Old Forest also shows greater heterogeneity of soil water contents than the Plantation. For example Sensors 5 and 7 (Figure 19) show relatively large fluctuations of soil water contents. In particular, Sensor 5, which is placed within the pine roots of a ~300 year Scots pine tree, has a range of soil water content change from 0.4 to 1, suggesting that rapid sub- surface flow occurs within the pine roots during winter and also for a moment during the summer, triggered by low intensity, long duration rainfall. On the other hand, Sensor 8, which is installed in peat below a sphagnum mound, is constantly highly saturated, indicating the presence of an available supply of water, as Sphagnum needs a relatively constant supply of water (Hayward and Clymo, 1982[2]). Sensors 5, 7 and 8. The soil water content measured by Sensors 5, 7 and 8 tend to lag behind rainfall events, suggesting that the vegetation and soil buffers rainfall infiltration for all these sensors. Sensor 6 (installed in sandy gravel) on the other hand is very responsive to rainfall (Figure 21), suggesting that rainfall infiltration rapidly infiltrates to the gravel layer.

In the plantation the soil water curves are more homogeneous, where for example Sensors 2 to 4, are located in course sand and gravel (Figure 16) and respond quickly to rainfall events and drain quickly (Figures 18 and 20). Fast draining soils mean that the soils are dominated by macropores, which have a lower ability to retain water. For all these locations infiltration rates are high, as also shown by the median Kfs value (and narrower range of Kfs values) in the Plantation. Sensor 1 (located in a peat depression) at the bottom of the slope does however remain almost saturated through the year, although it also responds to rainfall (Figure 18).

In answer to Hypothesis 1: It is to suggest that the forest root system creates macropores and increasing organic matter to optimise water flow and storage, when the gravel/sandy substrate below the forest (in this case moraine deposits) is highly permeable. The Old Forest however is a community of diversity where a combination of vegetation mounds, depressions and root systems develop a system of fast and slow infiltration rates. It is the dense undulating community of organic matter, blaeberry, bearberry, heather and sphagnum that creates storage and release of water, allowing a very stable system that does not simply allow infiltration, but also in the dense peat areas, water is stored and slowly percolates, when the peat is saturated.

Investigating hypothesis 2) springs develop within and around mature forests because their rooting systems interact with the soils in such a way as to form permanent and/or ephemeral perched water tables[edit]

Using the positive and negative feedback loops for spring formation shown in Figure 5, we will focus on positive feedbacks, i.e. feedbacks that create springs. Figure 31 divides the positive feedback factors into two categories: Physical environment and water flows. As described in Section 2 and summarised in Figure 14, the physical environment within the Cairngorms creates perched water tables because of glacial processes, depositing morainic mixed permeable material on top a more impermeable till layer. Glacio fluvial processes also forms differing layers of permeability, depending on, for example ponding or slow moving water depositing finer materials, forming intermittent impermeable layers and creating localised perched water tables. Such a physical environment provides the conditions for perched water tables.

Figure 31 Positive feedback loops for spring formation.

To ensure a constant spring flow, the environment needs to enable water accumulation for the storage of water and conditions that regulate water flow to ensure continual replenishment of water (Figure 31).

Water flows in the Plantation site[edit]

The dense plantation canopy intercepts more rainfall than the Old Forest (Figure 22) and therefore reduces the amount of rainfall that enters the ground in the Plantation. It is striking to have a difference of 134 mm less of rainfall in the Plantation than the Old Forest, when this does not include the winter rainfall, because the datalogger failed to function in the Plantation during most of the winter in 2015 to 2016. This however is 15 percent less rainfall from the total measured in the Old forest, which is similar to interception recorded in Scots pine plantations in the Mediterranean (Llorens, et al. 1997[3]). Therefore, there is a loss of water through interception, which reduces available water for springs to form.

What happens to this excess of water from rainfall that enters the soil in the Plantation site? According to the soil water content curves (Figure 18) the water infiltrates the soil relatively quickly as there is only a short lag between rainfall events and peak soil water contents and then the soil drains relatively quickly (Figures 18 and 20). Below the break of the slope, where the soil layer is thicker, soil moisture contents remain relatively high (Sensor 1 in Figures 18 and 20). This suggests that the gravel soils in the hillslope of the plantation site allow rainfall to infiltrate and drain towards the bottom of the slope, where the soil water enters the peat at the bottom of the slope, i.e. the bottom of the moraine depression (Figure 30). The high Kfs values (Figure 27) measured in the Plantation corroborate the relatively fast response of the soil water sensors in response to rainfall events. The significantly thinner organic soils and vegetation depth (Figure 23) in the Plantation infers that there will be less rainfall intercepted by understory vegetation and less storage of rainfall in the upper organic soil in comparison to the Old Forest. It is also interesting to note that overall the drainage channels have lower Kfs values, than the adjacent mounds, (Figure 29); this could facilitate surface water flow in drainage lines. These observations do not explain spring development in the moraine slope, but rather water flow is predominantly downslope towards the bottom of the moraine, creating an accumulation of water within the perched bog that seeps into the nearby river. The reason for the significantly thinner soils in the Plantation site in comparison to the Old forest, could be for a number of combined reasons, such as: 1) during the implementation of drainage and tree planting, the peat soils became disturbed and vulnerable to erosion by the increased amount of water flow through the newly installed drainage, 2) the loss of shrub canopy through the process of tree planting exposes the soil to erosion and as the plantation grows, the loss of understory biodiversity, causes the soils to become eroded particularly under high rainfall, 3) with increased infiltration rates, the soils are more highly drained, causing the water table to lower. The peat then becomes drier and oxidization of the peat occurs, lowering the depth of peat (Zanello, 1t al. 2011[4]). More investigation needs to be done to understand why the peats are thinner in the plantation site.

Because the moraine soils are highly permeable, it is difficult to find evidence that tree roots have an effect within this system, because no difference was found in Kfs samples near trees or further away from them, tending to suggest that the moraine substrate is the dominant driver for rainfall infiltration. In a previous study, it was found that a 40 year old Scots pine plantation in gravel soils did not have significantly greater infiltration rates compared to an adjacent grass area on the same gravel soils (Archer, et al. 2013[5]). More work is needed to be able to define the effect of tree roots in permeable soils.

The observations of the peat drying out and deep cracks forming in the peat during a period of little rain during October 2016 is an important observation, as the peat cracks create preferential flow, where water by-passes the peat matrix. This may cause flashy, large river flows, if heavy rainfall occurs after a period of drought in the Cairngorms.

Water flows in the Old Forest site[edit]

The greater capacity for the forest soils to retain water and the higher cumulative amount of rainfall (Figure 22) in the Old Forest, will cause higher soil water contents in the Old Forest soils in comparison to the Plantation site (Figures 18 and 19). The significantly greater vegetation depth creates greater rainfall interception and greater soil depths will increase rainfall storage in the Old Forest in comparison to the Plantation site (Figure 23). The significantly higher Kfs in vegetation mounds in comparison to depressions (Figure 29) shows that vegetation facilitates rainfall infiltration in the Old Forest site, but infiltration rates are lower in depressions. The soil moisture probes show a diverse range of response to rainfall (Figure 19), but there is no strong indication of increasing soil water content downslope, where for example, Sensor 8 which is upslope of the others (Figure 7), has consistently saturated soils, suggesting that there is a shallow supply of water upslope, to the other soil sensors (Figure 19). Overall the environment of the Old Forest site tends to gain and store water, which will facilitate constant spring flow within the moraine slopes. Even during the dry period in October 2016, when the peat soils were dry and cracking in the Plantation site, the peat soils in the Old Forest site remained relatively wet although the seepage within the Old Forest became dry and the spring below the site continued to flow.

Overall, the older mature Old Forest environment is one of diversity, where the undulating mixed understory of blaeberry, bearberry, heather and sphagnum species form a relatively thick layer above ground and below ground a dense layer of roots exists to at least 0.4 m, depending on the depth of organic soil material and permeability of substrate depth. To quantify specifically the effects of tree roots in this environment, requires a great deal of work, which is beyond the scope of this project, because the augering method did not penetrate the substrate deep enough to enable observation of deeper tree roots. Also, the conservation status of these rare old Scots pine trees and the surrounding environment prevents destructive digging to enable observation of the root system and to place instrumentations within and below the deeper root systems.

In answering hypothesis 2, it is important to question whether the two sites regulate water flow that facilitate a constant water supply for springs (as illustrated in Figure 31). As discussed in this section, the combination of a diverse understory canopy and deeper peat soils allows rainfall to be stored, which is slowly released into the sandy gravel substrate which drains to accumulate above finer more impermeable substrate to produce a spring within the moraine slope. The results in the Plantation site show the lack of understory vegetation and a propensity for water to infiltrate the soil and allow water to drain downslope, causing water to accumulate in the moraine depression.

Forest management and water distribution[edit]

The observations and measurements at the two sites question the management of drainage in the Plantation site. Ploughing drainage lines into the fragile peat layer may decrease peat depth on the moraine slopes. As there are no records of peat depth before planting, it is difficult to know the impact of draining and planting areas in the Cairngorms. Deep ploughing may also disturb thin layers of low impermeable substrate, reducing areas for perched water tables to form and therefore reducing seepages and springs.

The drainage lines in the Plantation site successfully direct water towards the moraine depression. This raises the question: why do the majority of windblown trees occur at the bottom of moraine depressions (Figure 8)? According to Mickovski and Ennos (2002)[6], the most important roots to provide anchorage are the tap and sinker roots. The growth of these roots may be repressed in waterlogged soils in the moraine depressions, suggesting that trees should not be planted in the waterlogged soils in moraine depressions. Certainly Scots pines grow well on the moraines as shown in the old Sots pine forests in the Rothiemurchus estate, where there are significantly more Scots pine trees growing on moraines than in their adjacent depressions (Figure 11). Vinke and Thiry (2008)[7] observed Scots pine in Belgium to extract water from the water table during the summer and drought period, which could suggest that the old Scots pine in Rothiemurchus, may avoid water logging during the winter by their elevated morainic position and during the drier summer months they may extract water from the nearby moraine depressions. Further investigations of water dynamics is needed to understand the possible water logging of mature Scots pine and the impact this may have on forestry in the Cairngorms.

Forests and flood management[edit]

The positive benefits of forests to attenuate flooding are through the greater water use by trees, the 'sponge effect', where i) the more organic soils absorb and store water, delaying water draining by sub-surface flows to streams and rivers and, ii) increased infiltration through root penetration into poorly draining soils and greater hydraulic roughness resisting overland flows in floodplains (Nesbit and Thomas 2006[8]). However, investigations on the effect of trees on hydrology in Scotland suggests that the main driver of runoff generation is soil type, which is more significant than vegetation effects in wet northern headwater catchments (Geris et al. 2014[9]). In terms of the Plantation site in this investigation, the fast response and low retention of highly permeable moraine soils are dominant factors allowing rainfall to infiltrate soils and the combination of drainage channels implemented before the planting of trees may have an overall greater impact on facilitating water downslope towards moraine depressions. To determine if the presence of roots in the Plantation site significantly increases water infiltration in such permeable soils, further investigation is needed.

In addition to this, the overall lower amount of rainfall reaching the ground in the Plantation, suggests that rainfall canopy interception does occur in the Plantation, reducing rainfall to enter the soil. The short duration of rainfall measurements in this study however could not provide the long-term data needed to record the less prevalent high rainfall intensity events that cause floods. Other studies in Scots pine plantations have observed decreased interception with increasing rainfall magnitudes (Llorens, et al. 1997[3]). Therefore, high rainfall intensities in the Plantation site could still reach the soil. If such rainfall coincided with dry cracked peat soils (as observed in the Plantation site in October 2016), high intensity rainfall would enter streams relatively quickly to facilitate fast rising waters in the Cairngorms.

As discussed in Water flows in the Old Forest site the Old Forest tends to act as a 'sponge' to rainfall within the moraine slope, storing water. Kurbatov (1968)[10] makes a distinction between forest peat, which he describes as being more aerated and similar to thick litter deposits, unlike peat in swampy anaerobic conditions, which remain saturated. For the soil water content to increase from 0.4 to 1 m3 m-3 as measured in the forest peat (Sensor 5 in Figure 19), the soil must be composed of 60% air filled pores. The greater diversity of Kfs values measured in the Old Forest, allows for a system to partition water into stored and infiltrating water, this combination retains rainfall in the moraine slope and therefore slows down water entering streams at the bottom of the moraine depressions.


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