Acid grassland

Overall vulnerability

Feature(s) assessed:

• Acid grassland

Special qualities:

• Internationally important and locally distinctive wildlife and habitats

Feature description:

Acid grassland is an extensive semi-natural grassland habitat that includes a wide range of grassland types within it. It includes upland acid grassland (unenclosed) and lowland acid grassland (enclosed). Upland grassland is often dominated by mat grass and wavy hair-grass, with a few broadleaved species including heath bedstraw and tormentil. It is an important habitat for breeding meadow pipits and skylarks. Enclosed land tends to be dominated by common bent and fescue and can be species-rich with high floristic diversity in good quality grassland. It may also be important for grassland fungi, with Waxcap and other mushrooms found in high abundance in some areas (see Waxcap Fungi).

Acid grassland is found on nutrient poor soils. These are generally free draining with an acidic pH range of 4 to 5.5. It occurs on top of insoluble rocks or superficial deposits such as sands, gravels and thin peat. In the PDNP, these conditions are often found on the clough sides or below the edges of many of the heathlands and blanket bogs. Approximately 8,500 hectares of acid grassland have been mapped in the PDNP, 6,500 hectares of this being priority habitat. Around 62% of this in the Dark Peak and 31% in the South West Peak.

Lowland acid grassland is a UK Biodiversity Action Plan (UK BAP) priority. Due to the wide range of grassland types this habitat encompasses, the value and condition of these habitats varies greatly. Nearly half (42%) of mapped acid grassland in the PDNP is found within Sites of Special Scientific Interest (SSSIs) and has therefore had its condition assessed.

How vulnerable is acid grassland?

Acid grassland in the PDNP has been rated ‘moderate’ on our vulnerability scale. This score is due to high sensitivity and exposure to climate change variables, coupled with a variable current condition and a high adaptive capacity.

The condition of acid grassland is variable, with some SSSI sites containing areas of high conservation value, but much acid grassland outside of SSSI designation in an unknown condition. The main impact on acid grassland will likely be a change in species composition in response to various climate change effects, either directly or in response to agricultural uses of these habitats. Livestock are a management tool for acid grassland which provide the opportunity for intervention to help this habitat to adapt to the changing climate.

Current condition:

Condition of SSSI sites varies between the uplands and the lowlands. In 2015 just 24% of upland acid grassland (unenclosed) in SSSIs in the PDNP was in favourable condition. The lowland acid grassland (enclosed) in SSSIs is in a better state with 93% in favourable condition. Outside of SSSI sites, the condition of acid grassland is variable.

Acid grassland vegetation structure is created and preserved by grazing. This maintains the short sward and prevents other larger plants from dominating. In the PDNP, unenclosed acid grassland is often species poor and much has probably originated through degradation of upland heath. Overgrazing is a common form of mismanagement, allowing species such as mat grass to dominate. Enclosed acid grassland is usually more diverse. Many acid grassland habitats have been lost completely to agricultural improvement.

Bracken can be a threat to acid grassland, where it is often found to spread and dominate over large areas. Many factors affect bracken spread, including depth of soil, slope aspect, altitude and steepness, with bracken generally favouring deep soils on shallow south facing lowland areas. Bracken spread has increased at a greater rate on north facing and higher altitude areas in recent years, suggesting some impact of warmer temperatures on bracken dominance.

What are the potential impacts of climate change?

Overall potential impact rating

Direct impacts of climate change

Climate effects are expected to have a high overall impact on acid grasslands, including direct and indirect effects. Temperature may affect acid grasslands directly. Higher average summer temperatures and warmer winters, combined with adequate rainfall, can cause faster or extended growth of species suited to warmer climates. This could negatively affect northern or cold adapted species in acid grassland that rely on lower temperatures. Data Certainty: Moderate

If prolonged drought periods occur as a result of climate change then upland acid grassland in south and east margins could be affected. Flushes within these habitats would be particularly affected due to their reliance on rainfall. If these dry out then diversity here could be lost. Uncompetitive and non-stress tolerant plants that have shorter roots, such as heath bedstraw, could suffer whereas deeper rooted and/or more stress tolerant plants could be favoured. Changes in species composition may occur, with drought tolerant perennials and ruderals able to recolonise bare patches becoming more common. Data Certainty: Low

Higher temperatures and summer droughts will likely benefit winter annual plants which could result in a change in vegetation classification of acid grassland. The richest acid grasslands in the PDNP are National Vegetation Classification (NVC) community U4. These are currently at the edge of their range requiring a mean annual maximum temperature of less than 27⁰C and over 800mm of precipitation per year. Warmer and drier summers could shift this towards a composition more similar to NVC community U1 which is richer in annuals, or wavy hair-grass community U2. Any such shift is likely to be accompanied by a decline in perennial species associated with the more species-rich grasslands such as mountain pansy, pignut, bitter vetch, devil’s bit scabious, betony, bilberry and heath milkwort. Data Certainty: Moderate

Higher winter temperatures could cause fewer frost days. Some annual plants depend on bare ground created by vegetation dieback for germination, a process known as frost heave. A reduction in the extent of this process could cause an imbalance between declining annual plants and perennial plants, resulting in a change of species composition. However, bare ground is created through other processes such as molehills and earthworms, as well as summer drought which is likely to become more common. The impact of a reduction in frost heave may therefore be negligible. Data Certainty: Moderate Waxcap fungi can occur in acid grassland habitat and are sensitive to drought conditions. Predicted drier summers in the PDNP could cause the reduction or loss in species such as these, decreasing the species diversity and value of acid grassland. Data Certainty: Low

Human behaviour change

Agriculture has already had significant impacts on acid grassland and the effects of climate change could increase this through the opportunity for agricultural intensification. Acid grassland often occurs on the moorland edge in the Dark Peak where it can be fairly dry. Less precipitation due to climate change could dry out these habitats further, making them more accessible for livestock and farm machinery. This could expose them to agricultural intensification. Data Certainty: Moderate

Conversely, extreme events causing flooding or water logging of acid grassland could limit the access that farmers use for managing the land. If areas are flooded or water logged then farmers may be prevented from moving stock. This change in grazing could impact the acid grassland negatively due to the reliance of grazing to preserve vegetation structure in acid grassland. Data Certainty: Moderate

Another potential change to grazing regimes in acid grassland may be caused by a loss in productivity of grasses. Hotter summers may reduce soil moisture levels, causing an increase in evapotranspiration and resulting in less grass growth. Grazing of some acid grassland could be reduced to abandonment or intensified to make up the shortfall in feed. As acid grassland is dependent on grazing levels to maintain it, both would result in grassland species composition changes, leading to habitat succession and a loss of this habitat type. While this could create a more diverse mosaic of habitats which could be more ecologically valuable, the acid grassland would still be lost. Data Certainty: Moderate

Conversely, there is a chance that the growth of grasses would be accelerated in future climates. Faster growing and more competitive species would be better placed to take advantage of this effect and spread at the expense of slower growing species. This could lead to an increase in stocking levels and therefore increased pressure on these habitats. Species diversity could decline and the species composition may move away from acid grassland habitat, or become a less valuable type of acid grassland. Data Certainty: Low

Invasive or other species interactions

Climate change could exacerbate the invasion of acid grassland by bracken, where soils are deep enough and are unaffected by drought. Because bracken favours conditions where there are fewer frosts, warmer winters could encourage growth, expanding current stands into acid grassland habitat, and in particular could result in a spread from more sheltered cloughs and valleys onto higher and more exposed land. Data Certainty: Moderate If nitrogen deposition is increased as a result of climate change then this could also favour bracken growth, adding to further losses of acid grassland as above. Data Certainty: Low The possibility of drier summers could help to partially mitigate both of the above issues however, as bracken does not thrive in drought conditions. Data Certainty: Moderate

Nutrient changes or environmental contamination

Species diversity could also be impacted by increased nitrogen input caused by higher water input into the system during wetter winters. In the Wardlow Hay Cop study, where nitrogen was added experimentally to study plots, changes in plant frequency were seen due to acidification of grassland soils. There was an increase in grasses at the expense of other species, such as broad-leaved plants and mosses. Waxcap fungi can also be sensitive to this increased grass growth. This study would suggest that if there is an increased nitrogen input due to climate change, then acid grassland habitat could become less diverse. Data Certainty: Moderate

Sedimentation or erosion

It is possible that extreme events, such as storms or extreme droughts, could cause erosion on acid grassland. Landslips have already occurred after major storms in PDNP acid grassland habitat, especially where it is found on steep slopes within cloughs. Increases in such occurrences could damage the soil structure, causing a loss of habitat and opening up further opportunities for invasive species to move in. Data Certainty: Low

Other indirect climate change impacts

Hotter summers, higher average summer temperatures and warmer winters may provide longer growing seasons. Flowering times could shift, with seed set occurring earlier in the year, and a phenological mismatch could result in population changes for some insect species. A shift in plant community composition may also follow. Data Certainty: Moderate

An indirect impact of climate change is an increase in likelihood of wildfires due to hotter drier summers, subsequent droughts and increased ignition sources as more people visit the PDNP. While acid grassland in good condition is not sensitive to fire, it can encourage the dominance of fire tolerant species, such as bracken, heather and wavy hair grass. This could further increase susceptibility to fire and cause a positive feedback loop of an increase in these species. Invasive species could also move into the bare ground created by the fires, threatening the acid grassland habitat. Data Certainty: Low

What is the adaptive capacity of acid grassland?

Overall adaptive capacity rating

Acid grassland has been given a high adaptive capacity rating because it can occur in a wide range of conditions and adaptation measures can be introduced, such as adjusting stocking levels as required.

The climatic envelope of lowland dry acid grassland habitat extends away from the PDNP to South East England, so should theoretically be possible for it to occur in the PDNP under all but the most extreme future climate scenarios. However, a loss or decline in some species, for example mountain pansy, may increase the fragmentation of the current extent. Data Certainty: High

The type and composition of habitat may change within the wider acid grassland classification, resulting in losses of higher conservation value habitats but preserving the overall habitat category. While acid grassland is generally of lower conservation value and supports few threatened plant species than other habitats assessed here, in the PDNP it is especially important for internationally important waxcaps and related fungi groups. Unenclosed grass moorlands are also important for predators such as short-eared owl and hen harrier. Longer established acid grasslands are likely to be more diverse and resilient to change. Data Certainty: High

Environmental stewardship options are currently available for the maintenance or restoration of species-rich semi-natural grassland. The habitat type can be found on land owned by a large variety of individuals and organisations. The impact of human responses to climate change – such as changes in stocking levels on acid grassland habitat is likely to have more of an impact than climate change directly. This means that it is possible to adapt stocking levels in ways that will not negatively affect acid grassland habitat, and it may be able to offset climate change at least in part. Data Certainty: High

Key adaptation recommendations for acid grassland:

Improve current condition to increase resilience

The current condition of a feature is an important factor alongside its sensitivity and exposure, in determining its vulnerability to climate change. These recommendations are aimed at improving the condition of the feature at present, therefore making it better able to withstand future changes to climate.

• Identify areas of refugia such as north facing slopes to target conservation efforts. Improve current condition to increase resilience: Targeted conservation efforts for important sites and at risk areas The current condition of a feature is an important factor alongside its sensitivity and exposure, in determining its vulnerability to climate change. These recommendations are conservation measures aimed at those sites that will have the biggest impact for this feature – either because they are particularly important for the feature or because they are most at risk from climate change.
• Ensure key sites are in appropriate management to maximize resilience.
• Where long established grasslands can be identified, these should be a conservation priority.
• Monitor extent of bracken on important sites and ensure appropriate management to prevent spread.

Improve current condition to increase resilience: Increase structural diversity to improve resilience at a landscape scale

The current condition of a feature is an important factor alongside its sensitivity and exposure, in determining its vulnerability to climate change. These recommendations focus on increasing the structural diversity of the area or habitat in which the feature is found. This can help to offset the effects of climate change on the feature, as well as to allow it to be in a better position to recover from future climate changes.

• Restoring the less valuable areas of upland acid grassland to heath; or scrub/woodland along edges of watercourses e.g. in upland cloughs. More trees and scrub could also reduce landslips.

Accept changes to feature

These recommendations are about adapting ways of thinking to be accepting of inevitable change. While some changes may be negative, this also presents a chance to seek out any positive opportunities that may be caused by climate change.

• Accept that some of this habitat may be lost or the structure of the habitat changed. It could become more diverse and create a mosaic of habitats, benefitting biodiversity.

Adapt land use for future conditions

These recommendations are adaptations to the way in which people use the land. Flexibility in land management - reacting to or pre-empting changes caused by the future climate - should afford this feature a better chance of persisting.

• Adapt grazing regimes to allow for more flexibility of timing and stocking density, to ensure they are responsive to new conditions.