Critical changes preceding a catastrophic shift
| Authors: | Ángeles G. Mayor, V Ramón Vallejo, Susana Bautista with Peter de Ruiter, Lia Hemerik, Violette Geissen, Jaap Bloem, Jacob Kéizer, Óscar González-Pelayo, Ana Isabel Machado, Sílvia Faria, Ana Vasques, Luna Morcillo, Rosario López-Poma, Azucena Camacho, Anna Urgeghe, Diana Turrión, Christel van Eck, Martinho Martins, Paula Maia, Alejandro Valdecantos, Jaime Baeza, Joan Llovet, David Fuentes, Giovanni Quaranta, Rosanna Salvia, Velia De Paola, Ioannis N. Daliakopoulos, Ioanna Panagea, Ioannis K. Tsanis, Michalakis Christoforou, Christiana Papoutsa, Dimitris Tsaltas, Kostas Andreou, Kyriakos Themistokleous, Giorgos Papadavid and Diofantos Hatzimitsis. |
| Editor: | Jane Brandt |
| Source documents: | Mayor et al. (2015) Identification of critical changes preceding catastrophic shifts: ecosystems affeced by increasing wildfire recurrence. CASCADE Project Deliverable 3.1a Mayor et al. (2017) Identification of critical changes preceding catastrophic shifts: ecosystems affeced by increasing grazing intensity and severe drought. CASCADE Project Deliverable 3.1b |
Observational and manipulative experiments to investigate responses to stress
This section of CASCADiS focuses on observational and manipulative field experiments to investigate changes in plant-soil ecosystems in response to external stress (i.e. increasing fire frequency and increasing grazing intensity, individually and each of them combined with severe drought), and to identify the most sensitive indicators to these changes.
For this purpose, two experiments have been conducted in the six CASCADE study sites.
- Measurements of soil quality and plant performance have been taken in three levels of stress determined by fire recurrence or grazing intensity depending on the site (stress-gradient experiment),
- and in a rainfall-exclusion experiment located in plots representing the stress level most likely to experience a tipping point (drought-stress experiment).
For more details see »Study sites and target species
1. Stress-gradient experiment
Assessment of changes in soils in response to increasing fire frequency
Since the mid of the last century fire recurrence (number of fire events that occur at a site in a given period of time) has increased in the Iberian peninsula and the overall Mediterranean basin.
For more details see »Impact of increasing fire recurrence on soil quality
CASCADE assessed the changes in organic soil carbon and nutrient status of two forest ecosystems affected by different levels of fire recurrence and where short inter-fire periods have promoted a transition from pine woodlands to shrublands.
For more details see »Measurements of stress gradient - fire
For more discussion of the short and long term effects of fire recurrence on soil nutrients and soil functioning see here
»Effects of recurrent fire on soil nutrients and functioning.
Assessment of changes in soil quality and plant performance in response to increasing grazing intensity
CASCADE assessed the effects of grazing-induced stress on soil quality and plant performance in order to see whether the plant-soil ecosystem moves in the direction of a catastrophic shift and assessed the differences of soil quality within and between plant patches to see whether and how facilitation occurs in plant patches.
For more details see »Measurement of stress gradient - grazing.
- The Messara and Randi sites showed increased soil quality under moderate grazing stress and decreased soil quality under high grazing stress. The other two sites, with a low or moderately low overall grazing pressure, appeared to be less sensitive to the grazing level.
- The Randi site showed reduced plant performance due to increased stress.
- All sites, but especially the Randi site, showed higher soil quality in the patch microsites compared to the interpatch microsites.
- In Randi, the increasing contrast between patch and interpatch soil conditions with increasing level of grazing points to the proximity of a shift into a degraded state.
For more details see »Effects of grazing stress on soil quality and plant performance.
2. Drought stress experiment - the combination of high fire frequency or grazing intensity with severe drought
Using rainfall exclusion roofs to induce the additional drought stress on sites already affected by moderate grazing and fire intensity CASCADE examined the effects on the soil plant system.
For more details see »Measurement of drought stress
- did not produce a consistent response across sites;
- proved the resistance of dryland plant-soil systems against severe drought
- proved the existence of significant roof artifacts in rainfall exclusion experiments, which may not be consistent across climatic gradients.
For more details see »Effects of drought stress on soil quality, plant performance and soil moisture
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