Understanding sudden shifts in dryland ecosystems ...

Authors: Tsanis, I. K. and Daliakopoulos, I. N.
Editor: Jane Brandt
Source document: Daliakopoulos, I. and Tsanis, I. (eds) 2014. Drivers of change in the study sites. CASCADE Project Deliverable 2.2. CASCADE Report 06. 59 pp.


Climate is a governing driver for all Study Sites, especially those that do not have significant vegetation cover. Water availability undoubtedly plays an important role, and while it is not addressed directly in this study, we consider that the use of the Aridity Index is a perfectly suited proxy. Results show that for the past 50 years, aridification has been very pronounced along imposing mountain ranges of the Mediterranean; the Cantabrian Mountains, the Pindus Mountain range and the Appennini Mountains. Three main reasons can be offered for this observation:

  1. As the meteorological station network becomes denser, areas that were previously harder to access, such as mountain tops, now enjoy more certainty in climatic observations; there is a possibility that earlier aridity values are overestimated thus creating negative trends (although datasets are normally corrected for such biases),
  2. The aridification may be part of a greater oscillation that will gradually recover and the current datasets are not long enough to draw consistent conclusions and
  3. Due to snow cover, these areas have traditionally been more humid, nevertheless slight temperature rises may affect them earlier than lowlands, rendering them more sensitive to climate change. The latter explanation is the least optimistic but it is also backed by previous observations of a receding snow cover in the mountain-ranges of Southern Europe.

The connection of vegetation with climate and water stress is well established at an annual basis, with widely used relationships estimating yields for one growing season. While this approach is straightforward, the NDVI analysis conducted here has shown that in several cases (e.g. Randi Forest and Messara), drought can have a creeping effect: multiannual periods of stress can cause non-linear breaks in the greenness signal. This behavior implies the existence of thresholds or resources buffers which once crossed lead to a different stable system. In all cases, the seasonal component of the NDVI after a break differs to some degree denoting possible phenological variations. In some cases, the magnitude of the change in the seasonal component can be subtle (e.g. Messara, Randi Forest) and in others more pronounced (e.g. Ayora, Várzea), thus making it easier to attribute it to its driving force.

Besides the simplifications introduced in the grazing model, it is shown that climate change can be expected to undermine the resilience of pastoralism ecosystems, especially under marginal management conditions. The expected consequences comprise a function of system parameters such as the land’s carrying capacity, the plant growth coefficient, the yield response factor and the biomass consumption coefficient. These parameters are strongly related to soil quality, which is under severe stress in arid areas. Results show that arid and semiarid areas will be more prone climate induced pressures in vegetation by the predicted increase in temperature. While the precise effects largely depend on local climatic particularities and specific grazing practices, simple models point to a decrease of the system’s stability as the ratio of actual over potential evapotranspiration decreases.

Until now, the cusp catastrophe framework is seldom used to describe forest composition instability due to fires. The most notable work has been done in the Great Lakes Region and incorporated fire frequency in a general "disturbance severity" concept. Here we show that Mediterranean dryland forests that suffer frequent fires fit well in the cusp catastrophe approach. The effects of fire frequency beyond the limits of the local ecosystem are evident in Ayora where shrubland gradually replaces pine forest, a conclusion also supported by the NDVI analysis. Here, a low fire resistance species was introduced in a fire prone environment, causing additional problems which are bound to escalate with climate change. The situation is less clear in Várzea where management decisions play a significantly more important role in shaping the landscape. Here the transitions to a less humid climate may eventually create a less favorable environment for the water intensive eucalyptus plantations, thus putting the sustainability of the current socioeconomic model of the region at risk.

Land use, land cover, production, consumption and disposal drive the relationship between social and ecological systems, by being embedded in social patterns and processes, such as demography, technology and culture. Therefore long term relationships are difficult to establish. Population growth, increase of demand, turn to tourism activities have in some areas led to the increase of irrigated land, changes in land use and the banishment of pastoralism activities to marginal lands (e.g. Messara and Randi forest). The socioeconomic backgrounds of all Study Sites follow patterns similar to those met in the desertification paradigm. Poor management decisions at individual, institutional and policy level can introduce perturbations that often accelerate the downward spiral of degradation and desertification. While addressing these interactions with simple trends has been considered, socio-economic factors, scenarios and policies are not taken into account in this report. Such relations are multi-dimensional, certainly in complex landscapes such as the Mediterranean where population, markets, technology of production, infrastructure development and accessibility, policies (and others) all play a role.

Quantification of several features of the human and natural driver interaction with the state and rate of land degradation still poses a challenge as feedbacks and interactions are often ambiguous. Here we have shown that features of the cusp catastrophe model can be identified in all the CASCADE Study Sites. Several of the features identified in these descriptions can be considered as catastrophe flags. In all cases, two or more stable states (bimodality) are present, possibly in different points in time. In all Study Sites, the transition from the degraded state to the conserved one is hindered to the extent that resources depletion and climate cannot be reversed, thus rendering the conserved state inaccessible. Furthermore, the NDVI analysis has shown that sudden jumps appear when the system is stressed to its limits, especially in the case of arid areas that have a smaller resource buffer. In Messara, an example of divergence is presented, with two neighboring locations beginning from a close starting point of vegetation health and having widely separated final states under the same driving conditions. Finally, in most cases, when degradation is present the transition to a conserved state is virtually impossible without substantial human intervention. This is more pronounced in the cases of Albatera and Ayora where restoration approaches have had limited success.

Concluding, a unifying framework of behavior that can fit the entire range of the Mediterranean drylands instability is an attractive prospective. Nevertheless, it is also critical to recognize that not all drylands are the same, that even within the Mediterranean climate can vary substantially and that the human factor has been significantly altering these parts for millennia. The cases presented here are a good set of examples where the cusp catastrophe framework applies but care must be taken when generalizing.


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