From theory to field observations

Modelling exercises, field observations and a variety of theoretical developments indicate that drylands may experience sudden shifts from functional to degraded states in response to gradual increases in human and climatic pressures. Positive degradation feedbacks between reduced plant cover and increased resource loss driven by runoff has been often claimed to trigger this kind of tipping-point dynamics, driving dryland ecosystems to sudden degradation. However, empirical support for the proposed control factors, processes and feedback mechanisms is scarce.

To assess the occurrence of non-linear, threshold dynamics and tipping points towards a degraded state in response to decreasing plant cover and to gain insights about the mechanisms underlying such dynamics, we followed a manipulative field experimental approach that mimicked a gradient of accumulated impact of pressure (grazing and wood gathering) on a dryland system. On a set of three large-size (∼ 300 m² each) experimental plots, which were previously inter-calibrated over a two-year period, we removed manually part of the vegetation cover to final values of approximately 45, 30, and 15% (Treatment plots IC45, IC30, and IC15, respectively). On these plots, we monitored resource loss (runoff and sediment yield), vegetation dynamics, bare-soil connectivity and soil-surface condition for ∼3.5 years.

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» Manipulative field experiment design and » Measurements and monitoring

What the observations showed

Our results demonstrate that the gradual decrease of (already) low vegetation cover in a dryland system causes a non-linear increase in the production of runoff and sediments, in the degree of rill development, and in the contribution of microtopography to erosion potential (LS-factor), with the change from 45 to 30% being critical for that increase, whereas no further significant increase was produced by a change from 30% to 15% cover. These results suggest that reduction of plant cover down to values ≤ 30% resulted in a particular degraded state, significantly different from the system that resulted from a reduction in plant cover down to 45%. It must be stressed that a severe drought naturally occurred in the area in 2013-2014, further challenging vegetation recovery in all plots, yet also contributing to certain homogenization of the pressure level between plots.

The capacity of the plots for producing runoff and sediments (i.e., for leaking resources) showed no sign of being reduced during the study period. Similarly, we found no evidence that pointed to the recovery of pre-disturbance vegetation cover for any of the plots. However, our results did also not provide evidence for the onset of any degradation loop that could further degrade the systems. The combined effect of the experimental removal of vegetation and a natural drought that occurred in the area drove the three experimental plots to a respective degraded state, with plant cover values that remained around 30-40% for the plot initially manipulated to have 45% plant cover, and around 20-30% for the other two plots (far from the pre-disturbance vegetation cover valus), and with poor soil functional conditions in all three plots. These degraded states showed certain stability over the study period. However, the assessment of their long-term stability falls beyond the timeframe of this work.

At the patch scale, larger bare-soil connectivity also implies larger inter-patch areas, which is beneficial for the performance of the downslope patch. Our study proved that runon inputs to individual patches increases with increased size of the upslope interpatch area, and that this relationship tends to a plateau, so that larger upslope areas beyond certain value do not further increase water gains. These results highlight the importance of the redistribution of resources and feedbacks that operate at the patch scale, which contribute to maintain patch productivity under very low plant cover, and could counterbalance net losses from the system, preventing or delaying the shift to a bare-soil state and promoting instead different communities and patterns with different overall resource availability and redistribution pattern.

It is worth mentioning that we found differences in the functional conditions of bare-soil interpatches as a function of the distance to vegetation patches (i.e., isolated bare-soil areas versus bare-soil areas next to vegetation patches), which supports the assumptions of dryland vegetation models that incorporate local facilitation as one of the critical processes that drive dryland vegetation dynamics and response to environmental change, yet the functional differences found in our work are not as contrasting as assumed by these models.

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» Manipulation experiment results

Despite the relatively short duration (∼3.5 years) of the monitoring period reported here, our results have provided critical findings and insights that are relevant for improving modelling and theoretical developments on dryland dynamics. From a management perspective, our results highlight the need for being conservative regarding the minimum vegetation cover that should be maintained through proper resource exploitation.

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» Discussion and conclusions

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