Work package 4

Title

Manipulative field mesocosm experiments on ecosystems dynamics

 

Objectives

  • Determine the impact of plant pattern in natural and non-natural communities on resource conservation and redistribution, assessing the potential for sudden shifts in ecosystem functioning and provision of services.
  • Assess the feedbacks between resource redistribution and plant pattern dynamics and the way in which these feedbacks are governed by plant diversity.
  • Assess the role of increasing pressure (e.g. water scarcity; grazing/browsing intensity, wildfires) in triggering threshold dynamics in ecosystem structure (plant pattern and diversity) and functioning.
  • Determine degradation reversal dynamics and thresholds as a function of plant colonization pattern and diversity

 

Description of work and role of partners

The objective of WB4 is to investigate how particular drivers (as identified in WB2) affect interactions in the plant-soil ecosystems that may cause sudden regime shifts. These studies will be carried out by means of controlled mesocosm experiments performed on a wide range of closed plots on artificial slopes. Key notions that underly these experiments are that spatial patterns in vegetation cover and decreased plant diversity may adversely affect soil and water conservation, creating feedbacks that eventually lead to degraded systems with a low plant diversity and biomass with losses in ecosystem services. The interpretation of the results obtained from the WB4 experiments will benefit from the inputs provided by WB3 on the mechanisms involved in the outcome of plant-plant interactions. On the other hand, WB4 results will help to understand the relative role of the factors responsible for WB5 field observations and will provide the needed inputs to fine-tuning and test WB6 models.

 

Task 1: Feedbacks between plant pattern dynamics and resource conservation
Eco-hydrological observations in dryland communities have suggested that coarsening plant spatial pattern (i.e., increasing bare-soil connectivity) and decreasing plant functional diversity have a negative effect on soil and water conservation, and thereby on general ecosystem functioning, which may trigger degradation feedbacks leading to poorly structured and dysfunctional ecosystems. However, at the plant patch scale, the coarsening of plant pattern results in a relative increase in resource inputs to the individual patches, which may increase patch productivity and growth and partially counterbalance the degradation loop. To disentangle the relative role of the factors involved in these feedbacks, and define the potential for tipping-point conditions leading to regime shifts, plant pattern and diversity will be independently manipulated in mesocosm experiments performed in small plots, creating a gradient of plant communities with varying levels of diversity and spatial distribution. Soil and water flows will be monitored in these plots, as well as plant growth and associated changes in plant pattern. This experiment will address Objectives 1 and 2, providing insights on both the feedbacks between plant pattern dynamics and resource conservation and the role of plant diversity in these feedbacks, and will focus on the provision of key services in dryland ecosystems: soil and water conservation and plant productivity. 

 

Task 2: Assessment of selected ecosystems

It is assumed that the increasing pressure of a number of driving factors, including controlled factors (e.g. grazing) and uncontrolled factors (e.g. droughts), can result in a relatively rapid collapse in ecosystem state to a degraded state that can be irreversible, but there is little experimental evidence on how these factors act, on the spatial distribution of their impacts and the associated effect on plant pattern, resource distribution, and threshold dynamics. In a set of large-size experimental plots, with a high, homogenously-distributed plant cover, different pressures (like grazing/browsing intensity) and water and nutrient availability will be manipulated in order to create gradients of biotic and abiotic responses, acting independently and in combination. The resulting changes in plant structure (diversity and pattern) and in soil-functioning indicators will be monitored and potential tipping points will be identified. Both biotic and abiotic pressures will be manipulated in order to achieve the complete degradation of the plant communities in the mesocosm trials. This experiment will address Objective 3.

 

Task 3: Experiments to trigger thresholds in ecosystem functioning

It is widely accepted that the trajectories of collapse and recovery differ, so that the decrease or cessation of the degradation pressure does not result in the recovery of the system. Barriers to restoring degraded systems may result from feedbacks between the degraded state and a variety of internal and external factors, making the degraded state highly resilient and resistance to restoration. A number of experimental plantations will be performed on bare-surface slopes, creating a range of plant communities with increasing values of plant cover in combination with a variety of plant patterns and diversity levels. Both plant cover/pattern dynamics and soil functioning indicators will be monitored in these mesocosms. This experiment, that will mirror the experiment performed in Task 2 and will be implemented on the same experimental slopes, allowing us to identify restoration thresholds and tipping points in the restoration path, and to quantify the distance between degradation and aggradations tipping points and the ecological resilience of degraded drylands. This experiment will address Objective 4.