The contribution of ecosystems to human well-being has received growing attention in the public and scientific domains. Following the multilateral treaty of the 1992 International Convention on Biological Diversity of the United Nations, the 2010 Conference of Parties (COP 10) formally engaged on a strategic plan to “take effective and urgent action to halt the loss of biodiversity in order to ensure that by 2020 ecosystems are resilient and continue to provide essential services”, explicitly recognizing the need for resilient ecosystems, able to withstand the increasing pressure from climate change and anthropization. More recently, in 2015, world leaders adopted the Sustainable Development Goals (SDG), which among others include climate action (SDG13) and life on land (SDG 15) to ensure conservation and restoration of vegetation and sustainable use of the related ecosystems services [1
The scientific community has provided evidence on the key role of ecosystems for human well-being [2
], and has discussed the economic valuation of ecosystem services to mainstream their value into the decision-making process, with the intent to support their conservation, restoration and sustainable use [3
]. The assessment and valuation of ecosystem services and their changes over time have been the focus of many scientific studies and projects from the global [6
] to the regional [8
] and the local scale [10
]. Terrestrial ecosystems in good ecological status have been found to [5
Protect from erosion and landslides.
Protect from inland flooding.
Buffer natural resources against drier and more variable climates.
Reduce risks and impacts of wildfires.
Protect from coastal hazards and sea level rise.
Moderate urban heatwaves and heat island effects.
Managing stormwater and flooding in urban areas.
However, how to measure terrestrial ecosystems’ ecological status and their ability to withstand the increasing pressure of climate change and anthropization—i.e., their resilience—is still an open question.
Data from satellite remote sensing have been increasingly adopted for assessing vegetation resilience and ecosystem services, as they can provide information on the land cover typology, extent and characteristics at different points in time, and from the local to the global scale [11
]. Remotely sensed data can be used to define vegetation indexes such as the Normalized Difference Vegetation Index (NDVI) that is strongly correlated with the photosynthetic activity of the plant canopies [13
]. The NDVI is frequently used to assess ecological responses of vegetation to climatic conditions and environmental changes [12
] and has often been adopted as proxy for ecosystem services [16
The NDVI was used as a key indicator in a wide range of ecosystem services including (according to the Millennium Ecosystem Assessment classification): provisioning services, such as energy, food, raw materials, water provisioning, and genetic resources; regulating services, such as climate regulation, disturbance regulation, erosion regulation, flood regulation, hazard regulation, pollination, water purification, and waste treatment; supporting services, such as biological refugia, habitat, primary production in sea and rivers; cultural ecosystem services, such as cultural heritage and recreation [21
Until recently the focus has been mainly on the assessment of the current flow of ecosystem services and their relative changes compared to past conditions [6
], which has been possible by the availability of satellite data time series. Given the importance of ecosystem resilience for the reliability of the provided services, several studies have addressed the resilience of ecosystems through indicators of ecosystem stability [23
]. However, additional research is needed to quantify the resilience of ecosystems to environmental change in spatially explicit assessments [25
Here, we focus on the estimation of terrestrial ecosystem resilience related to the stability of primary production, which is generally recognized as one of the main constituent of ecosystem resilience [26
]. We estimate vegetation primary production resilience by means of the NDVI and its inter-annual variability. More specifically, we adopt the recently proposed indicator for annual crop production resilience [27
]. This indicator is derived from the original ecological definition of resilience i.e., the measure of the largest stress that the system can absorb without losing its function [28
]. We apply the annual resilience indicator to remotely sensed NDVI, as proxy of vegetation primary production. Furthermore, we apply the annual resilience indicator to ground-based precipitation data, as proxy of annual green water resources (i.e., the water available to plants [29
We show and discuss three main results.
The functional quality of ecosystems is directly linked to the amount and reliability of services they can provide [43
]. With this paper, we enrich the set of analysis tools for assessing the quality and the stability of ecosystems through the resilience indicators that we propose to apply to green water resources and to vegetation primary production.
These indicators, formally derived from the ecological definition of resilience [27
], require relatively long time series (30 years and more) that, despite not available from the most frequently used satellite mission (e.g., MODIS, SPOT-VGT and Prob-V), can be extracted from AVHRR-based long-term archive as the one used in this study (i.e., GIMMS). This long-term approach may thus complement more complex resilience measures based on the engineering definition of resilience i.e., the ecosystem recovery time more suited for short time-scales analyses [45
]. Comparing these two different views would probably provide an interesting way forward to the understanding of ecosystems resilience and stability assessments.
Our analysis linking annual precipitation and vegetation productivity suggests a coherent relationship between the resilience indicators computed on precipitation and NDVI. The magnitude of the resilience indicator for vegetation was found larger than the one of precipitation. This may indicate the capability of vegetated ecosystems to maintain their functioning also in the presence of precipitation variability. Nevertheless, the spatial patterns of the two resilience indicators do coincide to a great extent. In regions with reduced agreement, as for instance in the high latitudes of the Northern Hemisphere, the reduced vegetation resilience as compared to the precipitation resilience may be due to temperature and/or radiation, not explicitly considered in this study. Further studies are needed to assess the robustness of these results including, for instance, the effects of temperature and radiation as well.
This analysis could also be further evolved by accounting for the delay between precipitation input and vegetation response, as well as different time-scales [45
], including different seasonalities such as double seasons and different periodicities as those found in the Southern Hemisphere and disentangling the differences between natural and anthropic vegetation cover, possibly isolating the effects of management and irrigation [49
This paper provides an alternative framework for the assessment of ecosystem resilience by proposing two indicators, one for green water resources and one for vegetation productivity, based on the ecological definition of resilience [27
Our global scale analysis strongly suggests a coherent relationship between these indicators over two wide ranges of biomes in the present climate. However, further studies are needed to quantify the robustness of these relationships in a changing climate, considering other definitions of resilience as well [23
]. Including temperature and radiation would be also needed to fully explain the observed spatial patterns in vegetation resilience.
Quantifying the resilience of ecosystems and ecosystem services is of upmost relevance when considering future climate change [50
]. Indeed, the variation of temperature and precipitation patterns caused by climate change is expected to affect the availability and seasonality of water resources and the growing season of vegetation in various regions of the world [54
], affecting biodiversity and the provisioning of ecosystem services [24
Our preliminary analysis of climate change projections from CMIP6 data displays alarming results. The increase of precipitation magnitude projected for the next future is counterbalanced by the concomitant increase of its variability, resulting in a general decrease of green water resource resilience at the global level.
In the future, it would also be interesting to carry on this analysis with an indicator of Blue Water Resources Resilience, which could be easily defined by computing the resilience indicator on the surface water budget given by precipitation minus evapotranspiration. This could be put in relation with soil moisture and river drought through the standardized Precipitation Evapotranspiration Index (SPEI [56
]) or the Standardized River Discharge Index (SRDI [58
]) to measure the reliability of freshwater resources for society and to help assessing the status of river ecosystems [60
]. This additional resilience indicator may be potentially suitable for river related ecosystem services.