Ten key issues for ecological restoration of territorial space

This study innovatively puts forward the three-stage restoration goals and cutting-edge key scientific issues of ecological restoration, as well as their relationships.

Ecological restoration is an effective measure to protect biodiversity, sustain ecosystem services and enhance social-ecological resilience.It is defined as the process of assisting the recovery of an ecosystem that has been degraded, damaged or destroyed [1 ,2 ].Multiple large-scale ecological restoration initiatives have been launched by international agencies and governments in recent decades, such as the UN Decade on Ecosystem Restoration (2021-2030), the Convention on Biological Diversity, the New York Declaration on Forests, and the Sustainable Development Goals [1 ].Extensive global ecological restoration priorities have made modest achievements; however they may not be efficient and lasting, and some of them have even failed, such as the Aichi Biodiversity Targets proposed by Global Biodiversity Outlook 5 ( https://www.cbd.int/gbo/gbo5/publication/gbo-5-en.pdf).For example, some restoration projects planted trees in a disorganized fashion, and in an inappropriate place, manner and time, resulting in negative outcomes such as tree death and reduced ecosystem diversity.Multiple social-ecological elements and processes need to be incorporated into the ecological restoration of territorial space.Therefore, how do we develop more comprehensive and phased efforts to reverse ecosystem degradation trends and achieve ecological restoration goals?Here, we innovatively put forward and systematize the three-stage restoration goals and cutting-edge key scientific issues of ecological restoration, as well as their relationships.
The core goal of ecological restoration is the harmonious coexistence of humans and nature, which requires ecology as the foundation and culture as the vein.However, current restoration usually focuses on greening with low efficiency, and the separation dilemma of nature and society.In fact, there are three main goals of ecological restoration, corresponding to the three stages of ecological restoration (Fig. 1 ).The first-stage goal is regreening, which can be achieved by increasing leaf area index, vegetation productivity and vegetation coverage.The second-stage goal is beneficiating.Increasing efficiency ranges from enriching ecological functions to enhancing ecosystem services, and to strengthening human well-being.The third-stage goal is revitalizing, which refers to enhancing the physical resilience of ecosystems and the cultural resilience of social systems, ultimately achieving sustainability of ecological restoration.After clarifying the threestage restoration goals, how to realize them can be tackled using the following 10 key scientific issues: reference state and size threshold before restoration, element coupling, trade-off efficiency, spatial connectivity, dynamic adaptation and scale cascade during restoration, and effectiveness evaluation, contribution distinguishment and cultural resilience after restoration.For each scientific issue, we systematically review the practical needs, existing or potential solutions, and research difficulties.size thresholds of ecological land have been proposed, for example, 12% of the natural ecological carrying capacity should be reserved for biodiversity in ecological footprint accounting; no less than 25% of China's territorial area should be designated as the ecological protection red line; and 30% of land and sea should be protected for global biodiversity by 2030, and half of the Earth by 2050.However, the current setting of size thresholds mainly relies on restoration expectations, subjective wi l lingness and financial capacity, while the dose-effect relationship, based on the resource and environmental carrying capacity, deserves more attention.Correspondingly, planetary boundaries and safe and just space were proposed as means of setting thresholds for critical planet biophysical processes [4 ].It is also reported that the goals of minimum areas under specific conservation were determined based on subjective thresholds or supply-demand relationships, e.g. at least 44% of the terrestrial area requires conservation attention to safeguard global biodiversity [5 ].
Moreover, the quality and location of ecological space to be protected or restored, and the regulation of social-ecological elements or pro-cesses beyond specific thresholds, cannot be ignored, and should be further explored.[11 ], and there is a growing concern for smaller patches in global conservation.Rewilding is one of ecological restoration's new approaches, aimed at reducing the human footprint and restoring ecosystem integrity [12 ].
The ecological network provides an effective means of enhancing ecosystem connectivity based on the habitat-corridor framework, which deepens the understanding of restoring complex systems and opens up new ways of measuring outcomes of ecological restoration [12 ].However, how to integrate controlled experiments and scenario simulations in order to develop new indicators for characterizing spatial connectivity, and how to design ecological corridors for enhancing functional connectiv ity w ithout other negative impacts such as species invasion and interference propagation, are sti l l critical challenges that need to be addressed.6. Dynamic adaptation .How should we dynamically adjust the phased goals of ecological restoration in the long-term restoration process of complex ecosystems under the context of adaptive management?
How do we identify the thresholds for regime shift?Earth's complex systems often have alternative stable states, and when external disturbances break through the critical threshold, the shifts wi l l occur between different stable states [13 ], so dynamic management is needed to enter a stable and controllable Earth system.Complex ecosystems require long-term restoration measures, and the time required for restoration increases with the complexity of phased goals.The phased goals of ecological restoration should be regulated by social-ecological feedback under the context of continuous monitoring.To maintain and optimize the stable equilibrium of the ecosystem, key variables of the social-ecological system need to be effectively regulated in order to avoid breaking ecological thresholds.Whether the energy inflow and outflow of the system are balanced, and whether the biogeochemical cycle is stable, can be used as criteria to determine whether the system has entered a steady state [13 ] The goal-cost-constraint framework can be adopted to systematically evaluate the efficiency of ecological restoration, and an indicator system can be established considering constraint conditions, so as to evaluate whether the objectives of ecological restoration are achieved with limited input or cost.However, the following key issues remain unclear: how should we balance comprehensiveness and specificity in determining evaluation indicators, especially considering the huge spatial differences in the social-ecological contexts of ecological restoration projects in China?What is the effectiveness difference between integrated and non-integrated ecological restoration (and how do we measure the difference)?How should we develop the effectiveness evaluation from quantification of ecosystem status to dynamic early warning in order to address the risk of future environmental change globally or locally?9. Contribution distinguishment .How do we determine the natural and anthropogenic contributions to the outcomes of ecological restoration?How do we distinguish the contributions of multiple sub-projects in an integrated ecological restoration project?It is acknowledged that the phenomenon of vegetation greening is often caused by a combination of ecological and socio-economic factors.Natural restoration benefits from natural ecosystem succession, and climatic warming and humidifying, while artificial restoration depends on artificial measures by way of reconstructing ecosystems, such as planting trees and grasses, as well as reducing or even removing negative human interventions to natural ecosystems.Statistical approaches and control experiments or simulations are usually used to quantitatively distinguish between natural and anthropogenic contributions [15 ].Moreover, it is important for the performance evaluation of ecological restoration projects to clarify the contribution of each specific sub-project, which can also be used for scheming horizontal ecological compensation between sub-project areas.One possible method is to identify the implementation time, implementation area and implementation object.In addition, the benefits brought by multiple There are also progressive and interrelated logical relations among these 10 key issues, which need to be considered in an integrated manner.In general, the goals set before restoration can guide restoration processes during restoration, which provide a ladder to restoration results after restoration.Restoration goals can be further used to verify restoration results, while the latter can be the feedback to the former.In the stage before restoration, the reference state and size threshold target are mutually restricted.During restoration, element coupling clarifies the element relationship, and tradeoff efficiency refers to the coordination of element relationship.Then ecological restoration can be conducted through achieving both connectivity in the spatial dimension, and adaptation in the temporal dimension.Correlated with the other four issues, scale cascade is aimed at the integration of multi-scale ecological restorations.In the stage after restoration, both effectiveness evaluation and contribution distinguishment are the judgment of restoration performance, focusing on integrated evaluation indicators and the contribution of restoration subprojects, respectively.Effectiveness evaluation is the premise of contribution distinguishment.Building cultural resilience is a low-cost measure to make ecological restoration effective in the long-term, and can also be regarded as a further enhancement of the anthropogenic restoration contribution, in order to achieve revitalization, the ultimate goal of threestage restoration.Relatively speaking, few studies have been conducted on reference state, size threshold, element coupling, dynamic adaptation and cultural resilience, covering all the three stages of ecological restoration.
Despite many restoration efforts implemented worldwide, the global ecosystem continues to degrade.This may be partly due to the lack of scientific recognition and induction of key issues in the ecological restoration of terrestrial space.Careful consideration of the three-stage restoration goals and 10 key issues proposed in this study is conducive to the spatial allocation, temporal deployment and benefit enhancement of ambitious restoration initiatives, and wi l l help them be organized and feasible globally and locally.It is also noteworthy that the proposed 10 key issues are not definitive, and it is hoped that they wi l l trigger researchers, managers and stakeholders to seek a deeper understanding and more systematic discussion on the ecological restoration of territorial space.Furthermore, integrated, ecosystem-based and multi-stakeholder-involved techniques are indispensable to the expected outcomes of ecological restoration.Various transdisciplinary approaches, as well as transboundary cooperation, should be developed to address the multidimensional governance challenges of complex social-ecological systems.Integrating top-down and bottom-up participatory approaches can enhance the practicality and efficiency of largescale ecological restoration projects.Al l in al l, ecological restoration science, technology and policy need to collaborate more closely in order to enhance ecosystem diversity, stability and sustainability.

3 Figure 1 .
Figure 1.Three-stage goals and 10 key issues of ecological restoration.