From Assessment to Action. Charting a Path for Urban Forest Restoration

The Drava Forest Park is a vital ecological and recreational area within the Varaždin-Ludbreg urban agglomeration in northwestern Croatia. This region is renowned for its rich cultural heritage, historical significance, and natural beauty, with the park providing an essential green corridor linking the two cities.

THE VARAŽDIN-LUDBREG AGGLOMERATION

This urban agglomeration combines the cultural vibrancy of Varaždin with the historical and spiritual significance of Ludbreg, creating a dynamic hub of tradition and modernity.

Known as the City of Baroque, Varaždin is celebrated for its well-preserved architecture and vibrant cultural life. Key landmarks include the Old Town fortress and the Varaždin Cemetery, renowned for its artistic design. The city hosts the annual Špancirfest, attracting numerous visitors with diverse cultural programs.

Ludbreg, often referred to as the Centre of the World, is known for a local legend suggesting it lies at the heart of concentric circles connecting it to other global cities. The town is also a significant pilgrimage site due to a Eucharistic miracle reported in 1411. The Batthyány Castle houses a chapel commemorating this event.

According to the 2021 census, Varaždin has a population of approximately 43,999 residents while Ludbreg counts 8,477 residents

THE DRAVA FOREST PARK

Situated along the Drava River, the Drava Forest Park is part of the Mura-Drava-Danube Biosphere Reserve and the Natura 2000 network, highlighting its ecological importance. The park encompasses diverse habitats, including floodplain forests, wetlands, and meadows, supporting a wide array of plant and animal species. It serves as a crucial green space for residents of Varaždin and Ludbreg, offering opportunities for recreation, education, and nature conservation.

However, the park faces challenges such as habitat degradation, invasive species, and the impacts of climate change. Addressing these issues is essential to preserve its biodiversity and maintain its role as a natural haven for the surrounding urban populations.

The first step in implementing InterACT Green is a comprehensive assessment of the forest’s health and ecosystem status. In this section, we will begin by outlining the methodology used by the Croatian Forest Research Institute—the partner responsible for this component of the project—to conduct the assessment. We will then present the main findings of the study, followed by an analysis of the dynamics that underlie and shape the current situation.

METHODOLOGY

The health assessment of the Drava Forest Park utilized advanced remote sensing techniques and field inspections to evaluate vegetation condition, tree health, and invasive species spread. The primary tools and methodologies applied in this assessment include:

• Visual Tree Assessment (VTA): The Visual Tree Assessment (VTA) method, developed by Mattheck and Breloer in 1994, is a widely accepted approach in forestry health evaluations for assessing tree vitality and identifying structural issues. This method involves three stages: an initial visual inspection, a detailed analysis, and diagnostic testing if necessary. VTA evaluates structural integrity and signs of disease or decay by examining anomalies in the trunk, canopy, and root systems. Through careful observation of trunk form, canopy density, and branch health, VTA identifies visible issues such as fungal growth, cavities, and branch dieback, which can indicate underlying health risks. This approach proved particularly valuable for assessing species in the Drava Forest Park, such as ash, which shows external signs of ash dieback, and spruce, where beetle infestations have led to visible canopy thinning and branch dieback.
• Multispectral Drone Imagery and NDVI Analysis: The assessment leveraged multispectral imagery captured by drones to conduct a large-scale analysis of vegetation health across the park. Multispectral imaging involves capturing data across various spectral bands, including red and near-infrared, which are essential for calculating the Normalized Difference Vegetation Index (NDVI). NDVI is a reliable indicator of vegetation health, with values ranging from -1 to +1, where higher values indicate dense, healthy vegetation, and lower values suggest stress or sparse vegetation.

In the Drava Forest Park, NDVI data helped pinpoint areas experiencing significant vegetation stress. Low NDVI values, below 0.5, marked in lighter colours, identified regions with thinning or degraded vegetation, which correlates with areas impacted by invasive species or where canopy loss has led to sparse ground cover. Conversely, high NDVI values closer to 1 highlighted healthy, dense vegetation, mostly in undisturbed sections of the park. This distinction allowed for a targeted approach in identifying zones in need of restoration or further monitoring.

• Field Surveys for Invasive Species Mapping: On-the-ground surveys complemented the remote sensing techniques by providing direct observations of the spread and density of invasive species, such as the box elder (Acer negundo). Field teams recorded the extent of invasive ground cover, particularly in areas with limited canopy, which inhibits natural regeneration of native species. These observations confirmed NDVI findings and informed the prioritization of areas for intervention.

ASSESSMENT RESULTS

The health assessment of Drava Forest Park, conducted by the Croatian Forest Research Institute, revealed several significant issues impacting forest health, particularly among specific tree species and in areas dominated by invasive vegetation.

Non-Native Species Decline

• Spruce Trees: The assessment indicates severe health deterioration among spruce trees, which were artificially introduced into the park. Due to the unsuitable habitat and local climatic conditions, many spruces are physiologically weakened and are succumbing to attacks by the spruce bark beetle (Ips typographus). These beetles primarily infest stressed trees, and their presence has led to widespread spruce dieback, necessitating the removal of affected trees.

Native Tree Health

• White Poplars: White poplars generally exhibit good health and are widely distributed throughout the park. However, many show signs of infestation by white mistletoe (Viscum album), a parasitic plant that attaches to branches and absorbs water and nutrients. The mistletoe presence is notable in both northern and southern areas, and if left unmanaged, it can gradually weaken the affected trees.
• Ash Trees: Ash trees in the park display symptoms of ash dieback disease (Hymenoscyphus fraxineus), with thinning crowns and dry branches. This fungal infection has resulted in extensive tree deterioration, with many ash trees expected to die in the coming seasons, necessitating eventual removal.

Invasive Species Proliferation

• The spread of invasive species, particularly box elder (Acer negundo), poses a significant challenge in the ground layer, especially in degraded sections where canopy thinning has occurred. These invasive plants outcompete native vegetation, complicating regeneration efforts and reducing the park’s biodiversity.

Structural Degradation and Bush-Dominated Vegetation

• The assessment found that, in areas with poor tree coverage, bushy vegetation has proliferated, leading to a simplified forest structure. This bush-dominated landscape lacks the vertical and horizontal structure necessary to support a diverse range of species. The imbalance favors invasive growth and limits the natural progression towards a mature, ecologically stable forest.

Limited Natural Regeneration

• Certain areas, particularly those overwhelmed by invasive ground cover and dense bushy growth, show little to no natural regeneration. The prevalence of invasive species and ground-level weedy vegetation inhibits the growth of young native trees, highlighting the need for active intervention to re-establish native flora and support biodiversity restoration.

 DYNAMICS UNDERLYING POOR HEALTH

The health decline of Drava Forest Park is influenced by a complex interplay of climate, invasive species, biotic and abiotic stressors, and human activities. Namely:

• Climate Change Impacts: Climate change has intensified environmental stressors on forest ecosystems, notably by increasing temperatures and altering precipitation patterns. This has led to more frequent drought conditions, which weaken trees and increase their vulnerability to pests and diseases. In recent years, prolonged heatwaves and irregular rainfall patterns, such as those experienced during the 2022 and 2024 summer seasons, have caused physiological stress, particularly on non-native spruce species, creating ideal conditions for pest outbreaks.
• Invasive Species Proliferation: Several invasive plant species, such as the box elder (Acer negundo) and black walnut (Juglans nigra), have spread throughout the park. These species often dominate areas with weakened native vegetation, further disrupting forest structure and biodiversity. In addition to plant invasives, the ash dieback fungus (Hymenoscyphus fraxineus) has had a devastating impact on ash populations, causing widespread dieback and tree mortality. This pathogen’s persistence is of particular concern due to its ability to rapidly spread and infect healthy trees.
• Pests and Pathogens: The spruce bark beetle (Ips typographus) and the oak web bug (Corythucha arcuata) have also contributed significantly to tree decline. Spruce, already weakened by unsuitable soil and climate conditions, is highly susceptible to infestation by the bark beetle. The oak web bug, an invasive pest from North America, has affected oak species by reducing leaf area, which decreases the tree’s photosynthetic capacity and overall health. Together, these biotic stressors have led to a marked decline in the vitality of native and non-native tree species alike.
• Human Impact and Historical Planting Practices: The introduction of non-native species, such as spruce and black walnut, was historically intended to enhance the park’s aesthetic value. However, these species lack adaptability to the local climate, making them vulnerable to pests and disease. Additionally, human activity, including foot traffic and soil compaction, has degraded soil quality in certain areas, further limiting natural regeneration. This historical and ongoing human influence has created an environment in which native vegetation struggles to thrive.
• Abiotic Stressors and Soil Degradation: Abiotic factors such as compacted and nutrient-poor soil, partly resulting from human interference, have also contributed to forest health issues. In compacted areas, soil aeration is limited, which hampers root development and nutrient uptake. These conditions favor invasive ground cover, which prevents natural regeneration of native tree species. The cumulative effect of these abiotic factors undermines the forest’s resilience, making it increasingly difficult to support a diverse and stable ecosystem.

The Drava Forest Park’s regeneration strategy is focused on prioritizing areas where intervention will most effectively restore the park’s ecological balance and biodiversity. Fifteen key plots have been identified for targeted regeneration based on specific criteria that reflect the forest’s most pressing challenges and the feasibility of long-term recovery. The main criteria for selecting these areas include:

• Stand Degradation: Areas where the vertical and horizontal structures of the forest have been significantly compromised, often due to the drying or natural death of trees, were prioritized. These degraded stands lack the layered complexity essential for supporting diverse wildlife habitats, making their restoration a priority.
• Health Condition: Particular attention was given to sections with drying coniferous species, notably imported spruce. Spruce, not native to this ecosystem, has shown high vulnerability to local conditions, leading to reduced resilience and increased susceptibility to disease and pests.
• Predominance of Bushy Vegetation Without a Tree Layer: Plots where dense, bushy vegetation dominates in the absence of a mature tree canopy were marked for intervention. Such areas lack the ecological structure that supports biodiversity and are prone to further ecological imbalance without active restoration.
• Abundant Occurrence of Invasive Species: Areas where invasive species have taken root extensively in the ground layer pose a serious threat to native flora. These invasive species compete aggressively with local plants, reducing biodiversity and inhibiting natural regeneration.
• Absence or Impossibility of Natural Rejuvenation: Weedy areas where natural forest regeneration is either absent or unlikely were also chosen for active intervention. Without intervention, these zones remain in a degraded state, unable to progress through natural succession to a mature forest ecosystem.

The intervention strategy involves removing invasive species, dead or unhealthy trees, and non-native coniferous species. By targeting these priority areas, the project aims to promote natural regeneration, enhance structural diversity, and facilitate the reestablishment of native flora. Over the next two years, approximately 7% of the forest area will be addressed through this phased restoration approach, setting the stage for a resilient and ecologically balanced Drava Forest Park.

The assessment of the Drava Forest Park conducted by the InterACT Green project provides valuable insights for cities embarking on similar efforts to restore and manage urban green spaces. While the project is at an early stage, several key lessons from the assessment phase can guide other cities facing comparable challenges.

• Comprehensive Health Assessment as a Foundation: The use of Visual Tree Assessment (VTA) and multispectral NDVI drone imagery has proven critical in building a detailed understanding of forest health, guiding the initial setup of targeted management practices. This underscores the value of employing diverse assessment tools early on to gain a clear picture of ecosystem health. For cities starting similar projects, investing in these tools can lead to more effective, data-informed strategies for forest management. In case of limited budgets, although drone imagery allows better quality and higher resolution data, the Copernicus Data Space Ecosystem Browser offers access to Earth observation and environmental data – provided by the Copernicus Sentinel constellations, contributing missions, Auxiliary engineering data, on-demand data and more – for a variety of applications, from environmental monitoring and disaster management to urban planning and agriculture.
• Setting Clear Criteria for Intervention: The prioritization criteria – such as identifying areas with stand degradation, high invasive species presence, and insufficient natural regeneration – offer a model for cities to allocate resources strategically. Focusing on high-impact zones from the outset can help maximize ecological benefits, even when resources are limited. Defining criteria based on local conditions enables cities to set focused intervention goals that can evolve as management practices develop.
• Early Invasive Species Control Measures: The assessment revealed a significant presence of invasive species, such as box elder and black walnut, highlighting the need for proactive management. Addressing invasive species early in the project can prevent further degradation and create conditions conducive to native vegetation recovery. Cities managing urban forests should consider establishing invasive species monitoring and control protocols as part of their foundational practices.
• Mitigating Human Impact from the Outset: Findings related to soil compaction and the challenges posed by non-native species underscore the importance of balancing recreational access with ecological health. Cities can benefit from planning early measures to minimize human impact, such as designating paths and protecting sensitive areas. Incorporating public awareness initiatives into management practices can help build community support for conservation efforts. In this direction, the PLATFORM currently under development within InterACT Green promises to play a crucial role.
• Preparing for Climate-Related Stressors: Although specific interventions are still in the planning phase, the assessment highlights the influence of climate change on tree health and pest populations, particularly among non-native species. Cities initiating urban green space projects should anticipate climate-related stressors by selecting resilient species and implementing adaptive management practices that can respond to shifting environmental conditions.

Overall, the findings from the Drava Forest Park health assessment provide a solid foundation for establishing targeted management practices within urban forests. While much work remains, the InterACT Green project’s approach offers a promising model for cities aiming to develop resilient, data-informed strategies for green space management, fostering ecological health and sustainability from the ground up.