Regeneration Status and Diversity of Woody Plant Species in the Priority Habitat 91E0* in Nestos, Greece ^†

Abstract

The priority habitat 91E0*, consisting of alluvial forests with Alnus glutinosa (L.) Gaertn. and Fraxinus excelsior L., represents a vital ecosystem along Europe’s riverbanks and floodplains. In Greece, the alluvial forests of the Nestos Delta are particularly notable for their unique composition and ecological importance. The present study aimed to assess woody species’ regeneration status and diversity in the priority habitat 91E0* in Nestos, Greece. In the studied area, 13 plant species belonging to 12 genera from 10 families were recorded. The most numerous families were Salicaceae (23.07%) and Moraceae (15.38%). The regeneration index (IR) for Amorpha fruticosa L. and Acer negundo L. exhibited a substantial decline, decreasing from 31.75 and 21.12 cm m⁻² to 2.07 and 2.6 cm m⁻², respectively. This intervention created space for expanding native tree species such as Cornus sanguinea L., Morus alba L., and Populus alba L. The results demonstrate that the regeneration of P. alba L. is currently the most extensive (31.1%) in the alluvial forest, with C. sanguinea L., M. alba L., and A. glutinosa (L.) Gaertn. also showing significant regeneration. In contrast, the regeneration of F. angustifolia Vahl remains very limited (0.21 cm m⁻²), indicating the necessity for targeted restoration efforts.

1. Introduction

The Mediterranean Basin ranks as one of the world’s most biodiverse regions and is the third most significant hotspot for plant diversity. It supports 25,000 plant species, with over half endemic [1]. However, this region is also one of the planet’s most threatened, classified as the fourth most significantly altered biodiversity hotspot [2] and the second in habitat area loss. Additionally, it has been identified as a climate change hotspot [3]. The Mediterranean climate is characterized by pronounced seasonality in temperature and precipitation. Overall, species in Mediterranean ecosystems experience significant water stress due to hot, dry summers and cool, wet winters [4].

The priority habitat 91E0*, consisting of alluvial forests with Alnus glutinosa and Fraxinus excelsior, is a critical ecosystem along Europe’s riverbanks and floodplains. These forests are essential for maintaining biodiversity, regulating water cycles, and providing ecosystem services such as flood control and wildlife habitats. In Greece, the alluvial forests of the Nestos Delta are particularly notable for their unique composition and ecological importance, including 22 habitat types (Annex I Dir. 92/43/EEC) [4]. Given the escalating impacts of climate change, particularly in regions like the Mediterranean Basin, the conservation and management of alluvial forests become increasingly vital. These ecosystems play a crucial role in retaining soil moisture and preventing soil collapse, particularly in areas adjacent to water bodies. The preservation of these areas is not only essential for maintaining biodiversity but also for safeguarding the hydrological stability of the landscape, which is increasingly threatened by changing climate patterns. Therefore, this study aims to evaluate the regeneration status and diversity of woody species in the priority habitat 91E0* in the Nestos Delta to quantify the conservation status of the alluvial forest.

2. Materials and Methods

2.1. Study Area

The study area is in SCI GR1150010 (Natura 2000 site) in the northeastern region of Greece and includes alluvial forests of the priority habitat 91E0 in the Nestos Delta. The alluvial forests are found in the floodplain areas and are influenced by the river’s hydrological dynamics, which create a unique environment for biodiversity. The region’s climate is humid, according to the UNEP Climate Zone Classification System [5]. Fifteen (15) circular permanent plots, with a radius of 15 m each, were established in May 2019. They were randomly and homogeneously distributed along the habitat (Figure 1).

2.2. Woody Plant Species

Data collection was a comprehensive process that took place in May and July 2023 across 14 of the 15 plots; 1 plot (plot #6) was inaccessible due to flooding. Within each of the accessible 14 plots, the composition and diversity of woody tree species (species richness) were recorded. The Index of Regeneration (IR) was calculated by multiplying the regeneration density (per m²) by the average regeneration height (cm), providing a metric of regeneration extent. This approach enables conclusions to be drawn regarding the condition of the forests and ongoing dynamic processes, validating their recovery and particularly assessing the long-term persistence of wood regeneration following targeted ecosystem restoration and conservation efforts.

3. Results

3.1. Woody Plant Species Composition and Diversity

In the studied area, 13 plant species belonging to 12 genera from 10 families were recorded (

Table 1. Woody plant species in the study area.

Species	Family
Acer negundo L.	Aceraceae
Alnus glutinosa (L.) Gaertn.	Betulaceae
Celtis australis L.	Ulmaceae
Cornus sanguinea L.	Cornaceae
Ficus carica L.	Moraceae
Morus alba L.	Moraceae
Platanus orientalis L.	Platanaceae
Populus alba L.	Salicaceae
Populus nigra L.	Salicaceae
Prunus cocomilia Ten.	Rosaceae
Robinia pseudoacacia L.	Fabaceae
Salix alba L.	Salicaceae
Sambucus nigra L.	Caprifoliaceae

). The most numerous families were Salicaceae (23.07%) and Moraceae (15.38%). In addition, 76.92% of plant species are native/non-range restricted, whereas 23.07% are alien/established providing valuable insights into the dynamics of plant distribution. According to the life form spectrum of the vegetation, Phanerophytes (92.30%) contributed the most to the total number of recorded species in the study area, followed by European–SW Asian (7.69%). The documented species chorological spectrum revealed that European–SW Asian species comprised 30.76% of the total flora, followed by East Mediterranean species (15.28%), Euro–Siberian (15.38%), and North American (15.38%).

3.2. Regeneration of Alluvial Wood

Our results indicate that the regeneration of Populus alba is the most extended (31.1%) in the alluvial forest (

Table 2. Average Index of Regeneration (IR) (cm/m²) of each tree species measured in all 14 permanent plots.

Species	IR (cm/m2)	IR %
Acer negundo L.	2.60	6.2
Alnus glutinosa (L.) Gaertn.	5.46	7.6
Amorpha fruticosa L.	2.07	5.8
Celtis australis L.	1.37	2.7
Cornus sanguinea L.	10.61	7.3
Ficus carica L.	2.87	4.6
Fraxinus angustifolia Vahl	0.21	0.3
Ligustrum vulgare L.	2.33	4.0
Morus alba L.	5.73	7.9
Populus alba L.	17.04	31.1
Populus nigra L.	1.50	3.0
Prunus cocomilia Ten.	2.67	5.4
Quercus robur L.	0.06	0.1
Robinia pseudoacacia L.	3.36	6.0
Rubus sp.	2.12	4.3
Sambucus nigra L.	1.27	0.9
Salix alba L.	1.30	2.6
Ulmus minor Mill.	0.06	0.1

). Cornus sanguinea L., Morus alba L., and Alnus glutinosa (L.) Gaertn. regeneration are also extended, whereas regeneration of Fraxinus angustifolia is very limited (0.21 cm/m²).

The average regeneration height was used to indicate the regeneration quality, attributing a greater height to a greater degree of affirmation. The comparison of the regeneration height among tree species measured in 14 permanent plots indicated that the tallest seedlings are the ones of Sambucus nigra (300 cm) and Robinia pseudoacacia, followed by Ficus carica (264 cm and 203 cm, respectively) (Figure 2).

By comparing the average regeneration height among various species on the two different banks (East and West) of the Nestos River, it appears that the average regeneration height is significantly higher on the west bank (p < 0.0001, t-Test) (Figure 3).

4. Concluding Remarks

Two years after the removal of alien species, the regeneration index (IR) for both A. fruticosa and A. negundo showed a substantial decline from 31.75 and 21.12 cm m⁻² to 2.07 and 2.6 cm m⁻², respectively. Concurrently, space was created to expand native tree species such as C. sanguinea, M. alba, and P. alba. The results indicate that the regeneration of P. alba is currently the most extended (31.1%) in the alluvial forest, with C. sanguinea, M. alba, and A. glutinosa regeneration also showing significant extension. In contrast, the regeneration of F. angustifolia remains very limited (0.21 cm m⁻²), signaling the need for targeted restoration actions.

Furthermore, the restoration initiatives have positively impacted the composition and regeneration dynamics of the alluvial forest in the Nestos Delta. Nevertheless, it is recommended that ongoing monitoring activities be systematically conducted to track the potential expansion of alien species and implement appropriate management and conservation measures. Additionally, restoration activities should be considered for Fraxinus angustifolia, which is not prominently present in the forest stands and exhibits limited regeneration on the west bank of the Nestos River.

Moreover, our project in the Nestos Delta effectively integrates multiple reforestation objectives by enhancing biodiversity through the restoration of native and keystone species, supporting economic utilization via sustainable harvesting and the development of non-timber forest products, and increasing carbon sequestration by prioritizing species with high carbon storage potential. Additionally, this project contributes to ecological stability by preventing soil erosion and maintaining hydrological balance, while fostering community involvement and collaboration with stakeholders to ensure long-term conservation and alignment with broader environmental and economic goals. This holistic approach ensures that the project benefits both the ecosystem and local communities.

Finally, proposed improvements and extensions to the restoration technique are described as follows: (a) Enhanced Monitoring and Adaptive Management: Systematic, continuous monitoring and adaptive management strategies are essential to track the re-expansion of alien species and respond to changes in forest dynamics, considering Greece’s evolving environmental conditions; (b) Restoration of Vulnerable Species: Targeted efforts should focus on increasing the regeneration of F. angustifolia and other underrepresented native species through planting, soil improvement, and protection measures; (c) Integration of Climate-Resilient Species: Incorporating climate-resilient species into restoration projects will help ensure the long-term stability and resilience of the forest ecosystem in the face of climate change; (d) Community Engagement: Involving local communities and stakeholders in restoration efforts can enhance conservation outcomes and support the expansion of restored areas; and (e) Research and Innovation: Ongoing research and innovation in restoration techniques, including the use of native species with high ecological value and new technologies like drones, can further improve restoration success.