The “Bosco Fontana” natural reserve includes the last remaining mixed floodplain forest in northern Italy and one of the most endangered ecosystems in Europe. Its effective management is hindered by the complexity of interactions of mixed-tree species and the influence of environmental factors on understory plant diversity. In this study we analyzed the patterns of natural evolution in semi-natural floodplain forest stands at Bosco Fontana with the aim of better understanding its current natural processes and dynamics. Stand structure, taxonomic and functional diversity, species composition, and leaf area index (LAI) of overstory and understory layers were surveyed in permanent plots over two inventory years (1995, 2005). The influence of environmental factors on understory plant diversity was assessed using Ellenberg’s indices for light, soil moisture, soil nutrient and soil reaction. Results indicated that overstory species composition varies according to the soil moisture, with hornbeam prevailing in xeric sites and deciduous oak species in mesic sites. Xeric sites showed high functional dispersion in both drought and shade tolerant traits, while it was significantly lower in both overstory and understory in the moist site. Functional dispersion of drought tolerance in the overstory and understory layers was positively correlated, while species richness was negatively correlated between the two layers. Diversity in the understory was mainly correlated with soil conditions. Understory LAI was positively correlated with overstory LAI in xeric and mesic plots, while no correlations were found in the moist plot. Overall, our results suggest that site conditions (soil conditions and water availability) are the major drivers of understory and overstory dynamics in the study forest. Hence, local site conditions and the understory should be carefully considered in the management of mixed floodplain forests.
Only less than 1% of temperate deciduous forests in Europe are undisturbed, free of logging, grazing, deforestation and other intensive uses (
Close-to-nature forest management requires a comprehensive knowledge of the stand structure, diversity and species interactions naturally occurring in forest ecosystems (
The composition of overstory species in the forest may influence understory vegetation diversity and composition (
Floodplain forests (one of the 14 categories under the European Forest Types classification framework -
The semi-natural reserve of “Bosco Fontana” is one of the last remaining floodplain forests in northern Italy (
The aim of the study was to investigate the structure, diversity, composition, and LAI of forest overstory and understory at Bosco Fontana from 1995 to 2005, as well as to assess the influence of the environmental factors on the main structural and diversity parameters. We analyzed how overstory and understory structure did vary according to soil and environmental conditions. Our specific goals are summarized by the following questions: (i) how do overstory and understory layers vary in taxonomic, structural and functional diversity among xeric, mesic and moist sites? (ii) what are the relationships between overstory and understory layers among xeric, mesic and moist sites? (iii) what is the relationship between understory structure and diversity related to environmental conditions?
Bosco Fontana is a State Natural Reserve in northern Italy (45° 12′ N, 10° 44′ E). The climate is continental, with long, cold winters and hot, humid summers. The average annual temperature is 13.2° C and the average annual rainfall was 658 mm (
A survey was conducted in three out of the six permanent plots, which were chosen as representative of the overstory and understory composition of the Natural Reserve. In particular, plots ID1, ID2 and ID3 reflect the xeric, mesic and moist regime variant of the oak-hornbeam forest, respectively.
The forest overstory survey was conducted in core areas (70×140 m) established within each of the three aforementioned plots, while understory was surveyed in a central area (2×100 m) nested within each core area (see below). Three additional 10×100 m plots (ID4, ID5, ID6) were established for the analysis of the overstory, each with a nested area of 2×100 m for the understory survey. The plots were categorized into three moisture regime variants (xeric, mesic, moist conditions) based on soil moisture measurements (
All vascular plants with height above 1.30 m and diameter above 5 cm were classified as overstory in both surveys (1995 and 2005). Each plant was identified at the species level and its diameter at breast height (1.30 m) and height measured. Stand structure, diameter distribution and species basal area contribution was compared between the two inventories to assess change in overstory structure and composition.
Within each monitoring plot, fifty 2×2 m permanent understory sub-plots were established along a linear transect at the center of each monitoring plot. The understory survey was conducted during periods of peak vegetation cover (July-August) in both 1995 and 2005. Each vascular plant was identified at the species level and its cover-abundance estimated using the Braun-Blanquet scale, which was converted to percentage cover values.
Taxonomic diversity at both overstory and understory level was measured as species richness (
where
where
Shade tolerance and drought tolerance were the functional traits considered in this analysis. Both these traits are relevant in terms of ecosystem dynamics and management (
For the overstory layer, species basal area was used as surrogate of abundance in the computation of both the overstory functional dispersion (
Leaf area index of both overstory and understory layers was estimated in a sub-set of three plots (one xeric, one mesic and one moist plot) according to
To evaluate the change in the overstory composition among sites, we compared the composition and the structure of the stands between the two inventory years (1995, 2005). Further, leaf area indices and the selected functional and taxonomic indices were compared among sites and dates. The significance of the relationships between overstory and understory attributes was tested using the Pearson’s
To explore the relationship between the understory and environmental conditions, we used Ellenberg’s indicators for light (
The studied plots generally showed a reverse-J diameter distribution (
Horn beam and oaks were the most dominant species, but their relative contribution varied according to the hydrological site conditions. The largest abundance of horn beam was observed in the xeric plots, where this species contributed to the total stand basal area between 50 to 64% in 1995 and between 49 to 52% in 2005; in the same plots the contribution of oak species varied between 25 and 46% in 1995 and between 34 to 38% in 2005. In mesic plots horn beam contributed to the total stand basal area between 28 to 47% in 1995 and between 28 to 38% in 2005, while the contribution of oak species varied between 33 to 63% in 1995 and between 32 to 49% in 2005. In the moist plot, more water demanding species (
The number of overstory vascular plant species found in the monitored understory plots (quintuplets) ranged from 6 to 17. Overstory functional dispersion (
The number of understory vascular plant species found in the monitored plots ranged from 4 to 20. Understory functional dispersion (
Understory diversity indices showed significant correlations with site environmental characteristics obtained from Ellenberg’s indicator values (
Analysis of overstory and understory relationship revealed a significant positive correlation in drought tolerance between the two layers (
Overstory LAI averaged (± SE) 4.1 ± 0.1, 4.4 ± 0.1 and 3.7 ± 0.1 in the xeric, mesic and moist plots, respectively. Understory leaf area index averaged 1.4 ± 0.1, 1.5 ± 0.1 and 1.1 in the xeric, mesic and moist plots, respectively. Understory LAI (
The studied stands were characterized by a complex and highly diverse structure, which is typical of floodplain forests, as indicated by the structural and diversity indices tested. However, our results revealed that site conditions are an important aspect in explaining species distribution patterns and their interaction. Species composition in the overstory layer appeared to vary according to soil water availability, being more similar in xeric and mesic plots, in which hornbeam and oaks are the dominant species, and favoring more hygrophilus species in the moist site. Such different overstory composition is reflected by the functional dispersion indices, which increased from moist to xeric conditions. A decreasing correlation between understory and overstory LAIs was also observed with respect to increasing soil water availability.
The significant and positive correlation observed in drought tolerance and leaf area indices between understory and overstory layers suggest a mutual influence on the structure and dynamics of the two layers in temperate broadleaf forests. Further, the positive correlation between the leaf area indices of the two layers in xeric and mesic plots is contrasting with previous studies conducted in managed forests, which reported an inverse relationship between overstory and understory LAI (
The high functional dispersion observed in the plots characterized by relatively drier conditions suggests that these stands are undergoing an early successional stage, in which ruderal species with wide ecological amplitude are exhibiting large competition for limited resources. In this context, the large dominance of
Based on our results, we conclude that local site conditions should be carefully considered in the management of mixed temperate floodplain forests. In the more resource-limited stands, a strong interplay between the overstory and understory layers may facilitate a fast recharge of nutrient pools (
FM designed the experiment and coordinated the work. FC, EM and MJF conducted the experiment, analyzed the data and wrote the paper. PM and AC collected the data. PC provided the conceptual support. FC was supported by the Project “ALForLab” (PON03PE_00024_1) co-funded by the Italian Operational Programme for Research and Competitiveness (PON R&C) 2007-2013, through the European Regional Development Fund (ERDF) and national resource (Revolving Fund - Cohesion Action Plan (CAP) MIUR). We are grateful to the anonymous reviewers for the useful suggestions that improved an early version of the manuscript.
Distribution of tree stem diameters at breast height in the studied plots in 1995 (top) and 2005 (bottom).
Mean values of functional dispersion (
Relationship between functional dispersion of drought tolerance (
Relationship between species richness (
Main stand characteristics of the studied plots in the two inventory years (1995, 2005). Standard errors are reported in brackets.
Plot | Hydrology | 1995 | 2005 | ||
---|---|---|---|---|---|
Basal area(m2 ha-1) | Height(m) | Basal area(m2 ha-1) | Height(m) | ||
ID1 | xeric | 19.65 | 14.9 (0.3) | 23.32 | 14.5 (0.4) |
ID2 | mesic | 23.59 | 14.9 (0.4) | 30.45 | 18.7 (0.6) |
ID3 | moist | 24.94 | 12.8 (0.2) | 31.93 | 16.5 (0.5) |
ID4 | xeric | 13.41 | 9.3 (0.1) | 18.16 | 12.4 (0.3) |
ID5 | mesic | 30.65 | 18.8 (0.6) | 20.86 | 14.7 (0.4) |
ID6 | mesic | 27.3 | 15.1 (0.4) | 26.73 | 15.1 (0.4) |
Basal area contribution (m2 ha-1) by each species surveyed in the overstory plots at the two inventory years (1995, 2005).
Species | 1995 | 2005 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
ID1 | ID2 | ID3 | ID4 | ID5 | ID6 | ID1 | ID2 | ID3 | ID4 | ID5 | ID6 | |
|
0.04 | 0.82 | 0.19 | 0.15 | - | - | 0.20 | 0.51 | 0.25 | 0.32 | - | - |
|
- | - | 3.00 | - | 2.23 | - | - | - | 2.72 | - | 2.12 | - |
|
0.05 | 0.04 | 0.65 | 0.02 | - | 0.21 | 0.57 | 0.17 | 0.96 | 0.77 | 0.42 | 0.43 |
|
9.76 | 11.00 | 6.46 | 8.61 | 8.44 | 7.60 | 12.04 | 11.60 | 7.04 | 8.93 | 7.97 | 7.58 |
|
- | 0.00 | - | - | - | - | 0.01 | 0.01 | 0.00 | - | 0.08 | 0.05 |
|
0.01 | 0.00 | 0.04 | 0.02 | 0.05 | - | 0.01 | 0.00 | 0.00 | - | - | - |
|
0.00 | - | 0.01 | - | - | - | - | - | - | - | - | - |
|
- | 0.01 | 0.00 | - | - | 0.14 | - | - | - | - | - | - |
|
- | - | 7.28 | - | - | - | - | - | 12.17 | - | - | - |
|
0.02 | 3.16 | 0.01 | 0.75 | - | 1.29 | 0.10 | 7.85 | 0.00 | 1.37 | - | 0.57 |
|
- | 0.01 | - | - | - | - | 0.07 | - | 0.01 | - | - | 0.00 |
|
- | - | - | - | - | 1.32 | - | - | - | - | - | 2.46 |
|
0.05 | - | 0.03 | - | - | 0.05 | 0.02 | - | - | - | - | - |
|
0.49 | 0.50 | 0.76 | 0.32 | 0.57 | - | 1.07 | 0.58 | 0.84 | 0.37 | 0.71 | - |
|
- | - | 0.03 | - | - | 1.45 | - | - | 0.04 | - | - | 2.51 |
|
- | - | - | - | 18.93 | 12.91 | - | - | - | - | 8.86 | 12.26 |
|
0.85 | 0.56 | - | 1.04 | - | - | 1.04 | 1.13 | 0.59 | 2.49 | - | - |
|
8.22 | 7.35 | 5.89 | 2.29 | 0.44 | 1.31 | 7.86 | 8.57 | 6.37 | 3.61 | 0.64 | 0.79 |
|
- | - | 0.07 | - | - | - | 0.02 | - | 0.16 | - | - | - |
|
0.03 | 0.00 | 0.03 | - | - | - | 0.05 | - | 0.01 | 0.08 | 0.02 | - |
|
0.12 | 0.12 | 0.06 | 0.20 | - | 0.94 | 0.09 | 0.03 | - | 0.23 | - | - |
|
0.01 | - | 0.42 | - | - | 0.08 | 0.19 | - | 0.77 | - | 0.03 | 0.07 |
Person’s
Attribute | Light(L) | Temperature(T) | Soil reaction(R) | Soil nutrient(N) | CombinedEllenberg Index |
---|---|---|---|---|---|
|
0.14 | 0.01 | 0.07 | 0.23* | 0.24** |
|
0.40*** | 0.51*** | 0.36*** | 0.30*** | 0.22* |
|
-0.29*** | -0.17 | 0.13 | 0.10 | 0.01 |
0.22** | 0.33*** | 0.35*** | 0.23* | 0.23* | |
Understory cover | 0.25** | -0.13 | -0.23* | -0.06 | 0.48*** |
|
0.13 | -0.04 | 0.06 | 0.20* | 0.53*** |
Tab. S1 - List of shade and drought tolerance trait values for overstory and understory vascular species.