-A natural exceptional ecosystem -
By Mia Marzi
"The public green areas in Venice's islands are not very extensive but they are most important for the life of a city which is called upon to cope with increasing pollution and yet which cannot expand its green areas.
By "public green areas" we mean the gardens and parks that are open to the public. They are not all of Venice's green heritage, but make up its largest concentrations and are thus considered the most significant and influential.
The survey of the current status of the gardens and parks of Venice's islands aims to capture in numbers the current tree species, and understand how plant life has adapted to living in the lagoon.
The public gardens in and around Venice that we have considered (Fig. 1) are owned by the administrations of the Venice Municipality (the Pinewood of S. Elena island, the Napoleonic and Biennale Gardens, the Groggia Gardens, and the Papadopoli Gardens), the Venice Province (Savorgnan Gardens) and the State, with concession for use by the Municipality (the Royal Gardens); their overall surface area is about 120,500 m2 divided as follows:
A brief analysis of the environmental factors is useful for defining the area where the plants grow and which conditions them.
Venice is on the sea, at latitudine 45° 27'N. Its main island is 7 km2: it is surrounded by its lagoon, which covers bout 550 km2 and is enclosed seawards by a coastal strip that stretches from the mouths of the Rivers Piave and Brenta.
In ancient times the city had areas of greenery and trees; these were found especially in the Dorsoduro district, where "... it the earliest centuries, the pirates infesting the lagoons easily took cover there" (Paoletti, 1839). Later, due to the need to survive the continuous barbaric raids, the inhabitants had to grow their own fruit and vegetables, thus changing the natural vegetation: "... The irregular vegetable gardens and vineyards scattered throughout the islands, were converted to growing crops. Every dwelling had its own vegetable patch; ..." (Paoletti, 1839); and then later: "The vegetable plots were converted into lovely gardens and the tart odours of garlic and onions were replaced by the scents of cedar, cypress, juniper and laurel ..." (Mutinelli, 1851). Thus, the gardens were enriched with both local species and ones from far afield, imported from the Orient from the Crusades, and the Navagero (former owner and ambassador to the Republic) "... soon went to that entrancing retreat where, with learned men, he conversed or even delved into the secrets of wonderful Nature: the trees, the grasses and flowers, diligently observing and increasingly enriching his curiosity with useful knowledge" (Mutinelli, 1851).
Generally, lagoons have similar origins and evolutions, being formed by the simultaneous action of rivers and the sea. The silts carried down by rivers are deposited, the sea receives and stratifies them and the wind and tides cause dunes to build up which delimits a wetland lagoon protected from the sea.
After this phase, the lagoon changes, becoming divided up into ponds and later, into marshlands; then, due to the continuous build-up of silty material, the lagoon bed emerges to form islands that are colonized by a vegetation which, in later stages become hygrophilous woods.
Not only the sands and silts but above all the vegetation and fauna are responsible for this latter evolution. These produce a large amounts of organic material that is constantly returned back into the ecosystem; if not taken out to the sea, this material contributes to a eutrophication of the lagoon, and this quickly turns into landfill. Among all the lagoons, the Venice lagoon is unique for its characteristics because, through the interventions of Venetians, its conditions of balance are comparable to those of a few centuries ago. They diverted the rivers which brought the most sediments and made them flow into the sea outside of the lagoon. Thus was stopped the normal evolution of this environment (Lorenzoni, 1974).
However, in recent times an indiscriminate exploitation of natural resources has taken place, causing a major increase in pollution (of urban, industrial, agricultural and other origin), exacerbated by the slow exchange of water among the semi-enclosed basins. Hence, substantial alterations are currently taking place to the lagoon's balance biological clearly impacting on the entire ecosystem.
The lands that form within a lagoon are mainly made up of fine sands and silts (Favero, Alberotanza & Serandrei Barbero, 1973). During works to buildt foundations, the existence was verified of sandy silt and sand banks. In some areas of the city, below 90 cm, a salty clay is found that tree roots avoid. As to the chemical composition of the soil, other studies highlight the high proportion of insoluble residue and limestone (25.93% calcium carbonate and 6.18% magnesium carbonate), which makes the edaphic (soil) environment saline. Also, there is a lack of organic matter (0.75%), which is important for most tree species develop.
The processing of temperature and rainfall figures over the twenty-year period of 1956-75 shows that the climate had a mean annual temperature of 13.8°C, with an excursion ranging from 3.4°C in January to 23.3°C in July (average monthly temperatures).
The average annual rainfall in that same period was 832 mm, with the highest fall of 98 mm in November and the lowest in January of 51 mm (1).
Another important climatic factor is the winds coming from the sea (the cold Bora from N-NE and the warm Sirocco from the SE) which negatively affect the most exposed areas such as the pinewood of S. Elena.
On Venice's historic main island, the area devoted to public parks is about 0.14 km2 (140,000 m2) and accounts for about 2% of that island.
(1) The values ere calculated on the basis of information taken from Hydrographic Annals (1956-1975) of the Hydrographic Office of the Venice Water Authority.
The green areas
The total extent of public parks is 120,500 m2. Adding about another 20,000 m2 between trees and small marginal gardens, Venice has a maximum of 140,000 m2 small (0.14 km2) of wooded or green areas.
Now, if we think that on 31 December 1984 the population of the main island was 87,936 inhabitants, each only had 1,59 m2 of green available!
Comparing this with other cities (Rome, Milan and Naples with their modest 2 m2/hab., Amsterdam with 10-12 m2/hab., London with 10-40 m2/hab., and Stockholm with 15-20 m2/hab.), the lack of greenery is even more striking.
On the other hand, for a variety of reasons, Venice does have real difficulties is creating parks: the thick clustering of housing, the inability to expand the green areas, and town-planning and heritage-preservation constraints.
In the last century, Venice's green areas held their ground; there were some reductions in private gardens, whether for new building or for expanding the public areas. The tree population was also largely unchanged although very little is known about the new plantings.
The current status of the gardens
The survey completed in 1982 made it possible to study the flora make-up of the gardens; the current status (1986) mostly corresponds to the study undertaken, except for the Savorgnan Garden, which has undergone significant changes since the census year and for which this survey has a particular historical significance.
Overall, the number of tree species exceeds that of shrub species (85 species out of a total of 143) whereas the number of shrubs is much higher than that of trees; shrubs also make up hedges and bushes, which are difficult to quantify so that the plans limit their scope to indicating the position of the trees only.
The study also has been able to highlight the predominantly exotic origin of all the plant species.
To give an overall view of the numbers and qualities of flora found in the gardens, we have compiled a summary table of the species surveyed in each garden. The trees and shrubs are listed alphabetically and beside them their numbers in each area is given.
The indigenous species have been identified using
FENAROLI & GAMBI (1976), FIORI (1923-29), KOHLHAUPT (1980) and ZANGHERI (1976).
For the exotic species, reference was made to HOOKER & JACKSON (1893).
The location of plantings is secondary to the choice of species, although the former do have importance if one wants to take advantage of the plays on perspective that are possible from planned planting.
The breath and depth of the visual field, studied by JOHNSON (1974), can also provide useful insights for correctly designing a garden. A right proportion should be found between the height of the trees and the distance between them, so that those entering the garden feel they are immersed in greenery. In the Groggia gardens, had the tallest trees been planted near the entrance and the shorter ones nearer the background, the impression would be of the boundaries being father away. Instead, by planting the taller species around the perimeter, the perspective is flattened.
Colours are also important for increasing the perspective effect: we know that warm colours (yellow and red) bring things closer while the cool shades (blue and grey) move them farther away. Plus, to create a good perspective we must have the foresight to put the right distance between the observer and the plants in order for the optical illusion to happen.
To obtain a play between light and dark, the foreground should have plants with a characteristic and compact shape (birch, olive); plants with an imposing and compact form (sycamore, oak, fir) are better behind them because they act as the background yet do not block the view.
Plants should be arranged close to each other as far as is possible for trees love each others' company; account may also be taken of the gardeners' "axiom" of always planting an uneven number of trees of the same species, unless an avenue is being created. A good practice is also to give precedence to a single form, using the others as a basis of comparison. Indeed, too much specimen variation creates a sense of disorientation and uncertainty.
Whether spontaneous or cultivated, the trees which inhabit our land and which have multiple functions (ecological, economic and landscaping) are not always considered and respected as is necessary.
Sometimes human intervention - albeit out of a love for nature, like when creating "green" areas in urban zone - forces nature to the point that even the most basic rules of plant ecology are overlooked. It is economically more viable to cultivate native species which, being already adapted to the typical environment, cost less in maintenance and treatment. Exotic species are instead more prone to attack by pests, and suffer earlier as the environmental conditions are not properly suited to their needs. Thus they may also more quickly lose their attractiveness.
With all the above findings and considerations, we think it useful to consider not so much reducing the exotic species but to give more consideration to the native ones. Exotic plants are mainly decorative but when choosing them, it is important to consider the environments they come from and where they vegetate. This basic concept is especially important if the introduction of new species is linked to specific problems, as in the case of reforestation.
There is also more need for sensitivity towards preserving Italy's tree population, as there are excellent reasons for enhancing their value rather than for neglecting the great natural heritage that Italy fortunately has.
In terms of its town planning and geographic location (the artificial Marghera industrial area), Venice should safeguard its botanical-forest heritage; although not very extensive, it acquires value not only for the "exchange" of gaseous pollutants, but mostly aesthetic, since these small and scattered areas are the only option for 'greenery', understood as 'urban integration', in such a particular city as Venice.