PROBLEMS ASSOCIATED WITH EXISTING WOODED AREAS IN URBAN PARKS AND RECENT RURAL-URBAN REFORESTING

Jordi Chueca i Abancó.

Head of Parks and Gardens of Terrrassa City ( Spain ). Post-graduate in landscape at the Agricultural College of Barcelona specialist in arboriculture

INTRODUCTION

Many articles have been written about the problems caused by ground compacting on arboreal masses.

We are aware of the effects of earth compacting. There are many studies on the density of the soil and the effect this supposedly has on trees, hut not a lot on the direct effect of the earth compacting process in wooded areas. Because of this lack of information, it occurred to us to check-out the state of those trees affected by the compacting process and compare the results to others which in theory did not suffer this process.

This is the first part of the study which deals with the thickening of the tree-trunk. We are concentrating on those wooded areas which have been in reserve, and their disappearance due to the compacting process. We will cover the characteristics of land on which this type of tree grows, especially with regard to apparent density of the soil. We hope to begin this work after summer with a study of the vegetative evolution of trees inoculated with root fungi.

This conference will concentrate on the most dominant species in parks and woods covered by our study - Pinus sp., and also Cedrus sp. However we will also study the effects of compacting on other trees, especially those of the leafy type. We would like to encourage people from other geographical areas with deciduous wooded areas to carry out similar studies, or to get in contact with us.

THE TREE - A BIOLOGICAL BEING

Trees form part ot living systems which are highly inter-related and any changes in any part of this system, affects the overall group. The compacting process is a major problem because it affects the root system, but also the fungi which live in and off the main roots and which are an essential part of the growing process. Compacting also affects the trees' natural reserves, as less absorption of nutrients will impede starch being synthesised, which will in turn cause a general decrease in growth. This-will be noticeable both in the trunk and the crown, and also in the roots. This could cause premature death of the trees.

What is a tolerable level of compacting ? There is no answer to this question, as there would be as many levels as there are different types of earth, trees and climates, etc.

For the purposes of this study, we located various areas hypothetically affected by Compacting in parks Iying less than 20 km. away from large cities with 4,000,000 inhabitants (parking lots, in rural-urban picnic areas, etc.). We were already aware of certain details relating to these areas, such as the year they were officially opened to the public, or when access was restricted to motor vehicles, etc.

We have also prepared a report on formerly private parks, to measure the effects of increased public use on tree growth. We counted the age rings in a decrement direction, and when a change in the sequence of growth was noticed, we checked against the date the park was opened to the public.

This method has been put into practice basically in the north-east area of Spain, mostly in the Metropolitan Region of Barcelona.

Population

The Metropolitan Region of Barcelona is densely populated with an average 13,07 inhabitants/ha, and some of its areas have a high level of saturation LS = (urban area + area for development/total area) x 100. For example, the figure for Barcelona is 174.32 Inhab/ha and an LS of 98.5% . in 1992, among the 162 municipal districts belonging to the Metropolitan Region of Barcelona, only 5 had more than 10 m2/inhab. of green zone, and not one of the 7 municipal districts with more than 100,000 inhabitants, surpassed 3.21 m2/inhab. which was the area allocated by Barcelona. This situation created the need to put the woods surrounding the city to intensive public use (some of these are classified as Natural Parks).

This increase in the intensity of the use of wooded areas has been detected mostly in the last 25-30 years, and has coincided with the beginning of a massive use of vehicles in our country.

Climate

We made a climatic analysis of Terrasa, a city in the vicinity of the areas studied in the present report. We encountered considerable variations due to the influence of the orography of the area and the proximity of the sea. The mean annual temperature was found to be 14.6º C, and the two extreme temperatures over the last 20 years were 42.6º C in July 1982 and - 9.59º C in January 1985. Over this period temperatures have surpassed 35º C during eleven summer seasons, and have descended lower than - 5º C during four winter seasons.

Mean annual rainfall is 664.2 mm, although the interannual variable is very high - reaching 1.060 mm in 1996, and 469 mm in 1970 which was the driest year. Another important factor to be taken into account is how rainfall figures are dispersed throughout the year - in the particularly rainy year of 1983 - 853.7 mm, the November figure was 456.1 mm. The average number of rainy days is 95, with rainfalls of over 0.1 mm on 78 of these.

The climate is Mediterranean - dry in the summer and winter seasons and rain, although sparse, in the spring. During the period between the end of the summer and the beginning of the autumn, there are intense rainfalls.

Soil

A variety of soil-types are encountered in this area, with clayey and conglomerate types of soil which predominate in the flatter areas, whereas in the higher mountainous areas, we find Palaeozoic and calcareous rocks, and Mesozoic clays.

Vegetation

The Mediterranean Forest is the most typical in this area and would naturally be made up of the following vegetation, depending to a certain extent, on which direction it faces and its altitude: Groves of holm oaks - Quercus ilex, Quercus pubescens, Arbutus unedo, Viburnum tinus, Rhammus alaternus, Ruscus aculeatus, Asparragus acutifolius, Coronilla glauca, Buxus sempervirens, Ilex aquifolium, Rosmarinus officinalis, Clematis vitalva, hedera helix, and others, but due to the exploitation to which it has been submitted, it has been substituted by the Aleppo pine Pinus halepensis, which because it allows more light to filter through, impedes the growth of the species mentioned above, resulting in the appearance of other species such as Erica arborea, Pistacia lentisclus, Euphorbia characias, etc.

OBJECTIVES OF THE STUDY

The general public are making increasing use of the existing green areas Iying in the surrounding areas of the great cities. These areas are mainly woods of the Pinus halepensis type, wooded parklands and parks on the rural-urban fringe. This is causing problems on a medium to long term basis, and we feel this could be remedied with adequate management.

The premature fall of some of these trees prompted us to initiate this study in order to find the cause for this occurrence.

First of all, a series of urban and rural-urban parks were selected, for which we already had the original data which was collected before the parks were created, to enable us to contrast the results obtained from tree inspection.

These data included:

- the year in which parks were opened to the public

- the year in which private parks were created

- the year in which picnic areas were created

- the number of licence plate numbers for vehicles in the Barcelona district.

METHODOLOGY AND WORK HYPOTHESIS

Samples were taken from fallen tree-trunks, and also cores of wood were extracted from trunks from nearby areas which were subjected to different types of everyday public use. The extractions were per,ormed with the aid of a Pressler drill. In this way, we learn about the different values for trees of the same species in nearby areas, but with distinctive soil conditions (covered with vegetation, eroded, compacted, damp, dry, etc.).

In order to make the comparisons, small pieces of the samples were placed with the fi-bres pointing upwards. They were then polished smooth and were examined with the aid of a binocular magnifying glass, making year by year note and contrasting changes in growth with climatological tables for the area. Many of the cores were rejected, es-pecially those which were rotten inside and from which the age of the tree could not be deduced and which we felt would lead us to an erroneous analysis. We were out to confirm that the results we obtained corresponded to our work hypothesis "edaphic problems should be reflected in the wood"

For the sample extraction and subsequent examination processes, those potential pro-blems which could derive from the readings obtained from the rings of wood were taken into account. as follows:

Tree growth is different in areas which are subjected to over-use (trunk and main branch torsion, etc), and also depending on whether it is a leafy or a resinous tree.

Trees grow less in woods which are close to autofelling areas (excessive density of wooded areas). it is not always easy to observP growth rings, and this obliged us to use binocular lenses which magnify betwee 20 and 60 times.

The changes in the growth rat of the rings could be produced by changes which had occurred in the surroun ng areas (partial felling of the forest, disappearance of a building, etch

These changes could also b due to some defect in the tree produced by pathogenic implications.

Diminished growth rates ar indications of periods of drought, intense rainfall or frost.

We performed sample extraction and subsequent analysis bearing in mind the factors mentioned above, avoiding trees with anomalous growth, areas which were too dense, areas close to irrigation channels, etc.

RESULTS

Pinus sp

Although we have spent many years in the study of various parklands, and we have ex-tracted countless samples of wood, at the moment, (May 1997) we are working on soil samples, which aher visual inspection, appear to be compacted. We feel that if we combine these results with those obtained from the wood samples the conclusions obtained could be useful to future administrators and investigators, We have especially chosen forests on the rural-urban fringe, close to major cities, with varying everyday use.

In the first example, we make a comparison between Pinus sp. from various zones, with the soil condition results obtained.

Graph 1: average rats ot growth of each tree being studied divided into two groups (Bt} = trees ina favourable situation, with no apparent edaphic problems M= Trees in an acute earth compacting area ( vehicles, excessive pedestrian use, etc.)

If we observe Graph no.1 we may appreciate the difference which exists between the average annual growth of both trees which suffer the compacting process, and those growing in more favourable areas.

The minimum average growth for trees in the B group is 0.19 cm. with a maximum growth of 0.71 cm. The minimum average growth for those trees in the M group is 0.14 cm. with a maximum growth of only 0.33 cm.

Nevertheless, these data could lead to error, because as they are average figures, they do not clearly reflect the compacting problem, as for instance, a tree With a high ave-rage growth figure may die off because of a compacting process which occurred only ten years earlier.

For the study of the Pinus species, the samples were divided into four groups:

trees under 60 years old, with no apparent compacting of the earth

trees under 60 years old, with apparent compacting

trees over 60 years old, with no apparent compacting of the earth

trees over 60 years old, with apparent compacting

 

Graph 2 : Evolution of trees over the last 50 years in soil with no apparent compacting, covered with grass or rich in Organic Mater.

We have grouped together trees growing in soil with no compacting (Graph 2) and trees on soil with considerable compacting (Graph 3), which pertain to the same area, but which suffer varying everyday use. We have observed a common type of behaviour which began 30 years ago in both groups. This phenomenon has been attributed to the massive invasion of vehicles, which enabled mass access to these areas which before-hand were almost inaccessible, and were only reached after long journeys by train or on foot.

The trees described in graph 2 (soil with no compacting), have maintained a regular growth rate over the last 40 years, and every ten years have grown on an average of 1 to 3.5 cm.

The trees described in graph 3 (compacted soil), had a normal growth rate during the first years of their life, but from the period 1957 to 1967, a progressive decrease in the growth rate is observed, due to the influence of motor Vehicles to which we have made reference above.

In both graphs we may observe that some trees have resumed normal growth over the last ten years (1987 - 1997), and in some cases this is due to the closure of roads to traf-fic in the areas we have studied, and in other cases, to a physiological reaction which is the way some trees announce their forthcoming death.

With a simple glance at the graphs, we may observe how the majority of those trees growing in compacted earth do not grow more than 1 cm/decade, whereas those which grow under more favourable conditions ( graph 2 ), grow from between 1.8 and 3.2 cm/decade.

Graph 3 : Evolution of tree growth over last 50 years in soil with apparent compacting.

Graph 4: Evolution of growth in trees over 60 years old over the last 50 years in soil with no apparent copmpacting, in some cases covered with grass or rich in Organic Mater.

Graph 5 : Evolution of growth in trees over 60 years old over the last 50 years in soil with apparent compacting.

When we observe trees which are over 60 years old, the argument repeats itself; those trees which have lived in optimum conditions, have a faster growth rate (graph 4), than those growing in compacted soil (graph 5). This has no bearing on the age of the tree. We have also noticed a general decrease in growth over the last 30 years.

CONCLUSIONS

a) The compacting process affects all trees in the same way. Any major increases is ground use are reflected in tree growth, whatever the age of the tree.

b) The combination of two factors - climatic conditions and excessive use by the pu-blic - plays an important part in the evolution of the future growth of trees.

c) on many occasions, by simply closing off roads to motor vehicles, an improvement is seen in tree iife-span.

d) By simply observing the exterior aspect of a tree, we get an idea of its real condition. We have made visual observations of a large number of trees before getting down to extracting the core. The following characteristics may be seen at a glance - the absen-ce of needles, scanty foliage, visible collar and roots due to loss of soil. In the latter case new soil is required around the base of the tree to increase stability. These are so-me of the aspects we observe before proceeding to a more detailed analysis 5

e) it should be a prime objective to continue with investigations designed to improve tree life-span and to save those which are in a very weak state.

f) The radicle root systems of those trees growing in compacted soil shrink and are vi-sible on the surface. This shrinking process minimises the trees' capacity to absorb nu-trients and reduces their capacity to form root fungus The loss of the layer of top-soil due to the compacting process and the intense rainfalls typical of the Mediterranean climate, quickly transform the surrounding soil which affect the tree in the following ways:

Loss of good quality top-soil - less nutrients for the tree

Loss of the total area of soil needed by the radiate root system

The radicle root sytem lose its capacity to bear the whole weight of the tree until the latter loses some of its branches and leaves.

These factors have a direct effect on the structural stability of the tree, causing prema-ture falling of branches and sometimes the entire tree.

The longest roots of the trees inspected in this study never reached deeper than 50 cm, for which we advise against the use of machinery used to air the soil which probe deeper than this level. We also strongly advise against ploughing around stressed radicle systems, as this will cause the few there are left to die. Although we plan to forge ahead with our work in this area, meanwhile, to improve soil condition, we recommend successive applications of composted mulch, not more than 10 cm deep, which helps to maintain the little humidity there is, and creates favourable conditions for root fun-gus to develop, factors which in some studies have proved to be of prime importance in tree conservation.

a) Future roads and parking lot planning for future rural-urban parks, should always ta-ke into account the possible influence traffic will have on the existing wooded areas.

b) Composted mulch may be obtained from fallen tree.trunks, and can be of enormous help to mitigate compacting when applied to the radicle root system.

c) The preparation of a easy to use protocol which would aid and improve our studies of wooded areas and allow us to ascertain their general condition, should be one of our aims in the future.

d) The anomalies we have detected in our study would have little importance were it not for the element of danger involved, in the event the tree falls. A tree is dangerous if there are people, buildings, vehicles, park benches, etc., nearby.

WORDS OF GRATITUDE

The present study, although still unfinished, has been possible thanks to the collabora-tion of the Agricultural College of Barcelona, especially Xavier Fabregas i Bargall6, Agriculturist and Professor of Landscape Gardening at this college; also my apprecia-tion goes to my friend Juanjo Martinez Sabate, Biologist and Agricultural Technician, and the ACSA Company, especially Francesc Faus, for his collaboration, and for believing in us and including this study within his R + D programme.

FINAL NOTE

In this article you have not a Cedrus sp. study, if you like more information, please contact wiht the autor.

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