Urban Vegetative Growth Project
Projects / Academic / Urban Vegetative Growth Project
1.1 To investigate the effect of conservation and neglect on plant succession within a variety of Inner London urban areas.
2.1 Relate the extent of neglect and management to the current plant ecology.
2.2 Investigate biological factors that may change with neglect/management and evaluate the effects of these factors on plant succession.
3.1 Neglect has a negative effect on bio-diversity
4.1 One site should give a fair idea of the effect of management on urban plant ecology since managed sites would be initially chosen for their suitability in supporting particular types of vegetation. Neglected urban areas are found with a wide range of characteristics which could affect their potential to support a plant succession. Hence it would give a better idea of the effect of neglect if several sites were chosen with differing urban characteristics. The characteristic most affecting the succession of vegetation is the cover of concrete usually in the form of tarmac for car parking or paving. The choice of neglected sites will hence be greatly affected by the extent and age of concrete cover which relates to degree of neglect. Three neglected sites were identified with one managed which it was at or near its climax.
5.1 The neglected sites were chosen to display different stages of plant succession dictated largely by the age of concrete cover, since vegetation would be killed by the initial laying of the concrete. The age of concrete was estimated by a builder familiar with the appearances of concrete over time. Trying to find the exact age of the concrete proved to be too time consuming and often impossible since the work had been completed so long ago. The managed site could act as a control with which to compare the differing areas of neglect.
The following areas were identified:
1. Neglected site Safeway car park 47- 49 Stamford hill
0 - 5 years since concrete laid - few cracks and structure generally intact. 2.Neglected site Car park between Aldam place and Abney park cemetery
10 - 15 years since concrete laid - fairly frequent cracks.
3. Neglected site Disused part of Leyton industrial village - Hackney marshes
20 - 25 years since concrete laid - very frequent cracks and crumbling concrete.
4. Managed site Springfield park
Managed regularly by park keepers - in close proximity to cafe
5.2 All measurements were taken in the field using the following apparatus:
Sharpened metal rod
5.3 Five sets of measurements were taken from each site and recorded.
The sites were chosen to keep as many other factors as constant as possible e.g. amount of sunlight, extent of surrounding vegetation, slope (if any), accessibility to public.
5.4 The locations for measurements were found by throwing the quadrat over my shoulder while closing my eyes and turning. This will introduce a random element which should give less biased results since the areas of bare concrete are accounted for. A systematic transect approach could prove inaccurate since most vegetation follows crack lines and the transect line would either hit or miss these lines of growth. The nine squares of the quadrant were approximated to 10% each and readings were taken to the nearest 5%. With the aid of a pocket plant guide ( Wild Flower Key - Francis Rose 1981) the percentage cover of all major species present was estimated and recorded.
5.5 The most common species were chosen to represent the effect of neglect since identification of every minor species would be inefficient regarding time consumption. A selection of common species from the apparent stages of succession at each site were chosen as indicator species and organised into category`s relating organism size and family. see Figure 1.1.
Fig 1.1 1. Lichens
3. Herbaceous plants - eg. weeds
4. Grasses, sedges and rushes
6. Fungi and mushrooms
7. Shrubs and scrub - Perennial /inc. Conifers
8. Saplings - evergreen / deciduous
9. Trees - evergreen / deciduous
5.6 By recording the number of organisms in each species in the quadrant the index of diversity can be calculated which would give a better comparison of diversity than the individual figures. Index of diversity (D) can be calculated using:
D = N(N-1) Where N = Total number of all species
n(n-1) and n = Total number of each species
The mean index of diversity can be calculated for each of the four sites:
Mean D = D1 + D2 + D3 + ......... + D10
5.6 Measurements for infiltration rate were taken to compare the amounts of water received and stored by the soil in each location. The infiltration can was placed on the ground in the middle of the measurement location and pushed a small distance into the soil (if possible). 100ml of water was poured into the can and the time taken for it to fully infiltrate, recorded. The infiltration rate was calculated in ml per second. Three measurements were taken from each location at different points within the quadrant. The mean was taken to account for the inevitable differences in infiltration.
5.7 To evaluate the depth of soil available for root structure the sharpened metal rod was pushed into the vegetated cracks in the quadrant. An average of five depths taken per location since the depth of cracks will vary along their length. Where the cracks penetrate the concrete completely the sand and gravel underlay would restrict root growth and impede the metal rod. In the case of the managed site this would not apply since plants would not be restricted to the depth of cracks in concrete.
6.1 Since the results were taken in October many of the less resistant species will have been killed by the cold. Plants were also no longer in flower so there is more room for human error in identifying species. This could lead to an inaccurate representation of the effect of neglect on plant succession. Photos of the site could not be taken until late November so even more of the population will have been killed by frost and covered by fallen leaves.
6.2 Quadrant squares were approximated to 10% each and readings taken to the nearest 10% which could cause some degree of accuracy but since the error in my judgement would amount to at least 5 -10%, it would not seem worthwhile to pursue any further degree of accuracy.
6.3 Measuring infiltration on concrete would seem inaccurate since most water would escape through the gap between the concrete and the bottom of the can rather than infiltrating through cracks. This would however account for the surface runoff on concrete surfaces which would leave little time for rainwater to infiltrate. Since surface runoff is negligible on managed soil except in circumstances of high rainfall the inaccuracy is, in practice, less than expected.
6.4 Infiltration rates would also change between summer and winter due to the ground freezing, or baking hard causing less infiltration and more surface runoff than during the seasons in-between. For a more accurate picture readings could have been taken all year round and an average taken.
6.5 Using a selection of indicator species in category‚??s may not give as good a picture of the succession but the identification of every individual species would be impractical and leave little time for other measurements.
6.5 Further limitations come from factors beyond my control such as the extent of human and /or animal contact with the site and how exposed the site is regarding weather conditions.
7: ANALYSIS AND INTERPRETATION
7.1 Data was collected from four locations;
Site 1. Neglected - concrete laid 0 - 5 yrs
Site 2. Neglected - concrete laid 10 - 15 yrs ago
Site 3. Neglected - concrete laid 20 - 25 yrs ago
Site 4. Managed - parkland
7.2 Data was recorded in the tables shown, figures 1.1 - 1.4?. Graphs and charts were
drawn to highlight any patterns and help identify trends. Figures 90-10325882??
7.3 Vegetation results are the most useful in determining the nature of plant successions in neglected urban areas. Site 1. shows characteristics of a primary succession with a large percentage cover of pioneer species such as lichens and mosses. ( Fig 1.5 & 1.6 ) The concrete will first have been colonised by lichens with extremely high environmental resistance to the exposed and vulnerable positions on bare concrete. As the numbers increase dust and dead organic matter accumulates in cracks forming a primitive soil. Species such as mosses will succeed the lichens with simple root systems in the shallow cracks. ‚??Vegetation alters the structure of the soil by adding organic matter which will hold water and nutrients......and so encourage further succession‚?Ě (Soils, Vegetation, Ecosystems - Greg O`Hare). The increasing accumulation of this modified soil in cracks encourages the growth of species that would previously have needed more water and nutrients. The following succession can be seen to have begun in site 1. with low percentage covers of small herbs and grasses (see fig 1.5 & 1.6 ). These larger species have more extensive root systems which very slowly burrow into cracks and widen them. This enables the accumulation of more soil and encourages the growth of more and larger successors.
7.2 Site 2. shows the climax of the next stage in succession with a wide variety of small plants, grasses and ferns. The index of diversity has increased as suspected ( Fig. 1.9 ) since both the range of species and total cover has increased. The faster growing shrubs and ferns are beginning to emerge but are not yet dominating. Due to the increased ground cover lichens are beginning to have compete with larger species for light and are hence declining. Cracks have now been physically eroded by weathering and root action to the extent that even the large root systems of shrubs can be supported. The root action of these larger plants accelerates the growth of cracks both in depth and width.
7.4 Site 3. shows signs of approaching a climax succession with shrubs and small plants now dominating. The age of the concrete and sustained weathering has led the structure to crumble and decay. Plants can now colonise a much larger area since they are no longer restricted to cracks in the establishment of root systems. This greater freedom regarding root structure has enabled the environment to support more demanding successors such as the saplings which require more water uptake and soil nutrients than the smaller predecessors. It can be seen from the high percentage cover and greater range of species ( Fig 2.5 ) that diversity has increased.
7.5 Site 4 has been managed and can be seen to have reached its climax state. Trees are the overall dominants in terms of canopy cover and root extent. The amount of light reaching the ground limits the smaller species which cannot compete. Trees also require more water and nutrient uptake so the proportion
As can be seen ( Fig. 3.1 ) the rate of colonisation increases as we progress from a primary succession to a managed climax state. Each succession improves the existing conditions making it more favourable for further species to colonise.
At site 1. the infiltration rate is fairly low ( Fig 2.4 ). This is partly due to the smooth nature of the concrete which leaves a very fine gap between the bottom of the can and the surface through which water can escape. Most water must escape through the small cracks ( Fig. 2.7 ) which takes a longer time. The environmental conditions this poses are fairly unfavourable regarding colonisation. Water will take a longer time to infiltrate and hence gather on the surface forming puddles which could drown any vegetation. The increase in overland flow could also wash away colonising vegetation especially on gradients.
The increase in infiltration rate in site 2. ( Fig 2.8 ) is due to the rougher, weathered surface of the concrete allowing more water to pass under the bottom of the can. The increase in the size of the cracks in site 2. ( Fig. 2.6 ) also contributes to this increase since a greater volume of water can flow into and through them into the ground below. Since more water is stored in the larger cracks and overland flow is decreased conditions are more favourable for plant colonisation.
The apparent decrease in infiltration on site 3. ( Fig. 2.8 ) can be explained by the absorbent properties of crumbling concrete which expands and stores moisture. The inceasing contact with the sand and earth under-layer allows more moisture to seep into the ground where much of it is retained. This larger amount of water stored in the soil can support more and larger plantlife which also has more surface area to colonise due to the concrete breakdown (Fig. 1.2 ). In the case of extreme rain however the ground could become waterlogged which may discourage less resistant forms of vegetation from the site.
Site 4. has been managed which in the case of the chosen site involves the occasional scattering of dead leaf litter from paths, to recycle the organic nutrients. This greatly improves the soil structure and ensures that too much water is not stored. Since conditions are more favourable the site can sustain much more demanding and dominant plantlife such as large trees. The infiltration rate at this site should be optimum and can be used as a control with which to compare the others.