Q1. WHY IS IT NECESSARY TO MATCH VEGETATION SURVEY TECHNIQUE TO HABITAT TYPE?
Studies in vegetation science involve collection of wide range of field data to suit specific management or research purposes. Plant assemblages growing in different habitats vary in their species composition, structural, functional and physiognomic characteristics (Chapman, 1976). In terms of physiognomy, vegetation exhibit differences in height, colour, shape, leaf size and growth form as described by earlier work of Du Rietz (1931) and Raunkier(1934). Based on the Raunkier's life form classification, plants are divided into phanerophytes, chamaephytes, hemicryptophytes, cryptophytes and therophytes.
Measuring biodiversity indices such as species richness or species diversity and other morphological investigations of Leaf Area Index (LAI), cover etc, requires the application of different survey techniques depending on the nature of vegetation and habitat type. Thus different approaches are needed to survey tropical or temperate grasslands, rainforests, heathlands or woodlands. In tropical rainforest for example, the vegetation is composed of tall, dense and stratified broad-leaved evergreen trees with epiphytes and climbers, while woodland is made up of tall to medium sized trees with small to needle-leaved canopy. As such methods of selecting sampling site, plot size, quadrat size as well as sampling pattern could vary.
During a vegetation survey a sample plot size must be determined, and this may vary from few meters to several hectares depending on the type of vegetation. Hence the question of minimal area concept which is defined by Westhoff and Van der Maarel (1978) as a "representative area or a sample of species of regular occurrence, and which is related to the total number of species in the stand" must be addressed. Therefore the area to be sampled must adequately represent the species composition of that community, and should also be large enough to represent the species abundance in that particular habitat.
Different minimal area values are suggested by ecologists like Rodwell et al., (2002), Westhoff and Van der Maarel(1978), Knapp(1984) for various range of species. Epiphytes, moss and lichen communities should have the minimal area of 0.1-0.4sq meter, 4-10 for open dune grasslands, 20-50 for heathlands,40-100 for woodlands and temperate scrubland, and 4000-10,000 for tropical rainforest communities.
Furthermore, different vegetation habitats require different quadrat sizes necessary for sampling techniques in vegetation survey. Quadrats are normally square shaped, but it can be circular, rectangular depending on the purpose of the investigation. Within a vegetation community different patterns are recognised which could be random, clustered or dispersed and this determine the size of quadrat to be used. For example, bryophytes and lichens communities should be sampled using a 0.5m0.5m, 1m1m-2m2m for grasslands and dwarf heath, 2m2m- 4m4m for shrubby heath, tall herbs and grassland, 10m10m for scrub, woodland shrubs, and 20m20m -50m50m or the use of plotless sampling for woodland canopies (Kent and Coker, 1992). Sampling design could be random, stratified, systematic or plotless which could be chosen to match up with the variation in species composition across various habitat types.
In conclusion, the purpose of the survey and the nature of habitat determine the methodology to be employed for a successful vegetation description and analysis.
Q2. TECHNIQUES IN SURVEYING TROPICAL RAINFOREST VEGETATION
There has been a great interest among ecologists to study the patterns and species composition and diversity in humid tropical forests (Gentry 1988; Wright 1992). Forest trees exhibit high degree of diversity and could easily be located identified and counted. This habitat is characterised by having about 80% of the world's biodiversity, with a rich flora arranged in layers or strata extending from 50-85m in height. It is dominated by broad-leaved evergreen trees; taller trees or emergents occupy the highest strata while epiphytes mosses and lichens are found at the forest floor or lower strata. Different techniques such as plotless sampling, and the use of nested quadrats could be used for measuring species diversity in this vegetation type.
An empirical study was conducted in the tropical rainforest in Panama in order to determine changes in species composition and diversity along a chronosequence by S.J Dewalt et al, (2002). The study area comprises of emergent trees with approximately 50m height of predominantly leguminosae and Bombacacea species.
The methodology involve the selection of eight stands of samples to represent replicates of secondary forest ranging from 20-100 years and another two stands to represent older trees. Sampling was done by selecting species contained in a nested quadrat of 160m10m transect. Also measurement of diameter at breast height (dbh) was done in each transect for the 16 contiguous quadrats. Understorey flora was also counted which include herbs, and small shrubs of = 1m tall and <5cm dbh) in 32 quadrats. Smaller plants like lianas of =0.5cm in diameter were also measured. The data was analysed using different procedures among which is the Importance Values Index (I.V) in order to determine species richness, sizes and presence along different chronological order. Calculation of I.V was done by dividing the total number of individual species by the total number of individual trees in the stand. Species dominance was also measured by dividing the total basal area of the species by the total basal area of all the individuals in the stand.
Similar technique was also used by Ojo (2004) in his study of Omo forest Reserve in Nigeria. In this, study 16 plots of 1hectare each were enumerated. Measurements of diameter at breast height(dbh) was taken across different species hierarchies, including small trees of about 5-20cm dbh measured at 12.5% of the sample plot, medium trees 20-40cm dbh measured at 25% of the plot, and large trees measured >40cm dbh at 100% of the plots. Simpson diversity Index (I) was used to calculate species diversity and density. Sorensen Similarity Index (SI) was also used to measure species composition during the survey period.
Q3. DECIDUOUS BROADLEAF WOODLAND VEGETATION SURVEY TECHNIQUES
Woodlands are open stands of trees of approximately 5m tall with most of their crowns touching each other. Their canopies cover about 30% of the surface area in which they grow, sometimes grasses can be present in this habitat. According to the classification of trees by Sykes and Lane (1996), four different groups are recognised in woodlands namely: Dominant trees with high crown and full reception of sunlight, sub-dominant, intermediate and suppressed trees.
The most appropriate method of measuring species diversity in woodland vegetation type is the use of 10m10m quadrat on a 100100m grid. In a survey of southern British woodland, Michael D. Morecroft et al., (2008) conducted a survey in the year 1993 and 1994 in order to record and determine the species distribution of Sycamore species among the native woodlands species. A random selection of 41 out of 294 sample plots of 1010m each on a 100m grid was done for continuous monitoring. A measurement of 10 different trees was carried out in order to determine their sizes and heights. Diameter at breast height (dbh) was measured using a diameter tape, while a hypsometer was used to measure trees' heights. Smaller vegetation species growing in the woods e.g shrubs and younger trees with diameter at breast heights of less than 0.5cm were also counted in 10 out of the sampled plots using a 400400mm quadrat.
Similarly, a 1010m quadrat on a 100100m grid was used to determine the changes in species composition of Wytham woods in south England from 1974-2002. Kirkby (2004) used a tape which was laid diagonally across the plot to estimate the percentage vegetation cover of different tree strata. Thus trees with canopy cover >2.5m high, shrubs cover of 0.5-2.5m high and the plants with lower cover < 0.5m high among different species can be measured.
Q4. GRASSLAND VEGETATION SURVEY TECHNIQUES
Chalk heath grassland habitat can be described as a species rich vegetation containing species such as common rock rose (Helianthemum numullarium), meadow oat grass (Avenuli pratense) etc. This vegetation is found in glacial gravels, sand, clay and ground-up chalk environments classified as U1, H1 and CG2 by the National Vegetation Classification (NVC).
Several methods for monitoring and measuring grassland vegetation has been developed such as the one suggested by Smith et al (1985). The use of nested sampling design has been developed to estimate species abundance and diversity in grasslands, because it enable simultaneous recording of species at different scales with high degree of precision. Schmida, 1984; Stohlgren et al., 1995 suggested the use of a sub-units within a fixed unit in order to measure species richness.
Critchley et al., 1998, conducted a study using a rectangular plot as a fixed unit consisting of 32 square sub-units in an 84 grid. This plot consists of sub-units called nests which are small cells of different sizes. Number of nests and cell sizes should be kept constant across different plot sizes. Counting and recording of plant species is done starting from the smallest cell to the largest cells. The process of counting is done systematically by recording a species only once, and any subsequent entry is done by recording the number of cell in which it first appeared.
The nested system provides an excellent technique in measuring grassland vegetation because of its ability to utilize an optimum scale at which measurement can be done. Although the major weakness of the use of this method is sometimes result in overestimation of values which its accuracy depends on the first recorded set values at the initial count, it became widely applied in Environmentally Sensitive Area's monitoring programmes in England and Wales(Critchley, 1997).
Permanent plot technique could also be used in the study of grasslands. Baba W. (2004) used this method to study changes in species composition in xerothermic grassland in Poland. 90 permanent plots of 1m size each was divided into 3 groups. Out of these, 30 plots were placed in 520m grassland patches dominated by carex and festuca rupicola left without management. Another 30 plots were placed in 3 different patches of grassland dominated by festuca rupicola, brachypodium pinnatum species which have been under management. The last 30 plots were placed in 3 different patches of grasses and shrubs. Using a scaled wooden frame he was able to determine the species cover in each plot for the 4-year study period.
Shannon-Wiener formula was used to calculate the species diversity in each plot in order to determine changes in species composition in both managed and unmanaged plots.
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