Tuatara: Volume 21, Issue 3, April 1975
A Method for Permanent Transects in Vegetation
A Method for Permanent Transects in Vegetation
A simple method is described for siting and recording permanent vegetation transects. Based on standard methods, it has the advantages that in forest plants can be relocated quickly and there is no need to set out boundary strings which are time-consuming. When necessary the recording of a transect can be effected by one observer.
Recording vegetation in permanently marked areas at suitable time intervals is the most direct method of measuring rates and trends of succession. As greater emphasis is placed on maintaining vegetation reserves in satisfactory condition, there will be an increasing demand for information on succession. The procedure outlined here has been in use for some years by the writer and has proved suitable for many different types of vegetation. It is modified from standard techniques and may have advantages in ease of use, particularly when only one recorder is available.
Siting of Transects
The siting of permanent transects usually depends on a subjective judgement influenced by the aims of the study. Two common objectives can be mentioned: 1. To record vegetation changes occuring at a particular site, e.g. a boundary between scrub and tussockland. The vegetation need not be typical of the surrounding area and the transect may include more than one type of vegetation. 2. To record the changes occurring within a particular type of vegetation. In this case the transects should be located in stands representative of the vegetation type and hence a vegetation survey is a necessary pre-requisite.page 82
Within a single type of vegetation much of the variation in rates and trends of succession will be associated with differences in site, particularly slope and position on the slope relative to ridge crests and valley bottoms. Thus some thought should be given to the site conditions of each transect so that future comparisons between similar sites in different vegetations, or contrasting sites in the same vegetation, can be made and the maximum amount of information gained.
Transects are orientated with the long axes parallel with the slope. On some steep slopes it may be easier to record a transect lying at right angles to the slope but in illustrating such a transect, features of growth form related to the slope may be lost.
Shape and Dimensions of Transects
Belt (rectangular) transects are faster to record and often give more information about the vegetation than do square quadrats of the same area (Clapham 1932). In the method described here, transect lengths of 20, 30 or 40 m are used according to the vegetation height, 30 m or more being most useful in forest. A width of 4 m is most satisfactory in forest and this can be reduced to 1 m or less in grassland. However, when a change from grassland to forest is anticipated, a standardised 4 m wide transect of fixed length will facilitate comparisons at a later date. The time needed for recording increases greatly with widths exceeding 4 m and furthermore the recorder is forced to enter the transect more frequently.
A graduated tape is stretched taut between two permanent marker posts (e.g. aluminium angle standards, waratah steel standards, galvanised iron pipes, tanalised pine posts) protruding 0.8 to 1 m above ground level. Unless in remote areas, the marker posts are best left unpainted so that they do not attract attention. The tape marks the centre-line of the transect and from it, offset distances to plants within each of the 2 m strips either side of the tape can be measured using a small measuring tape or, for greater speed, a 2 m long graduated pole or rod. Each plant recorded is identified and located according to two co-ordinates — distance along the centre-line tape and offset distance from the tape. This information can be plotted directly on a transect chart or transferred to a chart later.
Charting of low-growing vegetation within a 0.5 or 1 m wide transect is simplified by using graduated tapes along both sides of the transect. Vegetation boundaries can then be plotted within successive segments of the transect framed by two graduated rods or laths spanning the transect and of length equal to the transect width. For some plants the point of emergence of the main stem can be indicated; this is essential if the vegetation is to be followed through to the forest stage when charting of plant crowns will no longer be practical.
Recording of Plants in Transects
Trees and shrubs are classified into size classes as follows:
|Seedlings (sd)||0.05 - 0.3 m ht|
|Short saplings (ssp)||0.3 - 2.0 m ht|
|Tall saplings (tsp)||> 2 m ht and < 10 cm dbh|
|Trees||> 10 cm dbh|
The positions of trees and tall saplings only are located unless the position of a smaller plant is considered important. Of the tall saplings, only those plants having a diameter at breast height (dbh) > 2 cm are measured, breast height being taken as 1.4 m. The stem measurement made in the field is circumference at breast height (cbh) which can be converted to a dbh measurement subsequently if a circumference-diameter tape is not used. Sometimes it is of value to measure the cbh of trees or saplings immediately outside the transect where these can be re-located in the future.
Where major branching occurs below 1.4 m above ground level, the plant is treated as multiple-stemmed. In such plants identification and measurement of all stems increases the amount of growth data that can be gained and allows the growth of different parts of the same tree to be compared. With multiple-stemmed plants on sloping ground, the stem growing furthest up the slope is measured first and the remaining stems then measured in clockwise order. The stems of combines, e.g. rata + rimu, are measured separately whenever this is possible. Prostrate or leaning trunks are measured at a distance of 1.4 m from a point level with the centre of the trunk at its base. Dead trunks or limbs are not measured but their numbers, positions and sometimes condition are recorded.
|+ …||few, < 5 individuals|
|m …||many, 5 to 19 individuals|
|a …||abundant, 20 or more individuals|
|la …||locally abundant, groups of 20 or more individuals|