Relative Vegetation Cover 



Vegetation cover after trampling is expressed as a 

 proportion of the initial vegetation cover, with a correc- 

 tion factor (of) to account for changes occurring on the 

 control plots at the same time. Cover is based on the 

 sum of the coverages of all species, rather than a single 

 estimate of vegetation cover. It is calculated by (1) 

 summing the covers of all individual species to obtain 

 total cover and (2) calculating relative cover (RC) as: 



_ surviving cover on trampled subplots ^.^ 100% 

 initial cover on trampled subplots 



where 



_ initial cover on control subplots 

 surviving cover on control subplots 



Relative cover after trampling and after 1 year of re- 

 covery v^^as calculated for each trampling treatment. 



Relative Height 



Vegetation height data are also adjusted for changes 

 on controls. Relative height is calculated by (1) sum- 

 ming the heights and dividing the sum by the num- 

 ber of values greater than zero and (2) substituting 

 these mean height values for the cover values in the 

 formula for relative cover given above. Both relative 

 height after trampling and after 1 year of recovery 

 were calculated. 



Species Richness 



Richness is the number of different species occur- 

 ring on the two subplots in each lane. Means are pre- 

 sented, before and after trampling and after recovery, 

 for trampling intensities from to 500 or 700 passes. 



Species Composition 



Changes in composition are described by calculating 

 the floristic similarity of lanes before and after tram- 

 pling and after recovery. Sorensen's similarity indices, 

 based on cover, were used (Mueller-Dombois and 

 Ellenberg 1974). Similarity indices were calculated, 

 comparing conditions before trampling wdth (1) condi- 

 tions after trampling and (2) conditions 1 year later. 



Durability Indices 



Relative vegetation cover data were used to charac- 

 terize the relative durability of the vegetation types. 

 The durability of any vegetation type subjected to 

 trampling is affected both by its ability to resist dis- 

 turbance and its ability to recover. The terminology 

 for these different properties has not been consistent. 

 I use resistance when referring to the ability of a veg- 

 etation type to resist change when subjected to tram- 

 pling (Kelly and Harwell 1990; Kuss and Hall 1991; 



Sun and Liddle 1991); others have referred to this 

 property as inertia (Grime 1979; Orians 1975; 

 Westman 1978). I use resilience when referring to 

 the ability of a vegetation type to recover following 

 trampling (Grime 1979; Kelly and Harwell 1990; Kuss 

 and Hall 1991); others have referred to this property 

 as elasticity (Orians 1975; Westman 1978) or recov- 

 ery (Sun and Liddle 1991). I use tolerance when re- 

 ferring to the ability of a vegetation type to both re- 

 sist and recover from distxirbances such as trampUng. 

 An index of tolerance provides an overall indication 

 of vegetation durability. It does not indicate, however, 

 whether tolerance results from an ability to resist 

 damage, an ability to recover rapidly, or both. 



Indices of resistance, resihence, and tolerance were 

 developed. The index of resistance was the mean 

 expected relative vegetation cover after trampling, 

 for all possible levels of trampling between and 500 

 passes. Although only five trampling treatments were 

 applied, the responses define a curve of expected rela- 

 tive cover values between and 500 passes. The mean 

 of all these expected cover values (Y axis) provides an 

 index with a number of desirable attributes. It utiMzes 

 all the data collected; it provides a single index of re- 

 sponse to the range of treatments from to 500 passes; 

 it is weighted to account for the lack of a regular pro- 

 gression of tramphng intensities; and it remains rela- 

 tively constant regardless of trampUng intensity. This 

 mean relative cover value is equal to the proportional 

 area below curves that relate trampling intensity to 

 relative cover after trampling. It can be estimated by 

 calculating the area of a series of rectangles under- 

 neath the curve and dividing that area by the total 

 area of the graph. 



A similar index of tolerance is the mean expected 

 relative cover after 1 year of recovery, for aU levels 

 of trampling between and 500 passes. An index of 

 resilience can be obtained by (1) subtracting the mean 

 relative cover after trampling from the mean relative 

 cover aft«r 1 year of recovery (this provides a measure 

 of how much recovery occurred over the year) and (2) 

 dividing this by 100 percent minus relative cover after 

 trampling (this is the amount of recovery that could 

 possibly have occurred). 



Similar indices were derived using the relative 

 height data. These help quantify the ability of each 

 vegetation type to resist being flattened by trampling 

 and to recover its height afterward. 



Individual Species Responses 



For the most abundant individual species we cal- 

 culated relative cover. Calculations were identical 

 to those for total vegetation cover. For these species, 

 we also caloilated indices of resistance, resilience, and 

 tolerance. The responses of most species, however, 

 could only be described in relative terms. 



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