WITHIN-YEAR AND AMONG-YEAR VARIATION IN THE LEVELS OF HERBIVORY ON THE FOLIAGE OF TREES FROM A MEXICAN TROPICAL DECIDUOUS FOREST

Discrete (i.e., one-time) measurements of the levels of herbivore damage on the foliage of trees (16 species) were made for three consecutive years in the Chamela dry forest. There was a marked similarity in the overall levels of damage despite the fact that there were considerable climatic differences among years and that some of the species showed at least one significant among-year difference. In one of the three years of study, long-term measurements of damage (i,e., on individually marked leaves) were also made to assess within-year variation (by calculating the races of herbivory early and late in the wet season) and to assess the reliability of che discrete measurements. With this type of measurement the mean leaf area loss of the studied species was 17 percent. This value is higher than the values commonly reported for tropical forests. An outstanding example of defoliation was that of Jatropha standleyi, with 73 percent leaf area eaten. There was a very marked within-season variation in the rates of herbivory: during the early parr of the rainy season the mean race (0.352% leaf area eaten per day) was considerably higher chan lace in the season (0.097). Analyses of leaf nitrogen and water content early and late in the season indicated chat these components of the nutritional quality of the foliage decrease as the season progresses. The discrete measurements of herbivory underestimated the true (long-term) values of leaf damage. A discrepancy ratio long-term/discrete measurement varied considerably among species with an overall mean of 1.99. Thus, in general, discrete measurements underestimated herbivory by half. It appears that che major cause of che underestimates of the discrete measurements is that the leaves of some species are eaten completely and thus missed in a one-rime measurement. Another disadvantage of the discrete measurements is chat information on che timing of damage is lost. This study showed that folivory is nor only concentrated to the season of leaf availability, but chat it occurs within an even more restricted period.