Primary net productivity and biomass quality as drivers of herbivory rate at the regional scale.

ENRICO L.; DÍAZ S.

Chania (Creta), Grecia.

Simposio; 52 Symposium of the International Association for Vegetation Science,; 2009

International Association for Vegetation Science (IAVS)

There has been much discussion on the factors driving herbivory at the broad (regional to trans-regional) scale. There is no consensus about the main factor influencing herbivory rate, i.e. the percentage of ANPP consumed by herbivores (%H). According to some authors, %H depends mainly on ANPP, whereas according to some others, it is driven mostly by the quality of the plant biomass produced. We used a steep regional ANPP gradient in central-western Argentina to test the hypothesis of nested control of herbivory rate by ANPP and biomass quality. That is, ANPP is the main factor influencing %H at the regional scale, but differences in %H between ecosystems of similar ANPP are explained by the characteristics of the dominant plants. We sampled 9 ecosystems, including arid shrublands, grasslands, dry forests and savannahs. In each ecosystem we identified 10 plots subjected to both vertebrate and invertebrate herbivory, and 10 plots excluded from vertebrate herbivores. In each ecosystem, and in plots with and without vertebrate herbivores, we selected the 10 most abundant plant species, estimated their relative cover, and measured 7 leaf traits on them, as indicators of biomass quality: content of carbon (C), nitrogen (N), and phosphorous (P), C:N and N:P ratios, tensile strength (LTS) and specific area (SLA). In order to estimate herbivory rate, we tagged intact fully-expanded young leaves of the 10 dominant plant species, and measured % leaf area removed by vertebrates or invertebrates (%H-V and %H-I, respectively) during a standard period of time. We found no correlation between ANPP and %H either for all ecosystems considered together, or for woody ecosystems. Only in grasslands %H was related to ANPP. At the level of individual plant species, we found significant associations between herbivory rate and some indicators of biomass quality: positive correlations between %H-V and N content, P content and LTS, positive correlation between %H-I and N content, and negative correlations of %H-I with C:N ratio and LTS. Ecosystem-level biomass quality was not significantly correlated with %H, with the exception of P content and %H-V. Our preliminary results therefore do not fully support either the ANPP, biomass quality or nested-control hypotheses, suggesting complex interactions between herbivory controls at the within- and among-ecosystem scales.