The fracture toughness of sun-leaves of 42 tropical tree species growing in Singapore was measured along a ‘least tough’ path using an instrumented cutting technique. Punch-and-die (penetrometer) tests were also conducted. Intercostal material was analyzed for crude fibre and crude protein (total nitrogen concentration × 6.25) and the ratio of the two, the index of sclerophylly, calculated for each species. Leaf anatomy was examined and specific leaf area was calculated. Thirty-seven of the species were from three definable plant communities, namely mangroves, trema belukar (secondary forest on undegraded sites) and adinandra belukar (secondary forest on highly degraded soils).
There was a highly significant positive correlation between fracture toughness and index of sclerophylly for the 42 species (R2= 0.43, P < 0.001) brought about largely by a high correlation between toughness and crude fibre content. Fracture toughness was not correlated with lamina tissue dorsi-ventral thickness but high toughness was clearly associated with the presence of sclerenchyma bundle sheaths and sheath extensions. Analysis of the fracture path for 26 leaves of ten mangrove species showed a high correlation between the area fraction occupied by veins and fracture toughness if one species, Avicennia rumphiana, was excluded. The regression equation for the other nine species gave estimates of the fracture toughness of vein tissue and lamina matrix of 6053 and 327 J m−2 respectively. Punch-and-die test results only correlated well with the cutting tests when values were divided by lamina thickness.
The ratio of protein to fibre in a leaf (the reciprocal of the index of sclerophylly) has been established as a good predictor of leaf palatability and digestibility to generalist chewing folivores. The high correlation between fracture toughness and the index of sclerophylly for a large sample suggests that leaf toughness could be the factor used as a proximate cue for determining food quality in herbivore foraging strategies.
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