had higher % Her and weaker material traits than shade-tolerant species, suggesting that

they were maximizing height growth at the expense of safety and structure.

 As predicted, all species increased their mean E between 1 and 6 mos after leaf

expansion, although not always significantly (Figure 1-1). In contrast, there was no

pattern to the proportional increase in fracture toughness between T and T2 among

species, revealing that species do not necessarily increase toughness and stiffness

proportionally during ontogeny. Thus, for seven out of eight species in which stem fiber

content did not increase from T1 to T2, increases in stiffness and toughness over time

must have been caused by changes in stem anatomy, such as fiber distribution and

packaging, as opposed to increased fiber content (Hoffman et al. 2003), but further

anatomical and histological analyses are necessary.

Leaf Biomechanics

 Mean lamina and midvein toughness varied 30-fold among species, with values

from 71 to 395 J m -2 for laminas and 984 to 3475 J m-2 for midveins. In a study

performed on BCI with leaves from adult trees and understory saplings, Dominy, Lucas

& Wright (2003) reported considerably higher values for lamina and midvein toughness

than reported here. Nevertheless, for the three species used in both studies (A. excelsum,

C. elastic, and A. cruenta), the same ranking prevails: A. excelsum had the lowest

lamina and midvein toughness while A. cruenta had the highest. Although the

relationship was weaker than in stems, leaves of shade-tolerant species had higher

mechanical strength than leaves of shade intolerant species. Potentially, evolutionary

forces favoring selection of other leaf traits, such as photosynthetic capacity, vein

distribution, presence of secondary compounds, and water-use efficiency also influence

differences in leaf toughness among species (Choong et al. 1992, Wright et al. 2004).