Vertical structure of herbaceous canopies: the importance of plant growth-form and species-specific traits

Abstract

The factors determining herbaceous canopy architecture are poorlyunderstood, especially in natural and semi-natural plant communities. Inthis study, we tested three main hypotheses: (1) the structure of herbaceouscanopies can be explained by the vertical distribution of functional groupsdefined by leaf width and the presence/absence of leaves on upright stem;(2) the degree of canopy stratification is greater in habitats that experiencelower spatial heterogeneity in the supply of light (i.e., grasslands as opposedto forest herb layers); and (3) there is significant variation among specieswithin a growth-form, with respect to their vertical position in thecanopy. We used plant foliage height distribution data from 14 grassland and 13forest herbaceous communities to test these hypotheses. A general linear mixedmodel was applied to specify the proportions of total variance in the foliageheight, accounted for by the fixed effects of plants' basicgrowth-form properties (growth-form) and community type(forest/grassland), and by the random effects of sampling site, samplingpoint, and individual species. We were also interested in the correlation ofthedegree of the stratification with various community characteristics(productivity, other canopy properties, species richness, variation ofspecies' traits) and light availability. There was some evidence ofoverall canopy stratification according toplant growth-form, since plants with leafy stem were locatedsignificantly higher. However, such a pattern of two more or less distinctlayers (grasses + upright forbs and rosette forbs) occurred withconsistency only in grasslands (greater homogeneity in light). Thebetween-species variation within a growth-form was a highlysignificant predictor of canopy vertical structure in the 27 communities. Theproportion of total observed variance, explainable throughspecies-specific effects, was comparable to that caused bybetween-site differences. The effect of community horizontal pattern wasless obvious, but still significant. The site by site analysis revealed thatthe degree to which horizontalpatchiness explained variation in vertical canopy structure was negativelyrelated to the relative importance of species-specific effects, showingthat small between-species differences lead to a more obviouswithin-community horizontal pattern, and vice versa. The upper bound ofthe degree of foliage stratification, according to growth-form, wasrelated to the variability of species light requirements and to relative (tocommunity pool size) richness, indicating that certain aspects of canopyarchitecture might be explained through community species composition anddiversity pattern.