The role of leaf traits and bird-mediated insect predation on patterns of herbivory in a semiarid environment in central Mexico

<p class="p1"><strong>Background.</strong> In arid environments plants face aridity and herbivory, therefore it has been proposed that both are convergent selective forces. However the drivers of insect herbivory in these ecosystems remain poorly understood.</p><p class="p1"><strong>Question.</strong> Does insect herbivory vary in two plant associations subject to different levels of aridity? To what extent differences in herbivory are determined either by foliar traits, or predation by birds?</p><p class="p1"><strong>Study species</strong>. Citharexylum tetramerum, Viguiera pinnatilobata, and Solanum tridynamum.</p><p class="p1"><strong>Study site</strong>. We compared two-plant associations: the mezquital with a closed canopy and wettest conditions and the tetechera a dry place with an open canopy at the Zapotitlán Valley, México. </p><p class="p1"><strong>Methods</strong>. We evaluated leaf traits (N, C, water content, leaf strength and trichomes), herbivore insects abundance and the effects on herbivory when some predators are excluded.</p><p class="p1"><strong>Results</strong>. Herbivory was higher in the drier site (tetechera) than in the more humid one (mezquital) in one year but not in the second one. In both plant associations herbivory increased when predators of herbivores were excluded. Plants in mezquital had more water and nitrogen content than plants in tetechera. The later had higher carbon concentration and carbon:nitrogen ratio, leaf strength and density of trichomes. Abundance of herbivore insects and insect predation were higher in the most arid site. </p><p class="p1"><strong>Conclusions</strong>. Our results show that insect herbivory increases in the most arid site and that predation by birds plays a role as a driver of herbivory, while resistance to aridity seems to be the main driver of leaf structural characteristics.</p>

Leaf strength studies of pasture grasses II. Strength, cellulose content and sclerenchyma tissue proportions of eight grasses grown as single plants

1. Leaf strengths and cellulose contents of eight grasses grown as single spaced plants were compared over a period of 14 months. From some of the samples the proportion of sclerenchyma tissue in the leaves was compared with leaf strength.2. The leaf strengths of individual plants were measured and the strengths of the six strongest and six weakest plants in each species compared with their cellulose contents.3. There were significant differences between species, between seasons, and between regrowth ages within seasons in both leaf strength and cellulose content. There were also significant species × regrowth age interactions, particularly in leaf strength.

Leaf strength studies of pasture grasses: I. Apparatus, techniques and some factors affecting leaf strength

Leaf tensile-strength measurements have been made at Grasslands Division, D.S.I.R., over several years on a number of pasture grasses (Evans, 1964; Wilson, 1965). These measurements were prompted by the work of Beaumont, Stitt & Snell (1933) and Kneebone (1961), who attempted to relate leaf tensile strength to animal production value, and Edmond (1960, 1964), who considered that the toughness of pasture species may be a factor determining their tolerance to animal treading. The following is a description of the apparatus developed for this work and of the techniques employed.

Nutritive value and the genetic relationships of cellulose content and leaf tensile strength in Lolium

1. The leaf tensile strength of individual plants of a population of Lolium (multiflorum x perenne) perenne was measured on four occasions, October and December 1963 (160 plants) and January and March 1964 (73 of the 160 plants). All plants had been chosen for the good uniform vegetative growth they had exhibited over several seasons. The cellulose content of the 14 weakest and 14 strongest plants in October and of all 73 plants in March was determined.2. There was a wide range in leaf strength, maintained from October to March, within the population. In October all 14 weakest plants had lower cellulose content than the 14 strongest and from these two groups six plants of low leaf strength and low cellulose content and six of high leaf strength and high cellulose content were selected as parent plants for divergent selections. In March there was a positive highly significant (P < 0.001) phenotypic correlation between leaf strength and cellulose content (r = +0.58).3. Two diallel crosses were performed, one within the six parents of low leaf strength and low cellulose content and the other within the six high parents. Leaf strength and cellulose data was obtained from progeny and from parental ramets in October 1964 and high overall heritabilities for both characters (approx. 0.8) calculated from achieved advances and parent/progeny regressions. Selection for high leaf strength and high cellulose content was more effective than for low.