DIRECT AND INDIRECT EFFECTS OF CO₂, NITROGEN, AND COMMUNITY DIVERSITY ON PLANT–ENEMY INTERACTIONS

Abstract

Resource abundance and plant diversity are two predominant factors hypothesized to influence the amount of damage plants receive from natural enemies. Many impacts of these environmental variables on plant damage are likely indirect and result because both resource availability and diversity can influence plant traits associated with attractiveness to herbivores or susceptibility to pathogens. We used a long-term, manipulative field experiment to investigate how carbon dioxide (CO2) enrichment, nitrogen (N) fertilization, and plant community diversity affect plant traits and the amount of herbivore and pathogen damage experienced by the common prairie legume Lespedeza capitata. We detected little evidence that CO2 or N affected plant traits; however, plants growing in high-diversity treatments (polycultures) were taller, were less pubescent, and produced thinner leaves (higher specific leaf area). Interestingly, we also detected little evidence that CO2 or N affect damage. Plants growing in polycultures compared to monocultures, however, experienced a fivefold increase in damage from generalist herbivores, 64% less damage from specialist herbivores, and 91% less damage from pathogens. Moreover, within diversity treatments, damage by generalist herbivores was negatively correlated with pubescence and often was positively correlated with plant height, while damage by specialist herbivores typically was positively correlated with pubescence and negatively associated with height. These patterns are consistent with changes in plant traits driving differences in herbivory between diversity treatments. In contrast, changes in measured plant traits did not explain the difference in disease incidence between monocultures and polycultures. In summary, our data provide little evidence that CO2 or N supply alter damage from natural enemies. By contrast, plants grown in monocultures experienced greater specialist herbivore and pathogen damage but less generalist herbivore damage than plants grown in diverse communities. Part of this diversity effect was mediated by changes in plant traits, many of which likely are plastic responses to diversity treatments, but some of which may be the result of evolutionary changes in response to these long-term experimental manipulations.

Elevated CO2 Increases Constitutive Phenolics and Trichomes, but Decreases Inducibility of Phenolics in Brassica rapa (Brassicaceae)

“…Paoletti et al (2007) report increased trichome density in Quercus ilex growing near a natural CO 2 spring, but plants also were exposed to elevated levels of H 2 S and SO 2 . In contrast to our results, trichome density decreased under elevated CO 2 in Arabidopsis thaliana (Bidart-Bouzat et al, 2005;Lake and Wade, 2009), and did not change in birch (Riikonen et al, 2010), milkweed (Vannette and Hunter, 2011), or round headed bush clover (Lau et al, 2008). Given this variation in response, trichomes should remain a high priority subject for elevated CO 2 studies.…”

“…Despite being identified by Pritchard et al (1999) over a decade ago as a high priority subject, we are aware of only a handful of studies that address the effect of elevated CO 2 on trichomes. Existing studies report either a decrease (BidartBouzat et al, 2005;Lake and Wade, 2009) or no change (Lau et al, 2008;Riikonen et al, 2010;Vannette and Hunter, 2011) in trichome density under elevated CO 2 .…”

Will Elevated Carbon Dioxide Concentration Amplify the Benefits of Nitrogen Fixation in Legumes?

“…3). There is evidence that this also helps legumes avoid compensatory feeding at elevated [CO 2 ] (Karowe, 2007;Lau et al, 2008). However, additional folivory at elevated [CO 2 ] was reported in soybean, and the response was linked to increased sugar content and a compromised defense system.…”

Transgenerational effects of global environmental change: long-term CO2 and nitrogen treatments influence offspring growth response to elevated CO2

“…While maternal environmental eVects typically decrease over time and often become non-detectable after one or a few generations or even over the course of the oVspring life cycle (Roach and WulV 1987), genetic changes in response to novel environmental conditions are likely to persist longer, even if organisms are no longer exposed to that environment. Adaptive evolutionary changes are expected to reduce any negative Wtness eVects of the environmental perturbation; however, these evolutionary changes also have the potential to inXuence population dynamics (e.g., Yoshida et al 2003Yoshida et al , 2004, ecosystem processes (Wieneke et al 2004;Collins et al 2006), interactions with other community members (Snaydon and Davies 1982;Lau et al 2008), and, most importantly, the magnitude of response to the environmental change itself (Snaydon 1970;Snaydon andDavies 1972, 1982) (reviewed in Hairston et al 2005).…”