Nutrients and drought alter tri-trophic interactions in cool-temperate forests

Description

Herbivorous insects are the main consumers in forest ecosystems, and the complexity of trophic interactions varies with resource availability, heterogeneity among feeding guilds, and the presence of natural enemies. In this study, we evaluated the impact of nutrient supplementation and a drought event on insect herbivory in cool-temperate Nothofagus pumilio forests located in southern South America. We examine how these factors affect herbivory by various feeding guilds and the rate of parasitism in leaf miners. Previous research in this ecosystem has demonstrated that habitat type (dry versus wet) and soil nutrient levels play a role in the regulation of insect folivory by insectivorous birds. However, there is limited understanding of the influence of nutrients and drought, as well as their potential interaction with insect folivory and parasitism in controlling leaf damage in temperate Patagonian forests. This focus is relevant when considering that Patagonian forests, along with other forest ecosystems around the world, are being affected by episodes of extreme climatic variability, as well as the advancement of anthropic activities (e.g., livestock) which modify soil nutrient dynamic. Collectively, these factors may modify tri-trophic interactions in forest ecosystems. Specifically, we asked the following questions: (i) what is the effect of nitrogen addition and a drought event on the leaf quality, damaged leaf area, and frequency of damage to N. pumilio saplings caused by different insect guilds? (ii) What is the effect of nitrogen addition and a drought event on parasitism rates of different morphotypes of leaf miners? Our results showed that nitrogen addition increased the foliar nitrogen content but had no effect on total damage by arboreal insects. However, the different feeding guilds responded differently to nitrogen addition and these responses varied between average or dry year and within the growing season. Particularly, miner activity responded positively to nitrogen addition, increasing their incidence towards the late of the average year. The positive effect of N addition on miner activity did not cascade up to the trophic level of parasitoids, which varied with the climatic context and the growing season. These results suggest that tri-trophic interactions were mainly controlled by the climatic context since consumers and their natural enemy responses were more dependent on the climatic context and growing seasons than on plant quality. Overall, we highlight the role of fluctuating abiotic resources and the heterogeneity within each trophic level in structuring tri-trophic interactions in forest ecosystems.

Steps to reproduce

At the beginning of the growing season (October), 8 blocks of N. pumilio saplings (total = 47 saplings) were selected averaging 4 m in height and 6.5 cm in basal diameter. The blocks were formed with a minimum of 4-6 saplings each. Each block was then subdivided into 2 groups (i.e., patches) of 2-3 N. pumilio saplings. In each block, one sapling group received the nitrogen addition treatment, while the other group served as the control (hereafter referred to as + N and – N, respectively). Over three consecutive years, the N+ treatment consisted of a dose of 60 g N-urea m-2 year-1 applied directly to the soil surface and distributed in three applications (early spring: October, mid-summer: January and early autumn: March). During the average (2014) and dry year (2015), in the middle and late growing season, we harvested one fully insolated branch comprising ~ 400 leaves from each tree at ~2-3 m height. The leaves of the same block were then grouped according to N addition treatment. The branches and leaves were immediately taken to the laboratory and kept in a refrigerator for their conservation. Sampling was carried out on fully expanded leaves to avoid confusion that could generate changes in herbivory along with the stage of leaf development within the growing season. The leaves harvested did not include damaged leaves that fell prematurely due to early senescence. Close to the late of each growing season (average and dry years), we assess changes in foliage characteristics between the -N and +N treatments. We measured leaf size (mm2), toughness (punch resistance, gmm −2), water content (% fresh mass) and N content (%). We used subsamples composed of undamaged leaves and without signs of senescence. From the total leaves harvested per block, we took fresh leaves (n=20 leaves/treatment/block). From the total leaves harvested per block, we quantified herbivory on leaves chosen at random (n = 50 leaves /treatment /block). We estimated damage frequency (%) by insects as the number of damaged leaves in a sample. To estimate the leaf area damaged, we placed a clear 1-mm2 plastic grid over each leaf and recorded the area of intact and damaged (or missing) tissue. Also, we recorded foliar damage from various folivores guilds, including leaf miners, chewers, sap suckers and skeletonisers. To account for possible seasonal effects, we recorded insect damage in the middle and late growing season. To evaluate parasitism, we selected only leaves damaged by miners. All mined leaves were classified into three morphotypes: serpentine, blotch and linear miners. To assess parasitism rates (% parasitised mines), we dissected all leaf mines under the microscope (100×) within 2 months after field collection. We classified miner larvae as successful, parasitised, or dead from causes other than parasitism. All living larvae were incubated for 3 months; then, in the case that no insect had emerged, the larva was dissected to determine whether it had escaped parasitism.

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