DOI: 10.5061/dryad.280gb5mpb
title: Data from: Negative density dependence in the mortality and growth of tropical tree seedlings is strong, and primarily caused by fungal pathogens

description: Natural enemies have been implicated as agents of negative density
 dependence (NDD) in tropical forests, but their relative contributions to
 NDD, and thus to the maintenance of diversity, are largely unknown. We
 monitored the rates of survival and relative growth rates on seedlings for
 ten years in tropical moist forest in Manu National Park, Peru. We then
 experimentally manipulated the plots to exclude fungal pathogens, insects,
 small mammals, and large mammals for an additional 31 months to assess the
 influence of these natural enemies on density-dependent interactions among
 tropical seedlings. Fungal pathogens made the most important contribution
 to negative density dependence. The application of fungicide led to lower
 mortality rates, faster growth rates, and decreased species diversity.
 Other taxa of natural enemies had at most minor effects on seedling
 performance. Synthesis. We conclude that fungal pathogens are the
 strongest contributors to the widely observed NDD that occurs among
 seedlings. Moreover, the presence of fungal pathogens augments the species
 diversity of seedlings, indicating their critical contribution to the
 maintenance of species coexistence and the structure of tropical tree
 communities.
description: Site This study
 was carried out at the Cocha Cashu Biological Station (CCBS). CCBS is
 located in Amazonian South-East Peru in lowland tropical rain forest, at
 11°51’S, 71°19’W, 350 m elevation. This seasonal forest receives a mean of
 2167 mm of rain annually, and mean daily temperatures vary between 21.8ºC
 and 24.2º over the course of a year (Paine, 2007). The site is in a highly
 diverse and remote area of Manu National Park, with over 350 tree species
 with a diameter ≥10 cm DBH. It has experienced minimal hunting, and no
 logging or mining, during the last century (Hazelwood et al.,
 2020). Design
 Circular 1
 m<sup>2</sup> experimental plots were established in a random
 blocked design throughout a 4 km<sup>2</sup> area of mature
 floodplain rain forest. 24 plots were spaced between 5 and 10 meters apart
 in each of 24 blocks, avoiding trails and newly fallen trees, for a total
 of 576 plots. Within each plot, all woody seedlings ≥10 cm and &lt;
 100 cm in stem height were identified and tagged over eight censuses
 between 2003 and 2017. Height was measured on all seedlings as the
 vertical distance from the soil to the apical meristem. All understory
 shrubs and lianas were excluded. Owing to the widely dispersed blocked
 arrangement of the plots, it was not feasible to identify the adult trees
 surrounding them. Unfortunately, this precluded the assessment of the
 effects of adult conspecific and heterospecific density on seedlings.
 Censuses were carried out 269 to 1566 days apart (see Paine &
 Harms, 2009 for details). The experimental phase of the
 study began in October 2014, when we applied treatments to exclude fungi,
 insects, and mammals. Within each block, eight plots were randomly
 selected for the application of one of eight treatments: none (a control),
 fungi, insects, large mammals, all mammals, fungi and insects, all mammals
 and fungi, and all mammals and insects. The fungicide Amistar (Syngenta
 Ltd, active ingredient: azoxystrobin) provides a broad spectrum of
 protection against fungal attack, has low toxicity in non-target
 organisms, and was found to be effective by Bagchi et al. (2014). The
 insecticide Karate (Syngenta Ltd. active ingredient: lambda cyhalothrin),
 provides protection against a broad spectrum of insect herbivores, leaving
 low rates of residue and has low impact on non-target organisms.
 Pesticides were applied according to manufacturer’s instructions, mixing
 1.25 ml of pesticide with 1 litre of water, and applying 50 ml of the
 mixture to 1 m<sup>2</sup> plot with spray bottles. Pesticides
 were applied to treatment plots every 10 to 14 days, in equal amounts over
 31 months, with some treatment breaks when it was logistically impossible
 to apply treatments (max 1 month). Control plots were misted with an
 amount of water equivalent to that applied to pesticide plots.
 We
 excluded mammals from the study plots using 2×2 m wire mesh exclosures.
 These were 150 cm high and included a 50 cm buffer around each plot to
 reduce potential germination bias from perching birds. The ‘Large mammal’
 exclosures allowed the entry of small mammals through 15×15 cm holes cut
 into the base of the mesh. These were large enough to allow agoutis
 (<i>Dasyprocta </i>sp.) or smaller rodents to enter, but were
 too small for peccaries (<i>Pecari</i> and <i>Tayassu
 </i>spp.), deer (<i>Mazama americana</i>), or tapir
 (<i>Tapirus terrestris</i>). The ‘All mammal’ exclosures, on
 the other hand, were constructed flush to the ground and excluded all
 terrestrial mammals. Previous studies at the same site have shown this
 design of exclosures to be extremely effective at excluding terrestrial
 mammals (Beck et al., 2013; Paine et al., 2016).