Description of the six defoliated study sites and species for which seedlings were included.

<p>Land form, location, estimated defoliation level (in 2009), dominant species in the overstory (the three most abundant species based on stems ≥ 10 cm diameter at breast height; Baribault et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0167139#pone.0167139.ref027" target="_blank">27</a>]), and species for which seedlings were included are indicated. Site numbers generally correspond to the numbering in Baribault et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0167139#pone.0167139.ref027" target="_blank">27</a>], except for sites 12 and 13, which were merged into one site (site 12). acru = <i>Acer rubrum</i>; acsa = <i>Acer saccharum</i>; fagr = <i>Fagus grandifolia</i>; fram = <i>Fraxinus americana</i>; osvi = <i>Ostrya virginiana</i>; piba = <i>Pinus banksiana</i>; pogr = <i>Populus grandidentata</i>; prse = <i>Prunus serotina</i>; qual = <i>Quercus alba</i>; quru = <i>Quercus rubra</i>; quve = <i>Quercus velutina</i>; tiam = <i>Tilia americana</i>.</p

Linear mixed-effects model results of seedling height growth as a function of site, year and seedling height at the start of the growing season for six northern hardwood species.

<p>Per species, only models within two AIC_c (Akaike’s Information Criterion) units from the best model are included. The deviation from the best model in AIC_c units (ΔAIC_c) is indicated. <i>n</i> indicates the number of included seedlings per species. The marginal (m) <i>R</i>² (fixed effects only), and the conditional (c) <i>R</i>² (both fixed and random effects) are indicated. acru = <i>Acer rubrum</i>; acsa = <i>Acer saccharum</i>; fagr = <i>Fagus grandifolia</i>; fram = <i>Fraxinus americana</i>; osvi = <i>Ostrya virginiana</i>; prse = <i>Prunus serotina</i>.</p

Supplement 1. Data on tree dynamics during secondary succession and wood specific gravity in northeastern Costa Rica.

<h2>File List</h2><div> <p><a href="Chazdon_data.txt">Chazdon_data.txt</a> (MD5: 05f10c673d870ea6e647a56b3ea7b67d)          Permanent plot data</p> <p><a href="Chazdon_meta.txt">Chazdon_meta.txt</a> (MD5: 90757c3a3ffd54b096105814f2296c86)          Information on the plots</p> <p><a href="WSG_data.txt">WSG_data.txt</a> (MD5: 210cd495964e565b31d2efb1a0135bb8)          Data on wood specific gravity</p> </div><h2>Description</h2><div> <p>This supplement includes the raw data of the permanent sample plots (Chazdon_data.txt), along with information on the characteristics of the plots (Chazdon_meta.txt), as well as data on wood specific gravity (WSG) for 92 tropical tree species from La Selva, Costa Rica. </p> <p><i>Note:</i> Robin Chazdon should be contacted before using the data set, to sign a data-use agreement, and to receive the latest version of the data, as updates and corrections (e.g., to species names) are continuously made. </p> <p>Additional data on WSG is available from Plourde et al. (<i>in press</i>). Average, species-specific WSG was measured as the ratio of dry mass to green volume, from cores or discs of trees ranging from 5–60 cm DBH for each species. Green volume was measured with the water displacement method, then wood samples were dried at 103–105°C for 24–48 hours before dry mass was determined (Plourde et al., <i>in press</i>). See Plourde et al. (<i>in press</i>) for more details on the methodology.</p> <p>Additional information on some of the columns in Chazdon_data.txt:</p> <p>StemID – The first number is the site code. The three digits after the first hyphen refer to the subplot number (1-100). The digits after the second hyphen indicate the number of an individual tree within each subplot. If multiple stems are separated below 1.3 m height they are marked separately. Each StemID is unique.  </p> <p>DBH.1997-2013 – Diameter at 1.3 m height, or above buttresses or stem irregularities for each year.</p> <p>DBH2.2012-2013 – Second diameter measurement higher in the stem when the point of measurement was changed because of the formation of buttresses or stem irregularities.</p> </div

Predicted seedling height growth for seedlings of six northern hardwood species in response to a forest tent caterpillar outbreak.

<p>Back-transformed mean (± SE) predicted height growth per site, per year, while holding seedling height constant at a species-specific average across all sites and years. The outbreak occurred in 2009. Note the different scales on the y-axis. A) acru = <i>Acer rubrum</i> (<i>n</i> = 22–32, per site); B) acsa = <i>Acer saccharum</i> (<i>n</i> = 16–59, per site); C) fagr = <i>Fagus grandifolia</i> (<i>n</i> = 34–53, per site); D) fram = <i>Fraxinus americana</i> (<i>n</i> = 9–62, per site); E) osvi = <i>Ostrya virginiana</i> (<i>n</i> = 9–13, per site); F) prse = <i>Prunus serotina</i> (<i>n</i> = 13–21, per site).</p

Predicted log₁₀ relative change in total inorganic soil N, soil nitrate (NO₃^-) and soil ammonium (NH₄⁺) levels with two-unit support intervals in the peak outbreak year (2009) and the year after the outbreak (2010) relative to the non-outbreak year (2011).

<p>Sites are ordered by increasing level of defoliation. Changes were standardized by the average change observed in the six non-defoliated sites to account for annual climatic and other differences. Significant changes across years are indicated by deviations from zero: filled symbols indicate significant changes, open symbols indicate non-significant changes. A-C) Log₁₀ relative change from non-outbreak conditions to 2009 (<i>n</i> = 11–19, per site); D-F) Log₁₀ relative change from non-outbreak conditions to 2010 (<i>n</i> = 17–21, per site).</p

Back-transformed mean canopy openness (± SD) from 2009 to 2011 for six sites that were defoliated by forest tent caterpillar in order of increasing defoliation (<i>n</i> = 20 per site, per year).

<p>Canopy openness in 2011 was regarded as representative of non-outbreak conditions.</p

Appendix A. Species contributions to standing biomass and biomass dynamics.

Species contributions to standing biomass and biomass dynamics

A Forest Tent Caterpillar Outbreak Increased Resource Levels and Seedling Growth in a Northern Hardwood Forest

<div><p>In closed-canopy forests, gap formation and closure are thought to be major drivers of forest dynamics. Crown defoliation by insects, however, may also influence understory resource levels and thus forest dynamics. We evaluate the effect of a forest tent caterpillar outbreak on understory light availability, soil nutrient levels and tree seedling height growth in six sites with contrasting levels of canopy defoliation in a hardwood forest in northern lower Michigan. We compared resource levels and seedling growth of six hardwood species before, during and in the three years after the outbreak (2008–2012). Canopy openness increased strongly during the forest tent caterpillar outbreak in the four moderately and severely defoliated sites, but not in lightly defoliated sites. Total inorganic soil nitrogen concentrations increased in response to the outbreak in moderately and severely defoliated sites. The increase in total inorganic soil nitrogen was driven by a strong increase in soil nitrate, and tended to become stronger with increasing site defoliation. Seedling height growth increased for all species in the moderately and severely defoliated sites, but not in lightly defoliated sites, either during the outbreak year or in the year after the outbreak. Growth increases did not become stronger with increasing site defoliation, but were strongest in a moderately defoliated site with high soil nutrient levels. Growth increases tended to be strongest for the shade intolerant species <i>Fraxinus americana</i> and <i>Prunus serotina</i>, and the shade tolerant species <i>Ostrya virginiana</i>. The strong growth response of <i>F</i>. <i>americana</i> and <i>P</i>. <i>serotina</i> suggests that recurring forest tent caterpillar outbreaks may facilitate the persistence of shade intolerant species in the understory in the absence of canopy gaps. Overall, our results suggest that recurrent canopy defoliation resulting from cyclical forest insect outbreaks may be an additional driver of dynamics in temperate closed-canopy forests.</p></div

Average seedling height (± SE) per site, at the start of each year, for seedlings of six northern hardwood species.

<p>The outbreak occurred in 2009. Note the different scales on the y-axis. A) acru = <i>Acer rubrum</i> (<i>n</i> = 22–32, per site); B) acsa = <i>Acer saccharum</i> (<i>n</i> = 16–59, per site); C) fagr = <i>Fagus grandifolia</i> (<i>n</i> = 34–53, per site); D) fram = <i>Fraxinus americana</i> (<i>n</i> = 9–62, per site); E) osvi = <i>Ostrya virginiana</i> (<i>n</i> = 9–13, per site); F) prse = <i>Prunus serotina</i> (<i>n</i> = 13–21, per site).</p

Above-ground biomass of Neotropical secondary forests database

This database is the product of the 2ndFOR collaborative research network on secondary forests. The database contains aboveground biomass data (in Mg/ha) for 1334 secondary forest plots differing in time since abandonment. The plots belong to different chonosequence studies in the Neotropics. For a description of the database, see Poorter et al. 2016. Biomass resilience of Neotropical secondary forests. Nature doi:10.1038/nature16512