DOI: 10.5061/dryad.f1vhhmgtr
title: Data from: Tropical understory herbaceous community responds more strongly to hurricane disturbance than to experimental warming

description: The effects of climate change on tropical forests may have global
 consequences due to the forests’ high biodiversity and major role in the
 global carbon cycle. In this study, we document the effects of
 experimental warming on the abundance and composition of a tropical forest
 floor herbaceous plant community in the Luquillo Experimental Forest,
 Puerto Rico. This study was conducted within Tropical Responses to Altered
 Climate Experiment (TRACE) plots, which use infrared heaters under
 free-air, open-field conditions, to warm understory vegetation and soils
 +4 °C above nearby control plots. Hurricanes Irma and María damaged the
 heating infrastructure in the second year of warming, therefore, the study
 included one pre-treatment year, one year of warming, and one year of
 hurricane response with no warming. We measured percent leaf cover of
 individual herbaceous species, fern population dynamics, and species
 richness and diversity within three warmed and three control plots.
 Results showed that one year of experimental warming did not significantly
 affect the cover of individual herbaceous species, fern population
 dynamics, species richness, or species diversity. In contrast, herbaceous
 cover increased from 20% to 70%, bare ground decreased from 70% to 6%, and
 species composition shifted pre- to post-hurricane. The negligible effects
 of warming may have been due to the short duration of the warming
 treatment or an understory that is somewhat resistant to higher
 temperatures. Our results suggest that climate extremes that are predicted
 to increase with climate change, such as hurricanes and droughts, may
 cause more abrupt changes in tropical forest understories than longer-term
 sustained warming.
description: Tropical understory herbaceous community under experimental
 warming<br> Study site The study took place in
 the Luquillo Experimental Forest (LEF) in northeastern Puerto Rico
 (18°18′N, 65°50′W) near the USDA Forest Service Sabana Field Research
 Station. Both the control (n=3) and heated (n=3) plots were 4-m in
 diameter and hexagonal in shape. In heated plots, three infrared (IR)
 heaters (Model Raymax 1010, Watlow Electric Manufacturing Co., St. Louis,
 MO) were installed on crossbars at approximately 3.6 m from the ground.
 Warming treatments began September 28, 2016. Heated plots were warmed to
 maintain a 4.0 °C increase in hourly average temperatures to within ± 0.1
 °C compared to unheated plots, as sensed by IR thermometers. Warming
 treatments were stopped after 11.5 mont/phs of heating on September 5,
 2017, immediately before hurricane Irma, which passed north of Puerto
 Rico. Two weeks later, hurricane María struck the island as a category 4
 storm with sustained winds up to 250 km/hr and 500 mm of precipitation
 fell over 24 hours. While the concrete footings, posts, and cross-bars
 with the heaters were designed to withstand hurricane force winds, falling
 trees and branches during hurricanes Irma and María damaged portions of
 the heating and plot infrastructure. Tree trunks and large branches were
 removed from plots following the hurricanes, but leaf litter and small
 woody debris was kept intact. <b>Leaf cover:
 </b>Percent leaf cover of forest floor herbaceous plants (ferns,
 graminoids, forbs, and non-climbing herbaceous vines) was estimated in 12
 quadrats (0.75 × 0.75 m; 0.56 m<sup>2</sup>) arranged in rows
 around the center of each plot. Combined, the twelve quadrats covered a
 contiguous area that represented approximately 65% of each plot. The area
 omitted from sampling (35% of total plot area) was comprised entirely of
 plot edges. For the censuses prior to hurricane Irma (2015-2017)
 a<b> </b>digital image was taken of each quadrat approximately
 1 m above the forest floor. Taller non-herbaceous or woody cover (tree
 saplings) was pulled out of each photo frame. Shorter non-herbaceous cover
 (tree seedlings) that could not be pulled out of photo frames was
 digitally “masked” by coloring the green leaf area brown using a digital
 drawing application (You Doodle, v7.7.5, Digital Ruby, LLC). Total green
 leaf area of each digital image (masked and unmasked) was then estimated
 by processing images using Easy Leaf Area (ELA). Percentage of the total
 cover contributed by different herbaceous species was visually estimated
 for each image and multiplied by the total herbaceous cover to get the
 percent cover of individual species. Low woody cover (tree seedlings) was
 estimated as the difference in green leaf area between the unmasked and
 masked images. Bare ground (no woody or herbaceous cover) was calculated
 as 100 – total green leaf area (and therefore included leaf litter, woody
 debris, bare soil, and rocks). Herbaceous cover could not be estimated
 using digital images in the post-hurricane (2018) census due to the
 vegetation being a relatively tall multi-layered mat of vegetation.
 Instead, total percent herbaceous cover, woody cover, bare ground, as well
 as cover contributed by different species was estimated visually for each
 quadrat. Percent cover was measured four times over three years: twice
 before warming treatments began (October 2015 and August 2016), once after
 11 months of warming (August 2017), and once 12 months following hurricane
 passage with no warming (September 2018). This cover data is available in
 "TRACE_dryad_data". <b>Fern census:
 </b>In each quadrat, each individual fern leaf &gt;10 cm in
 length was identified to species, banded, and the petiole length, lamina
 length, and lamina width were measured. Each measured leaf was classified
 as fertile (spores present) or sterile (spores absent). In subsequent
 censuses, surviving and senesced fronds with bands were counted, and new
 leaves were banded and measured. Ferns were surveyed three times over a
 two-year period: a baseline survey 9 months before warming treatments
 began (October 2015), a pre-treatment survey one month before warming
 treatments began (August 2016), and post-treatment survey after 11 months
 of warming (August 2017). This cover data is available in
 "TRACE_dryad_data".
description: The percent cover of herbaceous species for each plot is the
 average calculated from 12 quadrats.