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.

Tropical understory herbaceous community under experimental warming
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.

Leaf cover: 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 m²) 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 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".

Fern census: In each quadrat, each individual fern leaf >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".

The percent cover of herbaceous species for each plot is the average calculated from 12 quadrats.