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Vertical stratification collapses under seasonal shifts in climate

Citation

Basham, Edmund; Scheffers, Brett (2020), Vertical stratification collapses under seasonal shifts in climate, Dryad, Dataset, https://doi.org/10.5061/dryad.dr7sqv9vk

Abstract

Aim: Tropical forests are vertically complex, and offer unique niche opportunities in the form of climate-, habitat-, and resource-gradients from ground to canopy. Rainforest species organize within this vertical spatial gradient and recent macro-ecological research suggests that the highest levels of vertical stratification occur in structurally complex and climatically stable tropical rainforests. Yet, patterns of verticality are typically derived across latitude, linking annual precipitation regimes to canopy utilization and vertical stratification of communities in space. However, although the tropics are not thermally variable, they do have strong seasonality in rainfall—a temporal component that is missed in macro-scale studies. We hypothesise that verticality of ecological communities responds to seasonality in climate, particularly rainfall.

Location: Sierra Llorona, Colón Province, Panama.

Taxon: Amphibians.

Methods: We performed 121 ground-to-canopy surveys across the wet and dry seasons for amphibians. We calculated species specific and community wide vertical height and abundance shifts between seasons, and separately test the importance of vertical height and season on the vertical distribution of species.  

Results: Our results show a clear downward shift of 5 m in height in amphibian communities from the wet season to dry season. We also observe significant changes in species composition across vertical strata in both seasons, driven by nestedness in the dry season (loss of species over height), and genuine turnover in the wet season (loss and addition of species over height).

Main Conclusions: The exploitation of canopy microhabitats and resources in the wet season resulted in complex patterns of stratification, whereas drying flattened the distribution and simplified the composition of arboreal communities. As such, pattern and process in the vertical dimension is not static in time but rather exists as a dichotomy with inverse patterns between wet and dry seasons. Complex, multidimensional distributions of diverse rainforest communities can be simplified by climatic constraints—an important consideration as anthropogenic climate change increases the magnitude of seasonal swings in temperature and precipitation.

Methods

Study area

In central Panama, we surveyed an Isthmian-Atlantic Moist Forest located within the Esteban Alphonso Lee Natural Reserve (Lat 9.358555 : Lon 79.7029; 499m a.s.l), which borders the Chagres and Portobelo National Parks. Here, we focused our sampling in older growth areas, with primary canopy tree species including Cedrela odorata, Vatairea erythrocarpa, Dipteryx oleifera, Manilkara zapota, Carapa guianensis, Ficus insipida, and Anacardium excelsum. Our sampling area was confined to an altitude range of 333 – 473m a.s.l (Figure S1). 

Vertical stratification of amphibians

We surveyed for frogs during the wet season (April – December) and dry season (January – March; Figure S2). Using survey methods in Scheffers et al. (2013) and Basham et al. (2019), we conducted vertical, ground-to-canopy surveys for amphibians, with each survey centred on a single canopy tree. Tree selection was randomized; however, each tree had to meet safety standards for arborist single-rope climbing (Jepson, 2000). We did not limit our selection by tree species, and were only limited in selection by safety concerns. We surveyed 45 trees for amphibians in the wet season, and 43 trees in the dry season, totalling 46 trees (Figure S1) (Panama Ministerio de Ambiente Permit SE/A-26-2018). Of trees surveyed in the wet season 4 were not resampled in the dry season and 1 tree surveyed in the dry season was not resurveyed in the wet season. Trees were surveyed on 1 - 3 occasions in each season resulting in 66 wet season surveys and 55 surveys in the dry season. For each tree we surveyed for 10 minutes at the ground, understory (2-4m), sub-canopy (approx. half the maximum height climbed), and canopy (maximum height climbed), with a 20 min roaming survey between the strata totalling to 60 minutes. The 10 minute ground survey consisted of searching through leaf litter and ground habitat in a 4 x 4 m area to standardise to the same search area available during climbing. In above-ground surveys, we searched for arboreal amphibians in tree holes, moss, epiphytes and other microhabitat structures (Heyer, Donnelly, Foster, & Mcdiarmid, 1994), and measured the height above ground of each individual using a laser distance metre (Leica Geosystems, Leica Disto D2; http://www.leica-geosystems.ca).

Vertical stratification of climate

Temperature loggers (Onset, HOBO- Pendant; https://www.onsetcomp.com/products/ data-loggers-sensors/temperature) were installed in sets of three per tree. One was hoisted into the canopy < 20 m, a second was hung in the understory at 2 m, and a third was placed 7 cm below the level of the soil. Each pendant was installed in a PVC pipe to protect it from direct sunlight and rain. We installed three arrays in August 2017 and collected the data in August 2018. From this data we gathered daily mean, maximum and minimum temperature. Rainfall data were sourced from the Agua Clara meteorological station (Meteorology and Hydrology Branch, Panama Canal Authority, Republic of Panama; Figure S1).

Usage Notes

Data usage statement

This data was collected by the Scheffers Lab at the University of Florida at great cost of effort and resources. If downloaded for use in publications we would appreciate a notification on its usage and would be glad to discuss potential research avenues and collaborations. Please use this email for contact : brett.scheffers@ufl.edu.

Surveys

This tab contains data relating to each sampling occassion, consisting of a vertical transect located on a tree. Tree, Season, and Date columns can be used to identify the sampling regime for each tree and each season. Location can be determined by the longitude and latitude columns which are included. Tree height, and the height to which the tree was surveyed are also included here.

Individual

This tab contains data relating to the individuals captured and used in this study. Tree, Season, and Date columns can be used to link the survey information to the individuals collected on each survey. Species names are listed, along with the height at which the individual was found. 

Rainfall

This tab contains data relating to monthly rainfall, sourced from the Agua Clara meteorological station (The Panama Canal Authority).

Temperature

This tab contains data relating to maximum, minimum, and mean daily temperatures taken from the soil, understory, and canopy. Data were collected from three trees, which can be linked to the survey tab which contains their coordinates. 

Funding

American Museum of Natural History