# Variation in Vessel Element Diameters and Densities Across Habitats at the Community and Species Levels in Southeast Florida

## TL;DR

This study examines how tree vessel structures vary in different habitats in Southeast Florida, revealing how species adapt to shaded versus sunny or swampy versus dry conditions.

## Contribution

The study provides novel localized cross-habitat comparisons of vessel element traits in South Florida, an understudied region.

## Key findings

- Shaded hammock species had lower vessel element densities than exposed pinelands, but no significant difference in diameters.
- Swamp species showed reduced vessel element diameters compared to scrub habitats.
- Variation in vessel element densities was consistently greater than variation in diameters across all surveys.

## Abstract

There is a rich but spotty history of research aimed at relating variations in wood structure to differences in growth conditions. Such work often centers around the structural details of vessel elements (VEs), which are hollow non-living conduits of water passage roughly comparable to pipes. The prominent characteristics of VEs are their diameters and densities (i.e., the number/mm2 across a stem). A broadly supported perception is a tendency for the association of comparatively small VE diameters and high densities with elevated hydraulic risk, and the reverse in relatively favorable drought-free conditions. This pattern, especially for VE diameters, is particularly well established comparing species across wetter to drier geographic regions. Comparisons across neighboring habitats sharing the same climate are limited, a deficiency addressed in the present study in South Florida, which is unstudied in this connection. Another weak area is the comparison of single species spanning different nearby habitats. The present study used microscopic examination of branchlet microtome cross-sections to address those two questions across habitats. The main findings are: 1. Shaded, well-drained hammock species on average had fewer VE/mm2 than exposed, poorly drained pinelands, but no significant mean difference in VE diameters. 2. Individual species across habitats displayed different modes of plasticity, some adjusting VE densities only, one species adjusting VE diameters only, and one adjusting both. Swamp species tended toward reduced VE diameters. Variation in VE densities consistently exceeded variations in VE diameters.

The study of woody dicot xylem structure in relation to habitats has a long but geographically incomplete history, generating generalizations and questions still in need of expanded data. One understudied area is localized cross-habitat studies under identical climate conditions. Also sparse are intraspecific cross-habitat data. Both of these weaknesses are addressed in the present project for unstudied S. Florida. Six surveys of woody dicot branchlet microtome cross-sections allowed the microscopic comparison of vessel element (VE) diameters and VE densities. The project took place in a small area within short timeframes per survey to assure near uniformity in weather and in seasonal growth cycles. The multispecies Initial Survey and single-species Ximenia americana Survey addressed the question of adjustments in VE diameters and/or VE densities in shaded vs. sunny habitats, finding significant downward average adjustment in VE densities in shaded hammock vs. open pineland habitats (112 shade vs. 182 VE/mm2 sun) but not in VE diameters. Single-species (Chrysobalanus icaco, Morella cerifera) surveys examined adjustments in VE diameters and VE densities in swamp vs. scrub (diameter mean: 42.5 µm swamp, 49.2 µm scrub; density means: 179.9 swamp, 154.0 scrub). Chrysobalanus icaco, having arguably the greatest environmental breadth and having the largest mean vessel element diameters in the project, was the sole species to adjust VE diameters only across habitats. Coefficients of variation in VE density exceeded those in VE diameters in every survey. This project sets the stage for future work in Florida and beyond aimed at isolation of environmental variables with respect to xylem traits and aimed at causal mechanisms, especially mode of xylem adjustment in relation to conductive risk.