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Data for: Fire history and weather interact to determine extent and synchrony of mast-seeding in rhizomatous scrub oaks of Florida

Citation

Pesendorfer, Mario et al. (2021), Data for: Fire history and weather interact to determine extent and synchrony of mast-seeding in rhizomatous scrub oaks of Florida, Dryad, Dataset, https://doi.org/10.5061/dryad.ht76hdrg7

Abstract

In disturbance-prone ecosystems, fitness consequences of plant reproductive strategies are often determined by the relative timing of seed production and disturbance events, but the role of disturbances as proximate drivers of seed production has been overlooked. We use long-term data on seed production in Quercus chapmanii, Q. geminata, and Q. inopina, rhizomatous oaks found in Southcentral Florida’s oak scrub, to investigate the role of fire history and its interaction with weather in shaping acorn production and its synchrony. Acorn production increased with the time since last fire, combined with additive or interactive effects of spring precipitation (+) or drought (–). Furthermore, multiple matrix regression models revealed that ramet pairs with shared fire history were more synchronous in seed production than ones that burned in different years. Long-term trends suggest that increasingly drier spring weather, in interaction with fire frequency, may drive a decline of seed production. Such declines could affect the community of acorn-reliant vertebrates in the Florida scrub, including endangered Florida scrub-jays (Aphelocoma coerulescens). These results illustrate that fire can function as a proximate driver of seed production in mast-seeding species, highlighting the increasingly recognized importance of interactions among reproductive strategies and disturbance regimes in structuring plant populations and communities.

Methods

Seed Production

Starting in 1988, we conducted annual acorn counts using 194 permanent posts throughout the ~3 km2 study area. The posts were located throughout a 50 m-grid with a few exceptions due to topography (mean inter-post distance between all posts: 1,186 m, range 36 – 3,054 meters, Fig. S1). At each post, the closest ramet of each species was located within 25 meters in each quadrant of the cardinal directions, marked with a permanent tag, the number of stems counted, the distance and bearing to the post measured, and maximum height recorded. Ramets can consist of multiple acorn-bearing stems, which emerge from a resprouting genet (Fig. S2). We counted all acorns in the ramet and used the log-transformed (ln (count +1)) number of acorns per stem (SAC) based on all stems (whether they carried acorns or not) as measure of annual seed production for analyses unless stated otherwise. While this measure does not account for individual stem age or size, it controls for the number of stems per ramet, which has a much stronger effect on number of acorns per ramet. For the analysis of masting behaviour, we only considered reproductive ramets that produced at least a single acorn at some point during the study period, thus excluding vegetative ramets. Because natural stem turnover can be quite rapid in rhizomatous oaks (e.g. ~ 10 years in Q. inopina), we moved tags to (emerging) stems in close vicinity following high-severity fires, making the assumption that they belong to the same oak genet, as individual rhizomes can reach up to 30 meters in diameter.

Fire history, habitat association, density, and weather data

Following all fire events, date and severity (based on vegetation damage; Table S1) were recorded for each post and assumed to apply equally to all oaks associated with the post. For analyses, we used year of fire and years since fire as measures of fire history and only considered high-severity, crown-consuming fires (severity 3; see Table S1 for definitions), which constituted the vast majority of events (> 90% of post-fire years, Fig. S3). Similarly, we used a post-based map to derive habitat association and elevation for each oak. To calculate pair-wise distance matrices for ramets, however, we projected the GPS location of each tagged ramet using the recorded distance and bearing to the post location. We extracted daily minimum and maximum of temperature, and precipitation estimates from a weather station located directly adjacent to the study tract, at the headquarters of the Archbold Biological Station (available at https://archbold-station.org/html/datapub/data/data.html). Furthermore, the daily Keetch-Byram Drought Index (KBDI, hereafter “drought index”), was recorded. The index, which ranges from 0 – 800 and increases for each day without rain and drops with precipitation events, is aimed to capture the dryness of soil and duff layers, thus targeting conditions that affect the occurrence and spread of wildfires.

Usage Notes

Only summary values provided. For individual-level data, contact mario.pesendorfer@yahoo.com.