DOI: 10.25349/d9x893
title: Spatial uncertainty in herbarium data: Simulated displacement but not error distance alters estimates of phenological sensitivity to climate in a widespread California wildflower

description: Herbarium records provide a broad spatial and temporal range with which to
 investigate plant responses to environmental change. Research on plant
 phenology and its sensitivity to climate has advanced with the increasing
 availability of digitized herbarium specimens, but limitations of
 specimen-derived data can undermine the inferences derived from such
 research. One issue that has received little attention is collection site
 uncertainty (i.e., error distance), a measure of confidence in the
 location from which a specimen was collected. We conducted comparative
 analyses of phenoclimatic models to determine whether spatial deviations
 of 2, 5, 15, or 25 km between recorded and simulated collection sites, as
 well as the error distance reported in digitized records, affect estimates
 of the phenological sensitivity of flowering time to annual temperature
 and precipitation in a widespread annual California wildflower. In this
 approach, we considered both spatial and interannual variation in climatic
 conditions. Simulated site displacements led to increasingly weak
 estimates of phenological sensitivity to temperature and precipitation
 anomalies with increasing distances. However, we found no significant
 effect of reported error distance magnitude on estimates of phenological
 sensitivity to climate normals or anomalies. These findings suggest that
 the spatial uncertainty of collection sites among specimens of widely
 collected plant species may not adversely affect estimates of phenological
 sensitivity to climate, even though real discrepancies and georeferencing
 inaccuracy can negatively impact such estimates. Collection site
 uncertainty merits further attention as a potential source of noise in
 herbarium data, especially for research on how plant traits respond to
 spatial and interannual climatic variation.
description: <em>Nemophila menziesii</em> herbarium data were
 downloaded from the Consortium for California Herbaria (CCH)
 (https://ucjeps.berkeley.edu/consortium/) and cleaned to remove duplicate
 specimens (collected on the same date within ~1 km of one another). We
 then selected specimens collected between 1901 and 2019 (N=1677), the
 period for which climate data was available. Climate data for our analyses
 were extracted from ClimateNA v6.40 (see http://climatena.ca/Help for more
 information). The file 'nemo_full_1901_2019.csv' contains all
 herbarium records and their associated data, plus climate data associated
 with each individual herbarium record. New datasets were generated using
 this primary .csv file to investigate the effects of collection location
 discrepancies on phenological sensitivity to climate. Specifically, new
 collection coordinates were generated for each digitized herbarium record
 by displacing the original coordinates of each by one of four distances
 (2, 5, 15, or 25 km) in a random direction (replicated 200 times). These
 data were used to construct phenoclimatic models whose results were
 compared to those of the original data set. See the README file for more
 details.
description: See README file. Contact Devin Gamble (dgamble@ucsb.edu) if you
 have questions about using these data or Rmd files.