DOI: 10.5061/dryad.fttdz096d
title: Demography and dynamics of Giant Kelp cohorts across four decades: Lessons for conservation and resilience planning

description: Kelp forests throughout many temperate zones are in decline due to various
 human stressors, chiefly marine warming. Conservation measures, including
 restoration, are presently of great interest and focus on both historical
 and novel methodologies. Of paramount importance for these efforts is an
 understanding of the mechanics of kelp decline to identify the factors and
 triggers leading to stepwise declines and thus support the development and
 spatial priorities of strategic intervention to facilitate resilience.
 Here, we utilized a unique dataset documenting the demographic dynamics of
 giant kelp, Macrocystis pyrifera, in response to multiple disturbances
 across &gt;40 years off San Diego (California, USA). The recruitment
 and life history of &gt;14,000 individuals were used to evaluate
 cohort structure, size, and longevity forced by algal community structure
 and disturbance. Cohort dynamics varied spatially by depth and study
 subregion, thus aiding the identification of areas to prioritize for
 intervention to foster resilience. Five algal assemblages were
 characterized, providing context for cohort dynamics in response to
 physical disturbances and sea urchin grazing. A trend of decreasing cohort
 size and resilience was observed over time, accentuated by the marine heat
 wave of 2014-15 (MHW), after which competition with understory canopies
 increasingly interfered with giant kelp cohort development and plant size
 structure. Cohort recruitment ranged on a continuum from discrete
 (‘pulsed’) to more gradual (‘trickled’) episodes. Pulsed cohorts mainly
 produced single cohort-dominated age stands punctuated by major
 disturbances. Pulsed events were more common than trickled recruitment,
 especially at deeper sites. Trickled cohorts resulted in relatively mixed
 age stands, especially when individual cohorts overlapped within sites.
 Trickled recruitment increased over time as understory dominance
 increased. Cohort longevity was highly variable among sites and among
 cohorts within a site, with high first-year mortality mostly due to
 warming, waves, or their combination. Longevity was inversely related to
 temperature and sea urchin density, and was greatest at deeper sites,
 especially after the MHW. The downward trend of single cohort dominance
 and individual plant size over time and its step downward after the MHW
 suggest that deeper areas should be prioritized for restoration.
 Regardless, understory canopies will increasingly dominate Southern
 California with continued warming.
description: # Demography and dynamics of Giant Kelp cohorts across four decades:
 Lessons for conservation and resilience planning Dataset DOI:
 [10.5061/dryad.fttdz096d](https://doi.org/10.5061/dryad.fttdz096d) ##
 Description of the data and file structure This dataset includes algal
 data collected across 20 sites off San Diego, CA, USA. The data were
 collected from 1984 to 2024 at depths between 8 and 21 meters along
 permanent leaded transect lines. The transect lines are described in the
 article text. Each site consists of 4 leaded lines (A, B, C, D) that are
 25 meters long and 4 meters wide. Data are recorded in 10 quadrats
 (sampling units) that are 5 meters along the line and 2 meters on each
 side of the line. Therefore, 400 square meters are sampled at each site
 along 4 transects that are each 100 square meters in area. Transect lines
 are oriented perpendicular to the shore, and ‘A’ lines are located at the
 northern end, with lines ‘B’, ‘C’, and ‘D’ located progressively to the
 south. The zero meter mark is located at the offshore end, while the 25
 meter mark is onshore. Quadrats ‘boxes’ are numbered 1-10 with the left
 side consisting of odd numbers (1, 3, 5, 7, 9 [offshore to onshore]), and
 the right side consisting of even numbers (2, 4, 6, 8, 10 [offshore to
 onshore]). Therefore, quadrats 1 and 2 are located next to each other, as
 is true for all other odd/even quadrat pairs. The data consists of 2 R
 data files (*.RData) as follows. ### Files and variables #### File:
 AlgaeBoxData.RData **Description:**  This data file consists of one object
 ‘tl’ with 19 fields as described below: Site = Name of study site (20
 sites) Bout = numerically indexed sampling bout BoutDate = Sampling bout
 date Line = Transect line name (A, B, C, or D) Box = Quadrat number (1-10)
 MacroAd = Number of adult *Macrocystis pyrifera* plants (adults with at
 least 4 stipes) MacroPA = Number of *Macrocystis pyrifera* pre-adults
 (plants with 2 stipes at least 1 meter tall) Stipes = Number of
 *Macrocystis pyrifera* stipes PteryN = Number of *Pterygophora
 californica* plants CystoN = Number of *Cystoseira osmundacea* plants EisN
 = Number of *Eisenia arborea* plants DictyP = Fractional cover of
 *Dictyopteris undulata* plants DesP = Fractional cover of *Desmarestia
 ligulata* plants ArtCorP = Fractional cover of Articulated Coralline Algae
 AgN = Number of *Agarum fimbriatum* plants BT = fractional cover of all
 other Brown Turf Algae RT = fractional cover of fleshy Red Turf Algae Lam
 = scaled cover of *Laminaria farlowii* (0 = none, 1 = trace cover such as
 bare stipes, 2 = 5 % cover, 3 = 10% cover, 4 = 25% cover, 5 = 50% cover, 6
 = 75% cover , 7 = 90%, to 8 &gt;90% cover) MacRecs = Number of *Macrocytis
 pyrifera* recruited juveniles (identifiable plants with single blades and
 early bifurcation) #### File: StipeData.RData **Description:** Contains
 one object named ‘stipes’ with 8 fields. Each row indicates a plant was
 found alive, and the number of stipes was recorded. NOTE: Each plant was
 followed over time and was identified in the data with an ID name that was
 unique within each transect line over the entire time series. Site = Name
 of study site (20 sites) Date = Survey Date Season = quarterly season
 (‘F’, ‘W’, ‘Sp’, ‘Su’) year.season = combined year and season (e.g.,
 ‘2006F’) Line = Transect line name (A, B, C, or D) Box = Quadrat number
 (1-10) Name = unique plant name ID within a transect line over time
 numStipes = Number of stipes counted on the plant at the time of the
 survey ## Code/software Data files are provided as R (open source) data
 files: R Core Team (2021). R: A language and environment for statistical
 computing. R Foundation for Statistical Computing, Vienna, Austria. URL
 [https://www.R-project.org/](https://www.R-project.org/). All analyses
 were conducted with standard packaged libraries within R as described in
 the manuscript.