Climate variability impacts on Orange County, CA grassland seedbank and plant community dynamics (2018-2020)

Description

Notes

Methods

Project 1: Seed Study – Field and Laboratory Protocols

1. Transect Design and Field Layout

   • Linear field monitoring was conducted along permanent 50-meter transects defined by logged GPS coordinates and anchored by rebar markers capped with white or red PVC pipes.
   • Measuring tape was pinned flat and straight using chaining pins from the 0m marker (start) to the 50m marker (end).
   • To preserve the canopy structure and prevent microhabitat trampling, field personnel walked at a minimum distance of 1 meter away from the line when laying out tape and traveling along the transect path.

2. Visual 1m² Quadrat Cover Assessments

   • Vegetation and ground cover evaluations were conducted inside ten discrete 1m² quadrats placed at alternating 5-meter intervals along the tape (starting on the left side from 0m–1m, alternating to the right side from 5m–6m, etc.).
   • Quadrats were evaluated as vertical rectangular prisms extending from the ground upward. All species rooted within or hanging over the prism area were identified to estimate absolute percent cover from a vertical bird's-eye perspective. Total cumulative vegetative cover values can exceed 100% due to overlapping structural strata (e.g., herbaceous understory beneath low shrub canopies).
   • Taxonomic observations incorporated standardized long-lived woody plant condition prefixes: Dead Shrub (D), Dead Part of Living Shrub (DPL), Crown Sprouting (CS), and Dead Crown Sprouting (DPCS). Species occupying <1% of the total area were explicitly entered as <1%.
   • Total ground cover inside each quadrat was visually estimated to equal exactly 100% across defined abiotic and micro-biotic surface categories: Litter (L), Bare Soil (B), Moss (M), Coarse Woody Debris (W), Cryptobiotic Crust (C), Fine Woody Debris (F), Thatch (TH), Rock (R), and Rooted Stems (S).

3. Point-Intercept Canopy and Height Measurements

   • High-resolution intercept records were compiled at 50 systematic points spaced exactly 1 meter apart along the tape line (from 0m to 49m).
   • A 170cm long, 0.5-inch diameter PVC dowel rod (graduated with 5cm marks) was dropped randomly near each meter mark.
   • Field operators recorded every unique plant species making physical contact along the vertical plane of the dowel or its imaginary upward linear extension. For individuals taller than 170cm, plant heights were calculated by extending the rod vertically and adding the offset value.
   • The absolute height of the tallest living individual within each functional group (Grass, Forb, Shrub, Tree) was logged at its intersection point. If a canopy intersection occurred on a woody shrub or tree consisting strictly of dead tissue, the height of that dead contact point was noted under a DPL or D modifier.
   • A single dominant ground cover letter category was assigned per point intercept based on the primary material making contact with the base footprint of the rod.

4. Field Soil Core Collection

   • Soil collections were localized to an open space approximately 1 meter away from the 0m transect start point in the direction facing away from the monitoring tape line to ensure zero disturbance to vegetation plots.
   • Seed Bank Cores (SC): Samples gathered specifically for seed bank analysis were driven into the ground using a weighted slide hammer until the core chamber was completely buried. Crucially, the surface litter and matted organic thatch layers were NOT cleared or brushed away prior to core insertion, capturing the complete seed profile. The extracted rings were emptied entirely into labeled bags for lab flotation.

5. Abiotic Site Characterization

   • Litter Depth Tracking: Fine-scale litter and thatch depth metrics were recorded at four intervals along the line (15 m, 25 m, 35 m, and 45 m) at a distance of 1 meter perpendicular to the tape. Litter layers were measured using standard rules/calipers down to the exposed mineral soil boundary.

6. Laboratory Sample Preparation and Volumetric Measurement

   • Field-collected soil core samples were transferred from paper collection bags into a 100 mL glass beaker using a funnel to measure and record the total initial dry sample volume (mL).
   • Samples were then transferred into a 400 mL plastic beaker for flotation processing.

7. Vacuum Flotation and Separation Procedure

   • Tap water was added to the sample until the water level reached approximately one inch above the sediment layer, carefully rinsing down the inner sides of the beaker.
   • The soil-water mixture was stirred vigorously to fully suspend organic materials. Large pieces of organic matter were pre-cleared manually using tweezers.
   • The water column holding the suspended buoyant organic material was slowly decanted into a Buchner funnel lined with an organza square filter. This funnel setup was mounted on top of a filter flask connected to an active vacuum pump.
   • A squeeze bottle was used to rinse any remaining fine organic fractions out of the residual heavy sediment. This washing step was repeated sequentially (averaging at least 6 iterations per sample) until the organic materials (appearing black against brown inorganic matter) were completely extracted.
   • The organza square capturing the organic material fraction was removed from the funnel and placed into a labeled petri dish.
   • The remaining heavy inorganic sediment fraction was transferred separately into a secondary organza square filter inside the Buchner funnel, washed to capture remnants, and placed into a separate labeled petri dish. Both organic and inorganic fractions were dried overnight for 24 hours.

8. Sifting and Sieve Layering

   • Once dried, the organic material fraction was emptied into the top layer of a mechanical sifter containing four discrete mesh openings to isolate size categories: 0.078 inches, 0.0394 inches, 0.0197 inches, and 0.0024 inches.
   • Samples were agitated via shaking until clean separation across layers occurred.
   • The separated materials from each individual sieve layer were transferred to four corresponding labeled petri dishes to streamline focus on expected species-specific seed sizes. Fine silt reaching the bottom pan was discarded into the inorganic material dish, as no target seeds penetrate the final fine mesh layer.

9. Microscopic Sorting, Identification, and Viability Testing

   • The isolated organic material in each petri dish was examined under a dissection microscope at 10X to 15X magnification to pick out target seeds.
   • Taxonomic identification was confirmed using reference seed mounts.
   • Seed viability was verified mechanically by probing each seed with the edge of a scalpel. Viable seeds were classified by a moist, white interior or by visible active germination structures. Non-viable seeds were identified by a dry, brown, or black interior or categorized as empty seed coats (hollow exteriors from past germination events).

10. Specimen Archiving

    • All materials were retained post-analysis: sorted seeds (viable and non-viable combined) were archived in a labeled microcentrifuge tube
    • The remaining organic matter was placed in a small Ziploc bag, and the inorganic sediment was placed in a medium Ziploc bag.
    • The microcentrifuge tube was placed inside the small organic bag, which was nested within the medium inorganic bag. These nested components were consolidated into a large master transect/year Ziploc bag and organized within long-term storage archival boxes.