Top-down and bottom-up controls limit woody encroachment into persistent temperate rainforest meadows
Elliott, Braden (2020), Top-down and bottom-up controls limit woody encroachment into persistent temperate rainforest meadows, Dryad, Dataset, https://doi.org/10.5061/dryad.5mkkwh731
These data describe soils, woody plant seedlings, and ungulate herbivory in and around temperate montane meadows in the Oregon Coast Range, USA. Meadows such as these are a global study system for the accelerating phenomenon of woody encroachment, but study this phenomenon into meadows in western Oregon has been conducted almost entirely in the western and High Cascades, with only two extant observational studies of grassy balds in the Coast Range. These data describe factors limiting woody encroachment into meadows in the Oregon Coast Range, including bottom-up control by soil properties, plant-plant interactions, and top-down control by large herbivores. I measured chemical and physical properties of soils (depth of organic layer; bulk density of top 3 cm of mineral soil; and mineral soil profiles: particle size distribution, pH, % total C, % total N) to a depth of 50 cm in meadow and forest. I recorded community, density, and proportion browsed for shrubs, conifers, and deciduous trees ≤2 m tall along transects from meadow into forest. I experimentally planted 20 Pseudotsuga menziesii (Douglas-fir) seedlings in each of five meadows (n = 100) and factorially manipulated aboveground neighboring plant presence and ungulate herbivore access. I found that meadow soils were lower in C and C:N; slightly lower in N, and similar in plant-available water (derived from particle size distribution) and pH relative to forest soils. Shrubs were most dense, but experienced the lowest browse pressure, near the meadow edge; while trees were sparse and varied by site—although at one site, browse pressure was heavier in meadow than forest. Seedling survival and growth varied by site, herbivory reduced growth, and total soil N best explained residual variation in seedling growth among sites. My findings indicate that ungulate herbivores exert top-down control on woody encroachment into temperate montane meadows, perhaps in concert with local N-limitation.
I conducted this study in the central-northern Oregon Coast Range, 45.0 to 46.2 °N and -123.1 to -124.5 °W, bounded by the Pacific Ocean and the Columbia, Willamette, and Salmon rivers in northwest Oregon, USA. The OCR is distinct from the Pacific coastal flats and the western edge of the Willamette Valley contained in the area described above, which are typically non-forested alluvial plains. This study focuses on the Coast Range ecoregion (EPA Level III Ecoregion 1), specifically the Volcanics and Mid-Coastal Sedimentary ecoregions therein (EPA Level IV Ecoregions 1d and 1g). Small, graminoid-dominated meadows occur sparsely in the OCR, accounting for a fraction of the grasslands which comprise approximately 5% land cover in the region. I sampled five meadows which share a mesic moisture regime but experience a range of abiotic conditions. These meadows are visible in aerial photography from the 1950s and are all sharply distinct from the adjacent forest, lacking a discernible ecotone in which herbs transition through intermediate growth forms to forest trees.
Field data collection
To evaluate the physical and chemical properties of forest and meadow soils, I sampled two soil profiles at each site to a depth of 50 cm, one in the meadow and one in the adjacent forest, each 25 m from the meadow edge. I recorded the depth of the organic layer; took a 226 cm3 bulk volume sample from the top 3 cm of mineral soil; and sampled 100 g per visually-distinct mineral soil horizon, subsampling within a horizon if necessary to ensure that no sample represented >15 cm horizon depth, resulting in approximately 5 sampled strata per profile. In total I recorded 10 organic layer depths, and took 10 bulk samples and 53 profile samples in mineral soil. I dried all fixed-volume bulk samples at 60°C overnight and recorded their dry mass. I air-dried all horizon samples and sieved them to 2 mm. To determine total C and total N content, I ground 6 mg subsamples and used a Carlo-Erba NA 1500 Series 2 gas analyzer (CE Instruments, Milan, Italy) for elemental analysis. To determine pH, I mixed 20 g of soil into a slurry with 40 mL of deionized water and recorded pH with a probe (VWR Scientific 8005). To determine particle size distribution, I used a Buoycous hydrometer method: I deflocculated 50 g of soil overnight in 100 mL of 5% sodium hexametaphosphate on a shaker, brought the resulting slurry up to 1L with deionized water in a graduated cylinder, mixed for 30 seconds to disperse particles, and took hydrometer measurements 40 seconds and 7 hours after mixing.
To describe the taxonomic community of, spatial pattern among, and browse pressure upon woody seedlings in and around meadows, I collected data along two transects approximately 100 m long at each site from the meadow center into the adjacent forest, along random bearings in the spring and summer of 2016. At each site I collected data for one transect in spring and another transect in summer, capturing synchronic browse pressure but not diachronic repeated browse damage. Every 5 m along each transect, I recorded all woody plants in the browse zone (≤2 m tall) by species within a 1 m2 quadrat, and whether each individual was browsed. I counted clonal shrubs per root collar visible at ground level. If any part of an individual plant was browsed I recorded it as browsed; if unsure whether a plant had been browsed (e.g., apical meristem was absent, but the cause of its absence was unclear), I recorded it as unbrowsed. I did not quantify mature (>2 m tall) woody plants, but I observed that they occurred almost entirely in forest. My authority for plant nomenclature is the Oregon Flora Project (http://oregonflora.org, accessed 13 September 2018).
To experimentally determine the effects of ungulate herbivores, plant-plant interactions, and soil conditions on woody encroachment, I planted 20 first-year Douglas-fir seedling plugs in each meadow in October 2015 and removed them in October 2016. These seedings were sourced from the same seed zone as my study sites, not inoculated with mycorrhizae prior to planting in the field, and spaced 2 m apart in a dispersed plot across the meadow interior. I randomly assigned five seedlings per meadow to each of four treatments: control, neighboring plant removal, browse exclosure, and both neighboring plant removal and browse exclosure. Every three months I removed all aboveground live plant material within 1m of each seedling for neighbor removals, and maintained a netted Vexar tube (5 cm diameter, 50 cm height) around each seedling for browse exclosures. I measured each seedling’s height at the beginning and end of the experiment.
Missing values from the P. menziesii seedling experiment are the result of herbivory, typically by ungulate herbivores (occasionally, I would find a seedling 1-10 m from the experimental plot, apparently transported by a large animal) but at least once by rodents (one seedling disappeared out of an undisturbed Vexar net tube overnight, and fresh rodent burrowing was apparent under and around its last known whereabouts).