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Experimental Design

Our experiment took place at three sites distributed across a latitudinal climate gradient in the Pacific Northwest from southwestern Oregon to central-western Washington (Table 1) from 2018 to 2020. The southern site, located in the Illinois River watershed, Oregon (42.2°N), experiences the warmest and driest summers and is a part of the Takilma cobbly loam soil series. The central site, located in the southern Willamette Valley near Eugene, Oregon (44°N), experiences intermediate climatic conditions, and has Hazelair silty clay loam soil. Finally, the northern site, located in the Puget Trough near Olympia, Washington, experiences the coolest temperatures and least-severe summer drought, and is on Cathcart medial-loam soil.

Our experiment capitalized on previous restoration efforts and climate manipulations that resulted in a range of starting conditions. Prior to our experimental burning and seeding, 60 plots of 7.1 m² across a latitudinal gradient underwent two restorations in 2009 and 2014. Plots were cleared with herbicide and seeded with native prairie species including native perennial grasses and native annual and perennial forbs, followed by experimentally applied warming, drought and irrigation from 2010-2012 and 2015-2018 (Pfeifer-Meister et al. 2013, 2016; Reed et al. 2019, 2021: https://doi.org/10.1002/ecy.3464). The most recent set of manipulations from 2015-2018 included four treatments: ambient, drought (60% of ambient precipitation), warming by 2.5 °C and warming + enough irrigation to offset the drying effect of the warming treatment, each replicated 5 times per site (Reed et al. 2019). All sites were initially dominated by perennial pasture grasses and experienced the same restoration and climate treatments, however, sites had very different trajectories with subsequent annual grass dominance in the south and perennial grass dominance in the north (Pfeifer-Meister et al. 2016; Reed et al. 2021c). Within sites, warming had a positive effect on introduced annual grasses mediated by a reduction in soil moisture, increasing the range of starting conditions at each site (Reed et al. 2021c), although we did not see significant differences in this study (Supplemental Figure 2). While heating and irrigation ceased in 2018, we retained the five drought treatment plots to assess whether post-burning trajectories were sensitive to precipitation amount.

Across the resultant range of variation in starting conditions, in 2018 we installed a fully-factorial experiment with site, burning, seeding, and drought as factors. To evaluate the effects of burning on functional group cover, we nested two 1-m² paired burned and unburned subplots separated by a 10 cm buffer within each of the 60 existing plots. These were established along a north-south axis in the western half of each plot. We burned between October and November 2018 using a 1-m² aluminum burn-box (Figure S1). We burned in low wind conditions between late morning to early evening. Fuels were ignited with wax paper or a propane torch, and we relit fires if they went out before all herbaceous fuels were consumed. Each site was burned on the earliest fall day permitted by local fire restrictions (Southern: November 3, Central: October 11, Northern: October 1). Fires removed all thatch and exposed mineral soil but did not cause mortality to most perennial species, which began to resprout quickly.

To assess the effects of starting composition drought and burning on seedling establishment, we further divided burned and unburned subplots into six 33 cm x 33 cm seeding microplots. We randomly assigned microplots to one of six seed mixes (Table 2): 1. control (no seed added); 2. Festuca roemeri alone (native perennial grass, NPG); 3. Danthonia californica alone (NPG); 4. Koeleria macrantha alone (NPG); 5. introduced perennial grasses (Schedonorus arundinaceus, Agrostis capillaris, Dactylis glomerata); or 6. mixed annual and perennial native forbs (Achillea millefolium, Agoseris grandiflora, Cammasia quamash, Clarkia purpurea, Collinsia grandiflora, Drymocallis glandulosa, Lomatium nudicale, Lomatium utriculatum, Acmispon americanus, Lupinus bicolor, Microseris lacinata, Plectritis congesta, Prunella vulgaris, Tritilea hyacinthea, Zigadenus venenosus). We included both native perennial grasses and introduced perennial grasses to compare their relative establishment rates, calculated as the probability of germination and surviving their first growing season. The forb mix, consisting of native species commonly used in restoration, was included to test the ability of burning to increase native diversity in different contexts of background grass dominance. Each microplot (excluding control) was seeded with a total of 8 g m^-2 of seed, divided evenly across species (either alone or in combination) by weight, within ten days of each burn. Each combination of site, burn treatment (subplot), and seeding (microplot) was replicated twenty times overall, with five replicates in each of the most recent climate legacy (warming, warming + precipitation) and within the two ongoing (ambient, drought) treatments.

Community measurements

To evaluate community trajectories, we estimated percent cover by functional group of interest (introduced annual grasses, perennial grasses, and forbs) in each 1 m² subplot at peak standing biomass in June 2019 at all sites. As our community measurements were focused on the functional annual-perennial transition we did not distinguish between species origin (native, introduced) for perennial grasses. This was repeated in 2020 to evaluate persistence of burn effects on seeded species, but only at the central site. To evaluate seedling establishment, we inventoried all individuals or culms (for perennial grasses) of seeded species in each 33 cm x 33 cm microplot at the peak of the growing season in June. Where seedlings were very abundant and evenly distributed within a microplot, we subsampled the center 11 cm x 11 cm of the microplot and weighted seedling abundance by proportion of total area covered. To calculate seedling establishment in 2019, we divided the number of individuals (or culms) by the original number of seeds added the previous Fall.