In many ecosystems, non-native grass species have been implicated in altering fire regimes, particularly where they invade areas with no native analogues (Brooks 1999, D’Antonio 2000, Brooks et al. 2004, Brooks 2008). In the Bald Hills, potential effects on the fire regime are complex, depending to a large extent on the degree to which the composition of the fuelbed has shifted with species invasions. Of the four non-native species that we studied, one of these, Harding grass, sustained high moisture content throughout the growing season and may act to delay ignition and dampen fire spread and intensity. In contrast, the non-native annual dogtail might decrease fuel moisture levels and increase their ignitability earlier in the fire season compared to most native perennial species. Two of the non-native species that we studied, sweet vernal grass and tall oatgrass, did not differ from the native species’ moisture contents.
The only annual species that we studied, the non-native dogtail, had significantly lower moisture content than all other study species across all dates except for the non-native sweet vernal grass and the native California oatgrass, which also had relatively low moisture content. These three grasses are relatively small-statured (Figure 2). Dogtail reached its maximum height early and flowered quickly, fitting with its sharp decline in moisture content early in the growing season. After this species senesced (at approximately the end of July), samples of this species contained primarily dead material. California oatgrass is unique among the study species because, when its seeds are mature, the dried culms detach from the still-green basal leaves and fall on the ground. This occurred between the 6 August and 5 September sampling dates, when the moisture content for this species decreased sharply (Figure 3). For the remainder of sampling dates, both basal leaves and detached culms were included in samples. With the exception of Harding grass, the moisture content of the four larger perennials (Figure 2; the native California brome, blue wildrye, California fescue, and the non-native tall oatgrass) did not differ significantly from each other.
Due to its substantial mass and the dead fuel it retains, California fescue is considered a highly flammable fuel in oak woodland understories, capable of burning with high intensity where it is dominant (Hastings et al. 1997). In the Bald Hills, this species is considered to be an exceptional fuel, contributing to the fast moving, low intensity fires that prohibit young conifers from invading (Engber et al. 2011). We did not find a difference in the moisture content for California fescue across all sampling months compared to the other robust perennial species (with the exception of Harding grass, which was significantly higher). California fescue’s reputation as an exceptional fuel source is partially dependent on its mass. Oak woodland understory dominated by California fescue in the Bald Hills had more than double the herbaceous mass of mixed grasslands, and a positive correlation was found between herbaceous fuel mass and surface fire temperatures in this ecosystem (Engber et al. 2011). Given the large amounts of dead thatch that this species retains, much of which is held close to the ground, it may have responded differently to the wetter than average year compared to the other species, and perhaps it had higher moisture content values than it otherwise would have. Likewise, the low variability in moisture content of this species may also be related to its growth form and the dead thatch it retains.
In stark contrast to the patterns in moisture content found in the seven other species, the non-native Harding grass had much higher moisture content than all other species throughout the growing season. Even on its driest sampling date, the moisture content of Harding grass was still more than double a probable value for moisture of extinction. One caveat to our findings is that limited pilot sampling on 28 June 2013 confirmed that the new Harding grass location we moved to in July was more mesic than our original Harding grass location. Mean moisture content for Harding grass at our initial location was 239 % compared to 290 % at the location we moved to (a 18 % difference relative to the new location). Harding grass was also sampled from two other locations in the Bald Hills that had mean moisture contents of 201 % and 203 %, a 31 % and 30 % difference relative to our new Harding grass location, respectively. On 25 September 2012, however, the relative difference between six of the grass species and Harding grass ranged from 84 % to 95 % (relative to Harding grass), and the relative difference between Harding grass and California fescue was 58 % (relative to Harding grass). It is unlikely that sampling in the more mesic location would negate the general pattern we reported, as the magnitude of difference between Harding grass and the other species was so large. Additionally, at our sampling location, Harding grass culms were noticeably greener late into the fall compared to the other co-occurring species. Although the inferences we are able to make from a single study location are somewhat limited, the patterns that we observed are still noteworthy.
The high moisture content of Harding grass may have implications for prescribed fire. In the Bald Hills, known Harding grass infestations totaled approximately 33 ha in 2012, with infestation densities ranging from scattered individuals to dense monocultures (S. Samuels, personal communication). Depending on the density and extent of Harding grass infestations, combined with burn-day weather conditions of wind speed and relative humidity, as well as topographic position, this species may act to dampen fire intensity. Because of the other threats it poses to native species, park managers are currently trying to eradicate this species from the Bald Hills. The potential impediments that this species may pose to prescribed burning may serve as additional motivation for managers to eradicate this species from fire-dependent ecosystems.
The relative dominance of non-native species in a fuelbed will have a significant impact on the overall contribution of the species on the fire regime. A survey of understory vegetation in the oak woodlands of the Bald Hills found that the mean cover of dogtail was 9 % (Livingston 2014). Comparatively, mean cover for tall oatgrass, sweet vernal grass, California brome, blue wildrye, and California fescue in oak woodland understories ranged from only 1 % to 4 %. California oatgrass occurred in woodland understory sampling rarely, and Harding grass was not detected in this survey (Livingston 2014). California oatgrass is one of the more common native grass species in the prairies of the Bald Hills, however, and Harding grass has primarily invaded prairies. Although its overall cover was low in the oak woodland understory survey, California fescue is dominant in several areas of the Bald Hills (Engber et al. 2011), and where it does occur, its cover is generally very high. The non-native species that we studied commonly invade other grasslands and woodlands ecosystems in the Pacific Northwest (Dennehy et al. 2011, Stanley et al. 2011) and elsewhere (Hitchcock 1971), making the results of this study relevant at broader geographic scales.
The non-native dogtail and the native California oatgrass were the first species to fall below a probable threshold for moisture of extinction, and the moisture content of dogtail was much lower than all other study species on 6 August. Depending on the density of dogtail, it may decrease fuel moisture levels and increase ignitability earlier in the fire season. Although the moisture content of California oatgrass was twice that of dogtail in August (21 % and 42 % respectively), California oatgrass had possibly cured sufficiently to be ignitable based on the more conservative values for moisture of extinction estimated for grass species (Burrows et al. 1991). Although still common in the Bald Hills, California oatgrass is patchily distributed in the prairies, and it is thought to be less dominant now than it was historically (cf. Davy 1902). Potential effects on the fire regime in the prairies of the Bald Hills caused by dogtail may be reduced because of the diminished dominance of California oatgrass. Although the species selected for this study are among the most dominant, there are several other grass species in the Bald Hills, including many other non-native annuals: oats (Avena L. spp.), bromes (Bromus L. spp.), rattail six-weeks grass (Festuca myuros L.), brome fescue (Festuca bromoides L.), and silver hair grass (Aira caryophyllea L.), and a few native annuals (e.g., Poa howellii Vasey & Scribn), that we did not study (Livingston 2014). If other annual species have moisture content values similar to those that we observed for dogtail, they might have an additive effect on the ignition potential of fuel early in the fire season. As our study was not comprehensive, speculation on the net effects of fuel moisture change with invasion is difficult.
The trend we observed in this study for the seasonal curing of grass is consistent with other research from the western United States. In quaking aspen (Populus tremuloides Michx.) woodlands, the moisture content of a composite of grass species steadily decreased until fall rains occurred and basal leaves of perennials greened up, and moisture content increased (Brown et al. 1989). Composite samples of grass and grass-like plants from the understory of upland Douglas-fir communities showed a similar drying pattern when early and late season moisture content was compared (Agee et al. 2002). Large differences in the moisture content among individual native herbaceous species have been reported from quaking aspen understories in Wyoming (Brown et al. 1989) as well as from the Great Lakes region (Loomis et al. 1979).
The relative changes in moisture content that we observed at two-hour intervals during our sampling window of 1130 hr and 1700 hr are consistent with relative changes reported for crown foliage and forest floor fuel moisture (Agee et al. 2002, Banwell et al. 2013). Our diurnal sampling showed that the early morning (between 0900 hr and 1100 hr) changes in fuel moisture were, however, dramatically greater than changes observed for these other fuel types. The moisture content of one-year-old foliage in the Pacific Northwest varied only 4 % to 12 %, depending on species, when sampled at two-hour intervals from 1000 hr to 1600 hr (Agee et al. 2002). Over a twenty-four-hour period in the Lake Tahoe Basin, moisture content of forest floor fuels (1 hr woody fuels, litter, and pine cones) underwent a similar 3 % to 11 % relative change in moisture content (Banwell et al. 2013). The large decrease in moisture content that we observed for three of our four study species (relative change ranged from 40 % to 84 %) likely resulted from the clearing of coastal fog often present in the morning at our study site, which increases nocturnal moisture content and slows morning drying. Not surprisingly, herbaceous fuels like grasses are much more responsive to ambient moisture.
A definitive value for moisture of extinction—the fuel moisture content at which fire will not spread, or when the probability of ignition is 1 %—has not been widely agreed upon for grasses (Dimitrakopoulos et al. 2010). Fire ignition and spread was difficult when moisture content approached 35 % for spinifex grass fuel in Australia, primarily hard spinifex (Triodia basedowii E. Pritz.) and soft spinifex (Plectrachne schinzii Henrard) (Burrows et al. 1991). Laboratory ignition tests for grass samples consisting primarily of Imperata cylindrica in Indonesia found moisture of extinction to be 40 % for dead grass and 44% for live grass (de Groot and Wang 2005). Recent field experiments in Greece estimated the moisture of extinction for slender oat (Avena barbata Link.) at 55.5 % (Dimitrakopoulos et al. 2010). Day of burn weather and topographic conditions also influence moisture of extinction, particularly wind speed, which has been shown to overwhelm ignition potential when fuel moisture is high (Dimitra-kopoulos et al. 2010). Morphological properties of individual species, including size of culms and leaves, as well as residual dead fuels (not part of our sampling) may also increase the probability of ignition and affect spread, particularly in the robust bunchgrasses (i.e., California fescue).
The potential effects that non-native grass species have on the fire regime in the Bald Hills are complex. The non-native annual dogtail may decrease fuel moisture levels, increasing the ignitability of fuels early in the fire season via its early senescence, as has been observed in other non-native grasses (Brooks 1999, Davies and Nafus 2012). In contrast, where Harding grass invades and occurs at high densities, it may serve as an impediment to fire throughout the fire season via its high, stable moisture content. Only one other invasive grass species, tall fescue, has been shown to have a dampening effect on fire in tallgrass prairies, and this is caused by its asynchronous physiology (McGranahan et al. 2013). Our results may be the first that identify a large magnitude difference in moisture content between cool season native and non-native grass species in a fuelbed. Although the effects of individual non-native species may be moderated by the characteristics and density of other species in the fuelbed, our study demonstrates their capacity for altering ecosystem processes and potentially complicating the restoration of fire-dependent ecosystems.