1. Introduction
Fire plays a critical role in terrestrial ecosystems across the globe, shaping ecosystem composition, structure, and function [
1,
2,
3,
4]. In western North America, fire exclusion has fundamentally altered the structure and function of fire adapted forests, particularly forest types such as ponderosa pine and dry mixed conifer forests historically characterized by frequent low and mixed severity fire regimes [
5,
6,
7]. Frequent fire in these forest types reduced overall stand densities and fuel loading [
5,
6,
7], while maintaining heterogeneous spatial patterns of vegetation structure that promoted resilience and moderated fire behavior [
8,
9,
10]. In combination with the changing climate, livestock overgrazing, and timber harvesting, fire exclusion in these forests has dramatically altered vegetation composition and structure, increased fuel loading and spatial continuity, and increased the risk of high severity fire. In response, fuel reductions via mechanical thinning and/or prescribed burning are widely applied in fire adapted forests to reduce wildfire severity and reintroduce fire as an ecological process [
11,
12]. National policies have emerged to incentivize changes away from prioritization of fire suppression to more holistic approaches to wildfire management [
13]. Yet, the pace and scale of fuel reduction treatments lags behind what is suggested to affect meaningful landscape change [
14], while persistent operational and administrative constraints hinder more widespread application of fuel reduction treatments and restoration of fire as an ecological process [
15,
16,
17].
Reducing the probability of high severity fire and extreme fire behavior is often the primary objective of fuel reduction treatments [
11], with concurrent goals to restore ecosystem composition, structure, and function. Increasing native species biodiversity is a common objective of ecological restoration [
18,
19,
20], following the central tenant that native biodiversity benefits from restoration of natural environmental conditions and processes [
21]. Most plant diversity in conifer forests of western North America is found in the understory [
22,
23], so understanding how understory plant communities respond to fuel reduction and forest restoration treatments is critical to evaluate restoration effectiveness. In fire adapted landscapes, plant species can persist via fire resistant and resilient traits [
24,
25,
26]. For example, some species resprout from belowground parts that survive a fire, others have seeds stimulated by fire resulting in species persistence even when the adults are killed by a fire, while yet others avoid fire by growing on sites that are less likely to have fire. Understory vegetation responses can be highly variable across forest types, time since disturbance, and disturbance intensities [
27], highlighting the continuing need to understand how understory vegetation communities respond to specific restoration treatments and forest conditions, especially if adverse understory responses potentially constrain application of fuel reduction and restoration treatments.
In the context of ecosystem restoration, prescribed fire can approximate the disturbance historically created by wildfires, but there are reasons to question this assumption. The first prescribed burn after decades of fire suppression and fuel accumulation may occur with higher severity and negative ecological effects than would be expected under the natural fire regime [
28,
29,
30]. Ecosystem restoration relies on reference conditions as a fundamental premise [
31], implying that prescribed burning frequency should approximate a forest’s historical fire return interval. However, knowledge of the appropriate fire return interval for a given forest type and location is often lacking. Fire scar data used to quantify historical fire return intervals are often not available for specific locations, while interpretation and inference from fire scar data have been a subject of debate [
32,
33]. Furthermore, climate change is likely to alter future wildfire frequency and severity [
34,
35], potentially reducing the relevance of historical fire regimes as reference conditions to guide future restoration objectives [
36] and adding uncertainty as to the appropriate prescribed fire return interval best suited for a given forest type, location, and desired outcomes.
In addition to uncertainties regarding appropriate prescribed fire frequency, operational considerations (e.g., air quality, weather, fuel loading and moisture, fire control, available personnel and resources, etc.) often constrain the seasonality of prescribed fires in the western United States to an early season after cessation of spring precipitation and snow melt, and a late season before the onset of winter precipitation. Yet in the Pacific Northwest, wildfires historically burned in the late summer and fall [
3]. Application of prescribed fire outside the historical wildfire season may have unintended consequences for understory vegetation, as season of burn can differentially stimulate or damage plants depending on species-specific traits and developmental stages. High moisture content in plant tissue during the spring can make plants more susceptible to fire [
37]. Additionally, many perennial plant species are most sensitive to fire when their carbohydrate reserves are lowest [
38], and the timing of this varies by species. Fires during the growing season can result in higher mortality and reduced biomass of grasses and forbs, in comparison to dormant season prescribed burns [
39,
40]. The seedbank response to fire can also be altered by seasonal changes in soil moisture, with seeds of some species being tolerant of fire if the soil is dry [
41], while others being stimulated by heat under higher moisture soil conditions [
42]. In addition to species adaption traits, the season of burning can influence fuel consumption and plant mortality, with early season burns often occurring in high fuel moisture conditions, resulting in reduced fuel consumption and fire severity [
43,
44], lower plant mortality [
45,
46], and more unburned patches where fire sensitive species are likely to persist [
44,
47]. Lastly, invasive species such as cheatgrass (
Bromus tectorum) pose a serious threat to ecosystem composition, structure, and function in many fire adapted forests in western North America [
48,
49]. Cheatgrass invasion is associated with changing fire frequency and seasonality [
50], so conversely the seasonality and frequency of prescribed fire may inhibit or exacerbate cheatgrass invasion [
51]. Taken in combination, it is unclear how changes in fire season and interval influence understory vegetation, and therefore restoration success.
This study focuses on how season and interval of prescribed burning influence the composition and compositional trajectories of understory vegetation, using a unique long-term (18 yr) experiment in ponderosa pine (
Pinus ponderosa) dominated forests of northeastern Oregon, USA. Previously, this experiment examined the effects of season and interval of prescribed burning on tree growth and mortality [
46,
52,
53], butterfly defoliation [
54], tree regeneration and fuels [
55], and understory vegetation [
56,
57,
58]. Focusing on functional groups, vegetation cover, and diversity, Kerns and Day [
57] found most native perennial functional groups resisted or recovered from different seasons or intervals of burning, but did not display strong responses to any specific combination of burn season or interval. Here we focus on overall vegetation composition, temporal trajectories, and specific indicator species of understory vegetation. First, we asked if species composition differs among treatments. Second, we asked whether compositional trajectories differ over time in response to spring or fall burning and 5- vs. 15-yr interval prescribed burning. Third, we asked if any individual species were indicators of different seasons and intervals of prescribed burning, and how did the importance of these indicator species vary over time.
4. Discussion
In this study with six years of vegetation measurement spanning 13 years, we found strong evidence that season of burn affected understory composition, and limited evidence that interval of burning mattered. However, compositional trajectory analysis shows that the combination of season and interval was important in determining overall compositional trends. Compositional trajectory analysis showed that after four burns the spring 5 year treatments were compositionally more similar to fall burns than 15 year spring reburns or unburned. However, only two pairwise comparisons (spring 5 yr vs. unburned control and fall 5 yr vs. unburned control) displayed strong evidence of diverging compositional trajectories, and no treatment pairs displayed strongly converging trajectories, suggesting understory community responses to seasonal and varying intervals of reburning are subtle, and initial entry burns were likely important drivers of initial trajectories. Indicator species analysis suggests, however, that species specific responses may be more nuanced, which may have been obscured by compositional analyses and the functional group approach of previous work [
58]. Indicator species analysis found increased importance in fall burns of early successional species that are typical “fire increasers” such as
Ceonothus velutinus, Ericameria spp., and
Elymus elymoides. At the same time, some early successional native forbs such as
Montia perfoliata and
Polygonum douglasii appear to have strong but very ephemeral and episodic responses to burning, although post burn pulses appear to have weakened through time. However, the ephemeral temporal trend of
Montia perfoliata was also somewhat present in unburned controls, suggesting other drivers such as climate, grazing, or seed source availability could play a role across treatments. The exotic species
Bromus tectorum increased in importance across all seasons and intervals of burning, which is a management concern due to this exotic grass’s negative influence on ecosystem composition, structure, and function [
49]. Below we pose multiple, non-exclusive explanations for the understory vegetation responses we found to season and interval of burn, and place our findings in the context of fire and fuel management of fire adapted forests in western North America.
Our results suggest that subtle differences in season of burn are important drivers of understory vegetation composition. Given the historical mean fire return interval of 10–18 years in the region [
7,
60], it is likely that native understory species are adapted to the range of fire intervals in this study, although we have some limited evidence that five year interval reburning may be too frequent. For example, pulses of short lived native early seral species (
Montia perfoliata and
Polygonum douglasii) characteristic of immediate post-fire years appear to be waning in their burn response after multiple reburns. Differences in understory vegetation due to season of burning are likely due to a combination of fire intensity and species-specific traits. Fall burns tended to have greater fuel consumption [
55], indicating high fire intensity.
Ceanothus velutinus in particular was strongly associated with fall burning, which may reflect a combination of vegetative resprouting and seed germination, both of which are encouraged by higher intensity fire [
77,
78]. One important deviation from season of burn differences in community trajectories is after multiple burns, spring 5 year burns trended towards fall burns in community composition. Only one indicator species (
Elymus elymoides) clearly displayed increased importance in spring 5 year burns. In combination with compound effects of multiple burns on surface fuels [
55], this may indicate that over decadal time frames multiple reburns may have comparable effects on understory vegetation to different seasons of burns. However, it is important to note that overall community trajectories for all treatments were rather similar; implying that a driver such as regional climate variability may have had a top-down effect on vegetation composition and obscured treatment effects [
57].
While this study found differences in understory composition in relation to season of burning, the results are subtle, consistent with prior findings at this site [
57]. There are multiple non-exclusive explanations for the subtle understory vegetation responses in this study. The study was conducted in a relatively low productivity ponderosa pine forest in northeastern Oregon, and multispecies dynamics can result from interactions between diversity, disturbance, and productivity [
79]. Other studies of prescribed burning have found comparatively weak vegetation responses in lower productivity southwestern ponderosa pine forests [
80], and stronger responses in productive mixed-conifer forests of the California Sierra Nevada [
81,
82,
83]. The lack of strong understory responses in our study may reflect an inherent limited capacity of compositional change possible in low productivity forests subject to decades of fire exclusion. This limited capacity to change could also reflect ecological inertia of forest composition, structure, and seed sources constrained by forest conditions that after initial thinning still deviated greatly from pre-settlement conditions [
84,
85]. Additionally, both the mechanical thinning, grazing and prescribed burning in our study may have homogenized environmental conditions. Mechanical thinning from below resulted in a fairly even spatial distribution of residual trees, and did not result in stands with high variation in canopy cover. Likewise, prescribed burns were very low severity, and applied with drip torches using a multi-strip head-fire pattern with an average flame length of approximately 60 cm. The operational implementation of very low severity prescribed burning was likely effective in consuming surface fuels [
55], but may not have promoted heterogeneous fire behavior and ecological effects (
Figure 5), and potentially limited active fire spread that is in part driven by fuel moisture that varies seasonally. Having said that, it is important to note the number, size, and geographic placement of vegetation plots was not designed explicitly for quantifying spatial heterogeneity.
Lastly, these stands have a long history of livestock grazing, and the plots used in this study continued to be grazed by cattle during this study. Livestock grazing can have short-term effects on vegetation response in ponderosa pine forests by reducing overall vegetation cover and altering the relative abundance of plant functional groups [
56]. In grassland ecosystems, long-term cattle grazing can affect the composition of the seed bank [
86], but it is unclear how strong a role the seed bank has in shaping understory vegetation responses in ponderosa pine forests [
87].
5. Conclusions
In the context of thinned ponderosa pine forests, we found that the season of prescribed burn subtly altered the understory vegetation community in our study. However, there is evidence that the combination of season and interval was important in determining overall compositional trends. Our findings suggest fall burning, which is more consistent with the seasonality of the historical fire regime in these forests, resulted in different understory vegetation compared to spring burning, likely associated with more early seral species. However, there was some evidence that native early seral species may be unable to continue to respond to repeated very frequent (5 year) burning. This would suggest fire managers should give more consideration to the seasonality and interval of prescribed burning in the context of desired outcomes. The 5 year fire regime tested in this study is at the low end of historical fire regimes in the area. While spring burning is less consistent with the seasonality of historical fire regimes, we found no specific negative impacts related to spring burning.
There is a call to increase the pace and scale of prescribed fire and fuel reduction treatments in many fire adapted forests of western North America that have experienced decades of fire exclusion. Our results, coupled with earlier findings from this project, demonstrate that season and interval of prescribed burning may not be a strong ecological constraint for implementing prescribed burn programs, although fire intensity and resultant severity remain important considerations. That is, strict adherence to mimicking historical fire seasons may not be necessary to achieve desired outcomes and avoid negative vegetative responses. However, an important caveat to our findings is that the thinning and prescribed burning applied in this study may have homogenized environmental conditions and fire effects, and this homogenization may have overridden season and interval effects. Mechanical thinning with spatially variable tree retention (sensu North et al. [
88]) and prescribed burning with more natural fire behavior are increasingly applied on fire adapted landscapes. An unanswered question is how understory vegetation will respond to different seasons and intervals of prescribed burning in more spatially complex forests with more active fire behavior. This may be of particular importance with respect to invasive species control, as variable tree retention and more active fire behavior may exacerbate exotic invasive species such as
Bromus tectorum, which has continued to increase throughout the study area with burning. Better integration of weed management into prescribed fire programs could mitigate such undesirable outcomes.