Soil moisture regime and canopy closure structure subalpine understory development during the first three decades following fire

Highlights

• Natural experiment of post-fire understory successional development over ~30 years.

• Study examined effects of soil moisture regime and developing tree canopies.

• Early understory composition reflected differences in soil moisture regime.

• After three decades, understory composition was related more to canopy closure.

• Limited tree canopy development may expose seral understories to climate warming.

Abstract

Subalpine coniferous forests are adapted to cycles of fire and successional development, but increasing fire frequency and severity are altering historical stand structure, composition, and plant diversity. For instance, conifer regeneration has become increasingly variable as a result of prolonged aridity following fire, but the potential cascading effects on understory community development remain virtually unexplored. We used a natural experiment to investigate the relationships among understory plant succession, soil moisture regime, and variable tree canopy development over 30 years following the 1988 fires in the Greater Yellowstone Ecosystem, U.S.A. In 1990, at each of two study areas, plots were established in different site types defined by burn status (burned or unburned) and soil moisture regime (mesic or xeric), determined using biophysical indicators. We asked: (1) Are differences in soil moisture regime associated with differences in understory community composition (species richness, diversity, and functional group representation) during the first three decades after fire? (2) Does the relationship between soil moisture regime and community composition change over time as subcanopy (height >137 cm) tree densities increase? We confirmed our original designations of soil moisture regime using in situ measurements of soil moisture. Over the first decade of succession, species richness and diversity were lower in xeric-burned than in mesic-burned plots. Nearly 30 years after fire at the south-aspect study area, where subcanopy tree densities were low, seral understories diverged by soil moisture regime. There, graminoid cover was 23-fold higher, and forb cover was 3-fold lower, in xeric-burned than in mesic-burned plots. In contrast, in the mixed aspect study area, subcanopy tree densities were markedly higher. There, the understory did not vary by soil moisture regime, converging successionally and becoming more similar to unburned forest communities. Our results suggest that site-specific soil moisture regimes structure the early trajectory of post-fire understory recovery, but the relationship diminishes as tree canopies develop over time. However, under a warming climate, variable tree canopy development may compound increasing aridity, particularly on xeric sites, resulting in decreases in plant diversity and forb cover, increases in graminoid cover, and the potential for altered successional dynamics.

Introduction

Understory plant communities are integral components of forest ecosystems worldwide, as they provide food and habitat for wildlife and comprise most of the plant diversity in some forests (Gilliam 2007). In subalpine and boreal coniferous forests, which experience infrequent stand-replacing fires, many understory plant species reestablish rapidly from residual on-site propagules or seed dispersed from unburned areas (Lyon, 1984, Stickney, 1986, Abella and Fornwalt, 2015) and contribute vital ecosystem services after fire, including soil stabilization and nutrient cycling (Swanson et al. 2011). Conifer regeneration, resulting in canopy closure over time, is a key driver of succession that also determines the rate of forest recovery (Turner et al., 2016, Hansen et al., 2018). As young conifers grow and increasingly shade the forest floor, they alter microclimatic conditions (e.g., air and soil temperatures, relative humidity, and soil moisture) and induce species turnover in the understory, shifting early, open-canopy communities to closed-canopy communities (Breshears et al., 1998, Anderegg et al., 2012). Conifer growth is constrained by short growing seasons (Pfister et al. 1977), and timeframes for forest recovery are highly variable, often requiring decades for early canopy closure (Romme et al. 2016) and centuries for mature canopy development (Peet, 1978, Alexander, 1987). However, increasing fire severity and prolonged aridity due to climate change have the potential to alter successional dynamics, with unclear consequences for the structure, community composition, and associated ecosystem services of the understory (Rocca et al. 2014, Stevens et al., 2015).

Plant communities recovering from fire may be particularly vulnerable to the effects of climate-induced moisture limitation, given that plants in early life stages are susceptible to water stress. For instance, conifer seed germination is largely dependent on sufficient moisture availability (Andrus et al. 2018), and young saplings often experience drought-induced mortality due to their shallow rooting habit (Hungerford and Babbit 1987). In the Rocky Mountains of the western United States, where fire frequency and soil moisture deficits have increased since the 1980s (Dennison et al., 2014, Abatzoglou and Williams, 2016), low and highly variable rates of post-fire conifer regeneration have been reported in areas that 1) experienced severe fire, resulting in large, arid burn patches; 2) experienced drought following fire; and 3) were located at the lower elevational limits of conifer growth (Donato et al., 2016, Harvey et al., 2016, Rother and Veblen, 2016, Stevens-Rumann et al., 2018). Over the near term, conifer regeneration is anticipated to become increasingly variable in distribution, with recent simulation studies predicting the failure of forests stands to reestablish under scenarios of accelerated warming (Hansen et al., 2018, Albrich et al., 2020). However, the potential cascading effects on the understory remain virtually unexamined. For instance, in subalpine forests of the southern Rocky Mountains, low conifer regeneration densities resulted in high light availability and persistently high herbaceous cover after three decades of post-fire succession (Coop et al. 2010). But whether variable conifer regeneration may interact with prolonged aridity and structure understory communities over the long term is unclear, and studies that examine these combined effects may provide insights with regards to climate change impacts on plant diversity.

Although soil moisture is predicted to decrease throughout the Rocky Mountain region under continued warming (Halofsky et al. 2018), the magnitude of the effect will depend on the biophysical setting of a given area, including its soil moisture regime (McLaughlin et al. 2017). Soil moisture in the Rocky Mountains exhibits high spatial and temporal variation (Alexander 1987). Winter snowfall is the primary source of moisture, which is released as snowmelt into the underlying soil substrate, followed by a peak in moisture availability during the summer growing season (MacMahon and Andersen, 1982, Harpold et al., 2015). Slope aspect is a major factor that determines soil moisture regimes, as it directly modifies the intensity of solar insolation. In the northern hemisphere, southerly and westerly slope aspects receive greater solar insolation and experience more xeric conditions than northerly and easterly aspects (Warren 2008).

Natural experiments that compare understory successional development under contrasting soil moisture regimes may reveal the effects of prolonged aridity following fire. For instance, Donnegan and Rebertus (1999), using a tree ring chronology that spanned 300 years of post-fire succession in Colorado, reported faster successional replacement of the dominant conifer species and higher tree densities on mesic, northerly slopes than on xeric, southerly slopes. Furthermore, previous studies of early-successional communities following volcanic eruptions on Mauna Loa, Hawaii have reported higher species richness, faster rates of soil nutrient accumulation, and more rapid community recovery on mesic, windward slope faces than on xeric, leeward slope faces, presumably due to lower water stress (Vitousek et al., 1992, Aplet et al., 1998).

We used a long-term natural experiment of subalpine forest succession to investigate understory community development in relation to different soil moisture regimes and variable conifer regeneration over three decades following the 1988 Yellowstone fires. This complex of large fires burned >250,000 ha within the Greater Yellowstone Ecosystem (GYE) and is considered a harbinger of future fire dynamics in the northern Rocky Mountains, given that fire behavior was driven primarily by extreme weather (e.g., severe drought and high winds) rather than fuels (Romme and Despain, 1989, Renkin and Despain, 1992). In 1990, D.F. Tomback established permanent plots in two study areas in the GYE (Fig. 1), which have been measured intermittently from 1990 to 2001 (Tomback et al. 2001). Site types (i.e., natural treatments) were defined by burn status (burned or unburned) and soil moisture regime (mesic or xeric), which was determined in the field using biophysical indicators (see below). We remeasured plots in 2016 (Henderson Mtn.) and 2017 (Mt. Washburn) to answer the following questions: 1) Are differences in soil moisture regime associated with differences in understory community composition (species richness, diversity, and functional group representation) during the first three decades after fire? 2) Does the relationship between soil moisture regime and community composition change over time as subcanopy (height >137 cm) tree densities increase? Our goals were to re-confirm our original designations of soil moisture regime using in situ measurements of soil moisture and characterize post-fire successional dynamics in the understory.