We compared surveys from 1881 with modern vegetation to test for changes in a big sagebrush landscape in a National Monument in New Mexico, USA. We found an increase in sagebrush over 140 years, which contrasts with the sagebrush decline occurring in the Great Basin. These results provide insights into different trends and processes affecting similar vegetation in different regions.

Abstract

Questions

Big sagebrush (Artemisia tridentata) ecosystems across the western United States have experienced many changes in ecosystem dynamics and vegetation composition over the last century due to livestock grazing, non-native species, and changing climate and fire regimes. We conducted the first systematic investigation of historical vegetation composition and vegetation change in a sagebrush landscape in the southwestern United States, asking whether sagebrush or grass dominated the landscape historically?

Location

The Rio Grande del Norte National Monument (RGDN), northern New Mexico, USA.

Methods

We combined General Land Office (GLO) surveys from 1881 with modern vegetation maps, field vegetation surveys, and sagebrush ages from growth ring analysis to test for changes in vegetation in the RGDN over the last 140 years.

Results

We found that big sagebrush presence across the study area increased significantly, from being present on 16% of section lines in 1881 to 79% in 2019, and only three section lines lost sagebrush presence during that period. Concurrently, the number of section lines with low grass index more than doubled since 1881, while moderate and high grass index declined. Grass declined equally in areas where sagebrush increased and areas with no change in sagebrush, suggesting that changes in both vegetation types were catalyzed by external factors, likely including overgrazing. The growth ring analysis of 93 sagebrush revealed a maximum age of 87 years and establishment in every decade since the 1930s, consistent with the GLO results.

Conclusions

The significant vegetation changes in the RGDN over the last century, including an increase of sagebrush, provide important context about the shifting mosaic of grasslands and shrublands relevant to current and future management and ecosystem dynamics.

Habitat mosaics due to shrub establishment in mixed conifer forest after fire creates high spatial heterogeneity at broader scales. However, reburned patches from high to high and low and moderate to low and moderate severities have less spatial heterogeneity. Under climate change-induced increase in wildfire within short return interval, conifer regeneration within large stand-replaced patches is uncertain.

Abstract

Aims

Prolonged fire suppression in conifer forests of the Sierra Nevada Mountain range, California, USA, has led to ingrowth of conifer seedlings, converting the open heterogeneous structure into uniformly dense and layered forest. The threat of a stand-replacing fire has increased because of fuel buildup in combination with rising drought and extreme heat frequency caused by climate change. With such high severity fire, there is also rising concern regarding conifer forest converting to shrublands as severe fire favors the establishment of large shrub patches altering landscape vegetation pattern and heterogeneity. However, a clear understanding of the effects of increased fire severity, size, and frequency on landscape-scale heterogeneity and postfire patch dynamics is lacking, which is critical in implementing restoration and forest management activities. Our aim was to understand multiscale dynamics and spatial heterogeneity patterns of conifer forests and chaparral shrublands due to repeated mixed-severity fire.

Location

A mosaic of burned and unburned patches spanning the boundary of Lassen and Plumas National Forests, CA, USA.

Methods

We used secondary geospatial landcover data classified by cover type before modern fires (1999) and after eight modern fires (2014). We calculated various landscape diversity and fragmentation metrics at patch and landscape scales using FRAGSTATS for comparison before and after fires.

Results

At the fire severity patch scale, high-severity fire reduced vegetation cover type heterogeneity by half, but reburning at low to moderate severity nearly doubled cover type heterogeneity. At the full landscape scale mixed-severity fire, including all burn severities, increased vegetation cover type heterogeneity. Fragmentation indexes confirmed that fire created larger patches of shrub and fragmented patches of conifer forest.

Conclusions

The effects of frequent large fire events on vegetation pattern and heterogeneity vary with the scale of analysis. Hence, heterogeneity and vegetation pattern change need to be evaluated at more than one scale to understand past and future ecological processes before prioritizing management actions for the conifer forests of the Sierra Nevada Mountain range.

We explored variation in plant community processes in sites spanning the latitudinal extent of the boreal biome of western North America. We found that the majority of variation in plant community composition and functional traits occurred within sites, suggesting that local-scale environmental gradients were more strongly driving plant community dynamics than the regional-scale climatic gradient across our sites.

Abstract

Questions

Rapid climate change in northern latitudes is expected to influence plant functional traits of the whole community (community-level traits) through species compositional changes and/or trait plasticity, limiting our ability to anticipate climate warming impacts on northern plant communities. We explored differences in plant community composition and community-level traits within and among four boreal peatland sites and determined whether intra- or interspecific variation drives community-level traits.

Location

Boreal biome of western North America.

Methods

We collected plant community composition and functional trait data along dominant topoedaphic and/or hydrologic gradients at four peatland sites spanning the latitudinal extent of the boreal biome of western North America. We characterized variability in community composition and community-level traits of understorey vascular and moss species both within (local-scale) and among sites (regional-scale).

Results

Against expectations, community-level traits of vascular plant and moss species were generally consistent among sites. Furthermore, interspecific variation was more important in explaining community-level trait variation than intraspecific variation. Within-site variation in both community-level traits and community composition was greater than among-site variation, suggesting that local environmental gradients (canopy density, organic layer thickness, etc.) may be more influential in determining plant community processes than regional-scale gradients.

Conclusions

Given the importance of interspecific variation to within-site shifts in community-level traits and greater variation of community composition within than among sites, we conclude that climate-induced shifts in understorey community composition may not have a strong influence on community-level traits in boreal peatlands unless local-scale environmental gradients are substantially altered.

Natural reforestation through secondary succession in marginal agricultural land is an important component of climate mitigation and adaptation, but the fundamental ecological processes promoting or constraining it are poorly understood. This study provides national-scale evidence that species accumulation in post-agricultural forest successions may be slowed by intense mammalian herbivory and low rates of biotic seed dispersal. Naturally reforesting post-agricultural land, South Island, New Zealand.

Abstract

Question

Natural reforestation is an important component of climate mitigation and adaptation, but the ecological processes promoting or constraining it are poorly understood. In this study we employ a stand reconstruction approach (which uses ages of extant trees to estimate year of establishment for each individual tree) to test for general trait-based effects on tree species arrival order in post-agricultural forest successions.

Location

Naturally reforesting post-agricultural landscapes throughout New Zealand.

Methods

Ages were obtained for 2434 individuals spanning 30 tree species across a nationwide network of 128 plots in 14 naturally reforesting post-agricultural sites. These ages were used to calculate individual-level arrival times (relative to the oldest individual in each plot). We estimated species-level arrival times by fitting linear mixed-effects (LME) regressions (with species identity as the fixed effect, and plots nested within sites as the random effects) to individual arrival time data. We used back-casting (where arrival time data are used to document individual-level presence in plots through time) to track annual changes in species abundance and community-weighted mean (CWM) trait values.

We used standardised major axis (SMA) regressions to examine the effect of traits related to resource use strategy, herbivory avoidance, seed dispersal and disturbance response on species-level arrival times. We used LME regressions to test for changes in CWM trait values with stand age.

Results

The earliest-arriving species had traits associated with herbivory avoidance, were abiotically dispersed and had short predicted dispersal distances. There was no evidence that traits linked to resource use strategy or disturbance response affected species arrival times. Every significant species-level relationship was recovered in community-level LME analyses.

Conclusions

Our findings suggest that mammalian herbivore control and enhancement of biotic (bird) seed dispersal may be key management interventions in realising the full climate mitigation and adaptation potential of natural reforestation in post-agricultural landscapes.