Hotter Drought Escalates Tree Cover Declines in Blue Oak Woodlands of California

Abstract

California has, in recent years, become a hotspot of interannual climatic variability, recording devastating climate-related disturbances with severe effects on tree resources. Understanding the patterns of tree cover change associated with these events is vital for developing strategies to sustain critical habitats of endemic and threatened vegetation communities. We assessed patterns of tree cover change, especially the effects of the 2012–2016 drought within the distribution range of blue oak (Quercus douglasii), an endemic tree species to California with a narrow geographic extent. We utilized multiple, annual land-cover and land-surface change products from the U.S. Geological Survey (USGS) Land Change Monitoring, Assessment and Projection (LCMAP) project along with climate and wildfire datasets to monitor changes in tree cover state and condition and examine their relationships with interannual climate variability between 1985 and 2016. Here, we refer to a change in tree cover class without a land-cover change to another class as “conditional change.” The unusual drought of 2012–2016, accompanied by anomalously high temperatures and vapor pressure deficit, was associated with exceptional spikes in the amount of both fire and non-fire induced tree cover loss and tree cover conditional change, especially in 2015 and 2016. Approximately 1,266 km2 of tree cover loss and 617 km2 of tree cover conditional change were recorded during that drought. Tree cover loss through medium to high severity fires was especially large in exceptionally dry and hot years. Our study demonstrates the usefulness of the LCMAP products for monitoring the effects of climatic extremes and disturbance events on both thematic and conditional land-cover change over a multi-decadal period. Our results signify that blue oak woodlands may be vulnerable to extreme climate events and changing wildfire regimes. Here, we present early evidence that frequent droughts associated with climate warming may continue to affect tree cover in this region, while drought interaction with wildfires and the resulting feedbacks may have substantial influence as well. Consequently, efforts to conserve the blue oak woodlands, and potentially other vegetation communities in the Western United States, may benefit from consideration of climate risks as well as the potential for climate-fire and vegetation feedbacks.

Trends in tree cover change over three decades related to interannual climate variability and wildfire in California

“…Drought and wildfire associated tree cover loss mostly surged in California during the last decade of the study period; a trend consistent with several other wildfire studies in the western United States (Parks and Abatzoglou 2020, Dwomoh et al 2021, Iglesias et al 2022. Our findings indicate that frequent hotter and drier climate in the future may trigger more rapid tree cover loss, catalyzed by more severe fires, with the potential of engendering ecosystem regime shifts across ecoregions.…”

“…The state's multi-year drought of 2012-2016 has been considered as one of the most severe droughts in over a century (Griffin and Anchukaitis 2014, Robeson 2015, Lund et al 2018. During that drought several millions of trees were severely affected leading to substantial dieoff (Byer andJin 2017, Stephenson et al 2018) and degradation of critical habitats (Dwomoh et al 2021).…”

“…This drought affected millions of trees, resulting in widespread crown defoliation, foliage die-back and massive die-offs (Byer and Jin 2017, Paz-Kagan et al 2017, Stephenson et al 2018, Keen et al 2020. Critical habitats of endemic tree species suffered largescale tree mortality during that warmer drought (Das et al 2020, Dwomoh et al 2021. Tubbesing et al (2020) estimate that approximately 1%-5% of the aboveground live tree biomass across California's forested lands died during the drought.…”

Witnessing history: comparison of a century of sedimentary and written records in a California protected area

“…3 a and 4 a), which may be related to Upper Lake Marsh’s greater proximity to upland Sequoia sempervirens forests, and Searsville Lake being surrounded by Quercus agrifolia woodland, as well as to potential differences in pollen delivery to the two locations. While we cannot directly tie the frequent fluctuations of oak during this time to specific ecological events, this period witnessed pruning for power lines at JRBP, and disease since at least the 1980s (Nona Chiariello, personal communication ), when blue oak ( Q. douglasii ) dieback became a regional problem, followed by Sudden Oak Decline in coast live oak ( Q. agrifolia ) and black oak ( Q. velutina ) (UC Berkeley Forest Pathology and Mycology Lab 2022 ), perhaps exacerbated by warming climates (Dwomoh et al 2021 ).…”

Conterminous United States Land-Cover Change (1985–2016): New Insights from Annual Time Series

“…This helped push the ascendancy of southern U.S. as the leading industrial forestry region in the nation [43], although its wood production and future trends have been well reported e.g., [44]. Towards the mid-2010s, tree-cover removal again mostly outpaced regrowth (Figure 3a), driven by increasing economic activities after the Great Recession [45], generally larger wildfire years [46] (Figure 4), and in at least one region wildfires during "hotter" droughts [47]. This second era (2012-2016) of net tree-cover loss rivaled the earlier one of the late 1980s and early 1990s.…”