The effect of shrub cover on conifer water-use patterns, growth and response to precipitation variability in the southern Sierra Nevada.

As wildfires increase in size and severity, large areas of forest are undergoing substantial increases in shrub cover. In forests where water is the limiting resource, the partitioning of soil water between shrubs and young trees may determine how shrubs affect tree growth and water-stress. Here we evaluated juvenile trees (average age = 32 years) of two dominant conifer species in the southern Sierra Nevada of California (Abies concolor (white fir) and Pinus jeffreyi (Jeffrey pine)) growing in the presence or absence of shrubs. The two shrub species included Arctostaphylos patula and Ceanothus cordulatus, a nitrogen-fixing species. We analyzed the δ2H and δ18O values of xylem water for both tree and shrub species to assess how shrub cover affects the water-uptake patterns of conifers and whether there is niche partitioning between trees and shrubs. We found that growing near shrubs did not have a significant effect on the water source dynamics of either tree species, with similar source water contributions calculated for conifers growing in both the presence and absence of shrubs. Using a tree-ring analysis of growth and δ13C from 2016 to 2021, a period of high precipitation variability, we found that shrub cover had a positive effect on tree growth while decreasing carbon discrimination, which may be due to increased nitrogen availability from Ceanothus cordulatus. Overall, our results suggest that growing in the presence of shrubs does not alter the water uptake patterns of white fir and Jeffrey pine and instead may have a positive effect on the growth rates of these species during both wet and dry years.

Conifer water-use patterns across temporal and topographic gradients in the southern Sierra Nevada.

Climate change is increasing the severity and duration of drought events experienced by forest ecosystems. Because water is essential for tree physiological processes, the ability of trees to survive prolonged droughts will largely depend on whether they have access to reliable water sources. While many woody plant species exhibit the ability to shift water sources between different depths of soil and rock water in response to changes in climate and water availability, it is unclear if Sierra Nevada conifers exhibit this plasticity. Here we analysed the δ18O and δ13C values of annual tree rings to determine the water-use patterns of large Sierra Nevada conifers during the 2012-16 California drought and 4 years before this drought event (2004-07). We analysed four species (Pinus jeffreyi Grev. & Balf. (Jeffrey pine), Pinus lambertiana Dougl. (sugar pine), Abies concolor (Gord. & Glend.) Lindl. Ex Hilderbr (white fir) and Calocedrus decurrens (Torr.) Florin (incense-cedar)) across a range of topographic positions to investigate differences in water-use patterns by species and position on the landscape. We found no significant differences in δ18O and δ13C values for the pre-drought and drought periods. This stability in δ18O values suggests that trees did not shift their water-use patterns in response to the 2012-16 drought. We did find species-specific differences in water-use patterns, with incense-cedar exhibiting more depleted δ18O values than all other species. We also found trends that suggest the water source used by a tree may depend on topographic and growing environment attributes such as topographic wetness and the surrounding basal area. Overall, our results suggest that the water source used by trees varies by the species and topographic position, but that Sierra Nevada conifers do not switch their water-use patterns in response to the drought. This lack of plasticity could make Sierra Nevada conifers particularly vulnerable to drought mortality as their historically reliable water sources begin to dry out with climate change.

Changing climate reallocates the carbon debt of frequent-fire forests.

Ongoing climate change will alter the carbon carrying capacity of forests as they adjust to climatic extremes and changing disturbance regimes. In frequent-fire forests, increasing drought frequency and severity are already causing widespread tree mortality events, which can exacerbate the carbon debt that has developed as a result of fire exclusion. Forest management techniques that reduce tree density and surface fuels decrease the risk of high-severity wildfire and may also limit drought-induced mortality by reducing competition. We used a long-term thinning and burning experiment in a mixed-conifer forest to investigate the effects of the 2012-2015 California drought on forest carbon dynamics in each treatment, including the carbon emissions from a second-entry prescribed fire that followed the drought. We assessed differences in carbon stability and tree survival across treatments, expecting that both carbon stability and survival probability would increase with increasing treatment intensity (decreasing basal area). Additionally, we analyzed the effects of drought- mortality on second-entry burn emissions and compared emissions for the first- and second-entry burns. We found a non-linear relationship between treatment intensity and carbon stability, which was in part driven by varying relationships between individual tree growing space and survival across treatments. Drought mortality increased dead tree and surface fuel carbon in all treatments, which contributed to higher second-entry burn emissions for two of the three burn treatments when compared to the first burn. Our findings suggest that restoration treatments will not serve as a panacea for ongoing climate change and that the carbon debt of these forests will become increasingly unstable as the carbon carrying capacity adjusts to severe drought events. Managing the carbon debt with prescribed fire will help reduce the risk of additional mortality from wildfire, but at an increasing carbon cost for forest management.