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1. Correcting tree-ring δ13C time series for tree-size effects in eight temperate tree species

   https://doi.org/10.1093/treephys/tpz138  

   Vadeboncoeur, Matthew A; Jennings, Katie A; Ouimette, Andrew P; Asbjornsen, Heidi (January 2020, Tree Physiology)

   Cernusak, Lucas (Ed.)

   Abstract Stable carbon isotope ratios (δ13C) in tree rings have been widely used to study changes in intrinsic water-use efficiency (iWUE), sometimes with limited consideration of how C-isotope discrimination is affected by tree height and canopy position. Our goals were to quantify the relationships between tree size or tree microenvironment and wood δ13C for eight functionally diverse temperate tree species in northern New England and to better understand the physical and physiological mechanisms underlying these differences. We collected short increment cores in closed-canopy stands and analyzed δ13C in the most recent 5 years of growth. We also sampled saplings in both shaded and sun-exposed environments. In closed-canopy stands, we found strong tree-size effects on δ13C, with 3.7–7.2‰ of difference explained by linear regression vs height (0.11–0.28‰ m−1), which in some cases is substantially stronger than the effect reported in previous studies. However, open-grown saplings were often isotopically more similar to large codominant trees than to shade-grown saplings, indicating that light exposure contributes more to the physiological and isotopic differences between small and large trees than does height. We found that in closed-canopy forests, δ13C correlations with diameter at breast height were nonlinear but also strong, allowing a straightforward procedure to correct tree- or stand-scale δ13C-based iWUE chronologies for changing tree size. We demonstrate how to use such data to correct and interpret multi-decadal composite isotope chronologies in both shade-regenerated and open-grown tree cohorts, and we highlight the importance of understanding site history when interpreting δ13C time series. 

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2. Evidence of Competitive Release Following Overstory Mortality in a Semi-Arid Piñon-Juniper Woodland

   Gonzalez, Corrie D (August 2023, University of New Mexico Publications in Biology)

   Extreme temperatures and severe drought events have led to widespread tree mortality worldwide. In semi-arid regions of the Southwest United States, these events pose a significant threat to piñon-juniper (PJ) woodlands. We studied the effects of piñon and juniper mortality on the growth and physiology of existing saplings in PJ woodlands by analyzing water status, photosynthetic activity, and tissue chemistry to gain insights into these impacts. Juniper saplings exhibited improved water status and water use efficiency in response to overstory mortality, whereas piñon saplings did not. Additionally, both piñon and juniper saplings exhibited increased photosynthetic rates, increased photosynthetic capacity, and enhanced growth rates. Our results suggest that saplings of both species responded similarly regardless of whether a mature piñon or juniper died. However, piñon saplings appeared to be more vulnerable to overstory mortality, likely due to the difference in hydraulic strategies between piñon and juniper This study enhances our understanding of the post-mortality recovery process in piñon-juniper ecosystems, providing valuable insights into the contrasting effects of piñon vs. juniper mortality as well as the distinct physiological responses exhibited by piñon and juniper saplings. 

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3. Mortality and Resprouting in California Oak Woodlands Following Mixed-Severity Wildfire

   https://doi.org/10.6084/m9.figshare.13554701  

   Ackerly, David D.; Kozanitas, Melina; Oldfather, Meagan; Papper, Prahlad; Clark, Matthew (January 2019, International oaks)

   null (Ed.)

   We quantified fire severity in the Tubbs Fire (Sonoma Co., CA, October 2017) across different vegetation types, and post-fire mortality and regeneration of tree species in permanent plots at the Pepperwood Preserve. The fire burned 14,895 ha, with > 25% in both medium and high severity. Chaparral and Pinus attenuata stands mostly burned at high severity, while other vegetation types experienced a fairly even distribution of fire severity. The fire killed 50% of saplings (dbh < 1 cm) and 27% of trees (dbh ≥ 1 cm), with higher mortality in high severity patches. Quercus agrifolia, Q. kelloggii, Arbutus menziesii and Umbellularia californica exhibited very high levels of topkill combined with basal resprouting. Pseudotsuga menziesii, which lacks resprouting ability, exhibited high mortality, especially in saplings at high severity. The results provide a baseline to examine potential vegetation change due to high-severity fire, especially in high-severity stands of P. menziesii. 

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4. Impacts of short-term feeding by spotted lanternfly (Lycorma delicatula) on ecophysiology of young hardwood trees in a common garden

   https://doi.org/10.3389/finsc.2022.1080124  

   Lavely, Emily; Iavorivska, Lidiia; Uyi, Osariyekemwen; Eissenstat, David M.; Walsh, Brian; Primka, Edward J.; Harper, Jeremy; Hoover, Kelli (December 2022, Frontiers in Insect Science)

   Spotted lanternfly (SLF; Lycorma delicatula White; Hemiptera: Fulgoridae) invaded the US from Asia and was first detected in 2014; currently, populations have established in 14 states primarily in the Northeast and Mid-Atlantic. It feeds voraciously on phloem sap from a broad range of host plants, with a preference for tree of heaven ( Ailanthus altissima [Sapindales: Simaroubaceae]), grapevines ( Vitis spp. [Vitales: Vitaceae]), and several common hardwood tree species. We evaluated the impacts of fourth instars and adults confined to a single branch or whole trees on gas exchange attributes (carbon assimilation [photosynthetic rate], transpiration and stomatal conductance), selected nutrients, and diameter growth using young saplings of four host tree species planted in a common garden. In general, the effects of adults on trees were greater than nymphs, although there was variation depending on tree species, pest density, and time post-infestation. Nymphs on a single branch of red maple ( Acer rubrum [Sapindales: Sapindaceae]), or silver maple ( Acer saccharinum [Sapindales: Sapindaceae]) at three densities (0, 15, or 30) had no significant effects on gas exchange. In contrast, 40 adults confined to a single branch of red or silver maple rapidly suppressed gas exchange and reduced nitrogen concentration in leaves; soluble sugars in branch wood were reduced in the fall for silver maple and in the following spring for red maple. Fourth instars confined to whole silver maple trees reduced soluble sugars in leaves and branch wood, and reduced tree diameter growth by >50% during the next growing season. In contrast, fourth instars in whole tree enclosures had no effects on black walnut ( Juglans nigra [Fagales: Juglandaceae]). SLF enclosed on tree of heaven at 80 adults per tree suppressed gas exchange after two weeks of feeding, but did not alter non-structural carbohydrates, nitrogen concentrations, or tree growth. Results suggest that moderate to heavy feeding by SLF on young maple saplings may impair tree growth, which could have implications for production nurseries and forest managers. 

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5. Understory plant communities show resistance to drought, hurricanes, and experimental warming in a wet tropical forest

   https://doi.org/10.3389/ffgc.2022.733967  

   Alonso-Rodríguez, Aura M.; Wood, Tana E.; Torres-Díaz, Jamarys; Cavaleri, Molly A.; Reed, Sasha C.; Bachelot, Benedicte (July 2022, Frontiers in Forests and Global Change)

   Global climate change has led to rising temperatures and to more frequent and intense climatic events, such as storms and droughts. Changes in climate and disturbance regimes can have non-additive effects on plant communities and result in complicated legacies we have yet to understand. This is especially true for tropical forests, which play a significant role in regulating global climate. We used understory vegetation data from the Tropical Responses to Altered Climate Experiment (TRACE) in Puerto Rico to evaluate how plant communities responded to climate warming and disturbance. The TRACE understory vegetation was exposed to a severe drought (2015), 2 years of experimental warming (4°C above ambient in half of the plots, 2016–2017 and 2018–2019), and two major hurricanes (Irma and María, September 2017). Woody seedlings and saplings were censused yearly from 2015 to 2019, with an additional census in 2015 after the drought ended. We evaluated disturbance-driven changes in species richness, diversity, and composition across ontogeny. We then used Bayesian predictive trait modeling to assess how species responded to disturbance and how this might influence the functional structure of the plant community. Our results show decreased seedling richness after hurricane disturbance, as well as increased sapling richness and diversity after warming. We found a shift in species composition through time for both seedlings and saplings, yet the individual effects of each disturbance were not significant. At both ontogenetic stages, we observed about twice as many species responding to experimental warming as those responding to drought and hurricanes. Predicted changes in functional structure point to disturbance-driven functional shifts toward a mixture of fast-growing and drought-tolerant species. Our findings demonstrate that the tropical forest understory community is more resistant to climatic stressors than expected, especially at the sapling stage. However, early signs of changes in species composition suggest that, in a warming climate with frequent droughts and hurricanes, plant communities might shift over time toward fast-growing or drought-tolerant species. 

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