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1. Drought and the interannual variability of stem growth in an aseasonal, everwet forest

   https://doi.org/10.1111/btp.12624  

   Hogan, J. Aaron ; McMahon, Sean M. ; Buzzard, Vanessa ; Michaletz, Sean T. ; Enquist, Brian J. ; Thompson, Jill ; Swenson, Nathan G. ; Zimmerman, Jess K. ( February 2019 , Biotropica)

   Linking drought to the timing of physiological processes governing tree growth remains one limitation in forecasting climate change effects on tropical trees. Using dendrometers, we measured fine‐scale growth for 96 trees of 25 species from 2013 to 2016 in an everwet forest in Puerto Rico. Rainfall over this time span varied, including an unusual, severe El Niño drought in 2015. We assessed how growing season onset, median day, conclusion, and length varied with absolute growth rate and tree size over time. Stem growth was seasonal, beginning in February, peaking in July, and ending in November. Species growth rates varied between 0 and 8 mm/year and correlated weakly with specific leaf area, leaf phosphorus, and leaf nitrogen, and to a lesser degree with wood specific gravity and plant height. Drought and tree growth were decoupled, and drought lengthened and increased variation in growing season length. During the 2015 drought, many trees terminated growth early but did not necessarily grow less. In the year following drought, trees grew more over a shorter growing season, with many smaller trees showing a post‐drought increase in growth. We attribute the increased growth of smaller trees to release from light limitation as the canopy thinned because of the drought, and less inferred hydraulic stress than larger trees during drought. Soil type accounted for interannual and interspecific differences, with the finest Zarzal clays reducing tree growth. We conclude that drought affects the phenological timing of tree growth and favors the post‐drought growth of smaller, sub‐canopy trees in this everwet forest.

   Abstract in Spanish is available with online material.

    

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2. Wood and water: How trees modify wood development to cope with drought

   https://doi.org/10.1002/ppp3.29  

   Rodriguez‐Zaccaro, F. Daniela ; Groover, Andrew ( March 2019 , PLANTS, PEOPLE, PLANET)

   Drought plays a conspicuous role in forest mortality, and is expected to become more severe in future climate scenarios. Recent surges in drought‐associated forest tree mortality have been documented worldwide. For example, recent droughts in California and Texas killed approximately 129 million and 300 million trees, respectively. Drought has also induced acute pine tree mortality across east‐central China, and across extensive areas in southwest China. Understanding the biological processes that enable trees to modify wood development to mitigate the adverse effects of drought will be crucial for the development of successful strategies for future forest management and conservation.

   Summary

   Drought is a recurrent stress to forests, causing periodic forest mortality with enormous economic and environmental costs. Wood is the water‐conducting tissue of tree stems, and trees modify wood development to create anatomical features and hydraulic properties that can mitigate drought stress. This modification of wood development can be seen in tree rings where not only the amount of wood but also the morphology of the water‐conducting cells are modified in response to environmental conditions. In this review, we provide an overview of how trees conduct water, and how trees modify wood development to affect water conduction properties in response to drought. We discuss key needs for new research, and how new knowledge of wood formation can play a role in the conservation of forests under threat by climate change.

    

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3. Drought survival strategies differ between coastal and montane conifers in northern California

   https://doi.org/10.1002/ecs2.4480  

   Robinson, Wallis ; Kerhoulas, Lucy P. ; Sherriff, Rosemary L. ; Roletti, Gabriel ; van Mantgem, Phillip J. ( March 2023 , Ecosphere)

   Increasingly severe and prolonged droughts are contributing to tree stress and forest mortality across western North America. However, in many cases, we currently have poor information concerning how drought responses in forests vary in relation to competition, climate, and site and tree characteristics. We used annual tree ring evidence of¹³C discrimination (Δ¹³C) and growth metrics to assess drought resistance and resilience for six conifer species at the intersection of several bioregions in northern California. Within each species' range in northern California, we collected competition and tree characteristics from 270 focal trees across sites that varied from wetter to drier habitat conditions (54 sites). Across sites, all six conifer species weathered the severe 2013–2015 drought with reasonably high resistance and post‐drought resilience. However, we found important differences in drought responses between coastal and montane species based on annual growth and Δ¹³C metrics. Broadly, the two coastal species showed consistent declines in drought resistance across successive drought years, whereas the four montane species maintained high drought resistance across drought years. More specifically, we found lower Δ¹³C and growth during drought years in coastal species, suggesting stomatal closure during drought with the potential for vulnerability to carbon depletion during long‐term drought. Conversely, Δ¹³C and growth were stable in montane species throughout the drought, which may contribute to hydraulic failure under increased drought frequency and/or severity. We also evaluated environmental factors that affect Δ¹³C using data from before and during the drought. These physiological models were consistent for the two coastal species, with a positive relationship between annual precipitation and Δ¹³C and a negative relationship between tree density and Δ¹³C. Conversely, the four montane models illustrated a greater importance of site conditions on drought responses for these species. Our findings show differential risk for drought stress across diverse conifers during severe drought. This work highlights the importance of site and tree characteristics in determining drought responses across cool, annually humid coastal habitats to seasonally dry montane habitats.

    

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4. Drought limits large trees in African savannas with or without elephants

   https://doi.org/10.1111/aec.13244  

   Jones, Maggie M. ; Fletcher, Robert ; Kruger, Laurence ; Monadjem, Ara ; Simelane, Phumlile ; McCleery, Robert ( December 2022 , Austral Ecology)

   Abstract Consumers, including megaherbivores and fire, are considered important limiting forces for woody plants and canopy closure in African savannas. However, climatic events like drought can also play a significant role in limiting trees and maintaining tree‐grass coexistence in savannas. The extent to which top‐down control (e.g. megaherbivores) and bottom‐up resource limitation through drought and competition interact to influence savanna tree mortality and woody structure is unclear. Here, we compared the change in the number of large trees before and after a severe drought in a savanna with elephants ( Loxodonta africana ) and one without elephants. Elephants and drought both limited the number of large trees at our sites, but contrary to our predictions, there was no interactive effect of these drivers on overall changes in tree densities. However, there was a synergistic effect on the dominant tree species, Senegalia nigrescens , such that tree loss post‐drought was greater where elephants were present compared to where they were absent. Hence, our results suggest that species‐specific differences in drought resistance, as well as density‐dependent factors, likely impact the severity of drought effects on savanna tree communities. In savannas, drought has the potential to exert strong control on tree survival and prevent canopy closure, thus partially filling the role of megaherbivores in limiting large trees when these consumers are absent. As drought severity and frequency are predicted to increase in the future, the influence of drought on savanna vegetation structure becomes increasingly important to consider. 

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5. Mitigating drought mortality by incorporating topography into variable forest thinning strategies

   https://doi.org/10.1088/1748-9326/ad29aa  

   Thomas, Anooja ; Kolb, Thomas ; Biederman, Joel A ; Venturas, Martin D ; Ma, Qin ; Yang, Di ; Dore, Sabina ; Tai, Xiaonan ( February 2024 , Environmental Research Letters)

   Drought-induced productivity reductions and tree mortality have been increasing in recent decades in forests around the globe. Developing adaptation strategies hinges on an adequate understanding of the mechanisms governing the drought vulnerability of forest stands. Prescribed reduction in stand density has been used as a management tool to reduce water stress and wildfire risk, but the processes that modulate fine-scale variations in plant water supply and water demand are largely missing in ecosystem models. We used an ecohydrological model that couples plant hydraulics with groundwater hydrology to examine how within-stand variations in tree spatial arrangements and topography might mitigate forest vulnerability to drought at individual-tree and stand scales. Our results demonstrated thinning generally ameliorated plant hydraulic stress and improved carbon and water fluxes of the remaining trees, although the effectiveness varied by climate and topography. Variable thinning that adjusted thinning intensity based on topography-mediated water availability achieved higher stand productivity and lower mortality risk, compared to evenly-spaced thinning at comparable intensities. The results from numerical experiments provided mechanistic evidence that topography mediates the effectiveness of thinning and highlighted the need for an explicit consideration of within-stand heterogeneity in trees and abiotic environments when designing forest thinning to mitigate drought impacts.

    

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