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1. Assessing Forest Level Response to the Death of a Dominant Tree within a Premontane Tropical Rainforest

   https://doi.org/10.3390/f12081041  

   Flores III, Manuel R.; Aparecido, Luiza Maria; Miller, Gretchen R.; Moore, Georgianne W. (August 2021, Forests)

   null (Ed.)

   Small-scale treefall gaps are among the most important forms of forest disturbance in tropical forests. These gaps expose surrounding trees to more light, promoting rapid growth of understory plants. However, the effects of such small-scale disturbances on the distribution of plant water use across tree canopy levels are less known. To address this, we explored plant transpiration response to the death of a large emergent tree, Mortoniodendron anisophyllum Standl. & Steyerm (DBH > 220 cm; height ~40 m). Three suppressed, four mid-story, and two subdominant trees were selected within a 50 × 44 m premontane tropical forest plot at the Texas A&M Soltis Center for Research and Education located in Costa Rica. We compared water use rates of the selected trees before (2015) and after (2019) the tree gap using thermal dissipation sap flow sensors. Hemispherical photography indicated a 40% increase in gap fraction as a result of changes in canopy structure after the treefall gap. Micrometeorological differences (e.g., air temperature, relative humidity, and vapor pressure deficit (VPD)) could not explain the observed trends. Rather, light penetration, as measured by sensors within the canopy, increased significantly in 2019. One year after the tree fell, the water usage of trees across all canopy levels increased modestly (15%). Moreover, average water usage by understory trees increased by 36%, possibly as a result of the treefall gap, exceeding even that of overstory trees. These observations suggest the possible reallocation of water use between overstory and understory trees in response to the emergent tree death. With increasing global temperatures and shifting rainfall patterns increasing the likelihood of tree mortality in tropical forests, there is a greater need to enhance our understanding of treefall disturbances that have the potential to redistribute resources within forests. 

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2. Spatial configuration and time of day impact the magnitude of urban tree canopy cooling

   https://doi.org/10.1088/1748-9326/ac12f2  

   Alonzo, Michael; Baker, Matthew E; Gao, Yuemeng; Shandas, Vivek (August 2021, Environmental Research Letters)

   Abstract Tree cover is generally associated with cooler air temperatures in urban environments but the roles of canopy configuration, spatial context, and time of day are not well understood. The ability to examine spatiotemporal relationships between trees and urban climate has been hindered by lack of appropriate air temperature data and, perhaps, by overreliance on a single ‘tree canopy’ class, obscuring the mechanisms by which canopy cools. Here, we use >70 000 air temperature measurements collected by car throughout Washington, DC, USA in predawn (pd), afternoon (aft), and evening (eve) campaigns on a hot summer day. We subdivided tree canopy into ‘soft’ (over unpaved surfaces) and ‘hard’ (over paved surfaces) canopy classes and further partitioned soft canopy into distributed (narrow edges) and clumped patches (edges with interior cores). At each level of subdivision, we predicted air temperature anomalies using generalized additive models for each time of day. We found that the all-inclusive ‘tree canopy’ class cooled linearly at every time (pd = 0.5 °C ± 0.3 °C, aft = 1.8 °C ± 0.6 °C, eve = 1.7 °C ± 0.4 °C), but could be explained in the afternoon by aggregate effects of predominant hard and soft canopy cooling at low and high canopy cover, respectively. Soft canopy cooled nonlinearly in the afternoon with minimal effect until ∼40% cover but strongly (and linearly) across all cover fractions in the evening (pd = 0.7 °C ± 1.1 °C, aft = 2.0 °C ± 0.7 °C, eve = 2.9 °C ± 0.6 °C). Patches cooled at all times of day despite uneven allocation throughout the city, whereas more distributed canopy cooled in predawn and evening due to increased shading. This later finding is important for urban heat island mitigation planning since it is easier to find planting spaces for distributed trees rather than forest patches. 

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3. Tree Transpiration and Urban Temperatures: Current Understanding, Implications, and Future Research Directions

   https://doi.org/10.1093/biosci/biaa055  

   Winbourne, Joy B; Jones, Taylor S; Garvey, Sarah M; Harrison, Jamie L; Wang, Liang; Li, Dan; Templer, Pamela H; Hutyra, L R (June 2020, BioScience)

   null (Ed.)

   Abstract The expansion of an urban tree canopy is a commonly proposed nature-based solution to combat excess urban heat. The influence trees have on urban climates via shading is driven by the morphological characteristics of trees, whereas tree transpiration is predominantly a physiological process dependent on environmental conditions and the built environment. The heterogeneous nature of urban landscapes, unique tree species assemblages, and land management decisions make it difficult to predict the magnitude and direction of cooling by transpiration. In the present article, we synthesize the emerging literature on the mechanistic controls on urban tree transpiration. We present a case study that illustrates the relationship between transpiration (using sap flow data) and urban temperatures. We examine the potential feedbacks among urban canopy, the built environment, and climate with a focus on extreme heat events. Finally, we present modeled data demonstrating the influence of transpiration on temperatures with shifts in canopy extent and irrigation during a heat wave. 

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4. Minneapolis-St. Paul, MN Canopy Temperature and Combined Urban Tree Index (CUTI), 2018-2021

   https://doi.org/10.6073/pasta/c03a0e57f1677fbf5cc65ebd4dc4a850  

   Wilkening, Jean V; Feng, Xue (May 2024, Environmental Data Initiative)

   This dataset includes a series of raster datasets of canopy temperature across Minneapolis-St. Paul, MN at a resolution of approximately 70 meters. There are data from five different warm, summer afternoons from 2018-2021. The canopy temperature data was derived from ECOSTRESS Land Surface Temperature and Emissivity data and high-resolution land cover data using a downscaling approach. Canopy temperature acts as a proxy of canopy health and function, with higher temperatures indicative of higher stress and lower function in the canopy. For each image, we also compute the combined urban tree index (CUTI), a metric we developed that provides an indication of relative benefits from urban canopy which arise from a combination of the amount and function/health of urban canopy (based on the canopy temperature). CUTI ranges from 0-1, with high values indicating areas likely experiencing greater canopy benefits due to greater canopy cover and better canopy health/function. 

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5. Understory plant diversity and composition across a postfire tree density gradient in a Siberian Arctic boreal forest

   https://doi.org/10.1139/cjfr-2020-0483  

   Paulson, Alison K.; Peña, Homero; Alexander, Heather D.; Davydov, Sergei P.; Loranty, Michael M.; Mack, Michelle C.; Natali, Susan M. (May 2021, Canadian Journal of Forest Research)

   null (Ed.)

   Cajander larch (Larix cajanderi Mayr.) forests of the Siberian Arctic are experiencing increased wildfire activity in conjunction with climate warming. These shifts could affect postfire variation in the density and arrangement of trees and understory plant communities. To better understand how understory plant composition, abundance, and diversity vary with tree density, we surveyed understory plant communities and stand characteristics (e.g., canopy cover, active layer depth, and soil organic layer depth) within 25 stands representing a density gradient of similarly-aged larch trees that established following a 1940 fire near Cherskiy, Russia. Understory plant diversity and mean total plant abundance decreased with increased canopy cover. Canopy cover was also the most important variable affecting individual species’ abundances. In general, tall shrubs (e.g., Betula nana subsp. exilis) were more abundant in low-density stands with high light availability, and mosses (e.g., Sanionia spp.) were more abundant in high-density stands with low light availability. These results provide evidence that postfire variation in tree recruitment affects understory plant community composition and diversity as stands mature. Therefore, projected increases in wildfire activity in the Siberian Arctic could have cascading impacts on forest structure and composition in both overstory and understory plant communities. 

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