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1. African savanna grasses outperform trees across the full spectrum of soil moisture availability

   https://doi.org/10.1111/nph.18909  

   Belovitch, Michael W.; NeSmith, Julienne E.; Nippert, Jesse B.; Holdo, Ricardo M. (April 2023, New Phytologist)

   Summary Models of tree–grass coexistence in savannas make different assumptions about the relative performance of trees and grasses under wet vs dry conditions. We quantified transpiration and drought tolerance traits in 26 tree and 19 grass species from the African savanna biome across a gradient of soil water potentials to test for a trade‐off between water use under wet conditions and drought tolerance.We measured whole‐plant hourly transpiration in a growth chamber and quantified drought tolerance using leaf osmotic potential (Ψ_osm). We also quantified whole‐plant water‐use efficiency (WUE) and relative growth rate (RGR) under well‐watered conditions.Grasses transpired twice as much as trees on a leaf‐mass basis across all soil water potentials. Grasses also had a lower Ψ_osmthan trees, indicating higher drought tolerance in the former. Higher grass transpiration and WUE combined to largely explain the threefold RGR advantage in grasses.Our results suggest that grasses outperform trees under a wide range of conditions, and that there is no evidence for a trade‐off in water‐use patterns in wet vs dry soils. This work will help inform mechanistic models of water use in savanna ecosystems, providing much‐needed whole‐plant parameter estimates for African species. 

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2. Post-Wildfire Regeneration in a Sky-Island Mixed- Conifer Ecosystem of the North American Great Basin

   https://doi.org/10.3390/f11090900  

   Kilpatrick, Mackenzie; Biondi, Franco (September 2020, Forests)

   null (Ed.)

   Information on wildfire impacts and ecosystem responses is relatively sparse in the Great Basin of North America, where subalpine ecosystems are generally dominated by five-needle pines. We analyzed existing vegetation, with an emphasis on regeneration following the year 2000 Phillips Ranch Fire, at a sky-island site in the Snake Range of eastern Nevada. Our main objective was to compare bristlecone pine (Pinus longaeva; PILO) post-fire establishment and survival to that of the co-occurring dominant conifers limber pine (Pinus flexilis; PIFL) and Engelmann spruce (Picea engelmannii; PIEN) in connection with site characteristics. Field data were collected in 40 circular 0.1 ha plots (17.8 m radius) randomly located using GIS so that half of them were inside (“burned”) and half were outside (“unburned”) the 2000 fire boundary. While evidence of previous burns was also found, we focused on impacts from the Phillips Ranch Fire. Mean total basal area, including live and dead stems, was not significantly different between plots inside the burn and plots outside the fire perimeter, but the live basal area was significantly less in the former than in the latter. Wildfire impacts did not limit regeneration, and indeed bristlecone seedlings and saplings were more abundant in plots inside the 2000 fire perimeter than in those outside of it. PILO regeneration, especially saplings, was more abundant than PIFL and PCEN combined, indicating that PILO can competitively regenerate under modern climatic conditions. Surviving PILO regeneration in burned plots was also taller than that of PIFL. By contrast, PCEN was nearly absent in the plots that had been impacted by fire. Additional research should explicitly address how climatic changes and disturbance processes may interact in shaping future vegetation dynamics. 

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3. Groundwater subsidizes tree growth and transpiration in sandy humid forests

   https://doi.org/10.1002/eco.2294  

   Ciruzzi, Dominick M.; Loheide, II, Steven P. (June 2021, Ecohydrology)

   Abstract As drought variability increases in forests around the globe, it is critical to evaluate and understand ecosystem attributes that ameliorate drought impacts. Trees in arid and semi‐arid ecosystems can sustain tree growth and transpiration during drought by accessing shallow groundwater, yet the extent to which groundwater influences forest growth and transpiration in humid environments has largely been unexplored. We quantified groundwater's influence on tree growth and transpiration in northern humid forests with sandy soils. We hypothesized that even in wet regions, soil droughts occur relatively frequently in forests with sandy soils and result in water stress and reduced tree growth. Further, we hypothesized these reductions in productivity are ameliorated if the forest can access shallow groundwater during dry conditions. We evaluated tree growth responses using tree cores inPinus resinosatrees and estimated forest groundwater use from diel water table fluctuations across sites covering a 1‐ to 9‐m depth‐to‐groundwater (DTG) gradient. In areas of shallow groundwater (DTG < 2.5 m), we observed twice as much tree growth and high, frequent groundwater use (up to 81% of non‐rainy summer days). Groundwater's influence on tree growth and transpiration declined as groundwater deepened along the DTG gradient in the range 1–5 m below land surface. These findings suggest that water provided by a shallow water table subsidizes evapotranspiration in humid forests and results in enhanced tree growth. Our research provides a basis for understanding the role of groundwater in conferring drought resistance in humid forests to help guide sustainable water and forest management decisions. 

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4. Exploring Environmental Factors That Drive Diel Variations in Tree Water Storage Using Wavelet Analysis

   https://doi.org/10.3389/frwa.2021.682285  

   Harmon, Ryan E.; Barnard, Holly R.; Day-Lewis, Frederick D.; Mao, Deqiang; Singha, Kamini (August 2021, Frontiers in Water)

   Internal water storage within trees can be a critical reservoir that helps trees overcome both short- and long-duration environmental stresses. We monitored changes in internal tree water storage in a ponderosa pine on daily and seasonal scales using moisture probes, a dendrometer, and time-lapse electrical resistivity imaging (ERI). These data were used to investigate how patterns of in-tree water storage are affected by changes in sapflow rates, soil moisture, and meteorologic factors such as vapor pressure deficit. Measurements of xylem fluid electrical conductivity were constant in the early growing season while inverted sapwood electrical conductivity steadily increased, suggesting that increases in sapwood electrical conductivity did not result from an increase in xylem fluid electrical conductivity. Seasonal increases in stem electrical conductivity corresponded with seasonal increases in trunk diameter, suggesting that increased electrical conductivity may result from new growth. On the daily scale, changes in inverted sapwood electrical conductivity correspond to changes in sapwood moisture. Wavelet analyses indicated that lag times between inverted electrical conductivity and sapflow increased after storm events, suggesting that as soils wetted, reliance on internal water storage decreased, as did the time required to refill daily deficits in internal water storage. We found short time lags between sapflow and inverted electrical conductivity with dry conditions, when ponderosa pine are known to reduce stomatal conductance to avoid xylem cavitation. A decrease in diel amplitudes of inverted sapwood electrical conductivity during dry periods suggest that the ponderosa pine relied on internal water storage to supplement transpiration demands, but as drought conditions progressed, tree water storage contributions to transpiration decreased. Time-lapse ERI- and wavelet-analysis results highlight the important role internal tree water storage plays in supporting transpiration throughout a day and during periods of declining subsurface moisture. 

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5. 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)

   Abstract 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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