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High productivity of fertilized loblolly pine ( Pinus taeda L.) plantations in the southern United States is related to increased leaf area index (LAI), but higher evaporative leaf surface area may increase drought vulnerability. To determine if the benefits of fertilization are affected by water availability or the effects of drought are exacerbated by fertilization, the interactive effects of throughfall treatment (ambient throughfall versus throughfall reduction) and fertilization treatment (no fertilization versus one-time fertilization) on a loblolly pine plantation were examined over five growing seasons. Enhancement of LAI and growth from fertilization was unaffected by throughfall treatment, and reductions in LAI, tree height, and stand volume increment in response to throughfall reduction were unaffected by fertilization treatment. Leaf-level stomatal conductance ( gS) was decreased and water use efficiency was increased by fertilization and by throughfall reduction. Lower gS was associated with decreased leaf predawn water potential in response to throughfall reduction. In contrast, lower gs in response to fertilization was associated with a reduction in the hydraulic allometry index, a measure of the ability of sapwood to supply water to leaves. These results suggest that fertilization may enhance LAI and growth even under mild or moderate drought. [ABSTRACT FROM AUTHOR]

Loblolly pine ( Pinus taeda L.) is the most important plantation species in the southeastern United States and therefore reductions in productivity associated with climate change may have significant economic impacts. To better understand the potential impact of drought on loblolly pine productivity, we studied the combined effects of reduced water availability and soil fertility on transpiration and canopy stomatal conductance ( G S ) over one year in an 8-year-old loblolly pine plantation by applying a throughfall treatment (ambient versus an approximate 30% reduction in throughfall) and a fertilization treatment (no fertilization versus one-time fertilization with N, P, K and micronutrients). We hypothesized that stomatal behavior would regulate water loss under a throughfall reduction treatment and the response would be greater under fertilization, due to an increase in leaf area index. Significant interactions between treatments indicated lower average monthly transpiration per unit ground area ( E G ) and per unit leaf area ( E L ), lower whole-plant hydraulic conductance, and lower average monthly midday G S in response to throughfall reduction only in the fertilized treatment, most likely due to increases in the leaf-to-sapwood area ratio in addition to isohydric regulation of water loss. In the throughfall reduction treatment but not the ambient throughfall treatment, reference G S (at vapor pressure deficit = 1 kPa, G Sref ) was significantly related to soil relative extractable water (REW) to a 0.6 m soil depth, and sensitivity of G Sref to REW was greater when combined with fertilization. These results indicate that the influence of drought on canopy-level processes of loblolly pine will be greater with fertilization. [ABSTRACT FROM AUTHOR]

Loblolly pine is the most extensively planted and productive commercial pine species in the southeastern United States, but future increases in drought frequency and intensity may affect plantation productivity. To better understand the potential impacts of reduced precipitation on loblolly pine productivity, the interactive effects of throughfall (ambient versus an approximate 30% reduction) and fertilization (no fertilization versus one time fertilization with 224 kg N ha −1 , 28 kg P ha −1 , 56 kg K ha −1 , and micronutrients) on growth, leaf area index (LAI), intercepted photosynthetically radiation (IPAR) and leaf-level physiology in a 7-year-old loblolly pine plantation were studied over two years. During a year with below average annual precipitation (831 mm), the throughfall reduction treatment decreased predawn leaf water potential, leaf gas exchange rates, needle elongation, basal area increment and stemwood production, and effects were independent of fertilization treatment. Leaf light-saturated net photosynthesis and stemwood production were on average 12% and 13% lower, respectively, in response to reduced throughfall treatment. During the following year with higher precipitation (1413 mm), reduced throughfall treatment decreased only leaf water potential. One-time fertilization had the largest impact on growth through increases in LAI, IPAR and total absorbed photosynthetically active radiation rather than changes in growth efficiency. Enhancement of growth by fertilization was independent of the reduced throughfall treatment in both years. [ABSTRACT FROM AUTHOR]

• We imposed a 3-year throughfall reduction experiment in a longleaf pine plantation. • Throughfall reduction had little effect on shoot and needle development patterns. • Development patterns varied over time and between branch axes. • Throughfall reduction had no effect on leaf area or litterfall dynamics. • Longleaf pine canopy development was resistant to reduced soil moisture. Reduced precipitation and, consequently, low soil moisture are known to limit tree growth and function by affecting shoot and foliage development, as well as canopy-scale leaf area dynamics and litterfall. Longleaf pine (Pinus palustris Mill.) is considered one of the most drought-resistant forest species in the southeastern U.S. and could serve as a pathway to increase drought resistance of forests in the region. Still, reduced precipitation and low soil moisture impacts on longleaf canopy phenology and growth are not well understood. Over three years (2017–2019), we determined the effects of a 40% throughfall reduction (TR 40) (relative to ambient throughfall treatment, TR 0) on shoot and foliage phenology and growth in a young (12–14 yr. old) longleaf pine plantation. Each year, we repeatedly measured shoot and needle lengths on primary and secondary branch axes of multiple trees in each treatment plot. We fit growth curves for each tree and branch axis to estimate shoot and needle growth rate, growth start and cessation dates, growth duration (growth start - growth cessation), and final shoot and needle lengths. At the plot level, we documented temporal patterns of leaf area index (LAI) and litterfall to determine whether branch-scale phenological and growth responses to reduced water availability corresponded with temporal changes in LAI and litterfall. We observed significant and consistent differences in shoot and needle elongation patterns between primary and secondary branches. Timing of needle development varied among years and was generally later each successive year. However, shoot elongation patterns were relatively consistent across years. Although soil moisture was lower under throughfall reduction, shoot and needle growth patterns were not affected. LAI and litterfall patterns were also not affected by throughfall reduction. Our results indicate that reductions in rainfall amount (without changes in rainfall frequency or timing) may have little impact on shoot and needle phenology, canopy development, and litter production in established longleaf plantations. [ABSTRACT FROM AUTHOR]

Long-term hydraulic acclimation to resource availability was explored in 3-year-old Populus deltoides Bartr. ex Marsh. clones by examining transpiration, leaf-specific hydraulic conductance (GL), canopy stomatal conductance (GS) and leaf to sapwood area ratio (AL:AS) in response to irrigation (13 and 551 mm year-1 in addition to ambient precipitation) and fertilization (0 and 120 kg N ha-1 year-1). Sap flow was measured continuously over one growing season with thermal dissipation probes. Fertilization had a greater effect on growth and hydraulic properties than irrigation, and fertilization effects were independent of irrigation treatment. Transpiration on a ground area basis (E) ranged between 0.3 and 1.8 mm day-1, and increased 66% and 90% in response to irrigation and fertilization, respectively. Increases in GL, GS at a reference vapor pressure deficit of 1 kPa, and transpiration per unit leaf area in response to increases in resource availability were associated with reductions in AL:AS and consequently a minimal change in the water potential gradient from soil to leaf. Irrigation and fertilization increased leaf area index similarly, from an average 1.16 in control stands to 1.45, but sapwood area was increased from 4.0 to 6.3 m² ha-1 by irrigation and from 3.7 to 6.7 m² ha-1 by fertilization. The balance between leaf area and sapwood area was important in understanding long-term hydraulic acclimation to resource availability and mechanisms controlling maximum productivity in Populus deltoides. [ABSTRACT FROM AUTHOR]