Drought promotes early leaf abscission regardless of leaf habit but increases litter phosphorus losses only in evergreens

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Caroline Dallstream, Biology, McGill University, Montreal, QC, Canada and Frida I. Piper, Centro de Investigación en Ecosistemas de la Patagonia (CIEP), Coyhaique, Chile

Background/Question/Methods
Plants resorb nutrients from leaves during senescence before they are abscised, and the quantity that remains after abscission is termed nutrient proficiency. Resorbed nutrients contribute substantially to plant nutrient requirements, representing important stores that are remobilized for new foliage, woody growth and reproduction. A global meta-analysis found similar nitrogen (N) proficiency between leaf habits, and greater phosphorus (P) proficiency for evergreens; however, it is unclear whether these differences are attributable to phylogeny or leaf habit. Additionally, drought can affect leaf abscission and resorption, but whether plants of contrasting leaf habits differentially respond remains unknown. We conducted an experiment for two years with potted saplings to compare leaf habit and drought effects on leaf abscission dynamics and nutrient resorption proficiency in two evergreen and two deciduous Nothofagus (Nothofagaceae) species from southern temperate forests (n = 8). Average N proficiencies were obtained for each period, and P proficiency for 2018-2019. In the 2018-2019 season, we additionally monitored abscission dynamics, relative growth rates and scaled up leaf proficiency to total litter nutrient losses. Linear mixed-effects models were used with leaf habit, watering and date (when appropriate) as fixed factors and species and block as random factors.
Results/Conclusions
Rain exclusion structures reduced sapling pots’ gravimetric water content by -18.4 ± 1.8% and -28.9 ± 1.8% (mean ± s.e.) relative to controls over the two experimental periods, Fall 2018 (February-April) and 2018-2019 growing season (October-March), respectively. Under control conditions, N and P proficiency did not differ between leaf habits. For both leaf habits, drought reduced relative growth rates in spring (p < 0.05) and accelerated leaf abscission dynamics (p < 0.001), but had no effect on N proficiency in either period. Contrarily, drought had a significant negative effect on P proficiency (p < 0.05). More specifically, drought reduced P proficiency in evergreen saplings by 45.4% (leaf habit x watering interaction, p < 0.05) which resulted in 84.6% greater litter P losses compared to controls (leaf habit x watering interaction, p < 0.05). The similar abscission dynamics but differing proficiency responses suggest that drought senescence was successful in reducing P losses only in deciduous saplings. If these results can be extrapolated to trees in field conditions, they suggest that the more frequent and intense droughts predicted for these species’ distributions could result in increased litter P losses for evergreens, potentially influencing nutrient cycling and altering species composition in southern temperate forests.