Fertilization increased leaf water use efficiency and growth of Pinus taeda subjected to five years of throughfall reduction

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 ([g.sub.S]) was decreased and water use efficiency was increased by fertilization and by throughfall reduction. Lower [g.sub.S] 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. Key words: loblolly pine, leaf area index, leaf water potential, hydraulic traits, drought. La productivite elevee des plantations de pin a encens (Pinus taeda L.) dans le sud des Etats-Unis est reliee a l'augmentation de l'indice de surface foliaire (LAI), mais une plus grande surface foliaire augmente l'evaporation et peut accentuer la vulnerabilite a la secheresse. Dans le but de determiner si les benefices de la fertilisation dependent de la disponibilite en eau ou si les effets de la secheresse sont exacerbes par la fertilisation, les effets interactifs de traitements impliquant la precipitation au sol (ambiante ou reduite) et la fertilisation (aucune ou une application) dans une plantation de pin a encens ont ete etudies pendant cinq saisons de croissance. L'amelioration de LAI et de la croissance due a la fertilisation n'etait pas influencee par les changements dans la precipitation au sol. La fertilisation n'a pas eu d'effet sur la reduction de LAI, de la hauteur des arbres et de l'accroissement en volume du peuplement en reaction a la reduction de la precipitation au sol. La conductance stomatique a l'echelle des feuilles ([g.sub.S]) a ete reduite et l'efficacite de l'utilisation de l'eau a ete augmentee par la fertilisation et la reduction de la precipitation au sol. La valeur plus faible de [g.sub.S] etait associee a la diminution du potentiel hydrique de base foliaire en reaction a la reduction de la precipitation au sol. Au contraire, la valeur plus faible de [g.sub.S] en reaction a la fertilisation etait associee a la reduction de l'indice d'allometrie hydraulique, une mesure de la capacite du bois d'aubier a approvisionner les feuilles en eau. Ces resultats indiquent que la fertilisation peut ameliorer LAI et la croissance meme en presence d'une secheresse legere ou moderee. [Traduit par la Redaction] Mots-cles : pin a encens, indice de surface foliaire, potentiel hydrique foliaire, caracteristiques hydrauliques, secheresse.
Introduction Along with increasing temperature, greater annual and seasonal variability in precipitation, more intense rain events, and longer interstorm periods are predicted for eastern North America (Swain and Hayhoe 2015; [...]