Integration of soil moisture, xylem water potential, and fall–spring herbicide treatments to achieve the maximum growth response in newly planted Douglas-fir seedlings

Abstract

Early in the establishment of Pacific Northwest Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantations, herbaceous vegetation can decrease seedling growth through competition for soil moisture during the dry summer months. This study was designed to statistically quantify soil moisture, seedling xylem water potential (J), vegetation community, and seedling growth response to six herbicide treatment regimes commonly applied over the first 2 years of establishment. When compared with the control, soil moisture and seedling J increased in response to reductions in competitive cover, allowing seedlings to extend productive growing time from 28 to 80 days. As a result, seedling volume growth increased from 56 cm 3 in the untreated control to greater than 250 cm 3 for the most intensive herbicide treatment regimes. Vegetation surveys revealed that treatment regimes had the potential to provide a disturbance, which could shift community composition from native to introduced species as the relationship decreased from 10:1 to 2:1. The most intense herbicide treatment regime reduced cover below 20%, retained soil moisture >30%, maintained predawn seedling J above -1.0 MPa, and decreased height to diameter ratio below 50, increasing the likelihood of successful plantation establishment.Résumé : Tôt, lors de l'établissement des plantations de douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco) dans le Pacific Northwest, la végétation herbacée peut réduire la croissance des semis à cause de la compétition pour l'humidité du sol pendant les mois secs d'été. Cette étude a été conçue pour quantifier statistiquement l'humidité du sol, le potentiel hydrique (J) du xylème des semis, la communauté végétale et la réaction en croissance des semis à six régimes de traitement avec des herbicides communément appliqués pendant les deux premières années d'établissement. Comparativement au témoin, l'humidité du sol et le potentiel hydrique des semis ont augmenté en réaction à la réduction de la compétition, ce qui a permis aux semis d'allonger leur période de croissance productive de 28 à 80 jours. La croissance en volume des semis a par conséquent augmenté de 56 cm 3 dans le cas du témoin non traité à plus de 250 cm 3 dans le cas du régime le plus intensif de traitement avec des herbicides. Les inventaires de végétation ont révélé que les régimes de traitement avaient la possibilité de provoquer une perturbation qui pouvait modifier la composition végétale en favorisant le remplacement d'espèces indigènes par des espèces introduites étant donné que le rapport entre les deux groupes d'espèces est passé de 10 : 1 à 2 : 1. Le traitement le plus intensif avec des herbicides a réduit le couvert en deçà de 20 %, a conservé l'humidité du sol au-dessus de 30 %, a maintenu le potentiel hydrique de base des semis au-dessus de -1,0 MPa et a diminué le rapport entre la hauteur et le diamètre à moins de 50, augmentant ainsi les chances que l'établissement de la plantation soit un succès.[Traduit par la Rédaction]

Effects of Vegetation Management on Wood Properties and Plant Water Relations of Four Conifer Species in the Pacific Northwest of the USA

“…At age five years, DF, WH, WRC and GF control trees at the CR site, had mean heights of 3.0, 2.3, 1.3 and 1.8 m, respectively, while the heights of DF and WRC at the CF site, were 2.3 and 1.3 m (data not shown), indicating that WRC and GF trees were shorter and, therefore, had lower leaf area and water demand. Even though it has been demonstrated that VM increases soil moisture and reduces seedling water stress on planted Douglas-fir [33,34] it is possible that the level of water stress experienced by GF and WRC control trees during the first five growing seasons was not strong enough to have a significant effect on iWUE, or that the effect on stomatal conductance was similar to the effect on the carbon assimilation rate, thus cancelling each other out.…”

Assessing Douglas-Fir Seedling Establishment Using Two Modified Forestry Reclamation Approaches in the Pacific Northwest

“…Less competitive understory species such as native species might provide similar erosion control benefits in the first year and not compete with seedlings as strongly for moisture [21]. Early establishment of understory cover in Douglas-fir plantations can decrease seedling growth because of competition for soil moisture during the dry summer months [22]. Water is a principle-limiting factor of Douglas-fir seedling growth [23].…”

Water Stress and Afforestation: A Contribution to Ameliorate Forest Seedling Performance During the Establishment

“…Herbaceous and/or woody vegetation which limits resource availability, hindering seedlings' expected growth and survival has been cited as one of the main causes of low plantation success (Smith et al, 1997;Tappeiner et al, 2007). Weed vegetation reduces the levels of water availability in the soil and affects negatively seedling water status (Dinger & Rose, 2009Löf & Welander, 2004;Picon-Cochard et al, 2006), representing the main biotic cause of water stress (Lamhamedi et al, 1998;PiconCochard et al, 2001). Differences in weed density (Florentine & Fox, 2003;Garau et al, 2008b) and growth forms Coll et al, 2004;Provendier & Balandier, 2008) may cause variations in water restriction.…”

“…stomatal closure), whereas others are delayed for hours or days, representing new capabilities and allocation patterns, providing some degree of resistance to the stress. Numerous studies demonstrated that tree seedlings responded to weed imposed water deficit by anticipating stomatal closure and showing a reduction in leaf water potential (Coll et al, 2004;Dinger & Rose, 2009Garau et al, 2008b;Picon-Cochard et al, 2001, 2006Provendier & Balandier, 2008;Rey Benayas et al, 2003;Watt et al, 2003). Even with high soil water availability, the presence of weeds produced a significant decrease in leaf stomatal conductance (Garau et al, 2008b;Watt et al, 2003).…”