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Comparative physiological responses of Rhododendron groenlandicum and regenerating Picea mariana following partial canopy removal in northeastern Quebec, Canada
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Franc¸ois He´bert, Nelson Thiffault, Jean-Claude Ruel, and Alison D. Munson
Abstract: In boreal black spruce (Picea mariana (Mill.) BSP) dominated stands, careful logging around advance regeneration contributes to protect the shrub layer while increasing light availability. Therefore, it may promote expansion of bog Labrador tea (Rhododendron groenlandicum (Oeder) Kron & Judd), which can impair black spruce growth by direct competition for nutrients and by alteration of soil biochemical conditions. Such harvesting approaches may not be suited for irregular, uneven-aged black spruce stands in the context of ecosystem-based management. Our objectives were to evaluate the effects of different intensities of partial harvest on critical resource availability and to compare the physiological and morphological responses of black spruce and Rhododendron to different intensities of partial harvest treatments. Two years after harvest, photosynthesis rate of black spruce was unchanged and was lower than that of Rhododendron, which was 63% higher after harvest. Nitrogen and water use efficiency were, respectively, 171% and 42% higher in Rhododendron than in black spruce after logging. Following harvest, black spruce physiology appeared to be regulated by the atmospheric humidity deficit, but for Rhododendron, physiology was regulated by light availability. The high plasticity of Rhododendron leaf traits could be indicative of the future advantage of this species in these stands following canopy removal associated with harvest. Re´sume´ : Dans les peuplements bore´aux domine´s par l’e´pinette noire (Picea mariana (Mill.) BSP), la coupe avec protection de la re´ge´ne´ration pre´e´tablie contribue a` prote´ger la strate arbustive tout en augmentant la disponibilite´ de la lumie`re. Ainsi, cette coupe peut favoriser l’expansion du the´ du Labrador (Rhododendron groenlandicum (Oeder) Kron & Judd), ce qui peut re´duire la croissance de l’e´pinette noire cause´e par la compe´tition directe pour les nutriments et de l’alte´ration des conditions biochimiques du sol. Dans un contexte d’ame´nagement e´cosyste´mique, de telles approches de re´colte peuvent ne pas convenir aux peuplements d’e´pinette noire irre´guliers et ine´quiens. Nos objectifs consistaient a` e´valuer les effets de diffe´rentes intensite´s de coupe partielle sur la disponibilite´ des ressources critiques et a` comparer les re´actions physiologiques et morphologiques de l’e´pinette noire et du Rhododendron aux diffe´rentes intensite´s de coupe partielle. Deux ans apre`s coupe, le taux de photosynthe`se de l’e´pinette noire est demeure´ inchange´ et e´tait plus faible que celui du Rhododendron qui e´tait 63 % plus e´leve´ apre`s la coupe. Les valeurs d’efficacite´ d’utilisation de l’azote et de l’eau du Rhododendron apre`s la coupe e´taient, respectivement, 171 % et 42 % plus e´leve´es que celles de l’e´pinette noire. Apre`s la coupe, la physiologie de l’e´pinette noire semblait controˆle´e par le de´ficit hydrique de l’atmosphe`re alors que la lumie`re controˆlait celle du Rhododendron. La grande plasticite´ des traits foliaires du Rhododendron pourrait eˆtre un indice que cette espe`ce serait avantage´e dans le futur par l’e´limination du couvert a` la suite d’une coupe dans ces peuplements. [Traduit par la Re´daction]
Introduction Bog Labrador tea (Rhododendron groenlandicum (Oeder) Kron & Judd, previously referred to as Ledum groenlandicum Oeder) is a widely distributed, light-intolerant, perennial understory shrub of the North American boreal forest (Jobidon 1995). In northeastern Quebec (Canada), Labrador tea (hereafter referred to as Rhododendron) is frequently found on coarse to medium-textured mineral soil with an
acid, mor humus between 20 and 60 cm where it is strongly associated with black spruce (Picea mariana (Mill.) BSP) overstories (Laberge Pelletier 2007). Rhododendron cover increases mainly through vegetative growth after disturbances such as fire and harvest that open the canopy, often resulting in slow growth of black spruce regeneration (Inderjit and Mallik 1996). This shrub competes directly with black spruce for nutrients (Munson and Timmer 1989) due to a high root nutrient uptake (Chapin and Tryon 1983) and com-
Received 4 December 2009. Accepted 9 June 2010. Published on the NRC Research Press Web site at cjfr.nrc.ca on 13 August 2010. F. He´bert, J.C. Ruel, and A.D. Munson.1 Centre d’e´tude de la Foreˆt, Faculte´ de Foresterie, Ge´ographie et Ge´omatique, Universite´ Laval, Que´bec, QC G1V 0A6, Canada. N. Thiffault. Ministe`re des Ressources naturelles et de la Faune du Que´bec, Direction de la recherche forestie`re, 2700 rue Einstein, Sainte-Foy, QC G1P 3W8, Canada, and Associate Member of the Centre d’e´tude de la Foreˆt. 1Corresponding
author (e-mail:
[email protected]).
Can. J. For. Res. 40: 1791–1802 (2010)
doi:10.1139/X10-124
Published by NRC Research Press
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Can. J. For. Res. Vol. 40, 2010
Can. J. For. Res. Downloaded from www.nrcresearchpress.com by 199.201.121.12 on 06/03/13 For personal use only.
Fig. 1. Conceptual representation of the Before-After-Control-Impact (BACI) design used for this study and details of the sampling approach in one plot. Each box represents a harvest plot. C, control; TT, temporary skid trail harvest; PT, permanent skid trail harvest; CPPTM, careful logging around advance growth (DBH > 13 cm); CPRS, careful logging around advance growth (DBH > 9 cm); PPFD, photosynthetic photon flux density; LMA, leaf mass per unit of area.
petes indirectly by (i) enhancing nutrient immobilization in humus (Damman 1971), (ii) altering soil chemical conditions that reduce soil nitrogen availability, and (iii) limiting mycorrhizal symbiosis, thus affecting black spruce growth (Inderjit and Mallik 1996). Since 1994, careful logging around advance growth (CLAAG or CPRS in French) is the principal logging method used to harvest Quebec’s boreal forest; the main objective of this approach is to favor black spruce advance regeneration while protecting soils (Anonymous 1994). However, by minimizing soil perturbation, CLAAG also contributes to protecting the shrub layer while increasing light availability to the understory plants (Harvey and Brais 2002). Several authors hypothesize that higher light availability could lead to a long-term encroachment of ericaceous shrubs by eventual edaphic modifications (e.g., Damman 1971; Mallik 1995). Sixty percent of the nonharvested area in northeastern Quebec is composed of stands characterized by an irregular
structure (Coˆte´ 2006). In the context of ecosystem-based management, partial cutting is considered a promising ‘‘near-nature’’ silvicultural approach to perpetuate this stand type in Quebec’s boreal forest (Bergeron et al. 1999; Ruel et al. 2007; Bouchard 2008). However, this approach can be problematic in stands that are susceptible to regeneration problems following canopy opening, such as black spruce dominated stands with a significant cover of ericaceous species. Specifically, we do not know how the light conditions created by partial cutting affect the response of black spruce and Rhododendron or how differential responses might impact subsequent succession on these sites. Plants respond to changing light conditions by morphological and physiological modifications that directly affect primary growth, mainly through changes in photosynthetic capacity and leaf resource allocation (Sims and Pearcy 1994). For example, higher light availability contributes to increased leaf thickness, specific leaf mass, and lower fine root to foliar ratio of salal (Gaultheria shallon Pursh.), an Published by NRC Research Press
Analysis performed on square root transformed data. Analysis performed on ln-transformed data. b
a
F 16.03 18.09 1.98 17.81 4.27 0.09 1.06 0.32 0.11 0.66 1.76 P 0.968 0.629 0.741 0.006 0.479
