Soil nutrition and temperature as drivers of root suckering in trembling aspen Erin C. Fraser, Victor J. Lieffers, Simon M. Landhäusser, and Brent R. Frey Abstract: In two separate studies, the suckering of trembling aspen (Populus tremuloides Michx.) roots was assessed in response to different daily maximum soil temperatures and in relation to available nutrients (CaSO 4 and NH 4 NO 3 ). In the first study, aspen root sections were incubated under high:low temperature regimes of 12:8, 14:8, 16:8, 18:8, or 20:8°C until 124 degree-days above 8°C had been reached. Daily maximum temperature did not affect the number of suckers produced per square centimetre of root surface area or per root section. However, more time was required to initiate suckers on root sections grown under the 12:8°C temperature regime compared with those grown at 20:8°C. Furthermore, when calculated from a base temperature of 5°C, the number of degree-days needed to initiate aspen suckers was not different across the temperature regimes. In the second study, CaSO 4 , NH 4 NO 3 , or distilled water was added to aspen root sections. While nutrients did not affect the number of suckers produced, the addition of CaSO 4 or NH 4 NO 3 significantly increased sucker dry mass. Nutrient addition appeared to stimulate sucker growth and mobiliza- tion of stored reserves to support this growth. Résumé : Le drageonnement du peuplier faux-tremble (Populus tremuloides Michx.) en réponse à différentes tempéra- tures quotidiennes maximum du sol et en relation avec les nutriments disponibles (CaSO 4 et NH 4 NO 3 ) a été évalué dans le cadre de deux études distinctes. Dans la première étude, des sections de racine de peuplier ont été incubées sous différents régimes de température maximum : minimum de 12 : 8, 14 : 8, 16 : 8, 18 : 8 ou 20 : 8 °C jusqu’au moment d’atteindre 124 degrés-jours au-dessus de 8 °C. La température maximum durant le jour n’a pas affecté le nombre de drageons produits par centimètre carré de surface racinaire ou par section de racine. Cependant, il fallait plus de temps pour initier la formation des drageons sur les racines conservées au régime de température 12 : 8 °C comparativement au régime 20 : 8 °C. De plus, lorsqu’il était calculé sur la base de 5 °C, le nombre de degrés-jours requis pour initier la formation des drageons était le même quel que soit le régime de température. Dans la seconde étude, CaSO 4 , NH 4 NO 3 ou de l’eau distillée a été ajouté aux sections de racine de peuplier. Bien que les nutriments n’aient pas affecté le nombre de drageons produits, l’addition de CaSO 4 ou de NH 4 NO 3 a significativement augmenté le poids sec des drageons. L’addition de nutriments semble stimuler la croissance des drageons et la mobilisation des réserves pour supporter cette croissance. [Traduit par la Rédaction] Fraser et al. 1691 Introduction The primary means of trembling aspen (Populus tremuloides Michx.) regeneration is through vegetative root suckers (Barnes 1966; Maini and Horton 1966a; Steneker 1976; Kemperman 1978). Initial sucker production and growth can be very rapid and under favourable conditions over 100 000 stems/ha establish in the first growing season (Schier and Smith 1979). In natural systems, stand-replacing disturbances such as wildfire typically lead to prolific aspen regeneration (Bartos and Mueggler 1981; Brown and DeByle 1987). Regeneration after harvesting of aspen stands, how- ever, has not always been as successful, especially on logged sites with vigorous grass competition or heavy machine traf- ficking (Darrah 1991; Bates et al. 1993; Landhäusser and Lieffers 1998). The increase in soil temperature that follows a major dis- turbance has been thought to be the major environmental condition necessary for sucker initiation (Horton and Maini 1964; Maini and Horton 1966b; Steneker 1974, 1976; Hungerford 1988). Controlled laboratory experiments have reported that the number of aspen suckers produced on a given root section increases with increasing temperature up to a maximum temperature of approximately 30°C (Horton and Maini 1964; Maini and Horton 1966b; Gifford 1967; Zasada and Schier 1973). However, several of these experi- ments used constant temperatures throughout the course of the experiment (Horton and Maini 1964; Maini and Horton 1966b; Gifford 1967), which may have exaggerated the in- fluence of temperature. Other studies utilized short (8– 10 cm) root segments (Maini and Horton 1966b; Gifford 1967; Zasada and Schier 1973), which may have affected sucker survival over the experimental period (Perala 1978). A number of field-based studies have also examined the relationship between aspen sucker densities and soil temper- ature. Hungerford (1988) noted a 4.5- to 15-fold increase in sucker numbers relative to controls following a 2–3°C in- crease in mean daily soil temperature (from 10–13° to 12– Can. J. For. Res. 32: 1685–1691 (2002) DOI: 10.1139/X02-080 © 2002 NRC Canada 1685 Received 23 October 2001. Accepted 14 April 2002. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 10 September 2002. E.C. Fraser, 1 V.J. Lieffers, S.M. Landhäusser, and B.R. Frey. Centre for Enhanced Forest Management, Department of Renewable Resources, 4-42 Earth Sciences Building, University of Alberta, Edmonton, AB T6G 2E3, Canada. 1 Corresponding author (e-mail: ecf@ualberta.ca).