Canopy gap disturbance and succession in trembling aspen dominated boreal forests in northeastern Ontario

Abstract

Canopy gaps play an important role in forest vegetation dynamics when fire return intervals are long. However, there is little known about the role of gaps in the development of forest stands that initially dominate following stand-replacing disturbance. We investigated gap disturbance during the breakup of trembling aspen (Populus tremuloides Michx.) stands at two scales: at the stand level we quantified gap fraction and gap size as stand development proceeds; at the gap level we determined causes of gap-maker mortality and evaluated resulting gap-maker structure and decay as stand development proceeds. We also evaluated the impact that gaps have on stand transition by quantifying the abundance and growth of juvenile trees in gaps of different sizes and ages. Ten stands between 60 and 120 years since fire in northeastern Ontario were sampled using line intersect transects. Gap fraction doubled (∼18%–36%) and mean gap size was more than four times greater (∼45–200 m2) over the time period. Standing dead gap makers in early states of decay were most frequent in young stands, whereas snapped gap makers in various states of decay were most frequent in old stands. Infection by fungal pathogens was the most frequent cause of mortality (56%) and was not related to time since fire. Balsam fir (Abies balsamea (L.) Mill.) was the most abundant juvenile recruit. However, transition probabilities for tree species were independent of gap type. These results indicate that gap creation is frequent during early stages of stand development in boreal forests; however, it is unlikely that successional trajectory is affected by their formation.

Northward migrating trees establish in treefall gaps at the northern limit of the temperate–boreal ecotone, Ontario, Canada

“…In southern boreal forests, 2° north of our study site, paper birch, a prototypical shade-intolerant pioneer species (Frelich and Reich 1995a), established at high abundances in treefall gaps of similar sizes as those found at Wolf Lake (»50-500 m 2 ) (Kneeshaw and Bergeron 1998;Hill et al 2005;de Römer et al 2007). Black spruce and balsam Wr [Abies balsamea (L.) Mill.…”

“…(Pinaceae)], two abundant canopy tree species of the boreal forest, also established in similar sized gaps thereby representing dominant overstory eVects in these studies. This has led Hill et al (2005) andde Römer et al (2007) to conclude that succession trajectories were unaVected by gap disturbances in the boreal forest. Additionally, red maples were absent both in gaps and the understory/canopy in each of these studies (Kneeshaw and Bergeron 1998;Hill et al 2005;de Römer et al 2007 represented over 75% of the total tree abundance and basal area in gaps of a hardwood mixed forest in New Hampshire (2°s outh of our study site), representing a dominant overstory eVect not observed at Wolf Lake.…”

Stand age structural dynamics of conifer, mixedwood, and hardwood stands in the boreal forest of central Canada

“…However, hardwoods in the canopy transition stage had a unimodal stand age structure with largely a single cohort of trembling aspen trees established after the disturbance and marginal amounts of conifer regeneration. Stands dominated by trembling aspen often developed a dense shrub layer of mountain maple (Acer spicatum) [35] and beaked hazel (Corylus cornuta) [36]. This was observed in all sampled hardwood stands in the canopy transition stage.…”

“…This was observed in all sampled hardwood stands in the canopy transition stage. Furthermore, dense shrub layers have been found to hinder understory conifer regeneration [35][36][37], thus contributing to the unimodal age structure in the canopy transition hardwoods.…”

The role of disturbances in forest regeneration

“…Niesprzyjaj¹ce czynniki meteorologiczne, np. susze, prowadz¹ do os³abienia drzew, a w konsekwencji do ich wiêkszej podatnooeci na grzyby (Drenkhan et Hanso 2004;Hill et al 2005;Bendel et al 2006). Uwa¿a siê, ¿e czynnikiem inicjuj¹cym powstawanie luk s¹ czêsto owady czy grzyby (Lundguist 2000, Lewis et Lindgren 2002), natomiast czynnikiem odpowiedzialnym za poszerzanie siê luk s¹ zazwyczaj patogeny grzybowe (Worrall et al 2005).…”