by Audrey Verreault | 30 January 2019
Published in Journal of Ecology 95: 458-467
1 In forest communities, species coexistence can be favoured by disturbance-related variations in light regime coupled with rank reversal in species performance. The objective of this study was to determine if a major canopy disturbance, resulting from an ice storm, would favour the coexistence of sugar maple (Acer saccharum) and beech (Fagus grandifolia) in an old-growth forest located near the northern limit of the species’ range.
2 The growth, density and frequency of occurrence of understorey stems were evaluated 7 years after the ice storm and compared with pre-disturbance values to determine if sugar maple, a slightly less shade-tolerant species, would be favoured over beech.
3 Although height and radial growth increased three- to five-fold, and > 70% of the 5–10 cm d.b.h. stems showed a release, sugar maple did not benefit more from the opening of the canopy than beech.
4 The inability of sugar maple to outgrow beech might be related to the high proportion of beech root sprouts at our site, suggesting that the potential for the disturbance-mediated mechanism to slow the competitive exclusion of maple might decrease as the importance of vegetative reproduction increases in beech.
5 Prior to the disturbance, sugar maple was more abundant and occurred in a larger proportion of the plots among small size classes, while beech dominated in the larger size classes. Although some increases in the density of both species were detected after the disturbance, the general trends of relative abundance and distribution of the species were not modified significantly.
6 Our results do not indicate that a single canopy disturbance such as the major 1998 ice storm significantly favoured sugar maple over beech. When a disturbance opens the canopy, competition for light in gaps may tend to maintain or reinforce a pre-existing hierarchy instead of reversing it, especially when interspecific differences in post-disturbance growth are small or absent, as in this study.
7 This study shows that the dynamics of sugar maple – beech communities are more complex than previously envisaged and that revised models should incorporate additional factors (e.g. vegetative reproduction) that might also play an important role in regulating community dynamics.
by Marie-Claude Boileau | 30 January 2019
Published in Northern Journal of Applied Forestry 18(4): 119-126
Experimental blocks were established in five regions of southern Québec to determine the response of hardwood stands to selection cutting. The blocks contain five control stands (no cut) and five treated stands, composed mainly of sugar maple in association with yellow birch and American beech. Treated stands were harvested using single tree selection to a residual density varying from 16.8 to 21.2 m2. Results obtained 10 yr after treatment demonstrate that the annual gross growth rate was not significantly different between treated and control stands. However, net annual growth rate was higher in four out of five cut stands, because the annual mortality rate was less in these stands than in controls. Cutting significantly enhanced the growth of stems with an initial diameter at breast height (dbh) of 10 to 28 cm and favored the development of sugar maple saplings in aIl blocks and yellow birch saplings in three blocks.
by Audrey Verreault | 30 January 2019
Published in Oikos 119 : 475-484
The belief that canopy gaps are important for the maintenance of tree species diversity appears to be widespread, but there have been no formal theoretical models to assess under what conditions gap phase processes allow coexistence. Much of the empirical research on niche differentiation in response to gaps has focused on evidence for an interspecific tradeoff between low light survival and high light growth. The objectives of this study are first to distinguish the possible mechanisms allowing coexistence based on this tradeoff, and second, to explore their limitations. We present a theory of forest dynamics driven by small-scale disturbances as a special case of the theory of coexistence in variable environments. We demonstrate that temporal and spatial heterogeneity in light conditions that results from canopy gaps can allow stable coexistence as a result of three previously documented general mechanisms: ‘relative non-linearity’, ‘the successional niche’ and the ‘storage effect’. We find that temporal fluctuations in light availability alone allow the stable coexistence of only two species. Spatial variation in disturbance synchronicity and intensity allows three species to coexist in a narrow parameter space. The rate of extinction is, however, extremely slow and there is transient coexistence of a larger number of species for a long period of time. We conclude that while the low light survival/high light growth tradeoff may be ubiquitous in forest tree species, it is unlikely to function as an important mechanism for the stable coexistence of several tree species.
by Audrey Verreault | 30 January 2019
Published in Oecologia 162: 153-161
Large-scale synchronous variations in community dynamics are well documented for a vast array of organisms, but are considerably less understood for forest trees. Because of temporal variations in canopy gap dynamics, forest communities—even old-growth ones—are never at equilibrium at the stand scale. This paucity of equilibrium may also be true at the regional scale. Our objectives were to determine (1) if nonequilibrium dynamics caused by temporal variations in the formation of canopy gaps are regionally synchronized, and (2) if spatiotemporal variations in canopy gap formation affect the relative abundance of tree species in the understory. We examined these questions by analyzing variations in the suppression and release history of Acer saccharum Marsh. and Fagus grandifolia Ehrh. from 481 growth series of understory saplings taken from 34 mature stands. We observed that (1) the proportion of stems in release as a function of time exhibited a U-shaped pattern over the last 35 years, with the lowest levels occurring during 1975–1985, and that (2) the response to this in terms of species composition was that A. saccharum became more abundant at sites that had the highest proportion of stems in release during 1975–1985. We concluded that the understory dynamics, typically thought of as a stand-scale process, may be regionally synchronized.
by Audrey Verreault | 30 January 2019
Published in Canadian Journal of Forest Research 38: 488-497
Crown openness (CO) of mature trees influences light transmission within the forest canopy. However, in modeling, this variable is often considered constant within species, and its potential regional variability is ignored. The objective of this study was to evaluate if CO values of yellow birch (Betula alleghaniensis Britt.), sugar maple (Acer saccharum Marsh.), and eastern hemlock (Tsuga canadensis (L.) Carrière) vary according to the following factors: (i) species, (ii) regional actual evapotranspiration(AET), (iii) tree size (i.e., diameter at breast height, DBH), and (iv) angle of transmission from zenith. To achieve this, CO was evaluated for 136 yellow birches, 109 sugar maples, and 68 hemlocks from different regions of western Québec, southern Ontario, and northern Michigan. Results showed that all of the studied factors affected CO. While dominant trees can intercept light laterally as well as vertically, smaller trees are more efficient at intercepting light vertically. Increasing AET is associated with more open crowns. Given its importance in light transmission in the understory, a better understanding of how CO varies between individuals, species, and regions is needed.
