Shade tolerance

1Severe winds are the predominant cause of natural disturbance in temperate forests of north-eastern and north-central North America. Conceptual models of the effects of wind disturbance have traditionally focused on the impacts of catastrophic disturbances and have painted a simple picture of how disturbance acts to maintain tree species diversity. These models ignore variation among species characteristics that could have important consequences for both resistance to and recovery from disturbance.2We integrated an empirically parameterized, mechanistic model of windstorm mortality (WINDSTORM) and a seed-mass-based dispersal and recruitment model into a spatially explicit, individual tree model of forest dynamics (SORTIE) in order to create simulated long-term ‘experiments’ designed to explore the sensitivity of forest composition and structure to species-specific resistance to and recovery from disturbance.3We found that species-specific variation in resistance to wind mortality interacted strongly with: (i) shade tolerance characteristics, (ii) the medium-term history of disturbances, (iii) the long-term average severity of the disturbance regime and (iv) seedbed substrate dynamics to influence tree population dynamics and successional trajectories.4We also examined how local and long-distance dispersal affect response to wind disturbance. Ignoring differences among local dispersal characteristics overestimates the importance of dispersal-limited species. Our results show that long-distance immigration maintains species coexistence only if the immigration rate is very high relative to local dispersal. Despite this, stand-scale models that ignore long-distance dispersal can underestimate population dynamics of dispersal-limited species.5Our results indicate that landscape-scale heterogeneity in structure and species composition in these forests is facilitated by synergisms between the stochastic nature of wind disturbance and complex interactions between species traits that govern: (i) resistance to disturbance, (ii) local dispersal and seedling establishment and (iii) competitive (i.e. growth/survival) traits that do not adhere to strict tradeoffs.6We conclude that wind disturbance has much more complex and variable effects on long-term dynamics of forest structure and composition than suggested by previous models. In particular, non-catastrophic disturbances can generate important variation in forest dynamics that can either lead to dominance by a single or small number of species, or facilitate species coexistence.Severe winds are the predominant cause of natural disturbance in temperate forests of north-eastern and north-central North America. Conceptual models of the effects of wind disturbance have traditionally focused on the impacts of catastrophic disturbances and have painted a simple picture of how disturbance acts to maintain tree species diversity. These models ignore variation among species characteristics that could have important consequences for both resistance to and recovery from disturbance.We integrated an empirically parameterized, mechanistic model of windstorm mortality (WINDSTORM) and a seed-mass-based dispersal and recruitment model into a spatially explicit, individual tree model of forest dynamics (SORTIE) in order to create simulated long-term ‘experiments’ designed to explore the sensitivity of forest composition and structure to species-specific resistance to and recovery from disturbance.We found that species-specific variation in resistance to wind mortality interacted strongly with: (i) shade tolerance characteristics, (ii) the medium-term history of disturbances, (iii) the long-term average severity of the disturbance regime and (iv) seedbed substrate dynamics to influence tree population dynamics and successional trajectories.We also examined how local and long-distance dispersal affect response to wind disturbance. Ignoring differences among local dispersal characteristics overestimates the importance of dispersal-limited species. Our results show that long-distance immigration maintains species coexistence only if the immigration rate is very high relative to local dispersal. Despite this, stand-scale models that ignore long-distance dispersal can underestimate population dynamics of dispersal-limited species.Our results indicate that landscape-scale heterogeneity in structure and species composition in these forests is facilitated by synergisms between the stochastic nature of wind disturbance and complex interactions between species traits that govern: (i) resistance to disturbance, (ii) local dispersal and seedling establishment and (iii) competitive (i.e. growth/survival) traits that do not adhere to strict tradeoffs.We conclude that wind disturbance has much more complex and variable effects on long-term dynamics of forest structure and composition than suggested by previous models. In particular, non-catastrophic disturbances can generate important variation in forest dynamics that can either lead to dominance by a single or small number of species, or facilitate species coexistence.

We examined the influence of small-scale gap disturbances on stand development and tree species coexistence in an old-growth Fagus sylvatica–Abies alba forest in the Dinaric Mountains of Bosnia–Herzegovina. The structure and composition of tree regeneration in gaps were compared to the forest as a whole, and the influence of gap size on the density and composition of regeneration was assessed. Transition probabilities were also calculated from gapfillers in different life stages to examine canopy replacement patterns. The structure and composition of tree regeneration were similar between gaps and the forest as a whole, and there was no relationship between overall regeneration density and gap size, indicating most individuals established prior to gap formation. Likewise, there was no strong evidence of gap-size partitioning for shade tolerant F. sylvatica and A. alba, although less tolerant Acer pseudoplatanus only recruited to taller life stages in larger gaps. Transition probabilities calculated from the seedling and sapling data suggest that most gaps will be captured by F. sylvatica, while probabilities based on pole-sized gapmakers indicate both A. alba and F. sylvatica will be maintained in the canopy. We suggest that gaps primarily play a role in reorganizing advance regeneration, and that coexistence of shade tolerant F. sylvatica and A. alba is more likely related to their differential ability to tolerate shaded understory conditions, particularly during larger life stages, rather than gap-size partitioning.

The study of forest landscape change requires an understanding of the complex interactions of both spatial and temporal factors. Traditionally, forest gap models have been used to simulate change on small and independent plots. While gap models are useful in examining forest ecological dynamics across temporal scales, large, spatial processes, such as seed dispersal, cannot be realistically simulated across large

Two Umbelliferous tall herbs – Laserpitium siler and Grafia golaka tend to form monodominant stands in the montane belt of the North Adriatic Karst after grassland abandonment. The germination and development of shade-tolerant species of further successional stages could be facilitated by favorable climatic conditions, measured in the canopy, but colonization by woody species was not perceived, except for Pinus sylvestris and Pinus nigra, which are not common trees in the area. The vegetation history of the site shows rather slow reforestation after abandonment and almost exclusively with pine woodland. All organs of both Umbelliferous species showed high allelopathic potential, measured as significantly inhibited germination and early development of test plants in bioassays. Under similar treatments, the germination rate of P. sylvestris was reduced, but further development of seedlings was stable. One possible explanation for the relative persistence of Umbelliferous monodominant ...

... Management Woody understory response to changes in overstory density: thinning in Allegheny hardwoods Ruth D. Yanai a,*, Mark J. Twery ... been well studied in Allegheny hardwoods (Grisez and Peace, 1973, Marquis, 1973, Marquis and Bjorkbom, 1982, Horsley and Marquis ...