Shade tolerance, canopy gaps and mechanisms of coexistence of forest trees

Abstract

Th e 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 diff erentiation in response to gaps has focused on evidence for an interspecifi c tradeoff between low light survival and high light growth. Th e objectives of this study are fi rst 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 eff ect'. We fi nd that temporal fl uctuations 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. Th e 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.

Carbon gain optimization in five broadleaf deciduous trees in response to light variation within the crown: correlations among morphological, anatomical and physiological leaf traits

“…One of the components of shade tolerance resides in the ability of individuals to op-timize carbon gain under low light environments, by maximizing light interception and minimizing carbon loss by respiration (WALTERS and REICH 1999), according to the carbon gain hypothesis (VALLADARES and NIINEMTES 2008). Light competition and inter-specific differences in shade tolerance are frequently important determinants of forest structure and dynamics (GRAVEL et al 2010). Nevertheless, the potential correlation between plasticity of morphological and physiological leaf traits and the tolerance to shade is still under discussion (ROBAKOWSKI et al 2003, PORTSMUTH andNIINEMETS 2007).…”

Variations in Herbaceous Composition of Dry Tropics Following Anthropogenic Disturbed Environment

“…However, disturbances in low severity usually cause little damage to overstorey trees 9 but affect ground vegetation directly by shifting the habitat available resources 10,11 . The disturbances with high severity generate relatively homogeneous resource availability while low to moderate severity disturbances partially remove the canopy and generally result in greater resource heterogeneity 12 . Anthropogenic disturbances in forests followed by livestock grazing and forest fire adversely affected the composition of herbaceous vegetation 13,14 , it is therefore imperative to conserve the herbaceous vegetation of these sites.…”

Role of Anthropogenic Canopy Gaps on Natural Regeneration Pattern of Monospecific Ceriops Tagal in Rakawa Lagoon, Sri Lanka

“…In the closed forest arrangement, the light availability reduced by the canopy cover prevents the further growth and survivability of recruited seedlings (Kneeshaw & Bergeron, 1998). Decreased in light intensity increases the mortality rate of shade intolerant species whilst canopy opening enhances the growth rate of shade intolerant species (Gravel et al, 2008(Gravel et al, , 2010) Ceriops tagal could be considered as shade intolerant species and light intensity through canopy gap increases the natural regeneration of Ceriops tagal species in Rakawa mangrove forest.…”

“…Recruited seedlings could be deteriorated due to lack of sunlight in closed canopy sites (Denslow, 1998;Gravel et al, 2010). Gap formation changes both biotic and abiotic settings with rapid increase of light penetration on the mangroves floor with the decline of dead mangroves and the growth of fresh faced mangroves (Aldrie & Norman, 2008) hence the height class 3 and 4 in the open canopy site showed mean densities of (27.3±3.9 m -2 ) and (10.6±2.6 m -2 ) respectively.…”