| Abstract: |
Gap size is critically important to ecological processes that drive forest dynamics within the gap, yet its threshold has never been explicitly defined. Consequently, gap sizes reported in the literature ranged from 4 m(2) to 2 ha, which makes comparisons among and synthesis of the published gap studies difficult. We suggested that the lower size limit be defined by the mean shadow length (SL) of canopy trees surrounding the gap (CTSG) at local 12:00 during growing season (GS), while the upper size limit be defined by considering the farthest impact of CTSG on growth of shade intolerant tree species, which was determined by the mean of SL at the initial and the final times when 30-min photosynthetic active radiation (PAR) is more than the light saturation point for shade intolerant tree species each day during GS.
The lower and upper limits of expanded gaps (the canopy gap plus the area extending to the bases of the canopy trees surrounding the gap) represented by gap diameter: CTSG height (R-D/H) were 0.49 and 3.49, respectively, for temperate forest areas. The lower limit of gap size is determined only by the location and the height of CTSG, which should be applicable worldwide. We also tried to provide a universal method for determining the upper limit of gap size without applying the observed PAR data, and using only sunshine duration, an easily obtained variable from meteorological stations worldwide. We suggest that expanded gaps may be classified as: small gap, 0.49 < R-D/H <= 1.0, medium gap, 1.0 < R-D/H <= 2.0; large gap, 2.0 < R-D/H <3.5 in temperate forests. (C) 2015 Elsevier B.V. All rights reserved. |