Distribution Of Canopy Gaps Research Articles

Mapping canopy gaps in an indigenous subtropical coastal forest using high-resolution WorldView-2 data

Invasive species usually colonize canopy gaps in tropical and subtropical forests, which results in a loss of native species. Therefore, an understanding of the location and distribution of canopy gaps will assist in predicting the occurrence of invasive species in such canopy gaps. We tested the utility of WorldView-2 (WV-2) with eight spectral bands at 2 m spatial resolution to delineate forest canopy gaps in a subtropical Dukuduku coastal forest in South Africa. We compared the four conventional visible-near-infrared bands with the eight-band WV-2 image. The eight-band WV-2 image yielded a higher overall accuracy of 86.90% (kappa coefficient = 0.82) than the resampled conventional four-band image that yielded an overall accuracy of 74.64% (kappa coefficient = 0.63) in pixel-based classification. We further compared the vegetation indices that were derived from four conventional bands with those derived from WV-2 bands. The enhanced vegetation index yielded the highest overall accuracy in the category of conventional indices (85.59% at kappa coefficient = 0.79), while the modified plant senescence reflectance index involving the red-edge band showed the highest overall accuracy (93.69%) in the category of indices derived from eight-band WV-2 imagery in object-based classification. Overall, the study shows that the unique high-resolution WV-2 data can improve the delineation of canopy gaps as compared to the conventional multispectral bands.

Spatial distribution of an undergrowth palm in fragments of the Brazilian Atlantic Forest

The occurrence of disturbed sectors may be as important as microscaleedge effects in tropical forests fragments. We considered the spatialdistribution dynamics of life stages (youngs, immatures, and adults) of theacaulescent, endemic palm Attalea humilis Mart. ex.Spreng.in fire-prone fragments of the Brazilian Atlantic forest to investigatepopulation responses to fragment size and fire occurrence. From 1996 to 1999,werecorded the number of individuals in different life stages in adjacent10×10-m plots in two perpendicular transects across each of threefragments of different sizes (1.6, 6.4, and 9.9 ha) in the ReservaNacional de Poco das Antas, Southeastern Brazil. A fire burnt thefragments studied in 1997. Application of Morisita's Index showed clumping atmost scales, with a marked reduction in clumping degree from youngs to adults,apattern not influenced by fragment size. After fire, clumping degree increasedtemporarily among the youngs. No association between life stages was detectedusing presence/absence data. Palm density concentrated in the most disturbedtransect arms, a pattern very little affected by fragment size or fire. Noconsistent relationship between palm density and distance from fragment edgewasdetected for any stage, fragment, or year. We hypothesize that short-distanceseed dispersal by scatterhoarding rodents results in discrete, low-densityclumps of youngs dissociated from reproductive plants. The spatial pattern oflater stages is probably conditioned by the distribution of canopy gaps in thelargest, more closed fragment. In the other fragments, the presence of largedisturbed sectors promotes the formation of large, continuous stands of palms.This pattern would be reinforced by fire, which is known to be recurrent onalready disturbed sites. For the species studied, large-scale variations in theforest structure (degraded vs. preserved fragment sectors) seems to be moreimportant than microscale edge effects.

A comparison of digital and film fisheye photography for analysis of forest canopy structure and gap light transmission

Due to the scarcity and high cost of conventional film-based hemispherical photographic systems, some forest scientists are now using multi-purpose, consumer-grade digital cameras for the analysis of forest canopy structure and gap light transmission. Although the low cost of digital cameras and direct capture of digital images appear to offer significant advantages over film camera systems, relatively little is known about their technical differences from an applications perspective. In this study, we compared the performance of a popular digital camera (Nikon Coolpix 950 with FC-E8 fisheye) with a conventional film camera (Nikon F with Nikkor 8 mm fisheye) under different stand structures and sky conditions. Our findings show that the Nikon Coolpix 950 digital camera produced hemispherical canopy photos with substantial color blurring towards the periphery of the exposure. We believe that chromatic aberration associated with the camera’s lens optics may be the source of this phenomenon; however, other factors may have also contributed to the diminished image sharpness. Color blur influenced (i) the size, shape, and distribution of canopy gaps; (ii) the accuracy of edge detection and the binary division of pixels into sky and canopy elements, and (iii) the magnitude, range, and replication of canopy openness, leaf area, and transmitted global radiation results. The Nikon Coolpix 950 produced canopy openness measures that were 1.4 times greater than film estimates in 22 of the 36 photo pairs. Cloud cover and sky brightness also influenced the spectral characteristics of the lateral chromatic aberration (halos), and thus had an added and unpredictable effect on canopy openness. Setting the Nikon Coolpix 950 to record in black and white, and shooting only under uniformly overcast skies will help to minimize the unpredictable effects of chromatic aberration. Nevertheless, we recommend a cautious approach when undertaking canopy measurements with the Nikon Coolpix 950, particularly when stands are dense and canopy openness falls below 10%. High-quality (1:4) JPEG compression had no significant influence on mean canopy openness; however, lower XGA and VGA image resolutions combined with 1:4 JPEG compression produced mean canopy openness results that were significantly lower than openness data extracted from uncompressed, full-resolution TIFF photos.

Spatial patterning in the canopy gap regime of a subalpine Abies‐Picea forest in the northeastern United States

Abstract. Spatial patterning in the disturbance regime of a forest affects the vegetation dynamics. Therefore, the distribution of canopy gaps was examined in detail for a subalpine Abies‐Picea forest in the northeastern United States. Gaps were not randomly distributed. The fraction of forest area in gaps and the abundance of gaps varied significantly with topographic position, elevation, and slope percent. On average, 15 % of the forest was influenced by gaps, but the gap fraction was greater near ridges (23 %) and near streams (27 %) than on the backslope (13 %). Also, gaps were larger and more abundant near streams and ridges. Gap fraction varied with elevation as well: more of the forest was disturbed at lower and higher elevations than at mid‐elevations. Significantly more of the forest on steep slopes (≥ 30 %) was under gaps. As a result of this patterning, some parts of the Abies‐Picea forest were predictably more disturbed than others. A remaining question is whether this patterning is sufficient to influence the regeneration environment and thus forest composition.