Paul M. Rich

Characterizing plant canopies with hemispherical photographs

Hemispherical (fisheye) canopy photography is a technique for characterizing plant canopies using photographs taken through an extreme wide-angle lens (Anderson, 1964). Typically, the viewing angle approaches or equals 180°. Hemispherical photographs can be taken either looking upward from within a canopy or looking downward from above a canopy. The resulting photographs serve as permanent records of the geometry of canopy openings. The geometric distribution of openings can be measured precisely and used to estimate potential solar radiation penetration through openings and to determine aspects of canopy architecture such as ground cover, leaf area index (LAI), and leaf angle distribution. Hemispherical photographs can be analyzed by hand using sampling grids; however, hand analysis is extremely tedious and generally impractical for large numbers of photographs. Digital image analysis techniques have recently been developed that allow for efficient analysis of large numbers of photographs (Chazdon and Field, 1987b: Rich, 1988, 1989; Becker et al., 1989).

Long-term study of solar radiation regimes in a tropical wet forest using quantum sensors and hemispherical photography

Abstract Daily photosynthetic photon flux density (PPFD) was monitored for 1 year in the understorey of a tropical wet forest along a transect extending from a treefall gap to the closed canopy at La Selva Biological Station, Costa Rica. Quantum sensors attached to data loggers were operated continuously. Hemispherical photographs were taken monthly above the sensors. An additional quantum sensor was used to monitor PPFD outside the canopy. Sensor measurements show large differences between gap and closed-canopy locations and high daily and seasonal variability at each sensor location, with notable increases in solar radiation in gap stations during September and April. The photographs demonstrate that seasonal variation in PPFD results primarily from shifts in the solar angle relative to canopy openings and secondarily from variation in PPFD levels outside the canopy (cloudiness). Photographs also demonstrate an overall decrease in PPFD with time, for the gap stations, owing to vegetation regrowth. Analyses reveal excellent agreement between sensor PPFD measurements and estimates from hemispherical photographs. Long-term monitoring of PPFD enables calibration of hemispherical photography to permit estimation of PPFD with a high degree of reliability.

Comparative analysis of microhabitat utilization by saplings of nine tree species in Neotropical rain forest

We examined the hypothesis that tropical rain forest tree species differ significantly in the degree to which their regeneration is associated with treefall gaps. Using static and dynamic analyses, we evaluated crown light environments and forest structure around 0.5-5 m tall saplings (N = 424) of two pioneer and seven nonpioneer tree species in old growth tropical wet forest at the La Selva Biological Station, Costa Rica. Canopy photographs were taken with a fisheye lens directly above all individuals. Each photograph was analyzed to calculate a Global Site Factor (GSF), which is analogous to the percentage of photosynthetically active radiation reaching a site relative to a totally open site (...)

Seasonal patterns of acid fluctuations and resource storage in the arborescent cactus Opuntia excelsa in relation to light availability and size

SummaryWe investigated relationships between light availability, diel acid fluctuation, and resource storage in the arborescent cactus Opuntia excelsa growing in western Mexico. We compared canopy and understory individuals from a deciduous forest as well as open-grown plants of the same approximate size as those in the understory. During the wet season light availability and daily fluctuations in titratable acidity (an index of carbon uptake) were lower in the understory than in unshaded habitats. In the dry season all plants had reduced levels of acid fluctuation, with the smallest individuals, regardless of habitat, showing the greatest reduction. These data suggest that light availability in the forest understory constrains carbon assimilation during the wet season, but that a factor associated with plant size, possibly water status, limits carbon gain during the dry season. Plants in all habitats remained physiologically active for at least five months into the dry season. We suggest that this was possible due to the maintenance of constant concentrations of water and nitrogen in the photosynthetically active chlorenchyma. Parenchyma in terminal cladodes showed a different seasonal pattern of resource storage; water content and nitrogen concentration were reduced from the wet to the dry season in the parenchyma. Using the parenchyma to supply photosynthetic tissues during times of reduced resource availability allows O. excelsa to assimilate carbon during times of the year when most other trees in the forest are leafless.