Comparing direct and indirect methods of assessing canopy structure in a northern hardwood forest

نویسندگان

  • Anne G. Rhoads
  • Steven P. Hamburg
  • Timothy J. Fahey
  • Thomas G. Siccama
  • Richard Kobe
چکیده

Several methods exist for measuring forest canopies following disturbance, and the biases and differences among them are unclear. We compared techniques for measuring the northern hardwood forest’s canopy structure at the Hubbard Brook Experimental Forest, New Hampshire, following the severe ice storm of January 5–10, 1998. Methods included leaf area index (LAI) using LI-COR’s LAI-2000, visual damage assessments based on tree branch loss, radiation estimates from hemispherical photographs, and LAI determined from litterfall. LAI-2000 measurements were not significantly related to visual damage class estimates, but were strongly correlated with radiation estimates from hemispherical photographs and average LAI values from litterfall. LAI from the LAI-2000 and litterfall differed on a pointby-point basis, but were similar at the stand scale. The LAI-2000 has the highest precision for large-scale measurements. Visual damage estimates appear adequate for assessing large-scale patterns of disturbance intensity in the northern hardwood forest, but the LAI-2000 is more accurate at quantifying canopy structure at large plot or stand scales. Hemispherical photographs may also accomplish this, but are better suited to characterizing the distribution of canopy gaps and light availability patterns over time. Litterfall provides accurate and precise measurements of small-scale LAI patterns in deciduous forests and reveals species-specific patterns. Résumé : Il existe plusieurs méthodes pour mesurer l’état du couvert forestier après une perturbation mais les biais et les différences propres à chacune de ces méthodes sont encore vagues. Nous avons comparé différentes techniques pour mesurer la structure du couvert de la forêt feuillue nordique à la Forêt Expérimentale d’Hubbard Brook au New Hampshire après l’importante tempête de verglas survenue du 5 au 10 janvier 1998. Les méthodes comprenaient la mesure de l’indice de surface foliaire (LAI) à l’aide du LAI-2000 de LI-COR, l’évaluation visuelle des dommages à partir des pertes de branches, l’estimation du rayonnement à l’aide de photographies hémisphériques et la détermination de LAI à partir de la chute de litière. La relation entre les mesures du LAI-2000 et les estimations visuelles de la classe de dommages n’était pas significative. Par contre, les mesures du LAI-2000 étaient fortement corrélées aux estimations du rayonnement à l’aide des photographies hémisphériques ainsi qu’aux valeurs moyennes de LAI déterminées à partir de la chute de litière. Les valeurs de LAI mesurées avec le LAI-2000 et celles obtenues à partir de la chute de litière différaient sur une base ponctuelle mais étaient similaires à l’échelle du peuplement. Le LAI-2000 a la plus grande précision pour les mesures à grande échelle. L’estimation visuelle semble adéquate pour évaluer les patrons à grande échelle de l’importance des dommages dans la forêt feuillue nordique mais le LAI-2000 est plus précis pour quantifier la structure du couvert à l’échelle des grandes places-échantillons ou des peuplements. Les photographies hémisphériques peuvent également satisfaire ce besoin mais sont mieux adaptées pour caractériser la distribution des trouées dans le couvert et l’évolution des patrons de disponibilité de la lumière dans le temps. La chute de litière fournit des mesures exactes et précises des patrons de LAI à petite échelle dans les forêts décidues et fait ressortir les patrons spécifiques à

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تاریخ انتشار 2004