A Comparison Between Plot-Count and Nearest-Tree Method in Assessing Tree Regeneration Features

Abstract

Assessing tree regeneration is important, in particular the proportion of saplings browsed due to the increasing number of ungulates. However, results of surveys differ depending on the method used. We investigated the differences between the plotcount method and the nearest-tree method for their use in tree regeneration inventories using simulations and field surveys at 11 study sites. The methods use differing references, i.e. number of trees vs. stand area. We focus in our comparison on aspects of practicability from a silvicultural point of view and not on comparing the estimators from a statistical point of view.For simulations, three artificial stands were generated taking gaps and tree clusters into account. Therein seven equidistant grids (36-484 grid points) were used for both methods. The plot-count was applied using circular sample plots with a radius of 1.5 m. The nearest-tree method was applied using maximum search distances of 1.13 and 3.99 m, referring to stocking goals of 2500 and 200 trees ha -1 , respectively. Two to five times more trees were evaluated for the plot-count method. In contrast to the nearest-tree method, the plot-count method does not account for unstocked stand area when evaluating the stocking goal. The estimated proportion of damaged trees (plot-count method) was larger than the estimated proportion of stand area occupied by 'damaged' trees (nearest-tree method). The same was the case for the variation but the estimators of both methods are asymptotically unbiased.For the 11 field surveys, results of 2 m circular sample plots are compared to the ones from the two nearest Abies alba saplings using a maximum searching distance of 10 m. Due to the large searching area, the density estimations of the nearesttree method were more precise for low sapling numbers. The estimated proportion of browsed saplings highly differed from the area occupied by browsed Abies alba saplings.The nearest-tree method is less laborious when measuring only the nearest tree independent of species and height class. Since an estimate of occupied stand area can be interpreted very intuitively, the nearest-tree method is particularly suitable to support silvicultural decision making in structured naturally regenerated stands.

Verbisseinfluss - Monitoring in den Buchenwäldern von Kirchberg SG: wissenschaftlicher Bericht zum Verbisseinfluss

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