Quantifying the effect of sampling plot size on the estimation of structural indicators in old-growth forest stands

Abstract

There is increasing awareness that structure-based indicators should be considered for assessing the biological value of late successional forests. In order to increase the unique habitat features critical for old-growth associated species, it is important to identify and rank candidate potential forest sites on the basis of their distinctive structural features. Data on living and deadwood components for the identification of old-growth condition are usually acquired in the considered forest stands by two sampling survey: (i) census performed in relatively large monitoring sites; (ii) network of small sampling units, on which inventory practices are usually based. Several authors argued that choosing between these survey strategies might have substantial effects on the values of common indicators of old-growth condition. Our study aims at (i) assessing the total estimate differences among old-growth structural indicators measured in field plots with different sizes, and (ii) defining the optimal sample size for the reliable assessment of such indicators. The study was carried out in six beech dominated forest stands on the Apennines range in Italy. In each stand, living and deadwood components were surveyed and geocoded in 1-ha square areas. Based on these dataset, circular plots with radii ranging from 4 m up to 20 m were then considered in order to quantify the effect of sampling plot size on the estimation of four structural indicators: (1) number of living trees; (2) number of large trees (dbh ⩾ 50 cm); (3) total deadwood volume; (4) number of deadwood elements (snags, dead standing trees; lying dead trees, lying deadwood) with dbh (or average diameter for lying deadwood) ⩾ 30 cm. We found that the size of the sampling plots should be at least 500 m2 in order to establish a database for the assessment of the investigated indicators. The census approach should be preferred to the sampling plot approach for old-growth forest stands smaller than 3–5 ha. The achieved results contribute to define assessment protocols for characterizing and ranking the degree to which forest stands approximate old-growth condition based on standardized indicators

Living and Dead Aboveground Biomass in Mediterranean Forests: Evidence of Old-Growth Traits in a Quercus pubescens Willd. s.l. Stand

“…A relict Quercus pubescens stand in Gennargentu (Sardinia, Italy) reached dominant heights ranging from 7 to 11 m [65]. Trees with DBH larger than 50 cm were 38.2 per hectare (87% of which were Quercus pubescens individuals) within the Fanuso wood, a value which is notably higher than the most frequently considered minimum threshold for European and Mediterranean forests, corresponding to at least 30 stems per hectare [38,66]. Such value exceeded the benchmark for temperate European old-growth forests [9], it is almost equal to that of other Mediterranean old-growth forests [66], and it is considerably higher than the average recorded in many forest reserves of France, including many oak species [67].…”

“…SDT and SN were dead trees higher than 130 cm with an intact or broken stem, respectively, whereas ST were lower than 130 cm. We also assessed the contribution of coarse deadwood, including all dead elements larger than 30 cm, due to its recognised ecological importance for biological diversity [38]. The volume of snags, coarse woody debris, and stumps was calculated with the following formula (adopted in the protocol of the PRIN 2007 project):…”

Comparing the Quantity and Structure of Deadwood in Selection Managed and Old-Growth Forests in South-East Europe

“…In addition, in two 50 × 8 m rectangular plots centered on the line transects, the diameters of stumps were measured at the ground and at the top, whereas the diameters of snags were measured at 1.30 m above the ground (DBH). Thus, the area measured around each single point was 736 m 2 , which is larger than the minimum plot area for CWD sampling (500 m 2 ) proposed by Lombardi et al [42]. In addition to size measurements, for each element of CWD, decay classes were also recorded, where the first class stands for fresh CWD and the fifth class represents very old, nearly decomposed CWD (for details, see [43]).…”

Deadwood distribution in European forests

“…In this study we consider data from subplot 2. Despite plot dimensions are not optimal for tree stand biodiversity evaluations, they are appropriate for estimations of total deadwood volume (Lombardi et al, 2015).…”