Relationships between coarse woody debris and understory vegetation in six forest ecosystems in British Columbia

Understory vegetation in relation to coarse woody debris (CWD) was studied in six forest ecosystems which were located in six biogeoclimatic zones of British Columbia. Sample plots were located in each ecosystem and transect lines were used to set vegetation sample quadrats. Size of CWD was measured and classified into three classes based on the degree of decay. Vegetation on each decay class of CWD and forest floor was sampled and compared within each ecosystem and among different ecosystems using principal component analysis and cluster analysis. Effects of log size and canopy closure on the understory vegetation were also studied by regression analysis. The stands' structure in terms of species, DBH (diameter at breast height), and crown canopy was also measured. In total, 245 plant species were identified in this study, including 8 tree species seedlings, 30 shrubs, 58 herbs and grasses, 8 ferns, 54 mosses, 28 liverworts (hepatics), and 59 lichens. All together, 169 species inhabited CWD one third of which were confined to it. CWD is particularly important to non-vascular plants, especially lichens (in a dry forest) and liverworts (in a moist forest) for 95% of the lichens arid 96% of the liverworts were found on CWD. CWD is also important to mosses, but the number of species and the abundance were also high on the forest floor. Among wood inhabiting (lignicolous) plants, lichens and liverworts are more sensitive to light, moisture, as well as rooting substratum than mosses. Lichens prefer dry and open forests and are more abundant on relatively fresh logs. Liverworts, in contrast, prefer moist and dense forests and are more abundant on medium or well decayed logs. As a habitat, CWD is an important site for tree seedling regeneration, especially well decayed ones. Results also showed that the abundances of different plant types (basically functional groups) were related to the degree of log decay. CWD has effects on the distribution and abundance of bryophyte growth-forms, e.g. mat type growth-forms and short turfs were more richly represented as wood-inhabiting species whereas weft type growth-forms were widely distributed on both CWD and forest floor, and their abundance increased as wood decay increased with the peak appearing on the forest floor. Analyses showed that vegetation/plant communities on CWD were significantly different from those on the forest floor in each of the six ecosystems. In general, forest floor vegetation was more different from each other than CWD vegetation among the study ecosystems. Evidence showed that log size and canopy closure had impact on species richness/diversity (number of species per unit area in this context) and the number of species was logarithmically correlated with log size and quadratically correlated with canopy closure. Usually, the larger the logs the more species were found, and there was a trend that this increase of species number diminished and then nearly leveled off when logs reached a certain size. However, the canopy - species relationship is different. The highest number of species was found under canopy closures of 65 - 75% and this number decreased with increase or decrease of canopy closure. It is concluded that CWD is an important functional element in forest ecosystems. It is important to understory vegetation, especially to non-vascular plants. Therefore, in forest operations, maintaining certain amounts of CWD in different decay stages is essential to maintain biodiversity in forest ecosystems.Forestry, Faculty ofGraduat