Threatened tree species across conservation zones in a nature reserve of North-Western Vietnam

The high diversity of tree species in tropical forests is driven by a large proportion of rare species. Rare species are vulnerable and threatened to extirpation and extinction when their habitats are destroyed. This study addresses the abundance of threatened tree species within three conservation zones with differing levels of protection in the Ta Xua Nature Reserve of north-western Vietnam. The objectives were (1) to assess differences in the abundance of red-listed tree species among the strictly protected core zone, the low intensity traditional forest use buffer zone and the forest restoration zone, and analyze whether they are related to human interference; (2) to analyze the tree community structure in the core zone and the buffer zone, and in case of differences, to identify the impact of important variables, such as timber use, NTFP use, tree diameter, tree rarity, and red-list status, on differences of tree community; and (3) to assess the abundance of an iconic tree species Fujian cypress (Fokienia hodginsii) and two other threatened tree species (Aglaia spectabilis and Quercus platycalyx), and to determine the regeneration status of these three target species. Firstly, 40 random sample plots were established in each conservation zone. Observed tree species richness decreased from 193 species in the core zone to 173 in the buffer zone and 135 in the restoration zone. In total, 18 red-listed tree species (IUCN and Vietnamese Red Lists combined) were detected in three conservation zones. 16 red-listed tree species were found in the core zone, 10 in the buffer zone, and five in the restoration zone. Most red-listed species, such as Fujian cypress (Fokienia hodginsii), reached their highest densities in the core zone, but one species (Quercus platycalyx) was quite abundant in the restoration zone. For some red-listed tree species, canonical correspondence analysis suggested relationships among the presence of footpaths, canopy closure and basal area, suggesting reduced abundance caused by human activities. Our data indicate that conservation effectiveness is related to the level of statutory protection afforded to a particular area, with full protection ensuring more robust conservation outcomes. Secondly, the patterns of tree community differences in the strictly protected core zone and the low intensity traditional forest use buffer zone were studied. We found that the forests in the core and buffer zones are rich in tree species (249 observed). Many of these tree species provide non-timber forest products (NTFPs) (48%) or valuable timber (22%). 79 tree species (32%) were rare in at least one of the zones and 18 species (7%) are red-listed. Overall tree density was not different in the two zones, but tree diameter and species richness were lower in the buffer zone. At the tree level, logistic regression analysis indicated that red-listed status, tree diameter, density of species and NTFP use (in order of reducing importance) were significant associated with the probability of tree absence from the buffer zone. More specifically, red-listed status, large diameter, and low density of conspecifics increased the probability of tree absence in the buffer zone but not the potential use as a NTFP. However, most NTFP species had different densities in the core and buffer zones, and this correlated with signs of human interference. At the species level, the density of species was the most important variable, and low density (species rarity) strongly increased the probability of species absence. Our results also indicate that rare and red-listed trees were depleted in the buffer zone. In consideration of conservation goals, the future monitoring of these species at the Ta Xua Nature Reserve and other protected areas is needed, and conservation measures most likely need to be improved. Finally, the population status of F. hodginsii and two other threatened tree species (Aglaia spectabilis and Quercus platycalyx) was assessed across the three conservation zones. Based on previous simple random sampling, we applied adaptive cluster sampling for trees with a diameter at breast height of at least 6 cm. In addition tree regeneration was assessed. In the core zone, the three target species were moderately rare, in that they were not among the 10% most common species, nor among the 50% rarest species. F. hodginsii and A. spectabilis were most abundant in the core zone, and much less abundant in the buffer and restoration zones. In contrast, Q. platycalyx had its highest density in the restoration zone. Regeneration of all three target species occurred in the core zone; however, there was little or no regeneration of F. hodginsii and A. spectabilis in the buffer and regeneration zones. Regeneration of F. hodginsii and A. spectabilis was mostly found in the vicinity of conspecific adult trees. Our data do not support conservation concerns regarding Q. platycalyx in used and secondary forests, which may influence its status in the next assessment of endangered species. In contrast, adult trees of F. hodginsii and A. spectabilis were mostly confined to the core zone, and regeneration of these species was absent or very rare in the buffer and restoration zones. For these two species, the core zone was thus the most important refuge, so continued conservation of this zone is important for the preservation of these species. In conclusion, our results indicate that conservation effectiveness is related to the level of statutory protection afforded to a particular area, with full protection ensuring more robust conservation outcomes. The use of logistic regression models allows evaluation of the conservation effectiveness in a given nature reserve over time and among other nature reserves and national parks, and also facilitates the development of conservation strategies by quantifying the effects of different forest management measures on the presence or absence of trees and species. Monitoring of forest resources with a focus on rare and red-listed species at the Ta Xua Nature Reserve and other protected areas is needed and conservation measures most likely need to be improved.