Demography of threatened tree species in Vietnam

Abstract

Demography of threatened tree species in Vietnam (Summary for the library) Effective conservation of threatened tree species requires information on natural dynamics and future prospects of populations of these species. Such information can be obtained from demographic studies. We investigated the demography of six threatened tree species in four protected areas in Vietnam: the broad-leaved Annamocarya sinensis, Manglietia fordiana and Parashorea chinensis, and the coniferous Calocedrus macrolepis, Dacrydium elatum and Pinus kwangtungensis. In a two-year field study, we quantified recruitment, growth and survival, and used these data to popularize models (matrix models). We also used these data and diameter growth rates obtained from tree ring series to construct stochastic population models. These models take into account unpredictable variation. We evaluated natural recruitment of the study populations by using two parameters: the population growth rates obtained from models, and the ability of populations to produce sufficient seedlings for replacement of adult trees. Our models project that populations of four species are maintainable in time, while those for two species (Annamocarya and Calocedrus) are declining. We then evaluated whether lifetime production of seedlings by an adult tree is sufficient to replace itself – an important prerequisite for population maintenance. For three of the six species lifetime recruitment was sufficient, but the three other species (Annamocarya, Manglietia and Pinus) required extremely long and unrealistic (480-100 y) reproduction period to produce sufficient recruits to replace themselves. These species thus do not produce sufficient seedlings to maintain populations. Our models show that stochastic (unpredictable) variation in vital rates due to environmental variation in rainfall and in tree location decreased population sizes. This unpredictable variation also led to higher extinction risk in our models. Our models project that Dacrydium and Parashorea do not face extinction within 300 y, while with this period of time the extinction rates of the four other species Annamocary, Calocedrus, Manglietia and Pinus would be 65%, 11%, 51% and 93%, respectively. Extinction risk was very sensitive to changes in the means of vital rates and initial population size than to changes in variation of vital rates. This result shows that good qualification of model parameters is crucial for realistic results. In a methodological study, we found that age estimates strongly increased with the increasing category number of matrices. For instance, we found only 83 y for Annamocarya at 60 cm diameter (at breast height) when we applied a matrix of 15 categories, while this increased to 135 y for a matrix of 90 categories. We suggest that narrow categories (1.5-3 cm diameter) are used in matrix models of trees to obtained best estimates of tree ages, population growth rate and elasticity. Combining the results obtained from+ the study, we concluded that the future prospects for populations of Dacrydium and Parashorea are good, but this is not the case for the four other species. We suggest that conservation measures include strict protection of individuals of populations, particularly for juvenile and adult trees, as these are most important for population maintenance. Other effective conservations include: improving growth of seedlings and juvenile trees, enriching populations with planted seedlings obtained from controlled germination and restoring habitat to increase populations.