25 June 2014, Mongabay news - Tropical forests boast high levels of biodiversity. But often, even if they  contain large numbers of species, the numbers of individuals of those species  can be quite small. Restricted geographic ranges, high habitat specificity, and  small local population sizes all contribute to the natural rarity of many tree  species. Anthropogenic activities such as selective logging can compound this  rarity by modifying habitats and altering the competitive balance among tree  species. National parks are created in part to mitigate human disturbances and  restore ecosystems to their previous, more natural states.

According to  a study published in mongabay.com's open access journal, Tropical  Conservation Science, previous logging in the forests of Vietnam's Xuan  Son National Park put rare tree species at risk by lowering the tree density,  tree size, canopy cover, tree species diversity, and tree regeneration compared  to un-logged forests.

To study how rare tree species in the limestone forest of Xuan Son National Park  in northern Vietnam fare after logging, the scientists selected five easily  identifiable tree species of high conservation concern to focus on. Diameter at  breast height, height, species identity, amount of tree canopy opening, and tree  density were recorded in 20 random sampling plots within both the previously  logged and un-logged areas. In addition, they measured soil and topographic  factors that also affect tree growth, such as slope, aspect, elevation, and soil  depth.

Three of the rare tree species studied (Excentrodendron  tonkinense, Cukrasia tabularis, and Garcinia fagraeoides) are  specialists for the limestone hills, and require very specific conditions to  grow. The other two species (Parashorea chinensis and Melientha  suavis) were generalists in that they did not show any specific trends based  on the ecological variables studied, and were more widely dispersed throughout  the forests.

The researchers found that while the more specialized rare  tree species were showing signs of regeneration in previously logged areas, the  rate of their regeneration was much slower than in unlogged forests. The  generalist tree species showed faster regeneration in both logged and unlogged  forests than did the specialists, although they were still slower to regenerate  in logged forests compared to unlogged.

The study attributes this  discrepancy in growth rate to a number of possible factors. First, the  specialist species prefer to grow in rocky places, which there were more of in  the unlogged forest plots. Second, the seeds of many tree species are dispersed  by animals, which logging activities tend to displace. Third, the researchers  hypothesize that loggers may have selectively targeted larger, seed-bearing  trees, thus limiting the abundance of the next generation.

Xuan Son  National Park only became a nature reserve in 1986. Illegal logging occurred in  the area until 2002, when more strict conservational measures were taken and the  area was declared a National Park. While the land was formerly logged and, as a  result, is still recovering, the paper does suggest that the area will continue  to improve as long as it keeps its national park designation.

In their  study, the researchers highlight the importance of rare species to ecosystems,  and stress the need for their protection in Xuan Son and other at-risk forests.
Source: http://news.mongabay.com/2014/0625-mann-trees-vietnam-tcs.html?n3ws1ttr