Fire Resilience of Native Tree Species in Montane Forest Ecosystem

Abstract: Montane forest ecosystems are important for watershed protection and as habitats for diverse organisms. They have been greatly modified by various land-uses. Therefore, restoring such ecosystems is a national priority. Fire disturbance is a major barrier for the success of forest restoration in northern Thailand, particularly during the dry season. Planting fire-resilient trees could increase restoration success. The objectives of this study were i) to quantify the effects of stem diameter on survival and resprouting ability of planted trees after fire and ii) to identify potential candidate tree species for restoring montane forest ecosystem in fire-prone areas.

After the accidental burning of 1-, 2-, 14- and 17-year-old restoration trial plots (planted in 2014, 2013, 2001 and 1998 respectively), during the summer of 2015 (April-May), data were collected 3 times (2, 18 and 30 weeks after the fire). Stem diameter, height and the number of resprouts of surviving trees were measured in 40 x 40 m sample plots. Survival after fire in the older plots (14- and 17-year-old plots) was high (95.3% and 98.6% respectively), whereas, in the younger plots (1- and 2-year-old plots) it was much lower (42.9% and 39.8%, respectively) Moreover, survival, throughout the monitoring, of younger trees declined more rapidly than that of older trees.

A chi square test, simple linear regression and generalized linear mixed models (GLMMs) all showed that the survival after the fire increased with increasing stem diameter. Bigger trees had thicker bark, which protected the cambium from fire, thus reducing mortality, stem necrosis and deformation. Trees with DBH or RCD > 40 mm had almost 100% survival after fire.

In general, bigger trees produced significantly fewer resprouting shoots (coppices) than smaller trees did. Although thick bark on larger stems protected buds from burning, it also inhibited resprouting by obstructing epicormic bud emergence. This might be associated with bud senescence, as the trees mature. Focusing on small trees (DBH or RCD <60 mm) the number of resprouting shoots significantly increased with stem size. This relationship is possibly associated with resource storage and therefore helps to support resprouting. Resprouting ability increases with stem size, until the trees reach their adult stage, after which it declines.

A suitability index was calculated from survival, number of resprouting shoots per tree, and stem relative growth rates. Twelve native tree species were recommended for restoring areas with high fire risk. Three species were categorized as excellent group (>75%) (Magnolia garrettii, Bischofia javanica and Ficus auriculata). Seven species were acceptable (60 – 74%) (Ficus hispida, Heynea trijuga, Prunus cerasoides, Castanopsis tribuloides, Protium serratum, Sarcosperma arboreum and Hovenia dulcis) and 2 species were marginal (<60%) (Ficus callosa and Cinnamomum longipetiolatum).

Three quarters of the recommended species have been identified as framework trees for forest restoration in northern Thailand. The exceptions were P. serratum, F. callosa and C. longipetiolatum. Even if seedlings of highly fire-resilient species are used for restoration projects, stakeholders should still implement fire-prevention measures, at least during the 2 consecutive years after planting, to keep the successional pathway on track, towards the ultimate goals of restoration.