Abstract: Over the past 20 years, Texas has experienced a number of severe droughts. The 2011 exceptional drought was the most severe one on the record. In this study, Forest Inventory and Analysis (FIA) data of four complete inventory cycles in the past 20 years (2001-2018) were used to examine carbon loss of forest stands affected by the drought in 264 plots in four national forests in East Texas. The temporal and spatial variations of forest carbon stock at the inventory cycle level and annual level before and after drought were analyzed. Random forest models were used to explore and predict the effects of drought severity (standardized precipitation evaporation index, SPEI), drought length, and forest stand factors (stand density, basal area and stand age) on carbon loss rates. Our results showed that the carbon loss in the four national forests increased significantly due to the 2011 severe drought and other associated natural disturbances, and tended to increase with the increases of SPEI. The carbon loss in the post-drought 9th inventory cycle increased significantly (91.45 t), being two times of that in the 8th inventory cycle before drought. In the four classification criteria (forest origin, tree size (DBH, diameter at breast height), tree height and tree species group), carbon loss increased with the increases of drought severity during the dry period. Compared with plantations (2.9%), the carbon loss in natural forest was greater (7.4%). The carbon loss of trees with smaller DBH (2.54 cm ≤ DBH <12.7 cm) and lower tree height (15 m ≤HT) was 18.7% and 7.9%, respectively. Pine trees had the lowest carbon loss (5.1%), which showed strong drought tolerance. Among different forest types, pine forests were less affected by the exceptional drought and had the lowest carbon loss rate (5.5%). The results of random forests models showed that the relative importance score of drought severity on carbon loss rate was the largest (9.2%) (P<0.01), and that the relative importance score of drought length was 8.1% (P<0.05). The relative importance of stand density, tree basal area, and stand age was 4.4%, 3.0%, and 1.3%, respectively. Compared to forest stand factors, drought is the dominant driving factor of carbon loss rate. When SPEI <-1.2, the carbon loss rate increased with the increases of drought severity. When drought length <2.2 or >11.0 months, the carbon loss rate is larger. Better understanding of forest dynamic and carbon stocks affected by drought and its driving factors provides scientific basis and reference information for a sustainable carbon forestry operational planning and management in the national forests in Texas and beyond.

Key words: carbon stock, drought, carbon loss, national forest, Texas