Xylogenesis is uncoupled from forest productivity

In drought-prone areas, xylem production and forest productivity are assumed to depend on the amount and timing of precipitation. However, few studies have addressed if xylogenesis and proxies of forest productivity as the Normalized Difference Vegetation Index (NDVI) are coupled and respond to moisture availability. In these areas, drought seasonality and timing could lead to bimodal growth patterns. We tested if xylogenesis was bimodal and coupled with climate and NDVI at the intra-annual scale in Pinus leiophylla from NW Mexico. We found a bimodal growth pattern with a major peak of production of radially enlarging tracheids in June and a secondary peak in October. The June peak preceded a coupled rise of the amount of cell-wall-thickening tracheids and the NDVI which peaked from September to October. The timing in the production of thickening and mature tracheids matched the NDVI pattern. This coupling suggests active carbon uptake after wet summer conditions and carbohydrate usage in tracheid maturation and cell-wall thickening. The production of radially enlarging tracheids, which determines the final ring width, was enhanced by warmer minimum temperatures whenever precipitation was abundant. In contrast, more mature latewood tracheids were produced in response to cool-wet summer conditions. The bimodal behavior of P. leiophylla radially enlarging tracheids illustrates xylem plasticity in response to seasonal drought. Xylogenesis and NDVI data were uncoupled. Therefore, the assessment of forest responses to climate warming in similar drought-prone areas must explicitly consider shifts in productivity and the timing of wood production.