The risks of xylem safety: when investing in cavitation resistance doesn't save trees from drought

As the earth’s climate warms, forest around the world – and particularly those in already arid climates – have been suffering increasing drought stress. During the “hot drought” of 2012-2016, over 1 million conifer trees across California’s Sierra Nevada mountains died. We investigated how trees that died differed from those that lived in how they built their wood. Surprisingly, pines that died tended to have narrower xylem tracheids (the water conducting cells in wood) with thicker walls – features that should have made them more resistant to hydraulic failure during drought. Surviving trees exhibited both a “slow and steady” growth pattern and less variability in their xylem hydraulic safety index from year to year. However, when pines close their leaf pores to reduce water loss, that prevents them from fixing carbon through photosynthesis. We hypothesized that spending more carbon on drought-resistant xylem might leave too little for other processes, such as producing the resin that defends pines from bark beetles. Preliminary data is consistent with this hypothesis.