UC Merced

Climate change is expected to drive shifts in forest species distribution to track ideal climatic conditions. The relative capacity for a tree species to persist under climatic stress is dependent on life history traits, such as growth, survival, and reproduction. Trees that produce large amounts of seed may be better able to colonize newly suitable habitats, while those that survive stress at current locations may persist longer than nearby competitors. These traits each represent distinct resource sinks, however. What remains unknown is how physiological modification in response to drought influences both survival and reproductive capacity. I analyzed growth, tree ring anatomy, and reproductive capacity in Pinus ponderosa and P. jeffreyi in the Sierra Nevada mountains of California, where the unprecedented 2012-2016 drought led to large-scale forest mortality. I found that trees that died during drought unexpectedly exhibited anatomical traits thought to confer drought tolerance, such as thicker walls in water-conducting xylem cells. Under drought, trees close stomata (pores in their leaf surface involved in gas exchange) to limit water loss, but at the expense of carbon (C) uptake. This leads to a theoretical expectation of C depletion in drought-stressed trees, particularly during prolonged (i.e., multi-year) drought. While direct evidence of this “C-starvation” has not been recorded in nature, my results point to a potential mechanism of the impact of C depletion on mortality. The sampled trees also experienced a high level of bark beetle (Dendroctonus spp.) attack, which is typically defended against in trees via the production of C-rich resin and other chemical defenses. Drought appears to have weakened sampled trees, and excessive allocation of available resources to drought defense may have depleted reserves necessary for fending off beetle attack. To quantify potential tradeoffs between drought defense and reproduction, I developed a novel technique to measure total lignin (a C-expensive material involved in xylem cell wall thickening) in tree rings, and found that trees that died had higher lignin content than living trees. To further explore these patterns, I modeled likelihood of tree mortality as a function of tree ring width (growth), xylem anatomy, competition, and climate. I first compared multiple commonly used drought metrics with ring widths from >800 trees from across the Sierra Nevada and found that drought metric choice influences interpretation of drought impacts. I then showed that trees that grew not only thicker-walled xylem cells, but also more variable growth rings and variable cells between years were more likely to die. Trees that grew the same amount each year, or grew rings with relatively constant xylem cell diameters and wall thicknesses, were more likely to survive drought, counter to hypothesized tradeoffs between growth and reproduction during drought. Finally, cone counts of measured trees show that ring width (growth) was the primary determinant of reproductive capacity, with trees that grew more producing more cones. These results demonstrate that tree response to drought is a function of variation in xylem anatomy and ring width, with the mechanism of mortality being associated with C depletion. Trees that are less responsive to climate and maintain fairly constant growth appear to be most likely to survive prolonged drought, and trees that grow large rings (with low variance between years) are more likely to reproduce. These results improve our understanding of whole-forest response to future climate change by demonstrating the importance of both cellular scale (xylem anatomy) and forest-scale (drought metrics and competition) variation in influencing drought-induced forest mortality.