Trees are uniquely long lived, and so their responses to the environment integrate climate across multiple time scales. Thus, the study of tree physiology requires an integration of approaches across scales, both spatial and temporal. Our research quantifies 1) global change impacts upon the memory of tree growth, such as growth legacies of past drought, and 2) investigates the physiological mechanisms of those impacts.
1Drought legacies and climatic memory in tree growth. While we often assume growth-climate relationships to be time-invariant, trees exhibit drought legacies—slow recoveries of growth sensitivities following drought—sometimes longer than 5 years. Recent papers have quantified the impact of La Niña events (ENSO cold-phase), compounded drought (multiple drought years in a row), and the broader implications of temporal variability in tree growth-climate sensitivity.
Trees experience regular legacies from La Niña and these legacies are largest in the heart of the North American Monsoon region (blue shading, left). Trees in the NAM region are highly dependent on summer monsoon precipitation to “rescue” them from these winter droughts. More here.
We also know climate change is increasing drought frequency. How does this shortening return interval impact tree recovery? We found “triple-droughts” and “double-droughts” are worse than a single drought year. That is, trees experience greater growth reductions and legacies. But, different populations responded very differently! More here
2NSC cycling under drought. So what types of dynamics alter tree growth responses to climate under drought?
Non-structural carbohydrates and their cycling may underlie memory in tree growth. For example, after a decade of rainfall exclusion at the Sevilleta LTER, piñon (P. edulis) trees have exhausted their oldest NSC reserves.
D14C of NSC shows these trees have younger carbon reserves than trees in a control.
(Left) “White rings,” – typically an indicator of severe carbon limitation in forested ecosystems experiencing periodic defoliation by insects – shown here in a tree experiencing 10 years of 45% rainfall exclusion (left of image).
Combined evidence from leaf level gas exchange, bole respiration rates, canopy and bole NSC concentrations, NSC D14C (age), leaf phenology, chlorophyll fluorescence, and tree rings, suggests tree NSC pools change slowly in response to drought. But long term drought induces severe carbon limitation, despite acclimation of leaf level photosynthetic rates (Amax) and reduction of bole respiration rates. We note that carbon limitation effects on defensive chemistry and heartwood investment remain poorly known. Defensive chemistry is a key focus of other researchers involved with this study, namely Amy Trowbridge and her lab group.
Future work within this study is exploring the terminal dynamics of NSC age and concentrations as trees die from bark beetle attack.
(Right) Tree killed in a Piñon ips beetle outbreak in 2020 at the Sevilleta LTER.
3Past work has explored the role of NSC in drought legacies from the 2012 drought in a network of 22 forested sites (piñon, juniper, and aspen) across the Southwest.
We sampled three tree species to understand how non-structural carbohydrate (NSC) concentrations respond to moisture stress.
Responses differed broadly across species and seasons in ways that sometimes reflected hydraulic strategy (e.g. isohydry). Dynamics in all three species suggested sink limitation was common, but respiratory depletion, and osmoregulation were important too.
More here.
Those NSC concentrations also had real consequences for tree recovery from the 2012 drought.
The climate memory length for both aspen (green) and piñon (black) was strongly related to sugar concentrations (top).
Moreover, NSC concentrations were also strongly related to the magnitude of drought legacies (bottom).
More here.
4Future work: What is the age of NSC used to recover from disturbance?
In old growth coast redwoods burned in the CZU Lightning Complex (2020) at Big Basin Redwoods State Park, I am part of a project that aims to answer this very question. By quantifying the age of carbon (with D14C) in epicormic sprouts, we are quantifying the age of the carbon these giants are drawing upon to rebuild their canopies. We found trees remobilized 50-100 year old carbon reserves to support resprouting.
Video from NAU-TV on our results.
Video from Save the Redwoods League on our project.
Future work: Drought mortality events are expanding in the western US and impacting previously considered stress-tolerant Cupressaceae. I am combining eco-physiological monitoring with tree-rings to understand how long-term climate change drought stress is leading to enhanced mortality in Colorado Plateau dominant Juniperus spp. and the neotropical arizona cypress (Hesperocyparis arizonica).
