How Might Redwoods Fare in a Changing Climate, and What Can We Do To Help?To answer these questions, we visited 45 locations (32 primary and 13 secondary forests) from California’s Monterey County to Oregon’s Curry County. We climbed 235 trees, measuring each one from base to top and extracting thin core samples. Redwoods, like other temperate-zone trees, store their growth histories in annual rings, and cores are a non-destructive way to read a tree’s story. We took core samples from trunks at regular height intervals because sampling only near ground level tends to underestimate growth rates (more biomass production occurs within crowns as trees enlarge with age). Overall, we sampled 1.2 million annual rings, which were crossdated by Cal Poly Humboldt dendrochronologist Allyson Carroll. These data were combined with intensive measurements and allometric equations to reconstruct tree size and productivity through time. Redwood performance was modeled as functions of tree attributes, landscape position, and climate. Funding for this work came from Kenneth L. Fisher (Chair in Redwood Forest Ecology at Cal Poly Humboldt) and the Save the Redwoods League (Redwoods and Climate Change Initiative, Phase 3). Our rangewide analysis was recently published in Forest Ecology and Management—526 (2022) 120573. What we learned is cause for both concern and hope.
Which Climatic Variables Affect Redwood Productivity?Redwood habitat suitability is generally dependent on soil water replenished through rain and fog drip as well as water absorption through foliage. The redwood range spans over 6° of latitude. Rain and summer fog are highest in the north and lowest in the south. Trees north of 40° are least drought-sensitive, making similar biomass in dry and wet years, while trees south of 37° are most sensitive. The extravagantly wet start to 2023 sets an unlikely stage for talking about drought, yet the climatic variable most related to redwood growth is a drought index encompassing both water availability and temperature variability.Drought sensitivity has recently been increasing throughout the redwood range. Southern trees experience problems earlier during multi-year droughts, and they recover more slowly from extreme drought than northern trees. Across the range, smaller and younger trees in secondary forests experience more growth suppression during extreme drought than larger and older trees in primary forests. In late 2022, when our analysis was published, the whole redwood range was once again in the midst of a multi-year drought. Abundant rain returned to the region in early 2023 with excessive precipitation and extreme winds presenting an entirely different challenge to redwoods.Regardless of precipitation, the redwood range will experience progressive drying due to global warming. Temperatures are highest in the south, lowest in the north, and rising steadily, especially at night. The drying power of air—vapor pressure deficit (VPD)—increases exponentially with temperature. High daytime VPD means trees need to close their leaf stomata earlier in the day to prevent damaging water loss. This limits photosynthesis, but such “source limitation” is mitigated by rising atmospheric carbon dioxide (CO2) levels. In today’s enriched atmosphere (CO2 currently 419 parts per million, was 317 in 1960), trees can partially close leaf stomata to reduce water loss and still absorb plenty of CO2 for photosynthesis. Nevertheless, heatwaves with extreme daytime temperatures can lead to treetop dieback, and another temperature effect directly inhibits radial growth of redwoods.Growing season minimum temperatures are increasing as nights become unusually warm. High nocturnal VPD creates problems in the layer of dividing cells where new wood is made (the cambium). Dry air at night prevents sufficient turgor pressure to develop in the cambium for cell division and enlargement. With this “sink limitation” too few cells are produced to make new wood, so sugar produced by leaves via photosynthesis must be stored or used elsewhere. Where? Roots and mycorrhizal fungi are a definite possibility, though the belowground biology of redwoods remains largely unexplored. Another major sink is indicated by the name of the tree itself—the wood is red because of heartwood chemicals that resist fungal decay. Heartwood fungicide is redwood’s superpower.Trees strike a balance between making new tissues and protecting them from corruption. We express this balance with the metric “growth efficiency”—the amount of biomass produced annually per unit leaf mass. Sink limitations due to warmer, drier nights reduce growth efficiency but may increase wood quality, because excess sugar is used to make fungicide, not tree rings. Coastal fog helps to lower VPD, and nighttime fog is one of the best predictors of redwood growth efficiency. During multi-year hotter droughts, redwoods in forests lacking sufficient nighttime fog will see the most growth inhibition, but again, their heartwood may become more durable. This could be a silver lining of climate change, though it is more complicated because young and old redwoods aren’t equivalent.A bigger tree makes more wood annually than a smaller tree because it has more leaves, and the older a redwood gets the greater its annual investment in fungicide. Heartwood production and fungicide investment are both higher in primary than secondary forests throughout the range. This means secondary forests are generally less effective than primary forests at long-term carbon sequestration, and the capacity of regenerating forests to sequester carbon in durable biomass may be overestimated. Considering 95% of current redwood forests are relatively young, the priority is clear—we need more big old redwoods on the landscape.
Elder TreesManaging redwoods as short-rotation crops squanders the potential of a species that can live for two millennia. Long-term carbon sequestration is one issue, and biodiversity is another. These two non-timber values are interconnected because decay-resistant heartwood creates long-lasting substrates for epiphytes (plants that grow on plants without parasitism), including giant fern mats and ericaceous shrubs in the wettest part of the redwood range. Vascular epiphytes like ferns and shrubs represent an endpoint in epiphyte community development. Tree structural complexity promotes biodiversity—the largest and oldest trees host the bulk of arboreal life in addition to being carbon-sequestration champions. We’ve gotten to know redwoods very well over the past few decades and have come to think of exceptional individuals as elder trees.
We choose the word “elder” with intention both figurative and literal. The word applies figuratively because respect for elders is a cherished value in most cultural traditions worldwide. The literal sense of the word is demonstrated by the data—of 235 study trees only 34 hosted vascular epiphytes, and their average age was over 1,100 years. The biggest, gnarliest, epiphyte-laden trees are precious individuals that deserve all the reverence implied by the term “elder.” With over 95% of redwood forests having been logged at least once, elder trees are now rare on the landscape. This reality becomes starker with each major fire, landslide, or flood event that causes attrition in the last remaining primary forests.