Matthew posted a link to this Grist article before, and I think it’s worthy of its own discussion.
The driving force here is that the rising global temperature is wiping out seedlings. In many spots around the U.S. West, summer temperatures are already high enough to cook young trees before they can develop thick protective bark. Others have become so dry that seedlings shrivel before their roots can grow deep enough to reach groundwater. Both circumstances can thwart forest regeneration. Mature trees can survive in these areas long after they stop reproducing. But when fires wipe out these forests and seedings can’t get a foothold, they are replaced with grasses and dense brush.
Some of us remember reforestation problems on dry sites before climate change (I think people would say it was occurring then, we just didn’t know about it.) Here is what we thought then… first you need seed. And ponderosa pines have intermittent seed crops. So grasses/forbs/shrubs may have reclaimed the site (sucked up the sun and soil moisture) by the time Mom and Dad ponderosa got together, not leaving any openings for baby trees to take hold. Then there are seed predators of various kinds. Then there’s soil moisture at the right time. And critters that munch on seedlings. You can go out to various sites today and see a range of ponderosa (and in my hood, Doug fir, regeneration) from none to lots.
Back in those days, it was OK, and even desirable, to assist seedlings in “getting a foothold.” As I’ve said numerous times, in the 80’s, we learned about seed collection, nursery practices, seedling treatment in transit, vermiculite slurries, vexar tubing, and so on. We used to travel around and examine cone crops and numbers of filled seed to select good sites to produce seed. The whole thing as down to.. dare I say… a science.
What was interesting to me is how different conceptions and research methods can yield the same, or different conclusions.
Here are some concepts (not observations):
(1) if forests won’t grow back without help, then we must just get used to fewer forests.
(2) the absence of natural regeneration (over some time period) means that trees won’t otherwise grow well if planted.
What physiologists and silviculturists (applied forest ecologists) used to tell us through logic and observation (seeds must come from somewhere), we now can validate via regression models.
While annual climate was an important driver of postfire regeneration, our findings also highlight that the nature of a fire event strongly influences postfire regeneration. For example, the combined relative influence of annual climate variables on tree recruitment in our boosted regression tree (BRT) models was 24% for ponderosa pine and 34% for Douglas-fir (SI Appendix, Fig. S3), while the relative influence of distance to seed source, which is largely determined by fire severity, was 32% for ponderosa pine and 21% for Douglas-fir (SI Appendix, Fig. S3). The importance of seed tree availability in determining postfire regeneration has been demonstrated across forest types in the western United States (e.g., refs. 17, 20, and 46),
TSW readers tend to be out and about in the woods. Take a look next time (in a place where there are parent trees producing seed) and check out whether there is natural regeneration. I did that in my neighborhood and came up with the above photo. I also live in the fringe of ponderosa pine habitat on the Front Range of Colorado. In this photo you can see parent trees, cones on the ground, and many seedlings. It’s something we can pay attention to, and maybe learn something by sharing what we see in different parts of the west.