- Author: Rob York
Adapting to climate change: Forests will try, but they can’t do it on their own
Article reviewed: Forest responses to climate change in the northwestern United States: Ecophysiological foundations for adaptive management
By D.J. Chmura, P.D. Anderson, G.T. Howe, C.A. Harrington, J.E. Halofsky, D.L. Peterson, D.C. Shaw, and J.B. St. Clair Published in the journal, Forest Ecology and Management (Vol. 261: 1121-1142).
The plot line: This is a review of the likely and potential effects that climate change will have on the physiology of trees in the western US. The authors discuss how these effects might influence forests at larger scales and also discuss the degree to which forests might be able to adapt to a changing climate. They focus on a changing snowpack and drought stress as important stresses that may lead to changing fire regimes and forest pest interactions. While significant impacts appear certain, they also note the tremendous uncertainty in predicting the details of how impacts will play out. They conclude that forests will not be able to adapt without management intervention. The recommended management actions that may help vulnerable forests adapt to climate change include density management, planting, and assisted migration.
Relevant quote: “Overall, density management should be the most effective [silvicultural] approach because of its ability to lessen drought stress, fire risk, and predisposition to insects and disease.”
Relevance to landowners and stakeholders:
If forest landowners are anything like me, they go through ups and downs when it comes to worrying about how climate change might influence their forest. For forest managers, it is arguably their responsibility to think in long time frames so it is therefore their responsibility to think about how climate change might influence the forests they manage. But landowners may not have that same incentive to think longer-term. I admit that sometimes my time frame only extends to the time at which I think I am going to sell the land or when I will no longer be able to physically work on it. This tends to make me rather blasé when it comes to worrying about climate change effects. But even for those like me that suffer this periodic short-sightedness, this review reminds readers that it is wise to address climate change impacts now. The uncertainty and complexity of how climate change will affect forests are frankly overwhelming. This review includes how climate change might influence factors of how forests grow:
- Carbon dioxide concentration (going to go up)
- Temperature (going to go up)
- Precipitation (not sure where it’s going)
- Drought (going to be more common and longer)
- Wildfire (going to be more frequent and severe, but might go down after a while)
- Insects and diseases (going to emerge in new locations and intensities)
Those are just 6 factors that we know are going to change (in uncertain ways), but there are probably more. Sometimes we can consider one factor individually and make a scientific guess about how it will affect forests. But the reality is that these factors will be interacting with each other to affect forests in completely uncertain ways. We really have no clue what the exact effects will be or how long they will take to occur. But we do know they will be a big deal socially, economically, and ecologically. As I’ve reviewed in previous posts, active adaptive management is really the only realistic management response to such a foreboding reality.
Relevance to managers:
True to the title of the paper, the review focused on the foundations for adaptive management so there are not many actual management recommendations. I think these are the primary foundations which can be drawn upon from this review with respect to constructing adaptive management plans:
- Inter-breeding populations are the scale at which plants can adapt, so management decisions are ideally done at a fairly local level
- The regeneration phase of trees is the most vulnerable to the impacts of climate change
- The abiotic changes that will most likely either directly or indirectly influence forests are drought stress, a shrinking snowpack, and an earlier timing of snow melt (I am thinking mostly of dry montane forests here)
- We have already seen climate change interact with existing pests to result in unpredicted epidemics (i.e. mountain pine beetles). Expect more of the same.
The authors very briefly suggest the following as possible management responses:
- Density management. Thinning forests makes individual trees more resistant to drought stress
- Planting. Because the regeneration phase is most vulnerable to failure
- Assisted migration. It was confusing, but I believe their emphasis was on within-species range migration
- Forest stand triage. Foresters should think of the different seral stages and structures that they manage for, and then consider which of these might be most vulnerable to climate change. For example, forests that have reserves where density is very high and fuel is also very high could be the most vulnerable. Because of the vulnerability of the seedling stage to changes in climate, young stands (or those in an understory re-initiation phase) might also be especially vulnerable.
Critique (I always have one, no matter how good the article is):
The management recommendations were not as thorough as I was hoping. They provided very detailed reviews of how climate change might influence forests differently in different parts of the western states. But management recommendations were not given with anywhere near the same level of detail. Assisted migration, molecular and genetic breeding, and gene conservation were mentioned as possible strategies. Given that many folks are very skeptical of these types of intervention (in my experience, some people think assisted migration is a capital offense), it would have been useful to provide some examples or perhaps bounds on how they should be used given the range of plausible ecophysiological responses to climate change.
- Author: Rob York
[Originally posted on www.foreststeward.com on Jan 28, 2011]
Article Reviewed: Changes in Climatic Water Balance Drive Downhill Shifts in Plant Species’ Optimum Elevations
By S.W. Crimmins, S.Z. Dobrowski, J.A. Greenberg, J.T. Abatzoglou, and A.R. Mynsberge, published in the journal, Science, Vol. 331, pp. 324-327
Plot line: The authors gathered data collected from the early 1930’s from surveys that measured the elevation where species occurred throughout the forested regions of California (north of about Tehachapi). They also gathered weather data from the same period to see what type of climate the species were occurring under. They then did the same thing (gathered plant and weather data) from the early 2000’s to see if there was a difference in where plants were growing and what climate conditions they preferred. They found that most species (72%) shifted downhill by on average 289 feet in elevation. Although average temperature has increased since the 1920’s (and especially since about 1950), there has also been an increase in precipitation which has actually resulted in a net reduction in drought stress at a given elevation. They conclude that species have shifted downhill in order to limit the amount of drought stress and to expect similar shifts to occur if the pattern of higher temperatures simultaneous with higher precipitation continues.
Relevant Quote: “Plant species in our study area appear to be tracking their climatic niche by shifting their altitudinal distributions downhill in response to decreased climatic water deficit.”
Relevance to landowners/stakeholders
Plants are finicky creatures. They have a certain set of climatic conditions under which they can thrive versus merely persist. This is kind of similar to how we set our thermostats at the temperature where we are most comfortable. Sure there is a wide range of temperatures in which we could survive, but we work best at a very precise range (my optimum is about 68F, but I could probably get by between 55 and 80). But how dry it is and how thirsty I am influences this range greatly. I’ll get heat stroke if I try to work when it is 85 degrees and dry outside, but if I am well hydrated or if it is raining, then I can get by and maybe even be more comfortable at a higher temperature. This study found that plants (many of which were trees) in mainly forested areas of California did what the authors call “niche tracking” as a response to climate change. Even though temperatures have increased recently (which may cause one to predict that species would retreat uphill to stay cool), there was also a corresponding decrease in “water deficit.” Water deficit accounts for changes in both water stress that comes from hot and dry weather as well as from alleviations of that stress that come from increases in precipitation. It appears that the trees (especially those at higher elevations to begin with) shifted downhill where it was hotter but also wetter. This makes sense from a physiology perspective since photosynthetic activity can increase with temperature (up to a point) as long as there are associated increases in water and nutrients. It also makes sense in the Mediterranean climate of the Sierra Nevadas, where annual droughts limit plant growth.
Relevance to managers
I think the primary relevance here is that there remains tremendous uncertainty in how forests will respond to climate change. This study suggests that forests have already responded and will continue to respond, but the details of the response are difficult to predict. As was discussed in a previous post, there is a lot of uncertainty in how precipitation in particular will change. And as this study confirms, the direction of change will have a very important influence in how forests respond.
This study demonstrates that it should not be assumed that species will generally shift uphill and to the north, as many have predicted. At the same time, however, I don’t think that this uphill/north scenario should be ruled out because of this study. Rather, managers need to hedge their bets against change in general, whether they are changes in species locations up, down, east, or west. In the end, it isn’t the fact that these plants have moved down a couple of hundred feet that is worrying. Of more concern is if some species may have not been able to adjust at all. Of even more concern is how disturbances such as fire or how exotic pests and pathogens will interact with these shifts. Uncertainty is the rule. Active Adaptive Management is perhaps the most reasonable solution.
Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:
From the graphs, it looks like there was a pretty severe drought in the 20’s and 30’s that puts a lot of leverage on the overall increase in precipitation and the decrease in water deficit. Just an observation.
I wasn’t convinced that the sampling bias was completely accounted for. The map of plots from the 30’s show the plots mostly in the central or southern portions of the study area, while the plots from the 2000’s are on average farther to the north. They corrected for differences in elevation and temperature between sample periods, but what about latitude? Species from farther north in California would be expected to occur at lower elevations, which would contribute to the temporal difference found. Perhaps I am missing how difference in latitude is accounted for (maybe elevation and temperature corrections inherently account for it).
I think their suggestion that there is a general assumption among scientists that temperature changes will be the primary factor that drives biotic changes is a slight overstatement. There are plenty of forecasters who have considered changes in precipitation as also being important (see this post, for example). Ecologists in dry western forests are especially aware of the importance of water deficit in driving change.
Lastly, they say that there was a widespread downward shift across the elevation gradient, but from the graph (figure 4), it looks like species that occurred below 750 m (2460’) did not shift down on average.
- Author: Rob York
[originally posted at www.foreststeward.com on Jan 7, 2011]
-> This post graciously provided by the Battles lab at UC Berkeley <-
Article Reviewed: Tree mortality in drought-stressed mixed-conifer and ponderosa pine forests, Arizona, USA
By J.L. Ganey and S.C. Vojta. Published in Forest Ecology and Management 261 (2011) 162–168
Plot line: This short paper documents tree death in northern Arizona between 1997 and 2007, a period of record drought in the southwest. The focus is on ponderosa pine and mixed conifer forest sites randomly distributed throughout National Forest lands along an elevation gradient from 1800 to 2800 m. Observations are of all dead trees over 20-cm diameter (at 1.37 m height) in an 1-ha plot; live trees were measured in nested 0.09ha plot located in the center of the larger plot. Mortality was characterized in two ways. Mortality per time period was calculated from tree counts at the beginning and end of a five year interval. The cumulative mortality, a measure of snag residency time, was defined as the ratio of standing snags to live trees at a single given point.
On the whole, the observed mortality patterns were more distinct in mixed conifer forests than in ponderosa pine. In 2002-2007, mixed conifer forests recorded a 200% increase over the previous five years, and ponderosa pine forest mortality increased 74% over the earlier period. Mortality during 2002-2007 was positively correlated with the proportion of live white firs in mixed conifer forests, but tree death was not related to elevation or stand density in either forest type. White fir and aspen, the least drought tolerant species in these communities, had the highest relative mortality, at 28% and 85% of individuals. However, Douglas fir, another among the less drought-tolerant species, showed very low mortality. The greatest number of tree deaths in all species occurred in the smallest size classes, but a larger proportion of the largest size classes on these sites died during the study. Cumulative mortality was also assessed with an eye toward structures for wildlife habitat. Mixed-conifer forests had median cumulative mortality rates of 21.1%, compared to 11.3% in 1995-2002. These mortality rates are quite high, but not unprecedented for comparable forests in the region.
Most of the snags showed signs of damage by forest insects, particularly bark beetles and western tent caterpillar. These pests probably acted in concert with a prolonged drought and extreme climate events, including the third driest year in the last 1400 years, 2002. The findings suggest that as forests experience more frequent and severe drought (as is predicted under most climate change scenarios), tree mortality may follow patterns similar to those observed here, with mortality disproportionately affecting species that are less drought tolerant than others the forest species mix.
Relevant Quote: “Mixed-conifer and ponderosa pine forests in northern Arizona have experienced high and accelerating drought-mediated tree mortality between 1997 and 2007. This mortality is altering species composition and size-class distributions in these forests rapidly.”
Relevance to landowners/stakeholders
Many people have a vested interest in our forests’ readiness for the new challenges that come with a changing climate. Studies like this one point to the ways in which our existing forests are unsuited to thrive in expected future conditions. It is also important to note that there is enormous variation in mortality for these forests. Some locations experienced almost total dieoff, while others were virtually unaffected. This serves as reminder that many of the factors that contribute to tree death are particularly difficult to predict at the individual and stand level.
These findings also may prompt some concern about habitat loss, since large trees died slightly more than other size classes. If aged and complex forests die, habitat for the Mexican Spotted Owl and other structure-dependent species might become scarcer.
Relevance to managers
Throughout the Southwest, management practices in the past century have contributed to inflated stand densities and an increase in white fir compared to historic norms. Because mortality was not density or elevation dependent in this study, it provides evidence that thinning cannot be expected to completely protect stands from the stresses of extreme drought and forest insects. The authors note that a number of other studies have also not found a relationship between density and mortality (including van Mantgem et al 2009 – reviewed on Forest Steward Feb 27, 2009), and that a better understanding of this interplay is needed, particularly with regard to mortality of small trees. Rather than focus solely on thinning as an insurance policy against climatic changes, they suggest a broader goal of resilience. Paths to resilience might include thinning treatments aimed at restoring more open stands, but should also consider management that favors species tolerant of drought and frequent fires. Such an approach might help alleviate competition for moisture in an increasingly arid future.
Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:
The analysis of whether mortality is related to stand density uses a density measurement that excludes trees less than 20cm in diameter. Trees under this size were not included in mortality counts. A more complete record of density and mortality dynamics in the smaller size classes would add an important component to the authors’ findings. Smaller tree mortality is quite likely the most density sensitive, so inclusion of these trees may help riddle out some density questions important for directing management of drought affected forests. It might also be helpful to see the total mortality broken down by tree species. This would illustrate how much of the overall mortality is comprised of drought intolerants like, white fir and aspen, as compared to other species.
It is also important to note that the definition of tree mortality used in this paper is based on changes in the number of live trees and the accumulation of standing dead trees in plots over the census interval. They do not track the fate of a cohort of marked trees through time as is typical in population ecology. Thus care must be taken when comparing mortality rates across studies that use different definitions of tree mortality.
The connection of drought to this mortality pattern might be improved by incorporating other mortality factors in the analysis. Comparisons with slope and aspect, for instance, have been used in other work, and the record of bark beetle and caterpillar irruptions might be useful as evidence this incident was not merely insect-caused.
