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.
[This post graciously provided by the Battles lab at UC Berkeley, and originally posted at www.foreststeward.com on Feb. 25, 2011]
Article reviewed: A rapid upward shift of a forest ecotone during 40 years of warming in the Green Mountains of Vermont.
By B. Beckage, B. Osborne, D.G. Gavin, C. Pucko, T. Siccama, and T. Perkins. Published in PNAS, Vol. 105, No. 11, pp. 4197-4202
The plot line: The goal of this study was to determine whether the location of the northern hardwood-boreal ecotone (NBE) in the Green Mountains of Vermont has shifted in the last 40 years. Furthermore, the study explored whether the shift was consistent with regional climate change over this period with the main prediction being that under a warming climate the NBE should shift upward. (Most simply, an ecotone can be defined as a zone of transition between two adjacent but different communities - in this case, the northern hardwood and boreal forests.) Their primary evidence to document a shift came from forest plots established along elevation transects in 1964 and resurveyed in 2004. Added support was introduced from remotely sensed data, competing models of forest change, observations from nearby climate stations, and cloud ceiling and lapse rate shifts. The forest inventory demonstrated that during the last 40 years, the abundance of northern hardwood species (primarily American beech and sugar maple) increased in the lower half of the NBE while boreal species (red spruce and balsam fir) decreased. This shift in dominance is consistent with remote sensed changes that indicate an upward shift in that NBE of approximately 100 m. The authors credit the ecotone shift to changes in climate, and attribute the faster than expected pace to increased canopy turnover at elevation range limits.
Relevant quote: “We propose that the upslope movement of the ecotone between northern hardwood and boreal species is the result of climatic change that has promoted the growth and recruitment of northern hardwoods at higher elevations.”
Relevance to landowners and stakeholders:
Eighty-percent of timberland in Vermont is family owned and divided into medium-sized land holdings (USDA Forest Service, Forest Inventory & Analysis, 2005). Depending on the elevation of their forestland, the findings from this study may come as a pleasant surprise to some landowners due to the species shifts that are in progress or will soon occur. In general, landowners can expect to see increases in northern hardwood species at the upper elevation limit, and decreases in boreal species at their lower limits.
Vermont has a strong forest-based recreation and tourism industry, particularly due to fall-foliage viewing (which brings in $200 million/year) (USDA, FIA 2005). Christmas tree sales (from tree farms) and maple syrup production add $22 million/year. Fall-foliage viewing of course relies on hardwood species. Certain hardwoods (e.g. maple) are also in demand for production of value-added items such as hand-crafted furniture. Given the rapid shift in species composition from boreal to northern hardwood in the forest ecotone as presented in this study, the economic and aesthetic consequences to landowners and tourists alike seem beneficial. Revenues from tourism would especially benefit public landowners (typically state and national parks) which make up 20% of all timber owners. On the other hand, although species shifts might be seen as beneficial, the overall high rate of species turnover may in fact hurt landowners in the near-term. The increases in basal area correspond to increases in density of smaller size classes, which at least in the beginning will not be as aesthetically pleasing or economically valuable as the original forest structure. Meanwhile, decreases in basal area correspond to mortality in all size classes.
Relevance to managers:
The authors warn that the shift in ecotone seen in the Green Mountains of Vermont is likely to occur in many other mountainous areas, since increased stressors such as pathogen and insect outbreaks and air pollution are affecting forest health across the Northeast. Managers working in forest ecotones should be aware of an increase in species-specific mortality and subsequent species composition shift. This study highlights the importance of long-term monitoring of forest ecotones.
Based on the results from this study, as climate change continues, we can expect NBE to continue migrating upslope and as the authors warn, “reach the high-elevation montane forest sooner than expected.”
Critique and/or limitation (there's always something, no matter how good the article is):
This study presents solid evidence of an elevational shift in NBE. However, there is not enough evidence that climate alone has caused this change. There are many different species-specific novel disturbances (e.g., red spruce decline, beech bark disease) that are promoting canopy turnover. Factors unrelated to climate are likely to be contributing to the ecotone shift, but in the current context of multiple novel stressors acting on the landscape, it is difficult to attribute cause to climate change.
The authors argue that boreal species have not moved down the mountain to fill in the gaps in the northern hardwood zone because the climate conditions have not favored this movement. An intriguing question is why hasn't balsam fir expanded to lower elevations. The answer might lie in climate or it might be that fir is restricted by insects and pathogens. Later in their discussion, the authors do state that increased canopy turnover (accelerated by novel disturbances) has sped up the primarily climate-driven ecotone shifts.
On a more technical note, the results from competing models of forest change over the period 1964-2004 are not as clear-cut as presented in the article. There is almost no difference between models that include the following terms: “Elevation * year” and “Elevation”. The added interaction with “year” presumably represents a climate signal, but the next best model which only includes an “elevation” term, is only marginally worse (difference in AICc is less than 2). The difference is minimal and should be interpreted with caution.
[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.
[originally posted on www.foreststeward.com on Dec. 17, 2010]
Article reviewed: Forward-Looking Forest Restoration Under Climate Change—Are U.S. Nurseries Ready?
By T.L. Tepe and V.J. Meretsky. Published in Restoration Ecology 2010. Vol. 18 issue 6
The plot line: This is an opinion article, but the authors also did some social science work that provides some data that are presented. They called up state and private nurseries across the country and asked them if they are preparing for or even thinking about climate change with respect to changing the species of seedlings that they offer. Only 20% of the state nurseries said they were thinking about climate change, but most (87%) did offer some seedlings from a fairly broad range that extended beyond their state’s borders. The authors’ answer to their own question, “are U.S. nurseries ready?” seems to be for the most part, “no.” Nurseries are somewhat prepared simply because traditional demands have led them to have species from different climate zones (I would also add that there simply aren’t very many of them so they have to cover wide areas). But this preparedness doesn’t come from specifically planning for future climate scenarios. The authors conclude that in the future, managers will likely want to plant a much wider variety of species than they currently do, and nurseries should prepare for facilitating management responses to climate change by incorporating climate change into their planning.
Relevant quote: “Restoration practitioners considering forward-looking restoration should consider plantings that use a reasonably broad diversity of species to accommodate a range of likely future climates rather than limiting plantings to species suited only to a single predicted future climate.”
Relevance to landowners and stakeholders:
Nurseries are a vital part of the forest management infrastructure, regardless of whether forests are managed intensively for timber or less intensively for other conservation objectives. Seedlings are often planted (“artificial regeneration”) following a disturbance such as a harvest or wildfire in order to establish a cohort of new trees. This article reminds us that the consequences of tree planting are long-term and profound. Trees are long-lived creatures that potentially modify the environment for animals and plants for centuries. A newly established tree (if it survives) is likely to grow in a climatic environment that is different than the one it evolved in. If we take a proactive approach to managing forests to be resilient to climate change, then paying close attention to which species we plant is critical.
Nurseries store and raise the seedlings that are ultimately planted. Without nurseries, we would not have the option of reforesting disturbed areas. While in some cases reforestation may not be needed to meet objectives, in many cases it is the primary way of achieving the goal of quickly establishing a forest following a disturbance such as a severe wildfire. Unfortunately, there appears to be a downward trend in the number of state nurseries. While private nurseries may be able to compensate in terms of meeting short-term demands for seedlings, they are much less likely to be thinking about climate change.
My bottom-line interpretation of relevance for landowners and stakeholders is to support the continuation of state nurseries. We are going to need them. The need seems especially important in southwestern states, where there are apparently hardly any nurseries. When high intensity wildfires occur in dry ponderosa pine forests, burned areas often have no sign of becoming forests again. Planting could help meet the goal of re-establishing forests in these areas, but it can’t happen without the nursery infrastructure in place.
Relevance to managers:
I am not so sure that we can ever expect nurseries to be the driving force behind being better prepared for reforestation needs in the face of climate change. It is managers and regeneration foresters who ultimately drive the demand for seedlings. If they are willing to pay for being more ready for climate change, wouldn’t a nursery then provide them with the type of seedlings they want?
The challenge for managers, however, is dealing with the extreme uncertainty in which seedlings will actually be better adapted to a future climate. The authors make an understated point that it is not just a changing climate but a changing disturbance regime that will shift where tree species will grow in the future.
The authors suggest a hedge-betting approach to dealing with uncertainty. There could be a wide range of species that will be adapted to the future climate. Rather than pick the one species that appears to be the most likely to survive, they suggest using a number of different possibilities based on the likely range of possible future conditions. This is essentially a form of active adaptive management, which I have discussed previously. It also brings up the significant risk of planting species outside of their ranges, which I have discussed previously. Finally, it also brings up how adaptable species might be to climate change just by staying put, which I have also discussed previously.
The bottom line relevance for managers is to take a good look at the seed zone map that is currently used and consider its relevance given the reality of climate change. Here is one for northern California. It was last revised in 1969. Surely this and other seed zone maps are out of date and they more relevance every year that goes by. In the forest that I manage, I have begun small trials where I have planted seedlings from different climatic zones. I have not planted species other than what are locally native, but I have tried to get seedlings from different zones that might be closer to the climate that will occur in my forest in the future. Seedling survival and growth will then be tracked over time. I’ll let you know the results in a couple of decades. Ideally, this will be done on much bigger scales and across large regions (akin to common garden experiments).
A number of websites were given for those interested in learning how climate might change locally. The Nature Conservancy's Climate Wizard looked to be the most interesting, although it doesn't capture the great uncertainty involved with projections.
Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:
The authors could have done more to recognize the importance of acting with caution when it comes to planting species outside their current zones. Rather than just discuss the planting of new species, they could have also discussed the planting of the same species from different climatic zones. Tree species often cover diverse climates and have high genetic diversity across their entire range.
Using state boundaries as a way to judge if nurseries have broad zones from which they get seeds is not very meaningful because the size and shape of states varies so widely. It would have been more meaningful to actually draw seed source zones around nurseries and see how well these zones overlapped with forest cover across the United States. But this would have obviously been a lot more work… perhaps it can be a future study./span>/span>