[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.