of Southeast Alaska Paul Alaback 1 solar and thermal energy, limitations to understory vegetation development

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1 Natural Disturbance Patterns in the Temperate Rainforests of Southeast Alaska Paul Alaback 1 Key points: Unique climate leads to lack of fire disturbances, low nutrient availability, low solar and thermal energy, limitations to understory vegetation development On well-drained sites productivity and nutrient cycling are often driven by infrequent high intensity windthrow disturbances Interactions of windthrow disturbance patterns and edaphic patterns result in highly structured ecosystems across a broad range of spatial scales Logging patterns generally contrast with historical patterns of disturbance creating long-term habitat degradation for many key wildlife species Conservation of biological diversity in the region will require maintaining complex structural and functional patterns at the landscape and stand scales as well as promoting adaptation to ecological changes resulting from climate change 1 Department of Forest Management, College of Forestry and Conservation, University of Montana, Missoula, MT palaback@forestry.umt.edu

2 Temperate rainforests are generally defined by their unique climate, including high annual rainfall and cool growing season temperatures (e.g., Alaback 1991, 1996). These climatic conditions lead to unique ecological conditions not only in terms of how it constrains species adaptations to a low energy, and nutrient-poor environment but also in terms of basic disturbance ecology (Deal 2007, Kramer et al. 2001, Veblen and Alaback 1996). While most temperate forests are characterized by their adaptations to drought stress, usually during the growing season, and to various intensities and frequencies of fire disturbances, temperate rainforests are unique in their lack of a characteristic natural fire disturbance regime (Gavin et al 2003, Agee 1993, Gutierrez et al. 2004). A general consequence of the lack of coarse-scale fire disturbances is the development of fine-grained mosaics at the landscape scale. The natural pattern of gap dynamics appears to be driven by a combination of disease, exposure to major storms, edaphic conditions, forest structure, and species characteristics, all of which vary depending on larger-scale patterns of ocean-land interactions which govern the intensity and frequency of storm systems across the region (Kramer et al 2001). Preliminary studies of gap dynamics in northern southeast Alaska suggest that most gaps are quite small (<100 m 2 ) and are formed mostly by the stem breakage of 1-2 dominant trees (Ott and Juday 2002, Hennon and McClellan 2003). Gaps occupy an average of 9% ( %) of a given forest patch and persist for at least 80 years. Assuming no changes in climate or wind patterns canopy patches persist an average of 200 to 900 years (Ott and Juday 2002). This amount of canopy gap is comparable to what has been reported for

3 similar forest types in British Columbia and Oregon (Lertzman et al. 1996; Taylor 1990: range %). Figure 1. Size class frequency of canopy gaps from three study sites in northern Alaska. Source: modified from Ott and Juday (2002). It is important to consider how canopy gaps are formed. Three principal mechanisms of tree death (gap-makers) are usually recognized: stem breakage, rootthrow, and mortality of dominant trees while standing. In the Ott & Juday (2002) study most gap makers were killed by stem breakage (69-95%). This is significantly higher than what has been reported for similar forests in British Columbia (Lertzman and Krebs

4 1991, Lertzman et al. 1996: 31-43%). Studies have shown that a high proportion of western hemlock trees which dominate the upper canopy layers are infected with heart rot decay fungi, weakening these trees (Hennon 1995, Hennon and McClellan 2003), suggesting that there is an important interaction between stem decay and gap formation in these forests, explaining the high proportion of stem breakage. Indeed in many cases trees with stem breakage first died standing then weakened and broke. In the most detailed study of tree mortality, trees mostly fell in the direction of mountain slopes, not in the direction of wind exposure, further suggesting a key role of decay fungi (Hennon and McClellan 2003). The less frequent but more intense storms that produce root throw have very different ecological implications than when gap makers are created by trees dying standing or from stem breakage. When root throw occurs mixing of organic and mineral fractions of the soil results in accelerated decomposition, and the creation of complex forest forest microtopography all of which tends to result in greater nutrient availability and greater long-term forest productivity (Bormann et al 1995, Kramer et al 2004). Larger scale windstorms also can occur in Southeast Alaska creating larger forest openings and even-aged or multi-aged forests. Recent work has helped clarify the patterns and scale of these disturbances (Kramer et al 2001, DeGayner et al 2005, Harcombe et al 2004). The strongest winds in Southeast Alaska generally occur in the fall and winter and come from a southeasterly direction (Nowacki and Kramer 1998). Larger scale windthrow disturbances generally occur on very specific topographic positions, (e.g. steep slopes that are directly exposed to southeasterly winds) so can be estimated with a simple model which considers elevation, slope, aspect, soils, and overall wind

5 exposure (Kramer et al 2001). In general approximately 15% to one third of the productive forest land is susceptible to this form of disturbance, and about one third of the land is protected from these wind storms, while the remaining area is a mixture of the two disturbance regimes. In the most wind-exposed sites catastrophic or stand-replacing storms generally occur every 1-2 centuries so that the forests tend to have younger, smaller trees and higher tree densities than in wind-protected sites (DeGayner et al. 2005, Kramer et al 2001). It should be noted that while classic old growth typically occurs in windprotected terrains, on wind-exposed terrains significant old-growth forests can also occur. Some authors have concluded from the data presented that old-growth forests primarily exist in wind-protected microclimates (Kramer et al 2001, DeGayner et al. 2005). Historically some of the most economically valuable old-growth occurred in windthrow exposed areas, especially when stands were able to develop two centuries or more following disturbance on nutrient-rich sites. The reason for this is simply that singlecohort stands when they develop at moderate densities typically retain higher densities of large trees with old growth characteristics than true all-aged stands (Poage and Tappeiner 2004). Examples of these areas include the highly contentious 3 high-volume old growth forests on the west coast of Admiralty Island, and many areas on Prince of Wales, such as 3 The most economically valuable forest on Admiralty Island were on the west coast both due to basic geomorphic and soils characteristics as well as disturbance history (fully exposed to windstorms off of Chatham Straight). They were the centerpiece of efforts to conserve old growth forests for wildlife and fishieries values for Southeast Alaska region in the 1970 s and 1980 s. While they were initially protected by the establishment of Admiralty Island National Monument and wilderness in 1980, ultimately many of these lands were transferred to Alaska Native Corporations, after which they were mostly clearcut harvested. Few wilderness areas or protected areas today include the highvolume, multiple cohort stand structure represented by these forests.

6 the central western coast. Since most of these areas have now been harvested it is difficult to accurately reconstruct the dynamics of these areas. It is important to recognize that even in this high latitude temperate rainforest many stand structures can lead to old growth characteristics including both stand replacing and small scale gapdynamics processes (Alaback 1982, Alaback and Juday 1989, Hanley and Brady 1997, Spies 2004). It is tempting to consider larger-scale windthrow disturbances as equivalent to human disturbances such as clearcuts. There is much evidence that while catastrophic windstorms can occur, they create a very different landscape pattern than what occurs with clearcut logging in the region (Table 1). For example in one study aerial photographs were examined across the Prince of Wales Island region to determine the extent of recent winthrow openings. Only an average of 1.6% of the forest area (or 1% of timber volume) had detectable canopy openings, and most of this was due to a single windstorm event (Harris 1989). These larger windthrow-caused openings generally range from 0.4 to 400 ha with a median of 2-8 ha (Nowacki and Kramer 1998). The largest of these openings are generally the result of multiple windthrow events which further emphasizes the long-term nature of these disturbances (Harcombe et al. 2004, Lertzman et al. 1996, Ott and Juday 2002). Analysis of three additional subregions within southeast Alaska (northeast Chichagof, southeast Chichagof and Kuiu islands) provided further corroborating data on the general pattern of windthrow disturbance across the Tongass from 1 to 17% per century, (Nowacki and Kramer 1998). Timber harvest in these same study areas across the Tongass from the 1950 s to the 1990 s show that rates of clearcut

7 logging were 4 to 10 times the natural rates of catastrophic (stand replacing) windthrow events (Nowacki and Kramer 1998, Figure 2). Figure 2. Overall pattern of windthrow and clearcutting disturbances over four study areas in Southeast Alaska. Adapted from Nowacki and Kramer (1998).

8 Table 1. Comparisons of landscape patterns of windthrow disturbance to clearcutting over the past century. Data from Nowacki and Kramer 1998, Kramer et al Windthrow Clearcut Median Size: (exposed slopes) 8 ha 20 ha (protected areas) < 0.01 ha 20 ha Median Retention 1 : 5-15% ~0% 2 Location: (exposed slopes) 1-10% 5% (protected slopes) <1% 20% 1 Proportion of original dominant and co-dominant trees that remain after disturbance. This represents legacy from the previous stand (Lindenmeyer and Franklin 2002) 2 Recent changes in Tongass Land Use Plan (USFS 1997) have proposed experimental variable stand retention in the range of 5-50%, but the vast majority of clearcuts on the landscape today continue to have minimal retention of dominant or codominant trees. An additional contrast in comparing natural patterns of stand replacing disturbances with clearcut logging is in the location of disturbances on the landscape and also the size of patches (Table 1). Since wind-disturbed habitats generally have smaller, and less economically valuable trees for timber it should be no surprise that you find timber harvesting today concentrated on protected or mixed disturbance regime landscape patches. On Kuiu Island an average of only 0-7% timber harvest occurred on forests with high wind exposure. On protected landscape patches by contrast 15 to over 20% was logged (Kramer et al 2001). This suggests that if logging continues in the locations and at the rates that have occurred in the recent past that the overall landscape structure will

9 continue to diverge from historical patterns. The ecological effects of changes in landscape patterns are made even more profound by considering that silvicultural prescriptions for cutover lands generally include harvest within 100 years after the first harvest. This management practice has the potential to permanently change the disturbance regime of these forests from long-term gap dynamics (with dominant trees persisting an average of years or more) to short-term catastrophic disturbance. A key ecological consequence of these shorter disturbance cycles is the elimination of latesuccessional habitats, and degrading habitat for wildlife species associated with these structures over a significant portion of their range in this region. One of the great challenges to conservation in Southeast Alaska is to retain a diverse and productive layer of understory vegetation under productive well-drained forest patches. It is more difficult to retain understory forage plants for species such as Sitka black-tailed deer here than in other forest regions due to the unique climatic and ecological setting. Forests are mostly dominated by western hemlock, or multiple canopy layers of hemlock and Sitka spruce which have extraordinarily dense thick canopies which can intercept as much as 99% of incoming sunlight (Alaback 1982, Tappeiner and Alaback 1989). These effects are magnified by low nutrient availability and a persistently cloudy, cool climate, so that there is little energy available for plants to grow, reproduce and defend themselves from herbivores. On well-drained sites where most clearcut timber harvest and catastrophic windthrow has taken place, understory plants generally are shaded out within 25 years of disturbance. While restoration practices such as thinning can temporarily improve some aspects of wildlife habitat, it is unlikely that such practices can restore the full complexity

10 of old growth forests (Alaback, unpublished data Hanley 2005, Lindenmeyer and Franklin 2002). Landscape context including patterns of snow deposition and wind exposure are further factors to consider in prioritizing areas for restoration (Doerr et al. 2005). More radical approaches such as promoting mixed canopies of alder and conifers may also be helpful in promoting more diverse understories on cutover lands (Deal 2007, Hanley 2005). By contrast preliminary studies on the effects of partial harvesting suggest that understory vegetation, and wildlife habitat values associated with it can often persist throughout the logging cycle (Deal 2007, Deal et al. 2001). [see chapter on partialharvesting in this volume]. One of the greatest challenges in developing a scientific approach to conservation in coastal temperate rainforests is reconciling the profound changes in temporal scale and pattern of timber harvesting with the long-term patterns of forest dynamics that have previously dominated these regions. The overall pattern of forest development has always been influenced by both short-term and long-term climatic patterns (Gavin et al. 2003, Lertzman et al 1996, Veblen and Alaback 1996). Climate change will clearly have direct implications on stand dynamics and wildlife habitat relationships in this high-latitude forest region (Alaback and McClellan 1993, Maguire et al. 2002). A well known example of ecological response to climatic changes is yellow cedar (Hennon et al. 2006). A key question will be if coarse-scale fire disturbances will become a more important factor in the disturbance ecology of the southern portion of Southeast Alaska, and how this may influence species interactions in the region, and if changes in wind storms will effect overall stand dynamics in the region. All of these uncertainties would suggest that conservation goals are most likely to be met by returning the region to a structure and

11 disturbance process more similar to its historical condition. Long lag-times characterized previous responses of these forests to climatic variation. Maintenance of stand and landscape legacies should therefore be useful in helping promote adaptation of this forest to future climatic and direct human-caused stresses. It would appear that all available scientific evidence on natural disturbance regimes points to the need to develop more ecologically sustainable forest practices which better emulate the structures and functions of forests that are maintained by natural disturbance events such as windthrow. Reflecting on what has been learned by studying natural disturbance regimes of Southeast Alaska over the past several decades, it is clear that one of the guiding principles for this region should be be developing heterogeneous structures at stand and landscape scales which promote connectivity. Complex patchy structure is what makes the temperate rainforests of Southeast Alaska and adjacent British Columbia unique, and appears to be be key to its ecological function as well. It has long been known that clearcut logging creates more homogenous, and a more pronounced and persistent depauperate successional stage than what results from gapdyanmics in old growth forests. Recent work on intense windthrow disturbances as reviewed here shows that the landscape pattern which results from clearcut logging is functionally more fragmented than what results from natural patterns of disturbance as well. A sustainable forest conservation strategy which better maintains ecological functions such as wildlife habitat productivity and overall biodiversity will require providing a high degree of ecosystem legacy following logging, similar to what occurs following natural disturbances. This could involve careful design of reserve areas within

12 and around timber harvest areas to promote connectivity of specific wildlife species, or it could involve an abandonment of clearcut logging entirely and the development of new silvicultural strategies, or a mixture of the two strategies. Clearly these partial-cutting silvicultural strategies will need to be tailored more closely to the specifics of each site than is required with clearcutting, including using precise models of windscapes, soils, and landscape context. Since the largest patches of highly productive forest habitats were logged in the past century, restoration of these habitats will be key to bringing the landscape back to a more productive structure and function as well. While conservation strategies that focus on legacy and landscape connectivity on the surface appear to be similar to what has been called for in many forest regions, the special challenge of Southeast Alaska will continue to be its geography [reference to chapter on biogeography]. As an island archipelago, with many isolated populations and habitats, species at their northern limit of range, and the vast majority of land area having unsuitable habitat for many wildlife species of interest, not to mention rugged topography and energy limitations from low nutrient availability, low solar radiation and occasional deep snowpack, it will be particularly challenging to promote functional connectivity in the face of climate change in this region. This would suggest than in Southeast Alaska managers will have to be even more skillful in developing landscape strategies than what would be required for regions that are inherently more connected and more productive. Rather than solely dwelling on the challenges, it is still important that we keep in mind the broader context for conservation in this region. Despite the significant impacts from resource development activities during the past century, Southeast Alaska remains unique in the extent of its globally significant conservation values and awe-inspiring

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