Bedrock Controlled Upland Forest

Description

Community phases within the Reference State follow classic successional trajectories. However, stands rarely became old growth because of periodic low severity disturbances, such as light surface fires and small to moderate windthrow events. An estimated rotation of such events is 130 years (MN DNR, 2014; MN DNR, 2005). This produced a patchwork of young and mature forests of mixed hardwood composition. Sugar maple and northern red oak are the most influential species and can even be co-dominant with paper birch and aspen in the young forest community phase due to their ability to accumulate as advance regeneration. However, if blowdown events are followed by a combination of drought and fire, quaking aspen and paper birch will be favored (Frelich, 1999; Landfire, 2007). As stands age, old paper birch and aspen are still present, but mostly give way to more shade tolerant species, such as younger generations of sugar maple. Historically, older forests had higher cover of coniferous species, including eastern white pine, white spruce, and balsam fir. Forests rarely grew older than 130 years before small to moderate scale regeneration disturbance occurred (MN DNR, 2014), but scattered super canopy trees of eastern white pine and white spruce may have persisted through these events and lived to old age.

Small to medium scale windthrow events would have provided habitat for forest interior wildlife, microsites for tree regeneration, and opportunity for some disturbance-adapted species to maintain themselves (such as beaked hazelnut; Kabrick et al., 1997 Landfire, 2007). Coupled with this is the accumulation of coarse woody debris in the way of snags and downed wood in various sizes and levels of decomposition (Hale et al., 1999).

Today, good examples of the Reference State are uncommon. However, some do exist in a few state parks or natural areas, mostly limited to northern populations within large intact landscapes having high biological significance. Post-settlement logging and contemporary forest management in part mimic early- and mid-successional dynamics, and are more common today.

Submodel

Description

The simplified forest state was a common state that followed the pre-settlement forests, and may be the most common state existing today. In general, forests on this ecological site were not completely cleared like other forests in the Great Lakes states (as in many coniferous forest types). This was largely due to the abundance of less desirable hardwoods, and partially because maple (and other hardwoods) could not be easily transported along waterways (Johnson et al., 2009). Instead, destruction of reference communities came in the way of selective logging of sought after species of adequate size (i.e., high-grading). This occurred in numerous pulses, with large eastern white pine and white spruce removed initially, which likely accounts for the limited occurrence and/or decline of these species today. In many cases, overstory vegetation turned into monotypic sugar maple stands; however, in other cases some level of diversity in the overstory was secured, although probably less than before. These two situations represent each of the community phases within this state. Like the reference state, these forests tend to be uneven-aged and, with natural succession or careful silviculture (e.g., retention of snags, removal of poor quality trees, etc.), it may be possible to restore some sites to reference conditions (Hale et al., 1999).

Communities in this state are a common occurrence on the modern landscape, particularly in private landholdings, which tend to be unmanaged. Today, depending upon the specific location, there may be early stages of earthworm invasion (e.g., Dendrobaena octaedra) as well as some elevated deer browse, but not enough to push it into the Invaded State.

Submodel

Description

Clearcutting in state 1, or more typically in state 2, will convert the community to an even-aged stand, which is an uncharacteristic age structure for this ecological site. However, community phases within this state can be similar to community phase 1.2 from the reference state, particularly in terms of stand structure. Communities in this state are most common in managed forest settings where forest managers often have goals of improving the sugar maple quality as well as providing better wildlife habitat for various game species, such as white-tailed deer and ruffed grouse (Bonasa umbellus; Tubbs, 1977). As the stand matures, opportunities develop for management and restoration to states 1 or 2.

There may be early stages of earthworm invasion (e.g., Dendrobaena octaedra) as well as some elevated deer browse in this state, but not enough to significantly alter vegetation or dynamic soil properties.

Submodel

Description

The Invaded State is the furthest removed from the Reference State and can transition here from either state 2 or state 3 following long-term heavy deer browse or advanced stage earthworm invasion from Aporrectodea spp. and/or Lumbricus spp. This state is more common throughout the southwestern part of this ecological site’s distribution, where habitat fragmentation and human development are prevalent. Stands in this state can be either even-aged following clearcutting, or uneven-aged following selective logging.

Herbivory by deer affects both woody and herbaceous vegetation by direct consumption of plant material. In areas of high deer densities sugar maple may become even more favored due to preferential browsing of other woody species, such as yellow birch (Rooney and Waller, 2003). Deer herbivory by itself has the potential to cause extirpation of the most preferred, palatable forb species, such as those in the lily family (Augustine and Frelich, 1998). In extreme cases, vegetation can become so sparse it is possible that changes in soil moisture, soil temperature, and dynamic soil properties may occur; for example, a reduction in soil organic carbon, which may result in a decline in soil moisture or an increase in soil temperature. Overall, elevated herbivory can result in distorted vegetation composition and structure in the forest understory (Alverson et al., 1988; Augustine and Frelich, 1998) and indirectly alter the trajectory of the entire forest ecosystem, thus creating novel, deer-induced natural communities affecting vegetation as well as wildlife patterns (Rooney and Waller, 2003; White, 2012).

This ecological site (and mesic hardwood forests in general) is particularly susceptible to earthworm degradation (Frelich et al., 2006). The type of leaf litter (e.g., sugar maple, American basswood, etc.) these forests produce has high nutritional value for earthworms compared to drier and less nutrient-rich pine and spruce-fir forests (Frelich et al., 2006; Godman et al., 1990). In previous states, the organic surface horizons may or may not have been affected by the epigeic (i.e., above the soil surface) Dendrobaena octaedra species of earthworm. This species does not by itself cause transition to the invaded state because it only affects the organic surface horizons, which happens by mixing the Oa (i.e., well decomposed) and Oe (i.e., partly decomposed) horizons, but leaving the Oi (i.e., recent litter) intact (Frelich et al., 2006). The advanced stages of earthworm invasion include the presence of D. octaedra as well as the deeper burrowing endogeic (i.e., beneath the soil surface) species in the Aporrectodea and Lumbricus genera, which cause the most significant dynamic soil property changes (Hale et al., 2006; Loss et al., 2013). Aporrectodea and Lumbricus species completely consume the organic surface horizons and incorporate that material into the upper mineral soil horizons (Frelich et al., 2006), producing an uncharacteristic bloated A horizon, along with mixing of any existing E horizons.

In earthworm-free forest soils, there tends to be a net increase in organic material on the soil surface (Great Lakes Worm Watch, 2013). By comparison, in the advanced stages of earthworm invasion, all of this organic material can be completely removed within 3-5 years, making the only input of organic material from new leaf litter each fall, which is quickly consumed, leaving bare soil at the surface by the next fall (Great Lakes Worm Watch, 2013). This process completely alters the nitrogen cycle (in which nitrogen is depleted by leaching) and produces a dense, pan-like layer similar to plowed agricultural soils (Frelich et al., 2006). Changes in dynamic soil properties, such as loss of the organic surface, along with higher bulk densities in the subsoil, produce drier growing conditions for plants, affecting the ability for characteristic native species to persist. The loss of the organic surface also can expose tree roots, potentially causing long-term effects on the life and/or health of trees. However, immature trees (i.e., saplings and seedlings) are likely to be the most at risk to root exposure. Sugar maple seedlings in particular decrease dramatically as a result of earthworm invasion (Hale et al., 2006). Forb seeds also are affected, as the duff layer provides insulation from hot and cold weather extremes and protection from predation by small mammals and birds (Great Lakes Worm Watch, 2013). Another negative consequence of advanced earthworm invasion is the alteration of important soil bacterial and fungal networks, especially symbiotic mycorrhizae, which facilitate essential water and nutrient uptake to many native plant species (Great Lakes Worm Watch, 2013).

Advanced earthworm invasion results in a physically and chemically altered plant rooting environment. Some species are able to handle these changes, while others are not. Pennsylvania sedge (Carex pensylvanica), one of the few non-mycorrhizal species, along with wild leeks (Allium tricoccum) and jack in the pulpit (Arisaema triphyllum), which produce toxic secondary chemicals hazardous to herbivores (and may also be avoided by earthworms), have been shown to increase in these situations (Frelich et al., 2006; Holdsworth et al., 2007). In contrast, other species like bigleaf aster, twisted stalk, and wild sarsaparilla tend to decrease (Holdsworth et al., 2007; Great Lakes Worm Watch, 2013). Although earthworms do not kill canopy trees, it is expected that long-term recruitment will be affected, particularly in the sapling stage. This may cause elevated sunlight to the forest floor, increasing the likelihood for dry-mesic, mid-tolerant species to establish (Frelich et al., 2006).

Overall, the combined effects of invasion by deer and earthworms can initiate an ecosystem decline syndrome that can negatively affect all parts of the ecosystem, from overstory structure, to forb diversity, soil properties, bacteria, fungi, insects, birds, reptiles, amphibians, and mammals. Sites near larger cities, heavily-used lakes, or other developed areas are particularly susceptible to the combination of deer and earthworm problems. Currently, we do not believe any community phases with advanced earthworm invasion can be restored. More research on this topic is needed.

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