Landscapes Forest and Global Change - ESA - Escola Superior ...

K. L. Martin & P.C. Goebel 2010. Impact of hemlock decline on successional pathways and ecosystem function
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group was also more abundant, largely due A. rubrum L. and a few scattered U. rubra Muhl. In
the sub-canopy, a slight increase in species richness (F = 3.87, P = 0.037, d.f. = 2) and diversity
(F = 3.10, P = 0.066, d.f.= 2) equated to slightly increased functional richness (F = 3.02, P =
0.070, d.f. = 2). Somewhat mirroring the canopy, B. lenta was more common in sub-canopy at
10 m while A. rubrum and Q. prinus were more frequent in the sub-canopy along the 50-m
transect. The sapling layer was the most depauperate with mean species richness below three
species. Saplings were overwhelmingly hemlock and were less abundant adjacent to the stream.
Following T. canadensis, A. rubrum and F. grandifolia were the most common species in the
sapling layers, but still at very low numbers (mean below 2 stems per plot).
4. Discussion
On the Unglaciated Allegheny Plateau of southeastern Ohio, T. canadensis dominates steep
ravines formed in sandstone bedrock. These ecosystems are characterized by low species and
functional diversity in the canopy, sub-canopy and sapling forest layers. While these forests
remain outside the current HWA invasion, the spread continues and will likely reach Ohio
within the next few years. The species that replace T. canadensis will be quite different
functionally. Sutherland et al. (2000) characterize T. canadensis as part of a group including
Acer nigrum Michx. f., A. saccharum Marsh., Carpinus caroliniana Walter, Fagus grandifolia,
Ostrya virginiana (Mill.) K. Koch , Oxydendrum arboretum (L.) DC, Tilia americana L. and T.
heterophylla Vent. These shade tolerant, long-lived species are currently sparse or absent in T.
canadensis ravines. Runkle and Whitney (1987) suggest that in this region, poor quality, low
pH soils may be responsible for the lack of these species typically dominant in mesic habitats of
the Central Hardwoods region such as Acer saccharum and Tilia spp. This functional group is
also challenged by mortality of F. grandifolia due to beech bark disease, caused by an
interaction between the pest Cryptococcus fagisuga Ling, and the exotic fungi Nectria coccinea
var. faginata Lohman, Watson, and Ayers and N. galligena Bres. This disease is currently
causing widespread mortality in the northeastern US and continues to spread westward.
HWA will shift ravine and riparian forests into a different ecosystem state, driven by different
forest functional groups. In Northern Hardwood Forests of the northeastern USA, T. canadensis
is being replaced largely by Betula lenta (Ellison et al. 2005; Orwig et al. 2008). B. lenta is
currently a minor component of T. canadensis ravines in Ohio, however, it is an opportunistic
species well adapted to exploit canopy gaps (Orwig and Foster 1998; Catovsky and Bazzaz
2002). Ford and Vose (2007) also identify Lirodendron tulipifera as a canopy replacement
species in the southern Appalachians. Sutherland et al (2000) group B. lenta, L. tulipifera as
relatively shade-intolerant and long-lived. Also included in this group are Juglans nigra and
Platanus occidentalis which occur infrequently at our plots, but respond well to disturbance.
Our results also indicate that HWA mortality may have differing impacts at local scales. At
upper slope positions, we found a trend of increasing species richness in the sub-canopy and
canopy, although sapling species richness remained low. This was largely due to increases in
species of Quercus and Carya. Species in these groups are responsive to disturbance (Small et
al. 2005) and may also exploit gaps created by the death of individual hemlock stems. Thus,
upper slope positions may transition differently than areas immediately adjacent to streams. As
we collect additional data across our mortality chronosequence, these patterns will be refined for
the central Appalachian region.
References
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2010, Instituto Politécnico de Bragança, Bragança, Portugal.