In the tropics, widespread deforestation and conversion of primary forests to
agricultural and pasture lands has resulted in losses of composition, structure, and functions
of forest landscapes. Deforestation in the tropics is typically preformed via slash-and-burn
practices; the byproducts from combustion have been identified as the second-highest form
of anthropogenically derived 'greenhouse-gases' (such as carbon dioxide) to the atmosphere,
and have been linked to the warming of the earth. Landscape-scale measures of species
composition and biomass structure of primary forests are important for two reasons: (i) they
provide accurate, land-based measures to predict what has been lost due to land-uses, and (ii)
they aid in the discovery of key factors which explain patterns in compositional and
structural diversity that are useful for defining conservation objectives. In this thesis, I
enumerate the landscape-level patterns in species composition and biomass and C structure
for 20-0.79 ha primary tropical forest stands within the region of "Los Tuxtlas", Veracruz,
Mexico. These 20 sites were selected to capture the variability in composition and structure
with respect to an array of environmental variables. These variables included a wide
elevational range (15-1280 m.a.s.l.), variable slopes (Range: 3-41% slope), 3 soil-types
(ash derived, lava flows, and weathered soils), a gradient of mean annual temperatures (~19.5-25.7��C), a broad precipitation range (2500 - 4000 mm year�����), a rainfall frequency range
(i.e. max rainfall in 24 hours; ranged 30->100 mm day�����), and 3 Holdridge Life Zones
(Tropical Moist Forest, Subtropical Wet Forest, and Subtropical Lower Montane Rain
Forest).
Species composition was highly correlated with the environmental variables,
particularly elevation. In general for plants ���10 cm dbh, site species richness declined at a
rate of ~2 species per 100 m rise in elevation. Forest sites located at similar elevations were
most similar in their species compositions as compared with sites separated by large
elevational differences. Despite the gradual change in species richness and composition,
four sub-regions, or forest environments, within Los Tuxtlas were identified that had
different species compositions and distinct combinations of elevation, soil-types, and
climates. These four sub-regions were described as community-types according to their
geographic location: Lowland-Reserve (LR), La Perla Plateau (LP), Volcanic Upslope (VU),
and Cloud Forests (CF). The LR, LP, and VU community-types were coarsely described as
Tropical Evergreen Forests (TEF's; INEGI 2001). All community-types corresponded with
classifications within the Holdridge Life Zone System; the LR community-type was classified
as Tropical (transition to Subtropical) Moist Forest; LP and VU community-types were
classified as Subtropical Wet Forest, and the Cloud Forest community-type was classified as
Subtropical Lower Montane Rain Forest. These community-types and Life Zones are useful
tools for conservation, as they represent unique forests that collectively capture much of the
variation in the species richness and compositional diversity of the Los Tuxtlas region.
Unlike species composition, the variability in forest structure among the 18 TEF
sites was not associated with the environmental variables of the Los Tuxtlas landscape. On
average, TEF's had a total aboveground biomass (TAGB) of 422 �� 17 Mg ha����� and 205 �� 8
Mg ha����� total aboveground carbon (C). The TAGB and C pools for Cloud Forests was
~18% lower than TEF's, and averaged 346 �� 1 and 168 �� 1 Mg ha�����, respectively. The
majority of this biomass difference was due to large trees within the forest structure. Cloud
Forests had generally fewer trees ���70 cm dbh, and a more even distribution of trees 30-70
cm dbh than TEF's. The biomass contribution of large trees (���70 cm dbh) accounted for
most, if not all, of the variation in TAGB and C for these tropical forests. The relatively high
TAGB and C pools implicates Los Tuxtlas forests as a significant pool of aboveground
biomass and C within the Neotropics. / Graduation date: 2002
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/31697 |
| Date | 02 May 2001 |
| Creators | Heider, Christopher |
| Contributors | Kauffman, J. Boone |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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