K. Mcgarigal et al., Cumulative effects of roads and logging on landscape structure in the San Juan Mountains, Colorado (USA), LANDSC ECOL, 16(4), 2001, pp. 327-349
In the southern Rocky Mountains of temperate North America, the effects of
Euro-American activities on disturbance regimes and landscape patterns have
been less ubiquitous and less straightforward in high-elevation landscapes
than in low-elevation landscapes. Despite apparently little change in the
natural disturbance regime, there is increasing concern that forest managem
ent activities related mainly to timber harvest and to the extensive networ
k of roads constructed to support timber harvest, fire control, and recreat
ion since the late 1800s have altered disturbance regimes and landscape str
ucture. We investigated the magnitude of change in landscape structure resu
lting from roads and logging since the onset of timber harvest activities i
n 1950. We found limited evidence for significant impacts in our study area
when all lands within the landscape were considered. The relatively minor
changes we observed reflected the vast buffering capacity of the large prop
ortion of lands managed for purposes other than timber (e.g., wilderness).
Significant changes in landscape structure and fragmentation of mature fore
st were, however, evident on lands designated as suitable timberlands. Roug
hly half of the mature coniferous forest was converted to young stands; mea
n patch size and core area declined by 40% and 25%, respectively, and contr
ast-weighted edge density increased 2- to 3-fold. Overall, roads had a grea
ter impact on landscape structure than logging in our study area. Indeed, t
he 3-fold increase in road density between 1950-1993 accounted for most of
the changes in landscape configuration associated with mean patch size, edg
e density, and core area. The extent of area evaluated and the period over
which change was evaluated had a large impact on the magnitude of change de
tected and our conclusions regarding the ecological significance of those c
hanges. Specifically, the cumulative impact on landscape structure was negl
igible over a 10-year period, but was notable over a 40-year period. In add
ition, the magnitude of change in landscape structure between 1950-1993 var
ied as a function of landscape extent. At the scale of the 228 000 ha lands
cape, change in landscape structure was trivial, suggesting that the landsc
ape was capable of fully incorporating the disturbances with minimal impact
. However, at intermediate scales of 1000-10 000 ha landscapes, change in l
andscape structure was quite evident, suggesting that there may be an optim
al range of scales for detecting changes in landscape structure within the
study area.