ENDRESS, BRYAN A. and DAVID L. GORCHOV.* Department of Botany, Miami University, Oxford, OH 45056 USA. - Effects of leaf harvesting and browsing on the demography of Chamaedorea radicalis Mart. (Arecaceae) palms in the El Cielo Biosphere Reserve.
Extracting Non-timber Forest Products is sustainable only if the
harvest does not cause negative population growth. Leaves of
Chamaedorea palms are harvested from Mexican forests and
exported for floral greenery. We examined the effect of alternative
harvest schedules, and livestock browsing, on demography of
Chamaedorea radicalis Mart., a dioecious understory palm, in El
Cielo Biosphere Reserve (23°08'N, 99°09'W), Tamaulipas, Mexico. In Jan. 1999 we
assigned 100 adult palms to each of 5 treatments: Control (no leaf
removal), 1X/year (marketable leaves removed each Aug.), 2X/year
(Aug., Feb.), 4X/year (Feb., May, Aug., Nov.), and Modified 4X/year
(max. 1 leaf per palm removed, and no palms defoliated). We calculated
stage transition probabilities and fecundities of adults over 1 year
(Aug. 1999 - Aug. 2000). We parameterized a stage transition matrix
for each treatment using these values and transition probabilities for
seeds, seedlings, and juveniles obtained from the same site.
Eigenanalysis of each matrices yielded l
(finite rate of increase) for each treatment. To quantify effects of
browsing by free-range livestock, we randomly assigned 2
100m2 plots to each of 3 treatments; No browse, 1X browsed
(Feb. 2000) by 1 burro for 15 min., and 2X browsed (Aug. 1999, Feb.
2000). The 4X harvest treatment reduced adult survival and
reproduction, and caused some adults to regress to "younger"
stages, resulting in l=1.00, while the
control was projected to grow with l=1.06.
Browsing elevated mortality of most stages. Incoporating this
mortality into the control's matrix, l
declined to 0.96 for 1X browse and 0.82 for 2X browse. Furthermore,
adults that survived defoliation by harvest or browse showed a
developmental response; their next leaf was frequently smaller than
previous leaves. In Jan. 2001 the youngest fully-expanded leaf on the
4X treatment averaged 34 cm long vs. 45 cm on controls (T=5.99,
df=136, P<0.001). Collectors only take leaves > 40 cm long, so
defoliated palms have a respite from harvest. Treatments are
continuing, allowing assessment of their impact beyond the first year.
Present results suggest occasional browsing more negatively effects
populations than periodic leaf harvest.
Key words: browse, Chamaedorea radicalis, leaf harvest, Mexico, palm, transition matrix