Impacts of feral cattle on native wet tropical forests of Kohala Mountain, Hawaii Island

Background/Question/Methods

Historically, feral cattle have been implicated in altering native forest structure and composition and, ultimately, in causing the decline of native Hawaiian tropical forests. However, few studies have quantified the causal factors (e.g., alterations in soil resource availability) driving this decline. This information is critical for the effective management of remnant native forests. We conducted comparative measurements in ‘ōhi‘a (Metrosideros polymorpha) dominated forest on the windward slope of Kohala Mountain, Hawai‘i Island, by establishing permanent plots (1000 m²) in an area with feral cattle for ≥100 years (n=9; cattle) and an adjacent area with no history of cattle (n=9; control) to examine the impacts of feral cattle on forest structure and composition, and soil nitrogen (N) pools and availability. Feral cattle may degrade native forest through: (i) destruction of native understory species and dispersal of nonnative vegetation; (ii) reduction of native seedling establishment; and (iii) alteration of pools and availability of soil nutrients (e.g., N). In all plots, we quantified forest composition and structure, total soil carbon (C) and N, inorganic N in soil solution, and foliar δ¹⁵N in two dominant canopy species as a proxy for soil N availability.

Results/Conclusions

Understory nonnative vegetation and disturbed soil comprised a greater percentage of ground cover in cattle plots (59 vs. 13%; P<0.01). Overstory structure was also impacted by cattle, with cattle plots exhibiting lower canopy cover (LAI 2.9 vs. 4.1 m²m^-2; P<0.05), less complex canopy stratification, and lower stand basal area (22.4 vs. 40.7 m²ha^-1; P<0.05). In addition, cattle plots contained a higher density of ‘ōhi‘a (1670 vs. 974 stems ha^-1; P<0.05), driven primarily by individuals in smaller size classes (<10 cm dbh). NO₃-N was higher in soils of cattle compared to control plots (3.34 vs. 1.73 μg ml^-1; P<0.05), and other indices of soil quality also decreased in cattle vs. control plots (C concentration 18.8 vs. 29.1%; C:N 16.7 vs. 22.4; P<0.05).  Foliar δ¹⁵N values were more depleted for both species in cattle plots (P<0.01), with no difference in total N, suggesting that cattle increased soil N availability. Taken together, these results support the hypotheses that feral cattle degrade native Hawaiian wet forest composition and structure, including increases in nonnative plant cover, and that this may be partially explained by increased soil resource availability. However, the presence of numerous small ‘ōhi‘a in cattle plots suggests that ecosystem restoration may be possible with the removal of cattle and control of nonnative plants.