Effects of thinning on throughfall in Canary Islands pine forest — the role of fog

Throughfall was monitored over a one-year period in a 48-year-old Pinus canariensis plantation on the northern side of the island of Tenerife, nine years after light thinning (mean 15% of basal area (BA)) or heavy thinning (mean 56% of BA). Three plots of each treatment (light thinning, heavy thinning, no thinning) were studied, using a randomized block design. Mean total throughfall over the year of study was about 2.0 times the incident rainfall in the control plots, about 2.2 times rainfall in the lightly thinned plots, and about 1.8 times incident rainfall in the heavily thinned plots. The high throughfall-to-rainfall ratios are as expected, given the importance of fog entrapment in these forests. The statistical analysis indicated that the observed differences in throughfall are attributable to the treatments, not to plot topography. Throughfall showed significant relationships with actual BA, surface roughness (as tree height variability) and leaf area index (LAI), all of which varied significantly among treatments (as expected). Our results are unexpected as heavy thinning led to a long-term decline, not increase, in throughfall. The explanation for this result is the importance of fog entrapment may mean that reducing LAI and surface roughness (height variability) has a negative effect on throughfall. The long period elapsed between thinning and throughfall estimation means that LAI of light thinning plots exceeded pre-thinning values, and then throughfall values.