Species and wildfire effects on the spatial pattern and scale of soil N and P pools in a Pinus canariensis forest

Background/Question/Methods

The spatial heterogeneity of plant resources plays a crucial role in the structure, composition, and productivity of terrestrial ecosystems. Several factors, such as plants and wildfires, can determine the spatial pattern of soil resources. The presence of a legume in a nitrogen (N)-limited ecosystem may create “islands of N fertility” and affect the phosphorus (P) availability. Wildfires may change the spatial distribution of soil resources, affecting nutrition and survival of colonizing plants. The objective of this study was to compare the effect of a pine (Pinus canariensis) and a leguminous shrub (Adenocarpus viscosus), and the effect of fire on the spatial pattern of soil N and P pools in a forest of the Canary Islands (Spain). We hypothesized (1) higher spatial dependence of NO₃-N beneath A. viscosus than beneath P. canariensis canopies, (2) higher spatial ranges of the studied variables under the influence of P. Canariensis, and (3) more clumped spatial patterns for all the variables shortly after the wildfire. To test these hypotheses, we collected soil samples under isolated mature individuals of each species one month before and one month after a wildfire, and we examined the spatial patterns of all the soil variables using geostatistical analyses.

Results/Conclusions

We did not find higher spatial ranges under the pine than under the legume canopy, but the spatial dependence of NO₃-N was twice as high beneath the legume as under the pine. Except for dissolved organic-N (DON), we observed an increased spatial dependence and range after the fire for all studied variables in the P. canariensis plots (large individuals). However, in plots with A. viscosus (smaller individuals), only PO₄-P and DON spatial patterns were affected by the fire. The results of this study indicate that these two species may have clearly different effects on the spatial distribution of soil N and P in this community.  In addition, the physical structure of the plant community may determine the new spatial structure of soil nutrients after fire with a more clumped distribution around large surviving trees and shrubs.