Fine root characteristics in kauri-dominated forests affected by Phytophthora agathidicida

Abstract

Fine roots (≤ 2 mm diameter) are a small but functionally important portion of belowground biomass in forest ecosystems, contributing to net primary productivity, carbon transfer, and nutrient cycling. Yet, fine roots are poorly quantified in Southern Hemisphere forests. Kauri (Agathis australis), an ecologically and culturally significant species, is threatened by the pathogen Phytophthora agathidicida (PA) which causes kauri dieback. Considering infection begins within living tissue of the fine roots, knowledge on fine roots is crucial to understanding tree responses to PA. Soil core samples were used to assess standing fine root density (mg cm-3) at 22 trees across six plots in the Waitākere Ranges Regional Park. Ingrowth cores (N = 12) were used to estimate fine root production (mg cm-3 year-0.5) and turnover (year-0.5). Tree and soil properties were assessed at each tree. Soil cores and ingrowth cores were processed by extracting fine roots and sorting them into four root groups: kauri, non-kauri, unidentified, and leftovers. Root chemical analyses (kauri roots only) included total C, total N, C/N ratio, and micronutrients. Mean total standing fine root density (16.94 mg cm-3 ± 3.59 mg cm-3) was greater than global estimates. Fine root production (7.41 mg cm-3 year-1 ± 1.48 mg cm-3 year-1) and turnover (3.45 year-1 ± 1.79 year-1) were within the range of global data. A positive correlation was found between standing fine root density and organic humic layer (Oh) temperature, and between fine root production and kauri fine root C/N. Standing fine root density was significantly greater in trees where PA had not been detected (P = 0.01). Fine root production and turnover was significantly greater (P = 0.03) in non-kauri fine roots than in kauri fine roots. The findings suggest that PA results in a reduction of kauri fine roots while non-kauri fine roots are not affected by the pathogen. This may have considerable effects on the functioning of kauri-dominated forest ecosystems.