Forest health and resilience modulated by soil microbes
Forest soil microbes play a critical role in the modulation of tree health and disease expression. Tree disease is expressed when environmental conditions favour pathogens and insect pests. The kauri forests of New Zealand are being compromised by a novel soil-borne pathogen (Fig. 1), and radiata pine forests are being blighted by an exotic foliar pathogen.
Disease impacts on trees could increase in the future through a range of interrelated ecological processes, from individual tree scale to entire forest ecosystems. These changes include:
- increasing atmospheric CO2 levels
- global temperature rise
- changes in global rainfall distribution
- increased deposition of nitrogen and sulphur generated by human activity
- habitat fragmentation and loss
- increased frequency of biological invasions and other processes threatening biodiversity
When viewed holistically, pathogens and plants exist within a tukutuku o te ora (web of life) – a wider environmental network of interacting species. In these networks, the interactions between species contribute to the natural regulation of disease. An intrinsic part of this web is the tree microbiota, which can be viewed as part of the tree’s whakapapa.These microbial communities (Fig. 2), the outcome of millions of years of coevolution with their partners, play a key role in long-term tree survival by modulating resistance to stressors, including pathogens.
Plant and ecosystem health is controlled by complex interactions between the plant, the environment, and associated communities of organisms (termed the “phytobiome”). Management strategies that acknowledge these complex interactions are more likely to be successful than those that only focus on one part of the interaction. Advances in genetic technology are helping unlock the complexity that exists in nature and deliver a profile/inventory of the diversity of natural samples. This new technology allows us to establish quantitative linkages between species diversity and functional diversity. Understanding these linkages, enables us to begin deciphering how the genetic machinery of the entire soil–microbe–plant system responds to changes and expresses a particular phenotype (including resistance to disease) under a given set of conditions (e.g. drought, exotic/pathogen invasion).
With human-driven global environmental climate change occurring with unnatural rapidity, the concern is that the short generation time of pathogens will allow them to adapt quickly to changes in distribution (e.g. by accidental introductions) and/or rapid changes in physiology (with or without host switches), while trees will adapt much more slowly due to their longer generation times. This inconsistency is predicted to result in phenotype-environment mismatches, with the resulting kaore i ōrite (imbalance) leaving trees more vulnerable to disease. However, there is one facet of a tree’s phenotype that may evolve as quickly as pathogens and thus have the ability to protect them – their soil microbiota. This raises two critical questions:
- Can the soil microbiota respond quickly enough to provide sufficient resilience to their tree hosts to maintain health when faced with pathogen pressures caused by climate change?
- If not, is there the potential to manage tree-soil microbiota in a way that will help them maintain the natural resilience to disease threats that their whakapapa provides?
These are questions best addressed by a Māori holistic viewpoint together with principles of ecology – notably that biodiversity increases the resilience and stability of ecosystem functions: mahi ngātahi – “resilience through collaboration” (Fig. 4).
STAN BELLGARD, MAJ PADAMSEE AND GWEN GRELET – LANDCARE RESEARCH