Inexpensive throughfall exclusion experiment for single large trees

Premise: Drought-induced tree mortality is an emergent threat to forests worldwide, particularly to large trees. Drought-manipulation experiments involving throughfall exclusion (TFE) tend to focus on large plots that can be expensive to establish and maintain and may be unsuitable for large trees or indigenous forests. We set out to establish a relatively inexpensive TFE method in a natural forest with large trees.

Methods: We designed a novel TFE method and installed it in the Waitākere Range of West Auckland, New Zealand, to study the southern conifer kauri (Agathis australis) under long-term simulated drought. We measured fluxes of water (sap flow) and carbon (stem increment and litterfall) as indicators of drought effects.

Results: Throughfall was cut off to a 22.25-m² area around individual boles, causing reduced soil moisture and reduced sap flow in droughted trees.

Discussion: Our new TFE method centered on individual, large trees in native forest and is highly customizable to fit other forest and species types. It can be used to assess physiological responses to drought of individual trees independent of stem size.