Tropical Montane Cloud Forests Have High Resilience to Five Years of Severe Soil Drought

TL;DR

Tropical cloud forests in Peru showed resilience to severe soil drought over five years, maintaining carbon storage despite reduced rainfall.

Contribution

A 5-year soil drought experiment in a Peruvian cloud forest reveals unexpected resilience and adaptive physiological responses.

Findings

Soil drought reduced gross primary productivity but not net primary productivity due to matching declines in autotrophic respiration.

Trees maintained hydraulic stability and increased wood density and starch storage to cope with drought.

Fruit production and nutrient uptake declined, signaling potential long-term impacts on reproduction and growth.

Abstract

Tropical montane cloud forests (TMCFs) are globally important ecosystems, acting as large carbon sinks and supporting exceptional biodiversity. However, climate‐driven declines in rainfall threaten these forests, but their responses to long‐term soil moisture deficit remain poorly understood. We implemented a 5‐year throughfall exclusion (TFE) experiment in a Peruvian TMCF, reducing soil moisture by 69.1% across a 0.09 ha plot. We compared the full carbon cycle budget, and surveyed tree physiological traits linked to hydraulics, metabolism and nutrients in the TFE plot and an adjacent, unmodified control (CON) plot. Soil drought reduced gross primary productivity by 4.24 ± 1.97 Mg C ha−1 year−1 but did not change net primary productivity because of an equivalent 3.38 ± 1.42 Mg C ha−1 year−1 decline in autotrophic respiration. Net ecosystem exchange also remained unchanged over 5 years of soil drought. Trees did not change xylem conductivity, hydraulic safety margins or photosynthetic capacity in the TFE, but did have 0.027 ± 0.011 g cm−3 denser wood and 4.58% ± 1.03% higher trunk starch concentrations. These results suggest that trees in TMCF avoid hydraulic failure and carbon starvation under sustained soil moisture drought via metabolic downregulation, resource conservation and non‐structural carbohydrate storage. However, reduced uptake of nutrients (nitrogen, phosphorus, calcium) and 90.6% ± 29.8% decline in fruit production may impact future growth and demography. Our findings demonstrate surprising resilience of TMCFs to sustained, severe soil drought but highlight potential impacts on nutrient cycling and reproduction under climate change. Understanding the impacts of soil drought in conjunction with other climatic changes (e.g., fog reduction, temperature increases) is needed to fully assess the resilience of TMCFs to climate change.

A 5‐year throughfall exclusion experiment in a Peruvian tropical montane cloud forest reduced soil moisture by 69%, leading to lower photosynthetic inputs and autotrophic respiration but maintaining net primary productivity and carbon sink capacity. Trees responded through metabolic downregulation, reduced fruit production, increased wood density, and starch accumulation while maintaining hydraulic stability.

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