“Understanding How Tropical Tree Crowns React to Environmental Changes: Insights from Scale-Dependent Temperature Responses” – Communications Earth & Environment

Temperature Differences in Leaves

We noticed that there are bigger temperature differences between species when we looked closely at leaves. After adjusting for crown structure influences, these differences became clearer. For instance, ‘ōhi’a leaves were about 0.59°C hotter than koa leaves, while pilo leaves were even hotter, averaging 1.26°C above koa leaves across various times of day. This indicates that leaf temperature variations are more pronounced than those at the crown level.

Focusing on ‘ōhi’a (M. polymorpha) and koa (A. koa), we found that direct sunlight caused ‘ōhi’a leaves to stay consistently warmer during the day. In our study area, around 60% of the canopy was made up of ‘ōhi’a crowns, while 24% consisted of koa. The temperature differences we measured varied throughout the day, but ‘ōhi’a maintained a higher average. This aligns with the fact that ‘ōhi’a leaves tend to face the sun more directly compared to the taller, more rugged koa crowns.

Pilo (C. rhynchocarpa) showed the greatest temperature difference compared to koa, highlighting that leaf-level measurements can reveal nuances missed at the crown level. The varying temperatures of leaves suggest that studying just the crowns can lead to underestimations of temperature stress that leaves experience, which is important for assessing their health and photosynthesis capabilities.

Trade-offs in leaf traits and crown structures impact how these trees manage temperature. For example, ‘ōhi’a and ‘ōlapa (C. trigynum) were similar in leaf temperatures, potentially due to ‘ōlapa’s larger leaves being balanced out by its rougher crown, which cools more effectively. While it seems that different tree traits contribute to temperatures, the overall crown temperatures showed less variation.

Variations Within Species

We also found differences in temperature variations within species. This means that not all tree crowns have the same temperature distribution. For instance, ‘ōhi’a presented different temperature patterns between leaf and crown levels. It showed that leaf temperatures varied less than crown temperatures, suggesting that while leaves respond consistently, whole crowns can vary significantly based on their structure.

Effects of Species on Temperature Differences

At the leaf level, species identity plays a bigger role in explaining temperature differences compared to the crown level. When we included environmental factors like sunlight and wind in our models, the influence of species increased significantly at the leaf scale. This illustrates how diverse species respond differently to their surroundings, particularly at the leaf level.

Drivers of Temperature Differences

Across the Laupāhoehoe forest, net radiation emerged as the most critical factor affecting temperature differences. Taller trees generally had temperatures closer to the ambient air, which means they coped with heat more efficiently. This influence was stronger for leaves than crowns, suggesting that the structure within the canopy plays a vital role in managing temperature.

While net radiation remained key, we found that tree height significantly impacted temperature differences. It became clear that how leaves are arranged and exposed to sunlight determines their temperature, rather than just the total number of leaves.

Our findings show that both individual differences in species and their collective interactions with environmental factors shape temperature responses within forests. These insights underline the need to study tree responses at various scales, from individual leaves to entire canopies. Understanding these dynamics is crucial for predicting how different species will cope with climate change.