Huge Contributions of Forest to the Present

Water Cycle

－ Findings from Recent Hydro-meteorological

Studies －

5th WWF in Istanbul

March 20, 2009

Makoto Tani

Kyoto University

http://www.shinrin-ringyou.com/forest_japan/jinkou_tennen.php

Forest influences on the water cycle are highly evaluated in general.

Science tends to reject the exaggerated effects. Debates are

continued for example about the flood control effect:

Introduction

In this presentation, we wish to reconsider the forest influences

from the following geographical classification.

- region with a high tectonic activity and heavy rainstorms

- a stable continent

Bradshow et al. (2007) made a statistical analysis and detected a forest influence on the flood frequency. Albert et al. (2009) reanalyzed the data and drew a negative result. Laurence (2007) raised a question about the forest effects on floods driven by extreme events such as typhoons.FAO-CIFOR (2005) suggested the mechanism of forest soil acting as sponges was negligible for these extreme events.

Due to a high tectonic activity, forest is exposed to threat of ground collapses.

Steep

landscape

Landslide

Findings from Regions

with a high Tectonic

activity

Runoff responses are highly controlled by the water storage behaviour within the soil layer.

Bare-land in a granite mountain

i

0 24 48 72 96 120 144 168 192 216.001

0.01

0.1

1

10

Bareland

Reforested

Tani, 2005 Storm runoff comparisonHou

r

Ru

no

ff

m

m/h

In our human history, we can often see severe forest uses resulting in

bare lands. An example occurred in Shogun era (1600-1868) in

granite mountains in Japan.

Once either forest-soil system or bare-land is established, each ground cover system persists stably. Soil produced by weathering is eroded every year on a bared hillslope, whereas thick soil is retained for long years on a forested hillslope.

A tree root system protects the soil from frequent erosion and contributes to a stabile river-runoff regime. 　

Weathering

Tectonicuplift

Heavy storm

Forest withsubsurface flow

Bareland withsurface flow

Terrace work needed

The relationship between tree and thick soil with water buffering potential may be interdependent.

A hard task with terrace work is needed for reforestation from the bare land to forest.

Biwako office of Ministry of Construction, 1985

Even on a hillslope with virgin forest, the soil layer is sometimes

eroded by landslide occurrences. However, the soil layer is again

developed because soil supplied from the surrounded area and

from the underlying weathered bedrock is trapped by tree roots

(Shimokawa, 1984). On the other hand, once a bare land is created, this bared

condition is fixed by the frequent soil erosion because the spatial

scale of bare land is larger than that of landslide and beyond one

unit of hillslope.

Forest and bare land are both sustainable under the same

climate.

What is important in regions with a high tectonic activity?

This strongly demonstrates an interdependent

relationship of forest and soil, and to evaluate a

threshold between the sustainability of forest

and falling down to bare land is the most

important for the forest and watershed

management.

Forest

Bare land

Threshold

Biome of the World http://www.digitalpencil.org/Projects_AllGrades/aroundtheworld/biomes/biomes.aspx?GS=431&GK=10&S=5&C=19&Sd=16&E=1を使用

67

8

13

2 Distance from seacoast km

Distance from seacoast km

Ann

ual rainfall m

mA

nnual rainfall

mm

12

3

67

8

Rain does not decease

Rain decrease

（ A. M. Makarieva and V. G. Gorshkov: Biotic pump of atmospheric moisture as driver of the hydrological cycle on land, Hydrol. Earth Syst. Sci., 11, 1013–1033, 2007. より引用 , 一部改変。）　

There are two types of rainfall

distributions along the distance from the

seacoast in stable continents. Why is the

humid climate for forest in inland area?

Findings from stable continents

The relationship between forest and humid climate may be interdependent.

Evapotranspiration is lower than rainfall on the land because the water balance shows the river runoff is their difference. Both vapor from the sea and evapotranspiration from the land are needed for much rainfall.

Distance from seacoast

Annual rainfall

The large rainfall amount in an inland area is strongly supported by the water recycle system driven by a large evapotranspiration from the forest.

Vapor from sea

River flow

Water cycle between land and sea

Rain>ET Rain<ET

Recycle system

Makarieva and Gorshkov, Global Change Biology 2007

The climate will become much dryer after a continental-scale deforestation because the dynamic water cycle stops.

Forests pump up deep water even in a dry season.

Dynamic pump Poor pump

Simulation result about the effect of

root depth on transpirationTanaka K. et al., JGR 2004

High evapotranspiration during a

late dry season in tropical monsoon

forest.

R

ainf

all

Eva

potr

ansp

irat

ion

mm month-1

Month0 3 6 9 12

100

200

300

400

Tanaka N. et al., Agric. For. Meteorol., 2008

Dry season

Max. ET

Hydrological studies support the recycle system.

A continuous evapotranspiration in a dry season supporting a

dynamic water recycle between land and the atmosphere is only

possible based on the pumping-up of water from deep zones by

tree roots.

Therefore, once forest is destroyed, the non-forest condition

is also fixed due to another dryer climate. Both types of

vegetations are sustainable under these different climate

conditions.

What is important in a stable continent?

This strongly demonstrates an interdependent

relationship of forest and climate, and to

evaluate a threshold between the

sustainability of forest and falling down to

non-forest condition is the most important for

the forest and watershed management.

Forest

Non forest

Threshold

The both relationships for the forest-soil system and the forest-climate system are interdependent and contribute to a present stable river-runoff regime and a present humid climate.

Although forest contributions are huge, forest products are continuously needed for humankind. Minimum disturbances by forestry cannot be avoided on earth.

Huge forest contributions

How to keep the huge contribution and the sustainable forest management is critical now.

Virgin forest Manmade forest Devastation

failure

SuccessWe are here

We should promote the sustainable forest management by making

active use of these scientific findings in order to maintain the present

water cycle.

Summary

A local-scale forest disturbance may cause severe floods.

A continental-scale forest extinction may destroy the humid climate.

The interdependent system between forest and its environment is a

key controller.

Forest products are needed for humankind. The most important

subject is how to keep a huge contribution of the interdependent forest

system to the present water cycle regardless of the increasing demand

of forest products.

We are standing at the threshold.

I greatly appreciate the support and preparation of this

presentation by Forestry Agency Japan.

Thank you

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