The reasons that caused land degradation are shown in the pie chart.

One problem here is that reason is closely synonymous with cause, and so it’s as though you’re saying:

• ..the causes that caused land degradation!”

Another problem is that reason has stronger collocates:

• The reasons for land degradation are shown in the pie chart.
• reason(s) + for + effect(s) noun
• The reasons why land became degraded are shown in the pie chart.
• reason(s) + why + effect(s) clause

Notice that in these last two examples there is no cause effect signal (cause). If you use a cause effect signal then you need a more suitable substitute word for the cause or reason:

• The factors that caused land degradation are shown in the pie chart.
• The conditions that led to land degradation are shown in the pie chart.

In your IELTS Task 1 essay you will go on to name and describe factors and conditions, and this is easier to do elegantly if you call them ‘factors’ and ‘conditions’ than if you call them ‘reasons’.